Realization of a scale of absolute spectral response using the National Institute of Standards and Technology high-accuracy cryogenic radiometer

Spectral supralinearity of silicon photodiodes with over-filled illumination in the near-infrared region

The spectral nonlinearities of silicon (Si) photodiodes (PDs) with over-filled illumination in the near-infrared region were investigated under reverse-bias and zero-bias voltage conditions. The supralinear behaviors and their wavelength dependences were clearly observed with over-filled illumination under both the reverse-bias and zero-bias voltage conditions at the selected wavelengths. Comparing these results with the nonlinearity results obtained in the under-filled illumination condition, it was evident that the maximum supralinearity values observed under the over-filled illumination condition were larger than those in the under-filled illumination condition, and the maximum value (∼10%) was obtained at the incident wavelength of 1060 nm. To validate the relatively large supralinearity value obtained under the over-filled illumination condition in the near-infrared region, the linearity factor and the photocurrent, as a function of the Si PD positions, and their wavelength dependences were examined under both the reverse-bias and zero-bias voltage conditions at all wavelengths. The linearity factors at the selected wavelengths, under both conditions, exceeded unity at all the PD positions; specifically, the values near the PD electrode were larger than those near the PD center, although the examined photocurrent decreased as the laser beam strayed off the beam detection area of the Si PD. These results highlighted that the relatively large supralinearity was generated under the over-filled illumination condition and that the minority carrier traps for the supralinearity-generating recombinations exist not only near the PD center but also near the PD electrode. These experimental results can facilitate the 3D theoretical modeling of Si-based photodetectors and aid the achievement of high-accuracy measurements with Si PDs in the fields of radiometry and radiation thermometry.

Invited Article: Advances in tunable laser-based radiometric calibration applications at the National Institute of Standards and Technology, USA

Recent developments at the National Institute of Standards and Technology's facility for Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) are presented. The facility is predicated on the use of broadly tunable narrow-band lasers as light sources in two key radiometric calibration applications. In the first application, the tunable lasers are used to calibrate the spectral power responsivities of primary standard detectors against an absolute cryogenic radiometer (ACR). The second function is to calibrate the absolute radiance and irradiance responsivities of detectors with uniform light sources, typically generated by coupling the laser light into integrating spheres. The radiant flux from the uniform sources is determined by the ACR-calibrated primary standard detectors. Together these sources and detectors are used to transfer radiometric scales to a variety of optical instruments with low uncertainties. We describe methods for obtaining the stable, uniform light sources required for low uncertainty measurements along with advances in laser sources that facilitate tuning over broader wavelength ranges. Example applications include the development of a detector-based thermodynamic temperature scale, the calibration and characterization of spectrographs, and the use of a traveling version of SIRCUS (T-SIRCUS) to calibrate large aperture Earth observing instruments and astronomical telescopes.

Robust Measurement for the Discrimination of Binary Coherent States

The discrimination of two nonorthogonal states is a fundamental element for secure and efficient communication. Quantum measurements of nonorthogonal coherent states can enhance information transfer beyond the limits of conventional technologies. We demonstrate a strategy for binary state discrimination based on optimized single-shot measurements with photon number resolving detection with a finite number resolution. This strategy enables a high degree of robustness to noise and imperfections while being scalable to high rates and, in principle, allows for surpassing the quantum noise limit (QNL) in practical situations. These features make the strategy inherently compatible with high-bandwidth communication and quantum information applications, providing advantages over the QNL under realistic conditions.

Three-element trap filter radiometer based on large active area silicon photodiodes

This paper shows the opto-mechanical design of a new filter radiometer built at the Physikalisch-Technische Bundesanstalt, Germany, for the accurate determination of the thermodynamic temperature of high-temperature blackbodies. The filter radiometer is based on a three-element reflection-type trap detector that uses three large active area silicon photodiodes. Its spectral coverage and field of view are defined by a detachable narrow-band filter and a diamond-turned precision aperture, respectively. The temperature of the filter radiometer is stabilized using a water-streamed housing and is measured using a thin-film platinum thermometer placed onto the first photodiode element. The trap "mount" has been made as compact as possible, which, together with the large active area of the chosen photodiodes, allows a wide field of view. This work presents the design of the filter radiometer and discusses the criteria that have been considered in order for the filter radiometer to suit the application.

Spectral supralinearity prediction of silicon photodiodes in the near-infrared range

A model describing spectral supralinearity for a silicon photodiode in the near-infrared region is presented. This theoretical model is based on the internal quantum efficiency model of the photodiode using Shockley-Read-Hall recombination, which depends on the inner structure parameters of the photodiodes. Comparing the experimental results with the theoretical calculation results, the model enables us to quantitatively predict the starting power level, shape, and wavelength dependence of the supralinearity for a silicon photodiode. This model contributes to high-accuracy measurements over wide optical power ranges and various incident wavelengths.

