Abstract

Single crystals of Hg₂Cl₂ are known to be highly birefringet optical material of technical importance for polarizing prisms and other optical components in the visible region. However, some fundamental physical properties of Hg₂Cl₂, such as its energy band structure, are not known up to now. Absorption spectrum in the vicinity of absorption edge (~3.9 eV) was measured recently /1/ showing a distinct structure, but unambiguous interpretation is difficult to be done due to the lack of any band structure calculation. Important complementary information can be obtained, however, from two-photon absorption experiments especially in systems possessing a center of inversion, what is the case of Hg₂Cl₂ crystals (D_2h point group for T < 185 K, D_4h for T > 185 K). Recently, we have reported for the first time observation of low-temperature two-photon absorption in Hg₂Cl₂ single crystals upon a pulsed dye-laser excitation and demonstrated the feasibility of its monitoring via a luminescence band at 396 nm (3.13 eV) /2/. In this contribution we wish to present and discuss two-photon exitation spectrum of the 396 nm emission band at T ~10 K in the region 4.16 ≤ 2hv ≤ 5.4 eV. The excitation spectrum is characterized by a gradual smooth increase starting from the low-energy side, a shoulder at 2hv ~4.8 eV and a dominating intense maximum at 2hv ~5.14 eV. Compared to the one-photon absorption spectrum /1/, one finds “alternating” maxima which are observed or in one-photon or two-photon experiments, but which are not common for the both cases. Conclusions are made on the symmetry of related Brillouin-zone points and wavefunctions.

© 1984 Optical Society of America