VOLUMETRIC SILICON META-OPTICS FOR COMPACT AND LOW-POWER TERAHERTZ SPECTROMETERS

A device including a stack of silicon meta-optical layers forming a meta-material comprising an input surface for receiving terahertz electromagnetic radiation, an output surface for outputting a plurality of beams of the electromagnetic radiation; and a spatially varying permittivity varying with sub-wavelength precision across a volume of the stack, wherein the spatially varying permittivity is configured to focus different spectral bands of the electromagnetic radiation into different spatially separated electromagnetic modes.

BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates to meta-optical devices and methods and systems using the same.

Description of the Related Art

The interaction of light with matter gives rise to spectral features that, like a fingerprint, can uniquely identify molecules. These spectral features propagate as electromagnetic waves, allowing molecular information to be characterized remotely by an instrument with sufficient spectral resolution. This technique is known as spectroscopy and is among the most critical techniques used in Earth, planetary, and astrophysics observations. The submillimeter-wave frequency range, spanning approximately 300 GHz to 3 THz, is a notoriously difficult range to work within. This lends it the name the “terahertz gap,” as the difficulties in scaling microwave technology to shorter wavelengths and scaling optical technology to longer wavelengths give rise to a frequency range wherein neither technology is adequate.

Certain facts of physics prevent the efficient scaling of technology to arbitrary frequency ranges. For example, non-superconducting metals inevitably become lossy as the frequency of operation increases. Many terahertz technologies today, including basic waveguides, rely on metallic structures and must accept the associated losses. State-of-the-art spectroscopy techniques involve heterodyning, which has as its basic ingredient one or more local oscillator signals, and it remains immensely challenging to both realize the required power over a broadband and scale such devices to many pixels. While progress is steadily being made on this front, new opportunities are arising for technologies that are uniquely suited for the terahertz range. Volumetric meta-optics are uniquely suited for the terahertz regime at present, given that 3D fabrication at higher frequency ranges remains complex. The present disclosure satisfies this need.

SUMMARY OF THE INVENTION

The present disclosure describes a device, comprising a stack of silicon meta-optical layers forming a meta-material comprising an input surface for receiving terahertz electromagnetic radiation, an output surface for outputting a plurality of beams of the electromagnetic radiation; and a spatially varying permittivity varying with sub-wavelength precision across a volume of the stack, wherein the spatially varying permittivity is configured to focus different spectral bands of the electromagnetic radiation into different spatially separated electromagnetic modes.

Novel functionalities of the volumetric meta-optical device can be used to implement a novel type of spectrometer which utilizes spectral or color routing. In some embodiments, the spectrometer has substantially reduced size, mass, and power consumption.

In addition to the functionality used in the spectrometer system described herein, volumetric devices can be optimized to control all properties of light simultaneously. Polarization, spectral, and spatial properties can all be controlled by a single, wavelength-scale device over broad bands and with high efficiency. Realistically these devices could be used for many more terahertz applications, and the applications described herein are intended as illustrative experimentation of their use in realizing innovative new terahertz instruments.

DETAILED DESCRIPTION OF THE INVENTION

Technical Description

FIGS.1A,1B,1C, and IF illustrate a device100, comprising a stack102of silicon meta-optical layers104forming a meta-material comprising an input surface106for receiving terahertz electromagnetic radiation108, an output surface110for outputting a plurality of beams112of the electromagnetic radiation; and a spatially varying permittivity varying with sub-wavelength precision across a volume of the stack. The spatially varying permittivity is configured to focus different spectral bands of the electromagnetic radiation into different spatially separated electromagnetic modes114.

For theFIG.1Fsimulations of transmission, there is assumed to be no oxide between the silicon layers of the volumetric meta-optical element, although there could be oxide between the layers in theory. In embodiments wherein the Si layers are directly bonded (Si—Si bonding), no oxide is present.

The meta-optics are made of multiple Silicon layers stacked directly on top of one another. At terahertz frequencies, the layers can be patterned and aligned with deep subwavelength accuracy. For the embodiments tested herein, the shapes of the layers are designed with adjoint-based inverse-design methods to identify an optimal shape for the full 3D device. The optimization procedure is general, allowing for arbitrary sets of inputs to be mapped to arbitrary sets of outputs. The input and outputs can be based on not only spectral properties, but also spatial and polarization properties, making this a promising platform for multi-functionality.

