Measurement of the absolute Stokes Raman cross sections of the longitudinal optical (LO) phonons of room-temperature GaP

Absolute first-order Stokes Raman cross sections (sigma(R)) of the longitudinal optical (LO) phonons of room-temperature GaP at 811 nm (1.53 eV) and 1112 nm (1.12 eV) have been measured, using pump/excitation wavelengths of 785 (1.58 eV) and 1064 nm (1.17 eV), respectively. A temperature-controlled, small-cavity blackbody source was used to calibrate the signal output of the Raman spectrometer system; this allows a direct measurement of the absolute Raman cross section without the need for the use of a known material as the reference standard. Measurements were made on a GaP single-crystal plate with (001) faces, using backscattering geometry. The measured values of sigma(R) are 3.0 +/- 0.6 x 10(-28) and 1.1 +/- 0.2 x 10(-28) cm(2) at 811 and 1112 rim, respectively, compared with the corresponding values of 2.8 x 10(-28) cm(2) and 0.7 x 10(-28) cm(2) extrapolated from the values measured at 647 nm (1.92 eV) by Calleja et al. in 1981, using then lambda(-4) wavelength dependence and decrease of Raman polarizability with wavelength due to resonance enhancement associated with the direct energy gap (E(0)) at 2.78 eV and the split-off energy gap (E(0) + Delta(0)) at 2.86 eV. (C) 2009 Elsevier Ltd. All rights reserved.