Preparation and characterization of sol-gel silica based neutral optical density coatings by the addition of graphite particles

Silica based coatings with various thicknesses (600-3000 nm) were prepared from the gellation of aqueous suspensions composed of colloidal silica particles, small graphite particles and inorganic binder materials, the latter were included to improve the coating structure. The graphite particles added in amounts of 0.8-3.6 wt.%, in the dried coating, absorb visible light with the same efficiency, which results in coatings with neutral optical density in this range of the spectrum. The optical density of the coatings can be changed from basically 0, in coatings without graphite, to more than 2 in thick coatings with 3.6 wt.% of graphite. The structural characterization of the coatings was carried out using X-ray diffraction, Raman scattering and atomic force microscopy.     

Scattering of light by laser fusion targets with small defects

The extended boundary condition method was used to determine the effect imperfections in small glass and metal shells have on scattered light at 10.6 microm. The results indicate that imperfections cause a shift in the locations of the minima in the differential scattering curve, a change in the extinction efficiency, and the presence of depolarized components for off-axis orientation of the object. Among these, the presence of and changes in the depolarized component are most sensitive to imperfections. We compute the depolarized scatter from shells with types I, II, and III defects and discuss the potential of light scattering as a characterization tool for laser fusion targets     

Tetragonal Single‐Crystalline Boron Nanowires with Strong Anisotropic Light Scattering Behaviors and Photocurrent Response in Visible‐Light Regime

Boron is a narrow‐bandgap (1.56 eV) semiconductor with high melting‐point, low‐density, large Young's modulus and very high refractive index (3.03) close to silicon. Therefore, boron nanostructures is expected to possess strong visible‐light scattering properties. However, photonic and optoelectronic properties of the boron nanostructures are seldom studied until now. In this paper, we have successfully prepared single‐crystalline boron nanowire (BNW) arrays with high‐density on Si substrate. All the BNWs are found to possess strong light‐scattering behaviors in the visible regime. Most of all, the scattered light is found to polarize along the longitudinal direction of the nanowire. They also have excellent second‐harmonic generation (SHG) properties under ultrafast laser irradiation. Further optoelectronic measurements show that an individual BNW device exhibits notable photocurrent responses in the visible‐light range at ambient conditions, which can be attributed to the strong coupling effect between individual BNW and the visible light. The maximum photoresponsivity of an individual BNW can reach up to 12.12 A W^–1 at a voltage of 10 V, and the response time is only 18 ms. Therefore, it unveils that the BNWs have a promising future in visible‐light communications and detections.     

Multiscale roughness in optical multilayers: atomic force microscopy and light scattering

We have previously shown that macroscopic roughness spectra measured with light scattering at visible wavelengths were perfectly extrapolated at high spatial frequencies by microscopic roughness spectra measured with atomic force microscopy [Europhys. Lett. 22, 717 (1993); Proc. SPIE 2253, 614 (1994)]. These results have been confirmed by numerous experiments [Proc. SPIE 2253, 614 (1994)] and allow us today to characterize thin films microstructure from a macroscopic to a microscopic scale. In the first step the comparison of light scattering and atomic force microscopy is completed by optical measurements at UV wavelengths that allow us to superimpose (and no longer extrapolate) the spectra measured by the two techniques. In the second step we extract multiscale parameters that describe the action of thin-film coatings on substrate roughness in all bandwidths. The results obviously depend on materials and substrates and deposition techniques. Electron-beam evaporation, ion-assisted deposition, and ion plating are compared, and the conclusions are discussed in regard to the deposition parameters. Finally, special attention is given to the limits and performances of the two characterization techniques (light scattering and atomic force microscopy) that may be sensitive to different phenomena.     