Thus, no matter the type of direct detectors used, and no matter the modes incident on the meta-optics, a high-performing solution can typically be found to maximize coupling over a broad frequency range.

First Spectrometer Embodiment

FIG.2A-2Cillustrate how a tunable resonator204(such as a Fabry-Perot cavity with a tunable length between mirrors) can be implemented as a high resolution spectrometer200when coupled with a sensitive direct detector210and receiving a broadband signal202,212. However, the resonator outputs a sequence of resonances214separated by a free-spectral range (FSR), and to avoid ambiguity these resonances must be either filtered out using a filter208, as illustrated inFIG.2A, which limits the bandwidth of the resonator to one FSR216, or must be spatially separated such that the system can simultaneously measure the different resonances. The traditional way of separating the resonances would be to use a blazed grating218, as illustrated inFIG.2B, but this necessitates a long propagation length between the grating and sensor array. Furthermore, the continuous dispersion217of a grating causes the coupling efficiency to the detectors to only be optimal at discrete frequencies, rather than over a continuous band.

The meta-optical device100described herein can be used to enhances resonator-based spectrometers and overcome these limitations.FIG.2Cillustrates the volumetric meta-optics element100replacing the grating and placed immediately after the FP cavity. A detector210array is in-turn placed immediately (approximately two wavelengths) after the meta-optics element, enabling an extremely compact spectrometer element. The coupling efficiency of the electromagnetic modes to the detectors can be optimized216for increased overlap with each of the direct detectors and in a much more compact volume that does necessarily rely on free space propagation to separate the spectral bins (the spatial separation of the spectral bins occurs primarily within the device rather than the free space outside the device, so that the detectors can be positioned much closer to the output of the device relative to the grating configuration).

FIG.3A-3Cillustrate the prototype system is a R˜104spectral resolution spectrometer300operating at 500-650 GHz, and based entirely on Silicon micro-fabricated parts. A 10{circumflex over ( )}5 spectra; resolution is possible with this technique through a combination of increasing mirror reflectivity and increasing cavity length of the resonator302.

The FP-cavity is made from two distributed Bragg reflector mirrors comprised of Silicon and void. The distance between the mirrors is scanned with a piezoelectric nano-positioner to scan the resonances over one FSR. The Si membranes (comprising alternating silicon and air layers configured as a Distributed Bragg Reflectors forming the cavity mirrors) are curved by a thin SiO2film, which causes the fundamental mode of the cavity to be compressed to a small Gaussian beam. The FSR of the cavity is 50 GHz, thus dividing the 500-650 GHz into three bands. The cascaded meta-optics device focuses these three bands onto three direct detectors.

AlthoughFIG.3Billustrates the resonator comprising curved mirrors, the cavity400can also comprise flat meta-surface mirrors402as illustrated inFIG.4. As for the curved mirrors, the mirrors each comprise a stack of alternating silicon and air layers forming a membrane comprising a Distributed Bragg Reflector (DBR). In the example of a flat metasurface mirror illustrated inFIGS.5A-5C, however, a first external layer of the DBR is patterned with holes to form the metasurface. The mirrors are coupled to a piezoelectric actuator scanning a separation between the membranes forming the mirrors of a Fabry Perot cavity.

FIG.5D-5Fillustrate a spectrometer500according to the first embodiment, comprising (e.g., silicon) lens502focusing electromagnetic radiation onto a meta-optically stabilized Fabry Perot cavity504comprising planar mirrors; frequency de-multiplexing using 3D meta-optics100, a waveguide array506coupling the different spectral bands507in different spatially separated electromagnetic modes outputted from the 3D meta-optical element, to each of a plurality of different Schottky detectors508, and wherein the output of the Schottky detectors is coupled to DC amplification and readout circuitry510. A piezoelectric nanopositioner512can be used to position one of the mirrors of the cavity. The waveguide array and Schottky detectors can be positioned in a split block514(metal block).

