Photoionization of atmospheric gases studied by time-resolved terahertz spectroscopy

Terahertz white-light pulses from an air plasma photo-induced by incommensurate two-color optical fields

We demonstrate plasma-THz generation in ambient air with two-color (ω-2ω) optical excitation using input pulses with a bandwidth supporting sub-20-fs duration, yielding a continuous bandwidth exceeding 100 THz with no significant roll-off and a pulse energy of 360 nJ at a 1-kHz repetition rate. The key aspect in achieving this performance is an optimized geometry of the second-harmonic generation (SHG) crystal, producing a 2ω-field detuned from the second-harmonic of the ω-field, which promotes both a high polarization bandwidth and optimal ω-2ω temporal overlap in the plasma, as supported by theoretical results.

Ultrafast laser-based spectroscopy and sensing: applications in LIBS, CARS, and THz spectroscopy

Ultrafast pulsed lasers find application in a range of spectroscopy and sensing techniques including laser induced breakdown spectroscopy (LIBS), coherent Raman spectroscopy, and terahertz (THz) spectroscopy. Whether based on absorption or emission processes, the characteristics of these techniques are heavily influenced by the use of ultrafast pulses in the signal generation process. Depending on the energy of the pulses used, the essential laser interaction process can primarily involve lattice vibrations, molecular rotations, or a combination of excited states produced by laser heating. While some of these techniques are currently confined to sensing at close ranges, others can be implemented for remote spectroscopic sensing owing principally to the laser pulse duration. We present a review of ultrafast laser-based spectroscopy techniques and discuss the use of these techniques to current and potential chemical and environmental sensing applications.