AU2017203205B1 - Frequency conversion of electromagnetic radiation - Google Patents

Abstract

A frequency conversion device, including an array of mutually spaced semiconductor islands composed of at least one III-V semiconductor compound and configured so that electromagnetic radiation of a first wavelength incident upon the semiconductor islands causes them to emit electromagnetic radiation of a second wavelength shorter than the first wavelength by a nonlinear frequency conversion process, wherein the semiconductor islands are supported by a transparent support such that the support is substantially transparent to radiation of the second wavelength, wherein at least the radiation of the second wavelength passes through the transparent support.

Description

FREQUENCY CONVERSION OF ELECTROMAGNETIC RADIATION

TECHNICAL FIELD

The present invention relates to frequency/wavelength conversion of non-ionising electromagnetic radiation, and in particular to a frequency conversion device, a frequency conversion process, and a process for producing a frequency conversion device.

BACKGROUND

There are many applications that require or at least benefit from the conversion of non-ionising electromagnetic radiation from one frequency/wavelength to a different frequency/wavelength, including imaging and detection applications. For example, night vision and thermal imaging devices are able to generate images in the visible light region from ambient or artificial radiation in the infrared region. However, despite the advantageous capabilities of existing frequency/wavelength conversion devices, there remains room for improvement in terms of their performance and also, in the case of night vision equipment, their bulkiness. It is desired, therefore, to overcome or alleviate one or more difficulties of the prior art, or to at least provide a useful alternative.

SUMMARY

In accordance with some embodiments of the present invention, there is provided a frequency conversion device, including an array of mutually spaced semiconductor islands composed of at least one III-V semiconductor compound and configured so that electromagnetic radiation of a first wavelength incident upon the semiconductor islands causes them to emit electromagnetic radiation of a second wavelength shorter than the first wavelength by a nonlinear frequency conversion process via photonic Mie resonances at the first and/or second wavelengths, wherein the semiconductor islands are supported by a transparent support such that the support is substantially transparent to radiation of the second wavelength, wherein at least the radiation of the second wavelength passes through the transparent support.

In some embodiments, the semiconductor islands are at least partially embedded in the transparent support. In some embodiments, the transparent support includes a transparent substrate attached to a layer of a transparent material in which the semiconductor islands are at least partially embedded. In some embodiments, the transparent substrate and the transparent material are both substantially transparent to radiation of the first wavelength and to radiation of the second wavelength.

In some embodiments, the refractive index of the transparent substrate is equal to or approximately equal to the refractive index of the transparent material. In some embodiments, the refractive index of the transparent substrate is different to the refractive index of the transparent material. In some embodiments, the transparent substrate is a glass and the transparent material is a polymer.

In some embodiments, the radiation of the second wavelength is simultaneously emitted in a forward direction and a backward direction relative to the direction of incidence of the radiation of the first wavelength.

In some embodiments, the radiation of the first wavelength is infrared radiation, and the radiation of the second wavelength is visible light.

In some embodiments, the semiconductor islands are in the form of cylinders with diameters and/or heights of the order of hundreds of nanometres.

In accordance with some embodiments of the present invention, there is provided a night vision device including of any one of the above frequency conversion devices.

In accordance with some embodiments of the present invention, there is provided a process for producing a frequency conversion device, including the steps of: forming, on a substrate, an array of mutually spaced semiconductor islands composed of at least one III-V semiconductor compound and configured so that radiation of a first wavelength incident upon the semiconductor islands causes them to emit radiation of a second wavelength shorter than the first wavelength by a nonlinear frequency conversion process via photonic Mie resonances at the first and/or second wavelengths, and wherein the substrate is substantially opaque to electromagnetic radiation of the second wavelength; attaching the semiconductor islands to a support that is substantially transparent to radiation of the second wavelength to provide a frequency conversion device wherein the array of mutually spaced nanometre-scale semiconductor islands is supported by the transparent support so that radiation of the first wavelength incident upon the frequency conversion device causes it to emit radiation of the second wavelength, and at least the radiation of the second wavelength passes through the transparent support.

In some embodiments, the process includes removing the substrate from the semiconductor islands.

In some embodiments, the semiconductor islands are at least partially embedded in the transparent support.

In some embodiments, the transparent support includes a transparent substrate attached to a layer of a transparent material in which the semiconductor islands are at least partially embedded. In some embodiments, the transparent substrate is a glass or MgF2 or BaF2 substrate, and the transparent material is a polymer.

In accordance with some embodiments of the present invention, there is provided a frequency conversion process, including directing radiation of a first wavelength onto an array of mutually spaced III-V compound semiconductor islands supported by a transparent support to cause the array to emit radiation of a second wavelength shorter than the first wavelength by a nonlinear frequency conversion process via photonic Mie resonances at the first and/or second wavelengths; wherein at least the radiation of the second wavelength passes through the transparent support.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein:

Figures 1 and 2 are perspective and side views, respectively, of a frequency conversion device in accordance with some embodiments of the present invention, including an array of III-V semiconductor islands on a transparent support and configured for nonlinear frequency conversion;

Figure 3 illustrates one mode of operation of the frequency conversion device of Figures 1 and 2, wherein radiation of a first wavelength incident upon the islands causes them to emit output radiation of a second wavelength shorter than the first wavelength by a non-linear frequency conversion process;

Figures 4 and 5 are schematic side-views of respective further embodiments in which the semiconductor islands are partially or completely embedded in a transparent support;

Figure 6 is a schematic side view of yet a further embodiment in which multiple layers of semiconductor islands are embedded in a transparent support;

Figure 7 is a flow diagram of an embodiment of a process for producing a frequency conversion device;

Figures 8 to 11 include schematic side-views of partially fabricated frequency conversion devices at different steps of the production process of Figure 7, together with corresponding scanning electron microscope (SEM) images showing details of the semiconductor islands;

