J. Bruce Johnson

Standoff Methods for the Detection of Threat Agents: A Review of Several Promising Laser-Based Techniques

Detection of explosives, explosive precursors, or other threat agents presents a number of technological challenges for optical sensing methods. Certainly detecting trace levels of threat agents against a complex background is chief among these challenges; however, the related issues of multiple target distances (from standoff to proximity) and sampling time scales (from passive mines to rapid rate of march convoy protection) for different applications make it unlikely that a single technique will be ideal for all sensing situations. A number of methods for spanning the range of optical sensor technologies exist which, when integrated, could produce a fused sensor system possessing a high level of sensitivity to threat agents and a moderate standoff real-time capability appropriate for portal screening of personnel or vehicles. In this work, we focus on several promising, and potentially synergistic, laser-based methods for sensing threat agents. For each method, we have briefly outlined the technique and report on the current level of capability.

Optimization of the electrodeposition parameters to improve the stoichiometry of In 2 S 3 films for solar applications using the taguchi method

Properties of electrodeposited semiconductor thin films are dependent upon the electrolyte composition, plating time, and temperature as well as the current density and the nature of the substrate. In this study, the influence of the electrodeposition parameters such as deposition voltage, deposition time, composition of solution, and deposition temperature upon the properties of In2S3 films was analyzed by the Taguchi Method. According to Taguchi analysis, the interaction between deposition voltage and deposition time was significant. Deposition voltage had the largest impact upon the stoichiometry of In2S3 films and deposition temperature had the least impact. The stoichiometric ratios between sulfur and indium (S/In: 3/2) obtained from experiments performed with optimized electrodeposition parameters were in agreement with predicted values from the Taguchi Method. The experiments were carried out according to Taguchi orthogonal array L27 (34) design of experiments (DOE). Approximately 600 nm thick In2S3 films were electrodeposited from an organic bath (ethylene glycol-based) containing indium chloride (InCl3), sodium chloride (NaCl), and sodium thiosulfate (Na2S2O3·5H2O), the latter used as an additional sulfur source along with elemental sulfur (S). An X-ray diffractometer (XRD), energy dispersive X-ray spectroscopy (EDS) unit, and scanning electron microscope (SEM) were, respectively, used to analyze the phases, elemental composition, and morphology of the electrodeposited In2S3 films.

Picosecond multiphoton STIRAP detection of gas phase species: a test with sodium

Laser detection technologies offer obvious benefits for the standoff detection of hazardous or energetic materials where safe detection at a distance is the goal. Of the many optical standoff detection methods available, multiphoton fluorescence techniques have been studied less extensively. Multiphoton fluorescence allows high selectivity relative to the background while preserving the larger signal of laser induced fluorescence (LIF). Using sodium vapor as a test system, we demonstrate that stimulated Raman adiabatic passage (STIRAP) is capable of providing more than a factor of ten improvement in population transfer efficiency to the final state when compared to stimulated emission pumping (SEP). The two sodium transitions used are the 3p (2P1/2) ← 3s (2S1/2) and 5s (2S1/2) ← 3p (2P1/2). The light used to couple the states was produced with two synchronously pumped OPG/OPAs pumped by the 355 nm light from a picosecond tripled Nd:YAG.

Limitations and guidelines for measuring the spectral width of a single pulse of light with a Fabry–Perot interferometer

We present a method of analyzing the output of a single pulse of light from a Fabry-Perot interferometer (FPI). Together with an independent measurement of the pulse width and shape, the analysis enables the determination of the linewidth, TBP, and, consequently, the degree of coherence of the individual light pulses. The analysis presented builds on the method presented by Marzenell et al. [Appl. Phys. B 71, 185-191 (2000)] by analyzing the ring pattern of the FPI.

Picosecond standoff multiphoton detection of gas phase species: initial results

In the implementation of laser-induced fluorescence (LIF) for the detection of vapor-phase organic compounds that accompany hazardous materials, multiphoton excitation offers a significant advantage over single photon methods. In particular, if the absorption spectra of unwanted background molecules overlap that of the target molecule, single photon LIF is plagued by false positives. Multiphoton methods alleviate this difficulty by requiring that the target molecule be in resonance with multiple molecular transitions. A promising multiphoton method is stimulated Raman adiabatic passage (STIRAP). This method involves a counterintuitive sequence of laser pulses which is capable of transferring 100% of the target molecules to the desired excited state from which fluorescence is to be observed. As a precursor to more complex molecules, we demonstrate the STIRAP technique on sodium vapor using the 3p (2P1/2) ← 3s (2S1/2) and 5s (2S1/2) ← 3p (2P1/2) transitions. This is the first time STIRAP has been achieved on a vapor using picosecond lasers. We produced light to couple the states using two synchronously pumped OPG/OPAs (pumped by the 355 nm light from a picosecond YAG). We measured the fluorescence from the 5s state to both 3p states (2P1/2, 2P3/2) and from both 3p states to the 3s state with monochromator using a gated CCD to eliminate Rayleigh scattered light. Our results indicate a four to five-fold increase in the transfer efficiency to the 5s state when the laser pulse that couples the 3p and 5s states precedes the laser pulse tuned to the 3p ← 3s transition.

