Patent Description:
In consumer and industrial applications there is an increasing need for ubiquitous sensors. For many of these applications optical sensing provides the best resolution and selectivity, but optical sensors are usually based on discrete optical elements and therefore costly.

Many sensing applications require measuring the light spectrum. This usually requires a dispersive element (eg a grating) and a detector or detector array, which are combined in a bulky (<NUM>-<NUM>) and expensive instrument. This is unacceptable for applications that require compact and cheap solutions - e.g. gas sensing for agriculture, monitoring of industrial processes, medical diagnostics. Moreover, there is a trade-off between resolution and size: Making the spectrometer smaller affects its resolution. Attempts at integrating the grating element on an optical chip typically result in a poor resolution (several nm). Additionally, producing large arrays of spectrometers for high-resolution hyperspectral imaging is presently not possible. A second application area is the measurement of mechanical motion at the picometer scale, and correspondingly force and acceleration.

While this can be done with optics (e.g. using interferometric methods), it usually requires complex and bulky systems.

Integrated spectrometer implementations are mostly based on arrays of filter elements, which limits the resolution, and rely on external detectors, resulting in a much increased packaging complexity and cost. In principle, the combination of a tuneable optical cavity and a photodetector can lead to an extremely compact spectrometer, particularly if the detector is integrated inside the cavity. However, for many applications high resolution is needed under a wide range of incident angles and over a wide spectral range. This can only be achieved by a wavelength-scale cavity combining low optical loss, wide free spectral range (FSR) and large tuneability. So far, tuneable microcavity detectors have achieved limited resolution and spectral range.

Examples of the available prior art can be found in the patent documents <CIT> and <CIT>, and in the articles "<NPL>; "<NPL>; and <NPL>.

What is needed is an optical sensor, having outstanding resolution and bandwidth, which is fully integrated and mass-manufacturable.

To address the needs in the art, a double membrane microspectrometer is provided as defined in claim <NUM>.

According to one aspect of the invention, the first electrode is connected to an amplifier and the second electrode is connected to a ground to form an integrated photocurrent detector, where the integrated photocurrent detector detects the photocurrent across the first intrinsic semiconductor layer according to the illuminating source directed on the pattern of holes. In one aspect, the combination of first and second pattern defines the resonance, where the first and second patterns of holes are identical and are configured to produce a resonance in a photocurrent, where the applied voltage alters a spectral position of the resonance, where a photocurrent dependence on the applied voltage outputs a measurement of a spectrum of the illuminating source. In one aspect, the applied voltage includes a frequency modulated applied voltage, where the photocurrent from the integrated photocurrent detector is output at the frequency.

In a further aspect of the invention the first pattern of holes in the first membrane and the second pattern of holes in the second membrane are configured to provide a photocurrent spectrum changing with wavelength on the scale of a small fraction of a wavelength, where the applied voltage between the second electrode and the third electrode changes the photocurrent spectrum by moving the membrane on the scale of at least a fraction of a nanometer, where a sequence of measureable photocurrents for different applied voltages is output for reconstructing a spectrum of the illuminating source according to a numerical procedure operated by an appropriately programmed computer. Here, the numerical procedure includes an optimization method of finding the reconstructed spectrum that fits the sequence of measureable photocurrents with least error. Further, the first hole-pattern and the second hole-pattern and the reconstructed spectrum are arranged according to an expected input spectrum according to compressive sensing techniques. In one aspect the compressive sensing techniques include a numerical procedure that reconstructs the expected input spectrum, where the reconstructed input spectrum includes a size that is larger than a size of the sequence of photocurrents.

In another aspect of the invention, at least one absorbing region is patterned within a region of the first membrane or the second membrane or outside a region of the first membrane and the second membrane, where measureable photocurrents of the at least one absorbing region are output according to different applied voltages, where a spectrum of the illuminating source is reconstructed according to a numerical procedure operated by an appropriately programmed computer. Here, the first hole-pattern and the reconstructed spectrum are arranged according to an expected input spectrum according to compressive sensing techniques.

According to one aspect of the invention, the first electrode is connected to an amplifier and the second electrode is connected to a ground to form an integrated displacement detector, where the integrated displacement detector detects a displacement between the first membrane and the second membrane, where the applied voltage between the second electrode and the third electrode actuates a position of the first membrane. In one aspect, a combination of an actuator and a sensor is configured to output feedback stabilization of the position of the first membrane. In another aspect, a combination of an actuator and a sensor is configured to map a spatial profile of a surface under test.