Demonstrating highly symmetric single-mode, single-photon heralding efficiency in spontaneous parametric downconversion

We demonstrate a symmetric, single-spatial-mode, single-photon heralding efficiency of 84% for a type-II spontaneous parametric downconversion process. High-efficiency, single-spatial mode collection is key to enabling many quantum information processing and quantum metrology applications.

Limits on the spatial resolution of monolithic scintillators read out by APD arrays

Cramér-Rao theory can be used to derive the lower bound on the spatial resolution achievable with position-sensitive scintillation detectors as a function of the detector geometry and the pertinent physical properties of the scintillator, the photosensor and the readout electronics. Knowledge of the Cramér-Rao lower bound (CRLB) can for example be used to optimize the detector design and to test the performance of the method used to derive position information from the detector signals. Here, this approach is demonstrated for monolithic scintillator detectors for positron emission tomography. Two detector geometries are investigated: a 20 × 10 × 10 mm(3) and a 20 × 10 × 20 mm(3) monolithic LYSO:Ce(3+) crystal read out by one or two Hamamatsu S8550SPL avalanche photodiode (APD) arrays, respectively. The results indicate that in these detectors the CRLB is primarily determined by the APD excess noise factor and the number of scintillation photons detected. Furthermore, it is shown that the use of a k-nearest neighbor (k-NN) algorithm for position estimation allows the experimentally obtained spatial resolution to closely approach the CRLB. The approach outlined in this work can in principle be applied to any scintillation detector in which position information is encoded in the distribution of the scintillation light over multiple photosensor elements.

Response of large area avalanche photodiodes to low energy x rays

For an experiment to study neutron radiative beta-decay, we operated large area avalanche photodiodes (APDs) near liquid nitrogen temperature to detect x rays with energies between 0.2 keV and 20 keV. Whereas there are numerous reports of x ray spectrometry using APDs at energies above 1 keV, operation near liquid nitrogen temperature allowed us to reach a nominal threshold of 0.1 keV. However, due to the short penetration depth of x rays below 1 keV, the pulse height spectrum of the APD become complex. We studied the response using monochromatic x ray beams and employed phenomenological fits of the pulse height spectrum to model the measurement of a continuum spectrum from a synchrotron. In addition, the measured pulse height spectrum was modelled using a profile for the variation in efficiency of collection of photoelectrons with depth into the APD. The best results are obtained with the collection efficiency model.

Internal quantum efficiency modeling of silicon photodiodes

Results are presented for modeling of the shape of the internal quantum efficiency (IQE) versus wavelength for silicon photodiodes in the 400 nm to 900 nm wavelength range. The IQE data are based on measurements of the external quantum efficiencies of three transmission optical trap detectors using an extensive set of laser wavelengths, along with the transmittance of the traps. We find that a simplified version of a previously reported IQE model fits the data with an accuracy of better than 0.01%. These results provide an important validation of the National Institute of Standards and Technology (NIST) spectral radiant power responsivity scale disseminated through the NIST Spectral Comparator Facility, as well as those scales disseminated by other National Metrology Institutes who have employed the same model.

Facility for spectral irradiance and radiance responsivity calibrations using uniform sources

Detectors have historically been calibrated for spectral power responsivity at the National Institute of Standards and Technology by using a lamp-monochromator system to tune the wavelength of the excitation source. Silicon detectors can be calibrated in the visible spectral region with combined standard uncertainties at the 0.1% level. However, uncertainties increase dramatically when measuring an instrument's spectral irradiance or radiance responsivity. We describe what we believe to be a new laser-based facility for spectral irradiance and radiance responsivity calibrations using uniform sources (SIRCUS) that was developed to calibrate instruments directly in irradiance or radiance mode with uncertainties approaching or exceeding those available for spectral power responsivity calibrations. In SIRCUS, the emission from high-power, tunable lasers is introduced into an integrating sphere using optical fibers, producing uniform, quasi-Lambertian, high-radiant-flux sources. Reference standard irradiance detectors, calibrated directly against national primary standards for spectral power responsivity and aperture area measurement, are used to determine the irradiance at a reference plane. Knowing the measurement geometry, the source radiance can be readily determined as well. The radiometric properties of the SIRCUS source coupled with state-of-the-art transfer standard radiometers whose responses are directly traceable to primary national radiometric scales result in typical combined standard uncertainties in irradiance and radiance responsivity calibrations of less than 0.1%. The details of the facility and its effect on primary national radiometric scales are discussed.

