Xiaoqing Jia

Suppression of superconductivity in epitaxial NbN ultrathin films

This paper studies the suppression of superconducting transition temperature (T(c)) of ultrathin NbN film. We fabricated epitaxial NbN superconducting thin films of thicknesses ranging from 2.5 to 100 nm on single crystal MgO (100) substrates by dc magnetron sputtering. We performed structure analyses and measured their electric and far infrared properties. The experimental results were compared with several mechanisms of the suppression of superconductivity proposed in the literature, including the weak localization effect, the proximity effect, and quantum size effect (electron wave leakage model). We found that the electron wave leakage model matches best to the experimental data

Nonlinear response of superconducting NbN thin film and NbN metamaterial induced by intense terahertz pulses

We present the nonlinear response of superconducting niobium nitride (NbN) thin film and NbN metamaterial with different thicknesses under intense terahertz pulses. For NbN thin film, nonlinearity emerges and superconductivity is suppressed with increasing incident terahertz electric field, and the suppression extent weakens as the film thickness increases from 15 to 50?nm. As the variation in intense terahertz fields alters the intrinsic conductivity in NbN, a consequent remarkable amplitude modulation in NbN metamaterial is observed due to the strong nonlinearity. Absorbed photo density in either NbN film or NbN metamaterial is estimated and used to understand the mechanism of nonlinear response. With a thicker NbN film element of 200?nm, the resonance of the metamaterial shows similar nonlinear modulation accompanied by a lower loss and a higher quality factor compared with a thinner NbN film element of 50?nm, which demonstrates the innovative implementation of strongly enhanced nonlinearity with thick superconducting film elements and the potential for novel applications using nonlinear metamaterial.

Enhanced slow light in superconducting electromagnetically induced transparency metamaterials

We characterized and compared the electromagnetically induced transparency (EIT) response of superconducting niobium nitride (NbN) and NbN–Au hybrid metamaterials. In our design, the two resonators in a unit cell have strong coupling and are directly excited under terahertz (THz) radiation. A stronger slow light effect was achieved using superconducting metamaterials than hybrid metamaterials. The enhanced slow light effect could be attributed to the remarkably low Ohmic loss and strong interaction of the resonators.

Terahertz superconducting metamaterials for magnetic tunability

We present the magnetic tunability of a metamaterial made from superconducting niobium nitride film. The inductive-capacitive resonance excited by a normally incident terahertz wave was found to be continuously modulated through an external magnetic field at temperatures below the superconducting transition point. A giant resonance modulation was observed due to a strong magnetic effect, where the variation of the magnetic field alters the intrinsic conductivity of the superconducting film. The high sensitivity of the metamaterial allows us to observe the temperature-dependent magnetic effect, and the magnitude of resonance modulation decreases with increasing temperatures. This work demonstrates that a strong magnetic effect could be implemented as an active control modality in superconducting integrated devices functioning at terahertz frequencies.

Diffractive microlens integrated into Nb 5 N 6 microbolometers for THz detection

We fabricated square diffractive microlens array with five staircases in the THz wave band for Nb5N6 microbolometers. With each microlens intergrated with an Nb5N6 microbolometer on the same substrate, an array chip was fabricated in the 4 inches silicon wafer. The lens exhibits good focusing and improves the coupling efficiency. The voltage response of the microbolometer integrated with diffractive microlens is 16 times higher than that of the microbolometer fabricated on silicon substrate. The microbolometers used as room-temperature detectors yield a good responsivity of 71 V/W and a noise equivalent power of 1.0 × 10-10 W/Hz. The diffractive microlens array features light weight, low absorption loss, and high resolution and can be mass produced using standard micro-fabrication techniques.

Demonstration of measuring sea fog with an SNSPD-based Lidar system

The monitor of sea fogs become more important with the rapid development of marine activities. Remote sensing through laser is an effective tool for monitoring sea fogs, but still challengeable for large distance. We demonstrated a Long-distance Lidar for sea fog with superconducting nanowire single-photon detector (SNSPD), which extended the ranging area to a 180-km diameter area. The system, which was verified by using a benchmark distance measurement of a known island, is applied to the Mie scattering weather prediction Lidar system. The fog echo signal distribution in the range of 42.3∼63.5 km and 53.2∼74.2 km was obtained by the Lidar system. Then the fog concentration and the velocity of the fog were deduced from the distribution, which is consistent with the weather prediction. The height of the sea fog is about two hundred meter while the visibility at this height is about 90 km due to the Earth’s radius of curvature. Therefore, the capability of this SNSPD-based Lidar was close to the theoretical limit for sea fog measurements for extremely high signal-to-noise ratio of SNSPD.

Terahertz narrow bandstop, broad bandpass filter using double-layer S-shaped metamaterials

In this study, double-layer S-shaped metamaterials (MMs) are analyzed by terahertz time-domain spectroscopy. These materials exhibit narrow bandstop and broad bandpass transmission properties at both horizontal and vertical electric-field polarizations. A 117% increase in the unloaded quality factor is experimentally observed for these materials. The center frequency is approximately 0.45 THz, with a 3-dB bandwidth of 0.52 THz from 0.20 to 0.72 THz at normal incidence. The measured average insertion loss is 0.5 dB with a ripple of 1 dB. These results show that double-layer S-shaped MMs are effective in designing tunable terahertz devices.

Selective coherent perfect absorption of subradiant mode in ultrathin bi-layer metamaterials via antisymmetric excitation

We demonstrate that the subradiant mode in ultrathin bi-layer metamaterials can be exclusively excited under two-antisymmetric-beam illumination (or equivalently, at a node of the standing wave field), while the superradiant mode is fully suppressed due to their different mode symmetry. Coherent perfect absorption (CPA) with the Lorentzian lineshape can be achieved corresponding to the subradiant mode. A theoretical model is established to distinguish the different behaviors of these two modes and to elucidate the CPA condition. Terahertz ultrathin bi-layer metamaterials on flexible polyimide substrates are fabricated and tested, exhibiting excellent agreement with theoretical predictions. This work provides physical insight into how to selectively excite the antisymmetric subradiant mode via coherence incidence.

Tunable electromagnetically induced transparency from a superconducting terahertz metamaterial

We demonstrate in this paper the tunable electromagnetically induced transparency (EIT) made from a superconducting (SC) niobium nitride (NbN) film induced by an intense terahertz (THz) field. As the variation of the incident THz field alters the intrinsic ohmic loss of the SC NbN film, the field-dependent transmittance is observed. To elaborate the role of the bright and dark modes, a hybrid coupling model is introduced to fit the experimental transmission spectra and extract the characteristic parameters of each mode. It is shown that the resonator for the bright mode is altered greatly due to strong direct coupling to the incident intense THz field, whereas the dark mode resonator has little interaction with the incident THz field via a weak near-filed coupling to the bright-mode resonator. This implies that we can partially control a mode or a part of metamaterial by introducing the intense THz field, which offers an effective manner to selectively control the electromagnetic property of the metamater...

Vortex ratchet effects in a superconducting asymmetric ring-shaped device

We investigate the vortex ratchet effects in a superconducting asymmetric ring-shaped NbN device. Through transport measurements, we find that the rectified dc voltages are significantly enhanced, and we observe time-dependent asymmetric voltage waveforms over a single cycle. Our vortex ratchet device operates over a wide range of temperatures, critical currents, and magnetic fields. We demonstrate that in this asymmetric structure giant ratchet effects are mainly caused by the collective behavior of vortices, which differs clearly from one-particle vortex effects studied in conventional vortex ratchet systems.