Figure-of-Merit Characterization of Hydrogen-Bond Acidic Sorbents for Waveguide-Enhanced Raman Spectroscopy.

The optical properties of several hydrogen-bond acidic sorbent materials are evaluated in situ to assess their suitability for waveguide-enhanced Raman spectroscopy (WERS) of vapor-phase organophosphonates. A number of characteristics critical to WERS are evaluated for each sorbent: infrared absorption, Raman spectral background, and the limit of detection for a test hydrogen-bond-basic analyte (dimethyl methylphosphonate, DMMP). We describe the chemical properties of the sorbents that differentiate their optical properties for sensing. Then, we introduce a sorbent figure-of-merit that quantifies these differences and provides a framework to assess the quality of newly developed sorbent materials.

Gas chromatography using a spin-coated stationary phase and a molded elastomer micro-channel.

Many different designs of microfabricated gas chromatography columns have recently been proposed and demonstrated. These designs either incorporate a stationary phase directly into the device which limits the versatility of the column as a separator, or require coating, which presents its own problems with determining the proper conditions for each different stationary phase a user may need. Here, we present a new approach: Uniformly spin coating a flat surface with the stationary phase and creating a column by pressing a lid, with micro-fabricated ridges, down onto the coated substrate. The lids are molded using commercially available epoxies so that when pressed onto a flat surface they create an airtight seal. The epoxy material is rendered inert by a thin layer of gold. We describe the fabrication and initial results from a standard, OV-1, stationary phase as proof of concept.

Time resolved characterization of Fabry-Perot quantum cascade lasers for use in a broadband "white light" source.

We report the time resolved characterization of Fabry-Perot quantum cascade lasers (FP-QCLs). We are developing a custom-built broadband laser source in the Mid-LWIR range by combining several high power FP-QCLs for a single snap shot application. This white light source would enable not only stand-off detection applications in a single snapshot but also new data collection modalities such as live, real-time chemical imaging, requiring extremely rapid measurements. In this study, the two FP-QCLs were operated in CW and pulsed modes with varying applied currents and diode temperatures to optimize the best laser operation condition to cover a broad spectral range including spectral features for the analytes of interest. To understand mode behavior of the FP-QCLs in a short period of time, the spectral output for each test condition was temporally resolved. Under most of the conditions, FP mode hopping was observed during the time evolution through the pulse length (3000 ns). Based on the time-resolved spectra, the ideal spectral characteristics for a single snap shot application are discussed, with respect to a broad spectral bandwidth, a flat-top power profile, and high spectral power density.

Reduction of speckle noise and mitigation of beam wander in tunable external cavity quantum cascade lasers using rotating diamond/KBr pellet coupled with multimode fiber.

We experimentally demonstrate speckle noise reduction and beam wander mitigation by using a rotating diamond/KBr pellet and a multimode fiber (MMF). As the diamond/KBr diffuser is rotated, the reflected speckle images that are captured by an infrared camera are temporally averaged. We demonstrate 78% speckle noise reduction by averaging 25 frames, which is within 80% of the theoretical contrast reduction. Large beam position fluctuations are also significantly suppressed by adding the MMF. This combination of beam wander mitigation and speckle reduction offers significant benefits for emerging optical technologies that use quantum cascade lasers as illumination sources.

Waveguide-enhanced Raman spectroscopy of trace chemical warfare agent simulants.

We report the measurement of waveguide-enhanced Raman spectra from trace concentrations of four vapor-phase chemical warfare agent simulants: dimethyl methylphosphonate, diethyl methylphosphonate, trimethyl phosphate, and triethyl phosphate. The spectra are obtained using highly evanescent nanophotonic silicon nitride waveguides coated with a naturally reversible hyperbranched carbosilane sorbent polymer and exhibit extrapolated one-σ detection limits as low as 5 ppb. We use a finite-element model to explain the polarization and wavelength properties of the differential spectra. In addition, we assign spectral features to both the analyte and the sorbent, and show evidence of changes to both due to hydrogen bonding.

Trace gas absorption spectroscopy using functionalized microring resonators.

We detect trace gases at parts-per-billion levels using evanescent-field absorption spectroscopy in silicon nitride microring resonators coated with a functionalized sorbent polymer. An analysis of the microring resonance line shapes enables a measurement of the differential absorption spectra for a number of vapor-phase analytes. The spectra are obtained at the near-infrared overtone of OH-stretch resonance, which provides information about the toxicity of the analyte vapor.

