"Preparation of micro- or nano-sized products"

The invention relates to a process for the preparation of xerogel and glass micro- or nano-sized fibres and capillaries.

Background of the invention

Optical fibres are used for transmitting light from one place to another. Fiber optics may be divided into three broad categories: glass, crystalline, and hollow waveguides. Conventional optical fibres consist of a solid or liquid core material surrounded by a solid cladding. Light can become trapped within the core by total internal reflection at the core/cladding interface. Most commonly used existing technology for optical fibre production involves the drawing of a fibre from glass using a draw tower. Another way of making such fibre involves the use of a nested set of concentric crucibles containing separate melts of the glass compositions that are to form the individual constituent layers of the fibre. The composite fibre is drawn from a set of concentric nozzles at the base of the crucibles. All these methods require high temperatures to fabricate the glass fibres and the diameter of the product is frequently difficult to control. It is also impossible to produce fibres of very small (<200 μm) diameter using these methods.

Hollow waveguides present an attractive alternative to solid-core optical fibers. (J.A. Harrington, 1990). Key features of hollow guides are: their ability to transmit wavelengths well beyond 20 μm; their inherent advantage of having an air core for high-power laser delivery; and their relatively simple structure, potential low cost, low insertion loss, no end reflection, ruggedness, and small beam divergence. Hollow-core waveguides may be divided into two categories: i) those whose inner core materials have refractive indices greater than one (leaky guides) and ii) those whose inner wall material has a refractive index less than one (attenuated total

reflectance, i.e. ATR, guides). Leaky or n>l guides have metallic and dielectric films deposited on the inside of metallic, plastic, or glass tubing. (Y. Matsuura and M. Miyagi, 1992). ATR guides are composed of dielectric materials with refractive indices less than one in the wavelength region of interest. (CC. Gregory and J.A. Harrington, 1993). Therefore, n<l guides are fiberlike in that the core index (n « 1) is greater than the clad index.

Hollow core waveguides have been fabricated using a variety of techniques. Some of the methods include physical vapor deposition of silver and dielectric layers on metallic substrates (US 5,005,944), sputtering of metallic, dielectric, and semiconductor films on a leachable mandrel followed by electroplating, (Miyagi et al, 1983) and liquid phase formation of coatings inside plastic tubing (Croitoru et al, 1990) and glass tubing (Abel et al, 1994). Most often the cross section of the guides is circular but early work by Garmire, et al. (Garmire et al, 1980) and more recently by Kubo, et al. (Kubo, 1994) on rectangular guides continues to be of interest. The advantage of the circular cross section is the ease of bending and the small overall size compared to rectangular or square cross section guides.

Glass capillary tubes useful for a thermometer, transportation of a fluid, sensors or other applications, have been produced directly from molten glass. A fine glass capillary tube having an inner diameter of from 0.1 to 0.2 mm useful as a ferrule for optical fibers in recent years, has been produced by hot stretching the glass capillary tube directly produced from molten glass as mentioned above. However, in the method for producing the capillary tube directly from molten glass, it is extremely difficult to produce stably with high precision a capillary tube having an inner diameter of less than 0.5 mm or an outer diameter being at least about 10 times the inner diameter. In the case of producing a finer capillary tube, the capillary tube produced directly from molten glass as mentioned above is used as the starting preform, whereby it is extremely difficult to bring the inner diameter to a level of about 1/10 of the outer diameter.

Microcavity resonators including one dimensional structures utilizing distributed Bragg reflectors, spherical microcavities, microdisk and microcylindrical laser emitters have been previously reported. In general, resonant cavities that can store and recirculate electromagnetic energy at optical frequencies have many useful applications, including lasing, high-precision spectroscopy, signal processing, sensing, and filtering. Optical microcavity resonators have quality factors (Qs) that are higher by several orders of magnitude, as compared to other electromagnetic devices. Maximum measured Qs as large at 10 ¹⁰ have been reported, whereas commercially available devices typically have Qs ranging from about 10 ⁵ to about 10 ⁷ . The highest Q resonances encountered in these microcavities are due to optical whispering-gallery-modes (WGM) that are supported within the microcavities.

