Absolute Reflectance Measurements Research Articles

Absolute EUV reflectivity measurements using a broadband high-harmonic source and an in situ single exposure reference scheme.

We present a tabletop setup for extreme ultraviolet (EUV) reflection spectroscopy in the spectral range from 40 to 100 eV by using high-harmonic radiation. The simultaneous measurements of reference and sample spectra with high energy resolution provide precise and robust absolute reflectivity measurements, even when operating with spectrally fluctuating EUV sources. The stability and sensitivity of EUV reflectivity measurements are crucial factors for many applications in attosecond science, EUV spectroscopy, and nano-scale tomography. We show that the accuracy and stability of our in situ referencing scheme are almost one order of magnitude better in comparison to subsequent reference measurements. We demonstrate the performance of the setup by reflective near-edge x-ray absorption fine structure measurements of the aluminum L2/3 absorption edge in α-Al2O3 and compare the results to synchrotron measurements.

(Invited) Complex Relative Permittivity of UV-C Transparent AlN

The optical absorption coefficients of high-quality 2-inch AlN substrates grown by physical vapor transport (PVT) were accurately calculated from transmittance and absolute reflectance measurements in the ultraviolet-visible (UV-Vis) range. Spatially uniform UV absorption coefficients of 13-23 cm-1 at 4.68 eV (265 nm) were demonstrated. This approach also provided accurate values for the reflection coefficients, allowing a determination of the complex refractive index and relative permittivity, which showed good agreement with previous reports. Values for the high-frequency and static permittivities obtained in this study were 4.22 and 7.84, respectively. The data specific to PVT AlN substrates obtained using this method are useful in the design of electronic and optoelectronic devices grown on these substrates.

Reflectometers for Absolute and Relative Reflectance Measurements in the Mid-IR Region at Vacuum.

We demonstrated spectral reflectometers for two types of reflectances, absolute and relative, of diffusely reflecting surfaces in directional-hemispherical geometry. Both are built based on the integrating sphere method with a Fourier-transform infrared spectrometer operating in a vacuum. The third Taylor method is dedicated to the reflectometer for absolute reflectance, by which absolute spectral diffuse reflectance scales of homemade reference plates are realized. With the reflectometer for relative reflectance, we achieved spectral diffuse reflectance scales of various samples including concrete, polystyrene, and salt plates by comparing against the reference standards. We conducted ray-tracing simulations to quantify systematic uncertainties and evaluated the overall standard uncertainty to be 2.18% (k = 1) and 2.99% (k = 1) for the absolute and relative reflectance measurements, respectively.

Hazes and clouds in a singular triple vortex in Saturn's atmosphere from HST/WFC3 multispectral imaging

In this paper we present a study of the vertical haze and cloud structure over a triple vortex in Saturn's atmosphere in the planetographic latitude range 55°N-69°N (del Río-Gaztelurrutia et al., 2018) using HST/WFC3 multispectral imaging. The observations were taken during 29–30 June and 1 July 2015 at ten different filters covering spectral range from the 225 nm to 937 nm, including the deep methane band at 889 nm. Absolute reflectivity measurements of this region at all wavelengths and under a number of illumination and observation geometries are fitted with the values produced by a radiative transfer model. Most of the reflectivity variations in this wavelength range can be attributed to changes in the tropospheric haze. The anticyclones are optically thicker (τ ~25 vs ~10), more vertically extended (~3 gas scale heights vs ~2) and their bases are located deeper in the atmosphere (550 mbar vs 500 mbar) than the cyclone.