In another aspect of the invention, the first semiconductor layer includes a p-type semiconductor layer and the second semiconductor layer includes an n-type semiconductor layer, or the first semiconductor layer includes an n-type semiconductor layer and the second semiconductor layer includes a p-type semiconductor layer.

According to a further aspect of the invention, the third semiconductor layer includes a p-type semiconductor layer and the fourth semiconductor layer includes an n-type semiconductor layer, or the third semiconductor layer includes an n-type semiconductor layer and the fourth semiconductor layer includes a p-type semiconductor layer, or the fourth semiconductor layer includes an intrinsic semiconductor layer.

In yet another aspect of the invention a lower part of the first membrane and an upper part of the second membrane are doped to form p-i-n diode.

According to a further aspect of the invention, the optical absorbing material includes quantum wells, quantum dots or bulk material.

In a further aspect of the invention, both membranes include the same periodic pattern of holes, forming a photonic crystal, with a defect, forming a cavity. In one aspect, the cavity is a modified L3 or H0 photonic crystal cavity.

In another example, the first membrane and the second membrane include an aperiodic pattern of through holes, where the first pattern of through holes is the same as second pattern of through holes or the first pattern of through holes is different from the second pattern of through holes.

In another aspect of the invention, the first membrane, the second membrane and the illuminating source are integrated on a chip, where the illuminating source can include a laser or a light-emitting-diode.

In a further aspect of the invention, an array of the double membrane microspectrometers are provided, where the array of double membrane microspectrometers are disposed in a linear or rectangular pattern, where the hole patterns in the double membranes are the same or different, where the array of double membrane microspectrometers are actuated separately or together, where an image is projected on the array of double membranes through an optical system, where each microspectrometer measures a light spectrum at a given position, where a set of all the light spectra forms a hyperspectral image.

Nano-optomechanical structures, such as photonic crystal cavities and micro-ring resonators, combine very high spectral resolution and large optomechanical coupling, resulting in exquisite sensitivity to nanoscale mechanical motion. This interaction between optical and mechanical degrees of freedom can be used to transduce pm-scale mechanical displacements into wavelength shifts and vice versa. This opens the way to a new generation of ultracompact optical sensors, if the required control and read-out can be integrated with the sensing part.

Presented herein is a nano-opto-electromechanical system (NOEMS) where the three functionalities of transduction, actuation and detection are fully integrated, resulting in a high-resolution spectrometer with a µm-scale footprint. This unique combination of functionalities is used to demonstrate a new method of resonance modulation spectroscopy, which provides sub-picometer wavelength resolution. Further presented is its application as displacement-to-photocurrent transducer, leading to the demonstration of optomechanical displacement sensing with integrated photocurrent read-out.

The optical sensors of the current invention offer outstanding resolution and bandwidth, and yet are fully integrated and mass-manufacturable. In one aspect, the current invention is directed to the measurement of optical spectra, for example for gas sensing and Raman spectroscopy or for the monitoring of laser lines (wavemeters) or to the detection of motion.

The invention includes an integrated sensing device, few tens of microns in size, capable of measuring the spectrum of incoming light. It is based on electromechanically-tunable photonic structures based on periodic or aperiodic hole-patterns in two moveable membranes. Light is coupled to the sensor from the top or the bottom or through one input waveguide, see for example <FIG>.

<FIG> shows a drawing of a microspectrometer with a photonic crystal cavity, contacts (middle and bottom) for the actuation, and contacts for the photodiode (middle and top), according to one embodiment of the invention.

<FIG> shows a scanning electron microscope image of the active area of a fabricated microspectrometer, having a periodic hole-pattern (photonic crystal) with a defect at the center (cavity) patterned into the double-membrane structure, according to one embodiment of the invention.

<FIG> show an overview of a microspectrometer sensor, according to one embodiment of the invention, where <FIG> shows a sketch of the device with contacts inside mesa structures, and visible cross-section with p-type and n-type doped layers and optical absorbers, such as quantum dots (QDs), embedded in the middle of the top membrane, where sensor actuation is enabled by applying a reverse bias voltage (VT) to the tuning diode (on the right side), and sensor readout is done by measuring the photocurrent of the photodiode (left side). According to one aspect of the invention, the first electrode is connected to an amplifier and the second electrode is connected to a ground to form an integrated photocurrent detector, where the integrated photocurrent detector detects the photocurrent across the first intrinsic semiconductor layer according to the illuminating source directed on the pattern of holes. In one aspect, the same pattern of holes are in both membranes, forming a photonic crystal cavity, and is configured to produce a resonance in a photocurrent, where the applied voltage alters a spectral position of the resonance, where a photocurrent dependence on the applied voltage outputs a measurement of a spectrum of the illuminating source. In a further aspect, the applied voltage includes a frequency modulated applied voltage, where the photocurrent from the integrated photocurrent detector is output at the frequency.

