W. W. Xu

Low-loss terahertz metamaterial from superconducting niobium nitride films

This paper reports a type of low Ohmic loss terahertz (THz) metamaterials made from low-temperature superconducting niobium nitride (NbN) films. Its resonance properties are studied by THz time domain spectroscopy. Our experiments show that its unloaded quality factor reaches as high as 178 at 8 K with the resonance frequency at around 0.58 THz, which is about 24 times that of gold metamaterial at the same temperature. The unloaded quality factor keeps at a high level, above 90, even when the resonance frequency increases to 1.02 THz, which is close to the gap frequency of NbN film. All these experimental observations fit well into the framework of Bardeen-Copper-Schrieffer theory and equivalent circuit model. These new metamaterials offer an efficient way to the design and implementation of high performance THz electronic devices.

Self-consistent electronic subband structure of undoped InAs/GaSb-based type II and broken-gap quantum well systems

Motivated by a very recent experimental work on investigating electronic properties of InAs/GaSb-based type II and broken-gap quantum well structures, in this article we present a simple and transparent theoretical approach to calculate electronic subband structure in such device systems. The theoretical model is developed on the basis of solving self-consistently the Schrodinger equation for the eigenfunctions and eigenvalues coupled with the Poisson equation for the confinement potentials, in which the effects such as charge distribution and depletion are considered. In particular, we examine the effect of a GaSb cap layer on electronic properties of the quantum well systems in conjunction with experiments and experimental findings. The results obtained from the proposed self-consistent calculation can be used to understand important experimental findings and are in line with those measured experimentally.

Second-order surface-plasmon assisted responsivity enhancement in germanium nano-photodetectors with bull’s eye antennas

The enhancement of photo-response in nanometer-scale germanium photodetectors through bulls eye antennas capable of supporting 2nd-order Bloch surface plasmon modes is demonstrated in theory and experiment. A detailed numerical investigation reveals that the presence of surface wave and its constructive interference with the directly incident light are incorporated into the main mechanisms for enhancing transmission through the central nanoaperture. With a grating period of 1500 nm, the area-normalized responsivity can be enhanced up to 3.8 times at 2 V bias for a 780 nm laser. It provides an easier fabrication path for ultra-short wavelength operations especially in devices using optically denser materials.

Spin energy enhanced by exchange interaction in a Rashba spintronic system in quantizing magnetic fields

This work was supported in part by the Australian Research Council, the NSF of China Grant No. 10664006, the Knowledge Innovation Program of the Chinese Academy of Sciences, and the NSF of Jiangsu Higher Education Institutions Grant No. 04KJB510066.

Electrically tunable terahertz metamaterials with embedded large-area transparent thin-film transistor arrays

Engineering metamaterials with tunable resonances are of great importance for improving the functionality and flexibility of terahertz (THz) systems. An ongoing challenge in THz science and technology is to create large-area active metamaterials as building blocks to enable efficient and precise control of THz signals. Here, an active metamaterial device based on enhancement-mode transparent amorphous oxide thin-film transistor arrays for THz modulation is demonstrated. Analytical modelling based on full-wave techniques and multipole theory exhibits excellent consistent with the experimental observations and reveals that the intrinsic resonance mode at 0.75 THz is dominated by an electric response. The resonant behavior can be effectively tuned by controlling the channel conductivity through an external bias. Such metal/oxide thin-film transistor based controllable metamaterials are energy saving, low cost, large area and ready for mass-production, which are expected to be widely used in future THz imaging, sensing, communications and other applications.

Low Noise Receivers at 1.6 THz and 2.5 THz Based on Niobium Nitride Hot Electron Bolometer Mixers

Low noise receivers based on superconducting niobium nitride (NbN) hot electron bolometer (HEB) mixers have been designed, fabricated and measured in the terahertz (THz) frequency range for applications in astronomy and cosmology. The NbN HEB mixer consists of a planar antenna and an NbN bridge connecting across the antennas inner terminals on high-resistivity Si substrates. Double sideband (DSB) receiver noise temperatures of 920 K at 1.6 THz and 1630 K at 2.5 THz have been obtained at 4.2 K without corrections. Effects of Parylene C AR coating on the lens have been studied; with such a coating a lowest DSB receiver noise temperature of 1135 K at 2.5 THz has been obtained at 4.2 K.

