The invention relates to optical devices with optical properties that can be tuned.

Light modulation functionalizes many wide-spread applications, from data storage to displays to lenses. Optical coatings, which are easy-to-manufacture multi-layered stacks of dielectric and metallic thin films (1), are the leading tools in achieving such

modulations. Such stacks utilize the principle of thin film interference, i.e. phase driven constructive and destructive interference of light waves, and enable a multitude of optical effects, such as anti -reflection, radiative emission, absorption and more. In their simplest form, optical coatings are Fabry-Perot type cavities that comprise a quarter wave thick dielectric C 4 _{n >} where l is wavelength and n is film’s real part of the refractive index), typically oxide, sandwiched between full or partial reflectors, which could be either metals, other dielectrics or their combinations (2).

However, such coatings, due to close to unity refractive index of the dielectrics, tend to be micron(s) thick, thereby material intensive. Moreover such coatings are passive, meaning they lack tunability due to their static material properties, which limits their usefulness in many potential applications, viz. as solid-state displays, artificial retinas and smart glasses. More recently, such limitations are overcome by relaxing the constraint that the materials comprising the substrate and the thin films be purely dielectric. Katz et al. pioneered ultra-thin optical coatings (3), comprising highly lossy (absorptive) thin films, typically semiconductors, on equally lossy substrates, such as metals. Hosseini et al.

included optical tunability in the coatings by utilizing lossy chalcogenide phase change material films (4, 5) which can reversibly be switched between two solid-state phases of contrasting refractive indexes. Crucially, tunability with lossy thin films comes at cost of increased stack complexity (5-7) and spurious effects from encapsulating layers (8). In addition, lossy films poorly perform due to their intrinsically reduced quality (Q) factor (from very high optical absorption) and function only on selected substrates which are equally lossy, such as metals (2, 3).

Therefore it is an object of the invention to provide improved tuneable optical devices.

According to an aspect of the invention, there is provided a tuneable optical device comprising: a layer of a tuning material in the solid state; and a substrate supporting the layer of tuning material, wherein: a refractive index of the tuning material is continuously tuneable within a range of values by heating of the tuning material, and the refractive index is stably retained after the heating of the tuning material has ceased; and a structure of the tuning material is amorphous at each of the values of refractive index within the range of values.

Thereby, there is provided a tuneable optical device with highly tuneable properties, and reduced stack complexity. This can allow optical devices to be

manufactured which are thinner and exhibit lower loss than existing devices. In addition, the optical properties can be continuously tuned, in contrast to many existing devices, which are restricted to a limited number of different optical states. Having the value of the refractive index stably retained after setting it through heating of the material allows devices to be manufactured which have low energy consumption, as energy is only required when switching states, and not to maintain a set state.

In an embodiment, a wavelength of light for which the tuneable optical device is maximally reflective is continuously tuneable by at least 1%. A minimum level of tunability ensures that a change in the optical properties of the optical device can be achieved is sufficient for particular applications.

In an embodiment, a reflectivity of the tuneable optical device at a wavelength of light for which the tuneable optical device is maximally reflective is greater than 90%.

High reflectivity in the region of the spectrum where the optical device is designed or tuned to be reflective is desirable to maximise performance of the device. For example, if the device is used as an optical filter, high reflectivity prevents leakage through the device of unwanted wavelengths of light.

In an embodiment, a transmissivity of the tuneable optical device at a wavelength of light for which the tuneable optical device is maximally transmissive is greater than 90%. High transmissivity in a region of the spectrum where the optical device is not reflective ensures that the optical device can perform its function of reflecting particular parts of the spectrum without affecting the remaining parts or causing high optical loss. This high selectivity reduces losses and improves the efficiency of the device.

In an embodiment, the tuneable optical device transmits less than 50% of ultraviolet light incident on the device. In many applications, blocking of ultra-violet light is desirable, as ultraviolet light can be harmful to humans, and cause damage to sensitive optical devices or electronics.

In an embodiment, the thickness of the layer of tuning material is less than 500nm. Thin coatings are advantageous, as they have reduced loss and have a reduced effect on the size, and weight of the object to which they are applied. Thinner coatings also require less material to fabricate, and so can be produced more quickly and with lower cost. A particular advantage of the type of device described herein is that they can achieve high reflectivity and tuneability while using a thinner layer of tuning material than existing types of device.

In an embodiment, a thickness of the layer of tuning material changes as the refractive index of the tuning material is tuned. Optical coatings function largely due to interference effects caused by reflection at the boundaries of the material layers.

Interference effects are significantly affected by the distance between the boundaries. By causing a change in the thickness of the layer of material, as well as a change in its refractive index, the interference effects in the device are further affected, and a larger change in the optical properties can be achieved.

In an embodiment, the thickness of the layer of tuning material decreases as the refractive index of the tuning material is tuned from a higher value to a lower value.

Linking the change in thickness to the change in refractive index in this way means that the two effects are additive, and cause an increased change in the optical properties.

In an embodiment, the tuning material comprises a chalcogenide glass. These materials have been found to be particularly suitable for these applications, as they can be manufactured to exhibit continuously tuneability of their refractive index.

In an embodiment, the tuning material comprises [A] _x [B] _y , wherein A is Ge; B is S, Se, or Te, preferably Se; and x and y are such that ^ / _x is between 2.5 and 3.5, preferably between 2.8 and 3.2, more preferably 3. Selenium-based chalcogenide materials, doped or alloyed with germanium, have been investigated in particular detail, and are found to have particularly beneficial properties in terms of tuneability and low optical loss.

In an embodiment, the device comprises a plurality of heaters configured to heat a plurality of different regions of the layer of tuning material. Having a plurality of heaters allows greater flexibility in the design and control of the device, for example to only change the properties of particular regions of the material layer while other regions remain static. In an embodiment, the tuneable optical device is configured such that a temperature and a duration of heating by each of the plurality of heaters are independently controllable. Independently controllable regions make possible particular types of application, such as displays, and provide greater flexibility in the design and control of the device.

In an embodiment, the layer of tuning material has a thickness which varies across the device, and the variation in thickness comprises a regular pattern. Having a thickness which varies in a regular pattern can allow the device to act as a metamaterial, or, for example, a diffraction grating, where regular differences in thickness cause an additional optical effect. The tuneability of the material refractive index then allows these patterned devices to have controllable transmissive or reflective properties.

In an embodiment, there is provided a temperature sensor comprising a tuneable optical device as described herein, the temperature sensor configured to measure an optical property of the tuneable optical device and calculate a temperature of the tuneable optical device using the measurement of the optical property. The dependence of the device properties on temperature can allow the device to be used as a temperature sensor, which may be convenient for monitoring the environment of the device or its performance.

According to an aspect of the invention, there is provided a method of tuning the optical properties of a tuneable optical device comprising a layer of tuning material in the solid state, and a substrate supporting the layer of tuning material, wherein a refractive index of the tuning material is continuously tuneable within a continuous range of allowed values by heating of the tuning material, and a structure of the tuning material is amorphous at each of the values of refractive index within the range of allowed values, the method comprising the steps of: selecting a final value of refractive index from the range of allowed values of refractive index; and setting the refractive index of the tuning material to the final value, wherein setting the refractive index comprises continuously tuning the refractive index of the tuning material from an initial value to the final value by heating the tuning material to a predetermined temperature for a predetermined length of time.

