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.
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 (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.
Integrated photonic systems require reliable, low power consumption phase shifters in order to scale circuit complexity. Micro-electromechanical systems (MEMS) are a viable candidate to provide low-power phase shifting, without the significant drawbacks of thermo-optic effects (high power usage and cross-talk). Previous works have demonstrated MEMS phase tuning through vertically displaced microbridges. However, these require > 40 V for a π phase shift [1] using gradient electric fields, though lower voltage requirements have been demonstrated for direct electrostatic actuation [2]. Ultra-low voltage (∼ 1 V) designs using horizontal slot waveguides have recently been demonstrated [3–5] although these methods require complex mechanical structures that must be released using vapor-HF or critical point drying post-processing. Here we present low-voltage phase tuning of vertically actuated beams that are released at wafer level with a vapor XeF2 etch completely in-house. Our process is carried out on an i-line photolithography stepper to define the waveguiding, metal, and MEMS structures. We use a sacrificial polysilicon layer between the SiN waveguide and the SiN beam. The XeF2 undercuts the beam, enabling a simple MEMS release process that does not undercut the waveguide. The movable SiN beam on top of the waveguide utilizes a single-sided anchor so that it resembles a wide and short single-clamped cantilever. This enables a phase shifter that is capable of a π phase shift with < 10 V and length < 100 μm. We measure the optical transmission versus applied voltage for multiple voltage sweeps and extract the phase shift per voltage at various wavelengths. We demonstrate reliable tuning over multiple sweeps with an average voltage of 7 V ± 0.5 V for a π phase shift. This phase shifter is central to the scalability of programmable photonic circuits, including quantum photonics.
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.
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.
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.
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.
Many components for free-space optical (FSO) communication systems have shrunken in size over the last decade. However, the steering systems have remained large and power hungry. Nonmechanical beam steering offers a path to reducing the size of these systems. Optical phased arrays can allow integrated beam steering elements. One of the most important aspects of an optical phased array technology is its scalability to a large number of elements. Silicon photonics can potentially offer this scalability using CMOS foundry techniques. A phased array that can steer in two dimensions using the thermo-optic effect is demonstrated. No wavelength tuning of the input laser is needed and the design allows a simple control system with only two inputs. A benchtop FSO link with the phased array in both transmit and receive mode is demonstrated.
Spontaneous parametric downconversion (SPDC) using periodically poled nonlinear optical crystals under the quasiphase- matching condition has found wide use in quantum optics. High efficiencies and good coupling to single-mode fibers resulted from using channel waveguides in crystals. It is often desirable to have a very narrow bandwidth for the signal and idler photons, but under the typical operating conditions, phase matching dictates the bandwidth of the SPDC to be of the order of <1 nm. This occurs because the co-propagating signal and idler photons are entangled, and an increase of the signal wave-vector is compensated by a decrease of the idler wave-vector. One way to reduce the bandwidth is by forming either external or internal cavities. Additionally, bandwidth reduction is possible without cavities when the signal and idler are counter-propagating, and the changes in the wave-vector with frequency are additive. To accomplish this a domain inversion on the wavelength scale is required. In this work, we experimentally demonstrate SPDC in one-dimensional KTP-based waveguides with sub-micron poling for forward and backward interactions. Some of the spectral features of the generated light are accounted for by mode coupling theory in periodically poled waveguides but other features are as yet not explained.
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.
Components for free space optical communication terminals such as lasers, amplifiers, and receivers have all seen substantial reduction in both size and power consumption over the past several decades. However, pointing systems, such as fast steering mirrors and gimbals, have remained large, slow and power-hungry. Optical phased arrays provide a possible solution for non-mechanical beam steering devices that can be compact and lower in power. Silicon photonics is a promising technology for phased arrays because it has the potential to scale to many elements and may be compatible with CMOS technology thereby enabling batch fabrication. For most free space optical communication applications, two-dimensional beam steering is needed. To date, silicon photonic phased arrays have achieved two-dimensional steering by combining thermo-optic steering, in-plane, with wavelength tuning by means of an output grating to give angular tuning, out-of-plane. While this architecture might work for certain static communication links, it would be difficult to implement for moving platforms. Other approaches have required N2 controls for an NxN element phased array, which leads to complexity. Hence, in this work we demonstrate steering using the thermo-optic effect for both dimensions with a simplified steering mechanism requiring only two control signals, one for each steering dimension.
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.
Many components for free space optical communication systems have shrunken in size over the last decade. However, the steering systems have remained large and power hungry. Non-mechanical beam steering offers a path to reducing the size of these systems. Optical phased arrays can allow integrated beam steering elements. One of the most important aspects of an optical phased array technology is its scalability to a large number of elements. Silicon photonics can potentially offer this scalability using CMOS foundry techniques. In this paper a small-scale silicon photonic optical phased array is demonstrated for both the transmitter and receiver functions in a free space optical link. The device using an array of thermo-optically controlled waveguide phase shifters and demonstrates one-dimensional steering with a single control electrode. Transmission of a digitized video data stream over the link is shown.
Silicon photonics provides the ability to construct complex photonic circuits that act on the amplitude and phase of
multiple optical channels. Many applications of silicon photonics depend on maintenance of optical coherence among the
various waveguides and structures on the chip. Other applications can depend on the modal structures of the waveguides.
All these application require the ability to characterize the amplitude and phase of individual optical channels. Fourier
imaging with high numerical aperture microscope objectives has been used to image the intensity of individual channels
of photonic structures in both real and Fourier space. In other work, holographic imaging of multimode fibers has
allowed modal decomposition. In this work we use interferometric microscopy to image the amplitude and phase of a
variety of silicon photonic structures. These include a multimode interference splitter and a multimode waveguide under
various excitation conditions.
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 have demonstrated a planar waveguide-based tunable integrated optical filter in indium phosphide (InP) with on-chip micro-electro-mechanical (MEMS) actuation. An air-gap Fabry-Perot resonant microcavity is formed between two waveguides, whose facets have monolithically integrated high-reflectivity multilayer InP/air Distributed Bragg Reflector (DBR) mirrors. A suspended beam electrostatic microactuator attached to one of the DBR mirrors modulates the microcavity length, resulting in a tunable filter. The DBR mirrors provide a broad high-reflectivity spectrum, within which the transmission wavelength can be tuned. The in-plane configuration of the filter enables easy integration with other active and passive waveguide-based optoelectronic devices on a chip and simplifies fiber alignment. Experimental results from the first generation of tunable optical filters are presented. The microfabricated filter exhibited a resonant wavelength shift of 12nm (1513-1525nm) at a low operating voltage of 7V. A full-width-half-maximum (FWHM) of 33 nm was experimentally observed, and the quality factor was calculated to be 46. Several improvements of the MEMS actuator, waveguide, and optical cavity design for the future devices are discussed.
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