Jonathan D. Suter

Dynamics of molecular emission features from nanosecond, femtosecond laser and filament ablation plasmas

The evolutionary paths of molecular and nanoparticle formation in laser ablation plumes are not well understood due to the complexity of numerous physical processes that occur simultaneously in a transient laser-produced plasma system. It is well known that the emission features of ions, atoms, molecules and nanoparticles in a laser ablation plume strongly depend on the laser irradiation conditions. We report the temporal emission features of AlO molecules in plasmas generated using a nanosecond laser, a femtosecond laser and filaments generated from a femtosecond laser. Our results show that, at a fixed laser energy, the persistence of AlO is found to be highest and lowest in ns and filament laser plasmas respectively while molecular species are formed at early times for both ultrashort pulse (fs and filament) generated plasmas. Analysis of the AlO emission band features show that the vibrational temperature of AlO decays rapidly in filament assisted laser ablation plumes.

Principles of Meniscus-Based MEMS Gas or Liquid Pressure Sensors

Pressure sensing using a trapped-gas volume-liquid meniscus interface offers several advantages over other microelectromechanical systems technologies for certain applications, including the potential for smaller footprints, harsh environment survivability, simple CMOS integration, and ease of fabrication. The small effective hydraulic diameter of microchannels can be exploited to produce gas/liquid interfaces that create menisci used to trap gas in sealed chambers. The pressure is monitored by optically or electronically measuring the displacement of the meniscus which behaves according to gas laws. This paper reports on the theory and realization of several fundamental concepts for this type of sensor, including the autocalibration of meniscus forces regardless of the sensor material or liquid; electrode integration for electronic interrogation in addition to optical measurements; simple repeatable manufacturing; and long-term drift. Hundreds of sensor devices were fabricated from silicon and glass and demonstrated positive pressure sensitivities of 42.5 μm/kPa near atmospheric pressure.

Characterization of a swept external cavity quantum cascade laser for rapid broadband spectroscopy and sensing.

The performance of a rapidly swept external cavity quantum cascade laser (ECQCL) system combined with an open-path Herriott cell was evaluated for time-resolved measurements of chemical species with broad and narrow absorption spectra. A spectral window spanning 1278 - 1390 cm(-1) was acquired at a 200 Hz acquisition rate, corresponding to a tuning rate of 2x10(4) cm(-1)/s, with a spectral resolution of 0.2 cm(-1). The capability of the ECQCL to measure < 100 ppbv changes in nitrous oxide (N(2)O) and 1,1,1,2-tetrafluoroethane (F134A) concentrations on millisecond timescales was demonstrated in simulated plume studies with releases near the open-path Herriott cell. Absorbance spectra measured using the ECQCL system exhibited noise-equivalent absorption coefficients of 5x10(-9) cm(-1)Hz(-1/2). For a spectrum acquisition time of 5 ms, noise-equivalent concentrations (NEC) for N(2)O and F134A were measured to be 70 and 16 ppbv respectively, which improved to sub-ppbv levels with averaging to 100 s. Noise equivalent column densities of 0.64 and 0.25 ppmv × m in 1 sec are estimated for N(2)O and F134A.

Hyperspectral microscopy using an external cavity quantum cascade laser and its applications for explosives detection

Using infrared hyperspectral imaging, we demonstrate microscopy of small particles of the explosives compounds RDX, tetryl, and PETN with near diffraction-limited performance. The custom microscope apparatus includes an external cavity quantum cascade laser illuminator scanned over its tuning range of 9.13-10.53 μm in four seconds, coupled with a microbolometer focal plane array to record infrared transmission images. We use the hyperspectral microscopy technique to study the infrared absorption spectra of individual explosives particles, and demonstrate sub-nanogram detection limits.

In situ non-destructive measurement of biofilm thickness and topology in an interferometric optical microscope

Biofilms are ubiquitous and impact the environment, human health, dental hygiene, and a wide range of industrial processes. Biofilms are difficult to characterize when fully hydrated, especially in a non-destructive manner, because of their soft structure and water-like bulk properties. Herein a method of measuring and monitoring the thickness and topology of live biofilms of using white light interferometry is described. Using this technique, surface morphology, surface roughness, and biofilm thickness were measured over time without while the biofilm continued to grow. The thickness and surface topology of a P. putida biofilm were monitored growing from initial colonization to a mature biofilm. Measured thickness followed expected trends for bacterial growth. Surface roughness also increased over time and was a leading indicator of biofilm growth.

Angle-resolved scattering spectroscopy of explosives using an external cavity quantum cascade laser

We present a study of the spectral and angular dependence of the diffuse scatter of mid-infrared (MIR) laser light from explosives residues on surfaces. Experiments were performed using an external cavity quantum cascade laser (ECQCL) tunable between 7 and 8 μm (1270 to 1400 cm-1) for surface illumination. A mercury cadmium telluride (MCT) detector was used to detect backscattered spectra as a function of surface angle at a 2 meter standoff. A ferroelectric focal plane array was used to build hyperspectral images at a 0.5 meter standoff. Residues of RDX, tetryl, and TNT were investigated on surfaces including a painted car door for angles between zero (specular) and 50 degrees. We observe spectral signatures of the explosives in the diffuse scattering geometry which differ significantly from those observed in transmission geometries. Characterization of the scattered light spectra of explosives on surfaces will be essential for understanding the performance of standoff explosives detection instruments and developing robust spectral analysis techniques.