Thereby, there is provided a method of tuning an optical device which allows its optical properties to be set to any value within a continuous range. This is advantageous over many existing devices, which can only be set to one of a limited number of different states.

In an embodiment, the predetermined temperature and predetermined length of time are dependent on the final value of refractive index. This provides a clear relationship between the desired value of refractive index, and the conditions which must be applied to the device to achieve that value.

In an embodiment, the structure of the tuning material remains amorphous during tuning of the refractive index from the initial value to the final value. By keeping the structure of the material amorphous, abrupt structural changes due to phase transitions are avoided, which permits continuously tuneability.

In an embodiment, heating of the tuning material comprises heating the tuning material using a laser. Using a laser to tune the material may be advantageous in some applications, for example where integrating a heater into the device is not practical or desirable.

In an embodiment, the tuneable optical device further comprises an electrical heater, and heating the tuning material comprises heating using the electrical heater.

Electrical heaters are a well-understood method for applying heat locally to the device, and are a convenient choice to control the properties of the optical device.

In an embodiment, the steps of selecting a final value of refractive index and setting the refractive index of the tuning material to the final value are repeated for each of a plurality of regions of the layer of tuning material. Setting the refractive index of a number of different regions allows greater flexibility in how the optical device is operated, for example by only changing the properties of some regions while leaving others static, or by setting different regions to have different properties.

In an embodiment, the method further comprises: heating the tuning material to a temperature above the melting point of the tuning material; and quenching the tuning material. Melting and quenching the tuning material provides one possible method of reversing a change in the refractive index of the tuning material caused by heating it while it is in the solid state. This allows a change in the optical properties of the tuneable optical device to be reversed, which may be particularly useful for applications such as displays or switchable optical filters.

In an embodiment, the method further comprises annealing the tuning material by: heating the tuning material to a softening temperature and not above the softening temperature, wherein the softening temperature is below the melting point of the tuning material, and a hardness of the tuning material at the softening temperature is below a first threshold value; and cooling the tuning material to a temperature at which the hardness of the tuning material is above a second threshold value. This method allows a change in optical properties of the device to be reversed, while keeping the tuning material in the solid state. This provides reversible tuneability without the need to accommodate a change in state of the material layers.

Throughout this specification, the terms‘optical’ and Tight’ are used, because they are the usual terms in the art relating to electromagnetic radiation. However, it is understood that in the context of the present specification that these terms are not limited to visible light. The invention can also be used with wavelengths outside the visible spectrum, for example infrared or ultraviolet light.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols represent corresponding parts, and in which:

Figure l is a side-schematic view of a tuneable optical device used in reflection;

Figure 2 is a side-schematic view of a tuneable optical device used in transmission;

Figure 3 is a side-schematic view of a tuneable optical device in which a layer of tuning material is used to alter the properties of a substrate acting as a waveguide;

Figure 4 is a side-schematic view of a tuneable optical device used in reflection, where the layer of tuning material has a thickness which varies across the substrate in a regular pattern;

Figure 5 shows measurements of the optical properties of tuneable optical devices;

Figure 6 shows measurements of the structural and optical properties of tuneable optical devices before and after tuning;

Figure 7 shows x-ray diffraction, Raman spectroscopy, and transmission electron microscope measurements of tuneable optical devices before and after tuning;

Figure 8 shows schematics of embodiments of tuneable optical devices suitable for particular applications, and measurements of the optical properties of the tuneable optical devices;

Figure 9 is a photograph of several tuneable optical devices with different thicknesses of tuning material layer, and different substrates;

Figure 10 shows reflectometry measurements of a prototypical prior art device comprising a layer of phase-change material; Figure 11 shows measurements of the optical properties of two tuneable optical devices with different substrates;

Figure 12 shows transmission electron microscope images of the interface between the layer of tuning material and the substrate in a tuneable optical device;

Figure 13 shows measurements of the reflectance and refractive index of several chalcogenide glasses as a function of wavelength;

Figure 14 shows schematics of Raman modes in GeSe, and Raman spectroscopy measurements of a tuneable optical device before and after tuning;

Figure 15 shows Raman spectroscopy measurements and optical micrographs of a tuneable optical device exposed to UV radiation;

Figure 16 shows images of Se-rich grains appearing in the layer of tuning material in some devices;

Figure 17 shows optical transmission measurements of a tuneable optical device before and after tuning;

Figure 18 shows simulations of the reflectance of an exemplary tuneable optical device as a function of wavelength as the thickness of its layer of tuning material is varied;

Figure 19 shows x-ray spectra of a tuneable optical device before and after tuning of its refractive index by heating;

Figure 20 is a flowchart of a method of tuning the optical properties of a tuneable optical device according to an embodiment; and

Figure 21 is a flowchart of a method in which the tuning material is heated to above its melting point and quenched after tuning of the refractive index of the tuning material to reverse the tuning of its optical properties.

Embodiments of a tuneable optical device 2 will be described with reference to Figures 1-4, which show schematically a layered structure in cross-section. The tuneable optical device 2 comprises a layer 4 of a tuning material in the solid state. The layer 4 of tuning material may also be referred to as a film, or a thin film, of the tuning material. The tuning material typically comprises an extremely loss-less chalcogenide thin film that is subwavelength thick and yet tuneable for creating accumulative interferences across a broadband spectrum. Many different materials may be used for the layer of tuning material. In an embodiment, the tuning material comprises a chalcogenide glass. The tuning material may comprise [A] _x [B] _y , wherein A is one or more element(s) selected from Al, Ga, Ge, As, Sn, Sb, S, and Se, and B is a group VI species, preferably S, Se, or Te. In an embodiment, x and y are such that the ratio ^ / _x is between 0.1 and 9. In an

embodiment, A and B are such that the tuning material is one of Sb _x S _y , Al _x S _y , Sn _x S _y ,

Ga _x S _y , Ga _x Gei- _x S _y , Ga _x Sei- _x S _y , Ge _x S _y , Ge _x Se _y , Ge _x Te _y , Se _x Xe _y , S _x Xe _y , As _x S _y , and A s _\ S e, and x and y are such that the ratio ^ / _x is between 0.1 and 9.

In an embodiment, A is Ge, B is S, Se, or Xe, preferably Se, and x and y are such that the ratio ^ / _x is between 2.5 and 3.5, preferably between 2.8 and 3.2, more preferably 3. A preferred tuning material is germanium selenide (GeSe ₃ ), and this material will be used as an example throughout the specification. However, the tuneable optical devices described herein are not limited to the use of GeSe ₃ as the tuning material.

Xhe optical device 2 further comprises a substrate 6 supporting the layer 4 of tuning material. Xhe substrate 6 may comprise a variety of materials, for example, silicon, platinum, gold, quartz, or aluminium foil. Where the optical device 2 is to be used in transmission, for example as an optical filter or smart window, the substrate 6 may be transparent. An example of such an embodiment is shown in Figure 2, where incident light 10 passes through the layer of tuning material 4 and the substrate 6. In other applications, the substrate 6 may be substantially or completely opaque, or may be reflective. An example of such an embodiment is shown in Figure 1, where the incident light 10 passes into the layer 4 of tuning material, but is reflected at the boundary between the layer 4 of tuning material and the substrate 6.

