Kinetic inductance and penetration depth of thin superconducting films measured by THz-pulse spectroscopy

Tunable terahertz electromagnetically induced transparency based on a composite structure superconducting metamaterial

We experimentally present a tunable electromagnetically induced transparency (EIT)-like response in bright-bright mode resonators. In contrast to previous studies, we used NbN film and a gold film composite structure metamaterial. A significant slow-light effect could be observed at the transmission window, and the maximum group index could reach 100. As a variation in temperature alters the intrinsic ohmic loss of superconducting NbN film, a temperature-dependent transmittance and slow-light effect were observed. To better illustrate the physical mechanism of the two modes, a hybrid coupling model was introduced to fit the experimental transmission spectra and extract the characteristic parameters of sub-resonators. We found excellent agreement with experimental results. Our results provide deeper insight into the metamaterial analogs of an EIT-like response and offer an alternative approach for engineering slow-light devices, bandpass filters, and switches/modulators at terahertz frequencies.

Electrically tunable terahertz switch based on superconducting subwavelength hole arrays

We experimentally demonstrate an electrically tunable superconducting device capable of switching the extraordinary terahertz (THz) transmission. The planar device consists of subwavelength hole arrays with real-time control capability. The maximum transmission coefficient at 0.33 THz is 0.98 and decreases to 0.17 when the applied voltage only increases to 1.3 V. A relative intensity modulation of 82.7% is observed, making this device an efficient THz switch. Additionally, this device exhibits good narrow-bandpass characteristics within 2 THz, which can be used as a frequency-selective component. This study offers an ideal tuning method and delivers a promising approach for designing active and miniaturized devices in THz cryogenic systems.

The dynamic process and microscopic mechanism of extraordinary terahertz transmission through perforated superconducting films

Superconductor is a compelling plasmonic medium at terahertz frequencies owing to its intrinsic low Ohmic loss and good tuning property. However, the microscopic physics of the interaction between terahertz wave and superconducting plasmonic structures is still unknown. In this paper, we conducted experiments of the enhanced terahertz transmission through a series of superconducting NbN subwavelength hole arrays, and employed microscopic hybrid wave model in theoretical analysis of the role of hybrid waves in the enhanced transmission. The theoretical calculation provided a good match of experimental data. In particular, we obtained the following results. When the width of the holes is far below wavelength, the enhanced transmission is mainly caused by localized resonance around individual holes. On the contrary, when the holes are large, hybrid waves scattered by the array of holes dominate the extraordinary transmission. The surface plasmon polaritions are proved to be launched on the surface of superconducting film and the excitation efficiency increases when the temperature approaches critical temperature and the working frequency goes near energy gap frequency. This work will enrich our knowledge on the microscopic physics of extraordinary optical transmission at terahertz frequencies and contribute to developing terahertz plasmonic devices.

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.

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.

Ballistic acceleration of a supercurrent in a superconductor

One of the most primitive but elusive current-voltage (I-V) responses of a superconductor is when its supercurrent grows steadily after a voltage is first applied. The present work employed a measurement system that could simultaneously track and correlate I(t) and V(t) with subnanosecond timing accuracy, resulting in the first clear time-domain measurement of this transient phase where the quantum system displays a Newtonian like response. The technique opens doors for the controlled investigation of other time-dependent transport phenomena in condensed-matter systems.