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.
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.
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.
The nonlinear optical response of photorefractive oxide materials was investigated using four- wave-mixing (FWM) techniques with laser pulses having durations in the pico- and sub- picosecond range. The specific materials studied are KNbO3, KTaO3, KTa1-XNbXO3 (KTN), SrXBa1-XNb2O6, and Bi2TeO5. In each case there are several different physical processes that contribute to the nonlinear signals and their origins are analyzed in the context of time intervals within and after the cross-correlation time of the two write beams pulses. The role of the nonlinear absorption in the four-wave mixing (FWM) processes is compared for the different materials. Variation in the build-up of the electro-optic photorefractive effect is discussed.