M. Danerud

Nonequilibrium and bolometric photoresponse in patterned YBa2Cu3O7−δ thin films

Epitaxial laser deposited YBa2Cu3O7−δ films of ∼50 nm thickness were patterned into detectors consisting of ten parallel 1 μm wide strips in order to study nonequilibrium and bolometric effects. Typically, the patterned samples had critical temperatures around 86 K, transition widths around 2 K and critical current densities above 1×106A/cm2 at 77 K. Pulsed laser measurements at 0.8 μm wavelength (17 ps full width at half maximum) showed a ∼30 ps response, attributed to electron heating, followed by a slower bolometric decay. Amplitude modulation in the band fmod=100 kHz–10 GHz of a laser with wavelength λ=0.8 μm showed two different thermal relaxations in the photoresponse. Phonon escape from the film (∼3 ns) is the limiting process, followed by heat diffusion in the substrate. Similar relaxations were also seen for λ=10.6 μm. The photoresponse measurements were made with the film in the resistive state and extended into the normal state. These states were created by supercritical bias currents. Measurem...

Optical mixing in a patterned YBa2Cu3O7−δ thin film

Mixing of 1.56 μm infrared radiation from two lasers in a high quality YBa2Cu3O7−δ thin film, patterned to parallel strips, was demonstrated. A mixer bandwidth of 18 GHz, limited by the measurement system, was obtained. A model based on nonequilibrium electron heating gives a good fit to the data and predicts an intrinsic mixer bandwidth in excess of 100 GHz, operating in the whole infrared spectrum. Reduction of bolometric effects and ways to decrease the conversion loss of the mixer is discussed. The minimum conversion loss is expected to be ∼10 dB.

Transient resistive photoresponse of YBa2Cu3O7−δ films using low power 0.8 and 10.6 μm laser radiation

Thin YBa2Cu3O7−δ laser deposited films were patterned into devices consisting of ten parallel 1 μm wide strips. Nonequilibrium picosecond and bolometric photoresponses were studied by the use of 17 ps full width at half‐maximum laser pulses and amplitude modulated radiation from an AlGaAs laser up to 10 GHz and from a CO2 laser up to 1 GHz. The time and frequency domain measurements were in agreement. The fast response can be explained by electron heating. The use of low optical power and a sensitive measurement system excluded any nonlinear transient processes and kinetic inductance changes in the superconducting state. At 1 GHz modulation frequency, the responsivity was ∼1.2 V/W both for 0.8 and 10.6 μm wavelengths. The sensitivity of a fast and spectrally broadband infrared detector is discussed.

9.6 μm wavelength mixing in a patterned YBa2Cu3O7‐δ thin film

Hot‐electron bolometric (HEB) mixing of 9.6 μm infrared radiation from two lasers in high‐quality YBa2Cu3O7−δ (YBCO) patterned thin film has been demonstrated. A heterodyne measurement showed an intermediate frequency (IF) bandwidth of 18 GHz, limited by our measurement system. An intrinsic limit of 100 GHz is predicted. Between 0.1 and 1 GHz intermediate frequency, temperature fluctuations with an equivalent output noise temperature Tfl up to ∼150 K, contributed to the mixer noise while Johnson noise dominated above 1 GHz. The overall conversion loss at 77 K at low intermediate frequencies was measured to be ∼25 dB, of which 13 dB was due to the coupling loss. The HEB mixer is very promising for use in heterodyne receivers within the whole infrared range.

High quality YBCO thin films - laser deposition, co-evaporation, and device fabrication

High quality superconducting YBa2Cu3O7-δ (YBCO) thin films were fabricated with ex situ co-evaporation on SrTiO3 and in situ laser deposition on (001) MgO substrates. The co-evaporated films gave a superconducting transition temperature, Tc ~ 90 K, and a critical current density, jc ~ 5 × 105 A/cm2 at 77 K after an ex situ post anneal at 890°C. For laser deposited films, Tc was above 90 K, and jc, about 106 A/cm2 at 77 K. Two kinds of Josephson weak links were developed. The first kind consisted of a 0.8 μm or wider constriction in a co-evaporated YBCO film. A lift-off stencil was used to pattern the film before the post anneal. The second kind of weak link utilized the inherent grain boundaries in the laser deposited YBCO film. A 70-130 nm wide strip in A1 was deposited across a 4-6 μm wide YBCO bridge. After a 10-20 min long anneal at 300-450°C, the A1 had diffused into the film. We believe that the diffusion occurs mainly along the grain boundaries. By the proximity effect, the normal metal weakened the superconductivity at the grain boundary, and a weak link was formed with IcRn ~ 0.9 mV at 77 K. Dry an wet etching were also used to pattern some structures.

Epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/(7-x) thin films on CeO/sub 2/ buffered sapphire for optical mixers

Optical mixers based on YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films deposited on CeO/sub 2/ thin film buffered sapphire substrates were developed and used for photoresponse measurements. Firstly, a 50 nm CeO/sub 2/ thin film was laser deposited on sapphire at a substrate temperature (T/sub s/) of 760/spl deg/C and an oxygen pressure (p/sub O2/) of 1.0/spl times/10/sup -2/ mbar, followed by an annealing for 15 min at 850/spl deg/C. Subsequently, a /spl sim/50 nm thick YBCO layer was laser deposited at T/sub s/=760/spl deg/C and p/sub O2/=1.0 mbar. Finally, a 150 nm thick Au film for contacts was laser deposited in situ. X-ray diffraction and /spl phi/-scan showed a high degree of c-axis and in-plane orientation. The critical temperature was 89 K and the critical current, j/sub c//spl sim/2.10/sup 6/ A/cm/sup 2/ at 77 K. Two diode lasers (/spl lambda/=1.56 /spl mu/m) were coupled into a single mode fiber, which was centered at a patterned mixer structure in the YBCO film. Optical mixing was detected up to 7 GHz at a temperature of 77 K.

Optical mixing in thin YBa/sub 2/Cu/sub 3/O/sub 7-x/ films

High quality, j/sub c/ (77 K)>10/sup 6/ A/cm/sup 2/, epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-x/ films of 50 nm thickness were patterned into ten parallel 1 /spl mu/m wide strips. The film structure was coupled to a single-mode fiber. Mixer response was obtained at 0.78 /spl mu/m using laser frequency modulation and an optical delay line. Using two semiconductor lasers at 1.55 /spl mu/m wavelength the beating signal was used to measure the photoresponse up to 18 GHz. Nonequilibrium photoresponse in the resistive state of the superconductor was observed. Bolometric response dominates up to 3 GHz, after which the nonequilibrium response is constant up to the frequency limit of our registration system. Using an electron heating model the influence of different thermal processes on the conversion loss has been analyzed. Ways of increasing the sensitivity are also discussed.<<ETX>>

A fast infrared detector based on patterned YBCO thin film

Detectors for infrared radiation ( lambda =0.85 mu m) were made of 50 nm thick YBa2Cu3O7- delta films on LaAlO3 and MgO or 60 nm thick films on NdGaO3. Parallel strips (1 mu m wide by 20 mu m long) were patterned in the films and formed the active device. These devices were designed to detect short infrared laser pulses by electron heating. The detectors were current biased into the resistive and the normal states. The response was studied in direct pulse measurements as well as by amplitude modulation of a laser. The pulse measurements showed a fast picosecond response followed by a slower decay related to phonon escape through the film-substrate interface and heat diffusion in the substrate. The frequency spectra up to 10 GHz showed two slopes with a knee corresponding to the phonon escape time.

High Tc superconducting IR detectors from Y-Ba-Cu-O thin films

A thin-film high-Tc superconducting multielement optical detector made of Y-Ba-Cu-O has been designed and evaluated using optical pulses from a diode laser (830 nm) and a Q-switched CO2-laser (10.6 microns). Different thin films have been tested. A laser deposited film showed the strongest response amplitude for short pulses and responded to an ultrafast, 50 ps wide pulse. Comparisons between dR/dT and response as a function of temperature indicated, however, a bolometric response.

Highly sensitive and wideband optical detection in patterned YBa2Cu3O7−δ thin films

We have measured the optical responses up to 18 GHz in patterned YBa2Cu3O7−δ thin films at various wavelengths by optical heterodyne mixing. The responsivity of the detectors is higher than 50 V/W below 1 Hz at various wavelengths and the responsivity is 20 mV/W at 780 nm and 150 mV/W at 1.55 μm wavelengths in the regime of the modulation frequency from 3 GHz to 18 GHz.