<FIG> show SEM images of an exemplary device with contacts for the two diodes, where <FIG> shows a zoom-in SEM image showing the active part of the sensor: a four-arm bridge of dimensions <NUM> x <NUM> containing a photonic crystal membrane suspended over the second, fixed photonic crystal membrane. The inset is an SEM image of the patterned L3 cavity design modified for high Q-factor and large free spectral range in a double membrane structure by displacing horizontally outwards and reducing the radius of six holes horizontally and displacing four holes vertically. <FIG> shows simulated optical mode wavelength dependence of membrane separation for two modes that are symmetric (S) or asymmetric (As) with respect to the out-of-plane direction. According to one embodiment, photon detectors are integrated within a chip as shown in <FIG>. In other embodiments, the photon detectors are placed outside the moveable structure or externally, in which case waveguides collect light out of the membranes (see <FIG>).

In one embodiment, a new method of scattering modulation spectroscopy is provided that is based on the mechanical modulation of the resonance and synchronised read-out, to improve the spectral resolution and the bandwidth in spectral measurements.

In a further embodiment, the invention provides "microspectrometers" that are tens of µm in size, with spectral resolution in sub nm range, and bandwidth of more than <NUM> if multiple output waveguide or multiple tuning regions are applied.

According to a further aspect of the invention, the optical absorbing material includes quantum wells, quantum dots or bulk material. In one aspect, the optically absorbing material has a broad (ideally flat) absorption spectrum, while the absorption of the combined structure, the double membrane and the optically absorbing material, is determined by the hole pattern and on the separation distance
The current invention integrates the actuation, sensing and read-out within a footprint of only 15x15 µm<NUM> and provides high resolution even under a large numerical aperture (NA) illumination. Sketches of some embodiments of the invention are shown in <FIG> and <FIG> that are based on an electromechanically tuneable, double-membrane photonic crystal (PhC), and a low-absorption active material (quantum dots). In one embodiment, two identical cavities in the two parallel membranes are evanescently coupled so that each original cavity mode splits into a symmetric (S) and an antisymmetric (AS) mode. The resonant wavelengths strongly depend on the separation d between the membranes, as shown in the simulated tuning curves of <FIG>. In the range of d ~ <NUM> the optical angular frequency shift per displacement Gω = dw/dx is in the range of 2π°-<NUM>/nm (dλ/dx = <NUM>/nm25). In one example, the lower part of the upper membrane and the upper part of the lower membrane are doped in order to form a p-i-n diode, where it is understood that the dopant types can be reversed between the two membranes. The distance between the membranes can be controlled by using electrostatic actuation provided by a reverse bias voltage VT across the p-i-n diode. Compared to in-plane capacitive tuning, this vertical-actuation structure offers larger capacitance, resulting in more efficient actuation, and smaller footprint. Moreover, it enables the actuation and sensing of out-of-plane motion, which is relevant for most nanometrology applications, such as atomic force microscopy. The upper membrane is configured as another p-i-n diode, the n-layer being common to both diodes, see <FIG>. In one example, a layer of InAs QDs, absorbing in the resonant wavelength range, is grown at the center of the upper membrane. Where the optical absorbers can be QD's, quantum wells or any bulk optical absorbing material. The modal absorption, and thereby the detector efficiency and the cavity loss, can be controlled by controlling the density of the QDs. In one example embodiment, the dot density is chosen so that the absorption contribution to the cavity loss does not limit the Q factor. In this example, it is estimated that the modal absorption is αmod = <NUM>-<NUM>, corresponding to an absorption-limited quality factor (Qabs) of <NUM>. In these examples, the PhC cavities are modified L3 or H0 cavities where the position and size of the holes close to the cavity center have been optimized to achieve at the same time a high quality factor and a wide spectral separation, as disclosed below.

