(en)An optical device includes a substrate, a metallic layer, a dielectric filling and a light collection element. The substrate has first and second opposite surfaces, and has a funnel-shaped cavity between a first aperture on the first surface and a second aperture on the second surface. The metallic layer covers an inner wall of the funnel-shaped cavity. The dielectric filling is disposed over the metallic layer and fills the funnel-shaped cavity. The light collection element is coupled to the second aperture, and is configured to collect light that impinges on the first aperture and is directed to the second aperture by the metallic layer and dielectric filling.

1.ApplicationNumber: US-201414249418-A
1.PublishNumber: US-2014321805-A1
2.Date Publish: 20141030
3.Inventor: YOSUB SHAY
ORENSTEIN MEIR

4.Inventor Harmonized: YOSUB SHAY(IL)
ORENSTEIN MEIR(IL)

5.Country: US
6.Claims:
(en)An optical device includes a substrate, a metallic layer, a dielectric filling and a light collection element. The substrate has first and second opposite surfaces, and has a funnel-shaped cavity between a first aperture on the first surface and a second aperture on the second surface. The metallic layer covers an inner wall of the funnel-shaped cavity. The dielectric filling is disposed over the metallic layer and fills the funnel-shaped cavity. The light collection element is coupled to the second aperture, and is configured to collect light that impinges on the first aperture and is directed to the second aperture by the metallic layer and dielectric filling.

7.Description:
(en)CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Patent Application 61/811,819, filed Apr. 15, 2013, whose disclosure is incorporated herein by reference.


FIELD OF THE INVENTION
The present invention relates generally to optical devices, and particularly to plasmonic antennas.
BACKGROUND OF THE INVENTION
Collection and focusing of light into small-size absorbers is applicable in many fields, for example in detection of single photons, enhancement of non-linear effects, fluorescence measurements, nano-lithography, among others.
Some focusing techniques use plasmonic effects for focusing light below diffraction level, i.e., into sub-wavelength apertures. Such techniques are described, for example, by Bozhevolnyi and Gramotnev, in “Plasmonics beyond the diffraction limit,” Nature Photonics, vol. 4, no. 2, 2010; and by Oulton et al., in “Confinement and propagation characteristics of subwavelength plasmonic modes,” New Journal of Physics, vol. 10, 2008, which are incorporated herein by reference.
Example plasmonic focusing devices have been proposed by Choi et al., in “Compressing surface plasmons for nano-scale optical focusing,” Optics Express, vol. 17, no. 9, 2009; by Ginzburg et al., in “Gap plasmon polariton structure for very efficient microscale-to-nanoscale interfacing,” Optics Letters, vol. 31, no. 22, 2006; by Yang et al., in “Efficiently squeezing near infrared light into a 21 nm-by-24 nm nanospot,” Optics Express, vol. 16, no. 24, 2008; by Verhagen et al., in “Nanofocusing in laterally tapered plasmonic waveguides,” Optics Express, vol. 16, no. 1, 2008; by Stockman, in “Nanofocusing of optical energy in tapered plasmonic waveguides,” Physical Review Letters, vol. 93, no. 13, 2004; by Ginzburg and Orenstein, in “Plasmonic transmission lines: from micro to nano scale with λ/4 impedance matching,” Optics Express, vol. 15, no. 11, 2007; by Vedantam et al., in “A plasmonic dimple lens for nanoscale focusing of light,” Nano letters, vol. 9, no. 10, 2009; and by Choo et al., in “Nanofocusing in a metal-insulator-metal gap plasmon waveguide with a three-dimensional linear taper,” Nature Photonics, vol. 6, no. 12, 2012, which are all incorporated herein by reference.
The description above is presented as a general overview of related art in this field and should not be construed as an admission that any of the information it contains constitutes prior art against the present patent application.
SUMMARY OF THE INVENTION
An embodiment of the present invention that is described herein provides an optical device including a substrate, a metallic layer, a dielectric filling and a light collection element. The substrate has first and second opposite surfaces, and has a funnel-shaped cavity between a first aperture on the first surface and a second aperture on the second surface. The metallic layer covers an inner wall of the funnel-shaped cavity. The dielectric filling is disposed over the metallic layer and fills the funnel-shaped cavity. The light collection element is coupled to the second aperture, and is configured to collect light that impinges on the first aperture and is directed to the second aperture by the metallic layer and dielectric filling.
