V. V. Pavlovskii

Terahertz Hilbert Spectroscopy by High-Tc Josephson Junctions

Hilbert spectroscopy is based on the frequency-selective detection of weak electromagnetic radiation by Josephson junctions described by the resistively shunted junction (RSJ) model. The YBa2Cu3O7-x grain-boundary Josephson junctions fabricated on twin-free NdGaO3 bicrystals were found to be close to the RSJ model. Laboratory prototypes of general-purpose Hilbert spectrometers were developed and characterized. The spectral bandwidth of Hilbert spectroscopy for any junction temperature between 30 and 85 K is as large as one frequency decade. The middle frequency of this bandwidth scaled with the characteristic frequency f c = (2e/h)I c R n of the Josephson junction and a total bandwidth of almost two orders, from 50 GHz to 4 THz, was covered by one Josephson junction at two temperatures. A spectral resolution δf/f of the order of 10-3 was realized in the terahertz range. The dynamic range of intensities of radiation which can be measured by Hilbert spectroscopy is close to five orders. With the help of Hilbert spectroscopy we measured the following emission spectra: Lorentz spectra of Josephson oscillations, spectra of high-harmonic content in commercial millimeter-wave oscillators, spectra of terahertz radiation from optically-pumped gas lasers and spectra of transition radiation from relativistic electron bunches at DESY (Hamburg).

Hilbert-transform spectroscopy with high-T/sub c/ Josephson junctions: first spectrometers and first applications

First laboratory prototypes of Hilbert-transform spectrometers have been developed. To meet the requirements of Hilbert-transform spectroscopy, high-T/sub c/ Josephson junctions with RSJ-like behavior have been fabricated on twin-free NdGaO/sub 5/ bicrystal substrates. The operation of the spectrometers has been demonstrated from 60 GHz up to 2.25 THz. The first out-of-lab application of Hilbert-transform spectrometers has been successfully demonstrated at the TESLA Test Facility linear accelerator at DESY (Hamburg). The spectra of coherent transition electromagnetic radiation from electron bunches have been measured and a bunch length of /spl sigma//sub t/=1.2 ps has been derived from these spectra. The market perspectives for Hilbert spectrometers have been estimated.

Signal and Noise Characteristics of Terahertz Frequency-Selective and Broadband High-

Josephson detectors based on YBa2Cu3O7-x [100]-tilt bicrystal junctions were fabricated and their frequency-selective and broadband modes of operation were studied at terahertz frequencies. The resistances of the [100]-tilt junctions were in the range of 1-200 Ohm and their characteristic voltages were up to 8 mV at 4.2 K. Values of the responsivity up to at radiation frequency of 0.7 THz were demonstrated for the low-resistance frequency-selective detectors at a temperature of 55 K. The responsivity values were found to be in accordance with the predictions of the RSJ model, when only thermal fluctuations are considered. But, the values of noise equivalent power (NEP) and power dynamic range measured in the same experiment at a modulation frequency of 2 kHz were found to be determined by 1/f noise of the junctions and equal to 3ldr10-13 W/Hz1/2 and 5ldr104, correspondingly. It was shown that the values of NEP of 5ldr10-15 W/Hz1/2 and power dynamic range of 106 could be reached in this mode if high-frequency modulation or pulsed radiation were used. Broadband classical detection in high-resistance [100]-tilt junctions was experimentally found to reach the terahertz range. It follows from numerical simulations that broadband detection by the [100]-tilt YBCO junction with the resistance of 300 Ohm might be characterized by NEP-values down to 3ldr10-15 W/Hz1/2 and a bandwidth of 1.5 THz. Josephson detectors based on YBCO [100]-tilt bicrystal junction are promising for various terahertz applications such as real-time spectral analysis of continuous or pulsed radiation sources and terahertz imaging for medical or security screening.

T_{c}

A prototype of a terahertz Hilbert-transform spectrum analyzer based on a high-T c Josephson junction integrated into a Stirling cooler has been developed. The detector response of YBa 2 Cu 3 O 7-x Josephson grain-boundary junctions to monochromatic radiation with the frequency f in the range from 60 GHz to 5 THz has been studied. Odd-symmetric resonances near the voltages V=hf/2e in the responses ΔI(V) of these junctions to radiation with different frequencies / have been observed in a decade of spectral range for any operating temperature between 30 to 85 K. Decreasing the junction temperature from 85 to 30 K by a Stirling cooler, the spectral analysis could be made in two decades of spectral range. A resolving power δf/f ∼ 10 -3 has been shown in the terahertz spectral analysis with the low-resistive Josephson junctions. As an example of application of this analyzer, an optimization of the tingle-line operation of a far-infrared optically-pumped CH 3 OH laser has been demonstrated.

