WO2014079450A1

WO2014079450A1 - Super-resolution near field imaging device

Info

Publication number: WO2014079450A1
Application number: PCT/DK2013/050365
Authority: WO
Inventors: Andrei Andryieuski; Radu Malureanu; Andrei Lavrinenko
Original assignee: Danmarks Tekniske Universitet
Priority date: 2012-11-23
Filing date: 2013-11-07
Publication date: 2014-05-30

Abstract

Super-resolution imaging device comprising at least a first and a second elongated coupling element, each having a first transverse dimension at a first end and a second transverse dimension at a second end and being adapted for guiding light between their respective first and second ends, each further comprising a dielectric center element and a metallic side-wall region, which surrounds the dielectric center element. The coupling elements are arranged in a matrix comprising a matrix material and the first ends of the coupling elements are located at or in a vicinity of the first side of the matrix and the second ends of the coupling elements are located at or in a vicinity of the second side of the matrix. The second transverse dimension is larger than the first transverse dimension. A microscope objective system and a microscope comprising the super-resolution imaging device are also claimed.

Description

SUPER-RESOLUTION NEAR FIELD IMAGING DEVICE

FIELD OF THE INVENTION

The present invention relates to a device for improving a resolution of optical microscopes. More specifically, the invention relates to a device for acquiring super-resolution images of samples.

BACKGROUND OF THE INVENTION

Optical microscopes are a very commonly used and versatile tool with application within a vast number of technical fields. A fundamental limitation of optical systems is the diffraction limit, which means that a dielectric lens is not able to resolve features which are smaller than the wavelength of light observed, divided by twice the numerical aperture of the lens. This limit may be pushed, e.g. by observing the sample under light with short wavelengths, e.g. blue light. This however, is not always possible, e.g. if the feature to be observed only has a small contrast to surrounding areas of the sample under such wavelengths.

An alternative technology capable of imaging a sample with a resolution higher than the diffraction limit is a scanning near-field optical microscope (SNOM) (see e.g Ash, et al. Nature 237, p.510-512, 1972). Such systems commonly use a metal-coated optical fibre, tapered to a thin tip for collecting radiation from the near-field of a sample. US 5,633,972 discloses such a SNOM system using an array of SNOM tips.

EP 0 185 782 discloses a waveguide device for an optical near-field microscope, i.e. a SNOM. The waveguide device is disclosed to comprise 2, 3, or 5 separate transparent bodies, i.e. waveguides, within the same waveguide device, such that light may be guided from an external light source onto a sample via one, central, transparent body (waveguide), and reflected light from the sample may be collected by one, two, or four other transparent bodies (waveguides) onto a number of optical detectors being directly connected thereto. The waveguide device is scanned across the sample to acquire an image, point by point.

WO 2008/092197 Al discloses a subwavelength optical/plasmon near-field channeling multi-core probe. The probe comprises an array of parallel metal wires acting as waveguides, clad with optical fibre material. The array is tapered from one end to the other, and the tapered end transmits light in plasmonic surface modes on an outside of the wires through the array to the opposite end . The number and spacing of the wires dictates the resolution obtainable with the probe.

Hence, an improved resolution in optical microscope images would be

advantageous. OBJECT OF THE INVENTION

It is a further object of the present invention to provide an alternative to the prior art.

In particular, it may be seen as an object of the present invention to provide a super-resolution imaging device that solves the above mentioned problems of the prior art with a relatively low optical loss and a high resolution.

SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a super-resolution imaging device for sampling an electromagnetic near field of a sample and transposing it as a far field image substantially corresponding to the sample near field, the near field having at least a first wavelength component. The device comprises at least a first and a second elongated coupling element. Each element has a first transverse dimension at a first end and a second transverse dimension at a second end and a longitudinal direction between the first end and the second end . Each of the first and second coupling elements are adapted for guiding light between their respective first and second ends. The first and second coupling elements each further comprises a dielectric center element, extending in the longitudinal direction throughout the coupling element from the first end to the second end, and a metallic side-wall region. The metallic side-wall region surrounds the dielectric center element along the longitudinal direction from the first end to the second end . The metallic side-wall region has an inner surface facing the dielectric center element, and an outer surface facing away from the dielectric center element. The coupling elements are arranged in a matrix comprising a matrix material, the matrix having a first side and a second side. The first and second coupling elements being arranged so that the first ends of the coupling elements are located at or in a vicinity of the first side of the matrix and the second ends of the coupling elements are located at or in a vicinity of the second side of the matrix, such that the first and second ends of the dielectric center elements are in optical communication with an exterior of the device. The second transverse dimension is larger than the first transverse dimension. In this way, when the device is within the near field limit (e.g. within 20-50nm) of the sample to be imaged, the first end of the first coupling element of the device may collect light from an area of the sample having a dimension on the order of the first transverse dimension. Likewise, the second coupling element may collect light from an area having a dimension on the order of the second transverse dimension. After transmission through the coupling elements, the light is coupled out at the respective second ends of the coupling elements. If light is input at the first end and output at the second end, the light spot corresponding to the coupling element has been enlarged at the output relative to the spot size at the input. The light at the second side of the device may then be imaged by conventional means, such as a microscope. Thus, each coupling element may be seen as providing a "pixel" of the near field image.

Thus, in an embodiment of the invention, the device is suitable for imaging of an electromagnetic near field of a sample.

In an embodiment of the invention, the far field image substantially corresponds to the near field of the sample in that the far field image has an optical intensity distribution which at least qualitatively corresponds to the near field sampled by the coupling elements.

In an embodiment, the first and/or second coupling element is an optical waveguide.

In an embodiment, the metallic side-wall region comprises a continuous metal layer extending along the dielectric center element.

In an embodiment, the metallic side-wall region comprises metal nano-particles so as to form a non-continuous layer.

In an embodiment, the device comprises a first distance between the first end of the first coupling element and the first end of the second coupling element, and a second distance between the second end of the first coupling element and the second end of the second coupling element, wherein a first ratio of the first distance to the first transverse dimension of the first coupling element is substantially equal to a second ratio of the second distance to the second transverse dimension of the first coupling element.

In an embodiment, the first ratio and the second ratio are equal to within 20%, such as within 10%, or even within 5%.

In an embodiment of the invention, the first ends of the first and second coupling elements, respectively, are substantially identical, and wherein the second ends of the first and second coupling elements are also substantially identical.

In an embodiment of the invention, the first coupling element and the second coupling element are substantially identical.

In an embodiment of the invention, the first transverse dimension of the first and/or second coupling element is about 200 nm or less, such as about 150 nm or less, or even about 110 nm or less, and wherein the first transverse dimension is about 10 nm or more, such as about 20 nm or more, or even about 50 nm or more. Coupling of radiation from the near field into the coupling elements is generally improved by increasing the first transverse dimension. In contrast, the resolution obtainable by the device is in general increased by decreasing the first transverse dimensions and thereby enabling that the first ends of the first and second coupling elements are located closer together. A first transverse dimension of these sizes is particularly advantageous as it provides a suitable trade-off between resolution and coupling of light to the coupling element.

In an embodiment of the invention, the first transverse dimension of the first and/or second coupling element is in the range of about 10 nm to about 200 nm, such as about 20 nm to about 150 nm, or even about 50 nm to about 110 nm. Coupling of radiation from the near field into the coupling elements is generally improved by increasing the first transverse dimension. In contrast, the resolution obtainable by the device is in general increased by decreasing the first transverse dimensions and thereby enabling that the first ends of the first and second coupling elements are located closer together. A first transverse dimension of these sizes is particularly advantageous as it provides a suitable trade-off between resolution and coupling of light to the coupling element. In an embodiment of the invention, the first transverse dimension of the first and/or second coupling element is about 50% or less of a wavelength of the wavelength component, such as about 30% or less, or even about 20% or less, and wherein the first transverse dimension is about 5% or more of the

wavelength, such as about 10% more, or even about 15% or more.

In an embodiment of the invention, the first transverse dimension of the first and/or second coupling element is in the range of about 5 % to about 50% of a wavelength of the wavelength component, such as about 10% to about 30%, or even about 15% to about 20%. In an embodiment of the invention, the second transverse dimension of the first and/or second coupling element is about 2 μηι or less, such as about 1.5 μηι or less, or even about 1.1 μηι or less, and wherein the second transverse dimension is about 0.2 μηι or more, such as about 0.3 μηι or more, or even about 0.5 μηι or more. A second transverse dimension of such dimensions is particularly

advantageous, as such a spot size is conveniently imaged by conventional means, such as a microscope.

