Ultrasensitive hot-electron nanobolometers for terahertz astrophysics

Abstract

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...

The electron-phonon relaxation time in thin superconducting titanium nitride films

“…N 0 is another important material parameter that determines the design and, consequently, the noise performance of MKIDs and hot electron bolometers (HEBs). 25 The electron densities of states for the three samples are estimated from experimental values of the resistivity and the electron diffusion constant using expression N 0 ¼ 1= e 2 qD À Á and are shown in Table I. These values of N 0 are greater than the values previously estimated.…”

“…10 , where V is the active volume of the detector, a sc is the kinetic inductance fraction, and Q i,max is the maximum quality factor of the resonator. For typical values of V % 0.01 lm 3 for HEBs, 25 the energy resolution de is about of 10 À3 aJ at 0.3 K and for MKIDs 28 with the values of V % 70 lm 3 , a sc % 1, Q i,max % 10 5 , Tc ¼ 1 K the value of de is order of 1.5 Â 10 À3 aJ at 0.1 K. These estimates for TiN films indicate that photon counting in the far-infrared and THz should be feasible for detectors at low temperatures.…”

Prospective Performance of Graphene HEB for Ultrasensitive Detection of Sub-mm Radiation

“…Our recent work has been focusing on the hot-electron TES 5 where a much lower thermal conductance than in a SiN membrane suspended TES could be achieved. This is due to the weak electron-phonon (e-ph) coupling in a micron-or submicron-size hot-electron Ti TES 6,7 . Using this approach, the targeted low NEP values have been achieved recently via direct optical measurements 8 .…”

THz/sub-THz bolometer based on electron heating in a semiconductor waveguide

“…This spectrum part, roughly (0.05-5) THz, is re− cently actively used in different areas of human activity, i.e., astronomy, THz ray imaging, security, biology, drugs and explosive material detection, etc. [1][2][3]. THz region spans the transition from radio electronics to photonics, and appli− cation of traditional electromagnetic radiation detectors, de− veloped for them, in the THz region is limited [3].…”

“…The appeared at electron heating changes of the superconductor resistance [2,5] and the elec− tron mobility in semiconductor [4,6,7], and the thermoelec− tromotive [8], as well as, effects in the HEMT channel [3] are used now for the radiation detector developing. In the represented article with this goal, the use of the electron− −hole pair concentration change in bipolar semiconductor is proposed.…”