气溶胶对远红外激光辐射的衰减规律研究

计算分析了10.6μm波长远红外激光辐射在不同能见度条件下的大气气溶胶中水平传输的衰减系数、有效传输距离以及斜程传输时的透过率变化规律,对比分析了远红外激光辐射在1.0g/m3的高浓度酸雾和油雾气溶胶中的传输能力。根据Mie理论计算了直径为0.5～40μm的水溶性大气气溶胶和尘状气溶胶粒子对10.6μm激光的散射效率因子、吸收效率因子和消光效率因子。结果表明:气溶胶的消光系数越大、大气能见度越低,大气气溶胶对远红外激光辐射的衰减越严重;在1.0g/m3的高浓度酸雾和油雾气溶胶中远红外激光辐射的有效传输距离只有20～50m。水溶性大气气溶胶和尘状气溶胶粒子对10.6μm激光辐射的衰减机理基本相同,其中散射作用居于主导地位并且平均直径大于5μm的气溶胶粒子对10.6μm远红外激光辐射具有显著的衰减作用。     

Light intensification modeling of coating inclusions irradiated at 351 and 1053 nm

Electric-field modeling provides insight into the laser damage resistance potential of nodular defects. The laser-induced damage threshold for high-reflector coatings is 13x lower at the third harmonic (351 nm) than at the first harmonic (1053 nm) wavelength. Linear and multiphoton absorption increases with decreasing wavelength, leading to a lower-third harmonic laser resistance. Electric-field effects can also be a contributing mechanism to the lower laser resistance with decreasing wavelength. For suitably large inclusions, the nodule behaves as a microlens. The diffraction-limited spot size decreases with wavelength, resulting in an increase in intensity. Comparison of electric-field finite-element simulations illustrates a 3x to 16x greater light intensification at the shorter wavelength.     

The influence and detection of processing induced defects in roving based composites

The fracture properties of composites are strongly dependent on defects originating from the heterogeneous structure of the composite, the imperfections of the processing methods and the properties of the raw materials. The influence of the glass content, the fibre distribution, the binders etc on the debonding, intra-sttrand crack growth and the final catastrophic crack growth through the inter-strand matrix rich regions has been studied for roving based composites, using a series of chopped strand mat, glass fibre/polyester laminates as model materials. The studies include measurements of stress-strain relations, acoustic emission, hysteresis, mechanical loss factor, laser light scattering, electron and visible light microscopy.     

Measurement system to determine the total and angle-resolved light scattering of optical components in the deep-ultraviolet and vacuum-ultraviolet spectral regions

An instrumentation for total and angle-resolved scattering (ARS) at 193 and 157 nm has been developed at the Fraunhofer Institute in Jena to meet the severe requirements for scattering analysis of deep- and vacuum-ultraviolet optical components. Extremely low backscattering levels of 10(-6) for the total scattering measurements and more than 9 orders of magnitude dynamic range for ARS have been accomplished. Examples of application extend from the control of at-wavelength scattering losses of superpolished substrates with rms roughness as small as 0.1 nm to the detection of volume material scattering and the study into the scattering of multilayer coatings. In addition, software programs were developed to model the roughness-induced light scattering of substrates and thin-film coatings.     

Pulsed Raman fiber laser and multispectral imaging in three dimensions

Raman scattering in single-mode optical fibers is exploited to generate multispectral light from a green nanolaser with high pulse repetition rate. Each pulse triggers a picosecond camera and measures the distance by time-of-flight in each of the 0.5 Mpixels. Three-dimensional images are then constructed with submillimeter accuracy for all visible colors. The generation of a series of Stokes peaks by Raman scattering in a Si fiber is discussed in detail and the laser radar technique is demonstrated. The data recording takes only a few seconds, and the high accuracy 3D color imaging works at ranges up to approximately 200 m. Applications for optical tomography in highly scattering media such as water and human tissue are mentioned.     

