Waveguide-enhanced Raman spectroscopy (WERS) efficiently collects Stokes-shifted scattering from target molecules in the evanescent field surrounding nanophotonic waveguides. By using a sorbent material as a top cladding, vapor phase analytes can be detected and identified at ambient densities as low as a few parts-per-billion. Previous demonstrations of vapor-phase WERS have used free-space optical components, such as microscope objectives and bulk Raman filters, to couple and filter light to and from the sorbent-clad waveguide. In this work we demonstrate a complete photonic integrated circuit (PIC) assembly that is packaged and fiber-coupled enabling us to measure WERS from trace vapor concentrations. The PIC comprises low-loss edge couplers from polarization maintaining single-mode optical fibers, sensing trenches with a sorbent top-cladding, and lattice filters for separation of the Stokes signal from the laser. The PICs are fabricated at AIM Photonics using the Silicon Nitride Passive PIC process with the TLX-VIS component library. Then, they are packaged into assemblies with permanent fiber-attach using fiber arrays. The sorbent is deposited in a thin, uniform layer in the sensing trench using one of two deposition techniques: nano-plotting and drip-coating. A laser wavelength of 785 nm enables the use of a compact spectrometer with a thermoelectrically-cooled silicon detector. Spectra are obtained with exposure times of a few seconds and show parts-per-billion detection limits for select vapors. This work successfully demonstrates the use of a compact Raman spectrometer integrated with a fully assembled PIC via optical fibers for the detection of low-density vapor-phase analytes.
Silicon-nitride-based photonic integrated circuits (PICs) can operate with low loss at visible and near-infrared wavelengths. This spectral range is essential for many applications in chemical and biological sensing, quantum sensing and networking, physical sensing, precision timekeeping, and augmented/virtual reality. At present, highquality silicon nitride PIC platforms optimized for operation in the visible are offered by low-volume custom foundries or by 200 mm silicon-based foundries. Both typically lack the minimum feature sizes and wafer throughput required for high-yield, high-volume operation at short wavelengths. In this work we describe a new component library and foundry process developed at AIM Photonics, a state-of-the-art PIC foundry. The TLX-VIS component library for the Silicon Nitride Passive PIC process is designed to operate in three bands at wavelengths from 500 nm to 1000 nm. A trench down to the primary waveguide layer is offered for sensing applications, and a dicing trench enables access to waveguide facets for low loss edge coupling. Propagation losses range from 0.2 dB/cm at 785 nm to 2 dB/cm at 532 nm. The component library is designed for both the TE00 and TM00 modes and includes broadband directional couplers, polarization rotators, edge and grating couplers, lattice filters, and high-Q ring resonators. The waveguides have small minimum bend radii (<100 μm) and low fluorescence, which is critical for applications in Raman sensing and quantum information. The component library and PICs are compatible with AIM Photonics’ Test, Assembly, and Packaging facility, enabling fully-packaged, fiber-attached assemblies.
Silicon nitride (SiN) has been receiving increased attention for photonic integrated circuits (PICs) due to its ultra-low optical losses, phase stability, and broadband transparency. However, SiN waveguides have a low thermo-optic coefficient and exhibit weak electro-optic effects. For this reason, most foundry-processed SiN PICs remain passive or exhibit inefficient tuning. In this work, we investigate polymer claddings to enhance the thermo-optic phase shifting in foundry-processed low-loss, thin core SiN PICs. We first develop a thermal testing setup and measure the response of standard foundry SiN / SiO2 waveguides. By taking advantage of the differing TE and TM modal overlap with the SiN core and SiO2 cladding, we extract the LPCVD-SiN thermo-optic coefficient as dnSiN / dT = 2.57 × 10 − 5 / ° C at λ = 1550 nm and dnSiN / dT = 2.82 × 10 − 5 / ° C at λ = 780 nm. We next consider SiN waveguides in which the top SiO2 cladding is replaced with a spin-coated thermo-optic polymer. The thin waveguide core (tSiN = 150 to 220 nm) enables a weakly confined mode with a large overlap with the top polymer cladding. Measurements at λ = 780 nm wavelength show up to a 12-fold improvement in the thermo-optic phase shift of these polymer-cladded SiN waveguides compared with SiO2 cladded devices while inducing negligible excess loss. Finally, we show broadband Mach–Zehnder interferometer measurements demonstrating thermo-optic tuning at visible wavelengths. The simple spin-coat post-processing of foundry SiN PICs in this work offers a potential path toward efficient optical phase shifting in low-loss SiN waveguides over a broad wavelength range