In some embodiments, the tuneable optical device may comprise one or more further layers. For example, the tuneable optical device 2 may comprise a reflective layer between the layer 4 of tuning material and the substrate 6. In an embodiment, the tuneable optical device 2 comprises a heater layer used to heat the layer 4 of tuning material. Xhe tuneable optical device 2 may also comprise a capping layer, or protective layer above the layer 4 of tuning material to protect it from damage during use of the tuneable optical device.

In some embodiments, the tuneable optical device 2 is configured such that light travelling through the device does not pass into or out of the device through the layer 4 of tuning material. Xhis can be used in applications such as tuneable optical waveguides. An example of such an embodiment is shown schematically in Figure 3. Incident light 10 passes through the substrate 6, which is configured to act as a waveguide. The layer of tuning material 4 is deposited onto the substrate 6, but light does not directly pass through the tuning material. Such an embodiment may be particularly useful where the waveguide acting as the substrate 6 forms part of a resonator, as tuning the optical properties of the layer 4 of tuning material can allow the resonant frequency of the resonator to be changed. This is discussed further in relation to Figure 8 below.

The tuning material may be deposited onto the substrate 6 using any suitable known techniques, such as RF-sputter-deposition. The exact thickness of the layer 4 of tuning material will depend on the specific application for which the optical device is to be used, and the range of wavelengths of light it is to be used with. However, in general the devices disclosed herein are able to provide the same level of tunability as prior art devices, but with smaller thickness. In an embodiment, the thickness of the layer 4 of tuning material is less than 500nm, optionally less than 400nm, optionally less than 300nm, optionally less than 200nm. This reduced thickness relative to prior art devices provides advantages of reduced stack complexity, and thereby manufacturing complexity, and reduced mass of the tuneable optical device.

A refractive index of the tuning material is continuously tuneable within a range of values by heating of the tuning material. This effect is due to dynamic interferences created in the thin films of, for example, germanium selenide, from non-volatile structural changes in its atomic structure. Such interferences are the result of a unique coupling between the structural and optical properties in these materials. Heating of the tuning material to tune its optical properties may be referred to as annealing or thermal annealing of the tuning material. The state of the tuning material after it has been deposited during manufacture, but before any tuning of its optical properties has occurred may be referred to as the as-deposited state. In an embodiment, the refractive index of the tuning material is settable by the heating of the tuning material to any value within the range of values. This provides for continuous tuning of the optical properties of the tuneable optical device, which makes the devices described herein substantially more flexible in their tunability than prior art devices using phase-change materials.

Exemplary devices display a range of colours, which are dependent on both the thickness of the layer 4 of tuning material and the underlying substrate. Thermal annealing of the as-deposited layer 4 of tuning material, for example using a hot plate under ambient conditions, brings forth a significant colour shift. This can be seen from Figure 9, which is a photograph of several exemplary devices comprising GeSe ₃ as the tuning material deposited onto different substrates. Figure 9 shows the devices before and after annealing at 350°C for 5 minutes in room conditions. The colouring and change in colour from annealing are substrate dependent.

Similar colour changes are observed in prior art devices using phase-change materials, which also commonly comprise chalcogenide glasses. In devices comprising phase-change materials, the colour change occurs due to a reversible amorphous- crystalline phase transformation of the thin film material.

In-situ reflectometry measurements of an exemplary device comprising GeSe ₃ as the tuning material were made on a heated stage. The onset for a change (optical reflection) is observed to occur at 257°C. The change is gradual and does not seemingly saturate with temperature. This is unlike in prior art devices using phase-change materials, where the change is instantaneous and bi-state. This can be seen from the measurements in Figure 10, which shows reflectometry measurements of such a prior art device comprising a GeiSbiTes thin film on a p-doped Si substrate. The instantaneous change in the reflection at 150°C seen in Figure 10 indicates the amorphous to crystalline phase transformation.

Examples of reflectometry measurements on a tuneable optical device are given in Figure 5a which shows the reflection change of a device comprising a 60nm GeSe ₃ thin film on a p-doped silicon substrate, as a function of temperature and time. The change is gradual and non-volatile. Reflection change is translated as colour change of the film, which is highlighted in the inset.

Figure 5b is a photograph of five different exemplary devices comprising GeSe ₃ films of varying thickness on p-doped Si substrate. The devices exhibit a range of colours depending on the thickness of the layer 4 of tuning material. One part of each device is thermally annealed under ambient conditions on a hot plate at 370 °C for 6 minutes. The corresponding change in reflected colour is quite dramatic. Thermal annealing of the devices beyond the onset temperature induces a significant change in the device colours, visible when incident‘white’ light is reflected back. The colour variations illustrate that such devices are suitable for a wide range of tuneable optical and photonics applications.

Specific example applications will be discussed further below. Figures 5c and 5d show reflectivity measurements on some exemplary devices comprising sputtered films of tuning material before and after thermal annealing, respectively. Distinct optical resonances, indicated by the peaks, that red shift

proportionally with the film thickness are evident in both figures. However, the resonances are blue-shifted by as much as 27 % from annealing, as shown in Figure 11. Figure 11 shows reflectivity measurements of a 90 nm thick GeSe ₃ film on silicon and platinum substrate. Annealing at 360°C in ambient conditions for 6 mins results in the blue-shift of the peak by ~27 %

Specifically, Figure 5c shows the reflection spectra of as-deposited GeSe ₃ thin films of varying thickness on a p-doped silicon substrate. The reflection spectrum displays peaks in reflectivity at particular wavelengths. These act as narrow-band filters, which allow the tuneable optical devices 2 to be used to reflect particular wavelengths of light. It is advantageous to provide high reflectivity at these peaks so that filters maximally reject the undesired wavelengths. In an embodiment, a reflectivity of the tuneable optical device 2 at a wavelength of light for which the tuneable optical device 2 is maximally reflective is greater than 90%, optionally greater than 95%, optionally greater than 97%, optionally greater than 99%.

The peak positions, which imply optical resonances, vary proportionally with the film thickness. Figure 5d shows the reflection spectra of the same films as measured in Figure 5c after thermal annealing at 370°C under ambient conditions for 6 minutes. The peaks blue shift significantly from heating, in accordance with the colour change observed after heating in the devices shown in Figure 5b. In an embodiment, a wavelength of light for which the tuneable optical device is maximally reflective is continuously tuneable by at least 1%, optionally at least 2%, optionally at least 5%, optionally at least 10%.

The magnitudes of these peak shifts are significant compared to that achievable in prior art devices, particularly for an optical cavity that comprises only a single thin film. Figure 5e shows a graph illustrating the magnitude of the peak shifts for each exemplary device shown in Figure 5b after thermal annealing of the as-deposited thin films at 370°C for 6 minutes in room conditions.

