Micro-plasma inspired THz liquid photonics

电磁频谱的远红外区域（0.3-10 THz）长期以来一直被认为是电磁频谱中最后剩下的科学空白，虽然尚未开发，但开发的时机已经成熟。由于各种原因，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。从固体、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。利用液态水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心：为什么物质中只有三种状态：固体、气体和等离子体，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，是否仅仅是由于太赫兹频率范围内的大吸收系数。太赫兹液体光子学可能是解决这一难题的关键项目之一。如果液体可以被用作太赫兹源，那么可以合理地预期液体可能具有独特的性质。液体具有高分子密度，接近固体，这意味着在一定区域内的光将与比气体等效横截面更多的分子相互作用。这使得液体非常适合研究高能量密度等离子体。液体产生THz的研究成功，将为THz界物质温压相图中THz源的物质相之谜提供最后的解答。 我们提出的太赫兹液体光子学的科学研究重点是通过使用飞秒激光诱导的微等离子体从精心选择的液体中产生太赫兹。与THz空气光子学中的环境空气相比，液体具有低得多的电离能（例如，水仅为6.5eV），但分子密度高3个数量级，这意味着在相同的电离体积中可以提供更多的带电粒子。与固体晶体相比，固体光源在产生太赫兹脉冲的过程中避免了相位匹配条件和晶体声子的损耗，从而大大限制了固体光源产生太赫兹脉冲的带宽。此外，液体中的击穿不是永久性损伤，其将通过电子-离子复合自然消除。液体流动性也可以为下一个脉冲提供新的相互作用区域。这些优点使液体成为一种很有前途的太赫兹光源。因此，研究水产生太赫兹波的机理是十分必要的。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的微等离子体在液体中进行太赫兹光子学的研究。特别是，我们建议在液体中产生宽带太赫兹波进行系统的研究，包括水溶液（正常和重水，盐水，糖水，酒精溶液），和超过20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方法产生太赫兹波。我们将测试不同的液体解决方案，从水膜和线到液滴和薄雾。该奖项反映了NSF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.