Measuring the complex refractive index of metals in the solid and liquid state and its influence on the laser machining

Abstract

Knowing the optical properties of metals in solid and liquid phase is important for understanding and optimizing material processing. But it is difficult to find reliable data, especially for the wavelength range in the near-infrared. Further there are several approaches of extending the Drude-model for optical properties of metals including temperature dependency and intra-band absorption, which can describe the qualitatively behavior of optical properties as a function of temperature and wavelength. Although these extended models can predict accurately the optical properties for some specific metals and wavelengths, in general they fail to predict accurate values. Comparing our own experimental results of gold and silver in the solid and liquid phase at near-infrared with extended Drude-models has revealed that even a combined extended Drude-model taking temperature dependency, anomalous skin effect and intra-band absorption into account, cannot predict the optical properties accurately. In general the temperature dependency of the optical properties of metals is much weaker than predicted by the various models. Additionally analyzing the refractive index and absorption coefficient of metals at 1.06μm and 10.6μm has shown a difference of approximately a factor of 10 between 1.06μm and 10.6μm causing a sharp absorption peak at 10.6μm compared to a broaden peak at 1.06μm. This factor of 10 is much larger than the difference of the optical properties between the solid and the liquid phase and has a great influence on the laser energy distribution absorbed in the metal during laser processing.