Since the original invention by Samuel P. Langley in 1878 5 , bolometers have gone a long way of improving the sensitivity and expanding the frequency range, from X-rays and optical/UV radiation to the submillimeter waves. The latter range contains approximately half the total luminosity of the Universe and 98% of all the photons emitted since the Big Bang 6 .Because the performance of ground-based THz telescopes is severely limited by a strong absorption of THz radiation in the Earth atmosphere, the development of space-based THz telescopes will be crucial for better understanding of the Universe evolution. Active cooling of primary mirrors on these telescopes will reduce the mirror emission below the cosmic background level (Fig. 1) and greatly expand the range of observable faint objects. The development of advanced detectors with background-limited sensitivity for such telescopes poses a significant challenge. Indeed, the photon flux N ph , which corresponds to the cosmic background fluctuations, is very weak: at ν > 1 THz, the photon flux in a diffraction-limited beam does not exceed 100 photons/s for typical extragalactic emission lines with ν/δν ~ 1000. The noise equivalent power (NEP) of a background-limited detector should be less than NEP ph = hν 2N ph ~ 10 -20 W/Hz 1/2 , which is a factor of 100 lower than that of state-of-the-art bolometers.Although new detector concepts are coming into play nowadays 7,8 , bolometers still have a great potential for achieving the most challenging goals. Realization of the ultra-high sensitivity requires an unprecedented thermal isolation of a bolometer. Indeed, in the fluxintegrating regime (the bolometric time constant τ >> N ph -1 ), the minimum NEP is determined by the thermal energy fluctuations in a bolometer, and the corresponding value ofG is controlled by the thermal conductance G between the bolometer and its environment. In a traditional (the so-called "geometrically isolated") bolometer, G is determined by the number of relevant phonon and photon "channels" (modes) participating in thermal transport between the sensor and its environment. It has been shown recently for both photons 3,9 and phonons 10 that the thermal conductance of a short single channel is determined by the universal value G Q = π Despite a relatively small size of this micromachined device, the heat capacity C was still rather large, which resulted in a slow bolometric response with the time constant τ = C/G =1-10 s.Here we present a novel approach that enables a significant increase of the bolometer sensitivity and, at the same time, reduction of its response time. Fast response in a well isolated bolometer requires a very small heat capacity C and, thus, the nanoscale dimensions of a sensor.To overcome the limitation of fast phonon exchange, we realized the hot-electron regime 11, , 12 13 in superconducting nanobolometers at sub-Kelvin temperatures. In this case, a weak electronphonon coupling, which governs the effective thermal conductance, dramatically improves the thermal isola...

This article reviews the state of rapidly emerging terahertz hot-electron nanobolometers (nano-HEB), which are currently among of the most sensitive radiation power detectors at submillimeter wavelengths. With the achieved noise equivalent power close to 10^{-19} W/Hz^{1/2} and potentially capable of approaching NEP ~ 10^{-20} W/Hz^{1/2}, nano-HEBs are very important for future space astrophysics platforms with ultralow submillimeter radiation background. The ability of these sensors to detect single low-energy photons opens interesting possibilities for quantum calorimetry in the mid-infrared and even in the farinfrared parts of the electromagnetic spectrum. We discuss the competition in the field of ultrasensitive detectors, the physics and technology of nano-HEBs, recent experimental results, and perspectives for future development.Comment: 16 pages, 12 figures, 128 reference

We analyze the performance of graphene microstructures as thermal photon detectors and deduce the range of parameters that define a linear response. The saturation effects of a graphene thermal detector that operates beyond the linear range are described in detail for a single-photon detector (calorimeter). We compute the effect of operating beyond this linear range and find that sensitive detection occurs for such non-linear operation. We identify the optimum conditions and find that single-photon detection at terahertz (THz) frequencies should be feasible.

We have measured directly the thermal conductance between electrons and phonons in ultra-thin Hf and Ti films at millikelvin temperatures. The experimental data indicate that electron-phonon coupling in these films is significantly suppressed by disorder. The electron cooling time τ ε follows the T -4 -dependence with a record-long value τ ε = 25 ms at T = 0.04K. The hot-electron detectors of far-infrared radiation, fabricated from such films, are expected to have a very high sensitivity. The noise equivalent power of a detector with the area 1 µm 2 would be (2-3)×10 -20 W√Hz, which is two orders of magnitude smaller than that of the state-of-the-art bolometers.

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