Enhanced RadOptic sensitivity in intrinsic bulk InP

RadOptic detectors are a promising method to record MeV pulsed radiation for inertial confinement fusion diagnostics. Because of the low radiation-induced refractive index change sensitivity of semiconductors, these detectors have poor responses to MeV gamma pulses. An enhancement of the radiation-refractive index change sensitivity in an InP sample is reported. The refractive index change of the InP sample was recorded by a plate interferometer. The probe laser had a wavelength of 1550 nm (0.8 eV). The excess carrier density was obtained using the refractive index modulation theory and evolution model. For the InP sample, the excess carrier density calculated using the refractive index change was much higher than the excess density calculated by the evolution model. This suggested that the refractive index change of the InP sample is more sensitive to the excess carrier density. The enhancement was attributed to the inter-conduction transition from the Γ valley to the X valley (Γ→X, 0.78 eV). In addition to enhancing the energy deposition efficiency, the radiation-refractive index change sensitivity of RadOptic sensors can also be enhanced by extra absorptions arising from inter-conduction band transitions.