SUBWAVELENGTH OPTICAL/PLASMON NEAR-FIELD CHANNELLING MULTI-CORE PROBE

This patent application claims a priority date from information disclosed in Australian Patent #2007 200373 entitled "Optical near-field channelling multi-core probe" with Mattias Aslund listed as inventor.

BACKGROUND

Typical optical microscopy, far-field light microscopy, cannot resolve distances less than the Rayleigh limit. The Rayleigh criterion states that two images are regarded as just resolved when the principal maximum (of the Fraunhofer diffraction pattern) of one coincides with the first minimum of the other [see Born, M. and Wolf, E. Principles of Optics. Cambridge University Press ό.sup.th ed. p.415 (1980)]. Currently, there are several possible methods for achieving resolution of spatial locations below the Rayleigh limit using light with wavelengths within and near the visible and near-IR regime. They include for instance: Confocal Microscopy and Near-Field Scanning Optical Microscopy (NSOM). Confocal Microscopy is a technique in which a very small aperture(s) is/are placed in the optical path to eliminate any unfocused light. This allows for a substantial increase in signal to noise ratio over conventional light microscopy. Also, it is possible to reduce the width of the central maximum of the Fraunhoffer pattern using a small slit or aperture. This, in turn allows a substantially enhanced resolution of 1.4 times better than the Rayleigh limit.

Near-Field Scanning Optical Microscopy (NSOM or SNOM), provides topographical information by scanning a metal coated light conducting probe with a sub wavelength aperture over a surface (aluminium is most often chosen as its skin depth for optical radiation is quite low, .about.13 nm at 500 nm). The probe can be used both for collection or illumination. Another approach is to use what are called "apertureless probes" [see Sanchez, Novotny and Xie "Near-Field Fluorescence Microscopy Based on Two-Photon Excitation with Metal Tips" Physical Review Letters VoI 82 20 pp 4014-4017 (1999)] where an evanescent plasmon wave is excited by bombardment with photons at the tip of a sharpened metal probe. Because the tip can be made very sharp (radii of 5 nm are achievable), resolutions can be correspondingly smaller. An associated problem with the "apertureless probes" is that the probe generates a white light continuum, which significantly decreases the signal to noise ratio.

Because the aperture size in a conventional probe is so much smaller than the wavelength of the excitation light and only an evanescent mode is supported resulting in very little light is transmitted through the aperture. Diffraction effects limit the effective collimated length from the aperture to less than diameter of the aperture. This, then, requires that the aperture be held below a maximum height above the surface of the sample. Ideally, a fixed height above the surface (usually less than 10 nm) is used for relative contrast measurements.

U.S. Pat. Nos. 5,973,316 and 6,052,238 issued to Ebbesen et al. of the NEC Research Institute, Inc. describe a NSOM device which employs an array of subwavelength apertures in a metallic film or thin metallic plate. Enhanced transmission through the apertures of the array is greater than the unit transmission of a single aperture and is believed to be due to the active participation of the metal film in which the aperture array is formed. In addition to enhancing transmission,

the array of apertures reduces scanning time by increasing the number of nanometric light sources.

An earlier patent, U.S. Pat. No. 5,633,972, issued to Walt and Pantano of Tufts college, describes a device which is functionally the same as Ebbesen's perforated metal film, a super resolution fibre array, where a large number of fibres are tapered down longitudinally and individually metal coated, only leaving a small aperture at the top of each metal cone.

All three of these latter patents suffer extensively from the large distance between the sub- wavelength apertures. U.S. Pat. No. 6,016,376 by Ghaemi et. al. at NEC Research Institute, Inc. describe an all dielectric waveguide/fibre bundle which has been tapered down together so that the sampling end of the waveguide taper presents an array of closely spaced sub-wavelength waveguides. Each of these waveguides samples the near-field and conveys that through the up-tapering region to the observation end, where it can be imaged using standard optics. This patent does not suffer from large aperture spacing, but it suffers from poor dielectric refractive index difference, which limits the minimum waveguide dimensions at the sampling end of the probe drastically.

