Chunhai Cao

Low loss and magnetic field-tunable superconducting terahertz metamaterial

Superconducting terahertz (THz) metamaterial (MM) made from niobium (Nb) film has been investigated using a continuous-wave THz spectroscopy. The quality factors of the resonance modes at 0.132 THz and 0.418 THz can be remarkably increased when the working temperature is below the superconducting transition temperature of Nb, indicating that the use of superconducting Nb is a possible way to achieve low loss performance of a THz MM. In addition, the tuning of superconducting THz MM by a magnetic field is also demonstrated, which offers an alternative tuning method apart from the existing electric, optical and thermal tuning methods.

Superconducting terahertz metamaterials mimicking electromagnetically induced transparency

We designed and fabricated planar terahertz (THz) metamaterials made from superconducting NbN films to mimic electromagnetically induced transparency (EIT) system. They are characterized using THz time domain spectroscopy over a temperature range from 8 to 300 K. High transmittance and large delay-bandwidth product at transparency window are demonstrated, which mainly arise from the enhanced coupling and decreased damping in superconducting state. The EIT-like spectral response could be tuned in a wide frequency range. By applying two dark resonators with different resonance frequencies coupled with a radiative resonator, we experimentally demonstrated the planar metamaterials mimicking four-level EIT system.

Tuning of superconducting niobium nitride terahertz metamaterials

Superconducting planar terahertz (THz) metamaterials (MMs), with unit cells of different sizes, are fabricated on 200 nm-thick niobium nitride (NbN) films deposited on MgO substrates. They are characterized using THz time domain spectroscopy over a temperature range from 8.1 K to 300 K, crossing the critical temperature of NbN films. As the gap frequency (f(g) = 2Δ0/h, where Δ0 is the energy gap at 0 K and h is the Plank constant) of NbN is 1.18 THz, the experimentally observed THz spectra span a frequency range from below f(g) to above it. We have found that, as the resonance frequency approaches f(g), the relative tuning range of MMs is quite wide (30%). We attribute this observation to the large change of kinetic inductance of superconducting film.

Extraordinary terahertz transmission in superconducting subwavelength hole array

We report the extraordinary terahertz (THz) transmission through subwavelength hole array in superconducting NbN film. As the temperature drops below the superconducting transition temperature, the transmission spectra experience distinct changes. The extraordinary transmission is greatly enhanced in superconducting state due to the enhancement of surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs). We have also observed temperature-dependent resonance frequency shift, which mainly depends on the coupling between SPPs and LSPs.

Experimental demonstrations of high-Q superconducting coplanar waveguide resonators

We successfully designed and fabricated an absorption-type of superconducting coplanar waveguide (CPW) resonators. The resonators are made from a niobium film (about 160 nm thick) on a high-resistance Si substrate, and each resonator is fabricated as a meandered quarter-wavelength transmission line (one end is short to the ground and another end is capacitively coupled to a through feedline). With a vector network analyzer we measured the transmissions of the applied microwave through the resonators at ultra-low temperature. The obtained loaded quality factors are significantly high, i.e. up to ∼106. When the temperature increases slowly from the base temperature (20 mK), the resonance frequencies of the resonators are blue shifted and the quality factors are lowered slightly. In principle, this type of device can integrate a series of CPW resonators with a common feedline, making it a promising candidate as the data bus for coupling distant solid-state qubits and the sensitive detector of single photons.

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.

Photon-induced thermal effects in superconducting coplanar waveguide resonators

We experimentally investigated the optical responses of a superconducting niobium resonator. It was found that, with increasing radiation power, the resonance frequency increases monotonically below around 500 mK, decreases monotonically above around 1 K, and exhibits a nonmonotonic behavior at around 700 mK. These observations show that one can operate the irradiated resonator in three temperature regimes, depending on whether two-level system (TLS) effects or kinetic inductance effects dominate. Furthermore, we found that the optical responses at ultra-low temperatures can be qualitatively regarded as a photon-induced thermalization effect of TLSs, which could be utilized to achieve thermal sensitive photon detections.

Photon-detections via probing the switching current shifts of Josephson junctions

X iv :1 40 8. 37 00 v1 [ co nd -m at .m es -h al l] 1 6 A ug 2 01 4 Optical responses of the switching currents in Al and Nb Josephson junctions Yiwen Wang, Pinjia Zhou, Lianfu Wei, 2, a) Beihong Zhang, Qiang Wei, Jiquan Zhai, Weiwei Xu, and Chunhai Cao Quantum Optoelectronics Laboratory, School of Physics, Southwest Jiaotong University, Chengdu 610031, China State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, China Research Institute of Superconductor Electronics, Department of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China

Nb5N6 bolometer operated at room temperature for detecting at 100 GHz

Onto a double layer, which is made of a Si substrate ( ρ> 1000 Ω·cm ) and a SiO2 layer 100 nm thick on top of it, a Nb5N6 thin film microbridge is deposited and integrated with an aluminum bow-tie planar antenna. With a SiO2 air-bridge further fabricated underneath the microbridge and operated at room temperature, such a combination behaves very well as a bolometer for detecting signals at 100 GHz, thanks to a temperature coefficient of resistance (TCR) as high as -0.7% K-1 of the Nb5N6 thin film. According to our estimations, the best attainable electrical responsivity of the bolometer is about -400 V/W at a current bias of 0.4 mA. The electrical noise equivalent power (NEP) is 6.9x10-11 W/Hz1/2 for a modulation frequency at 300 Hz and 9.8x10-12 W/Hz1/2 for a modulation frequency above 10 kHz respectively, which are better than those of commercial products (such as Golay cell and Schottky diode detectors). A quasi-optical receiver based on such a bolometer is constructed and measured.