Performance of high-density cast silicon carbide in the extreme ultraviolet

The normal-incidence reflectance of high-density cast silicon carbide (SiC) is evaluated in the extreme ultraviolet (EUV) spectral region. High reflectivity in the EUV is achieved. High reflectivity and the relatively low-cost manufacturing process make high-density cast SiC a promising mirror material for EUV applications.     

低镜面反射硅镀膜玻璃的微结构调制

通过反射率、透射率参量测试和高分辨电镜分析 ,研究了硅薄膜中微晶粒数量、尺寸和分布对镀膜玻璃镜面反射效果的影响。发现通过一定的热处理工艺可以调整硅膜中的晶粒状况 ,得到硅微晶粒数量众多、分布弥散且晶粒大小在数十至数百纳米的硅 /非晶硅复合薄膜 ,使入射光产生漫散射进而有效降低镜面反射所造成的光污染。     

Frequency switching multiwavelength Brillouin Raman fibre laser based on feedback power adjustment technique

A simple switchable multiwavelength Brillouin–Raman fibre laser (BRFL) was demonstrated. The laser was arranged in a half-open cavity configuration including a physical mirror device with an adjustable reflectivity at one side of the laser cavity. The impact of the feedback power adjustment on frequency switching was carried out by comparing the peak power difference between odd- and even-order Stokes lines. Up to 468 flat-amplitude lines with a 10?GHz frequency spacing and average optical signal to noise ratio (OSNR) of 33 dB were observed with mirror reflectivity values of ～15% up to 60% at a 1534?nm Brillouin pump power of 7?dBm and 0.9?W Raman pump power of 0.9?W. – Under the same pumping conditions, setting the mirror reflectivity at its OFF state (where reflectivity is nearly 0%) allows for up to 242 lines with 20?GHz spacing to be realized, with ONSR values of ～35?dB.     

乙烯对CVD法制备硅薄膜性能的影响

以硅烷和乙烯的混合气体的原料气,通过常压化学气相沉积方法在玻璃衬底上生长出了硅薄膜。结合光电子能谱、高分辨电镜以及薄膜抗腐蚀性研究,证实乙烯的引入使硅薄膜中形成一定数量的ＳｉＣ微晶粒。ＳｉＣ较高的化学稳定性使薄膜具有较强的耐碱性,而其较小的折射率使薄膜的镜面反射率较高低。含有ＳｉＣ的硅薄膜经碱液适当腐蚀后能降低镜面反射,进而有可能减少光污染现象。     

Frequency-dependent graded reflectivity mirror: characterization and laser implementation

We present a theoretical and experimental investigation of a new family of variable reflectivity mirrors, based on frustrated total internal reflection and interference effects. These mirrors are sensitive to frequency in the sense that their transverse reflectivity distribution changes significantly as a function of the frequency of light. The mirror reported here shows total power reflectivity changes of 50% within 8.0 GHz. The mirror was tested as the output coupler of an unstable resonator in a Nd:YAG laser working in the free-running regime. This configuration was compared with the standard stable multimode resonator configuration. The beam divergence was reduced by more than 1 order of magnitude and the output power was reduced by only 10%. The laser resonator mode competition that is due to the frequency-dependent mirror reflectivity distribution is discussed.     

Laser diode facet modal reflectivity measurements

A simple and accurate method for measuring the front facet modal reflectivity of a Fabry-Perot laser diode is presented. In this method, optical feedback from an external mirror of known reflectivity, R(ext), is used to alter the laser diode threshold current. The effect of the external mirror and front facet reflectivities on the threshold current then allows for a measurement of the front facet modal reflectivity of the laser diode and is theoretically and experimentally studied. This method was used to measure a facet reflectivity of R(2) = 0.0151(+0.0018/-0.0032) [R(2) = 0.00592(+0.00085/-0.00123)] for a commercially antireflection-coated facet of a laser diode with a center wavelength of 795 nm (935 nm). The results of the reflectivity measurements based on the threshold current as a function of the external mirror reflectivity are compared with the results of the reflectivity measurements based on modulation depth of the optical spectrum [IEEE J. Quantum Electron. QE-19, 493 (1983)].     

Investigation of aperiodic W/C multi-layer mirror for X-ray optics

Design, fabrication and characterization of aperiodic tungsten/carbon (W/C) multi-layer mirror were studied. W/C multi-layer was designed as a broad-angle reflective supermirror for Cu-Kα line (λ = 0.154 nm) in the grazing incident angular range (0.9-1.1°) using simulated annealing algorithm. To deposit the W/C depth-graded multi-layer mirror accurately, we introduce an effective layer growth rate as a function of layer thickness. This method greatly improves the reflectivity curve compared to the conventional multi-layer mirror prepared with constant growth rate. The deposited multi-layer mirror exhibits an average reflectivity of 19% over the grazing incident angle range of 0.88-1.08° which mainly coincides with the designed value. Furthermore, the physical mechanisms were discussed and the re-sputtering process of light-atom layers is accounted for the modification of layer thicknesses which leads to the effective growth rates. Using this calibration method, the aperiodic multi-layer mirrors can be better fabricated for X-ray optics.     

