长激光脉冲电离液体产生THz波

由于激光诱导物质(包括气体、团簇、液体和固体)电离可以产生高强度、宽谱线的太赫兹(THz)波,近些年来引起了人们的广泛关注。当强度超过电离阈值的激光脉冲聚焦到目标物质中,光电离产生电子和离子,在有质动力或电场的驱动下,这些带电粒子可以产生瞬态电流从而辐射电磁波,其光谱范围可以涵盖从微波到X射线。这些电磁波也可以用来表征和研究该目标物质。

在太赫兹频率范围内,激光诱导空气等离子体成为实验室最常用的太赫兹源之一。它可以通过双色光激发产生电场强度超过MV/cm量级的太赫兹波。人们也成功地证明了用激光脉冲电离液体(例如常温液态水)可以产生太赫兹波。利用液体的流动性可以为每个激发脉冲提供一个新的区域,这使得每一个脉冲都不会受前一个脉冲影响。由此,利用高强度、高重复频率的激光作为稳定的激发源成为了可能。

利用电离液体产生太赫兹波展现出许多与空气等离子体不同的特性。其中最值得指出的是,当把飞秒激光脉冲宽度展宽到亚皮秒量级时,液体所产生的太赫兹波也逐渐增强。而对于空气等离子体来说,往往越短的激光脉冲产生的太赫兹波越强。这个现象说明液体中的光电离过程与气体中的有所不同。

来自罗彻斯特大学的研究人员通过详细分析气体和液体中的电离过程,对液体“偏爱”长脉冲的现象进行了解释。研究成果发表在Advanced Photonics 2020年第1期上(Qi Jin, Yiwen E, Shenghan Gao, Xi-Cheng Zhang. Preference of subpicosecond laser pulses for terahertz wave generation from liquids[J]. Advanced Photonics, 2020, 2(1): 015001)。

通过计算电子密度,研究人员发现当激光脉宽增加时,虽然脉冲峰值功率相应降低,但液体中由电离产生的电子却增多。这是由于电子通过碰撞产生二级电离,从而导致电子密度呈指数式增长。对于气体来说,由于电子的平均寿命一般大于激光脉冲宽度,所以通常忽略由碰撞而导致的二级电离过程。

亚皮秒激光脉冲对电离液体产生太赫兹波

由电离液体产生太赫兹波是一个全新的研究方向。这项工作不仅揭示了辐射太赫兹波中激光脉冲宽度对激光诱导电离的影响,而且有助于进一步发展更强的液体太赫兹源。

Liquids prefer long laser pulse for radiating THz wave

Recently, laser-induced ionization in matter, including gas, cluster, liquid, and solid have attracted considerable interest in generating coherent, intense, broadband terahertz (THz) waves through nonlinear processes of laser-matter interaction. When a laser pulse with an intensity above the ionization threshold is focused into the target, photoionization creates electrons and ions, which can be driven by the ponderomotive force or electric fields, generating transient currents. These particles radiate electromagnetic waves covering the spectrum from microwaves to X-rays. Concurrently, radiated waves can be used to characterize target material.

In the THz frequency range, laser-induced air plasma becomes one of the most popular THz sources in the research laboratories. It can generate THz waves with a field strength over MV/cm level by two-color optical excitation. Recently, THz wave generation from ionized liquids, especially from liquid water, were also successfully demonstrated. Compared with a bulk liquid target, using a flowing liquid line shows its advantages, which provides a fresh area for each excitation pulse. Thus, the chaos caused by the previous pulse will not influence the next one.

In contrast to THz wave generation from air plasma, there are some distinct observations in THz wave generation from ionized liquids. The most important one is the preference of the excitation with a subpicosecond pulse, which means the photoionization process in liquids are different from that in gases. For a gas, the shortest pulse always generates strongest THz field. However, in liquids, longer pulse duration offers stronger THz emission.

Researchers from University of Rochester explain this preference in detail by discussing the ionization process in gas and liquid, respectively. Research results are published on Advanced Photonics, Vol. 2, Issue 2, 2020 (Qi Jin, Yiwen E, Shenghan Gao, Xi-Cheng Zhang. Preference of subpicosecond laser pulses for terahertz wave generation from liquids[J]. Advanced Photonics, 2020, 2(1): 015001).

By calculating the electron density, they find that the longer pulse duration generates more electrons in liquid. This is caused by the collision of electrons, which plays an important role in the ionization process. Herein, cascade ionization in the ionized liquid leads to an exponential increase in the number of electrons. For the gas case, however, the lifetime of electrons is always longer than the pulse duration, so the collision effect is always not considered.

Preference of subpicosecond laser pulses for terahertz wave generation from liquids

THz generation from ionized liquids is a new topic. This work not only unveils the influence of laser pulse duration on laser-induced ionization in radiating THz wave, but also contributes insight into the development of intense liquid THz sources.