Figures 12 to 15 are schematic side-views illustrating the formation of a transparent substrate on the semiconductor islands using a hot embossing step;

Figure 16 includes an SEM image of a test sample to illustrate the relationship between a polymer (BCB in this example) embedding layer, the semiconductor islands, and the original substrate (prior to its removal);

Figure 17 illustrates a step of the production process of Figure 7 in which the original opaque substrate is removed from the transparent substrate and partially embedded semiconductor islands;

Figures 18 and 19 are plan view SEM images of the resulting frequency conversion device and original opaque substrate, respectively;

Figure 20 is a schematic illustration of an experimental arrangement that was used to characterise the optical properties of the semiconductor islands of the frequency conversion devices;

Figure 21 is a schematic illustration of the linear spectral measurements of the semiconductor islands;

Figure 22 shows extinction spectra of individual semiconductor islands as measured experimentally and calculated theoretically, respectively, where the different colors correspond to different diameters of the islands, as indicated by the legend, and where the dashed lines show the spectral positions of the incident and the second harmonic frequencies;

Figure 23 is a graph of linear extinction and its multipolar decomposition as a function of the diameters of the semiconductor islands at the pump wavelength of 1556 nm, where the black solid line indicates the theoretical calculation, the dots indicate experimental measurements, and the coloured lines indicate the multipolar contributions as indicated in the legend;

Figure 24 is a schematic illustration of the nonlinear spectroscopy of single semiconductor islands;

Figure 25 is a bar chart of experimentally measured second harmonic generation (SFIG) efficiency (Psh/Pfw) from single semiconductor islands of different diameters at the pump wavelength of 1556 nm, where blue indicates forward radiation, red indicates backward radiation, and green indicates the sum of forward and backward;

Figure 26 is a bar chart of the backward-to-forward ratio of the second harmonic as a function of island diameter;

Figures 27 to 30 respectively show a calculated 3D pattern of far-field SH radiation, and front top and bottom views of the pattern, where the cones and inner circles indicate the experimentally accessible range of angles;

Figure 31 includes directionality (top row) and polarization (bottom row) diagrams of the SH signal in a forward direction, as measured experimentally and as calculated theoretically, where the arrows visualize the polarization states, and the incident beam is linearly polarized along the vertical direction;

Figure 32 is the same as Figure 31, but for a backward direction;

Figure 33 is a schematic illustration of the transmission measurements of an array of semiconductor islands;

Figure 34 includes transmission spectra of semiconductor island arrays as measured experimentally and calculated theoretically, respectively, where different colors correspond to different island diameters, as indicated by the legend;

Figure 35 includes experimentally measured backward back focal plane images of second and third harmonic generation from an AIGaAs semiconductor island with a 490 nm diameter;

Figure 36 is a schematic illustration of the polarization ellipse with the Stokes coefficients of ellipticity X angle and polarization-inclination angle ψ, where Ea and Eb are the main polarization axes (solid blue lines) of the polarization ellipse;

Figure 37 includes back-focal plane images of second harmonic signals in a backward direction after transmission through through six different polarizers: linear horizontal, vertical, two diagonal and two circular;

Figure 38 shows four components of the Stokes vector for backward directionality of the second harmonic signal;

Figure 39 shows the retrieved spatially-resolved degree of polarization, inclination and ellipticity;

Figure 40 is a graph of calculated scattering efficiency and multipole decomposition (up to fourth order) for AIGaAs semiconductor island with a diameter of 490 nm, and where the pump is set to be a plane wave polarized along the x axis;

Figure 41 includes three pie charts of the SH multipolar contributions calculated for three different island diameters: 340 nm (left), 490 nm (centre), and 640 nm (right);

Figure 42 shows the calculated SH directionality for an island diameter of 490 nm and an island height of 100, 300, and 500 nm, respectively;

Figure 43 is a bar chart of the efficiency of second harmonic generation polarization orientation with respect to the crystalline axis as a function of island diameter; and

Figure 44: First row: spatial profiles of the fundamental field (left), induced nonlinear current (center), and second-harmonic field (right) inside a semiconductor island of 490 nm diameter; near-field distributions of the fundamental (second row) and SH (third row) fields shown in three different cross-sections.

DETAILED DESCRIPTION

As shown in Figures 1 and 2, a frequency conversion device includes an array of mutually spaced semiconductor bodies or islands 102 supported by a transparent support 104. Although the shape of the islands 102 in plan view is typically circular as shown, they can alternatively be performed with essentially any desired shape. The islands 102 are composed of at least one III-V compound semiconductor, and are configured so that electromagnetic radiation 302 of a first wavelength λι incident upon the semiconductor islands 102 causes them to emit electromagnetic radiation 304, 306 of a second wavelength /½ that is shorter than the first wavelength, as shown in Figure 3, by way of a nonlinear frequency conversion process.

Compound semiconductors formed from combinations of elements from groups III and V of the periodic table (referred to in the art as "III-V semiconductors") are used because they have relatively large quadratic non-linear electromagnetic properties. In contrast, elemental semiconductors such as Si and Ge do not exhibit bulk quadratic nonlinearity due to their centro-symmetric crystalline structure.

The property of the support described as "transparent" in the context of this specification should be understood as meaning that the support is substantially transparent to radiation of the second wavelength. In some embodiments, the support is also substantially transparent to radiation of the first wavelength As will be understood by those skilled in the art, in practice no medium is completely transparent, and there will always be at least a small degree of loss when electromagnetic radiation passes through a medium, hence when the support is described herein as being transparent, it will be understood that this does not require the support to be completely transparent with zero insertion loss.