As a result of their small size and high cavity Q, interest has recently grown in potential applications of microcavities to fields such as electro-optics, microlaser development, measurement science and spectroscopy. By making use of these high Q values, microspheric cavities have the potential to provide unprecedented performance in numerous applications. For example, these microspheric cavities may be useful in applications that call for ultra-narrow linewidths, long energy decay times, large energy densities, and fine sensing of environmental changes, to cite just a few examples.

An intriguing potential application for such cavity structures is their use in optical modulation and switching. See for example (Villeneuve et al., 1996) and (Joannopoulos et al, 2000). The on/off switching functionality, for example, can be realized by shifting the centre frequency of resonances either towards or away from the signal frequency. To achieve a large on/off contrast ratio, however, the required frequency shift tends to be much larger than the width of a single resonance to achieve switching or modulation of optical signals and other applications. Moreover nonlinear materials proposed for these applications are expensive, device fabrication is arduous and performance requires high excitation intensities leading to fast degradation of materials in use.

Another example of the application of high Q microcavities, which is of considerable industrial interest, is tunable narrow-band optical filters based on micro-ring resonators (Vyatcharim, Gorodetskii et al. 1992; Little, Chu et al. 2005) and add/drop devices utilizing electrical thermo-optic tuning of modes in microtoroid resonators (Armani, Min et al. 2004). The complex structure of proposed devices and high- level technology involved in fabrication present a real challenge to manufacturers. (Little and Chu 2000).

Any improved method for the preparation of microcavity light emitting devices would have valuable potential.

Statements of Invention

According to the invention there is provided a process for the preparation of a micro-' or nano-sized product comprising the steps of:-

preparing a sol;

introducing the sol onto a substrate matrix; and

applying a vacuum.

In one embodiment of the invention the sol is introduced onto the substrate matrix before the gelation point of the sol has been reached.

In another embodiment of the invention a vacuum of between 1 and 1000 mbar is applied.

In one embodiment of the invention the product is dried at room temperature.

In another embodiment of the invention the product is further annealed at 400°C to 1000 ⁰ C for several hours.

Li one embodiment of the invention the substrate matrix comprises a porous substrate.

The substrate matrix may be selected from any one or more of porous silicon, microchannel glass, alumina and a polymer membrane. Preferably the polymer membrane comprises any one or more of polycarbonate, polystyrene, PVC, polyethylene and polypropylene.

In one embodiment of the invention the micro- or nano-sized product is in the form of a xerogel or glass fibre, tube or capillary.

In one embodiment of the invention the product has a diameter of from 50 nm to 200 μm. The product may have a diameter of less than 20 μm.

In one embodiment of the invention the product has a length of up to several (~10) millimetres.

In one embodiment of the invention the product is in the form of a microtube, microfϊbre or microcylinder having a quality factor (Q factor) of greater than 3000.

In one embodiment the process comprises coupling a microtube or microcapillary to a microsphere.

The invention also provides a microtube or microcapillary having a diameter of from 50 nm to 200 μm.

The invention further provides a glass or xerogel microtubes having a diameter of form 50 nm to 200 μm prepared by a process as hereinbefore described.

The invention also provides glass or xerogel microfibres prepared by a process of the invention.

The invention also provides a light emitting microtube or microcapillary having a diameter of from 50 nm to 200 μm.

The invention also provides a light emitting microtube or microcapillary having a diameter of from 1 μm to 10 μm.

The invention further provides a microfibre, microtube or microcapillary having a quality factor (Q factor) of greater than 3000, the quality factor (Q factor) may be greater than about 3500, in some cases greater than about 4000.

The invention also provides a micro-optical device comprising a microtube or microcapillary coupled to a microsphere.

The invention further provides use of a microfibre or microtube or microcapillary of the invention for any one or more of detectors of sub-micron objects, including biological pathogens, integrated optics, cavity quantum electrodynamics, nonlinear optics and optical communications, temperature detectors, bio- and chemosensors, microchannels for optically and spectral controlled cell growth (e.g. neurons to rewire damaged nerves), optical mode converters, optical polarization converters, components for micro-electrophoresis, light emitters and optical amplifiers and optical elements of quantum cryptographic systems.

The invention also provides use of a microfibre or microtube or microcapillary of the invention in single molecule detection or single nanoparticle detection.

