THz wave generation from solids, gases and plasmas has been demonstrated for decades. However, the generation of THz waves from liquids, especially liquid water or its plasma, has never been studied. One of the possible reasons is that liquid water has very strong absorption characteristics in the THz frequency range, causing it to be sworn off as a source for THz radiation. In fact, liquid water has been well studied as a source for various electromagnetic waves for over 10 years. For example, white-light generation and high-harmonic generation through nonlinear processes from ultrashort-pulse lasers focused into water contained in cells, jets, or droplets were previously reported. Recently, we demonstrate the generation of broadband THz waves from liquid water film excited by femtosecond laser pulses. 

For those interested on this topic we recommend reading:

Jin, Q., E, Y., Williams, K., Dai, J. & Zhang, X. C. Observation of broadband terahertz wave generation from liquid water. Applied Physics Letters111, 071103, doi:10.1063/1.4990824 (2017).

Jin, Q., Dai, J., E, Y. & Zhang, X.-C. Terahertz wave emission from a liquid water film under the excitation of asymmetric optical fields. Applied Physics Letters113, 261101, doi:10.1063/1.5064644 (2018).

E, Y., Jin, Q., Tcypkin, A. & Zhang, X.-C. Terahertz wave generation from liquid water films via laser-induced breakdown. Applied Physics Letters113, 181103, doi:10.1063/1.5054599 (2018).



Under intense pulsed laser excitation the very air that we breathe is capable of generation and detection of terahertz, THz, electromagnetic waves.The use of air, instead of nonlinear crystals or ultrafast antennas, as both the emitter and the sensor lead to development of THz systems with unprecedented broad bandwidth and high peak electric field with application in imaging, linear and nonlinear material characterization. Moreover, using this techniques, remote generation and detection of THz waves is feasible, therefore one can envision standoff detection of threat or hazardous substances.

We are interested in both the technical development and the theoretical comprehension of the generation and the detection process.The most efficient way of generating THz in air is to focus dual-color laser pulses, where the fundamental oscillation is mixed with its second harmonic. At the focus the intensity has to be high enough to ionize the medium making free electrons available. The dual color excitation creates an asymmetric transient transverse current radiating THz pulses in the far field.

In our research we put great effort in developing detection techniques with sufficient bandwidth and sensitivity to be combined with the air THz source. In this direction we implemented several schemes employing air as the THz detector too. From now on the air detector will be referred to as the "plasma sensor", since for those techniques to work the air has to be ionized creating plasma.The most sensitive of those detection schemes, providing high dynamic range and SNR, is the so-called ABCD, Air-Biased-Coherent-Detection. This technique is an implementation of heterodyne detection and requires the presence of two electrodes in proximity of the plasma sensor.More suitable to long distance detection are the techniques called REEF, Radiation Enhanced Emission of Fluorescence, and TEA, Terahertz Enhanced Acoustics. In those techniques the THz radiation amplitude and phase are encoded in the fluorescence and acoustic emission of the plasma sensor. Therefore, we can claim to be able to "see" and "hear" the THz wave.

For those interested on this topic we recommend reading:

Clough, B., J. M. Dai and X. C. Zhang, "Laser air photonics: beyond the terahertz gap." Materials Today 15(1-2): 50-58. (2012)

Fabrizio Buccheri, X.-C. Zhang,"Terahertz emission from laser-induced microplasma in ambient air",Optica, Vol. 2, Issue 4, pp. 366-369 (2015)



Developing intense table-top THz source is always one of our priorities. In spite of using ambiant air, we also explore other materials as THz emitters, such as metallic vapor and metallic film. For those who's interested on these projects please read:

Jianming Dai, X.-C. Zhang, "Terahertz wave generation from thin metal films excited by asymmetrical optical fields",Optics Letters, Vol. 39, Issue 4, pp. 777-780 (2014)

Xuan Sun, X.-C. Zhang, "Terahertz radiation in alkali vapor plasmas", Appl. Phys. Lett. 104, 191106 (2014)