Absolute Absorption Measurements in Optical Coatings by Laser Induced Deflection

Abstract

Absolute measurement of residual absorption in optical coatings is steadily becoming more important in thin film characterization, in particular with respect to high power laser applications. A summary is given on the current ability of the laser induced deflection (LID) technique to serve sensitive photo-thermal absorption measurements combined with reliable absolute calibration based on an electrical heater approach. To account for different measurement requirements, several concepts have been derived to accordingly adapt the original LID concept. Experimental results are presented for prominent UV and deep UV laser wavelengths, covering a variety of factors that critically can influence the absorption properties in optical coatings e.g., deposition process, defects and impurities, intense laser irradiation and surface/interface engineering. The experimental findings demonstrate that by combining high sensitivity with absolute calibration, photo-thermal absorption measurements are able to be a valuable supplement for the characterization of optical thin films and coatings.

Plasma-Enhanced Atomic Layer Deposition of HfO2 with Substrate Biasing: Thin Films for High-Reflective Mirrors

“…106,107 Also, there can be a small nonlinear absorption hidden by a strong linear absorption under intense laser irradiation. 108 An enlarged nonlinear absorption can result in two-photon absorption via intermediate defect levels. Since the optical band gap of SiO 2 is larger than HfO 2 , HfO 2 should be the major contributor to nonlinear absorption in the multilayer HfO 2 /SiO 2 mirror.…”

“…Since the optical band gap of SiO 2 is larger than HfO 2 , HfO 2 should be the major contributor to nonlinear absorption in the multilayer HfO 2 /SiO 2 mirror. 108 The nonstoichiometric defects might decrease by annealing the films, which improves stoichiometry; however, an appropriate annealing temperature should be used as the annealinginduced crystallization can also increase scattering losses. The atomic impurities in the films can be reduced using high purity precursors and optimized process conditions to prevent the incorporation of residual precursors and by-products into the films.…”

“…The sources of absorption in the coating can be intrinsic, such as defects in the form of impurities, nonstoichiometric defects in the form of oxygen vacancies, and extrinsic defects like inclusions in the form of nodules. , The shallow oxygen vacancy-related absorbing defects in HfO 2 are located a few eV below the conduction band. , Also, there can be a small nonlinear absorption hidden by a strong linear absorption under intense laser irradiation . An enlarged nonlinear absorption can result in two-photon absorption via intermediate defect levels.…”

Linear and Nonlinear Absorption of Titanium Dioxide Films Produced by Plasma Ion-Assisted Electron Beam Evaporation: Modeling and Experiments

“…Light absorption measurements in TiO 2 films were measured directly using a laser-induced deflection (LID) technique [30,31]. This technique belongs to an ensemble of photothermal techniques with a pump-probe configuration.…”

“…The calibration procedure itself is composed of measuring the probe beam deflection as a function of the electric power. Plotting the deflection signals versus electrical power ( Figure 4) gives a linear function that spans several orders of magnitude for electric power, and the calibration coefficient F CAL is defined by the slope of the linear function (including the zero-point, i.e., no electrical power means no probe beam deflection) [31]. From the LID deflection signal I LID (for the sample under investigation), the corresponding mean pump laser power P L , and the calibration coefficient F CAL , the coating absorptance (defined as the ratio of the absorbed and incident light intensity) is calculated by…”