Brief description of the drawings

The invention will be more clearly understood from the following description thereof given by way of example only, with reference to the accompanying drawings, in which:-

Fig. 1 is a sequential flow diagram showing the drawing of individual microtubules from the porous matrix as described in the invention;

Fig. 2 is a schematic representation of the preparation of porous silicon/micro(μ)-channel glass-rare earth doped xerogel composite;

Fig. 3 is (a) a cross-section of a xerogel-porous composite and (b) an optical image of a rare earth doped xerogel-porous silicon composite with 5OX objective;

Fig. 4 are SEM images of an aluminosilicate xerogel-porous silicon composite at (a) x 600 magnification and (b) x 400 magnification;

Fig. 5 is a photoluminescence spectrum showing the bands corresponding to Eu-doped in aluminium silicate xerogel fibres;

Fig. 6 are SEM images of aluminosilicate microtubes inside the micro- channel glass matrix: (a) cross-sectional view, (b) top view;

Fig. 7 are SEM images of aluminosilicate microtubes outside the matrix at: (a) x 450 magnification and (b) x 1000 magnification;

Fig. 8 (a) is a photoluminesence (PL) spectrum of an aluminosilicate microtube of the invention showing whispering gallery mode (WGM) and (b) its time-dependent PL intensity decay;

Fig. 9 is a PL spectra of free-standing single aluminosilicate microtubes separated from a glass micropore showing whispering gallery modes. The result of WGM identification based on Mie-theory is depicted on the top:

Fig. 30 shows the Quality factor obtained for WGM in spectrum presented in Fig. 4;

Fig. 11 is (a) PL spectra of single aluminosilicate microtube measured at different excitation intensities (a). Dependence of integrated PL intensity on excitation power (b);

Fig. 12 shows the time-dependent PL intensity decays measured from single aluminosilicate microtube demonstrating modification of PL lifetime and reversibility. Excitation power: 0.03 mW (1); 55.3 mW (2) and 0.03 mW (3);

Fig.13 is an optical image of microtube showing microspheres on the side (top). PL spectra of microtube showing Eu ⁺³ luminescence (from tube) and WGM from dye coated SiO ₂ microsphere on side of tube (bottom); and

Fig. 14 is an optical image showing a microsphere at the tip of the microtube (top). PL spectra of SiO ₂ microsphere at tip of microtube displaying waveguide modes of the microtube (bottom).

Detailed description

We have found an improved process for the preparation of xerogel and glass microfibres and microtubes or microcapillaries. The products of the process have a diameter of from 1 to 10 micrometers and a controlled length of up to several millimetres.

The products of the invention may be used as micro-sized light emitting devices and microsensors and have wide potential as detectors, sensors or components for optical communications. The materials and devices may be applied in photonics as optical fibres and waveguide and optical micro-resonators.

The luminescent glass and xerogel microfibres and microtubes have potential application as microcylindrical light emitting devices and optical communication components. Microcylindrical light emitting devices are promising for the development of super small lasers for networking with size comparable to that of the core of an optical communication fibre. By analogy with a spherical or disk microcavity, light circulates around the curved inner boundary of the microcylinder, reflecting from the walls with an angle of incidence always greater than the critical angle for total internal reflection, thus remaining trapped inside the resonator. Moreover, the total internal reflection allows the achievement of a mirror (walls) reflectivity near unity because there are only minute losses of light caused by evanescent leakage (tunnelling) and scattering from surface roughness. This suggests a low threshold for the onset of laser action and a smaller volume of active material with concomitant moderate energy requirements and the ability to pack the emitting devices into a small space.

The microtubes which are produced by this method demonstrate whispering gallery mode (WGM) lasing.

Briefly the process of the invention involves a gel sol being introduced onto a porous substrate before the gelation point of the sol is reached. A vacuum is applied in such a way that the sol infiltrates the pores of the substrate. The product is dried at room temperature and annealed, typically at 500°C for 2 hours.

The microfibres, microtubes or microcapillaries formed are subsequently separated from the substrate. Typically mechanical destruction (or cracking) of the matrix is carried out. However in some cases having microfibres or microcapillaries attached

to the substrate makes further processing easier. It can be difficult to process and control individual loose fibres of micron size. When the fibres are attached to the substrate like silicon (Si), for example, it may facilitate their further integration in various devices. Si-processing technology is very well developed (e.g. all microelectronics) and it would be easy to apply existing technological processes for fabrication of new devices based on the fibres attached to Si substrate as prepared by the process of the present invention.

