Work

Method for measuring the refractive index and extinction coefficient of liquid metals to determine the angular and temperature dependent laser absorptivity

Year: 2025

Type: article

Abstract: Accurate simulations of laser material processing techniques require realistic values of laser absorptivity as a function of angle of incidence and material temperature. The laser absorptivity can be calculated with the Fresnel equations, for which one needs the temperature dependent values of the refractive index n and extinction coefficient k of the material for the laser wavelength in question. However, there are hardly any published values for the n and k of metals at high temperatures, especially in the liquid state. This proof-of-principle paper presents a novel and simple experimental method for measuring the refractive index and extinction coefficient of liquid metals and thus their absorptivity as a function of angle of incidence and temperature for a short wavelength range. The method has been developed to determine n and k through the thermal emission over the curvature of a spheroidal droplet and is therefore not based on reflection measurements, which require a flat surface. The thermal emission was recorded using a linear polarizer, a bandpass filter and a camera. The method is demonstrated for liquid copper at a wavelength of 850 nm, and the results are in good agreement with published absorptivity values at a similar wavelength (1064 nm) and temperature range. It was found that the accuracy of the measurements in future work could be greatly increased by using spatial resolutions ≥ 1000 pixels/radius.

Source: Optics & Laser Technology

DOI: 10.1016/j.optlastec.2025.114353

Authors: Michael Sawannia, Julien Daligault, John Powell, Morgan Dal, Christian Hagenlocher, Thomas Graf