Evangelos Theocharous

Gold-black coatings for freestanding pyroelectric detectors*

We describe the process of depositing gold-black on thin, freestanding pyroelectric detector substrates and compare this with previous work documented in the literature. We have evaluated gold-black coatings on thin, freestanding pyroelectric detector substrates by means of scanning electron microscope, Fourier transform infrared spectrophotometer reflectance, and spectral responsivity measurements. Spectrophotometric measurements indicate that reflectance at normal incidence varies by less than 1% at wavelengths shorter than 2.5 µm and by less than 10% at 10 µm. These results are correlated with the spectral responsivity of the detector and demonstrate that radiation not reflected by the gold-black is absorbed by the detector element. We have evaluated gold-black coatings as a function of position at two wavelengths and found variations of less than 1% at 1.25 µm and less than 5% at 10.3 µm, which demonstrates that spatial uniformity can be coating dependent. Gold-black coatings exposed to a 193 nm wavelength excimer laser were evaluated by visual inspection for damage and determined to have a damage threshold of approximately 38 mJ cm−2.

Evaluation of a pyroelectric detector with a carbon multiwalled nanotube black coating in the infrared

The performance of a pyroelectric detector with a carbon multiwalled nanotube coating was evaluated in the 0.9-14 microm wavelength range. The relative spectral responsivity of this detector was shown to be flat over most of the wavelength range examined, and the spectral flatness was shown to be comparable to the best infrared black coatings currently available. This finding is promising because black coatings with spectrally flat absorbance profiles are usually associated with the highest absorbance values. The performance of the detector (in terms of noise equivalent power and specific detectivity) was limited by the very thick (250 microm thick) LiNbO3 pyroelectric crystal onto which the coating was deposited. The responsivity of this detector was shown to be linear in the 0.06-2.8 mW radiant power range, and its spatial uniformity was comparable to that of other pyroelectric detectors that use different types of black coating. The carbon nanotube coatings were reported to be much more durable than other infrared black coatings, such as metal blacks, that are commonly used to coat thermal detectors in the infrared. This, in combination with their excellent spectral flatness, suggests that carbon nanotube coatings appear extremely promising for thermal detection applications in the infrared.

Temperature and nonlinearity corrections for a photodiode array spectrometer used in the field

Temperature and nonlinearity effects are two important factors that limit the use of photodiode array spectrometers. Usually the spectrometer is calibrated at a known temperature against a reference source of a particular spectral radiance, and then it is used at different temperatures to measure sources of different spectral radiances. These factors are expected to be problematic for nontemperature-stabilized instruments used for in-the-field experiments, where the radiant power of the site changes continuously with the sun tilt. This paper describes the effect of ambient temperature on a nontemperature-stabilized linear photodiode array spectrometer over the temperature range from 5 °C to 40 °C. The nonlinearity effects on both signal amplification and different levels of radiant power have also been studied and are presented in this paper.

The partial space qualification of a vertically aligned carbon nanotube coating on aluminium substrates for EO applications

The fabrication of NanoTube Black, a Vertically Aligned carbon NanoTube Array (VANTA) on aluminium substrates is reported for the first time. The coating on aluminium was realised using a process that employs top down thermal radiation to assist growth, enabling deposition at temperatures below the substrates melting point. The NanoTube Black coatings were shown to exhibit directional hemispherical reflectance values of typically less than 1% across wavelengths in the 2.5 µm to 15 µm range. VANTA-coated aluminium substrates were subjected to space qualification testing (mass loss, outgassing, shock, vibration and temperature cycling) before their optical properties were re-assessed. Within measurement uncertainty, no changes to hemispherical reflectance were detected, confirming that NanoTube Black coatings on aluminium are good candidates for Earth Observation (EO) applications.

Absolute measurements of black-body emitted radiance

Near-ambient-temperature black-body sources are routinely used for calibration in terms of radiance of a variety of infrared instruments such as those used in remote sensing and thermal imaging. The black-body radiance is usually determined by reference to a measured temperature and a calculated effective emissivity. The temperature is measured with one or more contact thermometers positioned close to the emitting black-body surface. In this case traceability to the International System of Units (SI) is to the kelvin through the ITS-90. This paper describes an alternative, more direct method based on the use of absolutely calibrated filter radiometers. These filter radiometers form part of a new facility called AMBER (Absolute Measurements of Black-body Emitted Radiance) which has been designed to determine the radiance of an ambient-temperature black body with an uncertainty of about 0.1% (which corresponds to a radiance temperature difference of 25 mK at 4 mum) and a resolution of 0.001% (0.3 mK). The facility obtains its traceability to the SI directly through radiometric standards in the form of a cryogenic radiometer rather than through the ITS-90.

