Overcoat dependence of laser-induced damage threshold of 355 nm HR coatings

A series of HR coatings, with and without overcoat, were prepared by electron beam evaporation using the same deposition process. The laser-induced damage threshold (LIDT) was measured by a 355 nm Nd:YAG laser with a pulse width of 8 ns. Damage morphologies of samples were observed by Leica-DMRXE Microscope. The stress was measured by viewing the substrate deformation before and after coatings deposition using an optical interferometer. Reflectance of the samples was measured by Lambda 900 Spectrometer. The theoretical results of electric field distributions of the samples were calculate by thin film design software (TFCalc). It was found that SiO2 overcoat had improved the LIDT greatly, while MgF2 overcoat had little effect on the LIDT because of its high stress in the HR coatings. The damage morphologies were different among HR coatings with and without overcoats. (c) 2005 Elsevier B.V. All rights reserved.

Investigation on the properties of high reflective mirror prepared by ion-beam sputtering

The single-sided and dual-sided high reflective mirrors were deposited with ion-beam sputtering (IBS). When the incident light entered with 45 degrees, the reflectance of p-polarized light at 1064 nm exceeded 99.5%. Spectrum was gained by spectrometer and weak absorption of coatings was measured by surface thermal lensing (STL) technique. Laser-induced damage threshold (LIDT) was determined and the damage morphology was observed with Lecia-DMRXE microscope simultaneously. The profile of coatings was measured with Mark III-GPI digital interferometer. It was found that the reflectivity of mirror exceeded 99.9% and its absorption was as low as 14 ppm. The reflective bandwidth of the dual-sided sample was about 43 nm wider than that of single-sided sample, and its LIDT was as high as 28 J/cm2, which was 5 J/cm2 higher than that of single-sided sample. Moreover, the profile of dual-sided sample was better than that of substrate without coatings.

沉积温度对HfO2薄膜残余应力的影响

用电子柬蒸发方法制备了HfO2薄膜，根据镀膜前后基片曲率半径的变化，用Stoney公式计算了薄膜应力。讨论了沉积温度对薄膜残余应力的影响。结果发现，HfO2薄膜的残余应力均为张应力，应力值随沉积温度的升高先增大后减小，在280℃左右出现极大值。对样品进行了XRD测试，从微观结构上对实验结果进行了分析，发现微结构演变引起的内应力变化是引起薄膜残余应力改变的主要因素，HfO2薄膜在所选沉积温度60～350℃内出现了晶态转变，堆积密度随温度升高而增大。

Influence of substrate temperature on properties of MgF2 coatings

Thermal boat evaporation was employed to prepare MgF2 single-layer coatings upon both JGS1 and UBK7 substrates at different substrate temperatures. Microstructure, transmittance and residual stress of these coatings were measured by X-ray diffraction, spectrophotometer, and optical interferometer, respectively. Measurement of laser induced damage threshold (LIDT) of the samples was performed at 355 nm, 8 ns pulses. The results showed that high substrate temperature was beneficial to crystallization of the film. Above 244 degrees C, the refractive index increased gradually with the substrate temperature rising. Whereas, it was exceptional at 210 degrees C that the refractive index was higher than those deposited at 244 and 277 degrees C. The tensile residual stresses were exhibited in all MgF2 films, but not well correlated with the substrate temperature. In addition, the stresses were comparatively smaller upon JGS1 substrates. A tendency could be seen that the LIDTs reached the highest values at about 244 degrees C, and the films upon JGS1 had higher LIDTs than those upon UBK7 substrates at the same temperature. Meanwhile, the damage morphologies showed that the laser damage of the coating resulted from an absorbing center at the film-substrate interface. The features of the damages were displayed by an absorbing center dominated model. Furthermore, the reason of the difference in LIDT values was discussed in detail. (C) 2007 Elsevier B.V. All rights reserved.

Influences of oxygen partial pressure on structure and related properties of ZrO2 thin films prepared by electron beam evaporation deposition

ZrO2 thin films were prepared by electron beam evaporation at different oxygen partial pressures. The influences of oxygen partial pressure on structure and related properties of ZrO2 thin films were studied. Transmittance, thermal absorption, structure and residual stress of ZrO2 thin films were measured by spectrophotometer, surface thermal lensing technique (STL), X-ray diffraction and optical interferometer, respectively. The results showed that the structure and related properties varied progressively with the increase of oxygen partial pressure. The refractive indices and the packing densities of the thin films decreased when the oxygen partial pressure increased. The tetragonal phase fraction in the thin films decreased gradually as oxygen partial pressure increased. The residual stress of film deposited at base pressure was high compressive stress, the value decreased with the increase of oxygen partial pressure, and the residual stress became tensile with the further increase of oxygen pressure, which was corresponding to the evolution of packing densities and variation of interplanar distances. (c) 2007 Elsevier B.V. All rights reserved.

