| Abstract [eng] |
Photothermal response of various materials differs and precise measurement of surface absorption using the photothermal common-path interferometry (PCI) technique requires calibration samples made of investigative material. For this purpose, calibration samples of various substrate materials (glass and crystals) with a coated thin semi-transparent chromium layer were made and the photothermal response using 355 nm pump wavelength was investigated. Responsivity values of calibration samples made of glass (fused silica, borosilicate) and crystals (CaF2, YAG, β-BBO, LBO, crystalline quartz) were investigated in reflection and transmission modes. Further analysis and modelling showed the dependency of thermophysical parameters of material with measured responsivity value. It was found that responsivity in reflection mode mostly depends on thermal expansion, thermal conductivity, dn/dT and density of the material while in transmission mode responsivity mostly depends on thermal expansion, thermal diffusivity, specific heat and refractive index at probe wavelength. If compared responsivity values of fused silica and crystalline quartz in reflection (R for fused silica 3.66, crystalline quartz 14.24) and transmission (R for fused silica 9.65, crystalline quartz 1.74) modes, it's evident that not only the chemical composition of the material but also the crystalline state impacts photothermal responsivity. It is shown, that using the photothermal common-path interferometry technique with a high repetition rate (MHz) and high peak power laser source, it's possible to estimate not only linear absorption term but also a nonlinear response. For that reason, intensity-dependent nonlinear absorption of LBO crystal surface coated with anti-reflection coatings (Al2O3 and SiO2) was measured. From the results, it's observed that with increasing peak intensity from 3.1 to 4.7 GW/cm2, surface absorption losses increase from 1.04 to 2.68 % which could be attributed to two-photon absorption. |
