Christoph Rickers

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.

Optical characterization of hybrid antireflective coatings using spectrophotometric and ellipsometric measurements

A hybrid antireflective coating combining homogeneous layers and linear gradient refractive index layers has been deposited using different techniques. The samples were analyzed optically based on spectrophotometric and spectroscopic ellipsometry measurements under different angles of incidence in order to precisely characterize the coatings. The Lorentz-Lorenz model has been used to calculate the refractive index of material mixtures in gradient and constant index layers of the coating. The obtained refractive index profiles have been compared with the targeted ones to detect errors in processes of deposition.

Infrared-Reflective Coating on Fused Silica for a Solar High-Temperature Receiver

In concentrating solar power, high-temperature solar receivers can provide heat to highly efficient cycles for electricity or chemical production. Excessive heating of the fused-silica window and the resulting recrystallization are major problems of high-temperature receivers using windows. Excessive window temperatures can be avoided by applying an infrared-reflective solar-transparent coating on the fused-silica window inside. Both glass temperatures and receiver losses can be reduced. An ideal coating reflects part of the thermal spectrum (lambda>2.5 µm) of the hot absorber (1100°C) back onto it without reducing solar transmittance. Extensive radiation simulations were done to screen different filter types. The examined transparent conductive oxides (TCO) involve a high solar absorptance, inhibiting their use in high-concentration solar systems. Although conventional dielectric interference filters have a low solar absorption, the reflection of solar radiation which comes from various directions is too high. It was found that only rugate filters fulfill the requirements for operation under high-flux solar radiation with different incident angles. A thermodynamic qualification simulation of the rugate coating on a window of a flat-plate receiver showed a reduction of almost 175 K in mean window temperature and 11% in receiver losses compared to an uncoated window. For the configuration of a pressurized receiver (REFOS type), the temperature could be reduced by 65 K with slightly reduced receiver losses. Finally, a first 25-µm thick rugate filter was manufactured and optically characterized. The measured spectra fitted approximately the design spectra, except for two absorption peaks which can be avoided in future depositions by changing the deposition geometry and using in-situ monitoring. The issue of this paper is to share the work done on the choice of filter type, filter design, thermodynamic evaluation, and deposition experiments.

Design and manufacture of spectrally selective reflecting coatings for the use with laser display projection screens

To provide screens for laser projection that improve contrast, a spectrally selective reflecting filter was designed by using genetic algorithms to overcome the problem of unknown starting values. Colormetrics rather than fixed targets were used for evaluation. Various selective filters were deposited upon glass as well as upon solid and flexible plastic substrates by reactive mid-frequency magnetron sputtering. For process control, in situ spectroscopic ellipsometry was applied.

Process Technology, Applications and Potentials of Magnetron Sputtering Technology for Optical Coatings

Different magnetron sputter geometries and modes for the deposition of optical coatings are presented. Examples of AR- and filter coatings are given demonstrating the high potential of this technology for different applications.

Spectrally selective reflecting thin-film filters for laser display technology

To provide a contrast improvement for projection screens used with the laser display technology, a thin selective reflecting filter was designed. To overcome the problem of unknown starting designs and permit a new evaluation technique, an evolutionary algorithm was implemented. Due to the lack of known target spectra rather than known features of the target spectra, a novel evaluation procedure using color metrics was implemented. Different selective reflecting filters were deposited on glass as well as on plastic substrates by reactive mid-frequency magnetron sputtering. In-situ ellipsometry was used for process control and in-situ re-design of the filter.