Ultra-broadband, intense, coherent terahertz (THz) radiation can be generated, detected, and manipulated using laser-induced gas or liquid plasma as both the THz wave transmitter and detector, with a frequency coverage spanning across and beyond the whole “THz gap.” Such a research topic is termed “plasma-based THz wave photonics in gas and liquid phases.” In this paper, we review the most important experimental and theoretical works of the topic in the non-relativistic region with pump laser intensity below 1018 W/cm2.
THz liquid photonics is a new research frontier in laser-matter interaction community. We have successfully demonstrated THz wave generation from ionized liquids, including from liquid water, liquid nitrogen, and liquid gallium. Preferable to general targets, a flowing liquid line provides a fresh area for each excitation pulse, so the chaos and debris caused by the previous pulse will not influence the next one. This makes it possible of using a kHz repetition rate laser for excitation. THz wave generation from ionized liquids presents photoionization processes that are different from those in gases.
When an intense ultrafast laser pulse is focused into a free-flowing water film, broadband terahertz (THz) radiation is emitted through the interaction between strong laser fields and liquid water molecules. Experimental results show that THz waves generated from liquid water have very different characteristics in comparison to those from other THz sources such as solids and gases. The mechanism for the THz wave generation process is currently attributed to the plasma formation in the bulk water. This demonstration may find potential applications in biological imaging and provide new tools in the field of nonlinear laser-liquid interactions.
THz time domain spectroscopy has been largely applied on the measurement of semiconductor, electro-optic crystals,
and selected chemical, biological and explosive materials. The objective of this paper is to report THz gas photonics and
its applications, with an emphasis on remote sensing capabilities. The most recent results of using air (and selected
gases) as the emitter and sensor material for both generation and detection of broadband THz waves will be reported.
Air, especially ionized air (plasma), has been used to generate intense peak THz waves (THz field > 1.5 MV/cm) with a
broadband spectrum (10% bandwidth from 0.1 THz to 46 THz). THz-enhanced-fluorescence (TEF) and THz-enhanced
acoustic (TEA) techniques have been developed for remote sensing purpose. By "seeing" the fluorescence, or "hearing"
the sound, coherent detection of THz waves at standoff distance is feasible.
The significant scientific and technological potential of terahertz (THz) wave sensing and imaging has been attracted
considerable attention within many fields of research. However, the development of remote, broadband THz wave
sensing technology is lagging behind the compelling needs that exist in the areas of astronomy, global environmental
monitoring, and homeland security. This is due to the challenge posed by high absorption of ambient moisture in the
THz range. Although various time-domain THz detection techniques have recently been demonstrated, the requirement
for an on-site bias or forward collection of the optical signal inevitably prohibits their applications for remote sensing.
The objective of this paper is to report updated THz air-plasma technology to meet this great challenge of remote
sensing. A focused optical pulse (mJ pulse energy and femtosecond pulse duration) in gas creates a plasma, which can
serve to generate intense, broadband, and directional THz waves in the far field.
We report the first systematic study of broadband THz wave generation by using the focused femtosecond laser beams in ambient air. Generations of pulsed THz waves using air as the nonlinear media have been previously demonstrated by Cook et al. and Hartmut et al. We measured dependence of generated THz wave on the polarization, amplitude and phase of the individually controlled fundamental and second harmonic beams. Our results confirms that four-wave-mixing rectification is the major mechanism of THz wave generation with mixing the fundamental and the SH beams in air. This work is significant by providing the feasibility of standoff distance detection greater than 50 meters.
Based on the nonlinear ABCD matrix and the renormalized q- parameter for Gaussian-beam propagation, self-focusing in conjunction with a spatial gain profile for self-mode locking in a ring-cavity Ti:sapphire laser are analyzed. In the experiment, an astigmatism-compensated self-mode-locked ring-cavity Ti:sapphire laser is demonstrated, and self- mode-locked operation is achieved in both bidirection and unidirection with pulse durations as short as 36 fs and 32 fs, respectively. The experimental observations are in good agreement with theoretical predictions.
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