All of the peak shifts are non-volatile, meaning they can potentially last indefinitely after the thermal exposure is removed. This means that the refractive index of the layer of tuning material is stably retained after the heating of the tuning material has ceased. In an embodiment, the refractive index is stably retained for at least one month, optionally at least three months, optionally at least six months, optionally at least one year.

The optical transmission spectra of an exemplary device comprising a 72 nm GeSe ₃ thin film on a quartz substrate before and after annealing at 370°C in ambient conditions is shown in Figure 6a. The film is highly transmissive, and the transmission is observed to increase from thermal annealing. The annealing drives a non-volatile change, making the film more transmissive across all measured wavelengths. In an embodiment, a

transmissivity of the tuneable optical device at a wavelength of light for which the tuneable optical device is maximally transmissive is greater than 90%, optionally greater than 95%, optionally greater than 98%. In an embodiment, the tuneable optical device transmits at least 50% of infrared light incident on the device, optionally at least 70%, optionally at least 80%, optionally at least 90%.

The device comprising a film with a direct band-gap of 2.63 eV (derived from transmission measurements) is absorptive in the ultraviolet (UV), resulting in substantial drop in the otherwise high transmission of the quartz substrate. In an embodiment, the tuneable optical device transmits less than 50% of ultraviolet light incident on the device, optionally less than 30%, optionally less than 20%, optionally less than 10%.

The fact that the film is absorptive in the UV and transmissive in the visible and infra-red (IR) has important technological relevance and is discussed further later. It is further demonstrated by the measurements shown in Figure 17, which shows optical transmission through a 25 nm GeSe ₃ film coated quartz substrate. The film is very transmissive in the visible and infrared both before and after annealing, while the UV is significantly absorbed and blocked by the film.

Figure 6b illustrates the refractive indexes of an exemplary tuneable optical device comprising a 25 nm GeSe ₃ thin film in its as-deposited and annealed states. The wavelength dependent refractive index is a complex entity with a real part n (the measure of the phase velocity of a wave) and an imaginary part k (the measure of optical absorption, also called the extinction coefficient). In the visible region of the spectra (400- 800 nm), the n of the film peaks at 2.94, where k is equal to 0.5. The high n enables ultra- thin film thickness for resonances, down to ^/ i while the low k , that becomes zero at 540 nm, enables phase accumulation in the film without notable optical losses, thereby yielding high Q factors. Annealing causes a blue shift in the refractive indexes (both the n and k decrease), resulting in low optical absorption, in agreement with Figure 6a. In an embodiment, the refractive index (the real part, the imaginary part, or both) is continuously tuneable by at least 0.5%, optionally at least 1%, optionally at least 2%, optionally at least 5%, optionally at least 10%. In an embodiment, the size of the range of values of refractive index (the real part, the imaginary part, or both) over which the refractive index of the tuning material can be continuously tuned is at least 0.01, optionally at least 0.02, optionally at least 0.05, optionally at least 0.1, optionally at least 0.2.

In an embodiment, a thickness of the layer 4 of tuning material changes as the refractive index of the tuning material is tuned. This is due to a change in the physical volume of the material from annealing correlated with the change in refractive index. In an embodiment, the thickness of the layer 4 of tuning material decreases as the refractive index of the tuning material is tuned from a higher value to a lower value. Such a decrease can be seen in Figure 6c, showing typical atomic force micrographs of a GeSe ₃ thin-film device before and after thermal annealing at 370°C on a hot-plate, in ambient conditions.

In an embodiment, a change in the refractive index of the tuning material of 5% is accompanied by a change in the thickness of the layer of tuning material of at least 10%, optionally at least 15%, optionally at least 20%. In the specific example shown in Figure 6c, the thickness of the film shrinks by 22 % from thermal annealing beyond the onset temperature. This represents a significant change in thickness of the layer 4 of tuning material in such an embodiment.

In an embodiment, the percentage decrease in the thickness of the film is substantially independent of the unannealed film thickness. Exemplary measurements of such embodiments are shown in Figure 6d, which is a scatter plot of the percentage shrinkage of the GeSe ₃ thin-film of several exemplary devices as a function of its thickness. Shrinkage magnitude is independent of the film thickness. The inset of Figure 6d compares the percentage thickness shrinkage in an embodiment in which the layer 4 of tuning material comprises GeSe ₃ against prototypical chalcogenide glasses Ge?Sb?Te _5, Ge ₄ oTe ₆ o, and AglnSbTe used in prior art devices, deposited and annealed under similar conditions (9, 10). The shrinkage in GeSe ₃ is significantly higher than in the other systems.

In an embodiment, annealing driven changes in the optical properties of the thin films are dependent on the thickness of the film. This is shown for exemplary devices comprising GeSe ₃ in Figure 6e, which is a scatter plot highlighting the changes in optical properties of the GeSe ₃ films from annealing, as a function of film thickness. These values correspond to wavelengths at which the difference in the extinction coefficient & is the greatest. Unlike shrinkage, there is a clear dependency of the change in refractive indexes as a function of film thickness. The inset illustrates a comparison of maximum extinction coefficient in prototypical chalcogenide glasses used in prior art devices in the visible spectra, highlighting the very low magnitude of optical absorption in GeSe ₃ films.

Without wishing to be bound by theory, the inventors believe that the mechanism which permits the high magnitude of the peak shifts in the tuneable optical devices described herein can be explained by approximating the device as a Fabry-Perot type cavity (4). This is depicted in Figure 6f, which is a schematic illustrating the incident light from air being reflected from a device comprising GeSe ₃ films with thickness h and refractive index n. Optical resonances supported in the film (l) are a function of both film thickness and refractive index, and vary proportionally. The resonance condition is a function of the thickness of the layer 4 of tuning material and the refractive index of the tuning material, and at first approximation follows the relation l = h X 4n. Annealing results in a non-volatile decrease in both h and n of the film, which concomitantly switches the resonances supported by the film.

Thermal annealing by heating the layer 4 of tuning material results in a decrease in its refractive index, and also in some embodiments, the thickness of the layer of tuning material. The summation of these effects drives large blue shifts in the resonance conditions, as has been observed in measurements of exemplary devices, such as shown in the figures.

Similar devices, which do not represent embodiments of the tuneable optical devices disclosed herein, were made with lossy GeiSbiTes (GST), also a chalcogenide glass, thin films on an Si substrate. The absence of optical resonances, as well as the absence of thermally driven shifts in the spectra, was clearly observed. Measurements of these devices are shown in Figure 13, where Figures 13(a) and 13(b) show normalized reflection spectra of 20 nm and 37nm GST thin film on a p-doped Si chip, respectively.

No resonance peaks can be observed due to the lossy nature of GST. Annealing at 360°C also bring no significant change in the spectra. The films were sputter-deposited under exactly similar conditions as those used to deposit GeSe ₃ films for the tuneable optical devices. Figures 13(c) and 13(d) show a comparison of the real ( n ) and imaginary (k) components of the complex refractive index of GeuSe- against prototypical chalcogenide glasses: Ge22Sb22Te55, Ag5.4hi4.4Sb63.5Te25.6, and Ge4oTe ₆ o.

