Imaging and spectroscopy of optical transition radiation from thin foils irradiated by ultraintense laser pulse*

Optical transition radiation (OTR) by relativistic electrons emerging the rear surface of target conveys numerous information on laser-target interaction. Imaging of the radiation shows the spatial distribution of hot electron beams. Spectrum of OTR infers the temporal structure of relativistic electron bunches. In this contribution, we present the OTR images and spectra from foil target with various thickness (100 nm -- 10 um) irradiated by intense laser pulses up to 10$^{\mathrm{20}}$ W/cm$^{\mathrm{2}}$. The significant modulation on OTR shape and spectra are observed from nano-foils at extremely high intensity. Pre-pulses also introduce significant difference in OTR spectra. Detailed data and possible mechanism for such modulations will be presented.