In the publication ["Subwavelength Optical Imaging through a Metallic Nanorod Array" by A. Ono, J. Kato, and S. Kawata in Phys. Rev. Lett. 95, 267407 (2005)], the use of a parallel array of thin metal rods in a dielectric is presented in a theoretical calculation as a way of transferring sub-wavelength near-field images without focusing or magnification of the image using localised plasmonic polariton modes. In the publication ["Guiding, Focusing, and Sensing on the Subwavelength Scale Using Metallic Wire Arrays" by G. Shvets, S. Trendafilov, J. B. Pendry and A. Sarychev in Phys. Rev. Lett. 99, 053903 (2007)], the use of an array of thin metal rods where the transverse crossection is dimensionally linearly tapered in the longitudinal direction embedded within a dielectric is presented in a theoretical calculation as a way of focusing or magnification sub-wavelength near- field images using localised plasmonic polariton modes. This publication was received after the priority date of 30 ^th of January 2007 for AU Pat. #2007 200373 as a manuscript on the 5 ^th of February 2007 and published on the 3d of August 2007.

SUMMARY OF THE INVENTION The present invention contemplates sub-wavelength image magnification, translation or focusing devices, and the production methods thereof. These include production methods of image translation devices similar to that of Ono et. al.. It also includes both the device design (due to an earlier priority date) as well as the production method of image magnifying and focusing devices similar to that of Shvets et. al.. Specifically the invention takes the form of methods and apparatus that employ novel physical structures to provide an array of closely spaced sub-wavelength longitudinally, tapered or parallel, waveguides, which can expand, contract or translate the incident light, with significant amount of information of the near-field retained to, or translated from, dimensions which can be resolved satisfactorily with standard optics, including, but not limited to, e.g. lens and aperture configurations, CCD arrays, photographic plates, multi-cored optical fibres, etc.

Further, there is a range of detection methods whereby the optical image information and resolution of the near-field can be enhanced. These include, but are not limited to, as individually listed or in arbitrary combination: transverse scanning of probe with sub-pixel steps to build up an enhanced resolution image, longitudinal scanning of probe (in the z-direction, to and from

sample) to utilise e.g. sensitivity of plasmonic coupling of light from sample to different parts of the probe as a function of distance to extract depth information etc from the sample, sampling light emitting from the observation end of the probe of different angular portions to gather information of coupling efficiency from the nearfield to the probe as well as spectral transmission differences.

Further, the optical image information can also be used with image reconstruction and enhancing methods for increased information gathering ability and improved resolution of the near-field.

The preferred embodiment of the invention is for visible light, but it is implied through the whole invention that all forms and wavelengths of electromagnetic waves are included (e.g. UV, NIR, etc). Naturally, for other wavelengths different choices of dimensions, materials, electromagnetic wave manipulation methods and detection apparatus apply. Further, it is also implied that there is no limit to which materials or fabrication methods that can be used to embody the invention. The preferred embodiment is shown schematically in Figures 1 (longitudinal crossection) and 2 (transverse crossection). In this embodiment the device has a crossectional triangular (hexagonal) lattice of waveguides situated in a glass fibre/rod type host structure. In the figures 1-2 the metallic waveguides (e.g. silver or aluminium) are in black and the dielectric glass cladding in grey. The cladding is preferably in a material with low optical transmission loss e.g. polymer, glass etc.

Figure Ia shows the whole probe in longitudinal crossection. The measure 101 is the longitudinal length of the probe. Figure Ib shows a magnified portion of the observation end where 102 is the dielectric cladding and 103 is one of the metallic waveguides. Figure Ic shows a magnified portion of the sampling end. The device in Figure 1 shows a significant level of transverse crossectional tapering from the sampling end to the observation end. This probe is cleaved with significant longitudinal curvature retained, but it is implied that it can be cleaved with a more linear longitudinal tapering appearance. This nomenclature of Figure 1 assumes that the device is used for image magnification, however, it is implied that if the device is to be used for image focusing, the names "observation end" and "sampling end" swap places. If there is no crossectional tapering between the two ends, the device essentially functions as an image translation device, and the nomenclature is therefore arbitrary.

Figure 2a shows the whole probe in transverse crossection at the observation end. Figure 2b shows a magnified portion of the observation end. The measure 201 is the distance between two metallic waveguides and the measure 202 is the diameter of a metallic waveguide. Figure 2c shows a magnified portion of the transverse crossection of the down tapered sampling end. The measure 203 is the distance between two metallic waveguides and the measure 204 is the diameter of a metallic waveguide. Typically the dimensions of the 201 are optically resolvable by standard means whereas the dimensions of 203 are below the resolution limit.

