Subsurface Spectroscopy of Thermal Degradation Inside an Inert Plastic Bonded Explosive (PBX) Simulant Using Feedback-Assisted Wavefront Shaping.

We characterize the subsurface thermal degradation of an inert analog of high-explosive molecular crystals (Eu:Y(acac)3(DPEPO)) (EYAD) embedded inside of a plastic bonded explosive simulant using feedback-assisted wavefront shaping-based fluorescence and Raman spectroscopies. This technique utilizes wavefront shaping to focus pump light inside a heterogeneous material onto a target particle, which significantly improves its spectroscopic signature. We find that embedding the EYAD crystals in the heterogeneous polymer results in improved thermal stability, relative to bare crystal measurements, with the crystal remaining fluorescent to >612 K inside of the heterogeneous material, while the bare crystal's fluorescence is fully quenched by 500 K. We hypothesize that this improvement is due to the polymer restricting the effects of EYAD melting, which occurs at 400 K and is the primary mechanism for spectroscopic changes in the temperature range explored.

Single-Shot Standoff Hyperspectral Raman Imaging of a Chemical Warfare Agent Simulant.

We demonstrate single-shot standoff hyperspectral Raman imaging of liquid diisopropyl methylphosphonate at a standoff distance of 1 m using two different techniques: multi-bandpass filter imaging (MBFI) and fiber-bundle imaging spectroscopy (FBIS). We find that MBFI has good spatial resolution, but poor spectral resolution, due to the limitations of commercially available bandpass filters. On the other hand, we find FBIS to have excellent spectral resolution, but limited spatial resolution due to the relatively small number of fibers in a bundle. For FBIS, we also determine, for a 1 m standoff distance, a minimum pump fluence of 10 mJ/cm2 to obtain good single-shot spectra.

Absorption-invariant focusing efficiency for wavefront-shaping controlled reflection from absorbing disordered media.

We numerically model the influence of absorption on wavefront-shaping controlled reflection from absorbing disordered media and provide experimental verification of our model. We find that absorption modifies the reflection eigenvalue density, the average reflectance, and the reflection matrix element density. However, we also find that despite these effects, the efficiency of wavefront-shaping controlled reflection is invariant with absorption.

Monitoring sub-surface chemical reactions in heterogeneous materials using wavefront-shaping-assisted bidirectional focusing.

We have developed a bidirectional focusing microscope that utilizes feedback-assisted wavefront shaping to focus light inside a heterogenous material in order to monitor sub-surface chemical reactions. The bidirectional geometry is found to provide superior intensity enhancement relative to single-sided focusing, owing to increased mode control and long-range mesoscopic correlations. Also, we demonstrate the microscope's capability to measure sub-surface chemical reactions by optically monitoring the photodegradation of a Eu-doped organic molecular crystal embedded in a heterogeneous material using both fluorescence and Raman spectroscopy as probe techniques.

Burn Chamber Test of Ex Situ Thermal Impulse Sensors.

Recently, we reported on a novel ex situ thermal impulse sensing technique (based on lanthanide-doped oxide precursor nanoparticles) for use in structural fire forensics and demonstrated its functionality in small-scale lab-based tests. As a next step we have now performed a large-scale lab test at the US Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) Fire Research Laboratory using a burn chamber with three sand burners. In this test we demonstrate our technique's ability to determine the average temperature experienced by surfaces during the fire. While we successfully demonstrate our techniques accuracy, we also discover several previously unknown vulnerabilities. Namely, we find that: (1) our current method of embedding sensors in paint results in our sensor particles being difficult to recover (due to a large quantity of debris), (2) the current test panels have poor survivability, (3) debris from the fire tests interferes with excitation of dopant Dy ions (limiting our sensors' functionality), and (4) dispersal in paint results in suppression of the (metastable)tetragonal-to-monoclinic phase transition of ZrO2. To overcome these vulnerabilities we are evaluating new panel materials, paints, and lanthanide-dopants.

Nanoscale Ex Situ Thermal Impulse Sensors for Structural Fire Forensics.

We developed nanoscale ex situ thermal impulse (i.e., the temperature and duration of a heating event) sensors for structural fire forensics using a mixture of two lanthanide-doped oxide precursors (precursor Eu:ZrO2 and precursor Dy:Y2O3) that undergo irreversible phase changes when heated. These changes are probed using photoluminescence (PL) spectroscopy with the PL spectra being dependent on the thermal impulse (TI) experienced by the sensors. By correlating the PL spectra to different in-lab TIs, we are able to produce a spectroscopic calibration for our sensors. This calibration allows us to determine an unknown TI of a heating event using only the PL spectrum of the heated TI sensors. In this study, we report on the calibration of these sensors for isothermal heating durations up to 600 s and isothermal temperatures up to 1273 K. Using this calibration, we also demonstrate their ability to determine an unknown TI and demonstrate their functionality when dispersed into paint, which is heated in the presence of drywall.

