Mapping the nanoplasma regime using harmonics

Laser-irradiated microclusters exhibit a range of useful and scienti cally-relevant phenomena, including e cient absorption, harmonic generation, and anomalous dispersion. These properties make them useful for applications such as particle accelaration, nuclear fusion, and phase-matching for UV and X-ray sources. The generation of third harmonic light by intense illumination of pre-heated expanding clusters can be used as a way of mapping the nanoplasma regime, characterized by partial removal of the ionized cluster's electron core. We present measurements of this harmonic generation, showing a drop in expansion-related enhancement between probe intensity I[subscript pr] = 1.5 x 10¹⁶ W/cm² and I[subscript pr] = 1.0 x 10¹⁸ W/cm², where the harmonic generation vanishes. This is consistent with our model of cluster ionization, expansion, and polarizability, which we also use to estimate the cluster size distribution.