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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Picosecond standoff multiphoton detection of gas phase species: initial results

Jerald B. Johnson, Kevin Lyon, William D. Murry, Daniel R. Britton, Michael J. Johnson

Published Mar 6, 2008• 2 citations
Publisher

Abstract

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.

DOI
10.1117/12.785376

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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.
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.
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.
Calculations of STIRAP transfer efficiencies were made on sodium starting from the eight hyperfine states of the 3s (2S1/2) electronic ground state, passing through the eight hyperfine states of the 3p (2P1/2) and/or fourteen of the sixteen hyperfine states of the 3p (2P3/2) to the eight hyperfine states of the 5s (2S1/2) state. Linearly polarized light was used in calculations and experiment leading to six independent sets of coupled states (four sets of five states and two sets of nine states). Calculations were made for laser pulses several picoseconds in length and compared with experiment. Although the linewidth of the laser pulses used in both computation and experiment was a small fraction of the fine structure splitting between the 3p states, both fine-structure states were observed to contribute to the STIRAP transfer efficiency. A rich set of results was found including oscillations similar to the Rabi oscillations observed in two-state systems when the pump and Stokes pulses were tuned to wavelengths between the two 3p states.
Experimental measurements and calculations of stimulated Raman adiabatic passage transfer efficiencies were made on a sodium gas starting from the 32S1/23{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}32S1/2​ electronic ground state, passing through the 32P1/23{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{1/2}32P1/2​ and/or the 32P3/23{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{3/2}32P3/2​ to the 52S1/25{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}52S1/2​ 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 32P1/23\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}32P1/2​ and 32P3/23\phantom{\rule{0.16em}{0ex}}^{2}P_{3/2}32P3/2​ 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 32P1/23{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{1/2}32P1/2​ state, revealing evidence of quantum interference between the competing pathways.
We use a 28-level system consisting of the vibrational bend states of the HCN/HNC system as a model system for the investigation of population transfer across the isomerization barrier using counterintuitively ordered multiphoton (MP) laser pulses. This (m+n)-STIRAP (stimulated Raman-adiabatic passage) arrangement with m pump- and n Stokes-photons is found to be capable of driving HCN→HNC transition states selectively and in a robust manner, despite the presence of near-degenerate parallel processes of different photonicities. Employing Gaussian pulses with widths of 80 ps, the process of highest photonicity we demonstrate is (4+6)-STIRAP, and we investigate systematically the robustness against variations in pulse amplitudes and frequencies for all parallel processes with m=2 and 3, and n=1–6. We discuss the conditions under which selective population transfer is obtained. We also compare with (1+1)-STIRAP using overtone transitions to drive the same state-specific processes and conclude that the few-step sequential MP-STIRAP isomerization is a feasible alternative to the sequential (1+1)-STIRAP based method proposed by Kurkal and Rice [J. Phys. Chem. B 105, 6488 (2001)].
The interaction of two narrow band (Δν<100 MHz) UV light pulses of different intensity with a molecular three-level system is investigated experimentally. The laser frequencies are tuned to an up (pump) and a down (dump) transition sharing a common excited rovibronic S1 level whose population is probed by a transition to the ionization continuum and ion detection. The time sequence of the two pulses results either in a lambda type coherent stimulated Raman adiabatic passage or a stimulated emission pumping (SEP) process. When the first case is realized by a 6.4 ns delay of the low intensity pump laser from the high intensity dump laser pulse, a fourfold increase of the depth of the ion dips compared to the SEP experiment is observed. This is in line with numerical calculations of the level populations using a density matrix formalism including coherent effects. Rotationally resolved ion dip spectra of the 62 state of benzene are presented and demonstrate the high sensitivity of the coherent excitation process of this work for ion dip spectroscopy.