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 ( σ 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 σ R are 3.0± 0.6×10−28 and 1.1± 0.2×10−28 cm2 at 811 and 1112 nm, respectively, compared with the corresponding values of 2.8×10−28 cm2 and 0.7×10−28 cm2 extrapolated from the values measured at 647 nm (1.92 eV) by Calleja et al. in 1981, using the λ − 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 + Δ 0 ) at 2.86 eV.