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.
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
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.