沉积温度对LaF3薄膜性能的影响

abstract {LaF₃ single-layer coatings were prepared by thermal boat evaporation at the deposition temperatures of 189, 255, 277 and 321°C respectively. The crystal structures of the coatings were characterized by X-ray diffraction (XRD). A spectrophotometer was employed to measure its transmittance. Moreover, refractive index, extinction coefficient and cut-off wavelength were obtained from the measured transmittance spectral curve. The residual stress was evaluated by the Stoney's equation and optical interferometer. Laser induce damage threshold (LIDT) was performed by a tripled Nd:YAG laser system. The results show that the crystallization status becomes better with the deposition temperature increasing. Correspondingly, the grain size also gets larger. Meanwhile, the coatings become more compact and the refractive index increases. However, the absorption of coatings seriously rises and the cut-off wavelength drifts to the long wave. In addition, the residual stress also increases and the intrinsic stress plays a determinant role in the coating. The LIDT of the coating also enhances at high temperature.}

薄膜厚度对HfO2薄膜残余应力的影响

HfO2薄膜是用电子束蒸发方法制备的，利用ZYGO干涉仪测量了基片镀膜前后曲率半径的变化，计算了薄膜应力。对样品进行了XRD测试，讨论了膜厚对薄膜残余应力的影响。结果发现不同厚度HfO2薄膜的残余应力均为张应力，应力值随薄膜厚度的增加而减小，当薄膜厚度达到一定值后，应力值趋于稳定。从微观结构变化对实验结果进行了分析，发现微结构演变引起的本征应力变化是引起薄膜残余应力改变的主要因素。

Influences of CO2 laser annealing on mechanical and laser-induced damage properties of ZrO2 thin films

Zirconium dioxide (ZrO2) thin films were deposited on BK7 glass substrates by the electron beam evaporation method. A continuous wave CO2 laser was used to anneal the ZrO2 thin films to investigate whether beneficial changes could be produced. After annealing at different laser scanning speeds by CO2 laser, weak absorption of the coatings was measured by the surface thermal lensing (STL) technique, and then laser-induced damage threshold (LIDT) was also determined. It was found that the weak absorption decreased first, while the laser scanning speed is below some value, then increased. The LIDT of the ZrO2 coatings decreased greatly when the laser scanning speeds were below some value. A Nomarski microscope was employed to map the damage morphology, and it was found that the damage behavior was defect-initiated both for annealed and as-deposited samples. The influences of post-deposition CO2 laser annealing on the structural and mechanical properties of the films have also been investigated by X-ray diffraction and ZYGO interferometer. It was found that the microstructure of the ZrO2 films did not change. The residual stress in ZrO2 films showed a tendency from tensile to compressive after CO, laser annealing, and the variation quantity of the residual stress increased with decreasing laser scanning speed. The residual stress may be mitigated to some extent at proper treatment parameters. (c) 2007 Elsevier GmbH. All rights reserved.

Y2O3 stabilized ZrO2 thin films deposited by electron-beam evaporation: Optical properties, structure and residual stresses

This paper describes the preparation and the characterization Of Y2O3 stabilized ZrO2 thin films produced by electric-beam evaporation method. The optical properties, microstructure, surface morphology and the residual stress of the deposited films were investigated by optical spectroscopy, X-ray diffraction (XRD), scanning probe microscope and optical interferometer. It is shown that the optical transmission spectra of all the YSZ thin films are similar with those of ZrO2 thin film, possessing high transparency in the visible and near-infrared regions. The refractive index of the samples decreases with increasing of Y2O3 content. The crystalline structure of pure ZrO2 films is a mixture of tetragonal phase and monoclinic phase, however, Y2O3 stabilized ZrO2 thin films only exhibit the cubic phase independently of how much the added Y2O3 content is. The surface morphology spectrum indicates that all thin films present a crystalline columnar texture with columnar grains perpendicular to the substrate and with a predominantly open microporosity. The residual stress of films transforms tensile from compressive with the increasing Of Y2O3 molar content, which corresponds to the evolutions of the structure and packing densities. (C) 2008 Elsevier Ltd. All rights reserved.

沉积温度对氧化钇稳定氧化锆薄膜残余应力的影响

采用自制掺摩尔分数12%的Y2O3的ZrO2混合颗粒料为原料,在不同的沉积温度下用电子束蒸发方法沉积氧化钇稳定氧化锆(YSZ)薄膜样品。利用ZYGOMarkⅢ-GPI数字波面干涉仪对氧化钇稳定氧化锆薄膜的残余应力进行了研究,讨论了沉积温度对残余应力的影响。实验结果表明:随沉积温度升高,氧化钇稳定氧化锆薄膜中残余应力状态由张应力变为压应力,且压应力值随着沉积温度升高而增大;用X射线衍射仪表征了不同沉积温度下氧化钇稳定氧化锆薄膜的微观结构,探讨了薄膜微观结构与其应力的对应关系,并对比了纯ZrO2薄膜表现出的应力状态。

Bias-dependent recovery time of all-optical resonant nonlinearity in InGaAsP/InGaAsP MQW waveguide