New model for the internal quantum efficiency of photodiodes based on photocurrent analysis

A new expression for the internal quantum efficiency of a photodiode is presented. It is obtained from the analysis of the photocurrent generated within the diode, considering the power and the cross-sectional diameter of the incident beam. The model explains variations of the internal quantum efficiency with irradiance that are not explained by other existing models, although this experimental fact was already known. The well-known phenomenon of supraresponsivity is also explained with this model. Finally, we show the dependence of the internal quantum efficiency on the variables involved in the model.

Design and characterization of a photometer-colorimeter standard

A photometer and tristimulus colorimeter has been developed at the National Institute of Standards and Technology (NIST) to realize a color scale. A novel construction was developed to implement the spectral-responsivity-based scale with small uncertainty. The new device can be used as a reference illuminance and luminance meter as well. Temperature-controlled filter combinations, with 5-8 layers in one package, are used to match the responsivity of a silicon tunnel-trap detector to the CIE color-matching functions with small spectral mismatch values (f1'). Design considerations to extend the tunnel-trap detector with replaceable single and double apertures and changeable filter combinations are described. The design and fabrication of the filter packages and the dependence of the f1' values on the thickness of the filter layers are discussed. The colorimeter was characterized for angular, spatial, and spectral responsivity. An improved preamplifier can convert current to voltage in an 11-decade dynamic range with 0.01% uncertainty.

Response of a silicon photodiode to pulsed radiation

Both the integrated-charge and the peak-voltage responsivity of a 1-cm2 Si photodiode optimized for the extreme ultraviolet have been measured with 532-nm-wavelength pulsed radiation. The peak power of the optical pulse is varied from 35 mW to 24 kW with a pulse width of 8.25 ns. A decrease in responsivity is observed with increasing pulse energy, and a model is presented that accounts for the observed loss of responsivity. The integrated-charge responsivity decreases because the presence of photogenerated majority carriers increases the direct recombination rate. The peak-voltage responsivity is reduced because the electric susceptibility of the electrons and holes in the depletion region increases the capacitance of the device. The influence of an applied reverse bias on both responsivities is investigated. The integrated-charge responsivity is found to be identical, with a 1% uncertainty, to the cw responsivity of the device if the energy dependence is considered.

Quantum efficiency of silicon photodiodes in the near-infrared spectral range

The quantum efficiency of silicon photodiodes and factors that might be responsible for the drop in quantum efficiency in the near-infrared spectral range were analyzed. It was shown that poor reflectivity from the rear surface of the die could account for a decrease in Si photodiode quantum efficiency in near-infrared spectral range by more than 20%. The photodiode quantum efficiency was modeled with an appropriate representation for the carrier-collection efficiency dependence on the die penetration depth. A corrected analytical expression for calculating the photodiode quantum efficiency is given. Some methods to improve the quantum efficiency of silicon photodiodes in near-infrared spectral range are discussed.

Realization of the National Institute of Standards and Technology detector-based spectral irradiance scale

A detector-based spectral irradiance scale has been realized at the National Institute of Standards and Technology (NIST). Unlike the previous NIST spectral irradiance scales, the new scale is generated with filter radiometers calibrated for absolute spectral power responsivity traceable to the NIST high-accuracy cryogenic radiometer instead of with the gold freezing-point blackbody. The calibrated filter radiometers are then used to establish the radiance temperature of a high-temperature blackbody (HTBB) operating near 3,000 K The spectral irradiance of the HTBB is then determined with knowledge of the geometric factors and is used to assign the spectral irradiances of a group of 1,000-W free-electron laser lamps. The detector-based spectral irradiance scale results in the reduction of the uncertainties from the previous source-based spectral irradiance scale by at least a factor of 2 in the ultraviolet and visible wavelength regions. The new detector-based spectral irradiance scale also leads to a reduction in the uncertainties in the shortwave infrared wavelength region by at least a factor of 2-10, depending on the wavelength. Following the establishment of the spectral irradiance scale in the early 1960s, the detector-based spectral irradiance scale represents a fundamental change in the way that the NIST spectral irradiance scale is realized.

Interpolation of the spectral responsivity of silicon photodetectors in the near ultraviolet

We improve the methods used to interpolate the responsivity of unbiased silicon photodetectors in the near-ultraviolet region. This improvement is achieved by the derivation of an interpolation function for the quantum yield of silicon and by consideration of this function in the interpolation of the internal quantum efficiency of photodiodes. The calculated quantum-yield and spectral-responsivity values are compared with measurement results obtained by the study of a silicon trap detector and with values reported by other research groups. The comparisons show agreement with a standard deviation of 0.4% between our measured and modeled values for both the quantum yield and the spectral responsivity within the wavelength region from 260 to 400 nm. The proposed methods thus extend the predictability of the spectral responsivity of silicon photodetectors to the wavelength region from 260 to 950 nm. Furthermore, an explanation is proposed for the change in the spectral responsivity of silicon photodiodes that is due to UV radiation. In our improved quantum efficiency model the spectral change can be accounted for completely by the adjustment of just one parameter, i.e., the collection efficiency near the SiO(2)/Si interface.