However, we do note from Figure 5e that a linear relation between the degree of shift and materials property change for GeSe ₃ films does not exist. Without wishing to be bound by theory, the inventors believe this implies that non-linear optical effects likely play a role. The effect could also be associated with spurious compounds that form at the interface between the film and the substrate due to interfacial reactions. These can be observed on the TEM micrographs highlighting the interface between the film and the silicon substrate shown in Figure 12. Regardless, the fact that the films can be tuned significantly in a loss less fashion, can be applied inexpensively due to simple stack design directly for a range of optical and photonic applications (11).

In order to understand the observed changes in the optical and physical properties, the materials’ behaviour at the atomic and molecular scale was studied. It was found that unlike phase change materials, the aforementioned changes in the optical and structural properties in the GeSe ₃ thin films are not from simple amorphous to crystalline phase transition (10). Indeed, for the temperature the devices are annealed to, no crystallization event is observed on the X-ray diffraction and Raman Spectroscopy studies, or even at the nanoscale on TEM measurements. Figure 7a shows X-ray diffraction scans using Cu-K _a radiation of a 1 pm thick GeSe ₃ film on a silicon substrate, in as-deposited form, and after annealing at different temperatures of 290, 330, and 370°C. No signature peaks indicating crystallization are evident. Figure 7b shows TEM diffraction patterns of a 160 nm thick film in as deposited and annealed conditions. The halo diffraction pattern confirms the amorphous nature of the film. It is therefore a feature of embodiments of the tuneable optical device that a structure of the tuning material is amorphous at each of the values of refractive index within the range of values. The fact that the structure remains amorphous, rather than undergoing a distinct phase change at a particular temperature, contributes to the continuous tunability of the optical properties of the tuneable optical devices.

Without wishing to be bound by theory, the inventors hypothesize that the thin films of tuning material used in the tuneable optical devices undergo structural relaxation or reordering at the atomic scale, towards a more stable amorphous phase configuration upon annealing. This structural relaxation and reordering leads to the changes in optical properties and the changes in film thickness observed in some embodiments. Further discussion of the results leading to this conclusion is given under the heading“Structural experiments” below.

Exemplary applications of the tuneable optical devices disclosed herein are now discussed, which make use of the coupled optical and physical changes in the lossless GeSe ₃ thin films.

To this end, there is provided a method of tuning the optical properties of a tuneable optical device 2 comprising a layer 4 of tuning material in the solid state, and a substrate 6 supporting the layer of tuning material, wherein a refractive index of the tuning material is continuously tuneable within a continuous range of allowed values by heating of the tuning material. As discussed above, a structure of the tuning material is amorphous at each of the values of refractive index within the range of allowed values.

As shown in Fig. 20, the method comprises the steps of selecting S10 a final value of refractive index from the range of allowed values of refractive index, and setting S20 the refractive index of the tuning material to the final value, wherein setting the refractive index comprises continuously tuning the refractive index of the tuning material from an initial value to the final value by heating the tuning material to a predetermined

temperature for a predetermined length of time. In an embodiment, the structure of the tuning material remains amorphous during tuning of the refractive index from the initial value to the final value.

In an embodiment, the predetermined temperature and predetermined length of time are dependent on the final value of refractive index. A relationship between particular final values of refractive index and the necessary predetermined times and temperatures may be determined experimentally for different tuning materials. In an embodiment, the predetermined temperature is between 50 degrees Celsius and 1000 degrees Celsius, optionally between 100°C and 800°C, optionally between 200°C and 600°C, optionally between 200°C and 400°C. In an embodiment, the predetermined time is between 30 seconds and 15 minutes, optionally between 1 minute and 10 minutes, optionally between 2 minutes and 8 minutes.

Each shift is non-volatile, which means that the final value of the refractive index of the layer 4 of tuning material is stably retained after the heating of the phase-change material has ceased. In an embodiment, the refractive index is stably retained for at least one month, optionally at least three months, optionally at least six months, optionally at least one year.

In an embodiment, the steps of selecting a final value of refractive index and setting the refractive index of the tuning material to the final value are repeated for each of a plurality of regions of the layer 4 of tuning material. This is particularly suitable for applications such as displays or decorative layers, where it is advantageous to be able to control different regions of the layer 4 of tuning material separately, for example to display different colours in different regions of the layer.

This tunability can be used for artistic effects, or in a variety of other optical devices. According to embodiments of the disclosure, there is provided an optical filter, a decorative layer, a waveguide, an interferometer, or a window comprising a tuneable optical device as described herein. Devices incorporating the tuneable optical devices 2 may operate in transmission, in which case the substrate is chosen to be a material which is substantially transparent to the range of wavelengths of light over which the device is configured to operate. For example, the substrate may transmit over 90%, optionally over 95%, optionally over 99% of light in that range of wavelengths. Alternatively, the tuneable optical device may be configured to operate in reflection, in which case the substrate may be chosen to be reflective to the range of wavelengths of light over which the device is configured to operate, or one or more reflective layers may be added to the tuneable optical device 2. In an embodiment, the tuneable optical device further comprises a reflective layer between the layer 4 of tuning material and the substrate 6.

One application of the tuneable optical device 2 is a tuneable optical coating.

Figure 8a illustrates the blue shift in the peak position of a 60 nm thin film on p-doped Si substrate from annealing on a hotplate. The inset in Figure 8a provides a closer view of the 126 nm blue shift in the peak position. Accumulative peak shifts are achieved in a non volatile manner with select annealing temperature. This means that in at least some embodiments, each value of the refractive index between the initial value and the final value is within the range of allowed values, since the refractive index can be smoothly tuned by accumulative heating. Such a behaviour can be harnessed for multilevel reflective displays. It is observed that in contrast to phase change materials used in prior art devices, which are bi-state (either amorphous or crystalline), there exists a considerable dynamic range for tuning the peak position in the layer 4 of tuning material of the present tuneable optical devices, which may comprise, for example, GeSe _3. Essentially, this dictates modulation in the reflected colour, by simply heating the film to different temperatures.

The tuneable optical device 2 may further be used in temperature sensors, for example, where colours are modulated on thermal treatment (3). In an embodiment, there is provided a temperature sensor comprising a tuneable optical device 2 as described herein, the temperature sensor configured to measure an optical property of the tuneable optical device 2 and calculate a temperature of the tuneable optical device using the measurement of the optical property. The calculation may be performed using a predetermined or previously measured relationship between the temperature to which the tuneable optical device is heated and the relevant optical property of the tuneable optical device. The optical property may be the refractive index of the tuning material, i.e. the real part of the refractive index, the imaginary part, or a combination thereof.

Such temperature sensors have industrial applications for temperature sensing at high-temperatures, above room temperature. The sensors could function by illuminating the tuneable optical device 2 with a broad range of wavelengths of light, and measuring a change in the transmitted or reflected spectrum as a function of temperature. Alternatively, they could function by illuminating the device with a narrow range of wavelengths, and measuring a change in transmitted or reflected intensity. Other operational modes may also be possible, as could be readily devised by a person skilled in the art.

