Optical tuning and ultrafast dynamics of high-temperature superconducting terahertz metamaterials

TL;DR

This paper demonstrates ultrafast terahertz resonance tuning in high-temperature superconducting metamaterials by photoexcitation, revealing significant frequency and resonance strength modulation through changes in complex conductivity.

Contribution

It introduces a method for ultrafast resonance tuning in superconducting metamaterials using near-infrared laser pulses, with a detailed analytical model explaining the underlying mechanisms.

Findings

Resonance can be switched and tuned over a wide frequency range.

The tuning depends on the film thickness and photoexcitation fluence.

The resistance increase reduces resonance strength, while kinetic inductance affects frequency tuning.

Abstract

Through the integration of semiconductors or complex oxides into metal resonators, tunable metamaterials have been achieved by a change of environment using an external stimulus. Metals provide high conductivity to realize a strong resonant response in metamaterials; however, they contribute very little to the tunability. The complex conductivity in high-temperature superconducting films is highly sensitive to external perturbations, which provides new opportunities in achieving tunable metamaterials resulting directly from the resonant elements. Here we demonstrate ultrafast dynamical tuning of resonance in the terahertz (THz) frequency range in YBa_2Cu_3O_7-\delta (YBCO) split-ring resonator arrays excited by near-infrared femtosecond laser pulses. The photoexcitation breaks the superconducting Cooper pairs to create the quasiparticle state. This dramatically modifies the imaginary part of the complex conductivity and consequently the metamaterial resonance in an ultrafast timescale. We observed resonance switching accompanied with a wide range frequency tuning as a function of photoexcitation fluence, which also strongly depend on the nano-scale thickness of the superconducting films. All of our experimental results are well reproduced through calculations using an analytical model, which takes into account the SRR resistance and kinetic inductance contributed from the complex conductivity of YBCO films. The theoretical calculations reveal that the increasing SRR resistance upon increasing photoexcitation fluence is responsible for the reduction of resonance strength, and both the resistance and kinetic inductance contribute to the tuning of resonance frequency.