Luke Maidment

Stand-off identification of aerosols using mid-infrared backscattering Fourier-transform spectroscopy.

We show that aerosolized liquid chemicals can be identified from their optical absorption spectra by illuminating them with broadband mid-infrared pulses from an optical parametric oscillator and analyzing the backscattered light using Fourier-transform spectroscopy.

Singly and doubly resonant femtosecond optical parametric oscillators for precision spectroscopy from the near-to mid-infrared

Optical parametric oscillators (OPOs), pumped by Ti:sapphire and Yb-doped femtosecond lasers, provide unique capabilities to address a broad range of parameters of interest to precision spectroscopy. We review here a variety of OPOs under development that offer tuning from 1.5 to 13 μm, repetition rates from 100 MHz to 10 GHz and pulse durations from < 25 fs to a few picoseconds. Spectroscopic techniques revealing the individual frequency comb modes are discussed, along with dual-comb spectroscopy at 3 μm and from 6-8 μm.

Chemical detection using broadband femtosecond optical parametric oscillators in the 6-12-micron spectral fingerprint region

The broad and slowly varying spectral features of liquids and solids require broadband sources in the mid- to long-wave infrared for their detection and identification. We present here a range of measurements made using uniquely tunable femtosecond optical parametric oscillators, which have enabled stand-off Fourier-transform spectroscopy to be implemented across a large part of the spectral fingerprint region. In this way we have achieved active stand-off detection of liquids on surfaces, of powders and of airborne liquid particles in aerosol form. We discuss the optical parametric oscillator technology, the spectroscopy implementations and the detection capabilities and limitations of the techniques.

Infrared fingerprint-region aerosol spectroscopy

Mie theory predicts aerosols should exhibit different mid-infrared spectral signatures to bulk materials. We verify this experimentally using broadband pulses from a 7.1–8.85-μm femtosecond optical parametric oscillator, obtaining close agreement with theoretical calculations.

Compressive sampling for spectral imaging

Active spectral imaging in the mid-infrared can reveal absorption ‘fingerprints’ of different chemicals within a scene. The practicality of spectral imaging with active illumination is limited by the large number of images required (as the illumination wavelength or an interferometer mirror is scanned). Single pixel imaging uses the intensity reading from a single pixel detector while illuminating the scene with a series of speckle patterns, and reconstructs the image using a projection algorithm [1]. We have followed this approach, using an algorithm to reconstruct spectral information, rather than spatial.

Molecular spectroscopy from 5-12 μm using an OP-GaP OPO

We report a femtosecond optical parametric oscillator (OPO) based on the new semiconductor gain material orientation patterned gallium phosphide (OP-GaP) and being the first example of a broadband OPO operating across the molecular fingerprint region. OP-GaP crystals with lengths of 1 mm and several patterning periods were diced, polished, and antireflection (AR) coated for near- to mid-infrared wavelengths. We configured a synchronously pumped OP-GaP OPO in a 101.2-MHz resonator with high reflectivity from 1.15–1.35 μm, pumped with 150-fs pulses from a 1040-nm femtosecond laser (Chromacity Spark). The coating of one spherical mirror was optimized for transmission at the pump wavelength of 1040 nm and for high reflectivity at the resonant signal wavelength in a range from 1.15–1.35 μm, while the other spherical mirror collimated the idler beam emerging from the OP-GaP crystal and was silver coated to provide high reflectivity for all idler wavelengths. This collimated idler beam was output-coupled from the cavity by transmission through a plane mirror coated with high transmission for the idler wavelengths (5–12 μm) and high reflectivity for the signal wavelengths (1.15–1.35 μm) on an infrared-transparent ZnSe substrate. Idler spectra centered from 5.4–11.8 μm and extending to 12.5 μm were collected. The maximum average power was 55 mW at 5.4 μm with 7.5 mW being recorded at 11.8 μm. Details of Fourier transform spectroscopy using water vapor and a polystyrene reference standard are presented.

Stand-off identification of aerosols using mid-infrared backscattering Fourier-transform spectroscopy

We show that aerosolized liquid chemicals can be identified from their optical absorption spectra by illuminating them with broadband mid-infrared pulses from an optical parametric oscillator and analyzing the backscattered light using Fourier-transform spectroscopy.

Optical Frequency Combs Based on Femtosecond Optical Parametric Oscillators

The development of femtosecond frequency combs based on optical parametric oscillators (OPOs) in our group is summarized. Dual combs generated in a single OPO cavity have been showed applications in precise spectroscopy.

Stand-off detection of aerosols using mid-infrared backscattering Fourier transform spectroscopy

The spectrum of mid-infrared light scattered from an actively illuminated aerosol was used to distinguish between different chemicals. Using spectrally broad illumination from an optical parametric oscillator covering 3.2 – 3.55 μm, characteristic absorption features of two different chemicals were detected, and two similar molecules were clearly distinguished using the spectra of backscattered light from each chemical aerosol.