Waveguide-enhanced Raman spectroscopy (WERS) using nanophotonic waveguides has been used to demonstrate the detection of vapor-phase chemicals and liquid-phase biomolecules in water. The technique benefits from the fabrication processes and tolerances of CMOS foundries, but successful foundry-based WERS photonic integrated circuits (PICs) have only been demonstrated using excitation wavelengths of 1064 nm and 785 nm. Foundry-based PICS are beginning to operate with low loss at visible wavelengths, and WERS is uniquely poised to take advantage of this capability. Raman scattering cross-sections scale as λ−4, so a visible WERS platform could enable increased sensitivity, decreased exposure times, and/or decreased laser powers. However, increased fluorescence, increased waveguide loss, and decreased feature sizes make WERS in the visible challenging. Here, we demonstrate WERS using 300-mm foundry-based fabrication (AIM Photonics) with 633 nm and 785 nm laser excitation. We also show the successful operation and integration of other required components for a compact WERS system operating in the visible, including edge-couplers and lattice filters.
The concentration of small molecule biomarkers in human serum and saliva has been shown to be characteristic of viral disease and correlated with disease severity. Inexpensive point-of-care diagnostic methods to quantify and track these analytes would provide additional information beyond viral or antibody detection assays to guide diagnosis and therapy. Waveguide-enhanced Raman spectroscopy (WERS) enables the detection and identification of trace concentrations of dissolved analytes using a chip-scale photonic circuit based on long evanescent waveguides. Here, we describe WERS measurements of two biomarkers: glucose and urea. This proof-of-concept work will provide the basis for the development of handheld bio-marker detection systems based on packaged photonic circuits integrated with a laser source and detector.
Detection of threat materials is an important capability for the military and homeland security to protect soldiers and civilians. Waveguide-enhanced Raman spectroscopy (WERS), a photonic integrated circuit sensing methodology, is being developed for field detection of materials related to chemical warfare agents, explosives, and narcotic threats. Low-fluorescence silicon nitride spiral waveguides with long path lengths are used to obtain high signal levels with nearinfrared excitation (785 nm and 1064 nm). Compact single-mode-fiber-coupled spectrometers with high sensitivity are being utilized for detection of the Raman scattered light. Thermoelectrically cooled charged coupled device (CCD) or InGaAs detectors (-15 °C) provide for low-noise and high-quantum-efficiency spectral measurement. Performance comparable to that obtained with large benchtop spectrometers is observed. The spiral waveguides are coated with functionalized polymer sorbents suitable for concentrating relevant classes of threat materials in the evanescent field of the waveguide. The sorbents are deposited using piezoelectric microdispensers to allow for controlled deposition of thin films without the need for spin-coating. Raman chemical imaging microscopy is used to characterize the uniformity of the sorbent polymers on the waveguides. Library spectral matching can be used in combination with the selectivity of the sorbent materials to provide discrimination of the materials absorbed by the polymer coatings. The ultimate objective is development of a prototype handheld WERS sensor system suitable for defense and security applications in the field. WERS development and spectral measurements will be presented.
The development of a foundry-scale waveguide-enhanced Raman spectroscopy (WERS) platform is a vital for the widespead implementation of this analytical technique. In this work we analyze the waveguide material and fabrication processes offered by AIM Photonics with regard to their effectiveness for WERS, and other sensing techniques. Optical characterization of these materials via white light spectroscopy and fluorescence spectroscopy points to the designation of an optimal wafer composition comprising a thermal bottom oxide and an LPCVD silicon nitride waveguide. This optimal composition has no measurable fluorescence and a propagation loss of 3.2 dB/m at 1064 nm in the TM00 mode. In the c/l band, the optimal wafer build has as thermal bottom oxide, a PECVD silicon nitride waveguide, and is annealed. This build has a propagation loss of 8.1 dB/m at 1550 nm in the TE00 mode.