Challenges of infrared reflective spectroscopy of solid-phase explosives and chemicals on surfaces

Reliable active and passive hyperspectral imaging and detection of explosives and solid-phase chemical residue on surfaces remains a challenge and an active area of research. Both methods rely on reference libraries for material identification, but in many cases the reference spectra are either not available or do not sufficiently resemble the instrumental signals of light reflected, scattered, or emitted from real-world objects. We describe a physics-based model using the complex dielectric constant to explain what is often thought of as anomalous behavior of scattered or nonspecular signatures encountered in active and passive sensing of explosives or chemicals on surfaces and show modeling and experimental results for RDX.

Optically resonant subwavelength films for tamper-indicating tags and seals

We present the design, modeling and performance of a proof-of-concept tamper indicating approach that exploits newlydeveloped subwavelength-patterned films. These films have a nanostructure-dependent resonant optical reflection that is wavelength, angle, and polarization dependent. As such, they can be tailored to fabricate overlay transparent films for tamper indication and authentication of sensitive or controlled materials not possible with currently-known technologies. An additional advantage is that the unique optical signature is dictated by the geometry and fabrication process of the nanostructures in the film, rather than on the material used. The essential structure unit in the subwavelength resonant coating is a nanoscale Open-Ring Resonator (ORR). This building block is fabricated by coating a dielectric nanoscale template with metal to form a hemispherical shell-like structure. This curved metallic shell structure has a cross-section with an intrinsic capacitance and inductance and is thus the optical equivalent to the well-known “LC” circuit where the capacitance and inductance are determined by the nanoshell dimensions. For structures with sub 100 nm scale, this resonance occurs in the visible electromagnetic spectrum, and in the IR for larger shells. Tampering of the film would be visible though misalignment of the angle-sensitive features in the film. It is additionally possible to add in intrinsic oxidation and strain sensitive matrix materials to further complicate tamper repair and counterfeiting. Cursory standoff readout would be relatively simple using a combination of a near-infrared (or visible) LED flashlight and polarizer or passively using room lighting illumination and a dispersive detector.

Design of a dynamic biofilm imaging cell for white-light interferometric microscopy

Abstract. In microbiology research, there is a strong need for next-generation imaging and sensing instrumentation that will enable minimally invasive and label-free investigation of soft, hydrated structures, such as in bacterial biofilms. White-light interferometry (WLI) can provide high-resolution images of surface topology without the use of fluorescent labels but is not typically used to image biofilms because there is insufficient refractive index contrast to induce reflection from the biofilm’s interface. The soft structure and water-like bulk properties of hydrated biofilms make them difficult to characterize in situ, especially in a nondestructive manner. We build on our prior description of static biofilm imaging and describe the design of a dynamic growth flow cell that enables monitoring of the thickness and topology of live biofilms over time using a WLI microscope. The microfluidic system is designed to grow biofilms in dynamic conditions and to create a reflective interface on the surface while minimizing disruption of fragile structures. The imaging cell was also designed to accommodate limitations imposed by the depth of focus of the microscope’s objective lens. Example images of live biofilm samples are shown to illustrate the ability of the flow cell and WLI instrument to (1) support bacterial growth and biofilm development, (2) image biofilm structure that reflects growth in flow conditions, and (3) monitor biofilm development over time nondestructively. In future work, the apparatus described here will enable surface metrology measurements (roughness, surface area, etc.) of biofilms and may be used to observe changes in biofilm structure in response to changes in environmental conditions (e.g., flow velocity, availability of nutrients, and presence of biocides). This development will open opportunities for the use of WLI in bioimaging.

Progress towards prognostic health management of passive components in advanced small modular reactors

Sustainable nuclear power to promote energy security and to reduce greenhouse gas emissions are two key national energy priorities. The development of deployable small modular reactors (SMRs) is expected to support these objectives by developing technologies that improve the reliability, sustain safety, and improve affordability of new reactors. Advanced SMRs (AdvSMRs) refer to a specific class of SMRs and are based on modularization of advanced reactor concepts. Prognostic health management (PHM) systems can benefit both the safety and economics of deploying AdvSMRs and can play an essential role in managing the inspection and maintenance of passive components in AdvSMR systems. This paper describes progress on development of an experimental setup for testing and validation of PHM systems for AdvSMR passive components. The experimental set-up for validation of prognostic algorithms is focused on thermal creep degradation as the prototypic degradation mechanism. The test bed enables accelerated thermal creep aging of materials relevant to AdvSMRs along with multiple nondestructive evaluation (NDE) measurements for assessment of thermal creep damage. NDE techniques include eddy current, magnetic Barkhausen noise (MBN), and linear and non-linear ultrasonic measurements. Details of the test-bed design as well as initial measurements results for specimens at different levels of thermal creep damage are presented.