In some embodiments, a diameter of the second aperture is smaller than a wavelength of the collected light. In some embodiments, a ratio between a diameter of the first aperture and a thickness of the substrate is at least five. In an embodiment, the light impinging on the first aperture is non-polarized. In a disclosed embodiment, the dielectric filling includes Benzocyclobutene (BCB). In an embodiment, the metallic layer includes gold.
In another embodiment, the light collection element includes silicon. In yet another embodiment, the light collection element includes indium gallium arsenide (InGaAs). In still another embodiment, the light collection element includes a plurality of layers having respective progressively-increasing refraction indices. In an embodiment, the metallic layer and the dielectric filling are disposed on the substrate using an Integrated Circuit (IC) fabrication process.
There is additionally provided, in accordance with an embodiment of the present invention, an optical device including a metallic substrate and a dielectric filling. The metallic substrate has first and second opposite surfaces. The first surface has one or more first funnel-shaped cavities formed therein, and the second surface has one or more second funnel-shaped cavities formed therein facing the respective first funnel-shaped cavities, such that each first funnel-shaped cavity and a corresponding second funnel-shaped cavity penetrate the substrate from the first surface to the second surface. The dielectric filling fills the first and second funnel-shaped cavities.
In some embodiments, the optical device further includes a layer, which has a controllable transmission and is placed between the first funnel-shaped cavities and the second funnel-shaped cavities.
There is also provided, in accordance with an embodiment of the present invention, a method for fabricating an optical device. The method includes forming, in a substrate having first and second opposite surfaces, a funnel-shaped cavity between a first aperture on the first surface and a second aperture on the second surface. An inner wall of the funnel-shaped cavity is covered with a metallic layer. A dielectric filling is disposed over the metallic layer and fills the funnel-shaped cavity. A light collection element is coupled to the second aperture.
There is further provided, in accordance with an embodiment of the present invention, a method for fabricating an optical device. The method includes, in a metallic substrate having first and second opposite surfaces, forming one or more first funnel-shaped cavities in the first surface, and forming one or more second funnel-shaped cavities in the second surface, facing the respective first funnel-shaped cavities, such that each first funnel-shaped cavity and a corresponding second funnel-shaped cavity penetrate the substrate from the first surface to the second surface. The first and second funnel-shaped cavities are filled with a dielectric filling.
There is additionally provided, in accordance with an embodiment of the present invention, an optical processing method. The method includes collecting light using an optical device, which includes a substrate, having first and second opposite surfaces and having a funnel-shaped cavity between a first aperture on the first surface and a second aperture on the second surface, a metallic layer covering an inner wall of the funnel-shaped cavity, and a dielectric filling, which is disposed over the metallic layer and fills the funnel-shaped cavity. The collected light is directed by the optical device to a light collection element that is coupled to the second aperture.
There is also provided, in accordance with an embodiment of the present invention, an optical processing method. The method includes providing an optical device, which includes a metallic substrate and a dielectric filling. The metallic substrate has first and second opposite surfaces. The first surface has one or more first funnel-shaped cavities formed therein, and the second surface has one or more second funnel-shaped cavities formed therein facing the respective first funnel-shaped cavities, such that each first funnel-shaped cavity and a corresponding second funnel-shaped cavity penetrate the substrate from the first surface to the second surface. The dielectric filling fills the first and second funnel-shaped cavities. Using the optical device, light that impinges on the first surface is collected, and the collected light is emitted from the second surface.
The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a three-dimensional schematic illustration of a plasmonic horn antenna, in accordance with an embodiment of the present invention;
FIG. 2 is a schematic side cross-section of a plasmonic horn antenna, in accordance with an embodiment of the present invention;
FIG. 3 is a schematic side cross-section showing simulated performance of a plasmonic horn antenna, in accordance with an embodiment of the present invention; and
FIG. 4 is a three-dimensional schematic illustration of a partially-transparent metal sheet, in accordance with an embodiment of the present invention.



DETAILED DESCRIPTION OF EMBODIMENTS
Overview
Embodiments of the present invention that are described herein provide improved optical devices and associated methods. The disclosed devices perform efficient collection, direction and focusing of light onto a small, e.g., sub-wavelength, spot size.
In some embodiments, such an optical device comprises a three-dimensional funnel-shaped cavity formed in a substrate. The funnel-shaped cavity connects between an input aperture on one surface of the substrate and a smaller output aperture on the opposite surface. A metallic layer covers the cavity wall, and the cavity is filled with a dielectric filling material. A light collection element, also referred to as an absorber, is coupled to the output cavity.