In an embodiment of the invention, the second transverse dimension of the first and/or second coupling element is in the range of about 0.2 μηι to about 2 μηι, such as about 0.3 μηι to about 1.5 μηι, or even about 0.5 μηι to about 1.1 μηι. A second transverse dimension of such dimensions is particularly advantageous, as such a spot size is conveniently imaged by conventional means, such as a microscope.

In an embodiment of the invention, the second transverse dimension of the first and/or second coupling element is about 200% or less of a wavelength of the wavelength component, such as about 150% or less, or even about 100% or less, and wherein the second transverse dimension is about 20% or more, such as about 40% or more, or even about 60% or more.

In an embodiment of the invention, the second transverse dimension of the first and/or second coupling element is in the range of about 20% to about 200% of a wavelength of the wavelength component, such as about 40% to about 150%, or even about 60% to about 100%. In an embodiment of the invention, the first and/or the second coupling element has a length along the longitudinal direction of about Ιμηι or more, about 5μηι or more, such as about 25pm or more, or even about 50μηι or more, and wherein the length is about 150μηι or less, such as about 130μηι or less, or even about ΙΟΟμηι or less. Having a long coupling element aids in achieving a low loss arising from transverse expansion of light propagating through the element, but will instead increase propagation loss. A coupling element with a length in these ranges may in general be made long enough such that it may expand adiabatically so the light coupled in to the element at the first end will be gradually and without reflections be expanded in the transverse direction until reaching the second end. Thus the tapering loss may be minimized, while propagation loss may still be limited by keeping the length to a minimum.

In an embodiment of the invention, the first and/or the second coupling element has a length along the longitudinal direction in the range of about Ιμηι to about 150 μηι, such as about 25μηι to about 130 μηι, or even about 50μηι to about 100 μηη. Having a long coupling element aids in achieving a low loss arising from transverse expansion of light propagating through the element, but will instead increase propagation loss. A coupling element with a length in these ranges may in general be made long enough such that it may expand adiabatically so the light coupled in to the element at the first end will be gradually and without reflections be expanded in the transverse direction until reaching the second end. Thus the tapering loss may be minimized, while propagation loss may still be limited by keeping the length to a minimum.

In an embodiment of the invention, the first and/or the second coupling element has a length along the longitudinal direction of about 2 times a wavelength of the wavelength component or more, about 10 times or more, such as about 50 times or more, or even about 100 times or more, and wherein the length is about 300 times the wavelength or less, such as about 250 times or less, or even about 100 times or less. In an embodiment of the invention, the first and/or the second coupling element has a length along the longitudinal direction in the range of about 2 to about 300 times a wavelength of the wavelength component, such as about 10 times to about 250 times, or even about 50 times to about 100 times. In an embodiment of the invention, the coupling elements are adapted to support guiding of plasmonic modes at the inner surface of the metallic side wall, at least along a first length section extending from a point at or proximal to the first end. Plasmonic modes may generally be confined to a tighter transverse dimension than conventional index guided modes. Therefore, they are particularly suited for gathering light from the near field of the sample. Guiding plasmonic modes on the inner surface of the metallic side wall may further have the advantage that coupling between the first and second coupling element along the longitudinal direction may be minimized, especially compared to guiding plasmonic modes on an outside of a metallic wire.

In one embodiment, the coupling element comprises a tunneling section in close proximity to the first end. Thus, the light being collected from the near field tunnels through the tunneling section before reaching the first length section.

In an embodiment of the invention, the first length section has a longitudinal length being 50% or less, such as 20% or less, or even 10% or less of a total longitudinal extent of the coupling element, the longitudinal length being 0 % or more, such as 5% or more, or even 9% or more. Plasmonic modes commonly have higher propagation loss than index guided modes. Therefore, in one embodiment, the light propagating in plasmonic modes is rapidly coupled to an index guided mode, after being coupled into the coupling element.

In an embodiment of the invention, the first length section has a longitudinal length in the range of 0 % to 50%, such as 5% to 20%, or even 9% to 10% of a total longitudinal extent of the coupling element. Plasmonic modes commonly have higher propagation loss than index guided modes. Therefore, in one embodiment, the light propagating in plasmonic modes is rapidly coupled to an index guided mode, after being coupled into the coupling element.