In this embodiment, a method for performing spectroscopy comprises receiving, in a single resonator504, electromagnetic radiation after interaction with a target sample; scanning a cavity length of a single resonator across resonances of a free spectral range of the resonator; and coupling an output of the single resonator to the volumetric meta-optical device100that focuses the electromagnetic radiation outputted from a single resonator onto an array of detectors. The detectors508each have a sensitivity tuned or selected for a different one of the spectral bands and each output an output signal in response thereto. The output signals can be analyzed (e.g., using a computer) to determine the frequency response of the target sample to each of the different spectral bands.

Second Spectrometer Embodiment

FIG.6Aillustrates a spectrometer600according to a second embodiment and comprising an array of resonators602coupled to an output of the volumetric meta-optical element100. Each of the resonators comprises a cavity that can be detuned via an erosion or dilation of the cavity metasurface mirror features such that a longitudinal resonance roughly aligns with a spectral line of interest. Each of the resonators are configured to scan across the spectral line associated with the resonator.

FIG.6Afurther illustrates the spectrometer comprises a silicon lens604focusing electromagnetic radiation605from the target onto the frequency de-multiplexing using 3D meta-optics, each of the different spatially separated and non-overlapping electromagnetic modes606outputted from the meta-optics each coupled (via narrowband collimating metasurfaces) to a different one of a plurality of meta-optically stabilized Fabry Perot resonators602, a waveguide array606coupling the outputs from each of the resonators to a different one of a plurality of different Schottky detectors608, and wherein the output of the Schottky detectors is coupled to DC amplification and readout circuitry610.

FIG.6Billustrates detuning of the Fabry Perot resonators via the metasurface enables simultaneous/multiplexed detection of spectral lines and prevents phase-locking of the array.FIG.6Cillustrates an Electromagnetic mode114outputted from meta-optical device and how a specific longitudinal mode650(of possible modes652) of the resonator is selected by illumination by the electromagnetic mode114.FIG.6Eillustrates arbitrarily placed detection lines align with spectral lines654of interest.

Thus, a method for performing spectroscopy illustrated inFIG.6Acomprises focusing different spectral bands of electromagnetic radiation (received from a target sample) into different spatially separated and non-overlapping electromagnetic modes (using the volumetric meta-optical device described herein); collecting the modes on an array of resonators, wherein each of the resonators receives a different one of the electromagnetic modes focused by the volumetric device; and coupling the electromagnetic radiation outputted from the array of resonators to an array of detectors, so that each array received the electromagnetic radiation from a different one of the resonators (i.e., the nth detector receives the electromagnetic radiation from the nth resonator).

In one embodiment, each of the resonators are de-tuned from different known spectral line associated with that resonator. More specifically, the method further comprises, for each of the 1<i≤n resonators, scanning a cavity length of the ithresonator to scan a longitudinal resonant mode of the ith resonator across at least a portion of a linewidth of the ith spectral line; detecting the electromagnetic radiation outputted from the ith resonator on the ith one of the detectors; and generating the output signals in response thereto. The method can then further comprise analyzing the output signals to determine the frequency response of the target sample to the electromagnetic radiation at one or more (or each of) the frequencies of one or more of the spectral lines.

In another embodiment, the electromagnetic radiation is received after interaction with a target sample, and each of the resonators are de-tuned from a different known spectral line in a set of target spectral lines610(e.g., of molecules, atoms, gases, liquids, or other materials of interest, e.g., in an atmosphere), the method further comprising scanning cavity lengths of the resonators together, so that one longitudinal resonant mode650of each of the resonators602is only scanned across a spectral portion encompassing the one of the target spectral lines654associated with that one of the resonators, wherein the array of resonators collectively scans across all the spectral lines in the set (i.e., for 1 <i<n resonators, electromagnetic modes114, spectral lines, and detectors, a longitudinal mode of the ithresonator, selectively illuminated by the ithone of the electromagnetic modes114outputted from volumetric device100, is scanned across a spectral portion of the ithspectral line); detecting the electromagnetic radiation outputted from each of the resonators on an array of detectors608generating the output signals in response thereto (so that the ithone of the detectors detects the electromagnetic radiation from the ithone of the resonators); and analyzing the output signals to determine a response of the target sample to the electromagnetic radiation at one or more frequencies of the known spectral lines.

