Photothermal self-phase-modulation technique for absorption measurements on high-reflective coatings

We propose and demonstrate a new measurement technique for the optical absorption of high-reflection coatings. Our technique is based on photothermal self-phase modulation and exploits the deformation of cavity Airy peaks that occurs due to coating absorption of intracavity light. The mirror whose coating is under investigation needs to be the input mirror of a high-finesse cavity. Our example measurements were performed on a high-reflection SiO2-Ta2O5 coating in a three-mirror ring-cavity setup at a wavelength of 1064 nm. The optical absorption of the coating was determined to be α=(23.9±2.0)·10(-6) per coating. Our result is in excellent agreement with an independently performed laser calorimetry measurement that gave a value of α=(24.4±3.2)·10(-6) per coating. Since the self-phase modulation in our coating-absorption measurement affects mainly the propagation through the cavity input mirror, our measurement result is practically uninfluenced by the optical absorption of the other cavity mirrors.     

Low-Loss Dielectric Mirror with Ion-Beam-Sputtered TiO 2-SiO 2 Mixed Films

Ion-beam-sputtered TiO(2)-SiO(2) mixed films with 17% SiO(2) concentration were used as high-refractive-index layers in a multilayered-stack dielectric mirror. Experimental results indicated that total loss of the as-deposited mirror was 34% lower than that of the as-deposited conventional mirrors with pure TiO(2) films used as high-refractive-index layers. In addition, annealing reduced total loss of the mirrors. Although decreasing with an increasing annealing temperature, total loss of the conventional mirrors dramatically increased above ~200 degrees C annealing temperature, owing to increased scattering from an amorphous-to-crystalline phase transition in the TiO(2) films. In addition, total loss of the mirrors with the mixed films continuously decreased with an increasing annealing temperature up to 400 degrees C without the phase transition. Total loss was reduced 88% by means of decreasing absorption in the mixed films. Moreover, the annealed mirror with mixed films was better than both the as-deposited mirror and the conventional mirror with pure films in terms of laser-damage resistance.     

Simultaneous absorption, scattering, and luminescence mappings for the characterization of optical coatings and surfaces

An experimental setup, based on the laser-induced deflection technique, is developed to measure simultaneously the 244 nm laser absorption, scattering, and luminescence on optical components. The different techniques and methods that have been specifically developed to obtain both high resolution (micronic) and sensitivity (a few 10(-7) of the incident power for the absorption, 10(-8) for scattering and fluorescence) are presented. Different applications are then explored: the study of losses in deep UV multilayer coatings (HfO2/SiO2 mirrors) and the analysis of contamination defects on bare substrate and structural defects in optical coatings.     

非晶态铁电薄膜的铁电相转变

用ＡｒＦ脉冲激光在Ｐｔ／Ｔｉ／ＳｉＯ２／Ｓｉ衬底上合成了非晶态ＳｒＢｉ２Ｔａ２Ｏ９铁电薄膜,研究了铁电相转变。采用常规热退火、激光诱导和低温高压水热法使非晶态ＳＢＴ薄膜发生铁电相转变。     

Storage ring free-electron lasing at 176 nm--dielectric mirror development for vacuum ultraviolet free-electron lasers

Mirrors for storage ring free-electron lasers in the vacuum ultraviolet must provide adequate reflectivity and resistance against synchrotron radiation. The free-electron laser system at ELETTRA (Trieste, Italy) is targeted to lase in the spectral range between 155 and 200 nm. It was demonstrated that dense oxide multilayer coatings allow lasing down to 189.9 nm. However, pure oxide systems show significant absorption at lower wavelengths and cannot be employed below 189.9 nm. Fluoride stacks can be deposited down to 130 nm with high reflection values above 95%, but their resistance against the harsh synchrotron environment is poor. They rapidly degrade; lasing cannot be realized with this mirror approach. For the range between 170 and 190 nm, hybrid systems--combining fluoride and oxide materials--have been manufactured. With appropriate deposition procedures, mirrors achieve reflectance values up to 99% and an adequate radiation resistance simultaneously. A mirror based on a conventional fluoride stack protected by a dense silicon dioxide protection layer was deposited and successfully employed for free-electron lasing at 176.4 nm.     