Gas chromatography (GC) is a staple analytical technique used to separate chemical mixtures (analytes) prior to identification with a hyphenated technique, such as mass spectrometry or Fourier transform infrared (IR) spectroscopy. Traditionally, analytes elute through the GC separation column where they are detected when they exit. We have developed a technique to perform in situ IR spectroscopy during the process of separating the analytes along the GC column. This is achieved by spin coating the stationary phase onto a germanium prism and actively probing the stationary phase in an attenuated total reflectance configuration with a quantum cascade laser.. The GC column is formed by pressing a molded epoxy lid, with grooves that form the tubular column, onto the stationary phase coated prism.
The ability to rapidly detect hazardous airborne chemicals with high fidelity in a single point-detection system remains a significant challenge in a complex chemical background. Traditional Gas chromatography (GC) can significantly augment most detection technologies by separating complex mixtures for high fidelity detection, but with the disadvantage of requiring detection at the end of the GC column which adds a time disadvantage for any decision making process. Microfabrication of GC columns has reduced device footprint and power consumption, but the end-of-column detection paradigm remains. We present a rapid detection concept of in-column detection by probing the GC stationary phase which is coated on an IR transparent column substrate. The optical evanescent field interactions in the mid-infrared spectral region (US. Patent# 9,599,567) allows analyte detection along the column without having to wait for complete elution. These spectral signatures, collected at different points along the column, are analyzed by an algorithm to quickly identify components in a complex mixture. We present results with an ATR-based system that uses a focused tunable quantum cascade laser beam directed by galvo mirrors at points along a molded micro-GC column whose base comprises an optically transparent material coated with the stationary phase.
Waveguide-enhanced Raman spectroscopy (WERS) enables the detection and identification of trace concentrations of vapor-phase analytes using a functionalized chip-scale photonic circuit. Here, we show that WERS signal can be collected from part-per-billion levels of targeted analytes in a backscatter geometry, which, compared to forward-scatter, simplifies component integration and is more tolerant of waveguide loss and modal interference. In addition, we discuss our progress towards a compact Raman sensing system that incorporates a handheld spectrometer and chip-scale optical filters. We demonstrate that a handheld, thermo-electrically cooled spectrometer can be used for backscatter WERS with a comparable signal-to-noise to that of a liquid-nitrogen cooled benchtop spectrometer. Finally, we describe efforts to integrate the dichroic Raman filter on-chip using arrays of unbalanced Mach-Zehnder interferometers. Measurements show filter performance sufficient for integration with WERS: Transmission of >80% of the laser in the cross port and Stokes signal in the through port; and extinction of the laser by >20 dB in the though port and of Stokes signal by >8 dB in the cross port.
The need for ever-growing communications bandwidths has led to an interest in mode-division-multiplexed communications to increase the information carrying capacity of fiber-optic networks. More recently, mode-division multiplexed chip-scale photonic devices have been investigated as a means towards highly integrated photonic components and systems. To date, however, most chip-scale demonstrations have focused on fixed coupling and routing of individual waveguide modes on a chip. In this work we propose and investigate a new technique to dynamically couple and convert between different propagating waveguide modes via symmetry-breaking optomechanical near-field interactions. Silicon nitride waveguides (tSi3N4=175 nm) with air top cladding are fabricated and enable propagation of weakly-confined modes with substantial evanescent field near the waveguide surface. Suspended silicon nitride (tSiNx=200 nm) micro-electro-mechanical structures (MEMS) interact with the propagating mode’s evanescent field. However, the slight offset of the MEMS perturber with respect to the waveguide’s center axis leads to a symmetry breaking mode perturbation. This perturbation converts even propagating modes (e.g. TE0) to higher-order odd modes (e.g. TE1). We present various experimental techniques for characterizing the mode conversion including direct imaging, mode beating, and FFT spectrogram analysis. Simulation and experimental results demonstrate this new concept of using symmetry-breaking optomechanical near field interactions for mode coupling and conversion towards future mode-division multiplexing on a chip.
