Josephson effect in a Coulomb-blockaded SINIS junction

The problem of a Josephson current through a Coulomb-blocked nanoscale superconductor-normal-superconductor structure with tunnel contacts is reconsidered. Two different contributions to the phase-biased supercurrent I(?) are identified, which are dominant in the limits of weak and strong Coulomb interaction. Full expression for the free energy valid at arbitrary Coulomb strength is found. The current derived from this free energy interpolates between known results for weak and strong Coulomb interaction as the phase bias changes from 0 to π. In the broad range of Coulomb strength, the current-phase relation is substantially nonsinusoidal and qualitatively different from the case of semiballistic SNS junctions. The Coulomb interaction leads to the appearance of a local minimum in the current at some intermediate value of the phase difference applied to the junction.     

AC Josephson effect in the long voltage-biased SINIS junction

Theory of non-stationary coherent effects is developed for superconductor-normal-superconductor (SNS) structures with relatively strong normal scattering on S/N interfaces (interface resistance is large compared to intrinsic resistance of N metal). Analytical expressions are found for the time-dependent anomalous Green’s functions induced in the N region under the fixed-voltage-bias. The amplitude of the current oscillations is determined in non-equilibrium conditions. Non-stationary correction to the distribution function is calculated in high-temperature limit and found to be slowly decreasing with the temperature, leading to the dominance of the second-harmonic term in the Josephson current, I _s (t) ∝ sin(4eVt) at high temperatures and low voltage. The article is published in the original.     

Terahertz oscillator with vertical radiation extraction

A model of a laser that uses the GaAs/Al_yGa_{1 −y} As lattice nonlinearity to lase in the terahertz range is proposed. The laser mixes two-frequency near-IR oscillations in a vertical (i.e., arranged across the structure layers) Bragg cavity. The cw output at a wavelength of 10 μm may reach 0.5 to 5 μW.     

Terahertz pulse driven Josephson junctions

The voltage response of a Josephson junction to a pulsed terahertz current is evaluated in the limit of a negligible junction capacitance (overdamped limit). The time-dependent superconductor phase difference across the junction is calculated in the framework of the standard resistive shunted junction model by using a perturbative method. The pulsed current bias affects the time average value of the voltage across the junction and current steps are induced in the current–voltage characteristics for voltage values depending on the pulse repetition rate. The current step height is proportional to the square of the pulse time width (τ) to the period (T) ratio. A fast response detector for pulsed Terahertz radiation is proposed, with an expected responsivity of the order of 0.1 V/W and an equivalent noise power of about 3 × 10^?10 W/Hz^1/2.     

Terahertz heterodyne receivers based on superconductive hot-electron bolometer mixers

We consider recent results in development of hot-electron bolometer mixers. Special attention is paid to optimization of the contacts between the antenna and the active area of a superconducting film. An important result in the study of the parasitic effect of direct detection is obtained during the measurement of the noise temperatures by the hot/cold load method. The latest results of studies of the waveguide hot-electron bolometer mixers and their successful practical applications are considered. Progress in development of high-frequency (over 1.3 THz) heterodyne receivers for several important international projects is discussed and new submillimeter radio astronomy projects ESPRIT and SAFIR are described.     

渐变慢波结构的太赫兹过模表面波振荡器北大核心CSCD

模拟研究了一种在均匀慢波结构后增加渐变慢波结构段的太赫兹过模表面波振荡器,获得了输出功率和模式纯度的显著提高。首先根据S参数方法,研究分析了这种复合高频结构中TM_(01)模的谐振特性。然后采用2.5维PIC(Particle-in-cell)软件UNIPIC,模拟研究了振荡器输出性能随渐变慢波结构周期数的变化关系。结果表明:随着周期数的增加,输出太赫兹波中的TM_(02)和TM_(03)等高次模成分降低,TM_(01)模的功率比例增大至80%以上;选择合适长度的渐变慢波结构,能使得器件输出功率增大50%。     

渐变慢波结构的太赫兹过模表面波振荡器

模拟研究了一种在均匀慢波结构后增加渐变慢波结构段的太赫兹过模表面波振荡器, 获得了输出功率和模式纯度的显著提高。首先根据S参数方法, 研究分析了这种复合高频结构中TM01模的谐振特性。然后采用2.5维PIC（Particle-in-cell）软件UNIPIC, 模拟研究了振荡器输出性能随渐变慢波结构周期数的变化关系。结果表明：随着周期数的增加, 输出太赫兹波中的TM02和TM03等高次模成分降低, TM01模的功率比例增大至80%以上;选择合适长度的渐变慢波结构, 能使得器件输出功率增大50%。     

Submillimeter-wave Josephson spectroscopy

A Josephson high-temperature superconducting (HTSC) submillimeter-wave spectrometer is designed, built, and experimentally investigated. The integrated detection structure of the spectrometer includes a YBCO Josephson junction on a bicrystalline boundary, a double-slot or log-periodic antenna, and a low-inductance resistive shunt. The selective detector response and the response at an intermediate frequency of 1.4 GHz are measured under the action of a signal in the frequency range 350–1250 GHz. Three methods of spectroscopy are investigated using this setup: 1) a method of Hilbert spectroscopy with processing of the detector response is implemented; 2) it is found that for a wide Josephson line at intermediate frequency (IF) the response has the same form as the detector response, making it possible to obtain a spectrum and the width of the generation line from IF response measurements; 3) for a narrow Josephson line the IF response corresponds to the regime of conversion with self-pumping. A new method is proposed for calculating the emission spectrum. The method consists of simple shift, summation, and subtraction operations. The advantages of the method are simplicity, high sensitivity, and high resolution. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 5, 338–343 (10 September 1999)     

Semiconducting superlattice as a solid-state terahertz local oscillator for NbN hot-electron bolometer mixers

We present the results of our studies of the semiconducting superlattice (SSL) frequency multiplier and its application as part of the solid state local oscillator (LO) in the terahertz heterodyne receiver based on a NbN hot-electron bolometer (HEB) mixer. We show that the SSL output power level increases as the ambient temperature is lowered to 4.2 K, the standard HEB operation temperature.     