Investigation of light propagation and detection in human head for realistic settings under clinical conditions

Investigation of light propagation and detection in human head for realistic settings under clinical conditions

Haze and cloud structure of Saturn's North Pole and Hexagon Wave from Cassini/ISS imaging

In this paper we present a study of the vertical haze and cloud structure in the upper two bars of Saturn's Northern Polar atmosphere using the Imaging Science Subsystem (ISS) instrument onboard the Cassini spacecraft. We focus on the characterization of latitudes from 53° to 90° N. The observations were taken during June 2013 with five different filters (VIO, BL1, MT2, CB2 and MT3) covering spectral range from the 420 nm to 890 nm (in a deep methane absorption band). Absolute reflectivity measurements of seven selected regions at all wavelengths and several illumination and observation geometries are compared with the values produced by a radiative transfer model. The changes in reflectivity at these latitudes are mostly attributed to changes in the tropospheric haze. This includes the haze base height (from 600 ± 200 mbar at the lowest latitudes to 1000 ± 300 mbar in the pole), its particle number density (from 20 ± 2 particles/cm3 to 2 ± 0.5 particles/cm3 at the haze base) and its scale height (from 18 ± 0.1 km to 50 ± 0.1 km). We also report variability in the retrieved particle size distribution and refractive indices. We find that the Hexagonal Wave dichotomizes the studied stratospheric and tropospheric hazes between the outer, equatorward regions and the inner, Polar Regions. This suggests that the wave or the jet isolates the particle distribution at least at tropospheric levels.

Measurement of the absolute reflectance of polytetrafluoroethylene (PTFE) immersed in liquid xenon

The performance of a detector using liquid xenon (LXe) as a scintillator is strongly dependent on the collection efficiency for xenon scintillation light, which in turn is critically dependent on the reflectance of the surfaces that surround the active volume. To improve the light collection in such detectors the active volume is usually surrounded by polytetrafluoroethylene (PTFE) reflector panels, used due to its very high reflectance—even at the short wavelength of scintillation light of LXe (peaked at 178 nm). In this work, which contributed to the overall R&D effort towards the LUX-ZEPLIN (LZ) experiment, we present experimental results for the absolute reflectance measurements of three different PTFE samples (including the material used in the LUX detector) immersed in LXe for its scintillation light. The obtained results show that very high bi-hemispherical reflectance values (⩾ 97%) can be achieved, enabling very low energy thresholds in liquid xenon scintillator-based detectors.

Note: Visible reflectivity system for high-pressure studies

A near-normal visible reflectivity system was constructed for investigating the optical properties of materials at high pressure and temperatures. The system consists of focusing optics, spectrometers, and imaging cameras for use with a resistively-heated membrane-controlled diamond anvil cell (DAC). The DAC allows optical access on two sides: one dedicated to pressure measurement via fluorescence, the other side dedicated to absolute reflectance measurements. With this configuration, pressure and temperature may be controlled and measured without disturbing the sample position, permitting quantitative reflectivity measurements as a function of thermodynamic state variables. The system was tested on gold at high pressure conditions, and compared to data obtained on a commercial ellipsometer system at 1 bar.

Optical Nonlinearity for Few-Photon Pulses on a Quantum Dot-Pillar Cavity Device

Giant optical nonlinearity is observed under both continuous wave and pulsed excitation in a deterministically coupled quantum dot-micropillar system, in a pronounced strong-coupling regime. Using absolute reflectivity measurements we determine the critical intracavity photon number as well as the input and output coupling efficiencies of the device. Thanks to a near-unity input-coupling efficiency, we demonstrate a record nonlinearity threshold of only 8 incident photons per pulse. The output-coupling efficiency is found to strongly influence this nonlinearity threshold. We show how the fundamental limit of single-photon nonlinearity can be attained in realistic devices, which would provide an effective interaction between two coincident single-photons.

Extracting Infrared Absolute Reflectance from Relative Reflectance Measurements

Absolute reflectance measurements are valuable to the optics industry for development of new materials and optical coatings. Yet, absolute reflectance measurements are notoriously difficult to make. In this paper, we investigate the feasibility of extracting the absolute reflectance from a relative reflectance measurement using a reference material with known refractive index.

