Vyacheslav B. Podobedov

Raman scattering in La1−xSrxMnO3 single crystals (x = 0, 0.1, 0.2, 0.3)

Abstract We report polarized Raman spectra of La1−xSrxMnO3 crystals (x = 0, 0.1, 0.2 and 0.3) in the temperature range from 5 K to 300 K. The small distortion of the nearly cubic lattice of doped La1−xSrxMnO3 single crystals results in a structured phonon Raman spectrum which is composed of two parts with different contributions depending on the value of doping. The first part is assigned to the distorted non-cubic perovskite lattice and follows the selection rules for a tetragonal structure. This suggests the possible presence of a tetragonal structure in doped La1−xSrxMnO3 single crystals. The second part is mainly due to the density of vibrational states and is attributed to the second-order Raman scattering. The frequency of the A1g-like mode was found to be sensitive to doping but the observed shift is opposite to that expected from the difference in the atomic weight of Sr and La. This mode exhibits also an anomalously large shift of ∼20 cm−1 in the temperature range from 5 K to 300 K. A possible explanation of this behaviour is suggested.

Diagnostics of “colossal” magnetoresistance manganite films by Raman spectroscopy

Polarized Raman scattering by phonons is used to characterize thin films prepared by laser ablation of La1−xCaxMnO3 targets. It was found that, in the temperature range from 6 to 300 K, phonon spectra of La0.7Ca0.3MnO3 films exhibit observable differences from those in bulk materials (microcrystalline ceramics and single crystals). A significant difference was found in the spectra of “as-grown” films compared to those annealed in oxygen at 800 °C. The observed Raman peaks and their linewidths exhibit an irregular temperature dependence near Tc. A correlation of Raman data with magnetization of the sample was also found.

NIST traceable measurements of radiance and luminance levels of night-vision-goggle test-instruments

In order to perform radiance and luminance level measurements of night-vision-goggle (NVG) test instruments, NIST developed new-generation transfer-standard radiometers (TR). The new TRs can perform low-level radiance and luminance measurements with SI traceability and low uncertainty. The TRs were calibrated against NIST detector/radiometer standards holding the NIST photometric and radiometric scales. An 815 nm diode laser was used at NIST for the radiance responsivity calibrations. A spectrally flat (constant) filter correction was made for the TRs to correct the spectral responsivity change of the built-in Si photodiode for LEDs peaking at different wavelengths in the different test sets. The radiance responsivity transfer to the test instruments (test-sets) is discussed. The radiance values of the test instruments were measured with the TRs. The TRs propagate the traceablity to the NIST detector-based reference scales. The radiance uncertainty obtained from three TR measurements was 4.6 % (𝑘=2) at a luminance of 3.43 x 10-4 cd/m2. The output radiance of the previously used IR sphere source and the radiance responsivity of a previously used secondary standard detector unit, which was originally calibrated against an IR sphere source, were also measured with the TRs. The performances of the NVG test instruments were evaluated and the manufacturer produced radiance and luminance levels were calibrated with SI/NIST traceability.

Infrared spectral responsivity scale realization and validations

An InSb working standard radiometer, first calibrated at the National Institute of Standards and Technology (NIST) in 1999 against a cryogenic bolometer, was recently calibrated against a newly developed low-noise-equivalent-power pyroelectric transfer standard detector. The pyroelectric transfer standard, which can operate at the output of a monochromator, holds the newly realized NIST spectral power responsivity scale between 1.7 and 14 μm with an uncertainty of 1% (k=2). The InSb working standard was also measured at the National Physical Laboratory (NPL) of the United Kingdom in 1999. The less than 2% spectral power responsivity disagreements obtained on the InSb working standard (both from the 1999 NIST and NPL comparison and also against the pyroelectric standard) validate the three independently realized power responsivity scales and verify the long-term stability of the InSb working standard. The InSb working standard was also used in irradiance measurement mode to validate the previously determined spectral irradiance responsivity of four narrowband InSb radiometers that were applied to calibrate IR target simulators. The uncertainty of the present spectral irradiance responsivity scale held by the InSb working standard is 2.5% (k=2) in the 2 to 5.2 μm wavelength range.

Improving the performance of the NIST five axis goniospectrometer for measurements of bidirectional reflectance distribution function

The five axis goniospectrometer at the National Institute of Standards and Technology (NIST) measures the spectral reflectance of colored samples over a wide range of illumination and viewing angles. This capability is important for the colorimetric characterization of complex materials, such as gonioapparent coatings or retroreflective surfaces. To improve the efficiency of the goniometer, a broad-band source with a matrix-based stray-light corrected CCD based spectrometer was implemented. This new configuration offers a significant reduction in the measurement time allowing for the complete characterization of the goniodistribution of complex materials. Shorter measurement time reduces the load on the precise mechanical assembly, to ensure high angular accuracy over time. Special care was taken to extend the effective dynamic range of measured intensities in the multichannel detection mode to the values of 106 - 107 needed for the characterization of colored samples. The expanded uncertainty of the measured Bidirectional Reflectance Distribution Function (BRDF) for this new setup is about 0.5 % (k = 2) which is comparable to the uncertainty levels of the instrument operating with monochromatic illumination and a silicon photodiode. To validate the new system configuration, the measured BRDF or spectral reflectance factors (R) of test samples were compared with different instruments and we found an agreement of about 0.5 %.

