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This literature study examines previous studies of the optical properties of snow, and attempts to tie them together with studies on the effects of shading on photovoltaic solar panels. The study presents some information on the general properties of snow, and ice including geographic extent and some conditions of snow and ice formation. General optical properties of snow are examined, such as reflectance (albedo) and spectral transmittance. Common transmittance profiles for snow covers are also examined. The study also presents some commonly understood effects of shading on photovoltaic panels, both in the form of uniform shading (weak light) and partial shading. Other snow-related aspects of operating a photovoltaic system are also brought up, such as snow loads and the risks posed by snowmelt, particularly in regards to building-integrated or building-applied photovoltaics. Common methods of addressing snow-related challenges are summarized, both on a material and an architectural level. Lastly, suggested future research paths are presented.

We have experimentally investigated the ablation threshold in silicon as a function of temperature when applying ultrashort laser pulses at three wavelengths. By varying the temperature of a silicon substrate from room temperature to 320 C, we observe that the ablation threshold for a 3 ps pulse using a wavelength of 1030 nm drops from 0.43 J/cm 2 to 0.24 J/cm 2 , a reduction of 43%. For a wavelength of 515 nm, the ablation threshold drops from 0.22 J/cm 2 to 0.15 J/cm 2 , a reduction of 35%. The observed ablation threshold for pulses at 343 nm remains constant with temperature, at 0.10 J/cm 2 . These results indicate that substrate heating is a useful technique for lowering the ablation threshold in industrial silicon processing using ultrashort laser pulses in the IR or visible wavelength range. In order to investigate and explain the observed trends, we apply the two-temperature model, a thermodynamic model for investigation of the interaction between silicon and ultrashort laser pulses. Applying the two-temperature model implies thermal equilibrium between optical and acoustic phonons. On the time scales encountered herein, this need not be the case. However, as discussed in the article, the two-temperature model provides valuable insight into the physical processes governing the interaction between the laser light and the silicon. The simulations indicate that ablation occurs when the number density of excited electrons reaches the critical electron density, while the lattice remains well below vaporization temperature. The simulated laser fluence required to reach critical electron density is also found to be temperature dependent. The dominant contributor to increased electron density is, in the majority of the investigated cases, the linear absorption coefficient. Two-photon absorption and impact ionization also generate carriers, but to a lesser extent. As the linear absorption coefficient is temperature dependent, we find that the simulated reduction in ablation threshold with increased substrate temperature is linked to the temperature dependence of the linear absorption coefficient. Another factor influencing the ablation threshold is the wavelength dependence of the interaction with the excited electron plasma. This wavelength dependence can explain that we observe experimentally similar ablation thresholds for a wavelength of 1030 nm at 320 C and for 515 nm at room temperature, even though the linear absorption coefficient in the latter case is much higher.

A laser reflection method has been developed and tested for analyzing the etching of porous silicon (PS) films. It allows in situ measurement and analysis of the time dependency of the etch rate, the thickness, the average porosity, the porosity profile, and the interface roughness. The interaction of an infrared laser beam with a layered system consisting of a PS layer and a substrate during etching results in interferences in the reflected beam which is analyzed by the short-time Fourier transform. This method is used for analysis of samples prepared with etching solutions containing different concentrations of HF and glycerol and at different current densities and temperatures. Variations in the etch rate and porosity during etching are observed, which are important effects to account for when optical elements in PS are made. The method enables feedback control of the etching so that PS films with a well-controlled porosity are obtainable. By using different beam diameters it is possible to probe interface roughness at different length scales. Obtained porosity, thickness, and roughness values are in agreement with values measured with standard methods.

The effects of doubly ionized oxygen vacancies (V O 2+ ) on the electronic structure and charge distribution in rutile TiO 2 are studied by combining first-principles calculations based on density functional theory and experimental results from x-ray photoelectron and x-ray absorption measurements carried out in synchrotron facilities on rutile TiO 2 nanoparticles. The generalized gradient approximation of the Perdew-Burke-Ernzerhof functional has demonstrated its suitability for the analysis of the V O 2+ defects in rutile TiO 2 . It has been found that the presence of empty electronic states at the conduction band shifted ∼1 eV from t 2g and e g states can be associated with local distortions induced by V O 2+ defects, in good agreement with Gauss-Lorentzian band deconvolution of experimental O K-edge spectra. The asymmetry of t 2g and e g bands at the O-K edge has been associated with V O 2+ , which can enrich the understanding of studies where the presence of these defects plays a key role, as in the case of doped TiO 2 .

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