In an embodiment of the invention, the first length section has a longitudinal length being 40μηι or less, such as ΙΟμηι or less, or even 5μηι or less of a total longitudinal extent of the coupling element, the longitudinal length of the first length section being 0.2μηι or more, 0.5μηι or more, such as 2μηι or more, or even 5μηι or more. In an embodiment of the invention, the first length section has a longitudinal length in the range of 0.2 μηι to 40 μηι, such as 0.5 μηι to 10 μηι, or even 2μηι to 5μηι of a total longitudinal extent of the coupling element.

In an embodiment of the invention, the dielectric center element of the coupling elements are adapted to support dielectric waveguide modes, at least along a second length section extending from the second end. Waveguide modes may in general be propagated in the dielectric center with relatively low propagation loss when the transverse dimension of the waveguiding structure, here being the coupling element, is at least on the order of the wavelength of the light. In an embodiment of the invention, the second length section has a longitudinal length being 80% or less, such as 70% or less, or even 60% or less of a total longitudinal extent of the coupling element, the longitudinal length of the second length section being 10 % or more, such as 20% or more, or even 30% or more.

In an embodiment of the invention, the second length section has a longitudinal length in the range of 10% to 80%, such as 20% to 70%, or even 30% to 60% of a total longitudinal extent of the coupling element.

In an embodiment of the invention, the second length section has a longitudinal length being ΙΟΟμηι or less, such as 50μηι or less, or even 30μηι or less of a total longitudinal extent of the coupling element, the longitudinal length of the second length section being 5μηι or more, such as ΙΟμηι or more, or even 15μηι or more.

In an embodiment of the invention, the second length section has a longitudinal length in the range of 5μηι to ΙΟΟμηι, such as ΙΟμηι to 50μηι, or even 15μηι to 30μηι of a total longitudinal extent of the coupling element.

In an embodiment of the invention, the first coupling element is adiabatically tapered from the first end to the second end. In this way, the optical loss arising from the change in transverse dimension along the length of the coupling elements may be minimized.

In an embodiment of the invention, an intermediate transverse dimension of the first coupling element and/or the second coupling element monotonically increases in the longitudinal direction from the first transverse dimension at the first end to the second transverse dimension at the second end. In an embodiment of the invention, the first coupling element and/or the second coupling element has a substantially circular cross-section along its length, and wherein the transverse dimension is a diameter.

In an embodiment of the invention, the dielectric center element comprises a dielectric material comprising a glass material, such as chalcogenide glass. The use of chalcogenide glass is advantageous as this material has a relatively large refractive index and a relatively low optical attenuation. Furthermore, the

Chalcogenide glass can be photosensitive thus its manufacturing process can be done via Two-Photon Polymerisation (2PP). Also, it can be doped with various metals thus changing its refractive index values.

In an embodiment of the invention, the dielectric center element comprises a dielectric material comprising a polymer, such as PMMA, POM, IPL, IPG, SU8, or Az resist. Thus, a particularly efficient manufacturing process may be achieved.

In an embodiment of the invention, the dielectric center element comprises a dielectric material, the dielectric material being doped with a dopant material.

In an embodiment of the invention, the dopant material comprises nanoparticles.

In an embodiment of the invention, the dielectric center element comprises at least a first dielectric material and a second dielectric material.

In an embodiment of the invention, the metallic side wall comprises a metal chosen from the group of: Ag, Au, Cu, Al, or alkaline metals. The metallic side wall should preferably be configured to provide an optical attenuation as low as possible. This may be achieved by the abovementioned materials.

In an embodiment of the invention, the metallic side wall comprises a multi-layer metallic film. In this way, physical properties of the metallic side wall may be optimized. For instance, the optical properties such as loss may be improved.

Imaging device according to any one of the preceding claims, wherein the first and/or second coupling element comprises an anti-reflection region at the first end and/or at the second end. In this way, the anti-reflection region may work to maximize field coupling into and out of the coupling element. In an embodiment of the invention, the first and second coupling elements are arranged in the matrix so as to be substantially parallel at their respective first ends. Thus, light from the sample is collected in the same direction for both coupling elements. In an embodiment of the invention, the first and second coupling elements are arranged in the matrix so as to be substantially parallel at their respective second ends. In this way, light coupled out at the second end is emitted in the same direction. This improves the quality of image acquired.

