A. Sergeev

Ultrasensitive hot-electron kinetic-inductance detectors operating well below the superconducting transition

While most experimental studies of kinetic-inductance sensors have been limited so far by the temperature range near the superconducting transition, kinetic-inductance detectors can be very sensitive at temperatures well below the transition, where the number of equilibrium quasiparticles is exponentially small. In this regime, a shift of the quasiparticle chemical potential under radiation results in the change of the kinetic inductance. We modeled the noise characteristics of the kinetic-inductance detectors made from disordered superconducting Nb, NbC, and MoRe films. Low-phonon transparency of the interface between the superconductor and the substrate causes substantial retrapping of phonons providing high quantum efficiency and the operating time of ∼1 ms at ≈1 K. Due to the small number of quasiparticles, the noise equivalent power of the detector determined by the quasiparticle generation–recombination noise can be as small as ∼10−19 W/Hz at He4 temperatures.

Photon-noise-limited direct detector based on disorder-controlled electron heating

We present a concept for a hot-electron direct detector capable of counting single millimeter-wave photons. The detector is based on a microbridge (1 μm size) transition edge sensor made from a disordered superconducting film. The electron–phonon coupling strength at temperatures of 100–300 mK is proportional to the elastic electron mean free path l and can be reduced by over an order of magnitude by decreasing l. The microbridge contacts are made from a different superconductor with higher critical temperature Nb, which blocks the thermal diffusion of hot carriers into the contacts. The low electron–phonon heat conductance and the high thermal resistance of the contacts determine the noise equivalent power of ∼10−20–10−21 W/√Hz at 100 mK, which is 102–103 times better than that of state-of-the-art bolometers. Due to the effect of disorder, the electron cooling time is ∼10−1–10−2 s at 0.1 K. By exploiting negative electrothermal feedback, the detector time constant can be made as short as 10−3–10−4 s with...

Millisecond electron–phonon relaxation in ultrathin disordered metal films at millikelvin temperatures

We have measured directly the thermal conductance between electrons and phonons in ultrathin 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 τe follows the T−4 dependence with a record-long value τe=25 ms at T=0.04 K. 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 μm2 and the noise limited by fluctuations of the temperature are expected to be (2–3)×10−20 W/Hz, which is two orders of magnitude smaller than that of the state-of-the-art bolometers.

Subnanosecond photoresponse of a YBaCuO thin film to infrared and visible radiation by quasiparticle induced suppression of superconductivity

We observed subnanosecond photoresponse of a structured superconducting YBa2Cu3O7−δ thin film to infrared and visible radiation. We measured the voltage response of a current biased film (thickness 700 A) in a resistive state to radiation pulses. From our results we conclude a response time of about 90 ps and a responsivity of about 4×1010 Ω/J. We attribute the response to Cooper pair breaking and suppression of the superconducting energy gap induced by nonequilibrium quasiparticles.

High-gain quantum-dot infrared photodetector

An innovative idea in design of sensitive quantum-dot (QD) infrared photodetector is to use a structure with QDs surrounded by repulsive potential barriers which are created due to interdot doping. Spatial separation of the localized ground state and continuum conducting states of the electron increases significantly the photoelectron capture time and photoconductive gain. Large value of the gain results in high responsivity, which in turn improves detectivity and raises the device operating temperature.

Experimental studies of the electron–phonon interaction in InGaAs quantum wires

Electron-heating measurements are used to compare the form of the electron–phonon interaction in two-dimensional, and quasi-one-dimensional, InGaAs quantum wires. Evidence for a strongly enhanced interaction is found in the quasi-one-dimensional wire, and is suggested to result from the presence of the singularities in its electronic density of states. The Bloch–Gruneisen criterion is easily violated in this wire, and its energy-loss function is found to show a weak temperature dependence, which is argued to result from a saturation of scattering processes in the uppermost one-dimensional subband.

Inelastic electron–boundary scattering in thin films

Abstract Inelastic electron scattering at the interface is one of the basic processes of the electron–phonon interaction in thin films, nanostructures, and low-dimensional conductors. This scattering process determines the temperature-dependent resistivity and electron dephasing in a wide temperature range. It is also provides a new channel for energy transfer from the film electrons to the substrate phonons. Inelastic electron–boundary scattering allows us to explain the observed decrease of the Kapitza conductivity at the transition to the superconducting state and a value of the Kapitza conductance for pairs of materials with rather different acoustic impedances. It results in a nonequilibrium component of the photoresponse with a picosecond decay time proportional to the film thickness that has been recently observed in YBaCuO ultrathin films.

Influence of phonon trapping on the performance of NbN kinetic inductance detectors

Voltage and microwave photoresponse of NbN thin films to modulated and pulsed optical radiation reveals, far below the superconducting transition, a response time consistent with the lifetime of nonequilibrium quasiparticles. We show that even in 5 nm thick films at 4.2 K the phonon trapping is significant resulting in a quasiparticle lifetime of a few nanoseconds that is an order of magnitude larger than the recombination time. Values and temperature dependence of the quasiparticle lifetime obey the Bardeen-Cooper-Schrieffer theory and are in quantitative agreement with the electron-phonon relaxation rate determined from the resistive response near the superconducting transition. We discuss a positive effect of the phonon trapping on the performance of kinetic inductance detectors.

Processes of electron-phonon interaction in thin YBaCuO films

Abstract The ultrafast voltage response of YBaCuO films to laser radiation is studied and compared with previously investigated quasiparicles response to radiation of submillimeter wavelength range. Voltage shift under the visible light radiation has two components. Picosecond response realized as suppression superconductivity by nonequilibrium excess quasiparticles, response time is determined by quasiparticles recombination rate. Nanosecond response is probably due to bolometric effect.

Experimental study of superconducting hot-electron sensors for submm astronomy

Relaxation, noise, and spectral properties of micron-size hot-electron sensors made from thin Ti film are studied. Due to the small heat capacity of electrons, the devices are sensitive to single quanta of submm radiation. The sensors can be used for both hot-electron direct detectors (HEDD) and hot-electron photon-counters (HEPC) depending whether electron-phonon relaxation or electron outdiffusion is a dominating cooling mechanism. In an HEDD, the diffusion is blocked by Andreev contacts and the cooling rate is determined by the electron-phonon relaxation. The electron-phonon time in disordered films is long (/spl tau//sub e-ph//spl ap/0.16/spl times/T/sup -4/ /spl mu/s) providing an NEP/spl ap/10/sup -19/ W//spl radic/Hz at 0.3 K and NEP/spl ap/10/sup -20/ W//spl radic/Hz at 0.1 K. The output noise in micron-size bridges follows the predictions of the hot-electron model. In the diffusion mode, the relaxation time of 3 ns has been measured in a 3 /spl mu/m-long device. Smaller size HEPCs would be able to operate with the spectral resolution of 300 GHz at 0.3 K and 100 GHz at 0.1 K and with the photon counting rate in the GHz range. The spectral response of a prototype antenna-coupled Nb HEDD device has been measured and shown to be flat over the range 250-900 GHz.