The resulting optical device is flat and planar, e.g., having a thickness on the order of 100 nm. The funnel-shaped cavity has a very wide flare angle, e.g., from an input aperture of 15 μm to an output aperture of 5 μm, or from an input aperture of 1 μm to an output aperture of 10 nm or 100 nm. Nevertheless, the dielectric-filled funnel-shaped metallic structure exhibits extremely high light collection efficiency.
The physical mechanisms that govern the device operation are explained herein, along with example Finite-Difference Time-Domain (FDTD) simulation results. Such optical devices lend themselves to fabrication using conventional Integrated Circuit (IC) manufacturing processes.
The disclosed optical devices can be used in a wide variety of applications, such as in miniature detectors and detector arrays used in cameras or night vision equipment. When used in a detector array, the metallic layer also provides high isolation from neighboring devices, and thus reduces cross-talk.
Another possible use for such devices is in miniature optical sensors, such as sensors for detecting very small quantities of matter. The disclosed devices can also be used for enhancing non-linear optical effects, for example for use in optical switching or modulation. Other potential uses are in miniature and ultra-thin solar energy cells, in enhancement of optical emission and directivity, in fluorescence measurements, and in nano-lithography.
In other disclosed embodiments, a pair of dielectric-filled metallic funnels is connected back-to-back to form an hourglass-shaped light transmission element. An array of such elements may be fabricated in a metallic sheet, so as to produce a sheet that is optically partially-transparent yet electrically conductive and mechanically robust. Such an application is sometimes referred to as a “smart metal.” In some embodiments, a saturable absorber is placed between the two funnel-shaped antennas of each hourglass-shaped element. Such a structure provides a “smart layer” whose transmission properties can be controlled. Other uses for such arrays of hourglass-shaped elements are also suggested herein.
General Description
FIG. 1 is a three-dimensional schematic illustration of a plasmonic horn antenna 20 , in accordance with an embodiment of the present invention. Antennas having the structure of antenna 20 are used for focusing light from a micro-scale aperture into a micro- or nano-scale aperture using an extremely flat form factor. Such antennas can be fabricated using conventional Integrated Circuit (IC) fabrication processes.
Antenna 20 comprises a funnel-shaped metallic wall or layer 22 , e.g., made of gold. The funnel-shaped volume defined by layer 22 is filled with a dielectric filling 24 , e.g., Benzocyclobutene (BCB). A light collection element 26 , also referred to herein as an absorber, is coupled to the bottom aperture of the antenna. The light collection element may comprise, for example, an optical detector.
FIG. 2 is a schematic side cross-section of antenna 20 , in accordance with an embodiment of the present invention. In the present example, the antenna is fabricated in a suitable substrate 28 . A funnel-shaped cavity is formed in substrate 28 , and the cavity wall is plated with metallic layer 22 , in the present example formed of gold. The interior of the funnel-shaped cavity is filled with dielectric filling 24 , in the present example BCB.
In this embodiment, antenna 20 focuses incoming light from a first aperture 30 on a first surface of substrate 28 into a second aperture 32 on a second surface of the substrate. The thickness of substrate 28 , and thus of antenna 20 as a whole, is denoted 36 . Light collection element 26 is coupled to the second aperture, against the bottom of dielectric filling 24 .
In various embodiments, antenna 20 can be used for focusing light from a micro-scale aperture into a smaller micro-scale aperture, or into a nano-scale aperture. In other words, aperture 30 typically has a micro-scale area, whereas aperture 32 may have a micro-scale or a nano-scale area. Several examples of such antenna configurations are described below.
In one example embodiment, the diameter of aperture 30 is on the order of 15 μm, the diameter of aperture 32 is on the order of 5 μm, thickness 36 is on the order of 2.2 μm, and light collection element 26 comprises an indium gallium arsenide (InGaAs) absorber. In another embodiment, the diameter or aperture 30 is on the order of 1 μm, the diameter or aperture 32 is on the order of 100 nm or 10 nm, thickness 36 is on the order of 100-140 nm, and light collection element 26 comprises a 10 nm-thick silicon (Si) absorber. In such embodiments, the characteristic size (e.g., diameter) of aperture 32 is considerably smaller that the wavelength of the focused light. In other words, antenna 20 focuses the incoming light into a deep sub-wavelength aperture.