In an embodiment of the invention, the matrix material comprises a polymer. The matrix material should be chosen to provide a mechanical strength to the imaging device which is sufficiently large to withstand typical forces occurring during manufacturing, use or storage of the device.

In an embodiment of the invention, the matrix material is chosen to be optically absorbing at wavelengths in the visible range. This means that light transmitted through the matrix material from the first side to the second side of the imaging device experiences a significant optical attenuation, e.g. such as an attenuation of more than lOdB, such as more than 20dB, or even such as more than 30dB.

Preferably, the attenuation of light propagating through the matrix material should be at least as high as attenuation of light being transmitted through the coupling elements at the wavelengths of interest.

In an embodiment of the invention, the device comprises a number of first and/or second coupling elements, the number of coupling elements being arranged in the matrix so that the respective first ends of the coupling elements together form a pattern, and wherein the respective second ends of the coupling elements together form substantially the same pattern. Thus, the pattern at the second end will basically be an up-scaled version of the pattern at the first end. In this way, the near field of a sample coupled into the coupling elements at the first end will be imaged enlarged at the second end of the device.

In an embodiment of the invention, the pattern is a regular pattern, such as a rectangular pattern or a hexagonal pattern. Thus, each coupling element may act as a pixel to be imaged by conventional means at the second end. In an embodiment of the invention, the number of coupling elements is about 10 or more, such as about 50 or more, or even about 90 or more, the number being about 1000 or less, such as about 500 or less, or even about 150 or less.

In an embodiment of the invention, the number of coupling elements is in the range of about 10 to about 1000, such as about 50 to about 500, or even about 90 to about 150.

In an embodiment of the invention, the number of coupling elements comprises a first array of coupling elements, and wherein the device further comprises at least a second array of coupling elements, the second array of coupling elements also comprising substantially identical patterns at the first and the second ends, the first array and second array together forming a super-array of coupling elements. In this way, the imaging device may simultaneously sample multiple regions of the sample, corresponding to each array of the super-array. If the device is used in a scanning setup, wherein the device is scanned over an extended area of the sample, the simultaneous imaging capabilities achieved by use of a super-array may be used to increase the speed in sampling the extended area of the sample.

Thus, the above described object and several other objects are also intended to be obtained in a second aspect of the invention by providing a microscope objective system comprising a super-resolution imaging device according to any one of the abovementioned embodiments and an objective, wherein the first side of the imaging device is configured for being brought in proximity to the sample to be imaged and wherein the objective is configured for imaging a transmitted image of the sample displayed at the second side of the imaging device.

The objectives are further intended to be obtained in a third aspect of the invention by providing a microscope comprising the microscope objective system according to the second aspect.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE FIGURES

The device according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 is a schematic view of a second side of an embodiment of the device according to the invention,

Figure 2 is a cross-section of the embodiment of Figure 1 along the line A-A,

Figure 3 illustrates use of an embodiment of the device in an embodiment of the microscope objective system according to an aspect of the invention,

Figure 4 another embodiment of the device,

Figure 5 shows a simulation of a coupling element,

Figure 6a-c show simulations of a device according to the invention, and

Figure 7 illustrates production steps of making a device according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

Figure 1 shows an embodiment of a super-resolution device 100 according to of the invention. The device is seen from a second side, and comprises a number of coupling elements 102 embedded in a matrix 104. The coupling elements 102 are here arranged in a rectangular grid, of 5x5 elements. However, many other arrangements are also possible, such as a hexagonal grid, or even a random arrangement. Also, in other embodiments of the invention, a larger number of coupling elements may be present in the device. The skilled person will realize that the resolution obtainable from the device is directly linked to the number of coupling elements. Each coupling element has a second transverse dimension d, on the second side, as illustrated here, and a first transverse dimension at a first side (not shown. In this case, the second transverse dimension is a diameter. If the coupling elements are non-circular in cross-section, the transverse dimension may be taken to be a diameter of an imaginary circumscribed circle of the actual element. Each coupling element 102 comprises a dielectric center element 106, e.g. comprising glass or polymer. Surrounding the dielectric center element 106 is a metallic side-wall region 108. The metallic side-wall region 108 may comprise a metallic coating extending along the dielectric center element 106. Alternatively, the metallic side-wall region 108 may comprise metallic nano-particles disposed along center element 106, the particles being electrically isolated from each other. In one embodiment, the metallic side-wall region 108 may be patterned to improve transmission.