Reflectivity measurement inside small hollow mirrors using an x-ray tube

Small hollow x-ray mirrors are manufactured by several techniques. The mirror material—the reflecting surface itself—can be either a simple metal, such as Au or Ni, or a multilayer. The development of a manufacturing technique, however, requires a simple way of measuring the mirror reflectivity curves. This may be done using a pencil beam at the synchrotron, but it is rather difficult to make it in a laboratory using an x-ray tube. This paper describes the development of a method of carrying out absolute reflectivity measurements inside these mirrors using only a typical laboratory source. Thus, we provide a fast, convenient and inexpensive way to perform mirror characterization. The method was tested in a laboratory and verified in synchrotron experiments in the case of conical micromirrors, while tests for other mirror shapes are under way. The method utilizes only an x-ray tube, an x-ray camera and linear motorized stages. The tube and the camera are moving relative to the mirror in a predefined way taking images of the peak. Detected images are then automatically processed in order to obtain a reflectivity curve. A fully automatic measurement including automatic focusing is also possible and is under development. A typical manual measurement takes approximately 1–2 h for one reflectivity curve at present.

CloudSat Studies of Stratiform Precipitation Systems Observed in the Vicinity of the Southern Great Plains Atmospheric Radiation Measurement Site

Abstract The spaceborne W-band (94 GHz) radar on board the CloudSat polar-orbiting satellite offers new opportunities for retrieving parameters of precipitating cloud systems. CloudSat measurements can resolve the vertical cross sections of such systems. The radar brightband features, which are commonly present when observing stratiform precipitating systems, allow the vertical separation of the ice, mixed, and liquid precipitating hydrometeor layers. In this study, the CloudSat data are used to simultaneously retrieve ice water path (IWP) values for ice layers of precipitating systems using absolute radar reflectivity measurements and mean rainfall rates Rm in the liquid hydrometeor layers using the attenuation-based reflectivity gradient method. The retrievals were performed for precipitating events observed in the vicinity of the Southern Great Plains (SGP) Atmospheric Radiation Measurement Program (ARM) Climate Research Facility. The retrieval results indicated that IWP values in stratiform precipitating systems vary from a few hundreds up to about 10 thousands of grams per meter squared, and the mean rain rates were in a general range between 0.5 and about 12 mm h−1. On average, mean rainfall increases with an increase in ice mass observed above the melting layer; the corresponding mean correlation coefficient is about 0.35, although events with higher correlation as well as those with no appreciable correlation were observed. Horizontal advection, wind shear, and vertical air motions might be some of the reasons for decorrelation between IWP and Rm retrieved for the same vertical atmospheric column. A mean statistical relation between IWP and Rm derived from CloudSat retrievals is in good agreement with the data obtained from multiwavelength ground-based cloud radar measurements at the SGP site.

A new absolute diffuse reflectance measurement in the near-IR region based on the modified double-sphere method

A new method using integrating spheres for absolute reflectance measurements has been developed to establish a spectral diffuse reflectance scale in the near-IR (NIR) region. This method is a modified version of the double-sphere method developed in 1966, and the modification reduces the uncertainty due to the area measurement of the auxiliary sphere. In this new technique, the modified double-sphere method, a known absolute reflectance scale in the visible region derived from another absolute reflectance measurement is used to extend the absolute reflectance scale from the visible to the NIR region without requiring a strict area measurement. By using this modified double-sphere method, a spectral diffuse reflectance scale is established from 800 nm to 1600 nm with a relative expanded uncertainty (k = 2) of 0.48% to 0.54% depending on the wavelength. The same principle can be applied to determine a spectral diffuse reflectance standard in the ultraviolet region as well. In this paper, an outline of the modified double-sphere method is described with emphasis on its theoretical and instrumental features.

A sensitive, spatially uniform photodetector for broadband infrared spectrophotometry

We describe the design and performance of a liquid helium-cooled As:Si blocked-impurity-band photodetector system intended for spectrophotometry in the thermal infrared (2 to 30 mum) spectral region. The system has been characterized for spectral sensitivity, noise, thermal stability, and spatial uniformity, and optimized for use with a Fourier-transform infrared spectrophotometer source for absolute goniometric reflectance measurements. Its performance is evaluated and compared to more common detector systems used in this spectral region, including room-temperature pyroelectric and liquid-N(2)-cooled photoconductive devices.