As can be seen from the example dimensions above, antenna 20 has an extremely flat, planar form factor. Thus, the taper of the horn is highly non-adiabatic. Typically, the input aperture size (e.g., the diameter of aperture 30 ) is at least five times the antenna thickness (thickness 36 ). Generally, however, other suitable aperture-thickness ratios can be used. Notwithstanding the highly non-adiabatic taper, antenna 20 is highly efficient in capturing and transferring light from aperture 30 into absorber 26 , as will be explained and demonstrated below.
The example configurations of antenna 20 described herein are depicted purely by way of example. In alternative embodiments, any other suitable antenna configuration can be used. For example, the antenna dimensions (e.g., apertures 30 and 32 and thickness 36 ) may be set to any other suitable values. Filling 24 may be formed of any other suitable dielectric material. Metallic layer 22 may be formed of any other suitable metal. Any other suitable type of light collection element 26 can be used to collect and absorb the light focused by the antenna.
Moreover, the examples described herein refer to a conically-shaped antenna, by way of example. In alternative embodiments, the apertures 30 and 32 may have any other suitable shapes, e.g., square or rectangular. Metallic layer 22 and filling 24 may have any other suitable shape that connects an aperture on one surface of a substrate to a smaller aperture on the other surface of the substrate. In the context of the present patent application and in the claims, any such shape is regarded herein as “funnel-shaped.”
Principle of Operation
In a micro- to nano-scale antenna, the process of collecting and focusing light by antenna 20 can be divided into three different mechanisms, which generally operate at three different regions of the antenna. The incoming light, typically modeled as a non-polarized plane wave, is reflected by the slanted metallic layer 22 toward the interior of the funnel-shaped BCB-filled cavity. The reflected light is guided laterally along the flat BCB filling, bounded by the air and gold interfaces.
Typically, as the guided modes reach the mid-section of the antenna, the aperture narrows to a point in which all modes are at cut-off and should be back-reflected. For example, a linearly-polarized plane wave excites a dominant TE11 mode in a circular aperture. For this mode, the cut-off radius is given by a=p 11 ′λ/2πn, wherein p 11 ′=1.841 and n denotes the refractive index of dielectric filling 24 . For BCB, n=1.6, which for λ=845 nm yields a cut-off at ≅150 nm, which is approximately 30 nm above the antenna bottom (location of light collection element 26 ).
At this point, a second mechanism is dominant: The evanescent tail of the cut-off modes is in proximity to the high-refractive-index absorbing element (element 26 , in the present example made of 10 nm-thick silicon for which n=3.5). As a result, cut-off modes leak into the absorbing element, assisted by tunneling.
A 10 nm-thick silicon absorber is a poor absorbing material, virtually transparent. In order to enhance absorption, light should be slowed-down in order to allow efficient interaction with the absorbing silicon media. However, any light reaching the narrow bottom of the antenna will be back-reflected because of mismatch to free-space.
At this point a third mechanism is dominant: Dielectric filling 24 with surrounding metallic layer 22 at the bottom of the antenna creates a metal-dielectric interface, which was shown to be able to confine plasmonic modes at the interface. Moreover, the circular edge of the metallic layer is expected to help in momentum transfer needed for the coupling. The combination of these two conditions allows coupling of propagating modes to plasmonic resonances (PR). The circular geometry exhibits resonances of Whispering Gallery type, and lock incoming light until it is totally absorbed in element 26 .
In some embodiments, efficient transfer of light to element 26 is enhanced by forming element 26 from multiple layers having progressively-increasing refractive indices. This technique is demonstrated below for a micro- to micro-scale antenna, but it is also applicable to micro- to nano-antennas.
FIG. 3 is a schematic side cross-section showing simulated performance of a plasmonic horn antenna, in accordance with an embodiment of the present invention. In this example, the antenna focuses light from a micro-scale (e.g., 15 μm-diameter) aperture 30 into a smaller micro-scale (e.g., 5 μm-diameter) aperture 32 , and into an InGaAs absorbing element 26 .
InGaAs element 26 has a refractive index n=3.44, whereas dielectric filling 24 has a refractive index n=1.67. In some embodiments, an additional SiN layer (having n=2) and an additional InP layer (having n=3.167) are placed between dielectric filling 24 and the InGaAs absorber. As a result, the refractive index changes gradually, rather than abruptly at the boundary between element 26 and filling 24 . The graded change in refractive index reduces reflections and improves the transfer of light from the antenna to the absorber.
The specific layered structure of element 26 shown in FIG. 3 is chosen purely by way of example. In alternative embodiments, any other suitable choice of layers having gradually increasing refractive indices can be used.
