Anisotropic laser-induced damage threshold and residual stress of TiO2 sculptured thin films.

The residual stress and laser-induced damage threshold (LIDT) of TiO2 sculptured thin films prepared by glancing angle electron beam evaporation were studied. UV-Vis-NIR spectra and optical interferometer were employed to characterize the optical and mechanical properties, respectively. Optical microscopy and Raman spectra were used to observe damage morphology and analyze damage microstructure, respectively. It was found that the residual stress changed from compressive into tensile with increasing deposition angle. The LIDT was anisotropic with p- and s-polarization light, which was due to the anisotropic nanostructure and optical properties. Simultaneously, an optimum deposition angle for the maximum threshold of TiO2 film was about 60 degrees. The mechanism of laser-induced damage was thermal in nature. The process of thermal damage with crystallization is proved by Raman spectra.

Investigations on the catastrophic damage in multilayer dielectric films.

HfO2/SiO2 coatings are always fluence-limited by a class of rare catastrophic failures induced by a nanosecond laser with a wavelength of 1053 nm. The catastrophic damage in HfO2/SiO2 coatings behaves as the damage growth with repeated laser irradiation, and thus eventually limits the mirror performance. Understanding the damage processes and mechanisms associated with the catastrophic damage are important for reducing the occurrence of the catastrophic failure and allowing the HfO2/SiO2 coatings to survive at the high fluence required by high laser systems. The rough damage behavior of the catastrophic failure at the proper critical fluence is present. The pit and delamination in the catastrophic failure are investigated to find the possible reasons leading to the catastrophic failure. The experimental results indicate that nodular defect originated from the substrate easily incurs the catastrophic damage. The electric field enhancements of the pit and the substrate impurities may contribute to this phenomenon. The delamination is always present on the left of the pit when laser irradiates from left to right at oblique incidence, which may be related to the plasma plume toward the laser incidence.

Influence of composition and seed dimension on the structure and laser damage of nodular defects in HfO2/SiO2 high reflectors.

Seeds are the sources for forming nodular defects that largely limit the improvement of laser-induced damage threshold of 1ω laser mirrors in the nanosecond pulse regime. To shed more light of the composition and sizes of seeds on the associated structure of nodular defects and laser damage sensitivity, nodular defects were generated in 1064 nm HfO(2)/SiO(2) high reflectors with different sizes of absorbing Au and nonabsorbing SiO(2) nanoparticles located on the surfaces of substrates. The width dimensions, inner structures, and damage morphologies of nodular defects were characterized by an atomic force microscope, a field emission scanning electron microscope, and a focused ion beam. It was found that the composition and size both influenced the structure and the laser damage of nodular defects. The width of nodules from SiO(2) seeds were larger than that formed by the same size of Au seed. A nodule grown from a small seed generally tends to have a continuous and stable boundary. The ejection fluences of nodules generated from different size absorbing Au and nonabsorbing SiO(2) seeds were totally different. The results were interpreted from the aspects of absorption cross sections of seeds and mechanical stability of nodular structures.

Coupling effect of multi-wavelength lasers in damage performance of beam splitters at 355 nm and 1064 nm.

The coupling effect between a 355 nm laser and a 1064 nm laser in damage initiation and morphology formation was investigated on beam splitters. When extra 1064 nm pulse energy was low, 355 nm laser-induced damage thresholds (LIDTs) increased because of laser conditioning, and when 1064 nm pulse energy was high enough, 355 nm LIDTs decreased. Damage morphologies were also studied to explore the damage mechanism at respective wavelengths. For the entirely different electric field intensity distributions, 355 nm laser-induced damages were mainly from nanometer-sized absorbers at upper interfaces, while initiators for the 1064 nm laser were located at substrate-coating interface or substrate subsurface. Under simultaneous illumination, the sensitive defects were still the precursors, and damages also showed the representative damage characteristics induced by a single laser, namely, 355 nm laser-induced small pits and 1064 nm laser-induced large delamination. Further studies also showed that, although the 1064 nm laser fluence was kept unchanged, delamination area grew with the increase of pits, which were induced by the 355 nm laser. A possible mechanism was proposed to interpret the delamination area growth phenomenon.