In any case, the transparency of the support allows at least one of the radiation of the first wavelength and the radiation of the second wavelength to pass through the transparent support. However, in some embodiments, the emitted electromagnetic radiation can include radiation emitted in a direction that is emitted at an angle of more than 90° relative to the direction of incidence of the radiation of the first wavelength, as shown by 306 in Figure 3.

The values of the first and second wavelengths are determined by the composition of the compound semiconductor islands and their physical dimensions, allowing the semiconductor islands to be configured so that the values of the first and second wavelengths suit a particular application of interest. However, the composition of the compound semiconductor is chosen to support a nonlinear frequency conversion process, such as a harmonic generation process that effectively blue-shifts the incident radiation of the first wavelength to the output radiation of the second wavelength that is shorter than the first wavelength. In the described embodiments, the nonlinear frequency conversion process is a harmonic generation process; however, it will be apparent to those skilled in the art that other nonlinear interactions can be used in other embodiments to generate output radiation of the second wavelength that is shorter than the first wavelength.

The semiconductor islands can be formed of any III-V compound semiconductor provided that the lattice mismatch between the crystalline semiconductor and the corresponding transparent crystalline substrate is not so large that it causes high concentrations of defects in the semiconductor that effectively render the frequency conversion processes ineffective in a practical sense. Examples of such compounds include those based on GaN, GaSb, GaAs, InP, InAs and InGaAs. For example, the semiconductor islands may be composed of AlxGa(i-x)As or InxGa(i-x)As with xe [0, 1]. The orientation of the substrate (and thus the semiconductor islands epaxially grown on the substrate) is typically a major crystallographic orientation such as [100], [110], or [111].

The semiconductor islands can be freestanding and proud of the transparent support, as shown in Figures 1 and 2, or partially embedded, as shown in Figure 4, or completely embedded in the transparent support, as shown in Figure 5. In some embodiments, the array of semiconductor islands is a three-dimensional array, as shown in Figure 6. Although Figure 6 shows such an array as a stack of two layers of two-dimensional arrays of semiconductor islands, in other embodiments this stacking can be continued to provide a stack with three or more layers. The different layers can include islands composed of different compound semiconductors. Additionally, the semiconductor islands in each of the stacked layers can be completely embedded, as shown in Figure 6, or partially embedded. Additionally, the pitch and/or relative alignment of semiconductor islands in different layers can be arranged in a wide variety of different ways.

The frequency conversion devices described herein are thus able to up-convert or blue-shift non-ionising electromagnetic radiation at a wide range of input wavelengths and output wavelengths, as determined by the configuration of the individual compound semiconductor islands. Accordingly, by providing islands of different composition and/or physical dimensions in a single two-dimensional array or three-dimensional array, a single frequency conversion device as described herein can provide conversion over a wide range of desired input and/or output wavelengths.

In particular, where the described compound semiconductor islands are configured to blue-shift incident infrared radiation, a frequency conversion device as described herein can be used for thermal imaging or night vision. In some embodiments, the blue-shifting produces light at wavelengths in the visible wavelength region. In any case, by including compound semiconductor islands of different configurations selected to blue-shift different wavelengths of infrared radiation to respective different wavelengths of visible light, a colour representation of objects having different temperatures and/or emissivities can be generated, either directly (if the output wavelengths are in the visible region), or indirectly (in all cases) via standard amplification and image generation methods known to those skilled in the art (including those currently used in existing thermal imaging and night vision equipment).

Fabrication of Non-linear Electromagnetic Devices

The frequency conversion devices described herein can be manufactured by a production process such as that shown in Figure 7. The process begins at step 702 by selecting at least one desired input (or "first") wavelength and at least one desired output (or "second") wavelength. As described above, these wavelengths will generally be determined by the specific application for which the frequency conversion device is to be applied. In the described embodiments, the frequency conversion device is to be used for night vision or thermal imaging, and accordingly the first wavelengths are in the infrared region of the electromagnetic spectrum (and are about 10 pm or less), and the second wavelengths are in the visible region of the electromagnetic spectrum.

Having chosen the input and output wavelengths, at step 704, these are used to determine at least one corresponding configuration of the compound semiconductor islands, in particular the composition(s) and physical dimensions of the islands to support those wavelengths.

In the described embodiments, the semiconductor islands are in the form of cylinders composed of compound semiconductors with compositions of Alx Ga(i-X) As or Inx Ga(i-x)As, with X€[0, 1.0] or alloys thereof, i.e., ranging from GaAs to AlAs, InAs or InGaAs. In the case of Inx Ga(i-X) As compounds, it is found that islands composed of Ino.53Gao.47As provide the best performance, as it provides the smallest lattice mismatch with the InP substrate, and consequently produces the lowest defect density.

In the described embodiments, the dimensions of the islands are nanometer-scale ('nanoscale') dimensions of about 20 nm-10 pm). Appropriate physical dimensions for a given desired wavelength and compound semiconductor composition can be determined by simulation, using a computational electromagnetics software package such as COMSOL Multiphysics®, for example, as described at https://www.comsol.com/comsol-multiphysics.

At step 706, an array of compound semiconductor islands of the selected dimensions and composition are formed on a crystalline substrate that is compatible with epitaxial growth of the selected compound semiconductor. For example, in the case of Alx Ga(i-X)As compounds, the substrate can be a single-crystal GaAs wafer, and in the case of Inx Ga(i-x) As compounds, the substrate can be a single-crystal InP wafer. Unfortunately, compatible substrates have high refractive indices and are opaque to electromagnetic radiation in the wavelength ranges of interest (e.g., in the visible region). Growth of compound semiconductors on transparent substrates such as glass results in a high density of dislocations in the grown semiconductors, and therefore poor characteristics.