To demonstrate the resonant detection functionality, light from a tuneable laser (illuminating source) is coupled into the cavity from the top with a fixed bias on the actuation junction (<FIG>). The photocurrent spectrum corresponds to the cavity resonance, apart from a non-resonant background, and is a result of cavity-enhanced absorption. An experimental cavity linewidth for a symmetric fundamental photonic crystal mode as narrow as <NUM> pm (Q = <NUM>. 7x10<NUM>) was obtained utilizing the optimized cavity design from <FIG>. This corresponds to an order of magnitude improvement over previous reports in resonant cavity detectors. The device maps the combination of the incident spectral power density S(ω) and intermembrane distance d into a photocurrent signal <MAT>, where R is the responsivity (A/W) and Lcav(d, ω) the normalized spectral shape of the cavity resonance at frequency ω<NUM>(d). It can therefore be operated to sense either the spectrum of the incident radiation or the mechanical displacement by recording the photocurrent. In the spectrometer mode, the input spectral power density S(ω) is measured by actuating the membrane separation, d = d(VT), and for displacement sensing the membrane separation d can be deduced from the resonance frequency. As shown in <FIG>, tuning of an antisymmetric cavity mode by as much as <NUM> is obtained for a small applied voltage of <NUM> Volts, corresponding well to the simulated membrane tuning until the pull-in limit (<NUM>/<NUM> of the original distance) inherent to capacitive tuning. The mode used in this case is the fundamental antisymmetric mode of a H0 cavity optimized for high free-spectral range (FSR), where FSR is defined as the maximum wavelength range for which there is only the mode of interest. Large tuning range in this case comes at a price of a larger linewidth of <NUM> (Qexp = <NUM>. 9x10<NUM>), a result of a trade-off between the two parameters. The spectrometer operation is demonstrated for a cavity mode where both Q and FSR are sufficiently high (<FIG>), which is the case for the second antisymmetric mode (Y2-As) of the modified L3 cavity, with a calculated Q = <NUM>·<NUM><NUM> and FSR = <NUM>. For a number of fixed laser frequencies, a voltage sweep is made across the resonances. Peak positions were taken as calibration points, with which the wavelength scale (bottom) was converted to the voltage scale (top) in <FIG>.

The peak photodiode responsivity for the data in <FIG> is R ~ 3x10-<NUM> A/W. It is limited by the small absorptance (ηa = <NUM>), which can be increased without a large influence on the Q, as well as unoptimized coupling efficiency (ηc = <NUM>), which can be improved using a side-coupling scheme. The cavity photocurrent peak shown in <FIG> is superimposed on a non-resonant background caused by light that is directly absorbed in the top membrane. The limited stray light rejection ratio (typically <NUM>-<NUM> dB) may be detrimental when a small spectral feature must be measured on a broad background. A resonance modulation spectroscopy scheme is provided, which can at the same time suppress the effect of background absorption and dramatically increase the spectral peak position resolution. It is based on the small size and built-in actuation functionality of our NOEMS, which enables modulating the mode resonant wavelength at frequencies up to the MHz range. Applying a small modulation to the tuning voltage as VT = VDC + VAC cos(2πfmt), the cavity frequency ωcav is modulated around its central value ω<NUM>(VDC) and the photocurrent <MAT> at frequency fm, as measured using a lock-in amplifier, becomes: <MAT> where δωm= Gωδd is the frequency modulation depth (which we assume much smaller than the optical linewidth). Note that Lcav is assumed to be a Lorentzian Lcav(ω-ω<NUM>) of constant width, so that ∂Lcav/∂ω<NUM>= - ∂Lcav/∂ω. In the limit where S(ω) is much narrower than the cavity linewidth, <MAT> is proportional to the derivative of the cavity resonance lineshape. In the opposite limit of a slowly varying input spectrum, <MAT> is proportional to the derivative of the input spectrum <MAT> as immediately follows from Eq. (<NUM>) from the integration by parts. The output signal therefore exclusively results from spectral features at the mode frequency and any spectrally flat background is rejected. The principle is demonstrated experimentally for a narrow laser line in <FIG>, also showing a large improvement of the rejection ratio, from 10dB to 27dB, with values up to 30dB measured in other devices.