Waveguide-enhanced Raman spectroscopy (WERS) enables the detection and identification of trace concentrations of vapor-phase analytes using a chip-scale photonic circuit coated with a sorbent material. Previous demonstrations of WERS utilized a hydrogen-bond acidic hyperbranched carbosilane fluoroalcohol-based sorbent polymer and focused on detection limits for different nerve agent simulants. In this work, we examine the Raman spectra of a number of new sorbent materials obtained using WERS. By comparing the spectra pre-exposure to the modified spectra measured during analyte exposure, the effects of hydrogen-bonding on the sorbent and analyte molecules are observed. Changes to the Raman transition strength or frequency of individual lines due to analyte binding shed light on the partitioning of vapor-phase molecular agents into the sorbent, and can be used to design sorbent materials with even higher sensitivity. We examine two new types of sorbents: Fluorinated bisphenol-based materials that increase the steric bulk of the substituents ortho- to the hydroxyl group, designed to reduce self-binding; and carbosilane fluoroalcohol polymers synthesized with a novel hydrosilylation reaction. The WERS detection limits for these new sorbents are measured for nerve-agent simulants and compared to previous generation materials.
We report the design, fabrication, and measurement of waveguide lattice filters for use in integrated Raman- or fluorescence-based spectroscopy and sensing systems. The filters consist of a series of broadband directional couplers and optical delay sections that create an n-stage unbalanced Mach–Zehnder interferometer specifically designed to segregate pump light and redshifted signal light in the two output ports. We first report the design criteria for optimal filter performance. Then, we use these criteria with numerical beam propagation methods to design specific broadband couplers. The filters were fabricated by a photonic integrated circuit foundry and measured using white-light spectroscopy. We report both four-stage and eight-stage filters, with the eight-stage filter demonstrating a 190-nm-wide signal passband (1100 cm − 1) on the “through” port with <1.5 dB of ripple and a 17-nm-wide, 20-dB extinction band at the filter resonance.
A significant remaining challenge in chemical detection is the ability to rapidly detect with high fidelity a full suite of CWAs and TICs in a single point-detection system. Gas chromatography (GC) is a proven laboratory technique that can achieve the stated detection goal, but not at the required speed and not in a wearable (or even portable) form factor. Efforts in miniaturizing GCs yielded small devices, but they remain slow as they retain the end-of-column detection paradigm which results in long elution times of CWAs and TICs. We describe a novel concept of in-column detection by probing the sorbent coating (stationary phase) of a micro-GC column through optical evanescent field interactions in the long-wave infrared (“chemical fingerprint”) spectral region (U.S. Patent US9599567B2). Detection closer to the injection port ensures a rapid response for slow-eluting analytes. Although this results in poor separation (i.e. poor ability to identify chemicals), this is more than compensated by having full IR absorbance spectra at each location. This orthogonal spectral signature (along with GC retention times) is used in a powerful algorithm to quickly identify components in a complex mixture under conditions of incomplete separation. We present results with an ATR-based system that uses a focused tunable quantum cascade laser beam directed by galvo mirrors at points along a molded micro-GC column whose bottom wall is the sorbent coated ATR prism. Efforts are under way to further miniaturize this device by employing novel long-wave-IR photonic waveguides for a truly portable integrated photonic chromatographic detector of CBRNE threats.
We describe the detection of trace concentrations of chemical agents using waveguide-enhanced Raman spectroscopy in a photonic integrated circuit fabricated by AIM Photonics. The photonic integrated circuit is based on a five-centimeter long silicon nitride waveguide with a trench etched in the top cladding to allow access to the evanescent field of the propagating mode by analyte molecules. This waveguide transducer is coated with a sorbent polymer to enhance detection sensitivity and placed between low-loss edge couplers. The photonic integrated circuit is laid-out using the AIM Photonics Process Design Kit and fabricated on a Multi-Project Wafer. We detect chemical warfare agent simulants at sub parts-per-million levels in times of less than a minute. We also discuss anticipated improvements in the level of integration for photonic chemical sensors, as well as existing challenges.
We will review the state of the art for on-chip, Raman-based sensing using waveguides including our recent work with sorbent-coated waveguides for trace gas sensing showing parts-per-billion limits of detection. We will show that signal enhancements due to scattering that takes place in the evanescent field coupled with a thin hypersorbent polymer coating can yield Raman efficiencies which are nine orders of magnitude larger than traditional micro-Raman techniques. We will also discuss challenges with gas component discrimination and in moving toward a fully integrated photonic circuit architecture for handheld Raman-based trace gas sensors.
