Triplet-to-singlet conversion of 1s excitons in β-ZnP 2

Abstract

We observe the luminescence spectrum of the singlet exciton polariton ( E‖c) in β-ZnP 2 at 2 K under excitation into the lowest triplet exciton absorption ( E‖b) by measuring the excitation spectra. A sharp excitation peak is observed at the triplet exciton energy E t for each excitation spectrum, which appears through the anti-Stokes acoustic phonon scattering of the triplet excitons into the singlet exciton polariton states lying above 2 meV. The luminescence spectrum is obtained by plotting the intensities of the excitation peaks against the monitoring energy. The spectrum has similar shape to the singlet polariton luminescence. The excitation efficiency increases steeply again below the low-energy tail of the ordinary singlet luminescence. Since the singlet exciton polariton has the character of light in the low-energy region, the scattered polariton goes out easily from the crystal surface. As the monitoring energy is lowered, the excitation spectra have almost equidistant (∼0.14 meV) satellite peaks on both sides of the main peak at E t. The energy distance corresponds to the LA phonon energy connecting the light branch with the bottleneck in the triplet exciton polariton.