Atomistic Simulation of the Ion-Assisted Deposition of Silicon Dioxide Thin Films.

A systematic study of the most significant parameters of the ion-assisted deposited silicon dioxide films is carried out using the classical molecular dynamics method. The energy of the deposited silicon and oxygen atoms corresponds to the thermal evaporation of the target; the energy of the assisting oxygen ions is 100 eV. It is found that an increase in the flow of assisting ions to approximately 10% of the flow of deposited atoms leads to an increase in density and refractive index by 0.5 g/cm3 and 0.1, respectively. A further increase in the flux of assisting ions slightly affects the film density and density profile. The concentration of point defects, which affect the optical properties of the films, and stressed structural rings with two or three silicon atoms noticeably decrease with an increase in the flux of assisting ions. The film growth rate somewhat decreases with an increase in the assisting ions flux. The dependence of the surface roughness on the assisting ions flux is investigated. The anisotropy of the deposited films, due to the difference in the directions of motion of the deposited atoms and assisting ions, is estimated using the effective medium approach.

Investigation of the error self-compensation effect associated with direct broad band monitoring of coating production.

Computational manufacturing experiments are used to detect the types of optical coatings that are showing the presence of a strong error self-compensation effect in the coating production with direct broad band monitoring. It is shown that predictions made on the basis of these experiments coincide with the predictions of the previously developed rigorous mathematical approach to the investigation of the error self-compensation effect.

Error self-compensation mechanism in the optical coating production with direct broad band monitoring.

The main theoretical results related to the investigation of the error self-compensation mechanism associated with direct broad band monitoring of optical coating production are presented. The presented results are illustrated using the production of Brewster angle polarizer where this effect is especially strong. Specific properties of the design merit function required for the presence of the error self-compensation effect are discussed and the mechanism of thickness errors correlation by the direct broad band monitoring is described. It is also discussed how one can check whether a strong error self-compensation effect may be expected for a given coating design and specific parameters of the monitoring procedure that will be used for coating production.

Modified sequential algorithm for the on-line characterization of optical coatings.

We present a new algorithm for the on-line determination of thicknesses of deposited layers that can be used in the course of coating production with broadband optical monitoring. The proposed algorithm can be considered as a modification of the well-known sequential algorithm. The main idea of the new algorithm is to re-calculate thicknesses of some of the previously deposited layers along with the determination of the thickness of the last deposited layer. The algorithm implies analytical estimations that enable recalculating only those layer thicknesses that can be found with better accuracy than before. Simulation and computational manufacturing experiments confirm high accuracy of the proposed algorithm.

Design and production of antireflection coating for the 8-10 µm spectral region.

A special design procedure allowing to trap layer thicknesses inside specified limits is applied for designing of antireflection coating (AR) for the infrared spectral band of 8-10 µm. The obtained AR design has no too thick layers that may cause delaminating of the deposited AR coating. A special monitoring procedure taking into account wavelength positions of monitoring signal extrema is applied for coating deposition. The manufactured coating features excellent AR properties in the requested spectral region and possesses high mechanical stability.

High-dispersive mirrors for high power applications.

We report on the development and manufacturing of two different types of high-dispersive mirrors (HDM). One of them provides a record value for the group delay dispersion (GDD) of -4000 fs2 and covers the wavelength range of 1027-1033 nm, whereas the other one provides -3000 fs2 over the wavelength range of 1020-1040 nm. Both of the fabricated mirrors exhibit a reflectance of >99.9% and are well suited for intracavity applications. Mirrors of the second type have been successfully employed in a Kerr-lens mode-locked Yb:YAG thin-disk oscillator for the generation of 200-fs pulses with multi-10-W average power.

Robust synthesis of dispersive mirrors.

A synthesis technique allowing to obtain a set of robust designs is reported. The robust synthesis is based on simultaneous optimization of spectral characteristics of multiple designs located in a small neighborhood of a so-called pivotal design. Efficiency of this technique is demonstrated by the synthesis and successful experimental realization of a high dispersive mirror. The fabricated dispersive mirror covers 690-890 nm wavelength range and provides the dispersion of -300 fs2 at 800 nm.

Phase optimization of dispersive mirrors based on floating constants.

A novel floating constants phase-optimization technique is developed and applied to the design of dispersive mirrors. This technique reduces the dispersive mirror's sensitivity to layer thickness errors. To demonstrate the significant improvement in design stability, we theoretically and experimentally compare our new phase-optimization approach to the conventional one. The fabricated dispersive mirror has a reflectivity of >99.99% and provides an accurate dispersion control over a bandwidth of around 60 nm.

Double-angle multilayer mirrors with smooth dispersion characteristics.

We report the feasibility of precision broadband dispersion control with multilayer mirrors produced in a single coating run. Inherent fluctuations of the group-delay dispersion (GDD) are suppressed by using the mirrors at two different angles of incidence. With a specialized version of the needle optimization algorithm, we have designed the multilayer structure to yield a complementary pair with a resultant GDD substantially free from spectral oscillations characteristic of broadband chirped multilayers. Since the mirrors employed at two different incidence angles are produced in a single deposition run, their overall dispersion is more robust to errors in layer thicknesses than that of previous complementary mirror pairs manufactured in two different steps. This offers the potential for improving production yield and quality of femtosecond dispersion control. We have successfully used the first "double-angle" mirrors for compressing pulses to a duration of 4.3 fs.