Figures 8b and 8c demonstrate integration of the tuneable optical device where the tuning material comprises GeSe _3. Figure 8b is an example of an embodiment comprising a 170 nm thick GeSe ₃ thin film. In this example, the local structural modification in the GeSe ₃ films is done using laser scanning, and heating of the tuning material comprises heating the tuning material using a laser, in this case an 8 kW/cm ² , 532 nm laser. Optical absorption in GeSe ₃ films becomes notable for wavelengths < 540 nm. Therefore, an optical exposure at these wavelengths induces photo-thermal annealing, causing localized colour change of the film. As seen in the figure, the laser exposure results in photo annealing and a clear contrast in the colour can be noted after laser exposure. When the tuning material comprises materials other than GeSe ₃ , the most suitable wavelength of light for heating the tuning material using a laser will depend on the specific material chosen.

Figure 8c shows optical micrographs of a second device used as a solid-state colour pixel in a reflective display. The device is composed of an optical stack comprising 100 nm NiCr (heater)/200 nm SiN _x /20 nm SiCb/ l 00 nm Ag/45 nm GeSe ₃ , and has a resistance of 850W. This is an example of an embodiment in which the device further comprises a heater configured to heat a region of the layer of tuning material. In this embodiment, the tuneable optical device 2 further comprises an electrical heater, provided by the layer of NiCr, and heating the tuning material comprises heating using the electrical heater. The example of Figure 8c highlights the colour change between the amorphous and annealed states of GeSe ₃ films done electronically using a micro-heater (6). Herein, Joule heating is induced from microsecond current pulses (in this example, 2.5 V, 100ps square pulses) which anneal the films. For applications where transparency is crucial, such as filters, graphene can be used as the micro heater. With reversible switching, such devices could be used for multilevel optical data storage and solid-state reflective displays.

In an embodiment, the device comprises a plurality of heaters configured to heat a plurality of different regions of the layer of tuning material. Optionally, the device is configured such that a temperature and a duration of heating by each of the plurality of heaters are independently controllable. Such embodiments could be used to provide displays, either transparent or reflective. The fact that the refractive index of the tuning material is stably retained after heating has ceased makes for a very low-power display, because energy is only required to heat the material when it is switched. No power is required to maintain the display once it has been set. Correspondingly, there is provided a display comprising a plurality of pixels, wherein each pixel comprises a tuneable optical device as described herein.

Heating of the tuning material can be direct, for example through direct absorption of laser light by the tuning material, as in Figure 8b, or indirect through heating of an adjacent layer. For example, in an embodiment, the tuneable optical device 2 further comprises an absorbing layer, optionally wherein the absorbing layer is adjacent to the layer 4 of tuning material, wherein the absorbing layer is configured to absorb light at a predetermined wavelength. The wavelength of laser beam used to heat the tuning material can then be chosen more freely, as it only has to match a wavelength at which the absorbing layer absorbs light, and does not have to chosen to match the properties of the tuning material. Such a design may improve flexibility in the choice of tuning material and of laser wavelength for heating. The embodiment of Figure 8c demonstrates indirect heating using an electrical heater, where the electrical heating is of a layer below the layer 4 of tuning material in the stack. Depending on the choice of tuning material, the layer 4 of tuning material may also be heated by directly passing a current through the layer 4 of tuning material. As described above, heating of the tuning material may also be through conductive heating, by placing the tuneable optical device 2 onto a heated surface, such as a hot plate, with a controllable temperature.

Another direct application of the tuneable optical devices could be in optical filters, in particular towards filtering high energy electromagnetic radiation. Figure 8d illustrates photographs of transmissive filters comprising 72 nm thick GeSe ₃ films on quartz substrate, under both as-deposited and annealed states.

Figure 8d(i) highlights good transparency/transmission in the films, which increases significantly in the devices annealed at 370°C annealing in ambient conditions. The degree of transparency can again be accumulated as a function of temperature, with Figure 8d(ii) illustrating a relatively reduced transparency when the film is annealed to a lesser temperature (330°C). Similar devices annealed at 330°C in vacuum result in still larger transmission than un-anneal ed but smaller than the device annealed at 370°C

Most importantly, high energy radiation, such as UV can be significantly filtered or blocked by films as thin as 25 nm (see Figure 17) due to increased optical absorption from band-to-band to electronic excitations, while maintaining good visible and full infra-red transparency. UV radiation, in particular type UV-B and UV-C, harms not only biological systems, such tissues, but also many electronic and optical components, such as photodetectors, organic solar cells and lenses (28-30). Therefore, these embodiments are very relevant for applications such as photodetectors and lenses where UV could be degrading. Figure 8d(iii) demonstrates the wavelength dependent filtration of

electromagnetic radiation, which can be as high as 80%, while visible and infra-red can be appreciably transmitted.

In an embodiment, the layer 4 of tuning material has a thickness which varies across the device 2. In some embodiments, the variation in thickness comprises a regular pattern. This can be useful for producing devices that act as, for example, diffraction gratings to produce specific transmissive or reflective behaviour, or to filter narrow wavelength bands of incoming light. The tunability of the layer of tuning material can then mean that the selective optical properties of the device can be smoothly adjusted over a wide range. This is demonstrated by the embodiment shown schematically in Figure 4, where the layer 4 of tuning material includes regular reductions in its thickness. Incident light 10 reflected from each of the regions of tuning material will interfere with light from other regions, which has a different phase depending on the thickness of the region of the layer 4 of tuning material with which the light has interacted. This can be used to produce a diffraction grating, or other interference effects.

Another application utilizes the broadband transparency of GeSe ₃ thin films in spectrally selective optical (colour and absorber) filters (31, 32). Figure 8e shows simulated reflectivity spectra of a Fabry-Perot-type resonant cavity, formed with the structure shown inset in Figure 8e(II), which comprises a thin planar metallic layer (Ag 30 nm) coupled through the GeSe ₃ spacer and the bottom metallic layer (Ag 30 nm). The illumination source is normally incident onto the planar surface with arbitrary polarization. The transmission of the tuneable optical device 2 is plotted as a function of wavelength and spacer thickness in Figure 8e. The very sharp, thickness dependent optical resonances approach near unity transmission (near-perfect transmission) owing to the loss-less characteristic of the tuning material. Moreover the resonances can be shifted through tuning the state of the film by annealing, as seen in Figure 8e(II), which adds tunability to the filter through broadening the resonances. Similar behaviour is seen from simulated data of GeSe ₃ thin films of varied thickness on Pt substrate shown in Figure 18. Note the contour blue shift between as deposited and annealed states.

The zero optical absorption in the infrared spectra, yet tunability in the real part of the refractive index can further be exploited for a wide range of photonic devices including optical switches, photonic memories, and all-optical computers (11, 33-35). Chalcogenide glasses currently used for these applications induce high optical losses due to their intrinsic absorptive optical property (35) and photonic materials that can efficiently modulate light for the above applications over a broad band of wavelengths are elusive.