The sign-changing lineshape of the AC photocurrent amplitude also lends itself to the generation of an error signal for feedback-based stabilization. Similarly to frequency and wavelength modulation methods, the resonance modulation scheme allows for measuring the position of spectral lines with resolution much better than the linewidth. From the slope of the derivative curve at the zero crossing (inset <FIG>), the voltage-wavelength relation and the measured noise, a spectral resolution of <NUM> fm/Hz<NUM>/<NUM> is calculated, limited by the drift of the cavity resonance during the measurement time. This long-term drift, which produces resonant wavelength shifts in the picometer range over timescales of tens of seconds, is likely related to the adsorption of residual gases on the surface and in the holes of the PhC and temperature drifts. The intrinsic resolution, as limited by the electrical noise in the read-out, is estimated to be in the <NUM> fm/Hz<NUM>/<NUM> range. The background rejection provided by the resonance modulation scheme allows for measuring narrow absorption lines in a broad spectrum. This is demonstrated in <FIG>, where a hydrofluoric acid (HF) absorption line is detected, despite the fact that its linewidth (<NUM> pm) is about <NUM> times narrower than the cavity linewidth used in this experiment (250pm). The high peak position resolving power of our device would make it also useful for the read-out of temperature, index or pressure sensors that are based on spectral peak position determination.

In a further aspect, a microspectrometer is provided that is based on the aperiodic hole-patterns. These patterns, which could be the same or different in the two membranes, define a complex, multipeaked photocurrent spectrum with many sharp resonances which change with the membrane distance. A numerical reconstruction procedure is applied to get the original spectrum from the measured data, possibly using compressive sensing.

The first pattern of holes in the first membrane and the second pattern of holes in the second membrane are aperiodic and are configured to provide a photocurrent spectrum changing with wavelength on the scale of a small fraction of a wavelength (for example <<NUM>), where the first pattern of holes is the same as second pattern of holes or the first pattern of holes is different from the second pattern of holes, where the applied voltage between the second electrode and the third electrode changes the photocurrent spectrum by moving the membrane on the scale of at least a fraction of a nanometer, where a sequence of measureable photocurrents for different applied voltages is output for reconstructing a spectrum of the illuminating source according to a numerical procedure operated by an appropriately programmed computer. Here, the numerical procedure includes an optimization method of finding the reconstructed spectrum that fits the sequence of measureable photocurrents with least error. In one implementation, the first hole-pattern and the second hole-pattern and the reconstructed spectrum are arranged according to an expected input spectrum determined through compressive sensing techniques. In one aspect the compressive sensing techniques include a numerical procedure that reconstructs the expected input spectrum, where the reconstructed input spectrum includes a size that is larger than a size of the sequence of photocurrents.

Input light, such as laser or LED light, experiences multiple scatterings in the aperiodic hole patterns. Mechanical tuning can trigger an optical path changing during the scatterings, which leads to the changing of transmittance. In another aspect, the optical path also depends on the wavelength of light. Therefore, the transmittance array obtained by scanning the mechanical tuning, is a unique "finger print" of the wavelength. In a further aspect, during calibration the wavelength is scanned of the band of interest, collecting all the transmittance arrays, which form a matrix T. Any input spectra S is then reconstructed by solving the problem I=TS, where I is a transmittance array obtained by scanning the mechanical tuning. This problem can be solved by the method of compressive sensing. Although one output waveguide and one detector are enough for reconstruction of the spectra, multiple waveguides and detectors can also be applied, in which case the number of measurement channels (the length of transmittance array) are multiplied, providing a better resolution and broader bandwidth. Furthermore, the device can also be composed of cascaded multiple regions of aperiodic hole-patterns, in which the mechanical tuning is independent for each region. In this case, the total number of measurement channels of device is nm, where n is the number of measurement channels for single region, m is the number of regions. In another aspect of the invention, at least one absorbing region is patterned within a region of the first membrane or the second membrane or outside a region of the first membrane and the second membrane, where measureable photocurrents of the at least one absorbing region are output according to different applied voltages, where a spectrum of the illuminating source is reconstructed according to a numerical procedure operated by an appropriately programmed computer.

Some key aspects of the invention include broadband spectra reconstruction by mechanically tuning multiple scattering of light in aperiodically patterned media.

According to a further embodiment of the invention, multiple output waveguides and detectors are implemented to increase measurement channels. In one example, a bandwidth of more than <NUM> is provided. In another embodiment, cascaded multiple independent tuning regions are implemented, which broaden the bandwidth further.

<FIG>) shows an example pattern of holes designed to provide strong spectral changes in the photocurrent. <FIG> shows the calculated photocurrent (in arbitrary units, assuming the efficiency of the detector is independent of wavelength) as a function of wavelength (x-axis) and of the index in the membrane, where in this case the change in index of a single membrane is used to mimic the change in distance between two membranes, according to one embodiment of the invention.