Comparison of dispersive mirrors based on the time-domain and conventional approaches, for sub-5-fs pulses.

Dispersive mirrors based on time-domain approach are compared with mirrors resulting from conventional phase target designs. Phase targets have been applied to complementary-pair dispersive mirrors, used for sub-5-fs pulse compression. While the phase approach has hither to afforded the best performance for the shortest pulses, our new approach, based on time-domain targets and tailored for a specific input spectrum, appears to provide comparable performance for pulse compression for a pulse duration 4.6 fs. Experimental studies using dispersive mirrors made to both designs are described.

Elimination of cumulative effect of thickness errors in monochromatic monitoring of optical coating production: theory.

We propose an algorithm for correcting deposition termination levels that allows elimination of the cumulative effect of errors in previously deposited layers. For the application of this algorithm at least one monitoring signal extremum should be registered during a layer deposition. We also derive a theoretical relation for the estimation of errors in layer refractive indices based on the results of on-line monitoring measurements. At least two monitoring signal extrema are required for its application.

Optical coating design approaches based on the needle optimization technique.

Design approaches for optical thin films that recognize the key role of a design's total optical thickness are presented. These approaches are based primarily on the needle optimization technique but also utilize other optimization procedures. Using the described design approaches, an optical coating engineer can obtain a set of theoretical designs with different combinations of principal design metrics (merit function value, number of layers, and total design optical thickness); this extends opportunities for choosing the most practical and manufacturable design. We also show that some design problems have multiple solutions with nearly the same combinations of principal design metrics.

Band filters: two-material technology versus rugate.

We demonstrate theoretically and experimentally a band filter with two reflection and broadband transmission ranges, which was obtained with standard two-material technology. The fabricated filter has transmission and reflectivity characteristics better than those achievable with rugate technology.

Statistical approach to choosing a strategy of monochromatic monitoring of optical coating production.

We presents what we believe to be a new approach to choosing a sequence of monitoring wavelengths for monochromatic monitoring of optical coating production. The new approach is based on a preproduction estimation of expected levels of errors in thickness of layers of a deposited coating. It is demonstrated that the proposed monitoring strategy reduces the effect of accumulation of thickness errors. An advantage of the new monitoring strategy becomes especially noticeable when the number of monitored layers is equal to several dozens.

Spectroscopic ellipsometry of slightly inhomogeneous nonabsorbing thin films with arbitrary refractive-index profiles: theoretical study.

We develop a new approximation for the amplitude reflection coefficients of a slightly inhomogeneous thin film. This approximation incorporates exactly the interference effects at the substrate and the ambient interfaces. Interference effects inside the inhomogeneous film are incorporated in the Born approximation. We also develop a new approach to the reconstruction of the refractive-index profile from ellipsometric spectra. It is based on a physically sound parameterization of the refractive-index profile. The new approach is tested on the model reconstruction problem.

Phase properties of multilayers.

The phase shift upon transmission of a nonabsorbing multilayer is shown to be a monotonically decreasing function of the wave number, with an average slope proportional to the optical thickness of the coating. Two limiting situations of the phase shift upon reflection are examined: In one the phase monotonically increases with wavelength, and in the other the phase oscillates. The phase shift upon reflection is derivable from Kramers-Kronig-type relationships, provided the radiant reflectance and the Blaschke factors are known. Characteristic features of the refractive-index profile related to these factors are discussed.

Broadband high-reflection multilayer coatings at oblique angles of incidence.

The angular properties of various wideband high reflectors are investigated. The theory is developed for the design of high reflectors based on contiguous quarter-wave stacks for use at one oblique angle or for a range of angles of incidence. Numerical results are presented for several high reflectors designed to have a high reflectance in the 0.4-0.8-mum spectral region for use at 50 degrees and with angles of incidence ranging between 0 degrees and 50 degrees . A random error perturbation analysis shows that such layer systems can be produced experimentally.

Efficient refinement algorithm for the synthesis of inhomogeneous optical coatings.

An efficient first-order algorithm with an exact calculation of the gradients is described and applied to the refinement of inhomogeneous dielectric thin films. Numerical results calculated at normal and oblique angles of incidence are correlated with an important mathematical theorem. It is shown that, when appropriate conditions are satisfied, the synthesis process generates binary type solutions based on two materials, including classical multilayers.

Influence of small inhomogeneities on the spectral characteristics of single thin films.

It is well known that the spectral transmittance and reflectance of a thin film can be influenced by even small inhomogeneities or variations in its complex refractive-index profile. Formulas are derived that describe the theoretical variation of the spectral characteristics for small changes in the refractive index and the extinction coefficient of a homogeneous thin film. These formulas, accurate to the first order in the change in the complex refractive index, are compared with exact calculations for a number of different types of inhomogeneities. It is shown that specific qualitative features in the refractive-index profile of a nearly homogeneous thin film frequently can be determined from an examination of the change in the spectral transmittance and reflectance at normal incidence.