In one or more embodiments, the analyzing comprises determining at least one of an absorption or scattering of the electromagnetic radiation at one or more of the frequencies of the spectral line (e.g., for each of the i spectral lines), or comparing a line-shape of the electromagnetic radiation measured at the detectors with a line shape of the spectral line in a presence of a control sample. One or more computers670(e.g., comprising one or more processors and memories executing a program, or application specific integrated circuit, or field gate programmable array or circuit/circuitry) and motor514can be included in the spectrometer or coupled to one or more components of the spectrometer to perform analysis or scanning or other functionalities, or control the spectrometer, as described herein.

The volumetric meta-optics device can be placed directly on single-mode waveguide array that couples to the arrayed Fabry-Perot cavities in a tiled configuration.

Since all elements (including the detectors) can be tuned to a specific spectral line, scanning can be done in parallel to achieve multiplexed detection.

Method of Fabricating

Etching

FIGS.7and8illustrate a method of fabricating the meta-optical device.

The first step comprises photolithographically etching a plurality of regions of a silicon on oxide (SOI) wafer700using an oxide layer702in the wafer as an etch stop, so as to define a plurality of dies800each comprising a different one of the silicon layers704and a handle portion706of the wafer. For the devices fabricated and tested herein, the wafer was patterned with photoresist using photolithography and the etch pattern was transferred to the silicon using a Bosch process.

The silicon on oxide wafer comprises a layer of high resistivity float zone silicon (high resistivity for the terahertz electromagnetic radiation, e.g., resistivity greater than 10 kohm-cm). For the data presented herein, the silicon layers have a thickness of 40 microns, however this thickness can be optimized for different applications and frequencies of operation. The layers typically have a thickness less than a wavelength of the terahertz electromagnetic radiation in free space.

FIG.9illustrates each of the silicon layers are etched to comprise a thickness and a pattern of openings902through the thickness of the layer and are connected around their edge or perimeter to the handle portion. The openings are patterned to spatially tailor the permittivity as described herein. NOTE: the silicon is fully connected during optimization for mechanical robustness and does not rely on oxide layer for connectivity.

Next, the dies are separated and assembled one on top of the other so as to stack the silicon layers using the handle portions for alignment. The next step comprises bonding the silicon layers together using fusion bonding to form a stack of silicon layers. For the devices tested herein, the bonding comprises room temperature silicon to silicon direct bonding. In some embodiments, the handle portion of the bottom die in the stack has less of the handle wafer etched away. to provide increased rigidity to support the remaining silicon layers during the bonding process.

In order to position the silicon layers with the correct alignment, the areas of the dies are progressively designed to be smaller so that the dies can be assembled into a nested stack aligned by the handle portions.FIGS.10A and10Billustrates the layers can be positioned with +/−5 micron accuracy, and 2 micron accuracy is also possible.

FIG.11illustrates mounting the stack in a support mount (e.g., using pins inserted in through holes patterned in the dies).

Inverse Design

FIG.12andFIG.13illustrate how the pattern of openings (corresponding to the voids patterning the permittivity) is determined using an inverse design method. The inverse design method comprises using a gradient based optimization to iteratively find the optimal permittivity distribution of the meta-optical elements that focuses a transverse electric (TE) plane wave of the electromagnetic radiation received on the input face to a plurality of the electromagnetic modes/comprising the different resonances, e.g., different longitudinal modes of a single resonator, different detuned modes of a plurality of resonators, or different spectral bands (e.g., longitudinal resonant modes separated by a free spectral range).

The gradient can comprise the differential of a figure of merit, e.g., intensity at a point or a waveguide mode coupling) as a function of the permittivity of the meta-optical elements for each of the resonances/spectral ranges. The optimization uses a numerical method to iteratively find the void/opening size and distribution that maximizes the figure of merit, by changing the void/opening size and distribution at each iteration step until the figure of merit converges to a local optimum.

Possible Modifications and Variations

The volumetric meta-optical device described herein can be designed (e.g., as a lens using inverse design) for a variety of applications beyond spectroscopy.

