A difference-frequency based mid-IR broadband source for surface spectroscopy of explosives

Abstract

Abstract:We describe a 2.8 -4.4 µm tunable difference-frequency based mid-IR broadband source. With a FWHM of 170 nm centered at 3.3 µm, we have investigated the C-H stretch signatures of solid-phase trace explosives on surfaces.Mid-infrared (MIR) laser sources originating from femtosecond fiber lasers have gained recent attention as promising sources for molecular spectroscopy [1][2][3][4], combining broad spectral bandwidth and high resolution. In this study we also utilize the high brightness and power of a MIR laser to probe trace amounts of solid explosives on a surface using backscattered spectroscopy. A diagram of the MIR source is shown in Fig. 1(a). The 2.8 W amplified output of a Yb fiber laser, with 130 fs pulses and a 100 MHz repetition rate, is separated into two beams with a polarizing beamsplitter. A Raman-shifted soliton is formed in one arm by launching a portion of the power into a photonic crystal fiber (PCF). The soliton center wavelength can be tuned from 1.36 -1.66 m depending on the input power and polarization into the fiber [5]. MIR light is generated via simple difference frequency generation (DFG) in a 2-mm long fan-out periodically-poled lithium niobate (PPLN) crystal. The combination of the original laser output at 1.03 m and the Raman soliton results in an idler tunable from 2.8-4.4 µm, with a FWHM spectral width of ~170 nm, and average output power of up to 100 mW. Resulting MIR spectra are shown in Fig. 1(b). While this system has applications for high-resolution precision trace-gas spectroscopy [6], the low vapor pressure of explosives under typical environmental conditions makes the detection of outgassed molecules impractical for field applications [7]. We have thus begun to investigate the broadband spectroscopy of trace amounts of explosive residues deposited on surfaces, by examining the spectroscopic signature of MIR light scattered off the samples.By tuning the MIR laser center-wavelength to 3.3 µm the C-H stretch spectral region common to many organic molecules can be investigated. In particular, we have examined the high explosives RDX, HMX, Tetryl, and PETN. Starting with a solution of 1 mg explosive dissolved in 1 ml of solvent (typically a 1:1 ratio of methanol and acetonitrile), we deposit one drop at a time onto either a gold mirror or roughened aluminum plate, allowing the solvent to evaporate. Each drop contains ~ 20 µg of solid material, which allows us to simulate first-generation fingerprint quantities of ~ 20-200 µg/cm 2 [8]. An image of the prepared gold mirror is shown in the inset of Fig. 2(a).The optical system used to image scattered light from the explosive samples is shown in Fig. 2(a). The light from the laser system is directed at the solid sample, and a CaF 2 lens system is used to image scattered light to the input