Optical properties of a silicon-nanocrystal-based-microcavity prepared by evaporation

A silicon-based microcavity constituted by a SiO/SiO₂ multilayer and Si/SiO₂ Bragg mirrors was prepared by the evaporation technique. We give details about the microcavity realization process. The optical properties of the Si nanocrystals, obtained by the thermal dissociation of the SiO layers, were analyzed by continuous-wave and angle-resolved photoluminescence (PL) experiments. The results show a strong anisotropy of the light emitted by the silicon nanocrystals for a cavity constituted by mirrors with only two distributed Bragg reflectors (DBR).     

Liquid-phase broadband cavity-enhanced absorption spectroscopy measurements in a 2 mm cuvette

A novel implementation of broadband cavity enhanced absorption spectroscopy (BBCEAS) has been used to perform sensitive visible wavelength measurements on liquid-phase solutions in a 2 mm cuvette placed at normal incidence to the cavity mirrors. The overall experimental methodology was simple, low cost, and similar to conventional ultraviolet-visible absorption spectroscopy. The cavity was formed by two concave high reflectivity mirrors. Three mirror sets with nominal reflectivities (R) of R = 0.99, 0.9945, and 0.999 were used. The light source consisted of a high intensity red, green, blue, or white LED. The detector was a compact charge-coupled device (CCD) spectrograph. Measurements were made on the representative analytes, Ho(3+), and the dyes brilliant blue-R, sudan black, and coumarin 334 in appropriate solvents. Cavity enhancement factors (CEF) of up to 104 passes for the high reflectivity mirrors were obtained. The number of passes was limited by relatively high scattering and absorption losses in the cavity, of approximately 1 x 10(-2) per pass. Measurements over a wide wavelength range (420-670 nm) were also obtained in a single experiment with the white LED and the R = 0.99 mirror set for Ho(3+) and sudan black. The sensitivity of the experimental setup could be determined by calculating the minimum detectable change in the absorption coefficient alpha(min). The values ranged from 5.1 x 10(-5) to 1.2 x 10(-3) cm(-1). The limit of detection (LOD) for the strong absorber brilliant blue-R was 620 pM. A linear dynamic range of measurements of concentration over about two orders of magnitude was demonstrated. The overall sensitivity of the experimental setup compared very favorably with previous generally more experimentally complex and expensive liquid-phase cavity studies. Possible improvements to the technique and its applicability as an analytical tool are discussed.     

退火CdS/SiO2介孔组装体系光吸收的行为

用溶胶-凝胶法获得多孔SiO2载体，浸泡合成法得到不同复合量的CdS/SiO2块材料介孔组装体系。研究了复合量、气氛条件及退火温度对其光吸收特征的影响，发现吸收边随复合量增加往长波方向移动；在氮气和空气中退火时，随退火温度升高前者表现为红移而后者表现为蓝移，这归属于量子尺寸效应。     

Fabrication of channel waveguides from sol-gel-processed polyvinylpyrrolidone/ SiO(2) composite materials

Sol-gel-processed composite materials of polyvinylpyrrolidone (PVP) and SiO(2) were studied for optical waveguide applications. PVP is a polymer that can be crosslinked, so it is expected to have high thermal stability after crosslinking. However, thermal crosslinking and thermal decomposition of pure PVP take place around the same temperature, 200 °C, therefore pure PVP had a high optical propagation loss as a result of the absorption of the decomposed molecules after crosslinking. The incorporation of sol-gel-processed SiO(2) prevented the thermal decomposition of PVP and provided remarkably low optical propagation losses. The PVP/SiO(2)composite material also produced thick (>2-μm) crack-free films when the PVP concentration was 50% or higher. An optical propagation loss of 0.2 dB/cm was achieved at 633 nm in the 50% PVP/SiO(2) composite planar waveguide. Several aspects of the thermal stability of the waveguides were evaluated. The slab waveguide was then used for fabrication of channel waveguides with a selective laser-densification technique. This technique used metal lines fabricated with photolithography on the slab waveguide as a light absorbent, and these metal lines were heated by an Ar laser. The resultant channel waveguide had an optical propagation loss of 0.9 dB/ cm at 633 nm. This technique provides lower absorption loss and scattering loss compared with the direct laser-densification technique, which uses UV lasers, and produces narrow waveguides that are difficult to fabricate with a CO(2) laser.     