The metal waveguide does not need to be solid, it may be fabricated hollow as long as the optical thickness of the metal layer does not impede detrimentally on the functionality of the device. Further, it can be envisaged that the relative thickness of the metal compared to the spacing between the waveguides can change longitudinally, e.g. one may choose to use very pointy metal waveguides where the transverse area ratio between metal and cladding is very small toward the sampling end, and/or one may also choose to use a transition to very thin metal layers of the waveguides (near or significantly thinner than the skin depth of the metal) toward the observation end in order to create a softer gradual transition from guided waves to a non-guided propagation.

This definition applies throughout the whole document:

Further, it can also be envisaged that to render specific localised transmission properties of the probes, it may be required to be manufactured with different types of metals (or alloys) in different longitudinal or transversal positions, and that the metal layer themselves may also be comprised of a number of different layers of different metals.

There is no limitation in the invention to the number of waveguides other than that the number is larger than one. Further, there is also no limit in the crossectional distribution of the waveguides which may be e.g. a square lattice, chirped, a random distribution, concentrically distributed etc. The waveguides themselves may be of arbitrary crossectional shape and have variable crossectional dimensions e.g. round waveguides, oblong, slits, etc. The waveguides may also have different longitudinal tapering distributions in one or more axis. An example why this flexibility may be required is given here: it can be appreciated by someone skilled in the art that to achieve a desired phase distribution at the end of the tapered waveguides (to avoid dispersion effects) which is beneficial for e.g. imaging, the waveguides may be required to have different size/shape apertures and the waveguides may be required to have to have varying dimensional tapering distributions in crossection (i.e. different longitudinal and transversal "effective index" profile) to compensate for different path-lengths of the waveguides.

Fabrication methods for the probes:

One way to fabricate the device is to make use of optical fibre/rod type of structures with an array of longitudinal holes. Assuming that this type of structure is utilised, there are two main fabrication options: either taper probe first and deposit metal after, or deposit the metal first and tapering after. The listed metal deposition methods are the same for both cases.

Suggested metal deposition methods: Possible methods for metal deposition could be, but are not limited to, chemical vapour deposition (CVD), physical vapour deposition (PVD), wet chemical deposition of metals (e.g. water, silver nitrate, ammonia and sugar) or a pressure fill intrusion of a suitable metal/metal alloy with a melting point temperature below the glass softening temperature of the glass.

Deposition methods may rely on capillary action for liquid phase deposition, or may require a pressure differential between the ends of the holes of the array

Tapering:

Standard methods of tapering glass structures apply. These rely on differential pressures between holes and environment, a localised heat source and stretching and/or collapsing. The localised heating could be provided by, but is not limited to, CO2 lasers, electric arcs, flames, electrical filaments etc. Depending on the metal of choice, the metal could be melted or in gas phase whilst the glass fibre is being tapered. Tapering could be performed in an essentially two-step procedure, where e.g. holes with a thin layer of metal could be collapsed to thin rods using a low internal hole pressure and then tapered. If the deposition of the metal is to be perfomed after tapering, holes need to be open all the way through after tapering to allow flow of deposition compunds.

Optical surface fabrication:

The fibre may be cleaved, mechanically polished, laser-polished, focused ion-beam milled, chemically etched, plasma etched etc to a desired optical quality surface.

Another fabrication method avenue that can be envisaged is using standard photo-lithography, metal deposition and selective etching. However, these standard methods allow too many options to list here, and therefore only two methods which could realise the structure are listed here:

1. E-beam defined channels. An e-beam, (or focused ion beam, or focused laser source), could be used to selectively expose a thick layer of photo resist where barrier metal is supposed to be deposited. The beams must be incident in angles relative the substrate to create the funnelling effect. The resist is cured and the exposed regions are etched away. Metal is deposited into the void/voids, and the resist can be etched away if one so wishes or kept as waveguide material if sufficient transparent. If the resist is etched away, the reminding metal structure could be covered by e.g. SiO2 and the substrate could be selectively etched away.

2. Layers of photo masks. If there are issues in regards to e.g. void filling, one might have to deposit the metal structure in many steps using a range of photo masks with increasing/decreasing waveguide dimensions instead of a straight e-beam approach.