Josephson Detectors

The detector response of YBa2Cu3O7−x Josephson grain-boundary junctions to monochromatic radiation with the frequency f in the range from 60 GHz to 4 THz has been studied. Frequency-selective odd-symmetric resonances in the responses ΔI(V) of these junctions to radiation with different frequencies f have been observed near the voltages V=hf/2e in almost a decade of spectral range for any operating temperature in the range from 30 to 85 K. The spectral range of the selective detection has scaled with the IcRn product of the Josephson junction, reaching the range of 0.16–3.1 THz for a IcRn product of 1.5 mV. A resolving power δf/f of around 10−3 has been demonstrated in the selective detection by Josephson junctions. The high-frequency falldown of the amplitude of the selective response has been found to be proportional to exp[−P/P0], where P=(hf/2e)2/Rn is the power dissipated in the junction at the resonance and P0 is a characteristic power level. The values of P0 for our junctions were around 20 μW at 34...

Terahertz spectral analysis by ac Josephson effect in high-T/sub c/ bicrystal junctions

Abstract Hilbert spectroscopy is based on a frequency-selective detection of electromagnetic radiation by a Josephson junction described by the resistively shunted junction (RSJ) model. The YBa 2 Cu 3 O 7− x grain-boundary junctions fabricated on NdGaO 3 bicrystals were found to be close to the RSJ model. General-purpose Hilbert spectrometers were developed and characterized using voltage-biased low-resistance junctions. The spectral bandwidth of Hilbert spectroscopy was shown to be of several frequency decades for any junction temperature between 48 and 85 K. A total bandwidth from 6 GHz to 2.5 THz has been covered using one junction at different temperatures. The spectral resolving power f /δ f of Hilbert spectroscopy was found to be of three orders in the terahertz range. The dynamic range of intensities of electromagnetic radiation in Hilbert spectroscopy was shown to be of five orders.

Frequency-selective incoherent detection of terahertz radiation by high-Tc Josephson junctions

The results of the spectral analysis of a high-temperature superconducting (HTSC) log-periodic antenna integrated with a HTSC Josephson junction (JJ) and operating, in this case, as a test resonant system are presented. Current-voltage characteristics of the JJ and dependences of its differential resistance on the voltage across the JJ are measured at a temperature of 5 K by varying the JJ critical current by an external magnetic field in order to extend the operating frequency range of the spectral analysis. The low-frequency boundary of the spectral analysis, which is determined by temperature fluctuations, is lowered to 64 GHz. The equivalent circuit of an integrated device consisting of the JJ and the log-periodic antenna is analyzed. The real part of the antenna admittance is reconstructed from experimental data in the frequency range 65–1200 GHz. Experimental results are compared with the results of numerical calculation.

Hilbert spectroscopy from gigahertz to terahertz frequencies by high-Tc Josephson junctions

The results of studies of interaction between a bicrystalline YBa2Cu3O7–x Josephson junction and external planar resonators are presented. The square resonators of the millimeter wave band are fabricated with splits on separate substrates and were applied to the Josephson junction. Specific features related to excitation of resonance modes corresponding to one-half of the wavelength fitting in the average physical length of the resonator are observed in the voltage dependences of the differential resistance of the Josephson junction. Resonance frequencies (48.7 and 74.9 GHz) and Q factors (14 and 25) are obtained for two resonators with different dimensions. Thus, the possibility to determine the parameters of external passive components of microwave microelectronics from static electrical characteristics of the Josephson junction is demonstrated.

Extension of the frequency range of Josephson impedance spectroscopy

We have demonstrated the first applications of Josephson-effect-based Hilbert-transform spectral analysis using high-T/sub c/ Josephson junctions. YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary Josephson junctions have been fabricated on [110] NdGaO/sub 3/ bicrystal substrates for these measurements. At first, spectra of Josephson radiation have been measured by the Hilbert-transform technique in the range 60-800 GHz. A Lorentzian shape of the Josephson radiation and the possibility to obtain a spectral resolution as low as 0.1 cm/sup -1/ in the submillimeter-wave range has been demonstrated at liquid nitrogen temperature. Moreover, spectra of high-harmonic content of the radiation coming from commercial millimeter-wave oscillators have been measured in the spectral range from 60 to 450 GHz. A comparison of Hilbert-transform spectral analysis with conventional techniques is given.

Josephson spectroscopy for local diagnostics of planar resonator systems in the millimeter wave band

[100]-tilt YBa₂Cu₃O_7-x bicrystal Josephson junctions are good candidates to realize high values of characteristic voltages I_cR_n, which are expected due to large energy gaps in high-T_c materials. Here, we report on detailed electrical characterization of [100]-tilt YBa₂Cu₃O_7-x bicrystal Josephson junctions, which have the maximal I_cR_n-values up to 9.4 mV, for resistances R_n in the range from 0.9 to 400Ω. The I-V curves have been fitted well to those of the RSJ model with ac-coupled resistive shunts R_e. The first derivatives dV/dI versus V have demonstrated a reproducible set of resonances of two types. Numerical simulations of the dV/dI versus V for Josephson junctions coupled with external THz RLC-resonators have been made and both types of resonances have been fitted. A form of the I-V curves and resonances in the first derivatives dV/dI versus V have been attributed to intrinsic broadband and frequency-selective losses, due to interaction of Josephson oscillations with transverse and longitudinal IR-active phonons in YBa₂Cu₃O_7-x electrodes.