Field of the Invention

This invention relates to a waveguide structure for terahertz radiation.

Background to the Invention

The terahertz region (0.1 THz - 30THz) of the electromagnetic spectrum lies between the millimetre wave and the optical regions. Terahertz radiation has some special properties: it can penetrate many visually opaque objects and materials to enough depth to obtain information inside biological, chemical or complex composite objects. It is sensitive to many potentially harmful gases, and offers the prospect of their detection and identification. It is absorbed by molecules of water but is non-destructive for living cells and tissues with no real perceived health risks, owing to low power levels and the non- ionising nature of the radiation. Terahertz systems are being developed for a range of applications such as medical diagnostics, identification of pharmaceutical substances, and security screening.

To date, promising performance has been achieved by using optical free-space technology in which the terahertz radiation is generated by directing Titanium: Sapphire femto-second laser pulses onto a semiconductor sample. A disadvantage of this scheme is that the associated optical components are bulky and expensive. This would limit the potentially widespread commercial and scientific uptake of terahertz technologies.

InP/GaAs-based terahertz source and detection is proposed to address the problem of miniaturisation, but to achieve a fully integrated semiconductor based terahertz source it is necessary to develop methods to achieve terahertz wave-guiding within semiconductor material. Waveguides used in shorter wavelength optical systems are typically formed in planar materials by choosing the wave-guiding layer with higher refractive index than the surrounding materials. However, to apply this concept to the terahertz region, the dimension of the terahertz waveguide would be prohibitively large for modern molecular beam epitaxy (MBE) / metal organic vapour phase epitaxy (MOVEP) growth techniques.

Summary of the Invention

Accordingly, this invention provides a waveguide structure for terahertz radiation comprising a first layer of a first material having a positive dielectric constant and a second layer of a second material having a negative dielectric constant, the first and second layer sharing a common interface, wherein the layers are arranged such that terahertz radiation propagates along the interface by means of a surface plasmon.

Thus, according to the invention, a novel waveguide for terahertz application is proposed based on surface-plasmon effects. There exists a solution to the Maxwell Equations at the interface between two materials with positive and negative refractive indices (dielectric constants), respectively. An electromagnetic wave can propagate along the interface and exponentially decay away from the interface. This electromagnetic wave is called a surface-plasmon. The invention provides a terahertz waveguide based on a surface- plasmon which can be used with an all-optical terahertz system. The surface-plasmon waveguide is not limited by the dimension limitation imposed by MBE/MOVPE growth. Rather, it only requires an interface between two materials, which have positive and negative refractive indices respectively, and using metal or properly-doped semiconductor can achieve negative refractive index.

The negative refractive index may be achieved by appropriately doping semiconductor material. The second material may be highly-doped semiconductor material. The highly- doped semiconductor material is doped sufficiently highly that the semiconductor material has a negative dielectric constant. For example, the dopant concentration in the highly-

doped semiconductor material may be greater than 2 x 10 ¹⁶ cm ^'3 , in particular greater than 2 x 10 ¹⁷ cm ^'3 .

Alternatively, the negative refractive index may be achieved by a metal film. The second material may be a metal, for example gold.

The terahertz waveguide may be integrated with components for either the detection or generation of terahertz radiation. The terahertz waveguide may have dimensions chosen to modify the terahertz radiation mode for efficient coupling between terahertz components.

The terahertz waveguide may incorporate a material which can provide gain at terahertz frequencies to allow amplification of terahertz signals or laser operation at terahertz when placed in a suitable cavity. The terahertz waveguide may be combined with a grating for the purpose of wavelength selection.

There is also disclosed herein a double metal waveguide for the longer wavelength side of the terahertz region.

The waveguide structure may comprise a third layer of a third material having a negative dielectric constant. The first and third layer may share a common interface. The third material may be a semiconductor material sufficiently doped to produce a negative dielectric constant for the material. Alternatively, the third material may be a metal.

The semiconductor materials used in the first, second and/or third layers may be any suitable materials. The materials may be positively doped or negatively doped or may be intrinsic semiconductor. The semiconductor material may comprise InP, GaAs or other suitable semiconductor materials.

Brief Description of the Drawings Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a side view of a waveguide structure according to an embodiment of the invention;

Figure 2 is a cross-sectional view of a semiconductor device incorporating the waveguide structure of Figure 1;

Figure 3 is a cross-sectional view of an alternative semiconductor device incorporating the waveguide structure of Figure 1 ; Figure 4 is a plan view of the semiconductor device of Figure 2;

Figure 5 is a plan view of an alternative arrangement of the semiconductor device of

Figure 4;

Figure 6 is a plan view of a further alternative arrangement of the semiconductor device of Figure 4; Figure 7 is a plan view of a yet further alternative arrangement of the semiconductor device of Figure 4 for producing a small laser spot;

Figure 8 is a plan view of a yet further alternative arrangement of the semiconductor device of Figure 4 for producing a large laser spot; and

Figure 9 is a plan view of a yet further alternative arrangement of the semiconductor device of Figure 4 incorporating a grating;

Detailed Description of Embodiments

This invention describes applying the surface-plasmon concept to achieve a terahertz waveguide for all-optical terahertz generation, and its applications.