Application suggestion:

The probe can potentially be fitted inside a module which can be retro fitted/screwed onto existing microscope objectives (perhaps with minor adjustments), thus enabling nearly all existing microscopes to be converted into sub-wavelength, or near sub-wavelength, optical microscopes. A schematic of this design is shown in Figure 3, where 301 represent a microscope objective tube housing with a suitable lens configuration, 302 a module holding on to the sub- wavelength probe 303. Screwing the module 302 onto housing 301 could provide for focusing control. 302 could be fitted with piezoelectric devices to allow for translation of the probe 303 in the transverse and longitudinal directions.

Detailed specification:

A subwavelength-resolution optical imaging device for conveying light having a wavelength .lambda, emitted, reflected or transmitted by a sample, the device comprising: a coherent waveguide array comprising a plurality of metal optical plasmon waveguides disposed substantially coaxially along their lengths, each optical waveguide having a first optical waveguide end and a second optical waveguide end, and each optical waveguide comprising a metal core and a cladding of refractive index nl wherein: the coherent waveguide array includes an observation end comprising the first optical waveguide ends which collectively present an observation end face for at least one of introducing, conveying and emitting the light; the coherent waveguide array further includes a sampling end comprising the second optical waveguide ends, each second optical waveguide end being of reduced diameter in comparison to the first optical waveguide end and, preferably, tapered from the observation end so that the waveguides essentially stay in close relative proximity of their neighbouring waveguides (as is drawn schematically in Fig. Ia) to present a sampling end face, the core of each optical waveguide at the sampling end face having a cross-sectional diameter which is less than or equal to .lambda.; and wherein the values of nl, and the geometry of the metal waveguides and their the optical properties, the separation between the waveguides are selected so that at least a predetermined fraction of the light launched into plasmom ^' c surface modes localised to individual, or a small set of, metal waveguides at the sampling end of the coherent waveguide array are permitted to be conveyed through the optical waveguides to the observation end of the coherent waveguide array.

The subwavelength-resolution imaging device as mentioned above, wherein the waveguides in the coherent waveguide array are distributed in a rectangular cross-sectional format at least at the observation end or the sampling end.

The subwavelength-resolution imaging device as mentioned above, wherein the waveguides in the coherent waveguide array are distributed in a hexagonal (triangular) cross-sectional format at least at the observation end or the sampling end. The subwavelength-resolution imaging device as mentioned above, wherein the values of nl, the waveguides cross-sectional diameters at the observation end as well as the sampling end, waveguide lengths, waveguide cross-sectional geometry, the optical properties of the metal waveguides, the separation between the waveguides, the surface of the protecting tube around the device are selected so that light which is not being guided by individual waveguides (or a small set of waveguides deemed suitable to use for sub wavelength imaging) and interferes with sub wavelength imaging, experiences to a predetermined level larger relative losses than the light, which is guided by the waveguides and wanted for sub wavelength imaging.

The subwavelength-resolution imaging device as mentioned above, wherein an optically opaque mask is positioned on the observation end of the waveguide array so that unwanted light interfering with imaging that is propagating in the cladding of refractive index nl, is attenuated to a predetermined level, and the said mask has areas situated above (areas being slightly larger, same size or slightly smaller than the waveguides) the metal waveguides where the mask attenuates light less so that the light which is guided by the waveguides and wanted for imaging experiences less attenuation than the unwanted light.

An analysis technique of the subwavelength-resolution imaging devices of claims described as mentioned above, wherein the devices are scanned transversely and/or longitudinally with sub- pixel steps in order to extract additional information of the near field.

An analysis technique of the subwavelength-resolution imaging devices of claims described as mentioned above, wherein the light emerging from the observation end of the device is sectioned into a plurality of sample portions so that each sample contains, essentially, only light which emerged from the observation end of the subwavelength-resolution imaging device within a specified band of angles relative the longitudinal axis of the waveguide array, so that the light in those samples can be used separately as a function of longitudinal emission angle in order to extract additional information of the near field.

A method for making a coherent waveguide array with metal waveguides or claddings, comprising the steps of: obtaining an optically transmissive fibre/rod with a predetermined array of longitudinal air-holes in a predetermined configuration, geometry and dimensions; tapering the said fibre by stretching and heating it locally to a predetermined waist diameter and longitudinal tapering profile in such a way that the longitudinal holes are intact but reduced in dimensions in a predetermined fashion; and then depositing metal at predetermined places and thicknesses in the air-holes by means of chemical vapour deposition, physical vapour deposition or wet chemical deposition, potentially with thickness and transverse profile localised by means of a local temperature increase or UV- light exposure, or by pressure intrusion of molten metal; where the tapered fibre may be cleaved somewhere longitudinally to make metal deposition easier.

