Caihong Zhang

Plasmon-induced transparency in metamaterials: Active near field coupling between bright superconducting and dark metallic mode resonators

Structured plasmonic metamaterial devices offer the design flexibility to be size scaled for operation across the electromagnetic spectrum and are extremely attractive for generating electromagnetically induced transparency and slow-light behaviors via coupling of bright and dark subwavelength resonators. Here, we experimentally demonstrate a thermally active superconductor-metal coupled resonator based hybrid terahertz metamaterial on a sapphire substrate that shows tunable transparency and slow light behavior as the metamaterial chip is cooled below the high-temperature superconducting phase transition temperature. This hybrid metamaterial opens up the avenues for designing micro-sized active circuitry with switching, modulation, and “slowing down terahertz light” capabilities.

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.

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.

Terahertz nonlinear superconducting metamaterials

We investigate the nonlinear effect of a planar superconducting metamaterial made from niobium nitride (NbN) at terahertz frequencies. As the variation of the incident intense terahertz field alters the intrinsic conductivity in the NbN, a consequent giant amplitude modulation is observed due to the strong nonlinearities. The high sensitivity of the chosen metamaterial even allows observing the nonlinear behaviors at various temperatures, but the resonance modulation induced by the nonlinear effect was distinct from that induced by the heating effect. The presented results illustrate a clever implementation of strongly enhanced nonlinearities and thus may bring nonlinear metamaterials into novel applications.

Simultaneously efficient blue and red light generations in a periodically poled LiTaO3

Generations of efficient blue light at 447 nm and red light at 671 nm were achieved by frequency doubling and tripling of a diode-pumped, Q-switched 1342 nm Nd:YVO4 laser with a periodically poled LiTaO3 (PPLT). The blue light at 447 nm was generated by sum-frequency mixing of the fundamental at 1342 nm with the generated second harmonic at 671 nm. The first-order and third-order reciprocals of the PPLT compensated the phase mismatches of second-harmonic and sum-frequency processes, respectively, making them quasiphase matched. The resulting averaged blue light power of 51 mW and red light power of 207 mW under the averaged fundamental power of 500 mW indicate that the PPLT may be used to construct an all-solid-state blue and red dual wavelength laser.

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.

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.

Study on terahertz emission and optical/terahertz pulse responses with superconductors

Recent progress in terahertz technology has enabled precise investigation of the ultrafast dynamics of excited carriers, nonequilibrium state and nonlinear response of superconductors, resulting in the proposal of novel optoelectronic device applications based on such ultrafast perturbation of supercarriers in the terahertz frequency region. In this paper, we focus on exploratory research in the field of superconductor terahertz science and technology, and present a review of superconducting terahertz sources and the response of superconductors excited by ultrashort electromagnetic pulses, including optical pulses and high-intensity THz pulses.

Bandwidth tunable THz wave generation in large-area periodically poled lithium niobate

A new scheme of optical rectification (OR) of femtosecond laser pulses in a periodically poled lithium niobate (PPLN) crystal, which generates high energy and bandwidth tunable multicycle THz pulses, is proposed and demonstrated. We show that the number of the oscillation cycles of the THz electric field and therefore bandwidth of generated THz spectrum can easily and smoothly be tuned from a few tens of GHz to a few THz by changing the pump optical spot size on PPLN crystal. The minimal bandwidth is 17 GHz that is smallest ever of reported in scheme of THz generation by OR at room temperature. Similar to the case of Cherenkov-type OR in single-domain LiNbO₃, the spectrum of THz generation extends from 0.1 THz to 3 THz when laser beam is focused to a size close to half-period of PPLN structure. The energy spectral density of narrowband THz generation is almost independent of the bandwidth and is typically 220 nJ/THz for ~1 W pump power at 1 kHz repetition rate.

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.