Ultraviolet radiometry with synchrotron radiation and cryogenic radiometry

The combination of a cryogenic radiometer and synchrotron radiation enables detector scale realization in spectral regions that are otherwise difficult to access. Cryogenic radiometry is the most accurate primary detector-based standard available to date, and synchrotron radiation gives a unique broadband and continuous spectrum that extends from x ray to far IR. We describe a new cryogenic radiometer-based UV radiometry facility at the Synchrotron Ultraviolet Radiation Facility II at the National Institute of Standards and Technology. The facility is designed to perform a variety of detector and optical materials characterizations. The facility combines a high-throughput, normal incidence monochromator with an absolute cryogenic radiometer optimized for UV measurements to provide absolute radiometric measurements in the spectral range from 125 nm to approximately 320 nm. We discuss results on photodetector characterizations, including absolute spectroradiometric calibration, spatial responsivity mapping, spectroreflectance, and internal quantum efficiency. In addition, such characterizations are used to study UV radiation damage in photodetectors that can shed light on the mechanism of the damage process. Examples are also given for UV optical materials characterization.

Nonlinearity measurements of silicon photodetectors

Nonlinearities of the responsivity of various types of siliconphotodetectors have been studied. These detectors are based onphotodiodes with two sizes of the active area (10 x 10 mm(2) and 18 x 18 mm(2)). The detectorconfigurations investigated include single photodiodes, two reflectiontrap detectors, and a transmission trap detector. For all devices, the measured nonlinearity was less than 2 x 10(-4) forphotocurrents up to 200 muA. The diameter of themeasurement beam was found to have an effect on thenonlinearity. The measured nonlinearity of the trap detectorsdepends on the polarization state of the incident beam. Theresponsivity of the photodetectors consisting of the large-areaphotodiodes reached saturation at higher photocurrent values comparedwith the devices based on the photodiodes with smaller activearea.

Spectral reflectance of silicon photodiodes

A precision spectrometer was used to measure the spectral reflectance of a silicon photodiode over the wavelength range from 250 to 850 nm. The results were compared with the corresponding values predicted by a model based on thin-film Fresnel formulas and the known refractive indices of silicon and silicon dioxide. The good agreement at the level of 2 x 10(-3) in the visible wavelength range verifies that the reflection model can be used for accurate extrapolation of the spectral reflectance and responsivity of silicon photodiode devices. In addition, characterization of the photodiode reflectance in the ultraviolet region improves the accuracy of the spectral irradiance measurements when filter radiometers based on trap detectors are used.

Synchrotron-radiation-operated cryogenic electrical-substitution radiometer as the high-accuracy primary detector standard in the ultraviolet, vacuum-ultraviolet, and soft-x-ray spectral ranges

The accuracy of detector calibration in the UV, vacuum-ultraviolet, and soft-x-ray spectral ranges could be significantly improved by the use of the synchrotron radiation electrical substitution radiometer (SYRES) as the primary detector standard. The SYRES radiometer is optimized for use with spectrally dispersed synchrotron radiation as supplied by two monochromator beam lines in the radiometry laboratory of the Physikalisch-Technische Bundesanstalt at the Berlin electron-storage ring (BESSY). Wavelength ranges from 0.8 to 25 nm and from 35 to 400 nm are covered. The typically available radiant power of approximately 1-10 microW can be measured with the SYRES radiometer with a standard relative uncertainty of less than 0.2%. The spectral responsivity of qualified photodiodes for use as secondary detector standards is determined by direct comparison with the primary detector standard at an arbitrary wavelength. At present, the scale of spectral responsivity is realized with a standard relative uncertainty of well below 1% in the spectral ranges 0.8-3.5 nm, 5-25 nm, and 120-400 nm. We provide a comprehensive description of the SYRES radiometer and of the two facilities for detector calibration in the UV and vacuum-ultraviolet spectral ranges and in the soft-x-ray spectral range, respectively, and we discuss the achievable uncertainties in the calibration of detectors.

Calibration of a pyroelectric detector at 10.6 microm with the National Institute of Standards and Technology high-accuracy cryogenic radiometer

The National Institute of Standards and Technology (NIST) is establishing an infrared detector calibration facility to improve radiometric standards at infrared wavelengths. The absolute response of the cryogenic bolometer that serves as the transfer standard for this facility is being linked to the NIST high- accuracy cryogenic radiometer (HACR) at a few laser wavelengths. At the 10.6-microm CO(2) laser line, this link is being established through a pyroelectric detector that has been calibrated against the HACR. We describe the apparatus, methods, and uncertainties for the calibration of this pyroelectric detector.