As sol-processing is used, high temperatures are not required resulting in a more efficient process. As well as producing microfϊbres, the process also allows the production of microcapillaries or microtubes of controlled diameter. Obtaining products of controlled diameter cannot be done using existing fibre drawing technologies.

The process of the invention results in micro-tubes which have a quality factor at least an order of magnitude higher than that reported to date for analogous systems.

The quality factor (Q-factor) reflects how long a photon can be stored in the microcavity of a microcylinder before leaking out. The lasing threshold is somehow determined by the value of Q-factor. However the value of the Q-factor depends on the diameter of the micro-cylinder (Knight, Driver et al. 1994), index of refraction of the material and losses due to an imperfection in fabrication. Although, there are no publications on whispering gallery mode structure in a spectra of micro-cylinders or micro-tubes with sizes comparable to the products of the present invention, others have considered micro-disk cavities as something similar and in this case there are a few reports on the value of the Q-factor. For example, (Mohideen, Slusher et al. 1994) report a value of Q-factor as ~ 500 for a 5 micron diameter disk. (Mair, Zeng et al. 1998) report a Q-factor of ~ 100 estimated for 9 micron discs. In both cases samples were fabricated by molecular beam epitaxy (MBE). The maximum value of Q-factor reported to date for microcylinders of 10-30 micrometer diameter is 1100 (Gianordoli, Hvozdara et al. 1999). However the diameter of the microcylinders is 2-

3 times larger than that of the products of the present invention. In comparison, for the products of the present invention the Q-factor is estimated from experimental data to be as high as ~ 3200. Using the processing technology of the invention we can potentially achieve Q-factors of greater than about 4,000.

Although MBE (Molecular Beam Epitaxy) allows for more accurate control of composition of microcavities it fails to provide a high optical quality of the surface layer, which is crucial for applications utilising whispering gallery modes.

The whispering gallery modes (WGMs) of the microtubes or microcapillaries make them useful in applications such as detectors of sub-micron objects, including biological pathogens, integrated optics, cavity quantum electrodynamics, nonlinear optics and optical communications. The products of the invention may also be useful as temperature detectors (sensors) using the microcylinders, optical mode and optical polarization converters (which can be realized using two coupled microtubes). The products may further be useful in bio- and chemosensors, light emitters and optical amplifiers and optical elements of quantum cryptographic ■ ^■ systems. The product of the invention also has potential in single molecule or single nanoparticle detection. (R. A. Wallingford and A. G. Ewing 1988, S. Blair and Y. Chen 2001, Enderlein 1999, Brun and Wang 2000, Little and Chu 2000, Boyd and Heebner 2001, Smirnov et al. 2002, Arnold, et al. 2003, Armani, Min et al. 2004, Nam and Yin 2005, Jun and Guo 2006)

The invention will be more clearly understood from the following examples thereof.

Examples

Sol-gel processing

All common reagents were obtained from Sigma-Aldrich and were of at least 99% purity. Micro-channel glasses were received from State Optical Institute, St.

Petersburg, Russia. The micro-channel glasses have pore sizes of 7-8 μm and a thickness of 0.05 cm.

The preparation of Al ₂ O ₃ -SiO ₂ sol was carried out as previously described by Nogami et al. In brief, 5 A1 ₂ O ₃ .95 SiO ₂ (mol %) glass was prepared. The Si(OC ₂ H ₅ ) ₄ , TEOS, was first hydrolysed for 1 h at room temperature with a solution Of H ₂ O, C ₂ H ₅ OH and HCl in the molar ratios 1: 1: 0.0027 per mole of Si(OC ₂ Hs) ₄ . Al(OC ₄ H ₉ ^sec ) ₃ was added to this solution, followed by stirring for 15 min at 70°C. The resultant homogenous solution was hydrolysed by adding the mixed solution of H ₂ O, C ₂ H ₅ OH and HCl in the molar ratio 4: 1: 0.011 per mole of alkoxide. We have optimised that the conversion of this sol to gel takes place within 30 minutes of final stirring.

