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. Recently, several theoretical works have pointed out the possibility that laser produced plasmas radiate intense THz wave [1]. The laser produced plasma is one of the promising candidates for an efficient generator of the THz wave, the THz wave radiated from the laser produced plasmas, however, has not been well studied. So far, there are only a few reports regarding the THz radiation from laser produced plasmas in which solid and gas targets are used [2, 3]. Since gas targets usually exhibit low absorption of the laser energy, the THz radiation is expected to be weak. Although solid targets exhibit very large laser energy absorption, the efficiency of the THz radiation is limited by large mismatch of the impedance between the plasma and free space. It is well known that atomic clusters have large laser energy absorption [4]. In addition, since the average density of the plasmas produced from the cluster targets is low, the clusters will be transparent in the THz wave region. The laser produced plasmas with using the cluster targets, therefore, is expected to be one of the promising efficient THz sources. In this report, we present our study of THz radiation from the laser produced plasmas with using argon cluster targets. The radiated THz wave from the cluster targets was one or two order in magnitude stronger than that from the gas targets. We prove that the enhancement of the power of the THz radiation from the cluster targets originates mainly from large laser energy absorption by the clusters.