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IEEE Transactions on Applied Superconductivity

Nonbolometric NbN photodetector

Mark W. Johnson, A.M. Domino, Alan M. Kadin

Published Jun 1, 1995• 5 citations
Publisher

Abstract

There have been several recent reports of apparently nonbolometric optical responses in superconducting thin films. The photoresponse of 10 nm thin superconducting NbN meander lines on Si is measured at 670 nm using a diode laser modulated to more than 100 kHz. Several types of photoresponse are identified, some apparently due to the Si substrate. A fast photoresponse due to the NbN film with a responsivity of up to 125 V/W of absorbed power was observed. Results are analyzed in the context of a bolometric model, and the magnitude of the photoresponse is found to be too large for a purely bolometric response. Possible nonbolometric alternatives are discussed.>

DOI
10.1109/77.403119

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Softwaretechnik-Trends•Mar 1, 1999•1 citation
There have been several recent reports of apparently nonbolometric optical responses in superconducting thin films. The photoresponse of 10 nm thin superconducting NbN meander lines on Si is measured at 670 nm using a diode laser modulated to more than 100 kHz. Several types of photoresponse are identified, some apparently due to the Si substrate. A fast photoresponse due to the NbN film with a responsivity of up to 125 V/W of absorbed power was observed. Results are analyzed in the context of a bolometric model, and the magnitude of the photoresponse is found to be too large for a purely bolometric response. Possible nonbolometric alternatives are discussed.>
The photoresponse of 10 nm thick superconducting NbN meander lines is measured using an amplitude modulated infrared semiconductor laser operating at a wavelength of 1300 nm. The response time of the film is found to be less than 1 ns with a measured responsivity of up to 1500 V/W of absorbed power at 100 kHz. Thermal properties of the film are extracted from current–voltage characteristics using a self-heating hot spot model. At temperatures well below the superconducting transition, the magnitude of the photoresponse is found to be an order of magnitude too large to be purely bolometric, even when electron heating and effects due to intergranular weak links are taken into account. The photoresponse is seen to be bolometric near Tc. Other contributions to photoresponse are discussed, including kinetic inductance and the photofluxonic effect.
The change in the resonant frequency of YBa2Cu3O7−x meander lines under thermal and optical perturbations was used to determine whether pair breaking effects are induced either by bolometric or nonbolometric responses. The frequency-domain measurements were performed by monitoring the change of both the microwave resonance mode frequency and frequency width. The variation of the resonant frequency induced by two external perturbations shows a quadratic dependence on each perturbation in the range of 0.28Xc–0.56Xc. A perturbation to the fourth dependence is observed above 0.63Xc, where Xc is either the critical temperature or the critical optical pulse energy at which resonance can no longer be distinguished. The results suggest that the photoresponse of the superconducting meander line is bolometric due to laser heating.
The ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrathin (3.5 nm thick) NbN films deposited on sapphire by reactive magnetron sputtering into either a 5×10 μm2 microbridge or 25 1 μm wide, 5 μm long strips connected in parallel. Both structures were placed at the center of a 4 mm long coplanar waveguide covered with Ti/Au. The photoresponse was studied at temperatures ranging from 2.15 K to 10 K, with the samples biased in the resistive (switched) state and illuminated with 100 fs wide laser pulses at 395 nm wavelength. At T=2.15 K, we obtained an approximately 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electron heating effect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-speed response of NbN devices makes them an excellent choice for an optoelectronic interface for superconducting digital circuits, as well as mixers for the terahertz regime. The multiple-strip detector showed a linear dependence on input optical power and a responsivity R=3.9 V/W.
We report an effective quantum efficiency of 340, responsivity >200 A W -1 (>10 4 V W -1 ) and response time of 27±5 ps at temperatures close to the superconducting transition for NbN superconducting hot-electron photodetectors (HEPs) in the near-infrared and optical ranges. Our studies were performed on a few nm thick NbN films deposited on sapphire substrates and patterned into µm-size multibridge detector structures, incorporated into a coplanar transmission line. The time-resolved photoresponse was studied by means of subpicosecond electro-optic sampling with 100 fs wide laser pulses. The quantum efficiency and responsivity studies of our photodetectors were conducted using an amplitude-modulated infrared beam, fibre-optically coupled to the device. The observed picosecond response time and the very high efficiency and sensitivity of the NbN HEPs make them an excellent choice for infrared imaging photodetectors and input optical-to-electrical transducers for superconducting digital circuits.
We report time-resolved characterization of superconducting NbN hot-electron photodetectors using an electro-optic sampling method. Our samples were patterned into micron-size microbridges from 3.5-nm-thick NbN films deposited on sapphire substrates. The devices were illuminated with 100 fs optical pulses, and the photoresponse was measured in the ambient temperature range between 2.15 and 10.6 K (superconducting temperature transition TC). The experimental data agreed very well with the nonequilibrium hot-electron, two-temperature model. The quasiparticle thermalization time was ambient temperature independent and was measured to be 6.5 ps. The inelastic electron–phonon scattering time τe–ph tended to decrease with the temperature increase, although its change remained within the experimental error, while the phonon escape time τes decreased almost by a factor of two when the sample was put in direct contact with superfluid helium. Specifically, τe–ph and τes, fitted by the two-temperature model, were equal to 11.6 and 21 ps at 2.15 K, and 10(±2) and 38 ps at 10.5 K, respectively. The obtained value of τe–ph shows that the maximum intermediate frequency bandwidth of NbN hot-electron phonon-cooled mixers operating at TC can reach 16(+4/−3) GHz if one eliminates the bolometric phonon-heating effect.
We report on transient measurements of electron energy relaxation in NbN films with 300-fs time resolution. Using an electro-optic sampling technique, we have studied the photoresponse of 3.5-nm-thick NbN films deposited on sapphire substrates and exposed to 100-fs-wide optical pulses. Our experimental data analysis was based on the two-temperature model and has shown that in our films at the superconducting transition 10.5 K the inelastic electron-phonon scattering time was about (111}+- __.2) ps. This response time indicated that the maximum intermediate-frequency band of a NbN hot-electron phonon-cooled mixer should reach (16+41-3) GHz if one eliminates the bolometric phonon-heating effect. We have suggested several ways to increase the effectiveness of phonon cooling to achieve the above intrinsic value of the NbN mixer bandwidth.