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For a bi-layer structure consisting of a film deposited on a substrate, a new extraction method is proposed using which we can extract both the material parameters of the film and the thickness of the substrate from the measured Terahertz transmission through it, so long as the complex refractive index of the substrate is known beforehand. This method is applicable to a range of refractive indices of the film less than the refractive index of the substrate and a very wide range of layer thicknesses from 20 mu m to at least 200 mu m. It is very useful in some cases where the thickness of the substrate cannot be determined using conventional methods such as a Vernier caliper or micrometer screw.

Niobium nitride (NbN) nanowires are commonly used as photosensitive materials for superconducting nanowire single-photon detectors (SNSPDs). Their optical properties are the key factors influencing the performance of SNSPD. According to the experimental data and simulation results, in this paper we systematically study the optical properties of various niobium nitride nanowire detector device structures, and characterize the reflection spectra and transmission spectra of the following four device structures:1) Backside optical devices with SiO2 as the antireflection layer, 2) backside optical devices with SiN as the antireflection layer, 3) front-facing optical devices with Au+SiN as a mirror, and 4) front-facing optical devices with distributed Bragg reflector (DBR) as a mirror. The NbN films with different thickness are grown on the basis of the four device structures, and the absorption efficiencies of the NbN films with different thickness are observed. The relationships between the optimal NbN thickness and the optical absorption rate for different device structures are found as follows:The maximum absorption rate of NbN on the SiO2 antireflection layer is 91.7% at 1606 nm, while the absorption rates of the remaining structures at the optimal thickness of NbN can reach 99% or more. The absorption rate of the SiN device, the Au+SiN device and the DBR device are 99.3%, 99.8% and 99.9%, respectively. The measured results and simulation structure of DBR device are analyzed. These results are of significance for guiding the design and development of high efficiency SNSPD.

We have experimentally studied the surface modifications of Y-Ba-Cu-O (YBCO) thin films using CF4 plasma. The intensity of the plasma fluorination was controlled by changing the biasing voltage and the time of the plasma treatment. Microstructural analyses reveal that the oxygen content of the YBCO thin films was changed. Transport measurements of sufficient fluorinated YBCO films imply that the films changed totally into an oxygen-deficient semi-conducting state. From these experimental results, we believe that plasma fluorination is quite a useful method to form controllable a thin barrier layer in fabricating interface engineered junctions and to form a stable narrow weak-link region in fabricating planar superconductor-normal-superconductor junctions.

We investigate the surface structure and composition of a YBa2Cu3Oy (YBCO) thin film modified by CF4 plasma fluorination. In addition to the absorption of hydrocarbons, chemical reactions of the YBCO surface take place during CF4 plasma treatment. Various x-ray photoelectron spectroscopic data are reported and discussed. The existence of a thin barrier is confirmed, which homogeneously covers the edge of the base YBCO film in our interface engineering Josephson junction. Measurements of Auger electron spectroscopic data and the resistance versus temperature indicate that the barrier is a controllable-insulating layer.

Low-temperature scanning electron microscopy (LTSEM) is a promising measuring technique for probing the spatial distribution of the superconducting properties of the high-temperature superconducting (HTS) thin films. A theoretical analysis of the electron-beam irradiation on the HTS thin films in the LTSEM has been carried out. An inhomogeneously distributed grain array model has been applied in the analysis, and some numerical simulations have been carried out on the electron-beam induced voltage (EIV) signals in the LTSEM experiments. The comparisons of our numerical results with the LTSEM experimental data indicate that it is quite reasonable to use a two-dimensional Josephson junction array for stimulating the inhomogeneous HTS thin film sample. Our numerical results also show that the EIV signals are influenced by the electron-beam power used in the LTSEM, and a reduction of the electron-beam power is suggested in order to eliminate the errors in estimating the local values of critical temperature Tc and critical current Ic by the sample temperature and the bias current at which the first EIV signal occurs.

A YBa2Cu3O7-δ thin film is modified by a probe electric field of an atomic force microscope to form a ridge with the width of only a grain cell. The modification varies with the operation parameters of the bias voltage, the moving velocity of the probe and the ambient humidity. Energy dispersive spectroscopy analysis shows only oxygen deficiency in the modified YBCO thin film. As a result, the suppressed superconductivity was found in the junction crossing the ridge.