The micro-channel glass slides or porous silicon samples were cleaned by using acetone and dried. The micro-channel glass samples were placed in the chamber and a vacuum applied (29- 30 mbar) slowly during the initial stage. The "sol", just before its gelation point was poured onto these micro-channel glass samples and a vacuum assisted filtration helps in the formation of fibres inside the matrix. The pouring of the sol before its gelation point is important as this determines the amount of sol passing inside the matrix. Also, the vacuum assists in the formation of gel more easily inside the matrix. The composite is dried at room temperature and further annealed at 500°C for 2 hours heating time. Finally the micro-tubes are drawn outside from the matrix by a simple cut on the matrix. A sequential flow diagram of the procedure is depicted in the Fig. 1.

Preparation and characterization of xerogel microfibres

The rare earths (Er ³⁺ or Eu ³⁺ ) doped alumino-silicate xerogel were introduced to porous silicon according to the scheme shown in Fig. 2. The method of incorporation

of xerogel inside the porous silicon matrix was performed under vacuum. In brief, the rare earth doped aluminosilicate sol is introduced onto porous silicon samples before the gelation point and a vacuum [20 - 40 mbar] is applied such a way that the "sol" will infiltrate to the pores of silicon.

The incorporation of xerogel inside the porous silicon matrix by a vacuum method leads to fascinating results. The cross-section of the xerogel-porous silicon composite was examined under optical microscope and is presented in Fig. 3.

The optical image showed formation of fibres inside the porous silicon matrix. This was further confirmed by SEM (Fig. 4). The SEM images revealed xerogel fibres, which are formed on the surface of porous silicon samples. The fibres were pulled through the porous silicon matrix under vacuum.

The fibres have an approximate length of 100 μm and a diameter of 2-3 μm.

The luminescence studies on the fibres are presented in Fig. 5. The luminescence studies revealed the broad bands including peaks corresponding to Eu ³⁺ dopant in the fibres.

Preparation and characterization of xerogel microtubes or microcapillaries

The as-prepared aluminosilicate solgel-microchannel glass composite was annealed at 500°C/2hrs. The annealed microchannel glass matrix was subjected to a cut (as shown in schematic diagram, Fig. 1.) by using a diamond scribe which made it possible to draw the tubes. The SEM images of alumino-silicate microtubes outside the matrix are presented in Fig. 7. The microtubes, which are of more than 200 μm in length, can be seen from these images. Thus, the SEM images show the formation of high aspect ratio microtubules having smooth walled structures.

A cross-sectional SEM images of the microtubes is presented in Figs. 6a and 7b. The cross-sectional image shows that tubes are in hollow nature. The images also showed the uniform distribution of the fibres inside the matrix.

The total diameter of the tube can be calculated to 6-7 μm. The outer diameter is calculated to -10 μm and the inner diameter is ~ 3 μm.

Optical studies of the aluminosilicate microtubes

The luminescence studies were carried out in the micro-channel-xerogel composites using photo luminesence (PL) spectroscopy (Renishaw 1000 micro-Raman system) and fluorescent confocal microscopy (MicroTime 200 confocal microscope). The excitation wavelength was 514.5 nm from an Ar ⁺ laser (Laser Physics Reliant 150 Select Multi-Line. The 5Ox magnifying objective of the Leica microscope focused the beam into a spot of about 1 μm in diameter. The PL spectra of the aluminosilicate microtube accommodated in micropore is presented in Fig. 8(a). The emission spectra showed a broad luminescence ranging between 520 and 800 nm. The probable reason for the emission comes from the organic species trapped inside the gel matrix might have decomposed and created a C substitutional defect for silicon resulting in strong luminescence of thermally treated gels. The elemental analysis study on the prepared aluminosilicate gel under the same conditions as that for the microtube preparation and heated at 500°C for 2hours showed the carbon content and hydrogen content yields of 0.32% and of 0.56% respectively. A similar type of observation was reported before by W. H. Green et al.

PL decays were measured using time-correlated single photon counting (Time-Harp, PicoQuant). The samples were excited by 480 nm picosecond pulses generated by a PicoQuant, LDH-480 laser head contolled by a PDL-800B driver. The setup was operated at a 20-MHz repetition rate with an overall time resolution of ~ 150 psec.