Silicon photonics enables the development of optical components on a chip with the potential for large-scale optical integrated circuits that can be fabricated at the wafer-scale using foundries similar to those used in the electronics industry. Although silicon is a passive optical material with an indirect bandgap, reconfigurable devices have been demonstrated using thermo-optic effects (large phase shifts, but relatively slow with large power consumption) and carrier plasma dispersion effects (high-speed, but small phase shifts). We recently demonstrated a low-power approach for inducing large phase shifts (>2π) using a technique that we call micro-opto-electro-mechanical index perturbation (MOEM-IP). In this initial work we characterized silicon nitride waveguides in which the propagating optical mode’s evanescent field is vertically coupled to silicon nitride microbridges. This interaction leads to an effective index tuning that is a strong function of the waveguide-microbridge separation. We now extend our MOEM-IP approach to different configurations (i.e. in-plane coupling) and material systems (i.e. silicon-oninsulator). Mode perturbation simulations indicate that the MOEM-IP approach is widely applicable to many configurations and material systems enabling large effective index tuning (Δneffective>0.1) requiring microbridge displacements of only a few hundred nanometers. We also examine several device applications that take advantage of MOEM-IP. These include tunable optical filters using high-Q microring cavities and optical phased arrays that enable chip-scale beam steering in two-dimensions using low-power phase shifting enabled by MOEM-IP.
Highly evanescent nanophotonic waveguides enable extremely efficient Raman spectroscopy in chip-scale photonic integrated circuits due to the continuous excitation and collection of Raman scattering along the entire waveguide length. Such waveguides can be used for detection and identification of condensed-phase analytes, or, if functionalized by a sorbent as a top-cladding, can be used to detect trace concentrations of chemical species. The scattering efficiency is modified in guided-mode structures compared to unconfined, micro-Raman geometries. Here, we describe the theoretical framework for understanding the Raman scattering efficiency in nanophotonic waveguides, and compare these calculations to our measurements of trace gases in hypersorbent-clad silicon nitride waveguides.
We report long-wave infrared (LWIR, 5-15 μm) and mid-wave infrared (MWIR, 2.5 – 5 μm) differential absorption spectra of different nerve agent simulants and common solutes sorbed to poly(methyldi(1,1,1-trifluoro-2-trifluoromethyl- 2-hydroxypent-4-enyl)silane, HCSFA2, an NRL developed hypersorbent polymer. HCSFA2 is a strong hydrogen-bond acidic polymer which exhibits large gas-polymer partitions for a variety of hazardous chemicals with hydrogen-bond basic properties such as the phosphonate ester G-nerve agents or their simulants. The measured ATR-FTIR differential absorption spectra show complex fingerprint signal changes in the resonances for the sorbent material itself, as well as new resonances arising from chemical bonding between the solute or analyte and the sorbent or the solute itself being present in the sorbent.
The unique optical properties of porous silicon show it to be a promising material for imaging and spectroscopy in the
mid-infrared and long-infrared wavelength ranges. A tunable MEMS filter using porous silicon as a high-reflectivity
layer is proposed. Measurements on fabricated porous silicon-based distributed Bragg reflectors and Fabry-Perot etalons
are presented.
Sorbent materials are utilized in a range of analytical applications including coatings for preconcentrator devices,
chromatography stationary phases, and as thin film transducer coatings used to concentrate analyte molecules of interest
for detection. In this work we emphasize the use of sorbent materials to target absorption of analyte vapors and examine
their molecular interaction with the sorbent by optically probing it with infrared (IR) light. The complex spectral
changes which may occur during molecular binding of specific vapors to target sites in a sorbent can significantly aid in
analyte detection. In this work a custom hydrogen-bond (HB) acidic polymer, HCSFA2, was used as the sorbent.