According to the invention, two implementations exist that include a "photonic crystal cavity" having a periodic pattern of holes except for a defect in the center (see <FIG>), forming the cavity. In this example the same holes are patterned in both membranes. In another implementation an aperiodic pattern of holes, which may include a random pattern, may be different in the two membranes.

In another aspect of the invention, an array of double membrane microspectrometers is disposed, where an image is projected on the array with the use of a lens or equivalent optical system, where the microspectrometers are based either on a photonic crystal cavity or on aperiodic hole patterns, where the microspectrometers are actuated and read out in parallel or sequentially, where each microspectrometer of the array measures the light spectrum at a given position in the array, where the combination of all the spectra forms a hyperspectral image.

In a further aspect of the invention, a microspectrometer based on a photonic crystal cavity is illuminated with a spectrally-narrow source (for example a laser) with a wavelength close to the cavity resonance, where variations in the position of the first membrane produce a change in the photocurrent. The first electrode is connected to an amplifier and the second electrode is connected to a ground to form an integrated displacement detector, where the integrated displacement detector detects a displacement between the first membrane and the second membrane, where the applied voltage between the second electrode and the third electrode actuates a position of the first membrane. In one aspect, a combination of an actuator and a sensor is configured to output feedback stabilization of the position of the first membrane. In another aspect, a combination of an actuator and a sensor is configured to map a spatial profile of a surface under test.

To demonstrate the motion sensor functionality, displacement fluctuations due to the Brownian thermal motion of the upper membrane were measured through the photocurrent, which directly monitors the intracavity field. A laser, detuned from a high-Q cavity mode <FIG>, is coupled into the cavity and the photocurrent spectrum is measured by an electronic spectrum analyzer (ESA), see <FIG>. Transduced thermal motion of the fundamental flexural mode with a frequency ΩM/2π = <NUM> and quality factor QM≈<NUM> is observed, see <FIG>. By equating the observed resonant fluctuations to the thermal variance <xth<NUM>> = kBT/meffΩM<NUM>, with T = <NUM> and meff = <NUM> pg, obtained from FEM simulations (inset in <FIG>), the observed fluctuations can be converted to a displacement spectral density Sxx (<FIG> right axis). The measurement imprecision is estimated to be <NUM> fm·Hz-<NUM>/<NUM>. It is presently limited by un-optimized transduction and thermal noise in the read-out and could be improved to well below <NUM> fm·Hz-<NUM>/<NUM>. Demonstrated herein is the possibility of actuating and sensing the membrane displacement at the fundamental mechanical resonance by measuring the electro-optomechanical transfer function using the integrated detector, as disclosed below. This suggests that the device can be used as a self-sensing actuator with position stabilization at the pm scale over MHz-range bandwidths.

An integrated nanophotonic sensor is disclosed that embodies the unique features of direct wavelength/displacement detection via photocurrent, and independent voltage control of the optical and mechanical properties of the structure via electrostatic actuation. Demonstrated herein is a high-resolution microspectrometer and proved displacement sensing capabilities on a single device based on coupled patterned membranes. Furthermore, a resonance modulation spectroscopy method is provided, exploiting the electromechanical control of the mode wavelength to reject stray light and increase the spectral resolution well beyond the cavity linewidth. Owing to the ultracoinpact size (<NUM> x <NUM><NUM>) of the sensing element, this platform opens the way for mass production of multipurpose high-resolution sensors with embedded readout. The sensor can be easily applied to other material systems to cover different wavelength ranges (from the visible to the mid-infrared), and further developed for application in temperature, refractive index and electrical field sensing. In one example embodiment a III-V semiconductor platform is used for the device, which could be further exploited to integrate the light source, opening the way to fully-integrated optical and optomechanical sensors requiring no external optical connections.

Turning now to the methods of a sample structure. Here, a sample was epitaxially grown by Molecular Beam Epitaxy (MBE) and having two GaAs slabs with nominal thicknesses of <NUM> run (bottom) and <NUM> (top), separated by a <NUM> thick sacrificial Al<NUM>Ga<NUM>As layer. A <NUM> thick Al<NUM>Ga<NUM>As bottom sacrificial layer separates the membranes from the undoped (<NUM>) GaAs substrate. QDs (areal density <NUM> QDs/mm<NUM>, with ground-state absorption centered at <NUM> at room temperature) are grown in the middle of the upper slab in a Stranski-Krastanov growth process. The upper <NUM>-thick part of both membranes was p-doped, while the bottom <NUM>-thick part of the bottom membrane was n-doped (pupper = <NUM> x <NUM><NUM> cm-<NUM>, n = plower = <NUM> x <NUM><NUM> cm-<NUM>).

