Micro-plasma inspired THz liquid photonics

点击翻译按钮获取中文摘要

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.