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.     

金刚石线锯切割多晶硅片表面制绒工艺研

晶体硅片的制绒技术是太阳能电池制造工艺中的关键步骤。本研究以工业中酸制绒方法为基础, 研究了腐蚀时间、浓度对绒面结构以及反射率的影响。此外, 还采用金属催化化学腐蚀法进行制绒, 选用氢氟酸和硝酸银作为腐蚀液。而且对两种制绒方法效果进行了对比。研究获得的最优绒面结构及反射率结果的实验条件为: 氢氟酸浓度4.6 mol/L、硝酸银浓度0.02 mol/L, 室温下反应90 min, 得到的平均反射率为8%, 远低于目前多晶硅片制绒生产标准。     

Efficient acousto-optic Q switching of Er:YSGG lasers at 2.79-microm wavelength

A flash-lamp-pumped Er:Cr:YSGG laser at 2.79-mum wavelength has been acousto-optically Q switched. The Q-switched pulse energy and duration depend on pump pulse level and relative Q-switching time. Limits of single-pulse operation with the given acousto-optic diffraction efficiency have been determined. Resonator length, position of the Q switch, and output mirror reflectivity have been varied to obtain high pulse energy and the shortest pulse duration in the TEM(00) transverse laser mode. A maximum single-pulse energy of 27 mJ and a minimum pulse duration of 120 ns were obtained with an output mirror reflectivity of approximately 25%. The highest Q-switched single-pulse energy amounted to 52% of the free-running, fundamental mode output pulse energy.     

Apodizing holographic gratings for the modal control of semiconductor lasers

We first present the fabrication technique of apodizing holographic gratings. Gratings with a spatially variable reflectivity profile were obtained by the interference of two Gaussian beams on a glass plate covered with a photoresist. When the exposure time was short enough to avoid saturation of the photoresist, gratings with a quasi-Gaussian reflectivity profile for the beam reflected in the -1 order were produced; the reflectivity at the center could be as high as 71%, and the half-width of the reflectivity profile at the e(-1) position could be as small as 180 mum. Apodizing gratings were used as the end mirror of the external cavity of a broad-area semiconductor laser. Single longitudinal- and lateral-mode operation was observed over the full range of allowed injection currents.     

Effect of electroless etching parameters on the growth and reflection properties of silicon nanowires

Vertically aligned silicon nanowire (Si NW) arrays have been fabricated over large areas using an electroless etching (EE) method, which involves etching of silicon wafers in a silver nitrate and hydrofluoric acid based solution. A detailed parametric study determining the relationship between nanowire morphology and time, temperature, solution concentration and starting wafer characteristics (doping type, resistivity, crystallographic orientation) is presented. The as-fabricated Si NW arrays were analyzed by field emission scanning electron microscope (FE-SEM) and a linear dependency of nanowire length to both temperature and time was obtained and the change in the growth rate of Si NWs at increased etching durations was shown. Furthermore, the effects of EE parameters on the optical reflectivity of the Si NWs were investigated in this study. Reflectivity measurements show that the 42.8% reflectivity of the starting silicon wafer drops to 1.3%, recorded for 10 μm long Si NW arrays. The remarkable decrease in optical reflectivity indicates that Si NWs have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection.     

High-wavelength-resolution extreme-ultraviolet multilayer mirror

Multilayer mirrors with a system wavelength resolution (lambda/Dlambda) as high as 30-50 are required for the diagnostics of cosmic plasmas with temperatures of 1-20 MK. Such a high wavelength resolution can be realized by increasing the number of layer pairs contributing to the reflectance, by selecting less-absorbing materials for both the reflector and the spacer, and by decreasing the thickness of the reflector. We have fabricated a multilayer mirror tuned to 284 A with a silicon carbide reflector (20% thickness of the layer period) and an aluminum spacer and achieved lambda/Dlambda ~26.8 with a peak reflectivity of ~13.0%. This wavelength resolution is close to the value obtained with a numerical simulation and is considerably higher than the value obtained with the conventional Mo/Si multilayer.     

HfO2/SiO2 chirped mirrors manufactured by electron beam evaporation

A HfO2/SiO2 chirped mirror was manufactured by electron beam evaporation to increase the laser resistance. The hybrid monitoring strategy utilizing both monochromatic monitoring and quartz crystal monitoring was applied to the deposition compared to the single optical monitoring method. The coatings were characterized by transmission spectrophotometer and white light interferometry, and the experimental results showed that the chirped mirror monitored with the hybrid strategy possessed high reflectivity (>99.7%) and tolerable group delay dispersion oscillation (-50±20?fs2) in the spectra range of 740-860?nm.     