Typically, the formation step 706 involves standard semiconductor processing steps known to those skilled in the art, including epitaxial growth of a layer of the corresponding compound semiconductor on an opaque semiconductor substrate (possibly preceded by an intermediate or buffer layer, as described below), followed by deposition of a mask layer, patterning of the mask layer by lithography, selected area etching of the compound semiconductor layer, and removal of the remaining mask material. The specific details of the steps required to form compound semiconductor islands of a desired configuration are well within the capabilities of those of ordinary skill in the art.

In some embodiments, the semiconductor islands are at least partially decoupled from the substrate in order to weaken their attachment to the substrate. This can be achieved by growing an intermediate layer on the substrate, prior to growing the compound semiconductor from which the islands will be formed, with the intermediate layer being formed of a material that can be selectively removed in order to decouple the overlying semiconductor islands. Residual (but relatively weak) coupling forces (including Van der Waals forces) maintain the semiconductor islands at their originals locations. For example, where the semiconductor islands are composed of Alx Ga(i-X) As compounds with xe[0, «0.8], AlAs can be used as the intermediate layer as it is preferentially etched by HCI.

In all cases, having formed the compound semiconductor islands on the opaque substrate, at step 708 they are bonded to a transparent support, and at step 710 the opaque substrate is removed to provide the frequency conversion device in the form of mutually spaced semiconductor islands supported by the transparent substrate. In embodiments where the semiconductor islands have been decoupled, the opaque substrate can be removed by simply pulling it away from the semiconductor islands, because the decoupling step causes the bonding between these to be weaker than the bonding between the semiconductor islands and the transparent support. Otherwise, in the absence of the decoupling step, the opaque substrate can be removed by etching, for example. In the case of Inx Ga(i.X) As compounds on an InP substrate, for example, the substrate can be preferentially removed by HCI acid. In the case of AlAs compounds on a GaAs substrate, for example, the substrate can be preferentially removed by a citric acid/H202 solution.

As described above, the resulting frequency conversion device can be used at step 712 to blue-shift electromagnetic radiation.

Incidentally, although it would be possible to transfer the compound semiconductor layer to a transparent (e.g., glass) substrate and then pattern the bonded layer to form mutually spaced islands of the compound semiconductor, in practice due to poor adhesion and fragility of the transferred layer, it is not generally possible to form the islands in this way with high spatial resolution and smooth surfaces and edges, which are required characteristics to achieve high non-linear conversion performance.

Some embodiments of the present invention are now described in the context of frequency conversion devices configured to convert infrared radiation to visible radiation for thermal imaging or night vision applications. In these embodiments, the compound semiconductor composition was chosen to be AI0.2Ga0.8As of [100] crystallographic orientation normal to the plane of a two-dimensional array of islands of this compound, and the semiconductor islands were chosen to be cylinders or disks having various diameters in the range of 340-690 nm and a fixed height of about 300 nm so that the semiconductor islands would support Mie-type resonances at the input and output wavelengths (since the frequency conversion frequency is maximised when the semiconductor islands are resonant at both the input and output wavelengths). In the described embodiment, the islands were arranged on a square grid at a pitch or periodicity of 5 pm. However, in general, the islands can be arranged in any manner, including hexagonal lattice arrangements for high packing density, quasi-random arrangements, and arrangements that provide diffraction of the output radiation (e.g., to excite Fano resonances and enhance efficiency). For comparison, some arrays of islands of the same composition were formed at a pitch of 1 pm. A 20 nm AlAs sacrificial buffer layer was epitaxially grown on [100] GaAs wafers by metal-organic chemical vapour deposition (MOCVD), followed by a 300 nm layer of Alo.2Gao.8As, and finally a 5 nm GaAs capping layer to prevent oxidation of the Alo.2Gao.8As. A 400 nm SiOx masking layer was then deposited over the AI0.2Ga0.8As by plasma-enhanced chemical vapour deposition (PECVD). The mask layer was then patterned using electron-beam lithography and reactive ion etching using Cl2, Ar and H2 gases to remove all of the masking layer except for a square array of circular regions having the pitch and diameters indicated above.

The compound semiconductor regions exposed by the circular openings in the mask layer were then etched in an inductively coupled plasma (ICP) etching tool to remove all of the epitaxially grown compound semiconductors and a small amount of the GaAs substrate. As shown in the schematic diagram of Figure 8 and the scanning electron microscope (SEM) image of Figure 9, the resulting structure consists of the remaining GaAs substrate 802 with cylindrical pillars formed of layers of GaAs 804, AlAs 806, Alo.2Gao.8As 808, and Si02 810. In the described embodiment, the etching was performed in a plasma etching tool using Cl2, Ar, and H2 gases. Although other process gases and/or subtractive methods (e.g., ion-milling) can be used to achieve the same structure, the use of Cl2 as a purging gas results in a non-adhesive surface and thus assists with the subsequent removal of the semiconductor islands from the substrate, as described below.

The Si02 layer 810 and the AlAs layer 806 are then removed from each pillar by wet etching in 2% HF to produce the structure shown in the schematic diagram of Figure 10 and the SEM image of Figure 11, wherein the removal of the AlAs layer 806 produces an interface (represented by the dotted line 1002 in Figure 10) between the GaAs layer 804 and the AI0.2Ga0.sAs layer 808. As described above, if the AlAs layer 806 is completely removed (as shown in the SEM image of Figure 11), van der Waals forces keep the AI0.2Ga0.sAs layer 808 in place; otherwise, if the AlAs layer 806 is not completely removed, then any remaining part of that layer 806 can act to maintain the AI0.2Ga0.sAs layer 808 in place.

