Near-infrared refractive index of synthetic single crystal and polycrystalline diamonds at high temperatures

Abstract

We measured the refractive index n(T) and thermo-optical coefficient β(T) = (1/n)(dn/dT) of high quality synthetic diamonds from room temperature to high temperatures, up to 1520 K, in near-infrared spectral range at wavelength 1.56 μm, using a low-coherence interferometry. A type IIa single crystal diamond produced by high pressure–high temperature technique and a transparent polycrystalline diamond grown by chemical vapor deposition were tested and revealed a very close n(T) behavior, with n = 2.384 ± 0.001 at T = 300 K, monotonically increasing to 2.428 at 1520 K. The n(T) data corrected to thermal expansion of diamond are well fitted with 3rd order polynomials, and alternatively, with the Bose-Einstein model with an effective oscillator frequency of 970 cm−1. Almost linear n(T) dependence is observed above 800 K. The thermo-optical coefficient is found to increase monotonically from (0.6 ± 0.1) × 10−5 K−1 (300 K) to (2.0 ± 0.1) × 10−5 K−1 (1300 K) with a tendency to saturation at >1200 K. These β(T) values are an order of magnitude lower than those known for Si, GaAs, and InP. The obtained results significantly extend the temperature range, where the refractive index of diamond was previously measured.