Example Embodiments

The device, systems, and methods can be embodied in many ways including, but not limited to, the following (referring also toFIGS.1-15)

1. A device100, comprising:a stack102of silicon meta-optical layers104forming a meta-material comprising an input surface106for receiving terahertz (e.g., 0.3 THz-3 THz) electromagnetic radiation108, an output surface for outputting a plurality of beams112of the electromagnetic radiation; and a spatially varying permittivity varying with (e.g., sub-wavelength precision) across a volume of the stack, wherein the spatially varying permittivity is configured to focus different spectral bands of the electromagnetic radiation into different spatially separated electromagnetic modes114.

2. The device of clause 1, wherein the meta-optical elements each comprise a distribution of voids900comprising a shape and dimension patterning the spatially varying permittivity, wherein the sub-wavelength precision comprises a feature size of the voids that less than one or more wavelengths of the terahertz electromagnetic radiation.

3. The device of clause 1 or 2, wherein the spectral bands each comprise a resonance of a free spectral range (FSR) of a resonator602.

4. The device of clause 1 or 2 or 3, wherein silicon surfaces of the meta-optical layers are bonded together to prevent, suppress, or eliminate air gaps between the layers.

5. The device of any of the clauses 1-4, wherein:each of the meta-optical layers comprises an electromagnetic meta-surface comprising a thickness T and a two dimensional pattern902of voids through the thickness; andthe thickness is less than all the wavelengths of the terahertz electromagnetic radiation

6. The device of any of the clauses 1-5, wherein the thickness is less than or equal to a quarter of the longest of the wavelengths in free space.

7. The device of clause 6 wherein each of the meta-optical layers comprises a continuous piece of silicon906that is self-supporting.

8. The device of any of the clauses 1-7, comprising at least 4 of the silicon layers and wherein each of the silicon layers has a different pattern for the spatially varying permittivity.

9. A spectrometer500,600comprising a resonator504,602coupled to the stack102of meta-optical layers104of any of the clauses 1-8.

10. The spectrometer500of clause 9, wherein the spectral bands each comprise a different one of a plurality of free spectral range resonances of the resonator, wherein the input surface is coupled to the output of the resonator504to receive the electromagnetic radiation; and further comprising an array of direct detectors508coupled to the output surface each positioned to receive a different one of the electromagnetic modes

11. A spectrometer600comprising the device of any of the clauses 1-9, comprising:an array of direct detectors602or resonators608coupled to the output surface110of the metamaterial100and positioned to receive a different one of the electromagnetic modes.

12. The spectrometer600of clause 11, wherein each of the resonators602comprises a cavity detuned from a different spectral line and the resonators are configured to scan longitudinal modes of the resonator across the spectral line.

13 The spectrometer or device of any of the clauses 1-12, wherein the electromagnetic modes are spaced less than two of the longest one of the wavelengths apart in a lateral direction.

14. The spectrometer of any of the clauses 1-13, wherein the resonator comprises a coupled pair of membranes402(DBR air silicon) coupled by a piezoelectric actuator scanning a separation between the membranes.

15.FIG.14Amethod of making a meta-optical device, comprising:photolithographically etching (Block1400) a plurality of regions of a silicon on oxide wafer using an oxide layer in the wafer as an etch stop, to define a plurality of dies each comprising a different one of a plurality of silicon layers and a handle portion of the wafer, each of the silicon layers comprising a thickness and a pattern of openings through the thickness of the layer and connected at an edge to the handle portion;separating the dies (Block1402);assembling (Block1404) the dies to stack the silicon layers using the handle portions for alignment; andbonding (Block1406) the silicon layers together using fusion bonding to form a stack of silicon layers (Block1408), wherein the pattern of openings define a spatially varying permittivity of the stack configured to focus different spectral bands of electromagnetic radiation into different spatially separated electromagnetic modes.

16. The method of clause 15, wherein areas of the dies are progressively smaller so that the dies can be assembled as a nested stack aligned by the handle portions.