Determination of the infrared complex magnetoconductivity tensor in itinerant ferromagnets from Faraday and Kerr measurements

We present measurement and analysis techniques that allow the complete complex magnetoconductivity tensor to be determined from mid-infrared (11-1.6 {micro}m; 100-800 meV) measurements of the complex Faraday ({theta}{sub F}) and Kerr ({theta}{sub K}) angles. Since this approach involves measurement of the geometry (orientation axis and ellipticity of the polarization) of transmitted and reflected light, no absolute transmittance or reflectance measurements are required. Thick-film transmission and reflection equations are used to convert the complex {theta}{sub F} and {theta}{sub K} into the complex longitudinal conductivity {sigma}{sub xx} and the complex transverse (Hall) conductivity {sigma}{sub xy}. {theta}{sub F} and {theta}{sub K} are measured in a Ga{sub 1-x}Mn{sub x}As and SrRuO{sub 3} films. The resulting {sigma}{sub xx} is compared to the values obtained from conventional transmittance and reflectance measurements, as well as the results from Kramers-Kronig analysis of reflectance measurements on similar films.

MONOX: a characterization tool for the X-UV range

A new laboratory apparatus devoted to the characterization of various devices for the X-UV range (100-5000 eV), such as mirrors, diffraction gratings, spectrometers or detectors is described. The apparatus includes open x-ray tubes as x-ray sources, a two-crystal monochromator for wavelength selection and a goniometer. Various examples of its use are presented : dispersive mode where the radiation coming from the x-ray tube is dispersed by the two-crystal monochromator, spectrometric mode where the goniometer is used as a plane x-ray spectrometer and reflectometric mode where a selected wavelength is used to perform absolute reflectivity measurements.

Development of a new integrating sphere with uniform reflectance for absolute diffuse reflectance measurements

The design and concept of a new integrating sphere for absolute diffuse reflectance measurement are presented, together with an evaluation of the reflectance non-uniformity. The new integrating sphere was designed with emphasis on uniform reflectance over the sphere wall. SpectralonTM was employed as the inner material for the sphere wall. The integrating sphere with geodesic design was developed for the most appropriate specifications to realize the high reflectance uniformity. The evaluation of the reflectance non-uniformity of the sphere wall in the visible region revealed that the geodesic integrating sphere had a quite uniform reflectance with non-uniformity of about 0.16% on the average.

Absolute reflectance measurements by a modified cavity phase-shift method

This article reports on a modified cavity phase-shift (CAPS) method for accurate and reliable characterization of high reflectance mirrors. Our approach relies on using a directly modulated Fabry–Pérot laser to circumvent the difficulties encountered in previous attempts with the CAPS method. The Fabry–Pérot laser diode ensures a constant coupling between the probe laser and test cavity modes. This results in a stable beam intensity transmitted through the test cavity allowing for accurate measurements of the phase shift from which the absolute reflectance can be determined. The experimental arrangement presented in this article is versatile and easy to use. The method is nondestructive and especially suited for the characterization of distributed Bragg reflectors (DBRs) employed in vertical-cavity optoelectronic devices. A premium feature of this method is its capability to probe a relatively small area of less than 1 mm which can be positioned anywhere across the surface of the wafer. We demonstrate the use of the method by measuring the absolute reflectance of metalorganic vapor-phase epitaxy grown AlAs/GaAs DBRs for 1.3 μm vertical-cavity lasers.

変形シャープ・リトル法による高精度絶対反射率測定 (2)

変形シャープ・リトル法による高精度絶対反射率測定 (2)

Electronic structure of half-metallic materials: comparison of pyrite with doped manganite

Electronic structures of prototypical half-metallic materials, CoS 2, have been investigated by means of absolute reflectivity measurements. We have found a significant change of the optical spectra with decrease in temperature. Such a spectral change can be interpreted in terms of splitting of the up-spin (e g↑) and down-spin (e g↓) e g-electrons, or the reconstruction of the electronic structure in the half-metallic state. Temperature variation of electronic structures of CoS 2 will be compared with that of the doped manganites.