An experimental approach for growth rate determination of LCVD-written aluminium stripes

A new experimental approach for growth rate determination of microdimensional aluminium stripes written by laser-induced chemical vapour deposition (LCVD) was developed. The aluminium lines were obtained by pyrolysis of trimethylaluminium (TMA) on (100) silicon monocrystalline wafer using the focused beam of a copper bromide vapour laser. Quantitative determination of the deposit was performed by its chemical removal from the substrate into a solution and further analysis performed using inductively coupled plasma atomic emission spectroscopy. Using this analytical method, dependences of the aluminium quantity on the partial pressure of TMA and the laser power were obtained. The growth rate at direct writing with a pulsed visible laser was calculated on the basis of aluminium quantity determinations. It is demonstrated that the proposed experimental approach could be successfully used for kinetic studies of LCVD processes.     

Laser damage resistance of hafnia thin films deposited by electron beam deposition, reactive low voltage ion plating, and dual ion beam sputtering

A comparative study is made of the laser damage resistance of hafnia coatings deposited on fused silica substrates with different technologies: electron beam deposition (from Hf or HfO(2) starting material), reactive low voltage ion plating, and dual ion beam sputtering. The laser damage thresholds of these coatings are determined at 1064 and 355 nm using a nanosecond pulsed YAG laser and a one-on-one test procedure. The results are associated with a complete characterization of the samples: refractive index n measured by spectrophotometry, extinction coefficient k measured by photothermal deflection, and roughness measured by atomic force microscopy.     

Optical coatings for laser fusion applications

Lasers for fusion experiments use thin film dielectric coatings for reflecting, antireflecting and polarizing surface elements. Coatings are most important in neodymium-doped glass lasers. The most important requirements of these coatings are the accuracy of the average value of reflectance and transmission, the uniformity of amplitude and phase front of the reflected or transmitted light and the laser damage threshold. Damage resistance strongly affects the laser's design and performance. The success of advanced lasers for future experiments and for reactor applications requires significant developments in damage-resistant coatings for UV laser radiation.     

Laser intensification by spherical inclusions embedded within multilayer coatings

The initiation of laser damage within optical coatings can be better understood by electric-field modeling of coating defects. The result of this modeling shows that light intensification as large as 24x can occur owing to these coating defects. Light intensification tends to increase with inclusion diameter. Defects irradiated over a range of incident angles from 0 to 60 deg tend to have a higher light intensification at a 45 deg incidence. Irradiation wavelength has a significant effect on light intensification within the defect and the multilayer. Finally, shallow, or in the case of 45 deg irradiation, deeply embedded inclusions tend to have the highest light intensification.     

Standardisierte Charakterisierung optischer Komponenten

Standardized Characterization of Optical Components The scope of the present contribution is to present a plain review on prominent measurement techniques for the determination of quality parameters for optical coatings and laser components. Emphasis is imposed on standardized test methods for the spectral transfer characteristics, the optical losses induced by scattering and absorption, as well as for laser induced damage thresholds of coated optics. The fundamental principles of the measurement methods are considered und illustrated by examples selected from typical applications. This contribution is intended to introduce the measurement principles on a practical basis and to give indications for a deeper insight into the topic     

Low-level scattering and localized defects

We investigate the origin of low-level scattering from high-quality coatings produced by ion-assisted deposition and ion plating. For this purpose we use the polarization ratio of light scattering to separate surface and bulk effects that characterize the intrinsic action of the thin-film materials. In the first step the method is tested and validated at scattering levels greater than 10(-5). In the second step it is applied at low levels, and the results reveal some anomalies. To conclude, we perform a detailed analysis of scattering resulting from the presence of a few localized defects in the coatings.     

Ultrasound-modulated optical tomography of absorbing objects buried in dense tissue-simulating turbid media

Continuous-wave ultrasonic modulation of scattered laser light was used to image objects buried in tissue-simulating turbid media. The buried object had an absorption coefficient greater than the background turbid medium. The ultrasonic wave that was focused into the turbid media modulated the laser light that passed through the ultrasonic field. The modulated laser light that was collected by a photomultiplier tube reflected the local mechanical and optical properties in the zone of ultrasonic modulation. Objects buried in the middle plane of 5-cm-thick dense turbid media were imaged with millimeter resolution through the scanning and detecting alterations of the ultrasound-modulated optical signal. The optical properties of the dense turbid media included an absorption coefficient of 0.1 cm(-1) and a reduced scattering coefficient of 10 cm(-1) and were comparable with those of biological tissues in the visible and near-IR ranges. The dependence of the ultrasound-modulated optical signal on the off-axis distance of the detector from the optic axis and the area of the detector was studied as well.     