17. The method of clause 14 or 15, wherein the pattern of openings is determined using an inverse design method.

18. The device or any of the embodiments 1-14 manufactured using the method of any of the embodiments 15-17.

19.FIG.15illustrates a method of performing spectroscopy, comprising:focusing (Block1500) different spectral bands of electromagnetic radiation into different spatially separated and non-overlapping electromagnetic modes;collecting (Block1502) the modes on an array of detectors or resonators, wherein each of the first detectors or resonators receives a different one of the electromagnetic modes; andmeasuring (Block1504) an output signal from each of the detectors or the resonators and optionally analyzing the signal (Block1506).

20. The method of embodiment 19, wherein the electromagnetic radiation is received after interaction with a target sample, and each of the resonators are de-tuned from a different known spectral line in a set of target spectral lines610(e.g., of molecules, gases, or other materials of interest), the method further comprising:scanning cavity lengths of the resonators together, so that one longitudinal resonant mode650of each of the resonators602is only scanned across a spectral portion encompassing the one of the target spectral lines654associated with that one of the resonators, wherein the array of resonators collectively scans across all the spectral lines in the set (i.e., for 1<i<n resonators, electromagnetic modes114, spectral lines, and detectors, a longitudinal mode of the ithresonator, selectively illuminated by the ithone of the electromagnetic modes114outputted from volumetric device100, is scanned across a spectral portion of the ithspectral line);detecting the electromagnetic radiation outputted from each of the resonators on an array of detectors608generating the output signals in response thereto, (so that the ithone of the detectors detects the electromagnetic radiation from the ithone of the resonators); andanalyzing the output signals to determine a response of the target sample to the electromagnetic radiation at one or more frequencies of the known spectral lines.

20. The method of clause 18, wherein the electromagnetic radiation is received after interaction with a target sample, and each of the resonators are de-tuned from a different known spectral line, the method further comprising:for each of the resonators, scanning a cavity length of the resonator to scan a longitudinal resonant mode of the resonator across at least a portion of a linewidth of the known spectral line associated with the one of the resonators;detecting the electromagnetic radiation outputted from each of the resonators on an array of second detectors generating the output signals in response thereto; andanalyzing the output signals to determine a response of the target sample to the electromagnetic radiation at one or more frequencies of the known spectral lines.

21 The method of clause 19, wherein the analyzing comprises determining at least one of an absorption or scattering of the electromagnetic radiation at one or more of the frequencies of the known spectral lines, or comparing a line-shape of the electromagnetic radiation at the frequencies with a line shape of the spectral line in a presence of a control sample.

22 The method of any of the embodiments 19-21 performed using the device of one or more of the embodiments 1-14.

23 The method of any of the embodiments 19-21 wherein the resonators are not scanned in frequency ranges that do not comprise the frequencies of the spectral lines.

24 The method or device of any of the embodiments 1-23, wherein the spatial variation in permittivity is periodic or non-periodic.

25. The method or device of any of the embodiments 1-23, wherein the spatially varying permittivity causes scattering of the electromagnetic radiation within the volume of the stack to re-distribute the different spectral bands into the spatially separated electromagnetic modes.

26. The method or device of any of the embodiments 1-25, wherein the electromagnetic modes of outputted from the metamaterial selectively illuminate a longitudinal mode of the resonator.

27. The method or device of any of the embodiments 1-26, wherein the voids comprise or consist of or consist essentially of air (or an absence of the silicon).

28. The method or device of any of the embodiments 1-27, wherein the silicon layers comprise a spatially varying amount of silicon (presence or absence of silicon), spatially varying refractive index, and/or spatially varying presence of voids or openings, e.g., spatially varying across the two dimensional plane/area of the layer.

29. The method or device of any of the embodiments 1-28, wherein the spatially varying permittivity ε (e.g., electric polarizability of the dielectric material in the layer e.g., in response to the terahertz electromagnetic radiation) is tailored such that the metamaterial comprises a lens focusing different spectral bands of the electromagnetic radiation to spatially separated and non-overlapping locations.

30. One or more computers670(e.g., comprising one or more processors and memories executing a program, or application specific integrated circuit, or field gate programmable array) and motor514can be included in the spectrometer or coupled to one or more components of the spectrometer or device of any of the embodiments 1-29 to perform analysis or scanning or other functionalities described herein.

REFERENCES

Further information on inverse design methods can be found in the above references.

CONCLUSION