Micro-plasma inspired THz liquid photonics

电磁波谱的远红外区域（0.3-10太赫兹）一直被认为是电磁波谱中最后一个遗留的科学空白，这是一个尚未开发但开发成熟的领域。由于种种原因，这一领域显示出基础科学研究的巨大前景。众所周知，正常的宇宙物质由四种状态组成：固体、液体、气体和等离子体。固体、气体和等离子体产生太赫兹波已经被证明、使用和理解了几十年。从液态水中产生太赫兹波一直被认为是不可能的。这是一个科学上的好奇：为什么物质中只有固体、气体和等离子体三种状态可以用来产生太赫兹波，而液体却不能。在包括水在内的液体中，仅仅是因为太赫兹频率范围内的吸收系数大吗？太赫兹液体光子学可能是解决这一难题的关键项目之一。我们有理由认为，如果液体可以作为太赫兹源加以利用，它们可能具有独特的特性。液体的分子密度很高，接近固体的分子密度，这意味着在一定区域上的光会比同等截面的气体与更多的分子相互作用。这使得液体成为研究高能量密度等离子体的很好的候选者。对液体产生太赫兹的成功研究可能会完成太赫兹群落中物质温度-压力相图中太赫兹源的物质相谜题的最后一块。我们提出的太赫兹液体光子学的科学研究重点是利用飞秒激光诱导微等离子体从精心挑选的液体中产生太赫兹。在太赫兹空气光子学中，与环境空气相比，液体的电离能要低得多（例如，水的电离能仅为6.5 eV），但分子密度要高3个数量级，这意味着在相同的电离体积内可以提供更多的带电粒子。与固体晶体相比，在产生过程中避免了相位匹配条件和晶体声子吸收，这极大地限制了固体源产生的太赫兹脉冲的带宽。此外，液体中的击穿不是永久性的损坏，它将通过电子-离子复合自然消除。液体的流动性也可以为下一个脉冲提供一个新的相互作用区域。所有这些优点使液体成为很有前途的太赫兹源。因此，研究水产生太赫兹波的机理势在必行。将研究具有不同吸收系数（极性）、分子密度、光电离阈值和粘度的各种液体。我们将利用激光诱导微等离子体对液体中的太赫兹光子学进行研究。我们特别建议对液体中宽带太赫兹波的产生进行系统的研究，包括水溶液（正常和重水，盐水，糖水，酒精溶液）和20多种不同的液体（具有不同的极性和pH值）。将采用双光脉冲激发法产生太赫兹波。我们将测试不同的液体溶液，从水膜和线到水滴和雾。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The nontechnical description of the project The far-infrared region of the electromagnetic spectrum (0.3-10 THz) has long been considered the last remaining scientific gap in the electromagnetic spectrum, which is underdeveloped but ripe for exploitation. This field shows great promise of basic science research for a variety of reasons. It is known that normal universal matters are made of four states: solid, liquid, gas, and plasma. The generation of THz wave from solids, gases, and plasmas has been demonstrated, used, and understood for decades. To generate THz waves from liquid water has long been considered as impossible in THz community. This is a scientific curiosity: why can only three states in matter: solid, gas and plasma, be used to generate THz waves, but not liquid Among liquids, including water, is it just due to the large absorption coefficient in THz frequency range. The THz liquid photonics could be one of the key projects to place the remaining piece to the missing puzzle. It is reasonable to expect that liquids might have unique properties if they could be harnessed as THz sources. Liquids have a high molecular density, close to that of solids, meaning that light over a certain area will interact with many more molecules than an equivalent cross-section of gases. This makes liquids very good candidates for the study of high-energy-density plasma. A successful investigation in THz generation from liquids might complete the last piece of the matter-phase puzzle for THz sources in the matter temperature-pressure phase diagram in THz community. The technical description of the project Our proposed scientific investigation of THz liquid photonics focuses on THz generation from carefully selected liquids by using femtosecond laser induced micro-plasma. Compared with ambient air in THz air photonics, liquids have much lower ionization energy (for example, only 6.5 eV for water), but 3 orders higher molecular density, which means more charged particles can be provided in the same ionized volume. Compared with solid crystals, phase matching conditions and crystal phonon absorptions are avoided in the generation process, which significantly limit the bandwidth of the THz pulse generated from solid sources. Additionally, the breakdown in liquids is not a permanent damage, which will be naturally erased through electron-ion recombination. Liquid fluidity can also provide a fresh interaction area for the next pulse. All these superiorities make liquids a promising THz source. Therefore, it is imperative to investigate the mechanism of the THz wave generation from water. A variety of liquids with different absorption coefficient (polarity), molecular density, photo-ionization threshold, and viscosity will be investigated. We will conduct a study of THz photonics in liquids by using laser induced micro-plasma. Specially we propose to perform systematic study of the generation of broadband THz waves in liquids, include aqueous solutions (normal and heavy water, salty water, sugar water, alcohol solution), and over 20 different liquids (with different polarity and pH levels). Double optical pulse excitation method of THz wave generation will be used. We will test different liquid solutions, from water film and line to droplet and mist.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.