Stability of Superconducting Hot Electron Bolometer Receivers

Superconducting receivers based on large hot electron bolometer (HEB) bridges were constructed and they show low noise temperatures. Their stability has been characterized by the Allan variance measurements with either far-infrared laser or Gunn oscillator plus multiplier chain as the local oscillator. An Allan time of about 1 s has been obtained. Also demonstrated is the effectiveness of a stabilization scheme in which a radiation at a frequency much lower than the gap frequency of niobium nitride is used for improvement.

Magneto-infrared modes in InAs-AlSb-GaSb coupled quantum wells.

We have studied a series of InAs/GaSb coupled quantum wells using magneto-infrared spectroscopy for high magnetic fields up to 33T within temperatures ranging from 4K to 45K in both Faraday and tilted field geometries. This type of coupled quantum wells consists of an electron layer in the InAs quantum well and a hole layer in the GaSb quantum well, forming the so-called two dimensional electron-hole bilayer system. Unlike the samples studied in the past, the hybridization of the electron and hole subbands in our samples is largely reduced by having narrower wells and an AlSb barrier layer interposed between the InAs and the GaSb quantum wells, rendering them weakly hybridized. Previous studies have revealed multiple absorption modes near the electron cyclotron resonance of the InAs layer in moderately and strongly hybridized samples, while only a single absorption mode was observed in the weakly hybridized samples. We have observed a pair of absorption modes occurring only at magnetic fields higher than 14T, which exhibited several interesting phenomena. Among which we found two unique types of behavior that distinguishes this work from the ones reported in the literature. This pair of modes is very robust against rising thermal excitations and increasing magnetic fields alligned parallel to the heterostructures. While the previous results were aptly explained by the antilevel crossing gap due to the hybridization of the electron and hole wavefunctions, i.e. conduction-valence Landau level mixing, the unique features reported in this paper cannot be explained within the same concept. The unusual properties found in this study and their connection to the known models for InAs/GaSb heterostructures will be disccused; in addition, several alternative ideas will be proposed in this paper and it appears that a spontaneous phase separation can account for most of the observed features.

A terahertz detector operating at room temperature

A detector for terahertz(THz) operating at room temperature for detecting the THz signals is fabricated by using radio frequency (RF) magnetron sputtering and electronic beam lithography. This detector is composed of a planar logarithm periodic antenna of Al film and a microbolometer of Nb5N6 thin film, acting as both a radiation absorber and a temperature sensor, respectively which is fabricated on a high resistivity Si substrate with 100nm thick SiO2 layer. The best attainable responsivity of this device is over 100 volts per watt at 300K at a bias current of 2 mA,and the electrical noise equivalent powers (NEP) is as low as 3.98×10-10W•Hz1/2.These are good enough for many detecting and imaging arrays in THz frequencies.

Pulsed laser deposition of YBa2Cu3O7−δ/I/LaNiO3 trilayers (I = SrTiO3, CeO2 or Eu2CuO4) on (100) SrTiO3 substrates

To look at a quasi-particle injection into high Tc superconductors, YBa2Cu3O7−δ/insulator/LaNiO3 (YBCO/I/LNO) trilayers have been grown on (100) SrTiO3 substrates in situ by pulsed laser deposition (PLD), where, an insulator (I), SrTiO3 (STO), CeO2 (CO) or Eu2CuO4 (ECO) are used. All the deposited films are highly oriented. The surface morphology of LNO and the insulating films is investigated during fabrications after the films are made, revealing small roughness and low droplet density. The top YBCO layer exhibits good superconducting properties with a critical temperature above 82 K and a transition width as sharp as 1.5 K.