Parametric frequency transformation in a superconducting waveguide line with an integrated Josephson oscillator

The parametric frequency division in a coplanar waveguide line with an integrated single-contact rf SQUID (Josephson oscillator) is discussed. It is assumed that the oscillator is excited by pump pulses whose carrier frequency can be a multiple of the plasma frequency of the oscillator. It is shown that the Josephson oscillator excited at the pump frequency can induce frequency division by emitting subharmonics that are multiples of the fundamental frequency (fractional resonances). Parameters for which parametric frequency transformation occurs are determined. The possible generalization of this effect to the quantum case in which correlated microwave photons (entangled photon states) can be generated is discussed.     

Detection of fluxon oscillator radiation by a planar Josephson junction

A planar Josephson junction is used as a detector for the radiation emitted by a long Josephson junction oscillator dc-biased on a zero-field singularity. We observe Josephson current suppression and 300 μV radiation induced singularities in the current-voltage characteristic of the detector junction.     

Fluxon dynamics on the circular Josephson oscillator

A hamiltonian perturbation theory is developed for the perturbed sine-Gordon equation with periodic boundary conditions modelling the Josephson ring oscillator. Stationary fluxon velocities are determined as function of length, loss and bias parameters.     

Terahertz spectroscopy of DNA

The spectra of the degraded DNA of herring are studied using two methods of THz spectroscopy that employ a femtosecond laser and a frequency synthesizer based on a BWO and a high-Q cavity. An interpretation of the experimental results is presented.     

Selective measurements of superconducting qubit states by a nonlinear Josephson oscillator

Selective measurements of the states are studied for a single quantum system, viz, a Josephson qubit, by a nonlinear oscillator operating in the mesoscopic regime in which the number of quanta during measurements is varied from a few dozen to several hundreds. The quantum Monte Carlo method is used to simulate the dissipative dynamics of the qubit–oscillator system and the process of measurements of the qubit states from the change in the number of oscillator quanta. It is shown that for π-pulses in the recording of qubit states, discrimination of states is possible in single realizations (measurements), while statistical projective measurements can be made for the prepared superposition state.     

Sensitive wavelength-modulated photoacoustic spectroscopy with a pulsed optical parametric oscillator

With a laser-excited acoustic wave as the carrier wave and by modulation of the light wavelength of a multikilohertz-repetition-rate optical parametric oscillator at a lower frequency than the acoustic frequency, we demonstrate a wavelength-amplitude double-modulation technique and achieve an enhancement factor of 35 in sensitivity in photoacoustic trace gas detection with the technique.     

High-resolution Doppler-free molecular spectroscopy with a continuous-wave optical parametric oscillator

We present a reliable, narrow-linewidth (100-kHz) continous-wave optical parametric oscillator (OPO) suitable for high-resolution spectroscopy applications. The singly resonant OPO with a resonated pump is based on periodically poled lithium niobate crystal and features a specially designed intracavity etalon, which permits precise tuning to any desired wavelength in a wide range. We demonstrate Doppler-free spectroscopy of a rovibrational transition of methane at 3.39 mum.     

Terahertz spectroscopy of human sclera

Structural properties of human scleral tissues are investigated using polarization-dependent terahertz time-domain spectroscopy. Cross-linked tissue shows polarization-independent transmission properties, whereas normal tissue is polarization-dependent. This results from the different structural arrangements of the collagen fibrils that compose the human scleral tissues. In cross-linked tissue, collagen fibrils structurally form interlocking arrangements, unlike the regular parallel arrangement found in normal tissue. Our results demonstrate that terahertz spectroscopy is a powerful tool for investigating the structural characteristics of biological tissues and systems.     

Introduction to Terahertz Raman spectroscopy

This work, intended for advanced undergraduate students, explains the basic concepts and an actual scientific use of Terahertz Raman spectroscopy, displaying some of the benefits of this versatile technique for structural analysis of molecules building up materials since this technique has been increasingly used for molecular characterization in chemistry. A good didactic example of the experimentally applied temperature dependence of molecular vibrations according to the Boltzmann distribution is the ratio of anti-Stokes to Stokes Raman peak intensity used to calculate the temperature of a nylon 6,6 sample. The comparison of the temperature of nylon 6,6 calculated from the experimentally determined anti-Stokes to Stokes peak intensity ratio in the Terahertz region with the temperature set with the help of a heating-freezing stage is discussed. The limitations of Terahertz Raman spectroscopy are also part of this fundamental discussion.     

Terahertz spectroscopy of biological molecules

We study experimentally the absorption and refraction spectra for a number of amino acids, proteins and nucleic acids in the frequency range 0.1–3.5 THz. The characteristic absorption lines are revealed. The spectroscopy of water solutions of proteins and nucleic acids is carried out under conditions of total internal refraction. This method can be used for studies of the functional properties of biomolecules and the dynamics of their interaction.