Characteristics of plasma scalds in multilayer dielectric films.

Plasma scalding is one of the most typical laser damage morphologies induced by a nanosecond laser with a wavelength of 1053 nm in HfO(2)/SiO(2) multilayer films. In this paper, the characteristics of plasma scalds are systematically investigated with multiple methods. The scalding behaves as surface discoloration under a microscope. The shape is nearly circular when the laser incidence angle is close to normal incidence and is elliptical at oblique incidence. The nodular-ejection pit is in the center of the scalding region when the laser irradiates at the incidence angle close to normal incidence and in the right of the scalding region when the laser irradiates from left to right at oblique incidence. The maximum damage size of the scalding increases with laser energy. The edge of the scalding is high compared with the unirradiated film surface, and the region tending to the center is concave. Plasma scald is proved to be surface damage. The maximum depth of a scald increases with its size. Tiny pits of nanometer scale can be seen in the scalding film under a scanning electronic microscope at a higher magnification. The absorptions of the surface plasma scalds tend to be approximately the same as the lower absorptions of test sites without laser irradiation. Scalds do not grow during further illumination pulses until 65 J/cm(2). The formation of surface plasma scalding may be related to the occurrence of the laser-supported detonation wave.

Temperature field analysis of TiO2 films with high-absorptance inclusions.

To deduce the location of absorptive inclusions in thin films, temperature distributions in pure TiO(2) films and TiO(2) films with high-absorptance inclusions are analyzed based on temperature field theory. According to our theoretic simulations, the surface temperature rise increases when absorptive inclusions are incorporated into thin films and shows different values for different inclusions. With the increase of inclusion thickness, the surface temperature rise varies and has a maximum value. A potential method is presented to deduce the location of absorptive inclusion through calculating the surface temperature rise at two modulated frequencies, if it is possible to know in advance the inclusion material or to prejudge this from a thin-film deposition process.

Optimization design of an ultrabroadband, high-efficiency, all-dielectric grating.

More than 97% flat-top diffraction efficiency in the -1st-order TE polarization over a 110 nm wavelength range around 800 nm in an all-dielectric grating is designed by a simulated annealing algorithm and the Fourier mode method. Its band is near to the maximum bandwidth provided by a dielectric high-reflectivity mirror under the match layer. This result will provide a way for high-efficiency chirped-pulse amplification to be used in an ultrashort high-power pulse laser system based on all-dielectric gratings. Furthermore, an effective method for broadband high-efficiency all-dielectric grating design is presented in this Letter.

Effect of multiple wavelengths combination on laser-induced damage in multilayer mirrors.

The damage effect of the combined irradiation of 1omega and 3omega in multilayer films was investigated. The experiments were held in both the Laser Induced Damage Threshold (LIDT) mode and the damage probability mode. Moreover, the effect of the laser pre-conditioning was also discussed. It was found that with two wavelengths illumination simultaneously, the number of the sensitive defects still govern the damage probability of the samples, and the energy absorption of the defects to pulse laser is a basic process in causing damage. Additionally, correlative theory models were built to explain the experimental results.

Temperature field analysis of single-layer TiO2 films.

A method is presented to evaluate optical absorption at a random wavelength by calculating temperature distribution in single-layer TiO(2) films. Temperature distribution in single-layer TiO(2) films was analyzed based on temperature field theory. Through our calculations, optical absorption variation was obtained to be similar to that of surface temperature rise in films. The surface temperature rise depends on film thickness, refractive index, extinction coefficient, specific heat, and thermal conductivity. Furthermore, the optical absorptions of the same single-layer TiO(2) film at different wavelengths were deduced. As an example, the surface temperature rises were calculated for the 19 single-layer TiO(2) films, which had been prepared by 12 different laboratories for the annual meeting of the Optical Society of America in 1986. The results agree well with the measured optical absorptions.

Annealing effect on the laser-induced damage resistance of ZrO(2) films in vacuum.