The fabrication process begins with defining the vias for the contact pads for the two diodes of the device in two optical lithography steps followed by selective wet and dry etching steps to reach the bottom p-via and the middle n-via. In the p-via lithography step, the flexible four-arm bridges are also defined, which determine the stiffness of the top membrane. To prevent the stress-induced buckling of the bridge, stress release structures were implemented <FIG>. No arms are etched in the lower membrane, making it mechanically much less compliant than the upper one. In the third optical lithography step, contact pads for all three contacts are defined and metals are evaporated. After a lift-off step, <NUM> of Si<NUM>N<NUM> is deposited on the sample (hard mask), ZEP resist is spun and electron-beam lithography at <NUM> kV is performed to define the photonic crystal (PhC) pattern. After development, the PhC pattern is transferred onto the hard mask using RIE (reactive ion etching) with CHF<NUM>. Resist is then removed with oxygen plasma, and the PhC pattern is imprinted as an array of holes in both membranes using an Cl<NUM>-based inductively coupled plasma etching step. Release of the free-standing structure is done by selective wet etching of the sacrificial layer using a cold (<NUM>) HCl solution. To prevent membrane stiction due to capillary forces, supercritical drying in CO<NUM> is employed. Finally, the hard mask is removed by isotropic O<NUM>-CF<NUM> plasma dry etching.

In one example of a PhC cavity design, the light sensing double-membrane PhC cavity was designed to ensure small size (V ~ λ<NUM>), high Q and sufficiently large free spectral range, by modifying standard L3 (with <NUM> holes missing in the hexagonal PhC) and H0 (holes displaced around a position in the lattice) designs, as shown in <FIG>. In the chosen design of the L3 cavity used in the experiment in <FIG>, <FIG> and <FIG> the position and the radii of the closest six holes in the x-direction (displacements s<NUM>/a = <NUM>, s<NUM>/a = <NUM>, s<NUM>/a = <NUM>; radii r<NUM> = r<NUM> = r<NUM> = <NUM>·r, with r/a = <NUM> the radius of the holes in the PhC and a the lattice parameter), and the position of the four holes in the y-direction (h<NUM>/a = <NUM>) were optimized, providing a simulated Q factor of 4x10<NUM> for a cavity without the absorber, and a mode spacing of Δλ = <NUM> (from 3D Finite Element Method (FEM) simulations). In the case of the fundamental symmetric mode (Y1-S), the experimental result mentioned in the main text (<FIG>), provides the Q factor of Qexp = <NUM>. 7x10<NUM>, while the simulated value (with no absorber) gives a value of Qcold = <NUM>. The absorption losses were estimated to give Qabs ≈ <NUM>. The additional loss is attributed to scattering losses, patterning errors and other fabrication imperfections: Qfabr = (<NUM>/Qexp - <NUM>/Qcold- <NUM>/Qabs)-<NUM> = <NUM> x10<NUM>. In the case of the H0 cavity used for the experiment in <FIG>, by optimizing the position and radius of four holes, for a triangular lattice with r/a, = <NUM> and parameters sx = <NUM>, sy = <NUM>, and radii, rx/r = <NUM> and ry/r = <NUM>, a simulated mode spacing of Δλ = <NUM> and Q factor of <NUM>. 8x10<NUM> was obtained.

The peak responsivity is given by R = (e/hv)ηcηaηi (e elementary charge, h Planck constant, v light frequency, ηc coupling efficiency of light into the cavity mode, ηa fraction of cavity photons absorbed by the QDs, ηi internal efficiency of converting absorbed photons into collected electrons and holes). The internal efficiency is estimated to be close to <NUM>, no change of photocurrent was observed with applied reverse bias on the detector junction, indicating that carriers are efficiently extracted from the QDs. The current responsivity is limited by ηa and nc. The absorption can be increased further until the point when the absorption losses are comparable to the scattering losses, analogous to designs of Fabry Perot resonant cavity enhanced photodetectors. This would increase the sensitivity at the cost of decrease in the Q factor. This implies that the absorption and thus na can be increased by increasing the QD density or the number of the QD layers. The free space coupling currently employed is expected to have a low efficiency ηc due to the mismatch between the k-vector distribution of the incident field and the one of the cavity mode. It is known that coupling with on-chip waveguides can be much more efficient and lead to coupling efficiencies ηc larger than <NUM>%. Preliminary results of simulations show that similar coupling efficiencies can be expected when coupling waveguides to double-membrane cavities.

