Broadband phase-shifting mirrors for ultrafast lasers

Metal-dielectric phase-shifting multilayer optical elements have been developed, providing broadband, virtually dispersion-free polarization manipulation down to the few-cycle level. These optical elements are Ag/Al₂O₃ mirrors that operate in the spectral range from 500 to 100 nm, exhibiting reflectance higher than 95%, and a differential phase shift between the s- and p-polarization of about 90° distributed over four bounces. The mirrors have been designed, produced, and reliably characterized based on spectral photometric and ellipsometric data using a non-parametric approach as well as a multi-oscillator model. The optical elements were implemented into a few-cycle laser system, where they transformed linearly polarized few-cycle light pulses to circular polarization.

Ellipsometry-based study of glass refractive index depth profiles obtained by applying different poling conditions

Application of electric field and moderately elevated temperature depletes the side facing anode from alkali present in glasses. The change of composition of the treated glass results in variation of refractive index depth profile within the treated glass. Spectroscopic ellipsometry is employed for characterization of optical properties of glass treated in different conditions. The results of optical characterization are verified by secondary ion mass spectroscopy. It is found that the refractive index profile obtained from ellipsometry has a maximum value higher than the one of untreated glass. The obtained refractive index profiles are in very good agreement with concentration profiles.

Production of three-dimensional quantum dot lattice of Ge/Si core-shell quantum dots and Si/Ge layers in an alumina glass matrix

We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al2O3/Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells.

Deposition and spectral performance of an inhomogeneous broadband wide-angular antireflective coating

Gradient index coatings and optical filters are a challenge for fabrication. In a round-robin experiment, basically the same hybrid antireflection coating for the visible spectral region, combining homogeneous refractive index layers of pure materials and linear gradient refractive index layers of material mixtures, has been deposited. The experiment involved three different deposition techniques: electron-beam evaporation, ion-beam sputtering, and radio frequency magnetron sputtering. The material combinations used by these techniques were Nb(2)O(5)/SiO(2), TiO(2)/SiO(2), and Ta(2)O(5)/SiO(2), respectively. The spectral performances of samples coated on one side and on both sides have been compared to the corresponding theoretical spectra of the designed profile. Also, the reproducibility of results for each process is verified. Finally, it is shown that ion-beam sputtering gave the best results in terms of deviation from the theoretical performance and reproducibility.

Narrow-band reflector coatings: design procedures

A class of problem dealing with narrow-band reflectors under oblique light incidence has been selected, and a group of design procedures leading to satisfactory results is analyzed. Initially performance criteria and design restrictions are set. Various designs are then created and analyzed. Conclusions can then be drawn on the relative merits of the designs. Two different types of target function have been chosen. A global search for 50 or more layers with varied optical thicknesses between 0 and lambda/2 and a needle design method have been applied to the synthesis. Gradient and variable metrics have been used for further refinement. Results are compared, and it is shown that all the design methods used yield similar results for this problem. The issue of how to determine the best method is addressed.