Monolayer graphene bolometer as a sensitive far-IR detector

In this paper we give a detailed analysis of the expected sensitivity and operating conditions in the power detection mode of a hot-electron bolometer (HEB) made from a few {\mu}m$^2$ of monolayer graphene (MLG) flake which can be embedded into either a planar antenna or waveguide circuit via NbN (or NbTiN) superconducting contacts with critical temperature ~ 14 K. Recent data on the strength of the electron-phonon coupling are used in the present analysis and the contribution of the readout noise to the Noise Equivalent Power (NEP) is explicitly computed. The readout scheme utilizes Johnson Noise Thermometry (JNT) allowing for Frequency-Domain Multiplexing (FDM) using narrowband filter coupling of the HEBs. In general, the filter bandwidth and the summing amplifier noise have a significant effect on the overall system sensitivity. The analysis shows that the readout contribution can be reduced to that of the bolometer phonon noise if the detector device is operated at 0.05 K and the JNT signal is read at about 10 GHz where the Johnson noise emitted in equilibrium is substantially reduced. Beside the high sensitivity (NEP < 10$^{-20}$ W/Hz$^{1/2}$, this bolometer does not have any hard saturation limit and thus can be used for far-IR sky imaging with arbitrary contrast. By changing the operating temperature of the bolometer the sensitivity can be fine tuned to accommodate the background photon flux in a particular application. By using a broadband low-noise kinetic inductance parametric amplifier, ~100s of graphene HEBs can be read simultaneously without saturation of the system output.Comment: 9 pages. 6 figure, SPIE Astronomical Telescopes + Instrumentation, Montr\'eal, Quebec, Canada, 22-27 June, 201

Bolometric and non-bolometric radio frequency detection in a metallic single-walled carbon nanotube

We characterize radio frequency detection in a high-quality metallic single-walled carbon nanotube. At a bath temperature of 77 K, only bolometric (thermal) detection is seen. At a bath temperature of 4.2 K and low bias current, the response is due instead to the electrical nonlinearity of the non-ohmic contacts. At higher bias currents, the contacts recover ohmic behavior and the observed response agrees well with the calculated bolometric responsivity. The bolometric response is expected to operate at terahertz frequencies, and we discuss some of the practical issues associated with developing high frequency detectors based on carbon nanotubes.Comment: 11 pages (double-spaced), 3 figure

After the fence: Soil and vegetation condition in grazed, fenced and benchmark eucalypt woodlands of fragmented agricultural landscapes

Ecological theory predicts that vegetation changes caused by introduction of livestock grazing may be irreversible after livestock are removed, especially in regions such as Australia that have a short evolutionary exposure to ungulate grazing. Despite this, fencing to exclude livestock grazing is the major tool used for restoring biodiversity in remnant vegetation degraded by grazing in Australian agricultural landscapes. To characterise benefits and limitations of livestock exclusion for enhancing biodiversity in forb-rich York gum (Eucalyptus loxophleba subsp. loxophleba) – jam (Acacia acuminata) woodlands, we compared 29 fenced remnants from across the central Western Australian wheatbelt with 29 adjacent grazed remnants and 11 little-grazed ‘benchmark’ woodlands. We explored two hypotheses: (1) fencing to exclude livestock facilitates recovery of grazed woodlands towards benchmark conditions, and (2) after fencing, recovery of grazed woodlands to benchmark conditions is constrained by ecological or other limits. Our first hypothesis was supported, with fenced remnants more similar to benchmark woodlands in tree recruitment, exotic cover, native cover, native plant richness and plant species composition than grazed remnants were. Further, exotic cover decreased and frequency of jam increased with time since fencing (2–22 years). However we found no evidence for recovery of nutrient-enriched topsoils due to fencing. Our second hypothesis was also supported, with higher soil nutrients and exotic cover, lower native plant richness and different plant species composition in fenced compared with benchmark woodlands. Regression analyses suggested recovery of native species richness can be constrained by exotic species that persist after fencing, which in turn are more persistent at higher soil nutrient concentrations. We conclude that fencing to exclude livestock grazing is valuable for biodiversity conservation. However, consistent with ecological theory, additional interventions are likely to be necessary to achieve some conservation goals or to promote recovery in more productive (nutrient-enriched) sites

Energy resolution of terahertz single-photon-sensitive bolometric detectors

We report measurements of the energy resolution of ultra-sensitive superconducting bolometric detectors. The device is a superconducting titanium nanobridge with niobium contacts. A fast microwave pulse is used to simulate a single higher-frequency photon, where the absorbed energy of the pulse is equal to the photon energy. This technique allows precise calibration of the input coupling and avoids problems with unwanted background photons. Present devices have an intrinsic full-width at half-maximum energy resolution of approximately 23 terahertz, near the predicted value due to intrinsic thermal fluctuation noise.Comment: 11 pages (double-spaced), 5 figures; minor revision