Malcolm G. White

Planar electrical-substitution carbon nanotube cryogenic radiometer*

We have developed a fully-lithographic electrical-substitution planar bolometric-radiometer (PBR) that employs multiwall vertically-aligned carbon nanotubes (VACNT) as the absorber and thermistor, micro-machined Si as the weak thermal link and thin-film Mo as the electrical heater. The near-unity absorption of the VACNT over a broad wavelength range permits a planar geometry, compatible with lithographic fabrication. We present performance results on a PBR with an absorption of 0.999 35 at 1550 nm, a thermal conductance of 456 µW K−1 at 4 K and a time constant (1/e) of 7.7 ms. A single measurement of approximately 100 µW optical power at 1550 nm achieved in less than 100 s yields an expanded uncertainty of 0.14% (k = 2). We also observe an elevated superconducting transition temperature of 3.884 K for the Mo heater, which opens the possibility of future devices incorporating more sensitive thermistors and superconducting thin-film wiring.

A detector combining quantum and thermal primary radiometric standards in the same artefact

We present the concept of a dual-mode primary standard cryogenic detector, utilizing a predictable quantum efficient silicon photodiode, and demonstrate the behaviour of the detector from room temperature down to 30 K. The detector absorbs visible radiation generating either heat or photocurrent, dependent on the selected mode of operation. In effect, this detector links optical power to fundamental constants through the two different routes of operation in the one artefact. Forward biasing of the photodiode is used in lieu of resistive heating to provide the electrical substitution power. The detector has a thermal time constant of 50 s and a sensitivity of 1.39 K mW−1. Using an LED source, we measure equivalence between the two modes of operation of 1.5% at 50 K, limited principally by our knowledge of the wavelength of the emitted radiation of the source.

Planar hyperblack absolute radiometer

The absolute responsivity of a planar cryogenic radiometer fabricated from micromachined silicon and having carbon nanotubes, as the absorber and thermistor were measured in the visible and far infrared (free-field terahertz) wavelength range by means of detector-based radiometry. The temperature coefficient of the thermistor near 4.8 K and noise equivalent power were evaluated along with independent characterization of the window transmittance and specular reflectance of the nanotube absorber. Measurements of absolute power by means of electrical substitution are compared to the German national standard and the uncertainty of the radiometer responsivity as a function of wavelength is summarized.

Realization of an accurate and repeatable wavelength scale for double subtractive monochromators

We present the results and discussion of two methods for achieving a higher degree of wavelength accuracy of double grating monochromators. In particular, we assessed the benefits of using differential evolution curve fitting techniques to minimize wavelength uncertainties associated with the manufacture of lead screws in sine-bar driven monochromators. Absolute wavelength-scale uncertainties better than ?0.1?nm can be realized using this technique.We also report the realization of an accurate and repeatable wavelength scale, using a novel calibration technique, that is applicable to a wide range of monochromators. We present results demonstrating the robustness of the technique by realizing an absolute wavelength scale, across four grating sets from 250?nm to 1600?nm, with scale uncertainties within ?0.02?nm and repeatable to ?0.005?nm.

Carbon nanotube planar absorber cryogenic bolometer as a novel primary absolute standard detector for optical power measurements for fibre optics and photonics (Conference Presentation)

Primary standards of optical radiation total radiant flux are traditionally realized by absolute cryogenic radiometers [1] working on the principle of electrical substitution with a relative total uncertainty of 1e-4 in the power measurement. The current cryogenic radiometers though operate over a limited spectral range, usually from 350 nm to 800 nm and working with free space beam. For fibre optics telecom spectral range 1300 nm - 1650 nm this scale is then extended in several steps, typically via application of other standard detector systems such as spectrally flat room temperature pyro detectors [2] and spectrally dependent temperature stabilized solid state detectors [3], which adversely affects the scale accuracy by a factor of approximately one order of magnitude. The typical relative total uncertainty of state-of-the-art transfer standard fibre coupled detectors reaches 0.5 %. Recently published results on planar electrical-substitution carbon nanotube cryogenic radiometer (PCBR) [4] brought the opportunities for using these systems as new absolute primary standards in telecom spectral range directly in fibre coupled configuration. This shortens the traceability chain, with a potential improvement in the total uncertainty to below 0.1 %. CMI in collaboration with NIST are developing the first prototypes of fibre coupled PCBR systems. First both free space and fibre coupled measurements have confirmed radiometric The paper will present both the core physical parameters of these PCBR electrical-substitution systems and initial results including the currently achieved agreement of traditional transfer standards with the PCBR at the level of 0.2 %. The work reported in this abstract was partially funded by project EMPIR 14IND13 PhotInd. This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. References: [1] Martin J E, Fox N P and Key P J 1985 Metrologia 21, 147 [2] Lehman J., Theocharous E., Eppeldauer G., and Pannel C., “Gold-black coatings for freestanding pyroelectric detectors” Measurement Science and Technology, 14, 916-922, 2003 [3] E. Theocharous, M. Smid, T. Ward, N. Fox, “The establishment of an absolute infrared scale using cavity pyroelectric detectors”, in preparation. [4] N A Tomlin, M White, I Vayshenker, S I Woods and J H Lehman, Planar electrical-substitution carbon nanotube cryogenic radiometer 2015 Metrologia 52 376