Abstract

Matters are generally classified within four states: solid, liquid, gas, and plasma. Three of the four states of matter (solid, gas, and plasma) have been used for THz wave generation with short laser pulse excitation for decades, including the recent vigorous development of THz photonics in gases (air plasma). However, the demonstration of THz generation from liquids was conspicuously absent. It is well known that water, the most common liquid, is a strong absorber in the far infrared range. Therefore, liquid water has historically been sworn off as a source for THz radiation. Recently, broadband THz wave generation from a flowing liquid target has been experimentally demonstrated through laser-induced microplasma. The liquid target as the THz source presents unique properties. Specifically, liquids have the comparable material density to that of solids, meaning that laser pulses over a certain area will interact with three orders more molecules than an equivalent cross-section of gases. In contrast with solid targets, the fluidity of liquid allows every laser pulse to interact with a fresh area on the target, meaning that material damage or degradation is not an issue with the high-repetition rate intense laser pulses. These make liquids very promising candidates for the investigation of high-energy-density plasma, as well as the possibility of being the next generation of THz sources.

Highlights

  • We review recent studies of THz wave generation from flowing liquid targets as well as the ultrafast dynamics of liquids under intense THz excitation

  • This can be achieved through geometric focusing of intense laser pulses with lenses or curved mirrors

  • The results show that the THz signal generated from water is much stronger than that from ambient air

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Summary

Introduction

With tremendous advancements in laser technology, lightinduced ionization in matter including gases [1,2,3,4,5,6], clusters [7,8,9], liquids [10,11,12,13,14,15,16], and solids [17,18,19,20] has attracted considerable interest in generating coherent, intense, and broadband terahertz (THz) waves via nonlinear processes. The laser-induced air plasma has become one of the most popular THz sources, generating THz waves with a field strength over the MV/cm level under two-color (fundamental and its second harmonic beams) optical excitation [26, 27]. This opens up a new avenue of extreme THz science [28]. In 2000, it was reported by Cook and Hochstrasser [3] that the generation efficiency of THz waves from laserinduced gas-plasma is significantly enhanced under twocolor excitation This technology is a milestone in the THz community due to its capability of generating an intense THz pulse with a peak field of over MV/cm as well as an ultrabroad bandwidth. Terahertz liquid photonics shows a potential to develop new-type THz sources and offers a new perspective to investigate the process of laser-liquid interaction

THz Wave Generation from Air
THz Wave Generation from Water
Modulated
THz Wave Generation from Other Liquids
Pump 1
THz Wave Enhancement under DoublePump Optical Excitation
Ultrafast Dynamics in Liquids Excited by Intense THz Waves
M NaCl
Findings
Outlook
Full Text
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