We have investigated a resonant refractive nonlinearity in a semiconductor waveguide by measuring intensity dependent phase shifts and bias-dependent recovery times. The measurements were performed on an optimized 750-μm-long AR coated buried heterostructure MQW p-i-n waveguide with a bandedge at 1.48 μm. Figure 1 shows the experimental arrangement. The mode-locked color center laser was tuned to 50 meV beyond the bandedge and 8 ps pulses with peak incident power up to 57 W were coupled into the waveguide. Some residual bandtail absorption remains at this wavelength and this is sufficient to cause carriers to be photogenerated and these give rise to a refractive nonlinearity, predominantly by plasma and bandfilling effects. A Fabry-Perot interferometer is used to measure the spectrum of the light which exits the waveguide. The nonlinearity within the guide causes self phase modulation (SPM) of the light and a study of the spectrum allows information to be recovered on the magnitude and recovery time of the nonlinear phase shift with a reasonable degree of accuracy. SPM spectra were recorded for a variety of pulse energies coupled into he unbiased waveguide. Figure 2 shows the resultant phase shift measured from the SPM spectra as a function of pulse energy. The relationship is a linear one, indicating that no saturation of the nonlinearity occurs for coupled pulse energies up to 230 pJ. A π phase shift, the minimum necessary for an all-optical switch, is obtained for a coupled pulse energy of 57 pJ while the maximum phase shift, 4 π, was measured for 230 pJ. The SPM spectra were highly asymmetric with pulse energy shifted to higher frequencies. Such spectra are characteristic of a slow, negative nonlinearity. This relatively slow speed is expected for the unbiased guide as the recovery time will be of the order of the recombination time of the photogenerated electrons, about 1 ns for InGaAsP material. In order to reduce the recovery time of the nonlinearity, it is necessary to remove the photogenerated carriers from the waveguide by a process other than recombination. One such technique is to apply a reverse bias to the waveguide in order to sweep the carriers out. Figure 3 shows the effect on the recovery time of the nonlinearity of applying reverse bias to the waveguide for 230 pJ coupled power. The recovery time was reduced from one much longer than the length of the pulse, estimated to be about 1 ns, at zero bias to 18 ± 3 ps for a bias voltage greater than -4 V. This compares with a value of 24 ps obtained in a bulk waveguide.

Comparison of the coherence properties of superradiance and laser emission in semiconductor structures

The coherence properties of a transient electron-hole state developing during superradiance emission in semiconductor laser structures have been studied experimentally using a Michelson interferometer and Young's classic double-slit configuration. The results demonstrate that, in the lasers studied, the first-order correlation function, which quantifies spatial coherence, approaches unity for superradiant emission and is 0.2-0.5 for laser emission. The supercoherence is due to long-range ordering upon the superradiant phase transition. © 2012 Kvantovaya Elektronika and Turpion Ltd.

Fabrication and control of miniature McKibben actuators

This paper investigates the development of miniature McKibben actuators. Due to their compliancy, high actuation force, and precision, these actuators are on the one hand interesting for medical applications such as prostheses and instruments for surgery and on the other hand for industrial applications such as for assembly robots. During this research, pneumatic McKibben actuators have been miniaturized to an outside diameter of 1.5 mm and a length ranging from 22 mm to 62 mm. These actuators are able to achieve forces of 6 N and strains up to about 15% at a supply pressure of 1 MPa. The maximal actuation speed of the actuators measured during this research is more than 350 mm/s. Further, positioning experiments with a laser interferometer and a PI controller revealed that these actuators are able to achieve sub-micron positioning resolution. © 2010 Published by Elsevier B.V. All rights reserved.

Lineshape Analysis of the Beat Signal Between Optical Carrier and Delayed Sidebands

This paper presents the lineshape analysis of the beat signal between the optical carrier and the shifted and delayed side-bands produced by sinusoidal amplitude modulation. It is shown that the beat signal has a typical lineshape with a very narrow delta-peak superposed on a quasi-Lorentzian profile. Theoretical explanation for the appearance of this peak has been given based on optical spectral structure constructed by a large number of optical wave trains. It is predicted that the delta-peak is originated from the beat between the wave trains in the carrier and those in the delayed sidebands when their average coherence length is longer than the delay line. Experiments carried out using different delay lines clearly show that the delta-peak is always located at the modulation frequency and decreases with the increasing delay line. Our analysis explicitly indicates that the linewidth is related to the observation time. It is also suggested that the disappearance of the delta-peak can be used as the criterion of coherence elimination.

Interaction between the intrinsic second- and third-order optical fields in an Al0.53Ga0.47N/GaN heterostructure

We present an experimental demonstration of the interaction between the intrinsic second- and third-order optical fields in an Al0.53Ga0.47N/GaN heterostructure. The sample was deposited by metal-organic chemical vapor deposition on (0001) sapphire. The nonlinear optical coefficients of the sample, which were measured with a Mach-Zehnder interferometer system, quadratically increase with the applied modulating voltage, indicating the existence of the third-order optical field. The third-order signal was then detected by the Z-scan method and we calculated the built-in dc field on the AlGaN/GaN interface to confirm the strong interaction between the intrinsic second- and third-order optical fields. (c) 2008 American Institute of Physics.