Additionally or alternatively, in some embodiments the height or thickness of the antenna (thickness 36 ) is optimized such that high-order modes at their cut-off point are forward-scattered (assisted by tunneling) rather than back-reflected. This optimization may be performed, for example, by simulation.
Simulated Performance on Micro- to Micro-Antenna
The performance of micro- to micro-plasmonic horn antennas was evaluated using a Finite-Difference Time-Domain (FDTD) simulation. The simulated antenna configuration was the configuration shown in FIG. 3 above. The main results are reproduced below. The simulation results are provided in greater detail in U.S. Provisional Patent Application 61/811,819, cited above.
In the simulation, the light collection efficiency of the antenna was computed and compared to the efficiency of an air-filled antenna of the same dimensions. At a wavelength of λ=1550 nm, the collection efficiency of the air-filled antenna was on the order of 10%, whereas the BCB-filled antenna showed a high efficiency on the order of 72%. The average collection efficiency of the BCB-filled antenna over the range λ=800-1700 nm was 55%, indicating broadband behavior over the entire absorption spectrum of the InGaAs absorber.
The total absorption enhancement, of the InGaAs absorber integrated with the BCB-filled horn antenna, has increased by a factor of 5.5 at a wavelength of 1550 nm.
Simulated Performance on Micro- to Nano-Antenna
The performance of micro- to nano-plasmonic horn antennas was evaluated using FDTD simulation. Material constants, including losses, were taken from experimental data. The main results are reproduced below. The simulation results are provided in greater detail in U.S. Provisional Patent Application 61/811,819, cited above.
In the simulation, a monitor was placed below the modeled antenna (below absorber 26 , in the present example a silicon absorber) in order to measure the transmission of the structure. The total power absorbed in absorber 26 was determined by integrating a volumetric field monitor, and the absorption spectral dependence was calculated. The total enhancement is defined as the ratio between an absorber with antenna and a bare absorber, both illuminated by a linearly-polarized wave having a diameter of 1 μm.
The transmission spectrum was evaluated for two antenna geometries—An antenna collecting a 1 2 μm 2 to 100 2 nm 2 , and an antenna collecting 1 2 μm 2 to 10 2 nm 2 . For 1 2 μm 2 to 100 2 nm 2 light collection, the expected transmission is 1%, whereas the simulated results show a transmission of 38%. For 1 2 μm 2 to 10 2 nm 2 light collection, the expected transmission is 0.01%, and the simulated results show a transmission of over 20%. Furthermore, taking into account the collection aperture ratio of 1:10,000 (1 2 μm 2 to 10 2 nm 2 ) and approximately 10% transmission at λ=845 nm, one would expect a factor of 1,000 increase in absorption.
The simulation results, however, show an increase in absorption by a factor of 22,000. The effective collection efficiency is thus on the order of 200%, compensating for diffraction limit, even for a 10 nm absorber. This absorption enhancement is likely to be due to the Whispering Gallery plasmonic modes at the antenna bottom. This explanation was strengthened by a simulated comparison of the magnetic field profile between the antenna bottom and an infinite BCB-filled metallic cylinder.
The FDTD simulation was also used to examine the transmission for different output diameters (different diameters of aperture 32 ). The transmission spectrum can generally be divided into two regions: Above λ=1000 nm the results show a general increase in transmission with output diameter. The spectral peaks are also shifted towards higher wavelengths with as the output diameter increases. Below λ=1000 nm, on the other hand, the value for each peak does not show a clear trend and the spectral location of the peaks seems to remain constant.
The effect of the antenna thickness (thickness 36 ) on transmission and PR enhancement was also investigated. In the simulated example, different thicknesses were tested, for a constant output diameter of 100 nm. Here, too, the transmission behavior can be divided into the same two regions as in the previous case: At the high end of the spectrum the location of the transmission peaks remains unchanged, and there is an optimum point of approximately 50% transmission at a thickness of 140 nm. At the low end of the spectrum, there are two well-located lobes, opposite in behavior—As one lobe increases as a function of thickness, the other lobe decreases. The PR enhancement shows a very consistent dependency between thickness and location of peaks. An optimum is acquired for a thickness of 160 nm, exceeding a factor of 300 in enhancement.
The FDTD simulation was also used to examine different types of dielectric materials for implementing filling 24 . Two output diameters (100 nm and 10 nm) were simulated, while changing the refractive index of the antenna filling material. The antenna thickness was set at 100 nm. The transmission results for different dielectric filling materials were normalized to cancel Fresnel reflections, assuming perfectly index-matched interfaces.