Formation of light-emitting Si nanostructures in SiO(2) by pulsed anneals

Intense excimer laser pulses, flash lamp annealing and rapid thermal annealing were used to form Si nanocrystals in thin SiO(2) layers implanted with high doses of Si ions. The pulse durations were 20?ns, 20?ms and 1?s, respectively. Laser annealing produced light sources luminescing in the wavelength range of 400-600?nm. They were attributed to the Si clusters formed as a result of the fast segregation of Si atoms from the SiO(2) network. There were no indications of nanocrystal formation in the as-implanted layers after 20?ns laser pulses; however, nanocrystals formed when, before the laser annealing, the amorphous Si nanoprecipitates were prepared in the oxide layers. Evaluations show that the crystallization may proceed via melting. A photoluminescence band near 800?nm, typical of Si nanocrystals, was found after 20?ms and 1?s anneals. Calculations revealed that the annealing times in both cases were too short to provide the diffusion-limited crystal growth if one uses the values of stationary Si diffusivity in SiO(2). This points toward the existence of a transient rapid growth process at the very beginning of the anneals.     

气相、沉淀、稻壳SiO2/聚氨酯复合材料的力学性能和吸水性比较

比较了气相、沉淀和稻壳SiO2的物理性质,并利用上述3种SiO2与聚氨酯(PU)复合,考察了SiO2含量(1wt%,3wt%,5wt%)和种类对SiO2/PU复合材料的拉伸性能和吸水性的影响。结果表明:3种SiO2均能够提高SiO2/PU复合材料的力学性质,复合材料的拉伸强度、模量和断裂伸长率均随SiO2含量增加而增加。当SiO2含量相同时,拉伸强度和断裂伸长率的变化顺序为:气相SiO2/PU>沉淀SiO2/PU>稻壳SiO2/PU,拉伸模量的变化顺序为:气相SiO2/PU>稻壳SiO2/PU>沉淀SiO2/PU.对于同一种SiO2/PU复合材料,吸水性随SiO2含量和温度增加而增加,含量相同时,3种SiO2/PU复合材料的吸水性由低到高的顺序是:稻壳SiO2/PU<沉淀SiO2/PU<气相SiO2/PU。     

基于量子级联激光器的中远红外高反射率测量

利用光腔衰荡技术开展了中远红外波段的高反镜反射率测量研究.以中心波长9.7μm附近的脉冲量子级联激光器为光源,构建了高反射率测量实验装置.利用该装置对不同腔长下腔镜的反射率进行了测量,最终测定其反射率为99.9464%,测量重复性误差优于0.0014%.实验表明该测量装置重复性精度高,可用于中远红外高反镜反射率的精确测量.     

退火对CdS/SiO2介孔组装体系吸收边的影响

本文作者通过溶胶-凝胶法获得块材CdS纳米颗粒/介孔SiO₂组装体系(CdS/SiO₂)块体样品,室温下的拉曼谱中观察到CdS的两个特征峰;在氮气和空气气氛中退火处理后,发现吸收边位置随复合量和退火温度不同而移动。经计算,CdS颗粒粒径的理论值与其自由激子半径相当,说明吸收边的移动起因于量子限域效应。     

Single-mode tunable Ti:sapphire laser over a wide frequency range

We report a design of a tunable Ti:sapphire laser capable of operating in the range between 700 and 800 nm. The continuous, single-mode tuning is achieved by a pseudoexternal cavity consisting of highly reflective mirrors and a diffraction grating. The advantages of this laser include low operational threshold, a simple configuration that involves only four optical elements, and fine-tuning capabilities. The single longitudinal mode of operation was demonstrated at wavelengths between 695 and 725 nm and was limited by the choice of end mirrors in the laser cavity.     

A comparative study of the UV optical and structural properties of SiO2, Al2O3, and HfO2 single layers deposited by reactive evaporation, ion-assisted deposition and plasma ion-assisted deposition