A conventional planar waveguide is formed by choosing the refractive index of the waveguiding layer so that it is higher than the refractive indices of the surrounding materials. However, a surface-plasmon waveguide is not limited by this rule. Rather, it requires an interface between two materials, which have positive and negative refractive indices (dielectric constants) respectively. Using metal or properly doped semiconductor can achieve negative refractive index.

An embodiment of a waveguide structure according to the invention is shown schematically in Figure 1. In Figure 1 , layers 4 and 8 are the layers of properly doped semiconductor or metal, so that they can not only provide electric contacts with the semiconductor layer 6 between them, but also give negative refractive indices (dielectric constants). The semiconductor layers 4, 6, 8 are deposited on a semi-insulating substrate 2.

As a femtosecond pulsed laser beam (indicated by the large arrow in Figure 1) is shined onto the intrinsic area 6 of the PIN structure, terahertz radiation can be generated. The underlying physics is essentially the same as for lateral photoconductive emitters. As the generated terahertz radiation propagates, it is guided along the interfaces between the nanostructure 6 and the properly doped layers 4, 8, as indicated by the intensity curves in Figure 1. This structure has the major advantage over the conventional free-space terahertz generation system, e.g. Ti:Sapphire plus photoconductive materials (e.g. LT- GaAs) emitter, that it gives directionality for the generated terahertz radiation so that the generated terahertz beam can be coupled out to an optical fibre for remote users. The arrangement can also enhance the terahertz intensity. Because of the very nature of the surface-plasmon, the evanescent wave has a tail into the top doped layer 8 and air. This tail can be used for detection/sensing, e.g. gas or biomedical substance. The top doped layer 8 can also be etched to form a grating, similar to the conventional DFB (distributed feedback) laser, so that it can selectively reflect the generated terahertz wavelength(s). Such an arrangement potentially provides a "laboratory on a chip" system.

Depending on the wavelength of interest, it is also feasible to use a low-loss metal-metal waveguide, which can provide a confinement factor of nearly unity. To do this an etch- stop layer is grown. Low-temperature metallic wafer bonding and subsequent substrate removal using selective etching are normally used.

A cross-section of a semiconductor device incorporating the waveguide structure is shown in Figure 2. In Figure 2, the waveguide structure is provided with metal contacts 10, 12, 14. In the case of a metal-metal waveguide, the cross-section is shown in Figure 3. The waveguide can be a conventional ridge waveguide or a buried heterostructure waveguide.

Plan views of the semiconductor device (chip) are shown in Figures 4 to 9.

The top metallic contact 10 can be a stripe as in Figure 4 or multiple stripes with variable width depending on the mode size required, as shown in Figure 5. To reduce the loss related to the metallic contacts 10, two narrow metal stripes can be used for the contact 10, as shown in Figure 6.

Obviously the width of the mesa and the width of the metal stripe depend on the actual wavelength of interest. For example, for 60 μm emission wavelength, the waveguide width can be around 180μm.

This approach is also very flexible, if the mode size needs to be altered, for example to generate a large spot or a small spot. It can be realised by modifying the shapes of the top doped layer 8 and the metal contact 10, as shown Figures 7 and 8. Figure 7 shows the arrangement of the top layer 8 and the metal contact 10 to narrow the width of the laser spot. Figure 8 shows the arrangement of the top layer 8 and the metal contact 10 to widen the laser spot as the radiation propagates along the waveguide.

The top metallic contact 10 and the top doped layer 8 can be etched down to form a grating for targeted wavelength(s), as shown in Figure 9.

The proposed surface-plasmon and metal-metal waveguides not only can be used for terahertz emitter, but also could be used for an optically pumped all-optical terahertz laser with suitable gain material, which can work at room temperature. Such a system will have a major advantage over a terahertz quantum cascade laser, which normally works at cryogenic temperature. For this purpose, both the facets of the device can be coated. It is also feasible to use a photonic crystal structure to provide the feedback for the laser action.

In summary, a waveguide structure for terahertz radiation has a first layer 6 of semiconductor material having a positive dielectric constant and a second layer of semiconductor material 8 which is doped to have a negative dielectric constant. The first and second layers 6, 8 share a common interface and the layers are arranged such that terahertz radiation propagates along the interface by means of a surface plasmon.