In situ measurement on ultraviolet dielectric components by a pulsed top-hat beam thermal lens

A simple and sensitive mode-mismatched thermal lens (TL) technique with a pulsed top-hat beam excitation and a near-field detection scheme is developed to measure in situ the thermoelastic and the thermooptical responses of ultraviolet (UV) dielectric coatings as well as bulk materials under excimer laser (193- or 248-nm) irradiations. Owing to its high sensitivity, the TL technique can be used for measurements at fluences far below the laser-induced damage threshold (LIDT). We report on the measurement of both linear and nonlinear absorption of the UV dielectric coatings and bulk materials as well as the investigation of time-resolved predamage phenomena, such as laser conditioning of highly reflective dielectric coatings and irradiation-induced changes of a coating's various properties. The pulsed TL technique is also a convenient technique for accurate measurement of the LIDT of dielectric coatings and for distinguishing different damage mechanisms: thermal-stress-induced damage or melting-induced damage.     

Self-colored crosslinked cholesteric liquid crystalline solid films of hydroxypropyl cellulose

Solutions of lyotropic cholesteric liquid crystalline hydroxypropyl cellulose (HPC) in water were self-colored due to the selective reflection of visible light, depending on the solution concentration. It was attempted to apply colored coatings of the liquid crystalline aqueous HPC solutions. HPC solid films were cast from the liquid crystalline solutions at different conditions and the color of the films was controlled. The cast films were chemically crosslinked. The crosslinked cast films were self-colored blue, yellow, or orange, but not red. Circular dichroism study of the cast films revealed that some films exhibited the peak wavelength in the red region of visible light. Scanning electron microscopy also showed that some films had the cholesteric pitch corresponding to the red region of visible light. However, colorimetry of the films indicated that no red films were prepared. This may be due to the highly varied distribution of the pitch. Some problems for applying the HPC liquid crystalline solution as colored coatings still remained open.     

Surface plasmonic effect of nanoparticle-like silver nanostructure on the high responsivity of visible/infrared silver-based heterojunction photodetector

The fabrication of a silver (Ag) based photodetector on a silicon dioxide/p-silicon (SiO₂/p-Si) substrate using direct current (DC) magnetron sputtering is demonstrated. The proposed method deposits a nanoparticle-like Ag thin film that favours the photoconduction mechanism under light illumination at 468?nm and laser illumination at 660 and 980?nm. The thin film is characterized using scanning electron microscope (SEM), energy dispersive x-ray (EDX), x-ray diffraction (XRD), Raman scattering, photoluminescence (PL) and ultraviolet–visible (UV–VIS) analysis. Current–voltage (I–V) analysis and the calculated rectifying ratio (RR) suggests the establishment of good Schottky contacts for incident light/laser at 468, 660 and 980?nm, with good responsivity towards light and laser illumination in the forward and reverse DC bias regions. The responsivity increases as the wavelength decreases from 980?nm → 660?nm → 468?nm, with the highest responsivity of 213.7?mAW^?1 at 468?nm indicating better photoconduction at low light powers.     

Photon correlation and scattering: introduction to the feature issue

This feature issue of Applied Optics contains 31 research papers on photon correlation and scattering, many of which were presented at an OSA Topical Meeting that was held 21-23 August 2000 in Whistler, British Columbia, Canada. These papers focus on research in dynamic light scattering, surface light scattering, photon correlation, and laser velocimetry and their applications to physical, chemical, and biological processes.