A method for making a coherent waveguide array with metal waveguides or claddings, comprising the steps of: obtaining an optically transmissive fibre/rod with a predetermined array of longitudinal air-holes in a predetermined configuration, geometry and dimensions; and then depositing metal at predetermined places and thicknesses in the air-holes by means of chemical vapour deposition, physical vapour deposition or wet chemical deposition, potentially with thickness and transverse profile localised by means of local temperature increase or UV-light exposure, or by pressure intrusion of molten metal; and then taper the said fibre/rod by stretching and heating it locally to a predetermined waist diameter and longitudinal tapering profile in such a way that the longitudinal holes are intact but reduced in dimensions in a predetermined fashion and the metal inside the holes, which may have been melted or vaporised in the tapering process, also remains in a predetermined fashion.

A subwavelength-resolution optical imaging device for conveying light having a wavelength .lambda, emitted, reflected or transmitted by a sample, the device comprising: a coherent waveguide array comprising a plurality of metal optical plasmon waveguides disposed substantially coaxially along their lengths, each optical waveguide having a first optical waveguide end and a second optical waveguide end, and each optical waveguide comprising a metal core and a cladding of refractive index nl wherein: the coherent waveguide array includes an observation end comprising the first optical waveguide ends which collectively present an observation end face for at least one of introducing, conveying and emitting the light; the coherent waveguide array further includes a sampling end comprising the second optical waveguide ends, each second optical waveguide end being of essentially similar diameter in comparison to the first optical waveguide end where the essentially parallel waveguides stay in close relative proximity of their neighbouring waveguides to present a sampling end face, the core of each optical waveguide at the sampling end face having a cross-sectional diameter which is less than or equal to .lambda.; and wherein the values of nl, and the geometry of the metal waveguides and their the optical properties, the separation between the waveguides are selected so that at least a predetermined fraction of the light launched into plasmonic surface modes localised to individual, or a small set of, metal waveguides at the sampling end of the coherent waveguide array are permitted to be conveyed through the optical waveguides to the observation end of the coherent waveguide array; wherein the coherent waveguide array is fabricated using either the methods described in claims as mentioned above.

A fitting device for the devices described in claims as mentioned above which can be fitted onto microscope objective houses as is schematically shown in figure 3. A focusing mechanism of the devices described in claims as mentioned above enabled by the screwing of the fitting device onto the microscope objective housing.

An image resolution enhancement mechanism of the devices in claim as mentioned above whereby sub-pixel movement in any combination of the transverse or longitudinal direction is enabled by the fitting of a translation stage, which could be of a piezo electric material, onto any part of either the housing 301, fitting 302 or the coherent waveguide array 303.

References:

U.S. Patent Documents 5633972, Walt and Pantano of Tufts college 5973316 Oct., 1999 Ebbesen et al. 6016376 by Ghaemi 6040936 Mar., 2000 Kim et al. 359/245. 6052238 Apr., 2000 Ebbesen et al. 6236033 May., 2001 Ebbesen et al. 6285020 Sep., 2001 Kim et al. 6441298 Aug., 2002 Thio 136/250. 6818907 Oct., 2001 Stark Other References

1. Ebbesen, T. W., Lezec, H. J., Ghaemi, H. F., Thio, T. & Wolff, P. A. Extraordinary optical transmission through sub-wavelength hole arrays. Nature 391, 667-669 (1998).

2. Ghaemi, H. F., Thio, T., Grupp, D. E., Ebbessen, T. W. & Lezec, H. J. Surface plasmons enhance optical transmission through subwavelength holes. Phys. Rev. B 58, 6779-6782 (1998).

3. Kim, T. J., Thio, T., Ebbessen, T. W., Grupp, D. E. & Lezec, H. J. Control of optical transmission through metals perforated with subwavelength hole arrays. Opt. Lett. 24, 256-258 (1999).

4. Grupp, D. E., Lezec, H. J., Ebbessen, T. W., Pellerin, K. M. & Thio, T. Crucial role of metal surface in enhance transmission through subwavelength apertures. App. Phys. Lett.

77, 1569-1571 (2000).