Raman scattering from single crystal YBa2Cu3O7-δ in a magnetic field

A magneto-Raman study of a YBa2Cu3O7-δ single crystal(Tc = 92 K) was carried out at a resolution of 2 cm-1 over the temperature range 5–125 K. At temperatures belowTc we observed a slight narrowing of the 340 cm-1 Raman mode in a magnetic field of 5.15 T directed parallel to thec-axisof the crystal. The observed magneto-temperature dependences of the Raman continuum intensity measured in the low-frequency region, at ca. 40 cm-1, indicate contributions of electronic excitations arising from a pair-breaking process. Within the experimental uncertainty, the Raman intensity of the high-frequency continuum was observed to be independent of the magnetic field strength.

Performance of the NIST goniocolorimeter with a broad-band source and multichannel charged coupled device based spectrometer

The authors describe the NIST high-efficiency instrument for measurements of bidirectional reflectance distribution function of colored materials, including gonioapparent materials such as metallic and pearlescent coatings. The five-axis goniospectrometer measures the spectral reflectance of samples over a wide range of illumination and viewing angles. The implementation of a broad-band source and a multichannel CCD spectrometer corrected for stray light significantly increased the efficiency of the goniometer. In the extended range of 380 nm to 1050 nm, a reduction of measurement time from a few hours to a few minutes was obtained. Shorter measurement time reduces the load on the precise mechanical assembly ensuring high angular accuracy over time. We describe the application of matrix-based correction of stray light and the extension of effective dynamic range of measured fluxes to the values of 10(6) to 10(7) needed for the absolute characterization of samples. The measurement uncertainty was determined to be 0.7% (k = 2), which is comparable with similar instruments operating in a single channel configuration. Several examples of reflectance data obtained with the improved instrument indicate a 0.3% agreement compared to data collected with the single channel configuration.

Calibration of spectral responsivity of IR detectors in the range from 0.6 μm to 24 μm

We report the upgraded performance of the National Institute of Standards and Technology (NIST) facility for spectral responsivity calibrations of infrared (IR) detectors in both radiant power and irradiance measurement modes. The extension of the wavelength range of the previous scale, below 0.8 μm and above 19 μm in radiant power mode as well as above 5.3 μm in irradiance mode, became available as a result of multiple improvements. The calibration facility was optimized for low-level radiant flux. A significantly reduced noise-equivalent-power and a relatively constant spectral response were achieved recently on newly developed pyroelectric detectors. Also, an efficient optical geometry was developed for calibration of the spectral irradiance responsivity without using an integrating sphere. Simultaneously, the upgrade and maintenance of the NIST transfer standards, with an extended spectral range, were supported by spectral reflectance measurements of a transfer standard pyroelectric detector using a custom integrating sphere and a Fourier transform spectrometer. The sphere reflectance measurements performed in a relative mode were compared to a bare gold-coated mirror reference, separately calibrated at the Fourier transform Infrared Spectrophotometry facility to 18 μm. Currently, the reflectance data for the pyroelectric standard, available in the range up to 30 μm, are supporting the absolute power responsivity scale by the propagation of the reflectance curve to the absolute tie-spectrum in the overlapping range. Typical examples of working standard pyroelectric-, Si-, MCT-, InSb- and InGaAs- detectors are presented and their optimal use for scale dissemination is analyzed.

New night vision goggle gain definition

A new definition is proposed for the calibration of Night Vision Goggle (NVG) gains. This definition is based on the measurement of radiometric input and output quantities of the NVG. While the old definition used the “equivalent fL” which is a non SI traceable luminance unit, the new definition utilizes the radiance quantities that are traceable to the SI units through NIST standards. The new NVG gain matches the previous one as a result of the application of a correction coefficient originating from the conversion of the radiance to luminance units. The new definition was tested at the NIST Night Vision Calibration Facility and the measurement results were compared to the data obtained with a Hoffman Test Set Model ANV-126. Comparing the radiometric quantities of the Hoffman Test Set and those measured by the NIST transfer standard radiometer, indicates that the observed differences up to 15% were due to the calibration and experimental errors of the ANV-126 Test Set. In view of different spectral characteristics of luminophores that can be utilized in the NVG design, the simulation of the NVG output for gain measurement was performed. The NVG output was simulated with a sphere-based source using different LEDs and the measured gain was compared to that obtained with the ANV-126 internal luminance meter. The NVG gain uncertainty analysis was performed for the Type A, B, and C goggles.

Low-NEP pyroelectric detectors for calibration of UV and IR sources and detectors

Pyroelectric radiometers with spectrally constant response have been developed at NIST with the cooperation of a few detector manufacturers. The new devices have noise-equivalent-power (NEP) values less than 1 nW/Hz1/2 sufficiently low for use at the output of regular monochromators. Their response flatness is an order of magnitude better than that of filtered Si detectors and can be used to realize simple and low-uncertainty responsivity scales for the UV and IR wavelength ranges. For the first time, the UV irradiance responsivity of a pyroelectric detector has been determined. Based on spectral reflectance measurements of the black coating of the pyroelectric detector, the relative spectral response was determined between 0.25 μm and 30 μm. The relative response was then converted into spectral power and irradiance responsivities using absolute tie points from a silicon-trap-detector in the VIS range. In addition to the UV irradiance responsivity scale realization, the flat response between 1.6 μm and 2.6 μm was utilized and a constant irradiance responsivity was realized and applied as a reference scale for the Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS) facility of NIST. The spectral power responsivity of the low-NEP pyroelectric detector is the internal standard of the NIST VIS-IR detector calibration facility for the 0.6 μm to 24 μm wavelength range. The pyroelectric standard is used to calibrate other types of detectors for spectral responsivity using detector substitution. The flat-response interval of the pyroelectric standard, calibrated for irradiance responsivity, was also used to measure the integrated irradiance from UV LED sources without using any source standard. The broadband radiometric measurements can be applied to IR LEDs emitting low fluxes between 750 nm and 4300 nm. All pyroelectric detector based calibrations were performed with expanded uncertainties of about 2 % (k=2).