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.

Imaging studies of photodegradation and self-healing in anthraquinone derivative dye-doped PMMA.

We study photodegradation and self-healing of nine different anthraquinone-derivatives doped into PMMA using transmission imaging microscopy in search of structure-property relationships of the underlying mechanisms. We find that seven of the nine anthraquinone derivatives display partially reversible photodegradation, with 1,8-dihydroxyanthraquinone (Dantron/Chrysazin) having the best photostability and recovery characteristics of all dyes tested in this study. Based on these measurements we predict that a sample of 1,8-dihydroxyanthraquinone doped into PMMA with a concentration of 9 g l-1 will have a record setting irreversible inverse quantum efficiency of Bε = 4.56 × 109. Additionally, by considering the performance of the different anthraquinone derivatives and their structures, we develop three rules-of-thumb to qualitatively predict the photostability and recovery characteristics of anthraquinone derivatives. These rules-of-thumb will help guide future experiments and molecular modeling in discerning the underlying mechanisms of reversible photodegradation. Finally, we compare our results for disperse orange 11 dye-doped PMMA to the extended Correlated Chromophore Domain Model (eCCDM). While the eCCDM correctly predicts the behavior of the reversible decay component, it fails to correctly predict the behavior of the irreversible degradation component. This implies further modifications to the eCCDM are required.

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.

Imaging studies of temperature dependent photodegradation and self-healing in disperse orange 11 dye-doped polymers.

Using confocal transmission imaging microscopy, we measure the temperature dependence of photodegradation and self-healing in disperse orange 11 (DO11) dye-doped (poly)methyl-methacrylate (PMMA) and polystyrene (PS). In both dye-doped polymers, an increase in sample temperature results in a greater photodegradation rate and degree of degradation, while also resulting in a slower recovery rate and larger recovery fraction. These results confirm the temperature dependence predictions of the modified correlated chromophore domain model (mCCDM) [B. R. Anderson and M. G. Kuzyk, Phys. Rev. E 89, 032601 (2014)]. Additionally, using quantitative fitting of the imaging data for DO11/PMMA, we determine the domain density parameter to be ρ = 1.19 (±0.25) × 10(-2) and the domain free energy advantage to be λ = 0.282 ± 0.015 eV, which are within the uncertainty of the values previously determined using amplified spontaneous emission as the probe method [S. K. Ramini et al., Polym. Chem. 4, 4948 (2013)]. Finally, while we find photodegradation and self-healing of DO11/PS to be qualitatively consistent with the mCCDM, we find that it is quantitatively incompatible with the mCCDM as recovery in DO11/PS is found to behave as a stretched (or double) exponential as a function of time.

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.