Microscopic and Spectroscopic Studies of Thermally Enhanced Electrospun PMMA Micro- and Nanofibers.

Carbon nanofibers (CNFs) have been incorporated into poly(methyl methacrylate) (PMMA) through electrospinning. The resulting micro- and nanofibers have been characterized by Scanning Electron Microscopy (SEM), which confirmed fiber formation and demonstrated a core-sheath structure of the PMMA fibers. Thermogravimetric Analysis (TGA) was used to obtain the thermal properties of the materials, indicating an enhancement in the thermal properties of the composite fibers. In addition, Fourier Transform Infrared Spectroscopy (FTIR) was utilized to investigate the interactions of PMMA micro- and nanofibers with CNFs, demonstrating the preferred sites of intermolecular interactions between the polymer matrix and the filler.

Waveguide micro-opto-electro-mechanical resonant chemical sensors.

We demonstrate a silicon micro-opto-electro-mechanical sensor based on mass-loading of a chemo-selective polymer coated onto a microbridge. The sensor is probed optically using an on-chip waveguide Fabry-Pérot interferometer for high resolution displacement and resonant frequency measurement. The mechanical resonator is designed with paddles to simplify chemo-selective polymer deposition and to minimize any strain effects from the polymer during analyte sorption. Water vapor sensing experiments indicate mass-loading with minimum humidity detection of DeltaRH = 0.25% corresponding to a measured limit-of-detection of 68 parts-per-million. The sensor response time constant is 30 s with minimum frequency noise of 58 Hz at 149.4 kHz resonance. The measured mass-loading resolution is 4.6 picograms. We extract the chemo-selective polymer's partition coefficient and confirm that strong sorption and mass-loading occurs. Various device improvements are discussed, including scaling up to large single-chip sensor arrays, increasing mass-loading resolution by adjusting the device geometry, and using other chemo-selective polymers with larger partition coefficients. These improvements suggest parts-per-billion limit-of-detection levels for a variety of toxic chemicals.

Infrared microthermography of microfabricated devices.

We report a new experimental apparatus for infrared microthermography applicable to a wide class of samples including semitransparent ones and perforated devices. This setup is particularly well suited for the thermography of microfabricated devices. Traditionally, temperature calibration is performed using calibration hot plates, but this is not applicable to transmissive samples. In this work a custom designed miniature calibration oven in conjunction with spatial filtering is used to obtain accurate static and transient temperature maps of actively heated devices. The procedure does not require prior knowledge of the emissivity. Calibration and image processing algorithms are discussed and analyzed. We show that relatively inexpensive uncooled bolometer arrays can be a suitable detector choice in certain radiometric applications. As an example, we apply this method in the analysis of temperature profiles of an actively heated microfabricated preconcentrator device that incorporates a perforated membrane and is used in trace detection of illicit substances.

A solvolysis model for 2-chloro-2-methyladamantane based on the linear solvation energy approach.

Solvolysis/dehydrohalogenation rates of 2-chloro-2-methyladamantane (CMA) in 15 hydrogen-bond acidic and/or basic solvents are studied. The rates of reaction in these solvents have been correlated with the solvation equation developed by Kamlet, Abraham, and Taft. The linear solvation energy relationship (LSER) derived from this study is given by the following equation: log k = -5.409 + 2.219 + 2.505alpha(1) - 1.823beta(1) where , alpha(1), and beta(1) are the solvation parameters that measure the solvent dipolarity/polarizability, hydrogen-bond acidity (electrophilicity), and hydrogen-bond basicity (nucleophilicity). A high correlation coefficient (r = 0.996, SD = 0.191) was achieved. The cavity term, which includes the Hildebrand parameter for solvent cohesive energy density, delta(H), was not found to be statistically significant for this reaction substrate. The resulting equation allows calculated rates of reaction in other solvents and provides insight into the reaction pathway. In a previously reported correlation for another tertiary chloride, tert-butyl chloride (TBC), the coefficients for alpha(1) and are significantly larger and the coefficient for is statistically significant. In addition, the coefficient for beta(1) in the TBC correlation is positive, rather than negative, indicating that the transition states for TBC and CMA are significantly different. These results demonstrate why the uses of simple solvolytic correlation methods may be invalid even for comparisons of similar type substrates, e.g., tertiary chlorides. Also, these results provide confidence in the use of multiple linear regression analysis for predicting solvolytic rates in additional solvents.