Absolute linearity measurements on a PbS detector in the infrared

The nonlinearity characteristics of a commercially available thin-film photoconductive PbS detector were experimentally investigated in the infrared using the National Physical Laboratory detector linearity characterization facility. The deviation from linearity of this detector was shown to be significant even for relatively low values of radiant power incident on the active area of the detector. For example, the linearity factor was approximately 0.8 when 0.6 μW of radiant power at a wavelength of 2.2 μm was illuminating a spot of 1 mm in diameter on the active area of the PbS detector. These figures demonstrate the poor linearity characteristics of this detector and provide a warning to other users of PbS detection systems. The deviation from linearity was shown to be a function of the size of the spot being illuminated on the detector active area, as well as the wavelength of the incident radiation. The deviation from linearity was shown to be a function of irradiance illuminating the detector for irradiance values lower than 1 μW mm−2.

High-accuracy, infrared, spectral responsivity scale

This paper describes the establishment of a high-accuracy, infrared, spectral responsivity scale in the 8 µm to 12 µm atmospheric window through the calibration of specially designed HgCdTe/sphere detectors. The scale has been established using intensity-stabilized laser radiation from a grating-tuned CO2 laser directly with the primary standard cryogenic radiometer of the UK National Physical Laboratory (NPL). The factor-of-10 improvement in the uncertainty over the NPLs previous spectral responsivity scale in this spectral region has been achieved mainly through the use of improved transfer standard detectors and the direct calibration of these detectors against the cryogenic radiometer. This paper describes these new transfer standards, the intensity-stabilized CO2 laser facility, its use with the NPL cryogenic radiometer and the improved methodology.

Comparison of the performance of infrared detectors for radiometric applications

NPL, the UK National Standards laboratory, is currently engaged in a program of work to improve the accuracy of measurements in the infrared region of the spectrum (1 to 20 micrometers ) particularly in the atmospheric windows, with a target best uncertainty of around 0.1%. This work includes the identification of the most appropriate detector type on which to maintain and disseminate the new scale. Transfer standard detectors should ideally be large in area (up to 10 mm diameter), spatially uniform, linear, have high D* values over a wide spectral range and have good stability/ageing characteristics. A number of detector technologies have been investigated and the characteristics of these devices are described together with an assessment of their suitability as transfer standards.

Latest measurement techniques at NPL for the characterization of infrared detectors and materials

In its role as the national standards laboratory for the UK, the National Physical Laboratory (NPL) maintains, develops and disseminates, amongst others, the UKs detector spectral responsivity scale and material spectrometric scales (regular, hemispherical and angular reflectance and transmittance). In order to carry this work out detectors, materials, methods and facilities are continually under development at NPL. This paper will present the latest measurement techniques used at NPL that are applicable for the characterisation of infrared detectors and materials. NPL has extensive calibration capabilities, making use of grating and FT spectrometers and tuneable lasers, covering a wide spectral range, catering for single element, array, sub-pixel resolution and photon counting devices. As well spectral responsivity, detector spatial uniformity and linearity measurements are available. The UK spectrometric scales are maintained from 200 nm to 56 μm and include regular, hemispherical and angular reflectance and transmittance scales, and artefacts for the wavenumber and ordinate calibration of mid-infrared spectrometers.

Low optical power reference detector implemented in the validation of two independent techniques for calibrating photon-counting detectors.

We introduce a technique for measuring detection efficiency that is traceable to the primary standard, the cryogenic radiometer, through a reference silicon photodiode trap detector. The trap detector, used in conjunction with a switched integrator amplifier, can measure signals down to the 0.1 pW (3 x 10⁵ photons second-1) level with 0.1% uncertainty in a total integration time of 300 seconds. This provides a convenient calibration standard for measurements at these levels across the optical spectrum (UV - near IR). A second technique is also described, based on correlated photons produced via parametric down-conversion. This can be used to directly measure detection efficiency in the photon counting regime, and provides a route for expanding the formulation of the candela in terms of photon flux to enable it to address the needs of emerging quantum optical technologies and applications. The two independent techniques were cross-validated by a comparison carried out at 702.2 nm, which showed agreement to within 0.2%.