By modifying some structural characteristics, the annealing process can have considerable effects on the optical performance and laser-induced damage resistance of ZrO(2) thin films deposited by electron-beam deposition. Annealing at increased temperature gives rise to an increase of refractive index, the evolutions of packing density, and the structure order of the films due to the removal of adsorbed water in advance, material crystallization, and phase transformation. Thus, the combined effects of greatly strengthened endurance, crystal structure ordering, and stress transition after the annealing leads to an increase of the laser-induced damage threshold in a vacuum environment from 12 to 16 J/cm(2) (at 1064 nm, 12 ns pulse duration, and 1-on-1 testing mode).

Analysis and design of transmittance for an antireflective surface microstructure.

In order to easily analyze and design the transmittance characteristics of an antireflective surface called the 'moth-eye structure', the validity of both scalar diffraction theory and effective medium theory is quantitatively evaluated by a comparison of diffraction efficiencies predicted from both simplified theories to exact results calculated by a rigorous electromagnetic theory. The effect of surface microstructure parameters including the normalized period and the normalized depth has been determined at normal incidence. It is found that, in general, when the normalized period is more than four wavelengths of the incident light the scalar diffraction theory is useful within the error of 5%. Besides, the effective medium theory is accurate for evaluating the diffraction efficiency within the error of less than 1% when the higher order diffraction waves other than zero order wave is not to propagate. In addition, the limitation of scalar diffraction method and effective refractive index method is dependent on not only the normalized period of surface profile but also the normalized groove depth.

Method for characterization of repetitive frequency laser resistance of optical coatings.

The characterization of repetitive frequency (S-on-1) laser resistance of optical coatings was investigated experimentally based on damage pattern divided into slight damage and severe damage. It was discovered that, when only the slight damage pattern was counted, a peak would be seen in the damage probability curve, and the laser fluence at this peak was stable as pulse sequence S changed. The stable laser fluence at this peak, therefore, could be employed to characterize the repetitive frequency laser resistance of optical coatings. This method is different from the method proposed by the International Organization for Standardization [ISO 11254-2 (2001)] and has the advantage of simplicity in practical applications. According to our analysis, it is a method that can obtain the functional damage threshold of optics.

Influence of plasma treatment on laser-induced damage characters of HfO2 thin films at 355 nm.

HfO(2) thin films were deposited by e-beam evaporation, and were post-treated with plasma under different flow rate ratios of argon to oxygen. By measuring the surface defect density, weak absorption, laser-induced damage threshold (LIDT) and damage morphology, the influence of the flow rate ratio of argon to oxygen on the laser-induced damage characters of HfO(2) thin films were analyzed. The experimental results show that plasma treatment is effective in reducing the surface defect density of thin films. Compared with the as-grown sample, the absorption reduction is obvious after plasma treatment when argon and oxygen flow rate ratio is 5:25, but the absorption increases gradually with the continued increase of argon and oxygen flow rate ratio. LIDT measurements in 1-on-1 mode demonstrate that plasma treatment is not effective in improving LIDT of the samples at 355 nm. Damage morphologies reveal that the LIDT is dominated by nanoscale absorbing defects in subsurface layers, which agrees well with our numerical simulation result based on a spherical absorber model.

Nonpolarizing guided-mode resonance filter.

A normal-incidence nonpolarizing guided-mode resonance filter is designed. There are two waveguide layers and one grating layer in the filter. By adjusting the distance between the two waveguide layers, the same resonance wavelength for both TE and TM polarization can be achieved. An antireflection design method is also used to decrease the sideband reflection of the filter. The results show that the filter has high reflection, more than 99.9% at 500 nm, and the FWHMs of TE- and TM-polarized light are 2.16 and 0.15 nm, respectively.

Optical properties and microstructure of Ta2O5 biaxial film.

This study investigates the optical properties and microstructure of Ta(2)O(5) film deposited with the glancing angle deposition technique. The tilted nanocolumn microstructure, examined with scanning electron microscopy, induces the optical anisotropy of thin film. The optical properties of thin film are characterized with an inverse synthesis method. Based on the Cauchy model, the dispersion equations of optical constants of film are determined from the transmittance spectra measured at normal and oblique incidence over 400-800 nm. The starting values derived with an envelope method quicken the optimization process greatly. The dispersion of the principal indices N(1), N(2), and N(3) and the thickness d of thin film are presented statistically. A good agreement between the measured optical properties and theoretical calculation is obtained, which validates the model established for thin film produced by glancing angle deposition.