Figure 8f illustrates an example of an embodiment in which the tuneable optical device comprises an optical resonator, and a resonant wavelength of the optical resonator is dependent on the refractive index of the tuning material. Figure 8f is a scanning electron micrograph of a photonic race track resonator made from SiN _x on a wafer. By placing a strip of 150 nm thick GeSe ₃ thin film on the resonator (highlighted by arrow) and thermally annealing it step-wise, the resonance frequency of the resonator can be efficiently tuned without affecting the Q-factor. Figure 8g and h illustrate that peak shifts can be achieved in the resonance frequency of as much as 6 nm without compromising the Q factor, and in a non-volatile manner. The shifts are the result of the change in effective refractive index of the propagation modes that are confined in the resonator. Crucially, this demonstration exemplifies the use of loss-less GeSe ₃ films for corrective optics, where post-fabrication modifications through GeSe ₃ thin film deposition and thermal treatment can tune the photonic component, thereby attaining a desired resonant frequency by suitable thermal treatment.

Reversible crystallization of GeSe alloys, and other suitable tuning materials, is possible across a wide composition range (36-40). The modulation of properties driven by structural changes, as discussed herein, can augment the otherwise conventional phase change characteristic of chalcogenide glasses. This can potentially usher a wide array of applications, due to the broadband loss less characteristic of the GeSe ₃ thin films. This may include solid-state displays (5), smart windows (5,11), transmissive optical filters (41), multi-level optical data storage, and tuneable holograms (42).

It can also be advantageous to reverse the tuning of the optical and structural properties of the material, to return the properties of the layer 4 of tuning material back to a state close to that before any tuning was performed. This can be achieved by further heating of the tuning material.

In an embodiment, as demonstrated in Fig. 21, the method of tuning the optical properties of a tuneable optical device 2 further comprises heating S30 the tuning material to a temperature above the melting point of the tuning material, and quenching S40 the tuning material. Melting and reforming the layer 4 of tuning material by rapidly quenching it returns its structural state to a state close to the as-deposited state, allowing the refractive index of the tuning material to be tuned again through annealing of the tuning material by heating.

Heating of the tuning material to a temperature above the melting point of the tuning material may comprise heating by any suitable means. In an embodiment, heating S30 the tuning material above its melting point comprises heating the tuning material by passing an electrical current through the layer 4 of tuning material or another layer of the tuneable optical device 2, optionally a layer adjacent to the layer 4 of tuning material. Alternatively, laser heating of the layer 4 of tuning material, or another layer of the tuneable optical device, optionally a layer adjacent to the layer of tuning material, may be used, as described above for the heating used to tune the optical properties.

Quenching the tuning material involves some form of active cooling of the tuning material. This increases the rate at which the material cools relative to the cooling rate achieved by simply removing the source of heating. Quenching may be achieved by applying a cooling fluid to the tuning material. Alternatively the cooling fluid may be applied to another layer of material within the tuneable optical device (for example, the substrate) such that heat can be removed from the tuning material by conduction through the one or more intervening layers. Cooling fluids may include water, mineral oil, or gases such as air or nitrogen. Other suitable cooling fluids can be chosen by the skilled person depending on the specific tuning material and application for which the tuneable optical device is used.

In some situations, or for some choices of tuning material, it may not be necessary to melt the material in order to return its optical properties close to an as-deposited state.

In such cases, it is sufficient to heat the material to a temperature below the melting point and subsequently allow it to cool. In an embodiment, the method of tuning the optical properties of a tuneable optical device 2 further comprises annealing the tuning material by heating the tuning material to a softening temperature and not above the softening temperature, wherein the softening temperature is below the melting point of the tuning material, and a hardness of the tuning material at the softening temperature is below a first threshold value; and cooling the tuning material to a temperature at which the hardness of the tuning material is above a second threshold value.

This represents an alternative way to return the properties of the tuning material to a state close to the as-deposited state, such that the tuning of the optical properties of the tuning material can be reversed. Although the softening temperature is below the melting point of the tuning material in this embodiment, it is still likely to be above any

predetermined temperature used for tuning of the refractive index of the tuning material. The softening temperature may be related to a softening point of the tuning material. In an embodiment, the softening temperature is within 25% of the softening point of the tuning material, optionally within 10%, optionally within 5%, optionally within 1%, optionally equal to. The first threshold value in such a case will be the value of hardness at the softening point. The hardness of the tuning material defined in terms of any suitable measure of hardness, for example Vicat, Rockwell, Vickers, Shore, or Brinell hardness.

The cooling of the tuning material may be performed passively, by simply removing the source of heat once the tuning material has reached the softening

temperature. Alternatively active cooling may be performed to more rapidly reduce the temperature of the tuning material. Active cooling may be performed by quenching, as described above. The second threshold value of hardness may be related to the value of hardness at room temperature. In an embodiment, the second threshold value is within 50% of the hardness of the tuning material at 25°C, optionally within 25%, optionally within 10%, optionally within 5%. As mentioned above, the hardness may be defined using any suitable measure of hardness.

The devices disclosed herein allow a notable decrease in the refractive index of a layer 4 of tuning material, and in some embodiments a significant decrease in the physical volume of the film, from thermal annealing. A potential explanation for this is intrinsic thermally activated structural changes. The lossless characteristic of the tuning material, and the coupled opto- structural changes, make the devices particularly suited for a variety of different applications, broadly relating to corrective optics.

In particular, the use of GeSe for optical applications has some obvious benefits, which are less-achievable with currently used devices based on lossy phase change materials, viz. smaller losses enable sharp resonance peaks for vivid colours and selective optical-filters, high refractive index and broadband transparency negate the limitation on choice of substrate, and, importantly, devices can be built from a single thin-film of GeSe ₃ , eliminating fabrication challenges associated with complex multi-layer optical devices. Structural Experiments

Without wishing to be bound by theory, the inventors hypothesize that the thin films of tuning material used in the tuneable optical devices 2 disclosed herein undergo structural relaxation or reordering at the atomic scale, towards a more stable amorphous phase configuration upon annealing.

The fact that the optical properties, which are a function of nearest neighbour stoichiometry, change from annealing indicate that the atomic rearrangements also involves formation of new bonds. Indeed, this is substantiated on the Raman spectroscopy measurements. Figure 7c highlights Raman spectra of a 90 nm thin film of GeSe ₃ in as- deposited, thermally annealed (370°C) and laser annealed conditions (5 mW/532 nm/spot size 1 pm). The spectra are recorded at low laser power to negate photo-annealing.

Signature Raman peaks (12, 13) of GeSe alloy are evident and represent various vibrational modes of the alloy, as seen in Figure 14. Figure 14(a) shows schematics of active Raman modes in GeSe alloy. Figures 14(b) and 14(c) show Raman spectra of a 90 nm GeSe ₃ film and 125 nm GeSe ₃ film (respectively) on p-doped Si substrate, before and after annealing at 370°C for 6 minute in room conditions. Notably, the 262 cm ^-1 peak that corresponds to Se-Se bonds get diminished and the overall intensity of the film decreases.

Structural blocks of GeSe alloys are understood to be Ge centred tetrahedrons and chains and rings of Se. The 194 cm ¹ corresponds to ETH vibration modes of the corner sharing GeSe4/2 tetrahedrons and the companion peak at 211 cm ^-1 represents the ES breathing vibrations of the edge-shared Ge2Se8/2 bi-tetrahedrons. The peak at 262 cm ^-1 corresponds to the Se-Se bonds, and suggest that predominant configuration of Se in the form of Se-8 rings. The thermal annealing is observed to induce to a decrease in the Se-Se peak intensity. This however occurs with an increase in the ES peak intensity (also see Figure 14).