Decays were measured to 6000-8000 counts in the peak and reconvoluted using nonlinear least squares analysis (FluoFit, PicoQuant), using an equation of the form: I(t) cc ∑a,. exp(-t / τ _f ), where Tj are the PL decay times. The pre-exponential

factors ctj, were taken into account by normalisation of the initial point in the decay to unity. The quality of fit was judged in terms of a χ ² value (with a criteria of less than 1.1 for an acceptable fit) and weighted residuals. The T; and α; parameters were used then to calculate the average lifetime, τ _av :

The data from our time-resolved PL measurements (Fig. 8 b) are indicative of strong contribution of defect states in recombination dynamics. The PL of a single aluminosilicate microtube accommodated in micropore shows distinct multi- exponential decay. Sum of at least three-exponential functions is required to achieve satisfactory fit to the decay data (Fig. 8 b) yielding reasonable plot of weighted residuals, y; ' value 1.1 with corresponding lifetimes in the nanosecond time scale (Table 1), Table 1 shows the multiexponential fit parameters to the observed PL of single aluminosilicate microtube accommodated in micropore.

Table 1

In contrast to the broad, featureless PL band in the spectra of tubes embedded in a micro-porous glass matrix, the emission spectra of a single "free standing" microtube separated from micropore matrix exhibit very sharp periodic structure (Fig. 9). The observed peaks are the so-called whispering-gallery mode (WGM) resonances: a

result of coupling of electronic states in aluminosilicate gel and photon states of microtube.

Optical micro-resonators

The invention also provides a process for the preparation of silicate and glass micro- sized tubes or capillaries and coupling them to microspheres. The microtubes are coupled with optical quality microspheres giving new types of micro-optical devices. These devices can be applied in photonics as optical micro-resonators and optical amplifiers.

Preparation of microtubes and microtube-microsphere devices

As described above preparation of microcapilaries involves a gel sol being introduced onto a porous substrate before the gelation point of the sol is reached. A vacuum is applied in such as way that the sol infiltrates the pores of the substrate. The product is dried at room temperature and annealed at 500°C or at higher 800°C for several hours. This process results in the formation of microtubes or microcapillaries inside the substrate.

The substrate containing microtubes has been wetted by a dispersion of silica or ethidium bromide coated melamine resin microspheres of 2.2 μm in diameter (Particle Size Standard, Microparticles GmbH, Berlin) in water added on the top of the porous structure. After drying at ambient temperature for 12 hrs the microtubes coupled to microspheres have been subsequently separated from the substrate by a mechanical destruction (or cracking) of the microchannel glass matrix.

Individual microtube-microsphere devices have been used in our optical studies.

Optical studies of the microtube-mkrosphere devices

The luminescence studies were carried out in the micro-channel-aluminosilicatε composites using PL spectroscopy (Renishaw 1000 micro-Raman system). The excitation wavelength was 457nm nm (Laser Physics Reliant 150 Select Multi-Line). The 50x magnifying objective of the Leica microscope focused the beam into a spot of about 1 μm in diameter. The PL spectra of the Eu ⁺³ doped aluminosilicate microtubε with two ethidium bromide coated melamine resin microspheres is presented in Fig. 13 (bottom). The emission spectra showed a broad luminescence ranging between 520 and 800 nm. There is an overlap OfEu ⁺³ emission coming from microtubes and photonic modes caused by dye emission from the microspheres. The microtube acts as both an optical waveguide and rnicroresonator exciting the photonic modes of microspheres, enhancing the signal and providing improved signal collection even though the microsphere is located far away from excitation/detection spot.

In another experiment the non-luminescent silica microsphere has been placed at the tip of the luminescent aluminosilicate microtube doped by interstitial carbon from the organic species trapped inside the silicate matrix (Fig. 14). The excitation beam was focused on the microsphere. This resulted in the broad a broad luminescence between 550 and 750 nm. PL band in the spectrum exhibit a sharp periodic structure (Fig. 14 bottom). The observed peaks are the so-called waveguide modes: a result of coupling of photon states in the aluminosilicate microtube and microsphere. In this case the microsphere acts as a microscopic focusing device providing efficient coupling of photonic modes of the microsphere and waveguide modes of microtube cavity.

Fig. 13. is an optical image of microtube showing microspheres on the side (top). PL spectra of microtube showing Eu ⁺³ luminescence (from tube) and WGM from ethidium bromide coated melamine resin microsphere on side of tube (bottom). The microtube was excited at 457nm.

Fig.14 is an optical image showing a microsphere at the tip of the microtube (top). PL spectra of SiO ₂ microsphere at tip of microtube displaying waveguide modes of the microtube (bottom).

The invention is not limited to the embodiments hereinbefore described which may be varied in detail.

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