After this step, the AI0.2Ga0.sAs layer/disks 808 are attached to a transparent support. In the described embodiments, the transparent support is a composite structure or assembly consisting of a transparent polymer on a planar transparent substrate/superstrate. In some embodiments, the transparent support is formed and attached by spin-coating a thin (4 pm in some embodiments) polymer layer on the sample, curing the polymer, and bonding it to a thin transparent substrate/superstrate, as shown in Figure 12. The thickness of the polymer layer can then be reduced, typically but not necessarily to the height of the semiconductor islands by a hot embossing step, wherein the polymer layer is heated and made to flow around the semiconductor islands by applying pressure to the superstrate as shown in Figure 14 to produce the structure shown in Figure 15. For the purposes of illustration, the SEM image of Figure 16 shows an array of Al0.2Gao.8As islands, some of which are embedded in a BCB polymer layer of the same (400 nm) height/thickness.

As an alternative, in some embodiments the attachment is achieved by first bonding the polymer and superstrate layers, and then attaching the resulting composite support to the semiconductor islands, as shown in Figure 13, before performing the hot embossing step described above and shown in Figure 14.

In either case, the hot embossing step can also be used to simultaneously cure the polymer. For example, where the polymer is BCB, it needs to be cured at a temperature of about 300°C, and consequently the hot embossing step can be performed at that temperature or higher in order to simultaneously cure the BCB layer. Once cured, BCB acts like a glass and can be heated to substantially higher temperatures (e.g., up to at least 500°C) without melting, which can facilitate the addition of one or more further layers (including BCB layers).

In various embodiments, the transparent substrate and the polymer layer can have the same, similar, or different refractive indices, depending on the application. In some embodiments, the substrate is a glass substrate, and the polymer is benzocyclobutene (BCB), with equivalent refractive index to glass, allowing the BCB and glass to act as a composite waveguide. In some embodiments, the substrate is a MgF2 substrate (with a refractive index of about 1.3). In other embodiments, the substrate is a BaF2 substrate or a quartz substrate. In some embodiments, the polymer is PolyDiMethylSiloxane (PDMS). Many other suitable transparent substrates and polymers and combinations thereof will be apparent to those skilled in the art in light of this disclosure.

Finally, the remaining portion of the original opaque substrate 1702 is removed by peeling or otherwise pulling it away from the transparent superstrate, as shown in Figure 17, to provide the frequency conversion device 1704. The Cl2 gas treatment of the opaque substrate surface described above is optional, but the inventors have found that it facilitates this separation by reducing the adhesion of the transparent support to the opaque substrate. Figures 18 and 19 are SEM images of the resulting frequency conversion device 1704 and the opaque substrate 1702 (which can be discarded).

The resulting frequency conversion device 1704 is in the form of a two-dimensional array of compound semiconductor islands (in the form of cylinders or 'disks' in this example) partially embedded in a transparent superstrate. Optionally, the semiconductor islands can be completely embedded within the transparent substrate in a variety of different ways, but most simply by adding (e.g., by bonding or forming in situ (e.g., by spin coating and curing)) a second transparent substrate/supersrate to cover the exposed surfaces of the semiconductor islands. Optionally, a second hot embossing step can be performed to remove any gap between the second transparent substrate and the semiconductor islands. A three-dimensional, array of semiconductor islands can be formed by bonding two or more single-layer frequency conversion devices together. The bonding can be achieved by simply arranging a stack of singlelayer devices, typically in a wafer alignment tool to determine the relative locations of the islands in different layers, and using the tool to apply heat and pressure to the stack in order to achieve bonding. In some embodiments, different etch mask layouts are used to form the different layers and thus can provide lateral offsets between the islands in different layers, if desired.

Optical Characterisation of Non-linear Electromagnetic Properties

The electromagnetic behaviours of the manufactured frequency conversion devices and of single islands were characterised using a variety of different optical methods commonly used by researchers in the field. Figure 20 is a schematic diagram showing an experimental configuration that was used to characterise two-dimensional arrays and individual islands of AI0.2Ga0.sAs. To measure the non-linear generated output radiation, including second harmonic generation ("SHG"), from individual semiconductor islands, a single island is placed in the focal spot of two confocal air objective lenses: an Olympus LCPIanNIR (0.85 NA, lOOx, infrared) for focusing of the FW, and an Olympus MPIanFLN (0.9 NA, lOOx, visible) for collection of the output radiation. This corresponds to collection angles for the emission patterns within approximately 58° and 64° in air, which corresponds to collection angles of approximately 44° and 40° in glass and BOB.

The diameter of the focused pump laser beam is measured by performing knife-edge experiments and ensuring that the pump beam is close to a diffraction limit of 2.2 pm. The substrate side faces the visible objective. Thus, the objective lens of the Olympus MPIanFLN collects the output radiation from an individual island in the forward direction, and the Olympus LCPIanNIR lens collects the output radiation in the backward direction. The pump laser is a pulsed Er3+-doped fiber laser (~500 fs, repetition rate of 5 MFIz) operating at a wavelength of 1556 nm. At the laser output, a quarter-wave plate and a half-wave plate were used to control the output polarization, and two cooled CCD cameras were used to detect the output radiation. In the forward direction, a notch filter blocks the pump laser. In the backward direction, a dichroic mirror is used in front of the objective lens to direct the backward-directed output radiation onto the camera.

Linear characterisation

The extinction spectra of individual AIGaAs nanodisks were measured using the same equipment in a confocal configuration, using a white-light source (fiber-coupled tungsten halogen light bulb) and two spectrometers: a Princeton Instruments Acton SP 2300 monochromator with Andor DU490A-1.7 InGaAs array detector for infrared wavelengths, and an Ocean Optics 65000 for wavelengths in the visible region. Transmission spectra were measured through a disk and a field diaphragm, and the extinction cross-sections were calculated using an approximate relation of In(l-T), where T is the measured transmission, normalized to the transmission of the substrate.

