Time Domain Spectroscopy of Terahertz Radiation from Argon Clusters Irradiated by Intense Femtosecond Laser Pulses

The electromagnetic wave ranging from the millimeter wave to gamma ray is radiated from plasmas produced by irradiating solid and gas targets with high intensity optical pulses. The THz wave radiation from the plasmas produced by femtosecond laser pulses has been intensively studied by many groups [1-3] these few years since it is one of the promising intense THz radiation sources. Solid THz emitters such as nonlinear crystals and photoconductive antennas are easily damaged by the intense optical pulses. However, there is no limitation on intensity of the optical pulses producing plasmas. Thus, the intense THz radiation from the plasmas is expected by using the intense optical pulses. So far, we have investigated the radiation from argon clusters irradiated by the intense femtosecond optical pulses by using the liquid He cooled InSb hot electron bolometer that is of course unable to observe the waveforms of the THz radiation. From the results we obtained so far, the THz radiation from the argon clusters is gained by the large laser energy absorption of the clusters. However, the mechanisms of the THz radiation from the laser produced plasmas are unclear at present. In order to clarify the radiation mechanism, it is important to observe waveforms of the THz pulses radiated from the plasmas. Thus, we carried out the measurements of the THz radiation from the argon clusters by using the time domain spectroscopy with electro-optic (EO) sampling technique.