For an output diameter of 100 nm, transmission is distinctly red-shifted and broadened for increasing refractive index. An optimum occurs for n=1.3, exceeding 60% of transmission. For an output diameter of 10 nm, the red-shift is more moderate, reaching a maximum of 47% for n=2. The PR enhancement results both show red-shifted response for increasing of refractive index, with rising values of enhancement. A slight offset towards the blue of the peaks location is evident in the 10 nm diameter case.
The fact that transmission and PR enhancement peaks do not overlap in wavelength and are far from optimum, emphasizes the potential for enhancement factors achievable with appropriate optimization of the structure. For illustration, an antenna with 10 nm output diameter and n=2 filling, combining 47% transmission with 35-fold PR enhancement can yield a 165,000-fold enhancement in absorption.
Another set of simulations was performed for a 10 3 nm 3 silicon absorber. The results show that proper optimization of the antenna can potentially enhance the Quantum Efficiency (QE) of such an absorber from 10 −8 to 10 −3 . In order to enhance QE, an InGaAs absorber was tested. In this configuration, the thickness of InGaAs element 26 was increased to 100 nm instead of 10 nm. Thickness 36 was set to 140 nm, reaching QE of 16% at λ=1054 nm.
Back-to-Back Plasmonic Horn Antennas
In some embodiments, a pair of metallic, dielectric-filled horn antennas such as antenna 20 is connected back-to-back to form an hourglass-shaped transmission element. An array of such elements may be fabricated in a metallic sheet, so as to produce a sheet that is optically partially-transparent yet electrically conductive and mechanically robust. Such an application is sometimes referred to as a “smart metal.”
FIG. 4 is a three-dimensional schematic illustration of a partially-transparent metal sheet 40 , in accordance with an embodiment of the present invention. Sheet 40 comprises a sheet 44 of metal, e.g., gold. An array of multiple hourglass-shaped elements 42 is fabricated in sheet 44 .
The insets at the bottom of the figure show the details of an example element 42 . Each hourglass-shaped element 42 comprises a pair of funnel-shaped cavities connected back-to-back. The two funnel-shaped cavities are formed facing one another in the opposite surfaces of the sheet. Together, the pair of cavities penetrates the sheet from one surface to the other.
Each funnel-shaped cavity is filled with dielectric filling 24 , e.g., BCB, and generally has a structure similar to that of antenna 20 described above. The dielectric filling extends contiguously from one surface of sheet 40 to the other. (Although the figure shows element 42 as having a metallic wall 22 as in antenna 20 , in practice this wall is formed inherently from the metal of sheet 44 .)
The structure of element 42 was investigated using FDTD simulation. The simulation investigated a 200 nm-thick hourglass-shaped element (100 nm per funnel-shaped cavity), having a 100 nm output diameter. Results show that the light collection efficiency of element 42 (i.e., antenna 20 terminated by another identical antenna 20 ) exceeds the light collection efficiency of antenna 20 terminated with an absorber element 26 . The simulated element 42 reached peak transmission of 46% at λ=1087 nm.
As can be appreciated, sheet 40 is on one hand partially optically-transparent (by virtue of optical properties of elements 42 ), but on the other hand electrically-conductive (since its electrical continuity is broken only in very small apertures at the apexes of the funnel-shaped cavities). The mechanical rigidity of sheet 40 is not considerably degraded by elements 42 , and is similar to that of the original metal sheet. A structure such as sheet 40 can be used, for example, for electromagnetic shielding purposes.
In other embodiments, a saturable absorber (not shown in the figure) may be placed between the two funnel-shaped antennas of each hourglass-shaped element 42 . By controlling the transmission properties of this absorber, the transmission properties of the entire sheet can be controller, so as to provide a “smart layer” structure.
In alternative embodiments, the symmetric structure of element 42 can be exploited in various transmit-receive configurations, such as for exciting a molecule from one direction and performing spectroscopy in the other direction.
In another example embodiment, an element such as element 42 is placed above a pumping fiber-coupled laser, and an active media is placed between the funnel-shaped cavities. This structure can be used for creating enhanced emission and coupling to free-space or fiber on the other side of the structure. This configuration can be used, for example for non-linear effect enhancement.
Although the embodiments described herein mainly address optical light collection and focusing, the methods and systems described herein can also be used in other applications, such as in collecting Radio Frequency (RF) waves or at THz frequency ranges.
It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.