There are many different matrix materials which may be used, as the function of the matrix is primarily to provide mechanical strength to the structure. However, it is advantageous if the matrix material is optically absorbing at the desired wavelength(s) of observation. In that case, propagation of "false light", i.e.

electromagnetic radiation transmitted through the device but outside the coupling elements may be suppressed. As the coupling elements 106 are adapted to guide electromagnetic radiation within the element, a high optical loss in the matrix outside the element has limited or even no effect on the attenuation of the desired radiation along the coupling elements. In some cases, the matrix material may even comprise a metal, so that the metallic side-wall region of one coupling element extends to the metallic side-wall region of a neighboring coupling element.

Figure 2 illustrates a cross-section along the line A-A in Figure 1 (only showing the five center-most coupling elements). The device 100 is positioned so that the first side 120 is within the electromagnetic near field of a sample 124 to be examined. Radiation from the near field is then transmitted through the coupling elements 106 to the second side 122 from where it may be observed e.g. with a

conventional microscope. The embodiment shown here also comprises a glass substrate 126 on the second side 122, but other embodiments of the invention do not comprise the substrate. The coupling elements 106 preferably widen adiabatically so as to enable a smooth expansion of the field from the first end 127 to the second end 128, thus requiring a certain length. To minimize the propagation loss experienced by the field propagating along the coupling elements, the coupling elements should, however, preferably be as short as possible.

Figure 3 illustrates an embodiment of the microscope objective system 130 according to the invention. The super-resolution imaging device 100, e.g. as illustrated in Figures 1 and 2 is held in front of a microscope objective 132. Thus, electromagnetic radiation from the near field of the sample 124 is coupled through the device 100, effectively resulting in a magnified image being shown on the second sidel22 of the image acquired at the first side of the device 100. Since the device must be accurately and reliably placed in the near field of the sample 124, i.e. 20-50nm from the sample, it may be advantageous to mount the device 100 and/or the sample 124 on a motorized stage. The device 100 and the objective 132 are here shown as separate units, which may be a way to upgrade an existing objective. However, the inventors also envision that the device and objective may be integrated in a single unit for more convenient handling.

Figure 4 shows another embodiment of the super-resolution imaging device 140 according to the invention, as seen from the second side. This embodiment comprises number of arrays 142 of coupling elements (here 2x2 are shown), e.g. each corresponding to the embodiment of the device 100 shown in Fig. 1. The number of arrays is arranged in a super-array. This embodiment may be advantageous for use in a scanned setup, wherein the device is scanned over the surface of the sample to assemble an image. Each array 142 will transmit and magnify a sub-image of a part of the sample, the super-array thus transmitting a super-image comprising a number of such sub-images. Each sub-image will ideally be true to scale, in the sense that distances within the sub-image are a scaling of the distance between the corresponding sample features. In contrast, the super-image will not necessarily be true to scale with regards to distances between features found in different sub-images. This possible error in the imaging between sub-images of a super-image may subsequently be corrected by postprocessing, e.g. in a computer. After a super-image has been acquired, the sample and/or the imaging device is stepped with respect to each other, e.g. by a distance corresponding to a field of view of one sub-array, where after the subsequent super-image is acquired. This procedure may then be repeated until the desired area of the sample has been imaged, at which point the acquired super-images are post- processed to assemble a compound image of the sample. In this way, acquisition of the image may be parallelized to increase the recording speed. To aid the post-processing, the device 140 may comprise an indicator 144 corresponding to an interface between the individual sub-arrays within the super- array. The indicator may for instance be a visual indicator which may be seen with the microscope used for acquiring the image. Example 1

Figure 5(a) a nd (b) show a simulation performed in the com mercial software package CST m icrowave stud io® (CST MWS) to eva luate transm ission through a n em bodiment of a single coupling element according to the invention. Figure 5(a) shows a cross section of the geometry 150 studied, i .e. a dielectric cone 152 in a metallic surrounding 154. The cone is 2 μηι long, sta rting with a circular hole 150 nm in diameter (first tra nsverse dimension in the first end 127) and end ing with a circula r hole 700 nm in diameter (second transverse dimension in the second end 128). The metal is taken to be silver a nd the d ielectric to be silica (n = 1.5) . The system is solved in the frequency doma in in a tetrahedra l mesh . Excitation is with a point d ipole.