HCSFA2 exhibits a high affinity for hazardous vapors with hydrogen-bond (HB) basic properties such as the G-nerve
agents. Using bench top ATR-FTIR spectroscopy the HFIP hydroxyl stretching frequency has been observed in the mid
wave infrared (MWIR) to shift by up to 700 wavenumbers when exposed to a strong HB base. The amount of shift is
related to the HB basicity of the vapor. In addition, the large analyte polymer-gas partition coefficients sufficiently
concentrate the analyte in the sorbent coating to allow spectral features of the analyte to be observed in the MWIR and
long wave infrared (LWIR) while it is sorbed to HCSFA2. These spectral changes, induced by analyte-sorbent
molecular binding, provide a rich signal feature space to consider selective detection of a wide range of chemical species
as single components or complex mixtures. In addition, we demonstrate an HCSFA2 coated microbridge structure and
micromechanical photothermal spectroscopy to monitor spectral changes when a vapor sorbs to HCSFA2. Example
ATR-FTIR and microbridge spectra with exposures to dimethylmethylphosphonate (DMMP – G nerve agent simulant)
and other vapors are compared. In a generic form we illustrate the concept of this work in Figure 1. The results of this
work provide the potential to consider compact detection systems with high detection fidelity.
A new type of a resonator defined by two or more mode-converting gratings in a waveguide is proposed
and analyzed. It is shown that the proposed structure can exhibit narrow resonances similar to Fabry-Perot
cavities but has an advantage of being a four-port device and thus is capable of serving as an add-drop filter
in various integrated optical circuits.
Micromachined waveguide Fabry-Perot cavities are demonstrated. The devices are fabricated in silicon-on-insulator
using a cryogenic dry-etch process, enabling large aspect ratios with high verticality and low surface roughness
(⩽10 nm). Details of the process development are presented with emphasis on our specific device application. The
Fabry-Perot cavities consist of shallow-etched rib waveguides and deep-etched silicon/air distributed Bragg reflector
(DBR) mirrors. The high-index-contrast mirrors enable large reflectance with only a few mirror periods. High Q-factor
(Q≈27,000) and large finesse (F≈500) were measured. We demonstrate thermo-optic tuning over &Dgr;&lgr;=6.7 nm and also
examine modulation of the cavity (f=150 kHz). Future improvements and application areas of this device are discussed.
We describe experimental results from micromechanical resonators coated with chemoselective polymers that detect
chemical vapors from volatile organic compounds or explosives using all-optical interrogation. The shift in
the resonant frequency of a gold microbeam is read-out using photothermal actuation and microcavity interferometry.
For detection of toluene vapor, response times of less than 5 seconds are achieved for vapor concentrations
as low as 60 ppm. For detection of TNT vapor, concentrations as low as 10 ppb are detected in 100 seconds. An
analysis of the measured frequency noise in these sensors shows that it is dominated by thermal-mechanical fluctuations
at the fundamental flexural mode. Our measurements thus indicate that thermal-mechanical frequency
noise is the primary intrinsic detection limit for typical resonant-frequency MEMS biosensors or chemical vapor
sensors.
Since 1998, the Naval Research Laboratory has been developing modulating retro-reflectors (MRR) for free-space optical (FSO) communication links. Using an MRR is beneficial for asymmetric links containing one node with weight or power constraints, such as ground-to-air links. An MRR configuration shifts most of the power, weight, and pointing requirements onto one node.
To close an MRR link at any significant distance, the area of the modulator must be fairly large. Using a large area limits the data rate, since capacitance is proportional to area. Resistance also depends on device size, although not in the same manner. To increase the data rate, we designed a top metal contact that diminishes the effect of the semiconductor resistivity.
Using this new top contact design decreases the rise time from approximately 2.6 μs to 60 ns using the same wafer structure. However, increasing the data rate increases the power input and results in self-heating, which changes the optimal operating wavelength of MQW modulators.
Accordingly, a new coupled quantum well structure is designed. The new design lowers the required drive voltage, which in turn lowers the power consumption, increases yield, and simplifies drive circuitry. The lower power input also reduces the modulation rate dependence of the wavelength.
We report absolute measurements of thermal-mechanical noise in microelectromechanical systems. The measurements are made possible with a simple, high resolution optical technique that has a displacement resolution on the order of hundreds of femtometers per root Hz at frequencies of tens of kHz. The measured noise spectrum agrees with the calculated noise level to within 25%, a discrepancy most likely due to uncertainty in the effective dynamic mass of the vibrating bridge. These measurements demonstrate that thermal-mechanical noise can be the dominant noise source in actuated microelectromechanical devices. This noise will become even more pronounced as the size of mechanical devices continues to shrink.
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