Calculation of femtosecond pulse laser induced damage threshold for broadband antireflective microstructure arrays.

In order to more exactly predict femtosecond pulse laser induced damage threshold, an accurate theoretical model taking into account photoionization, avalanche ionization and decay of electrons is proposed by comparing respectively several combined ionization models with the published experimental measurements. In addition, the transmittance property and the near-field distribution of the 'moth eye' broadband antireflective microstructure directly patterned into the substrate material as a function of the surface structure period and groove depth are performed by a rigorous Fourier model method. It is found that the near-field distribution is strongly dependent on the periodicity of surface structure for TE polarization, but for TM wave it is insensitive to the period. What's more, the femtosecond pulse laser damage threshold of the surface microstructure on the pulse duration taking into account the local maximum electric field enhancement was calculated using the proposed relatively accurate theoretical ionization model. For the longer incident wavelength of 1064 nm, the weak linear damage threshold on the pulse duration is shown, but there is a surprising oscillation peak of breakdown threshold as a function of the pulse duration for the shorter incident wavelength of 532 nm.

Mechanism initiated by nanoabsorber for UV nanosecond-pulse-driven damage of dielectric coatings.

A model of plasma formation for UV nanosecond pulse-laser interaction with SiO(2) thin film based on nanoabsorber is proposed. The formalism considered the temperature dependence of band gap. The numerical results show that during the process of nanosecond pulsed-laser interaction with SiO(2) films, foreign inclusion absorbing a fraction of incident radiation heats the surrounding host material through heat conduction causing the decrease of the band gap and making the initial transparent matrix into an absorptive medium around the inclusion. During the remainder pulse, the abosorbing volume of the host material is effectively growed and lead to the formation of the damage craters. We investigated the experimental damage craters and compared with theoretical prediction. The pulselength dependence of damage threshold was also investigated.

Preparation of high laser induced damage threshold antireflection film using interrupted ion assisted deposition.

Single layers and antireflection films were deposited by electron beam evaporation, ion assisted deposition and interrupted ion assisted deposition, respectively. Antireflection film of quite high laser damage threshold (18J/cm2) deposited by interrupted ion assisted deposition were got. The electric field distribution, weak absorption, and residual stress of films and their relations to damage threshold were investigated. It was shown that the laser induced damage threshold of film was the result of competition of disadvantages and advantages, and interrupted ion assisted deposition was one of the valuable methods for preparing high laser induced damage threshold films.

Thermodynamic damage mechanism of transparent films caused by a low-power laser.

A new model for analyzing the laser-induced damage process is provided. In many damage pits, the melted residue can been found. This is evidence of the phase change of materials. Therefore the phase change of materials is incorporated into the mechanical damage mechanism of films. Three sequential stages are discussed: no phase change, liquid phase change, and gas phase change. To study the damage mechanism and process, two kinds of stress have been considered: thermal stress and deformation stress. The former is caused by the temperature gradient and the latter is caused by high-pressure drive deformation. The theory described can determine the size of the damage pit.

Extended bidirectional reflectance distribution function for polarized light scattering from subsurface defects under a smooth surface.

By introducing the scattering probability of a subsurface defect (SSD) and statistical distribution functions of SSD radius, refractive index, and position, we derive an extended bidirectional reflectance distribution function (BRDF) from the Jones scattering matrix. This function is applicable to the calculation for comparison with measurement of polarized light-scattering resulting from a SSD. A numerical calculation of the extended BRDF for the case of p-polarized incident light was performed by means of the Monte Carlo method. Our numerical results indicate that the extended BRDF strongly depends on the light incidence angle, the light scattering angle, and the out-of-plane azimuth angle. We observe a 180 degrees symmetry with respect to the azimuth angle. We further investigate the influence of the SSD density, the substrate refractive index, and the statistical distributions of the SSD radius and refractive index on the extended BRDF. For transparent substrates, we also find the dependence of the extended BRDF on the SSD positions.