The linear extinction spectra of individual islands was measured in a linear transmission arrangement, as shown schematically in Figure 21. Figure 22 includes two graphs: the left-hand graph shows the measured extinction cross-sections as a function of wavelength for semiconductor cylinders of different diameters ranging from 340 to 670 nm and a fixed height of 300 nm, and the right-hand graph shows the corresponding theoretical predictions of the rigorous coupled wave analysis (RCWA) numerical simulation method described in Hugonin, J. P.; Lalanne, and P. Reticolo, Software for Crating Analysis; Institut d'Optique: Orsay, France, 2005. The two vertical dashed lines in both graphs indicate the fundamental wave ("FW") wavelength and the second harmonic ("SH") wavelength, respectively. The shaded region in the left-hand graph indicates the wavelength range not covered by the spectrometers.

The measurements shown in Figure 22 demonstrate a pronounced size-dependent resonance at the FW wavelength and multiple resonances at the SH wavelength, and are in good agreement with the numerical simulations shown in the right-hand graph. The discrepancies between the experiments and the theoretical predictions are believed to be due to fabrication imperfections and the finite numerical aperture of the measurement apparatus described above.

Figure 23 shows the extracted scattering cross-sections at the FW as a function of the diameter of the semiconductor islands, where the dots represent experimental results, and the solid curves represent the numerical simulations. Multipolar decomposition was performed using the polarization currents induced inside each semiconductor island, and reveal that the resonant profile of the linear scattering, which is maximal for disk diameters of 400-500 nm, is essentially determined by a magnetic dipole excitation ("MD") and an electric dipole excitation ("ED") in each island, playing a dominant role at the pump FW wavelength (1556 nm). Some minor contributions of quadrupoles tend to grow slightly when the disk diameter is increased.

The linear measurements applied to single semiconductor islands as shown in Figures 21 and 22 were also applied to 100 pm x 100 pm arrays of the islands, as represented schematically in Figure 33, to produce the measured and simulated zero-order forward scattering spectra shown in Figure 34 for a variety of island diameters. As with the single island measurements, the experimentally measured spectra are in good agreement with numerical calculations.

Second Harmonic Generation

It is noted that the highest extinction is achieved when the amplitudes of the electric dipole and magnetic dipole become equal. In other words, the highest extinction is observed for islands that satisfy the generalized Kerker condition. At the SH wavelength (778 nm), higher-order multipoles are excited in the islands. These two resonant conditions at the FW and the SH wavelengths are responsible for SHG enhancement in the islands. However, a more-sophisticated dependence of the SHG efficiency on the sizes of the semiconductor islands is expected when the spatial overlaps of the resonant modes at the FW and the SH fields are taken into account. These results suggest that there are optimal sizes of individual islands to maximise the efficiency of SHG from single islands.

The nonlinear response of individual islands supported by transparent supports was measured in both forward and backward directions (relative to the incident radiation) for various island diameters, using linear (vertical) polarization of the pump laser at a 45° angle of incidence relative to the crystalline axes of the islands, as shown in Figure 23, in order to maximize the non-linear tensor component. A laser beam with an average beam power of ~1 mW is focused by an infrared wavelength objective (NA = 0.85) to a diffraction limited spot of 2.2 pm, resulting in a peak intensity of ~7 GW/cm2. Another visible wavelength objective (NA = 0.9) collects the SH emitted by the semiconductor island in a forward direction, while the focusing objective collects the SH radiation in a backward direction. The SH signal is detected by the two cooled CCD cameras, calibrated with a power meter.

The results of the SHG measurements from single AI0,2Ga0.8As islands of different diameters are shown in Figure 24 as values of SHG efficiency derived from the sum of the measured forward and backward SH signals. The overall dependence of the efficiency on the island size is complex due to the large number of higher-order modes that exist in the SH frequency band. The most efficient SHG is observed for the island with a diameter of 490 nm, having a conversion efficiency as high as 8.5 x 10-5.

Importantly, as shown in Figure 25, the directionality of the second harmonic emission can be determined by selecting the dimensions of the semiconductor island. For example, for cylindrical island diameters of about 400 nm, the SH radiation is mostly backward, while for diameters of 500-600 nm, the backward-to-forward ratio remains close to unity, with a slight domination of the backward SH emission. At larger disk diameters, the backward-to-forward ratio peaks again; however, for these larger disks, the dependence becomes very sensitive to the island diameter due to the many higher-order multipoles that contribute to the SH scattering. The SH efficiency (Figure 24) and backward-to-forward ratio (Figure 25) are both generally sensitive to the geometry of the semiconductor islands. In particular, geometrical parameters can be tuned to bring high efficiency in overlap with comparable backward-to-forward ratio (as shown). Additionally, it is possible to simultaneously achieve high efficiency and unidirectionality.

An important feature of the measurements is that the SH radiation pattern can be characterised in both forward and backward directions as well as in transverse momentum space. The data, however, suggest that the experimental measurement apparatus is capturing only a small portion of the total SH radiated power due to the finite numerical apertures of the objectives.

To estimate the total efficiency of the radiated SH power, the nonlinear response of the semiconductor islands was simulated numerically using the finite element method solver in COMSOL Multiphysics in the frequency domain. In these simulations, each semiconductor island is assumed to be embedded in a homogeneous medium having a refractive index equal to that of the glass substrate. The material dispersion of the compound semiconductor is taken from COMSOL tabulated data. The second-order nonlinear susceptibility tensor of the [100] oriented Alo.2Ga0.8As, possessing a zinc blende crystalline structure, contains only off-diagonal elements X^with / Φ] φΙ<. Thus, in the principal-axis system of the crystal, the /th component of the nonlinear polarization at the SH frequency is given by: ριί2ω) =ε0Χ$Ε}ω)Είω) (1)

An undepleted pump approximation is assumed and two coupled steps are used to calculate the radiated SH power. First, linear scattering at the fundamental wavelength is simulated. To emulate the experimental conditions more accurately, the semiconductor island is excited by a focused monochromatic Gaussian beam, polarized along the [110] direction. The bulk nonlinear polarization given by Eq. 1, induced inside the island, is then employed as a source for the next electromagnetic simulation at twice the frequency, to obtain the generated SH field.