Hydrostatic optical cell for temperatures below 350 K and pressures to 400 MPa

A compact optical pressure cell tested at pressures up to 400 MPa is presented. The cell has been used within the temperature range 10–350 K. The cell has a small mass of 0.2 kg which facilitates rapid changes in temperature. A new high-pressure window seal is presented which allows the cell to be mounted directly in vacuum with no leak over its entire temperature and pressure range. A method of attaching the cell to common coldhead-type cryostats and closed-cycle refrigerators is presented.

Stimulated Raman adiabatic passage in sodium vapor with picosecond laser pulses

Experimental measurements and calculations of STIRAP transfer efficiencies were made on a sodium gas starting from the 3S1/2 electronic ground state, passing through the 3 P1/2 and/or the 3P3/2 to the 5 S1/2 state. The lasers used in the experiments had a pulse width of several picoseconds and were close to the Fourier transform limit. Although the linewidth of the laser was much smaller than the spin orbit splitting between the 3P1/2 and 3 P3/2 states, Experiments and calculations reveal that both 3p states play a role in the transfer efficiency when the lasers are tuned to resonance through the 3P1/2 state, revealing evidence for quantum interference between the competing pathways.

Morphological and compositional analysis of electrodeposited indium (III) sulfide (In 2 S 3 ) films

In the last few years, notable progress in understanding the growth mechanism of thin solar films deposited by numerous techniques have been made. Electrodeposition continues to be a complex deposition technique that can lead to low-quality material regions (crack) in the semiconductor material. Such cracks form porous zones on the substrate and diminish the heterojunction interface quality of a photovoltaic (PV) cell. In this paper, electrodeposition of In 2 S 3 films was systematically and quantitatively investigated by varying the electrodeposition parameters including bath composition, current density, deposition time, and deposition temperature. Their effects upon the film growth mechanism, composition, and morphology were studied with the help of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and fracture and buckling software (digital image analysis). In addition, the effect of different glass-substrates (Mo, ITO, and FTO) and annealing treatments upon the performance of the electrodeposited In 2 S 3 film was analyzed. Furthermore, the Taguchi Method was used to determine the optimal electrodeposition parameters and study their influence upon the morphological and compositional properties of In 2 S 3 films.

Stoichiometric control via periods of open-circuit during electrodeposition

Electrodeposition can enable stoichiometric control of deposited samples through variation of electroplating potential. We demonstrate an in-situ technique for deposit analysis and stoichiometric control by interspersing periods of open-circuit during deposition. Opening the circuit in an organic Cu-In-S plating bath allows greater incorporation of Cu, In, and/or S into deposited films, based upon the open-circuit voltage the film/electrolyte interface is allowed to achieve. With the same deposition potential, samples can be made to vary from highly Cu-rich to highly In-rich through selection of an appropriate open-circuit voltage limit.

STIRAP on sodium gas as a function of argon buffer gas pressure

The standoff detection of energetic materials via laser-induced fluorescence of vapors has received relatively little attention due to spectrally broad fluorescence emission from aerosols and unwanted background molecules. This unwanted broad emission can obscure fluorescence from the molecule of interest. When multiphoton excitation is used, the problem can be avoided by blue-shifting the emission from the target molecule relative to the unwanted broad emission. As a precursor to the detection of explosives, we demonstrate coherent multiphoton excitation via stimulated Raman adiabatic passage (STIRAP) on sodium vapor in an argon buffer gas as a function of argon pressure. Results indicate that STIRAP can be performed in a buffer gas at atmospheric pressure with a minimal eduction in STIRAP efficiency. The 15 ps long light pulses used for the pump and Stokes pulses were produced by two synchronously pumped OPO/OPAs tuned to the 3p (2P1/2) ← 3s (2S1/2) transition for the pump pulse and the 5s (2S1/2) ← 3p (2P1/2) for the Stokes pulse.