Figure 5(b) shows the ca lculated absolute electric field through the ca lculation doma in . The triang le just outside the first end 127 of the cone ind icates the source. The field is clea rly seen to propagate through the coupling element to the second end 128, primarily within the d ielectric. Calculations show that the transm ittivity from source to output (second end) is -72dB. In a com parative calculation, transm ission along a metal nanowire (not shown) with a constant diameter of 150nm and length of 2μηι was found to be a bout -84d B. Most of the loss comes from reflection from the input at the first side. The light intensity that is coupled into the device reached the second side with only 40% reduction .

Further optim ization of the structure is likely to im prove the transm ittivity even further. Reducing reflection of the incom ing light may thus significantly im prove the over-a ll performance of the device.

A critica l issue with the cone is the eva nescent nature of waves at the first end (input). Before the diameter reaches a critical diameter, the propagation has a high loss. We may say that no propagating modes are a llowed in that region so the field observed is tunneling through the "ba rrier". Thus, a trade-off is to make the transition from the subcritical d iameter to above critica l as fast as possible (at the shortest d istance) keeping at the same time dense packaging of the tips. Example 2

Figure 6a-c shows another sim ulation, in this case of a 1x5 array 160 of coupling elements. Each coupling element has the same overall geometry as described above in Example 1, except that the surface on the first side 120 has been retracted a distance of 300nm from the first ends 127 of the coupling elements, in order to minimize unwanted field coupling between the coupling elements.

Figure 6b shows the calculated absolute electric fields, when the device is excited with a point dipole near the first end of the central coupling element. Some excitation is observed also in the neighboring coupling elements, but only to a minor degree. This may be due to the disposition of the coupling elements (not in a plane, but oriented towards the dipole).

In a practical device, a plane disposition would typically be preferred due to both manufacturing and use of the device. In the case of cones parallel with each other, as illustrated in Figure 6c, the excitation in the neighboring cones is basically zero.

Figure 7 illustrates a method of manufacturing a super-resolution imaging device, according to the invention. The method comprises:

• Two-photon polymerisation (2pp) for defining the coupling elements

o Deposition 170 of the 2pp material on a thin glass substrate (such as a 140microns thick glass slide)

o Polymerise the 2pp material where it is needed to define the dielectric center elements

o Develop 171 the material such that the only part remaining are the dielectric center elements

• Optionally perform other aligned 2pp exposures 172 for defining extra polymer layers for better field confinement

• Electroless deposition of metal 173 for covering the dielectric center

elements so as to fabricate the metallic side-wall regions

o Perform treatment of surfaces for enhancing the adhesion between the metal and the polymer

o Actual deposition of the metal by reducing the metallic salt at the dielectric center element's surface

o Rinsing of the structure

• Optionally deposit other metals 174 to obtain multilayered side-walls

• Cover the structure with the matrix material 175. For instance, this can be done by spinning (if the material is a polymer or similar), evaporation or sputtering (in case of oxide or other materials). • Etch the matrix material 176 until the metallic tip of the coupling elements emerge.

• Etch the metallic tip 177 to allow light coupling from the near field to the coupling element.

· Optionally, perform aligned exposure 178 for defining the patterned areas at the surface to reduce coupling between the elements and help in coupling from free space into each element

o Resist spin

o Electron-beam exposure

o Etching of the matrix material using the resist as mask or filling up the resist or depositing some material and then remove the resist

• Optionally, deposit or pattern an anti-reflection coating 179.

o Perform multi-layer deposition of non-patterned layers (classic anti- reflection coating or deposition and aligned exposure for metamaterial-based antireflection coating