Growing requirements for the optical and environmental stability, as well as the radiation resistance against high-power laser radiation, especially for optical interference coatings used in the ultraviolet spectral range, have to be met by new, optimised, thin-film deposition technologies. For applications in the UV spectral range, the number of useful oxide thin film materials is very limited due to the higher absorption at wavelengths near to the electronic bandgap of the materials. Applying ion-assisted processes offers the ability to grow dense and stable films, but in each case careful optimisation of the deposition process (evaporation rate, substrate temperature, bombarding gas, ion energy and ion current density) has to achieve a balance between densification of the layers and the absorption. High-quality coatings and multilayer interference systems with SiO₂ as the low-index material can be deposited by various physical vapour deposition technologies, including reactive e-beam evaporation, ion-assisted deposition and plasma ion-assisted deposition. In order to improve the degradation stability of dielectric mirrors for use in UV free-electron laser optical cavities, a comparative study of the properties of SiO₂, Al₂O₃ and HfO₂ single layers was performed, and was addressed to grow very dense films with minimum absorption in the spectral range from 200 to 300 nm. The films were deposited by low-loss reactive electron-beam evaporation, by ion-assisted deposition using a ‘Mark II’ ion source, and by plasma ion-assisted deposition using the advanced plasma source. Optical and structural properties of the samples were studied by spectral photometry, infrared spectroscopy, X-ray diffraction and reflectometry, as well as by investigation of the surface morphology. The interaction of UV radiation with photon energy values close to the bandgap was studied. For HfO₂ single layers, laser-induced damage thresholds at 248 nm were determined in the 1-on-1 and 1000-on-1 test modes as a function of the deposition technology and film thickness.     

Toward resistant vacuum-ultraviolet coatings for free-electron lasers down to 150 nm

Research and development are currently trying to run a storage ring free-electron laser down to 150 nm with robust optics. Vacuum-ultraviolet fluoride optics with protected oxide layers and enhanced metallic mirrors are investigated.     

Discretely tunable single-mode lasers on GaSb using two-dimensional photonic crystal intracavity mirrors

We report on single-mode emitting coupled cavity ridge waveguide lasers on the GaSb material system in the 2?μm spectral range using two-dimensional (2D) photonic crystals (PhCs). Eight rows of 2D PhCs lateral to the ridge waveguides act as intermediate mirrors and are used to create two coupled cavities. This leads to preferential emission at one single longitudinal mode in the emission spectrum with side mode suppression ratios of 30-35?dB. Monolithic integration of high reflectivity 2D PhC back mirrors allows the realization of cavity lengths as short as 300?μm with threshold currents as low as 18.5?mA while reaching output powers well above 18?mW. Under variation of driving current the lasers exhibit both discrete and continuous tuning behavior over a wide current range very well explicable by simulation of the sub-threshold spectra, rendering the devices especially interesting for multi-gas sensing by absorption spectroscopy.     

Structural and optical properties of silicon nanocrystals grown by plasma-enhanced chemical vapor deposition

Silicon nanocrystals (Si-nc) embedded in SiO2 matrix have been prepared by high temperature thermal annealing (1000-1250 degrees C) of substoichiometric SiOx films deposited by plasma-enhanced chemical vapor deposition (PECVD). Different techniques have been used to examine the optical and structural properties of Si-nc. Transmission electron microscopy analysis shows the formation of nanocrystals whose sizes are dependent on annealing conditions and deposition parameters. The spectral positions of room temperature photoluminescence are systematically blue shifted with reduction in the size of Si-nc obtained by decreasing the annealing temperature or the Si content during the PECVD deposition. A similar trend has been found in optical absorption measurements. X-ray absorption fine structure measurements indicate the presence of an intermediate region between the Si-nc and the SiO2 matrix that participates in the light emission process. Theoretical observations reported here support these findings. All these efforts allow us to study the link between dimensionality, optical properties, and the local environment of Si-nc and the surrounding SiO2 matrix.     

Al2O3/SiO2 and HfO2/SiO2 dichroic mirrors for UV solid-state lasers

HfO₂/SiO₂ and Al₂O₃/SiO₂ multilayers to be employed as high reflectance end mirrors in Cerium-doped fluoride solid-state lasers were produced by radio frequency sputtering. The components were designed to have high transmittance at the pumping wavelength and high reflectance in a wavelength band corresponding to the active medium emission. A photoacoustic beam deflection technique and inspection of the irradiated area under a microscope were used to measure the laser induced damage threshold of the mirrors at the pumping wavelength. These coatings were tested in a laser cavity.     

Stability of a laser cavity with a two-mirror multipass system

An investigation is made of the stability of a laser cavity in which one of the nontransmitting mirrors is a multipass system consisting of two spherical mirrors. The stability of the cavity depends strongly on the number of passes of the beam in the multipass system, the configuration of the mirrors, and the constriction of the beam entering the system. A stable cavity configuration ensures low diffraction losses in the system and efficient lasing. Pis’ma Zh. Tekh. Fiz. 25, 33–38 (November 12, 1999)