5. Sonnichsen, C, Duch, A. C, Steininger, G., Koch, M. & Plessen, G. V. Launching surface plasmons into nanoholes in metal films. App. Phys. Lett. 76, 140-142 (2000).

6. Sandoz, P., Giust, R. & Tribillon, G. Multi-aperture optical head for parallel scanning near field optical microscopy. Opt. Commun. 161, 197-202 (1999).

7. Grupp, D. E., Lezec, H. J., Thio, T. & Ebbessen, T. W. Beyond the Behte Limit: Tunable Enhanced Light transmission Through a Single Sub-Wavelength Aperture. Adv. Mater. 11, 860-862 (1999).

8. Strelniker, Y. M. & Bergman, D. Optical transmission through metal films with subwavelength hole array in the presence of a magnetic field. Phys. Rev. B 59, 12763-

12766 (1999).

9. Raether, H. Surface Plasmons on Smooth and Rough Surfaces and on gratings (Springer- Verlag, 1988).

10. Lezec, H. J. et al. Beaming light from a subwavelength aperture. Science 297, 820-822 (2002).

11. Jung, L.S., Campbell, C.T., Chinowsky, TM, Mar, M.N. & Yee, S.S. Quantitative Interpretation of the Response of Surface Plasmon Resonance Sensors to Adsorbed Films. Langmuir, vol. 14, No. 19, 5636-5648 (1998).

12. Thio, T., Pellerin, K.M. & Linke, R.A. Enhanced light transmission through a single subwavelength aperture. Optics Letters, vol. 26, No. 24, 1972-1974(2001).

13. B. T. Schwartz, R. Piestun, "Total external reflection from metamaterials with ultralow refractive index," J. Opt. Soc. Am. B, 20, 12.

14. B. T. Schwartz, R. Piestun, "Waveguiding in air by total external reflection from ultralow index metamaterials," Appl. Phys. Lett., 85, 1.

15. Z. Yuan, N. H. Dryden, X. Li, J. J. Vittal, R. J. Puddephatt, "Chemical vapour deposition of copper or silver from the precursors [M(hfac)(C=NMe)][M=Cu,Ag;hfac=CF ₃ C(O)CHC(O)CF ₃ ], J. Mater. Chem., 5,

16. 2, pp. 303-307, 1995. 17. Itsuki, H. Uchida, M. Satou, K. Ogi, "Properties of a new organo silver compound for

MOCVD", Nuc. Instr. And Methods in Phys. Res. B, 121, pp. 116-120, 1997.

18. J. Foord et al., J. Chem. Soc. Chem. Commun., 11, 1990

19. J. Glass et al., J. Phys. Chem. Solids, 57, 563, 1996

20. Ono, J. Kato, and S. Kawata, "Sub wavelength Optical Imaging through a Metallic Nanorod Array", Phys. Rev. Lett. 95, 267407 (2005)

21. G. Shvets, S. Trendafilov, J. B. Pendry and A. Sarychev, "Guiding, Focusing, and Sensing on the Subwavelength Scale Using Metallic Wire Arrays", Phys. Rev. Lett. 99, 053903 (2007)

Figure captions (note, in all figures: glass is grey, air is white and metal is black): Naming nomenclature of the ends of the probe assumes that the probe is used to magnify optical nearfield to dimensions resolvable by standard optical means. If the probe is used to focus incident light below the diffraction limit, the names of the ends of the probe can be interchanged.

Fig. 1, Longitudinal crossections of the probe

Fig. Ia, longitudinal crossection of the whole probe. The measure 101 is the longitudinal length of the probe.

Fig. Ib shows a magnified portion of the observation end where 102 is the dielectric cladding and 103 is one of the metallic waveguides.

Fig. Ic shows a magnified portion of the sampling end.

Fig. 2, Transverse crossections of the probe.

Fig. 2a shows the whole probe in transverse crossection at the observation end. Fig. 2b shows a magnified portion of the observation end. The measure 201 is the distance between two metallic waveguides and the measure 202 is the diameter of a metallic waveguide.

Fig. 2c shows a magnified portion of the transverse crossection of the down tapered sampling end. The measure 203 is the distance between two metallic waveguides and the measure 204 is the diameter of a metallic waveguide.

Fig. 3, Schematic of module design allowing for fitting of probe to existing microscope objective tube housings

Microscope housing containing suitable lens system is represented by 301, the module is 302 and the sub- wavelength probe is 303.