Confocal two-photon spectroscopy of red mercuric iodide

One- and two-photon fluorescence of red mercuric iodide single crystal was studied using confocal laser scanning microscopy and spectroscopy at room temperature. There is a clear difference in the fluorescence spectra for one- and two-photon excitation. With two-photon excitation, a fluorescent band is found at the band gap and its central wavelength shifts toward lower energy with increasing depth below the surface, whereas the fluorescent band for one-photon excitation remains invariant with depth. There is an approximately 70 μm thick defect transition layer near the surface and the defect concentration decreases approximately linearly from the surface to the bulk. The band-gap energy at room temperature is determined as 2.12 eV, which is consistent with previous studies.