The disk size providing the maximum SH (d = 490 nm) was chosen, and the three-dimensional SH far-field radiation pattern was calculated, as shown in Figure 26. The non-linear scattering is governed by the interference of an electric quadrupole and higher-order nonlinearly generated multipoles (up to / = 4), leading to the suppression of the forward SH radiation, as shown in Figure 26. The side, top, and bottom views of this same radiation pattern are shown in Figures 27 to 29, respectively. The shaded area in Figure 27 depicts the forward and backward collection angles of the SH signal in the experimental configurations described above. These collection angles are also indicated by the inner circles in the forward and backward far-field radiation images in Figure 28 and 29. Clearly, the energy collected in the experiment is less than the total generated SH. By integrating the amount of SH emitted within the numerical aperture of the objective lenses, it is estimated that only about 30% of the total SH energy is experimentally collected in the forward and backward directions. As such, the total generation efficiency is estimated to be three times larger than the measured collection efficiency, thus exceeding the prior art record value of 10-4. This high efficiency provides a solid ground for the use of the nonlinear semiconductor arrays described herein as functional elements for beam and polarization shaping.

The radiation patterns are measured by building back-focal plane (BFP) images of the SH radiation pattern by adding a pair of confocal lenses between the objective lenses and the cameras, in both forward and backward directions. The top left images in Figures 30 and 31 respectively show the radiation diagram captured by the objective lenses on the basis of their numerical aperture in forward (Figure 31) and backwards (Figure 32) directions. From these BFP images, it can be concluded that the SH radiation in the normal directions (the (0,0) point of the BFP images) is zero, as recently predicted theoretically. Zero SH emission in normal directions here originates from the symmetry of the nonlinear bulk x(2) tensor, and is therefore insensitive to geometry. As a result, zero SH emission is observed for all of the semiconductor islands that were fabricated as described above.

Third Harmonic Generation

To further support these findings, third harmonic generation ("THG") from the same islands was measured. The third harmonic relies on the x(3) nonlinear tensor, and in contrast to the SH radiation pattern has a radiation maximum in normal directions. Although the third-order nonlinear term of AlxGa(i-x)As non-linear polarization is much weaker than the second-order non-linear term, it is nevertheless non-zero, and third harmonic generation (THG) signals from AlxGa(i-x)As islands can be observed. The THG is expected to be similar to the THG from other well-studied materials like silicon and germanium. Therefore, in terms of the radiation pattern, the THG signals is non-zero in the direction normal to the disks axis. This characteristic is in contrast to the SHG radiation pattern, which is reflected in the doughnut shape of back-focal plane (BFP) images because of zero diagonal components of the second-order tensors. This difference is clear from the forward and backward normalised intensity images shown in Figure 35, demonstrating that the measured doughnut shapes of SHG are indeed characteristic of the SH emission.

This is an important finding for arrays of the semiconductor islands, because the interference of emission from multiple islands of an array will result in lower radiation efficiency from the zeroth order SH beam. It is also noted that surface second-order nonlinearities can in principle result in normal SH radiation for specific excitation; however, surface SHG is not pronounced in these experiments, and the bulkx(2) is the dominant nonlinear contribution.

Polarisation properties of the Second Harmonic

Even more intriguing is the polarization state of the observed far-field doughnut beam. To test the polarization properties of the SH radiation from the semiconductor islands, the spatially resolved polarization states of the BFP images were retrieved using the Stokes formalism. The Stokes coefficients provide a complete description of the light polarization state in terms of its total intensity Itot, (fractional) degree of polarization p, polarization inclination angle ψ, and the ellipticity angle Λ". The ellipticity tan(X) is defined as the ratio of the two axes of the polarization ellipse (see Figure 36), and the polarization inclination is described by the angle between the main polarization axis and the x-axis of the laboratory coordinate system.

Experimentally, the Stokes parameters are found by measuring light transmission through a set of six different polarizers: linear horizontal, vertical, two diagonal and two circular polarizers realized by different orientations of the quarter-wave plate and a linear polarizer. The set of measurements for the backward directionality of SH emission from a disk with diameter of 490 nm are shown in Figure 37. 7

The next step is to retrieve the Stokes vector M - ^ , where V.

I = H + V=Da + Db = L + R Q = H - V U = Da - Db V = L - R

Here, H is the transmission through horizontal polarizer, while V, Da, Db, L, R are the transmissions through vertical, two diagonal, left- and right-circular polarizers, respectively. A set of four back-focal plane images forming the Stokes vector are shown in Figure 38.

Next, the coefficients are calculated as follows:

Jq2 +u2 + v2 p =-1- 1 ip = -arg((? + iU) 1 ( V \ X = - arctan , 2 \JWTQt)

The resulting coefficients are shown in Figure 39.

We observe vector-beam formation at the SH frequency, as shown with arrows in Figures 31 and 32. In particular, nearly perfect radial polarization of the SH can be observed in the forward direction. In the backward direction, the polarization state is more complex, with polarization inclination having radial structure and ellipticity ranging from nearly circular to linear, as shown in Figure 32. The polarization of the SH beam was calculated numerically, and is shown in the top right-hand images in

Figures 31 and 32. Some differences between theory and experiment can be observed. These differences can be attributed to the slight non-uniformity of the device, in particular because the BCB does not fully cover the semiconductor islands.