Collectively, these indicate the intrinsic structural changes on annealing includes an increased formation of Ge-Se heteropolar bonds at the expense of homopolar Se-Se bonds (14). On the contrary an intense optical exposure is observed to induce segregation effects in such low dimensional films, instead of structural ordering. Figure 7c illustrates that an intense laser exposure does not favour reconfiguration of Se-Se bonds, instead it gives rise to a new peak at 302 cm ^-1 . This peak corresponds to a vibration mode of strained Ge-Ge bonds (15). Similar changes, but of smaller magnitudes are also observed in UV illuminated films, as seen in Figure 15(a), which shows Raman spectra of the film optically exposed to continuous UV exposure. Ge-Ge segregation is evident at 302 cm ¹ . Optical exposure to pulsed UV laser is found to ablate the thin films, as seen in Figure 15(b), which shows optical micrographs of a GeSe ₃ film on exposure to pulses of UV laser. At high power the film ablates (see inset) and at even higher power the Si sample degrades.

The optical and thermal exposure of thicker films (1 pm), however, causes preferential conversion of the heteropolar bonds to homopolar bonds, in line with literature: although there is a discernible hump at 302 cm ^-1 , thicker films are found to be more resilient to segregation effects. This is seen in Figure 7d, which shows a Raman spectra of a 1 pm thick GeSe ₃ thin film before and after thermal and optical exposure. Similar results as in Figure 7c can be noted, however the film is more resilient to Ge-Ge bond formation under optical exposure. The contrast in the material’s behaviour to the optical and thermal response probably arise from beyond thermal effects, such as electronic excitations. The segregation effects are intensified in thinner films, likely due to increased disorder (16) and interfacial stress effects (17) from the substrate.

The effect of the annealing environment on the microstructure of a thin film is illustrated on the Raman spectra in Figure 7e, which shows the Raman spectra of a 75 nm thin film annealed to 370°C, in room, vacuum, and argon environment. Similar structural changes as observed in Figures 7b and 7d can be noted, highlighting the structural changes are intrinsic. Similar observations as in Figure 6c stand, however, for devices annealed in vacuum (10 ⁶ torr) and in argon atmosphere (10 ⁶ torr) the 302 cm ^-1 peak becomes apparent. However, we note that this peak emerges not from thermal annealing, instead from the laser exposure during Raman measurements.

The devices annealed in vacuum and argon are found to be more susceptible to photo-annealing than the devices annealed in ambient conditions, likely due to longer thermal exposure (the thermal time constant of the furnaces are several minutes). From X-ray photoelectron spectroscopy measurements we deduce the chemical changes, which are induced in the films from thermal treatment. The Ge-3d core-level peak of the as- deposited thin film is centred at 31.1 eV, which is line with literature.

This is demonstrated by Figure 7f, showing XPS spectra of the prominent Ge 3d and GeO _x peak in the film, in the as deposited and annealed conditions. Annealing results in a peak shift in the 3D peaks and an increase in the intensity of the GeO _x peak. Both are indicative of a change in Ge oxidation state. There is an accompanying shoulder peak at 33.2 eV, which is suggestive of GeO _x. Annealing is observed to induce slight peak shifts, but more notably an increase in the GeO _x intensity, which is indicative of surface oxidation (18).

Figure 7g highlights the changes in the Se-3d peak from annealing, and shows that a blue shift in the binding energy is also observed in Se; indicative of an increase in the Se oxidation state, likely due to the increased Ge-Se bonds. No signatures of Se-0 oxides are observed on the spectra. Along these lines, the intensity of the Oxygen-1 s peak is observed to increase from annealing, as seen in Figure 7h). The increase in its intensity is indicative of larger surface oxidation.

The fact that the oxygen concentration in the film increases even when annealing in high vacuum and under argon indicates that the surface of film is likely enriched with oxygen that diffuses from the bulk of the material during thermal treatment, and that oxygen plays a role in the structural changes of the film. The formation of GeO _x and re ordering of the amorphous lattice to a more stable disordered state from decreased concentration of Se-Se bonds, perhaps explain the change in the optical properties of the film after annealing.

These results are further complemented by the decrease in the GeSe peak intensity of annealed devices on the Raman spectra (see Figure 14), suggestive of Ge being oxidized. Based on the results, we hypothesize that the GeSe ₃ films undergo thermal bleaching upon annealing, independent of environment (room/vacuum/argon) that results in the dynamic changes discussed above (19).

The optical exposure of thin films are also found to result in similar changes. Contrary to our observations, photo-darkening effects have been reported in similar alloy systems (20), which were synthesized using other techniques. Alongside, conversion of photo-darkening effects into photo-bleaching have also been reported in GeSe films (21, 22). Therefore, changes the material undergoes likely depends on the starting as-deposited state of amorphous structure, which varies with the method of deposition.

On the basis of the above observations, it is conclusive that the thermal treatment relaxes the amorphous film towards a more entropy driven disordered (equilibrium) state (14, 23, 24). Amorphous chalcogenide glasses are characterized by severe atomic disorder (25), where disorder implies wrong bonds (mainly homopolar), structural defects, and dangling bonds. Concomitantly, these defects result in several localized states at the edge of the conduction band, which minimizes the optical band-gap. Annealing (both thermal and photo) likely provides sufficient activation energy for breakage of wrong bonds, as well induce atomic re-ordering in the short and medium range. These effects likely result in annihilation of the localized states, and enlarges the optical band-gap. Indeed, on the Raman spectra a drastic decrease in the Se-Se density from annealing, while an increase in the Ge-Se bonds (26,27) is observed.

The significant shrinkage in film thickness is also a consequence of the structural changes, as well as free-volume reduction. While volume changes in S based glasses are well studied, changes in Se glasses are scarce. Some literature associates the densification to void annihilation that originate from porous deposition (19). However, the fact that we observe similar volume contraction in sputtered thin films suggest that the material change is intrinsic and likely associated to both bond re-ordering and mass transport. Indeed, thin films of other chalcogenide glasses of similar thickness deposited under exact conditions do no undergo similar volume change on annealing (see inset in Figure 6d). Furthermore on multiple TEM cross-sections, we find good homogeneity and uniformity in the films under both as-deposited and annealed states (see inset Figure 7b). We also find circular shaped grains scattered on the annealed films, independent of the annealing environment. We find these grains are both Se rich, and are amorphous as the rest of the film using energy dispersive spectroscopy and TEM, respectively. This is seen from measurements shown in Figure 16. Figure 16(a), top panel, illustrates circular topological features scattered in annealed samples (independent of annealing environment). These features are rich in Se as revealed on the energy dispersive X-ray maps of a 90 nm thick film shown in Figure 19 before (a) and after (b) thermal annealing at 370°C. The stoichiometry between Ge:Se is 1 :3. Figure 16(b) shows TEM diffraction spectra of such a region, revealing this region is amorphous as the rest of the film.

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