Abstract
The integration of ambient air plasmas as source and sensor in terahertz time-domain techniques allows spectral measurements covering the elusive terahertz gap (0.1–10 THz), further increasing the impact of those scientific tools in the study of the four states of matter. In this article we describe the experimental study of the terahertz emission from a laser-induced plasma of submillimeter size. The main direction of emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. We show that laser pulse energies lower than 1 μJ are sufficient to generate measurable terahertz pulses from ambient air. This significant decrease in the required laser energy will make plasma-based terahertz techniques more accessible to the scientific community.
Highlights
Terahertz (THz) time-domain techniques, such as time-resolved THz spectroscopy (TRTS) [1] and THz time-domain spectroscopy (THz-TDS) [2], are becoming widely used tools to study the dynamics and equilibrium properties of the four states of matter [3,4,5,6]. In most studies those techniques were implemented using solid-state THz emitters and sensors, inorganic nonlinear crystals or photoconductive antennas, which limited the spectral coverage of the measurements to a few THz
For nonequilibrium studies, it allows simultaneous access to correlated dynamics associated with different excitation channels having spectral signatures in separate frequency regions
From an application standpoint, one should consider the presence of THz spectral signatures in chemicals relevant to security applications, e.g., explosives and drugs, across the entire THz gap [8,9]
Summary
Terahertz (THz) time-domain techniques, such as time-resolved THz spectroscopy (TRTS) [1] and THz time-domain spectroscopy (THz-TDS) [2], are becoming widely used tools to study the dynamics and equilibrium properties of the four states of matter [3,4,5,6]. The requirement for the air to be ionized together with the use of low-NA focusing limits the realization of THz plasma sources with low repetition rate amplified ultrafast laser systems (with millijoule pulse energies). Other features of interest are that the THz waves are radiated almost orthogonally to the laser propagation direction, and that the generation volume is subwavelength compared to the emission wavelength (1 THz 300 μm).
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