A tunable superconducting half-wavelength coplanar waveguide resonator (CPWR) with Nb parallel nanowires ∼ 300 nm in width embedded in the center conductor was designed, fabricated, and measured. The frequency shift and the amplitude attenuation of the resonance peak under irradiation of 404-nm pulse laser were observed with different light powers at 4.2 K. The RF power supplied to such a CPWR can serve as current bias, which will affect the light response of the resonator.

Superconducting nanowire single-photon detectors (SNSPDs) with a composite optical structure composed of phase-grating and optical cavity structures are designed to enhance both the system detection efficiency and the response bandwidth. Numerical simulation by the finite-difference time-domain method shows that the photon absorption capacity of SNSPDs with a composite optical structure can be enhanced significantly by adjusting the parameters of the phase-grating and optical cavity structures at multiple frequency bands. The absorption capacity of the superconducting nanowires reaches 70%, 72%, 60.73%, 61.7%, 41.2%, and 46.5% at wavelengths of 684, 850, 732, 924, 1256, and 1426 nm, respectively. The use of a composite optical structure reduces the total filling factor of superconducting nanowires to only 0.25, decreases the kinetic inductance of SNSPDs, and improves the count rates.

In this paper, we study the response properties of multi-photon of NbN superconductor nanowire in superconducting single photon detector (SSPD). We measure the NbN nanowire device's DC characteristics and detection probability for single and multi-photon light pulse signal at a temperature of 3.5 K. The measured results show that the superconducting transition current of superconductor nanowire decreases as light irradiation intensity increases. The photon number detected by SSPD is derived from the slope of detection probability versus light intensity. We find that the detected photon number increases as superconducting nanowire bias current decreases. Moreover, based on quantum optics and hotspot theory, we analyze the mechanism of the multi-photon response of superconducting nanowire semi-quantitatively. This result may be of benefit to understanding SSPD and developing the SSPD with the capability of resolving photon number.

Considering that in a magnetic resonance system, if the detection coil contributes dominantly to the system noise, the performance of the whole system can certainly be improved by switching to a detection coil made of a high-temperature superconductor, and using YBa2Cu3O7 thin films on 25?25?mm2 LaAlO3 substrates, we have prepared two kinds of detection coils: single-coil and two-coil. Encouragingly, their quality factors are measured to be Q>2500 for two-coil (at 22.566?MHz and 77?K) and Q>5500 for single-coil (at 92.3?MHz and 77?K), respectively. Here, we describe the details of the design, fabrication and testing of the coils.

In this paper,the fabrication details and optimization of micro-fabrication process are presented for developing superconducting nanowire single-photon detectors (SNSPD). Besides,the device failure analysis is also introduced. With those methods,we successfully fabricated high-quality SNSPDs whose maximum system efficiencies were up to 30% for 660 nm wavelength and 4.2% for 1550 nm wavelength according to the single-photon detection experiment. At the dark count rate of 10 c/s,the detection efficiencies were 20% (660 nm) and 3% (1550 nm) with the SNSPD fabricated with above mentioned methods.

The complex refractive indices and the dielectric function of GaN for frequencies ranging from 0.25 to 1.22 THz are obtained using THz time-domain spectroscopy. The real part of the dielectric function first decreases from 0.25 to 0.42 THz and then oscillates from 0.42 to 1.22 THz, whereas the imaginary part of the dielectric function is oscillating within the whole range of frequency. The simple Drude model is extended to take into account the effect of defects on the dielectric function. The extended model is in agreement with the experimental data.

Using terahertz time domain spectroscopy （THz-TDS）, THz transmission spectrum of the array with sub-wavelength apertures on the Nb thin film has been studied in the temperature range of 5—300 K. The extraordinary transmission is observed and the peak positions are in agreement with the computer simulation technology simulation results. The transmission amplitude increases with the decreasing temperature.

This paper reports the process for fabricating Nb/Al-AlOx/Nb tunnel junction using rf-magnetron sputtering. The S-I-S sandwich structure is prepared without breaking vacuum. A detailed AES and XTEM analysis of the Nb/Al-AlOx/Nb sandwich structure shows the good quality of Al barrier. Typical l-V characteristic curves of tunnel junction are also given.