Temperature-dependent Sellmeier equations of nonlinear optical crystal Na3La9O3(BO3)8

• Thermal refractive index coefficient expressions were derived for the first time.
• Temperature-dependent Sellmeier equations were fitted accurately.
• Thermal refractive index coefficients change from positive to negative values.
• PM-I directions for THG at different temperatures were calculated and measured.

The principal refractive indices of Na3La9O3(BO3)8 (NLBO) crystal in the wavelength range of 0.363–2.325 μm were accurately measured by using the minimum deviation method within the temperature range from 23.5 °C to 160 °C. We derived the expressions of thermal refractive index coefficient as a function of wavelength that could be used to calculate the principal refractive indices at different wavelengths. The temperature-dependent Sellmeier equations were also derived and used to calculate the phase-matching (PM) angles for a frequency conversion device based on NLBO crystal at different temperatures. We found that the thermal refractive index coefficients of NLBO crystal changed from positive to negative values with the increase of wavelength. In addition, the phase matching conditions for third harmonic generation (THG) at different temperatures were also investigated.