In the experimental setup, light from a tuneable laser (Santec TSL-<NUM>) was coupled into the cavity from the top through a 50x objective (NA = <NUM>). All laser powers indicated in the main text are values incident on the sample. The two diodes were contacted using two adjustable RF probes. In <FIG> and <FIG>, the photocurrent was measured as a voltage drop on a <NUM> kΩ load resistor (R in <FIG>). For the measurements in <FIG>, photocurrent was amplified using a transimpedance amplifier (A = 5x10<NUM> V/A). All measurements were performed at room temperature. The thermal noise measurements in <FIG> were performed under vacuum conditions (p < <NUM>-<NUM> mbar) to suppress viscous air damping.

The wavelength resolution in <FIG> is determined by measuring the current noise in the readout when laser is on resonance with the cavity, δInoise = <NUM> pA·Hz-<NUM>/<NUM> (measured using the lock-in-amplifier) and the slope of the derivative curve at the zero crossing S<NUM> = <NUM>µA/V, from which the voltage accuracy is calculated to be δVT = dInoise/S<NUM> =<NUM>µV·Hz-<NUM>/<NUM>, With the mode wavelength tuning rate being <NUM>/V, this voltage accuracy can be translated into a (peak position) resolution of <NUM> fm when measured in <NUM> bandwidth, a value three orders of magnitude smaller than the linewidth. As previously mentioned, this value is limited by the long-term drift of the cavity resonance. The fundamental noise limit, in the case where no drift is present, would be determined by the thermal noise of the load resistor (~ <NUM> pA·Hz-<NUM>/<NUM>) and photon shot noise (~ <NUM> fA·Hz-<NUM>/<NUM>), both being orders of magnitude lower.

<FIG> show an array of the double membrane microspectrometers disposed in a linear or rectangular pattern (see <FIG>), where the hole patterns in the double membranes are the same or different, where the array of double membrane microspectrometers are actuated separately or together. As shown in <FIG> an image is projected on a rectangular array of double membranes through an optical system, such as a lens, where each microspectrometer measures a light spectrum at a given position, and a combination of all the light spectra forms a hyperspectral image.

The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. For example, the membranes could be made of different materials, including semiconductors, dielectrics or metals, the optically absorbing material could be different or positioned differently, for example above or below the membranes or within the second membrane or on the side of the membranes, the actuation of the first or the second membrane could be realized through a metal capacitor or by piezoelectric or inductive forces, or only parts of the first or the second membrane could be actuated.

Claim 1:
A double membrane microspectrometer, comprising:
a) A first membrane comprising a first doped semiconductor layer, a first intrinsic semiconductor layer, and a second doped semiconductor layer, wherein said first intrinsic semiconductor layer is disposed between said first doped semiconductor layer and said second doped semiconductor layer, wherein said first intrinsic semiconductor layer comprises an optically absorbing material disposed therein, wherein said first membrane comprises a first pattern of through holes disposed perpendicular to said first membrane semiconductor layers, wherein said first membrane comprises lateral support arms disposed to support said first membrane;
b) a second membrane comprising a third doped semiconductor layer and a fourth semiconductor layer, wherein said fourth semiconductor layer comprises an intrinsic semiconductor layer or a doped semiconductor layer, wherein said second membrane comprises a second pattern of through holes disposed perpendicular to said second membrane semiconductor layers, wherein said first membrane is separated from said second membrane by a first insulating bridge layer disposed proximal to the ends of said lateral support arms, wherein said first optical membrane is supported above said second membrane by said lateral support arms, wherein an absorption spectrum of said optically absorbing material is dependent on a separation distance of said first membrane from said second membrane;
c) a first electrode disposed on said first doped semiconductor layer;
d) a second electrode disposed on said second doped semiconductor layer; and
e) a third electrode disposed on said third doped semiconductor layer, wherein light from an illuminating source directed across said first pattern of holes is absorbed in said optical absorbing material in said first intrinsic semiconductor layer, wherein a photocurrent is output between said first electrode and said second electrode for detection, wherein a voltage applied across said second electrode and said third electrode is disposed to move said first membrane to alter a photocurrent between said first electrode and said second electrode, wherein said photocurrent alteration corresponds to the optical spectrum of said illuminating source.