Interface engineered ultrashort period Cr-Ti multilayers as high reflectance mirrors and polarizers for soft x rays of lambda = 2.74 nm wavelength

Cr-Ti multilayers with ultrashort periods of 1.39-2.04 nm have been grown for the first time as highly reflective, soft-x-ray multilayer, near-normal incidence mirrors for transition radiation and Cherenkov radiation x-ray sources based on the Ti-2p absorption edge at E = 452 eV (lambda = 2.74 nm). Hard, as well as soft, x-ay reflectivity and transmission electron microscopy were used to characterize the nanostructure of the mirrors. To achieve minimal accumulated roughness, improved interface flatness, and to avoid intermixing at the interfaces, each individual layer was engineered by use of a two-stage ion assistance process during magnetron sputter deposition: The first 0.3 nm of each Ti and Cr layer was grown without ion assistance, and the remaining 0.39-0.72 nm of the layers were grown with high ion-neutral flux ratios phi (phiTi = 3.3, phiCr = 2.2) and a low energy Eion (ETi = 23.7 and ECr = 21.2), ion assistance. A maximum soft-x-ray reflectivity of R = 2.1% at near-normal incidence (approximately 78.8 degrees) was achieved for a multilayer mirror containing 100 bilayers with a modulation period of 1.379 nm and a layer thickness ratio of tau = 0.5. For a polarizing multilayer mirror with 150 bilayers designed for operation at the Brewster angle, 45 degrees, an extinction ratio, Rs/Rp, of 266 was achieved with an absolute reflectivity of R = 4.3%.     

Measured limits to contamination of optical surfaces by elastomers in vacuum

We have monitored the reflectivity of mirrors that were exposed to a fluoroelastomer (3M-Fluorel 2176) and a room-temperature vulcanizing silicone rubber (RTV-615) in vacuum. The 95% confidence limit on the decrease of mirror reflectivities was less than 0.35 ppm/week for Fluorel and <0.29 ppm/week for RTV-615.     

Far-field intensity distribution of a beam generated by a resonator with a phase-unifying mirror

The far-field intensity distribution (FFID) of a beam generated by a phase-unifying mirror resonator was investigated based on scalar diffraction theory. Attention was paid to the parameters, such as obscuration ratio and reflectivity of the phase-unifying mirror, that determine the FFID. All analyses were limited to the TEM00 fundamental mode.     

Analytic solution for the peak reflectivity of an asymmetric mirror

Asymmetric Bragg reflectors have been shown to optimize mirror performance in strained-layer material systems. Although the theory behind the reflectivity of symmetric mirrors has been well studied, little is known about asymmetric mirror designs. We present a closed-form solution for the peak reflectivity of an asymmetric mirror that results from applying a tanh substitution. This elegant technique has been shown to yield a markedly simplified calculation of the peak reflectivity of a symmetric mirror. These analytic expressions will be useful in mirror design by providing a straightforward way to compare the trade-offs between asymmetric and symmetric mirror designs.     

Making an omnidirectional reflector

The effect of having a finite number of layers on the design of omnidirectional reflectors was investigated. It was shown that the structure should be finished with a low-index layer having a thickness larger than a quarter-wave to increase reflectivity, whereas layers below may remain of quarter-wave optical thickness at normal incidence angle. This general trend has been used for designing and realizing two a-Si-SiO(2) (amorphous silicon and silicon dioxide) omnidirectional reflectors in the near-infrared range on a silicon and a silica substrate, respectively. Owing to the decrease of absorption of recrystallized silicon as compared with a-Si in the visible range, the transmissivity of the structure realized on silica substrate was dramatically increased in the visible range upon annealing, whereas the high reflectivity and the omnidirectional effect were maintained in the near-infrared range.     

Production and performance of multilayer-coated conical x-ray mirrors

A method of fabricating replica figured x-ray optics with integral multilayer coatings is presented. With the intact electroforming multilayer process (IEMP) technique, we sputter multilayers onto a reusable superpolished mandrel, electroform nickel over the multilayers, and remove the multilayer-coated nickel shell intact from the mandrel. This approach offers advantages over more traditional, original, and segmented-replica fabrication techniques, including low cost; compatibility with a wide range of mirror designs, diameters, and focal lengths; simple integration with multilayer sputtering processes; and the ability to produce complete shells of revolution. The fabrication of W/Si multilayer-coated 10-cm-diameter conical x-ray mirrors is described, as are reflectivity measurements at 10 and 30 keV. The measured reflectivity of the IEMP multilayers at the 10-keV primary Bragg peak was 17%. Measurements of multiple points on the cone showed multilayer uniformity to within a few percent around the mirror.