Researchers find liquids can enable high-power terahertz radiation

October 31, 2017 //By Jean-Pierre Joosting
Scientists at the Tata Institute of Fundamental Research (TIFR), Mumbai have devised a high power radiation source in the much sought after terahertz (THz) region of the electromagnetic spectrum. The study was done in collaboration with laboratories in Greece and France and is described in the journal Nature Communications.

High power terahertz radiation has typically been available from large, complex machines like Free Electron Lasers. Compact sources, relying on semiconductor antennas and special crystals excited by visible/infrared femtosecond laser pulses, have very limited energy outputs, typically in the nanojoule (billionth of a joule) level or lower. They are not useful for many applications. High power femtosecond lasers have however, excited terahertz emission that are a thousand times larger (microjoules) from a plasma formed in air, under special conditions [1].

For a long time, researchers in this area have believed that liquids could not give out significant terahertz radiation, because they would efficiently reabsorb whatever was generated. Yet, this is where the TIFR researchers proved successful. In their experiments, they irradiated common laboratory liquids like methanol, acetone, dicholorethane, carbon disulphide and even water, with moderate energy femtosecond laser pulses, ionizing the liquid and forming long plasma channels called filaments. They measured energies as high as 50 microjoules, thousands of times larger than the energies emitted by most existing sources and 10-20 times larger than those produced from air.

(a) A high power laser pulse ionizes and creates plasma in a common laboratory liquid like acetone or dichloroethane or even water. The plasma can be seen as a long line along the length of the tube containing the liquid (see inset). This line is called a filament and it radiates, among others, copious terahertz radiation. (b) Integrated terahertz energy obtained from various liquids and air. (c) Power spectrum of terahertz obtained from the filamentation of acetone, ethanol and air. Image courtesy of Indranuj Dey, G. Ravindra Kumar.