The nonlinear generation of vector beams from the semiconductor islands can be intuitively understood by the excitation of Mie-type multipoles at the wavelengths of the input and output radiation. In the simplest exemplary case, a vector beam of radial polarization can be emitted by an electric dipole oriented along the optical axis of the disk antenna. In a more complex situation, as in the examples described herein, higher-order multipoles are excited at the SH wavelength. The superposition of these multipolar contributions governs the output polarization state. This can be engineered for a specific application.

The radiation patterns and polarization states of SH emission from the compound semiconductor islands described above demonstrate that nonlinear conversion efficiencies exceeding 10-4 can be achieved, so the described nanostructures can be applied used to provide functional nonlinear devices at the nanoscale. In particular, nonlinear nanoscale light sources emitting vector beams with a desired polarization state, e.g., radial polarization, have been experimentally demonstrated. These results open new avenues for novel nonlinear imaging, as well as applications such as bright fluorescent markers for bio-imaging, or constituent elements for efficient nonlinear holograms (which can be used as security devices, for example).

Multipolar decomposition of these spectra support the attribution of the observed spectral resonances in the linear regime to excitation of Mie-type multipoles. Polarization currents were used for this task, and the island parameter that provided the best performance was 490 nm, with corresponding results shown in Figure 40.

Multipolar decomposition in the non-linear regime of SH fields was also performed, for cylindrical islands with diameters of 340 nm, 490 nm and 640 nm. The relative contributions of different multipoles into the SH for these three diameters are shown in the respective pie charts of Figure 41.

The strong dependence of SH directionality on cylindrical island height is demonstrated by the directionality diagrams of Figure 42 for cylindrical semiconductor islands of 490 nm diameter and heights of 100 nm, 300 nm and 500 nm.

The strong dependence of SH efficiency on the relative orientation of the in-plane crystalline axis to the orientation of pump polarization is shown in Figure 43.

Finally, distributions of the near-fields at both the pump wavelength and the second harmonic wavelength are shown in Figure 44.

Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims (17)

1. CLAIMS:

   1. A frequency conversion device, including an array of mutually spaced semiconductor islands composed of at least one III-V semiconductor compound and configured so that electromagnetic radiation of a first wavelength incident upon the semiconductor islands causes them to emit electromagnetic radiation of a second wavelength shorter than the first wavelength by a nonlinear frequency conversion process via photonic Mie resonances at the first and/or second wavelengths, wherein the semiconductor islands are supported by a transparent support such that the support is substantially transparent to radiation of the second wavelength, wherein at least the radiation of the second wavelength passes through the transparent support.
2. 2. The frequency conversion device of claim 1, wherein the semiconductor islands are at least partially embedded in the transparent support.
3. 3. The frequency conversion device of claim 2, wherein the transparent support includes a transparent substrate attached to a layer of a transparent material in which the semiconductor islands are at least partially embedded.
4. 4. The frequency conversion device of claim 3, wherein the transparent substrate and the transparent material are both substantially transparent to radiation of the first wavelength and to radiation of the second wavelength.
5. 5. The frequency conversion device of claim 4, wherein the refractive index of the transparent substrate is equal to or approximately equal to the refractive index of the transparent material.
6. 6. The frequency conversion device of claim 4, wherein the refractive index of the transparent substrate is different to the refractive index of the transparent material.
7. 7. The frequency conversion device of any one of claims 3 to 6, wherein the transparent substrate is a glass or MgF2 or BaF2 substrate, and the transparent material is a polymer.
8. 8. The frequency conversion device of any one of claims 1 to 7, wherein the radiation of the second wavelength is simultaneously emitted in a forward direction and a backward direction relative to the direction of incidence of the radiation of the first wavelength.
9. 9. The frequency conversion device of any one of claims 1 to 8, wherein the radiation of the first wavelength is infrared radiation, and the radiation of the second wavelength is visible light.
10. 10. The frequency conversion device of any one of claims 1 to 9, wherein the semiconductor islands are in the form of cylinders with diameters and/or heights of the order of hundreds of nanometres.
11. 11. A night vision device including the frequency conversion device of any one of claims 1 to 10.
12. 12. A process for producing a frequency conversion device, including the steps of: forming, on a substrate, an array of mutually spaced semiconductor islands composed of at least one III-V semiconductor compound and configured so that radiation of a first wavelength incident upon the semiconductor islands causes them to emit radiation of a second wavelength shorter than the first wavelength by a nonlinear frequency conversion process via photonic Mie resonances at the first and/or second wavelengths, and wherein the substrate is substantially opaque to electromagnetic radiation of the second wavelength; attaching the semiconductor islands to a support that is substantially transparent to radiation of the second wavelength to provide a frequency conversion device wherein the array of mutually spaced nanometre-scale semiconductor islands is supported by the transparent support so that radiation of the first wavelength incident upon the frequency conversion device causes it to emit radiation of the second wavelength, and at least the radiation of the second wavelength passes through the transparent support.
13. 13. The process of claim 12, including removing the substrate from the semiconductor islands.
14. 14. The process of claim 12 or 13, wherein the semiconductor islands are at least partially embedded in the transparent support.
15. 15. The process of claim 14, wherein the transparent support includes a transparent substrate attached to a layer of a transparent material in which the semiconductor islands are at least partially embedded.
16. 16. The process of claim 15, wherein the transparent substrate is a glass or MgF2 or BaF2 substrate, and the transparent material is a polymer.
17. 17. A frequency conversion process, including directing radiation of a first wavelength onto an array of mutually spaced III-V compound semiconductor islands supported by a transparent support to cause the array to emit radiation of a second wavelength shorter than the first wavelength by a nonlinear frequency conversion process via photonic Mie resonances at the first and/or second wavelengths; wherein at least the radiation of the second wavelength passes through the transparent support.