Devices according to the invention may also be manufactured in other ways. Some of these steps are optional, or may be combined with other steps. Further, it is noted that a sequence of the steps is not necessarily as stated here, but may be changed in a number of ways.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. Super-resolution imaging device for sampling an electromagnetic near field of a sample and transposing it as a far field image substantially corresponding to the sample near field, the near field having at least a first wavelength component, the device comprising :

at least a first and a second elongated coupling element, each element having a first transverse dimension at a first end and a second transverse dimension at a second end and a longitudinal direction between the first end and the second end, each of the first and second coupling elements being adapted for guiding light between their respective first and second ends, the first and second coupling elements each further comprising :

a dielectric center element, extending in the longitudinal direction throughout the coupling element from the first end to the second end, and

a metallic side-wall region, the metallic side-wall region surrounding the dielectric center element along the longitudinal direction from the first end to the second end, the metallic side- wall region having an inner surface facing the dielectric center element, and an outer surface facing away from the dielectric center element,

the coupling elements being arranged in a matrix comprising a matrix material, the matrix having a first side and a second side, the first and second coupling elements being arranged so that the first ends of the coupling elements are located at or in a vicinity of the first side of the matrix and the second ends of the coupling elements are located at or in a vicinity of the second side of the matrix, such that the first and second ends of the dielectric center elements are in optical

communication with an exterior of the device, and

wherein the second transverse dimension is larger than the first transverse dimension.

2. Imaging device according to claim 1, wherein the first transverse dimension of the first and/or second coupling element is about 200 nm or less, such as about 150 nm or less, or even about 110 nm or less, and wherein the first transverse dimension is about 10 nm or more, such as about 20 nm or more, or even about 50 nm or more.

3. Imaging device according to any one of the preceding claims, wherein the second transverse dimension of the first and/or second coupling element is about 2 μηι or less, such as about 1.5 μηι or less, or even about 1.1 μηι or less, and wherein the second transverse dimension is about 0.2 μηι or more, such as about 0.3 μηι or more, or even about 0.5 μηι or more.

4. Imaging device according to any one of the preceding claims, wherein the first and/or the second coupling element has a length along the longitudinal direction of about 5μηι or more, such as about 25μηι or more, or even about 50μηι or more, and wherein the length is about 150μηι or less, such as about 130μηι or less, or even about ΙΟΟμηι or less.

5. Imaging device according to any one of the preceding claims, wherein the coupling elements are adapted to support guiding of plasmonic modes at the inner surface of the metallic side wall, at least along a first length section extending from a point at or proximal to the first end.

6. Imaging device according to claim 5, wherein the first length section has a longitudinal length being 40μηι or less, such as ΙΟμηι or less, or even 5μηι or less of a total longitudinal extent of the coupling element, the longitudinal length of the first length section being 0.5μηι or more, such as 2μηι or more, or even 5μηι or more.

7. Imaging device according to any one of the preceding claims, wherein the dielectric center element of the coupling elements are adapted to support dielectric waveguide modes, at least along a second length section extending from the second end.

8. Imaging device according to claim 7, wherein the second length section has a longitudinal length being ΙΟΟμηι or less, such as 50μηι or less, or even 30μηι or less of a total longitudinal extent of the coupling element, the longitudinal length of the second length section being 5μηι or more, such as ΙΟμηι or more, or even 15μηι or more.

9. Imaging device according to any one of the preceding claims, wherein the first coupling element is adiabatically tapered from the first end to the second end.

10. Imaging device according to any one of the preceding claims, wherein the first and/or second coupling element comprises an anti-reflection region at the first end and/or at the second end.

11. Imaging device according to any one of the preceding claims, wherein the matrix material comprises a polymer.

12. Imaging device according to any one of the preceding claims, comprising a number of first and/or second coupling elements, the number of coupling elements being arranged in the matrix so that the respective first ends of the coupling elements together form a pattern, and wherein the respective second ends of the coupling elements together form substantially the same pattern.

13. Imaging device according to claim 12, wherein the number of coupling

elements is about 10 or more, such as about 50 or more, or even about 90 or more, the number being about 1000 or less, such as about 500 or less, or even about 150 or less.

14. A microscope objective system comprising a super-resolution imaging device according to any one of the preceding claims and an objective, wherein the first side of the imaging device is configured for being brought in proximity to the sample to be imaged and wherein the objective is configured for imaging a transmitted image of the sample displayed at the second side of the imaging device.

15. A microscope comprising the microscope objective system according to claim 14.