Initial tamper tests of novel tamper-indicating optical physical unclonable functions.

Optical physical unclonable functions (O-PUFs) are materials containing a large number of randomly distributed degrees of freedom that can be probed optically. We develop tamper-indicating O-PUF seals using polyurethane adhesives with dispersed nanoparticles, where authentication is performed by use of wavefront-shaping controlled reflection. We test the material's and authentication method's tamper-indicating ability using tampering attacks including: mechanical, thermal, and chemical attacks, with the results demonstrating the material/method's robust tamper-indicating ability. Additionally, we demonstrate that authentication depends strongly on the microscopic distribution of nanoparticles within the polymer, which is unfeasibly difficult to clone. These results are the first demonstration of the validity of this technique for use as tamper-indicating seals.

Microgenetic optimization algorithm for optimal wavefront shaping.

One of the main limitations of utilizing optimal wavefront shaping in imaging and authentication applications is the slow speed of the optimization algorithms currently being used. To address this problem we develop a microgenetic optimization algorithm (µGA) for optimal wavefront shaping. We test the abilities of the µGA and make comparisons to previous algorithms (iterative and simple-genetic) by using each algorithm to optimize transmission through an opaque medium. From our experiments we find that the µGA is faster than both the iterative and simple-genetic algorithms and that both genetic algorithms are more resistant to noise and sample decoherence than the iterative algorithm.

Self-healing organic-dye-based random lasers.

One of the primary difficulties in the implementation of organic-dye-based random lasers is the tendency of organic dyes to irreversibly photodecay. In this Letter, we report the observation of "reversible" photodegradation in a Rhodamine 6G and ZrO2 nanoparticle-doped polyurethane random laser. We find that during degradation, the emission broadens, redshifts, and decreases in intensity. After degradation, the system is observed to self-heal leading to the emission returning to its pristine intensity, giving a recovery efficiency of 100%. While the peak intensity fully recovers, the process is not strictly "reversible", as the emission after recovery is still found to be broadened and redshifted. The combination of the peak emission fully recovering and the broadening of the emission leads to a remarkable result: the random laser cycled through degradation, and recovery has a greater integrated emission intensity than the pristine system.

Spectroscopic observation of neutral carbon during photodissociation of explosive-related compounds in the vapor phase.

We perform time-resolved laser-induced fluorescence measurements of mononitrotoluenes (MNTs) and dinitrotoluenes (DNTs) in nitrogen and air. We observe the multipeak emission spectrum of NO and find that the emission peak intensity in the 247-248 nm range is stronger than expected compared to the other NO emission peak intensities. This increased emission intensity is believed to be due to neutral carbon [C(I)], which has a strong emission peak at 247.85 nm. By comparing the ratios of integrated emission peak intensities with those expected from the Franck-Condon factors for NO, we are able to identify samples that exhibit C(I) emission. We show that the DNTs exhibit C(I) emission for gate delays of 1500 ns and beyond, while the MNTs exhibit C(I) emission for gate delays of only up to about 500 ns. Carbon deposits in the analysis chamber confirm the presence of C. Ambient NO in air enhances the observed NO+C(I) signal from MNTs and DNTs.

Photoemission spectroscopy and atomic force microscopy investigation of vapor-phase codeposited silver/poly(3-hexylthiophene) composites.

Nanocomposite matrices of silver/poly(3-hexylthiophene) (P3HT) were prepared in ultrahigh vacuum through vapor-phase codeposition. Change in microstructure, chemical nature, and electronic properties with increasing filler (Ag) content were investigated using in situ XPS and UPS, and ambient AFM. At least two chemical binding states occur between Ag nanoparticles and sulfur in P3HT at the immediate contact layer, but no evidence of interaction between Ag and carbon (in P3HT) was found. AFM images reveal a change in Ag nanoparticles size with concentration which modifies the microstructure and the average roughness of the surface. Under codeposition, P3HT largely retains its conjugated structures, which is evidenced by the similar XPS and UPS spectra to those of P3HT films deposited on other substrates. We demonstrate here that the magnitude of the barrier height for hole injection (epsilon(v)(F)) and the position of the highest occupied band edge (HOB) with respect to the Fermi level of Ag may be controlled and changed by adjusting the metal (Ag) content in the composite. Furthermore, UPS reveals distinct features related to the C 2p (sigma states) in the 5-12 eV regions, indicating the presence of ordered P3HT, which is different from solution processed films.