Exploring dynamics in the far-infrared with terahertz spectroscopy.

Recent development of the terahertz time domain spectroscopy (THz-TDS) and its application to solids have been reviewed. This spectroscopy is unique in that the time-domain wave forms are measured at first and the complex optical constants are deduced directly by the Fourier transformation of them without resort to the Kramers-Kronig analysis. Various types of the THz-TDS systems are briefly described. Applications of the THz-TDS to various solids, i.e., semiconductors, superconductors, polymers, photonic crystals, and so on are also presented to demonstrate how widely this spectroscopy is applicable to characterization of solids.

Terahertz Time-Domain Spectroscopy of Solids: A Review

The mechanism of Cooper pair formation and its underlying physics has long occupied the investigation into high temperature (high-Tc ) cuprate superconductors. One of the ways to unravel this is to observe the ultrafast response present in the charge carrier dynamics of a photoexcited specimen. This results in an interesting approach to exploit the dissipation-less dynamic features of superconductors to be utilized for designing high-performance active subwavelength photonic devices with extremely low-loss operation. Here, dual-channel, ultrafast, all-optical switching and modulation between the resistive and the superconducting quantum mechanical phase is experimentally demonstrated. The ultrafast phase switching is demonstrated via modulation of sharp Fano resonance of a high-Tc yttrium barium copper oxide (YBCO) superconducting metamaterial device. Upon photoexcitation by femtosecond light pulses, the ultrasensitive cuprate superconductor undergoes dual dissociation-relaxation dynamics, with restoration of superconductivity within a cycle, and thereby establishes the existence of dual switching windows within a timescale of 80 ps. Pathways are explored to engineer the secondary dissociation channel which provides unprecedented control over the switching speed. Most importantly, the results envision new ways to accomplish low-loss, ultrafast, and ultrasensitive dual-channel switching applications that are inaccessible through conventional metallic and dielectric based metamaterials.

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A Superconducting Dual‐Channel Photonic Switch

We have developed a supercurrent distribution imaging system for high Tc superconductive thin films and demonstrated the visualization of the supercurrent distribution in the vortex-penetrated YBa2Cu3O7−δ thin film strips. The terahertz (THz) radiation and detection system with a scanning femtosecond laser was employed to visualize the distribution. The imaging system utilizes the principle that the femtosecond optical pulses excite THz radiation into the free space by optical supercurrent modulation, and the radiation amplitude is proportional to the local supercurrent density at the optically excited area. Prior to the observation of the supercurrent distribution, we studied optical excitation effects on the vortices trapped in the strips, calibration of the current density from the THz radiation amplitude, temperature dependence of the THz radiation properties, etc. The laser power dependence of the THz radiation in the remanent state revealed that the excitation with powers larger than the relatively ...

Terahertz radiation imaging of supercurrent distribution in vortex-penetrated YBa2Cu3O7−δ thin film strips

The design and optimization of very broadband integrated lens antennas (ILAs) constitutes one of the future trends in lens antenna field. To this end we investigate numerically the radiation performance of millimeter wave ILAs coated with multiple anti reflection layers. We propose lens structures of moderate size (four wavelengths in diameter at the center frequency) and made from a dense dielectric material (ceramic). They are illuminated by two kinds of on-axis primary sources, namely a dielectric-loaded metallic waveguide and a patch antenna. This enables to assess the role of the lens illumination law on the performance of broadband ILAs. In particular, we demonstrate that ILAs coated with three stacked quarter wavelength matching layers exhibit a very broadband promising features. First their radiation characteristics remain very stable over a large frequency band: a 36% relative bandwidth is achieved using dielectric-loaded waveguide feeds. Secondly very high values of aperture efficiencies (beyond 91% over a 21% bandwidth) are obtained using printed feeds. The truncation effects of the ground plane and substrate of planar feeds upon the beam characteristics are also studied. We conclude that they must be taken into account at the very first stages of the design process of ILAs.

Very Broadband Extended Hemispherical Lenses: Role of Matching Layers for Bandwidth Enlargement

We demonstrate power enhancement of terahertz (THz) radiation from YBCO thin films. An ultra-wide-band electromagnetic pulse, which contains frequency components over 3 THz, was excited by ultrafast supercurrent modulation with femtosecond optical pulse irradiation. The maximum THz radiation power emitted into free space was achieved up to 0.5 /spl mu/W with the combination of a bow-tie antenna and an MgO hyper-hemispherical lens. We discuss the radiation power dependence on temperature, excitation power, and bias supercurrent.

Enhanced THz radiation from YBCO thin film bow-tie antennas with hyper-hemispherical MgO lens

A superconducting optical flux-trap memory with a femtosecond pulsed laser is proposed and then demonstrated. The magnetic flux is trapped in a simple YBa2Cu3O7−δ thin film loop with a polarity controlled by means of optical supercurrent modulation. After removing the external bias current, the trapped fluxes are detected by observing electromagnetic pulse radiation 60 cm away from the sample, which is excited by modulating a persistent current that circulates in the loop. The mechanism for the readin of the magnetic fluxes is discussed in relation to the current distribution in the memory.

Control of magnetic flux in a YBa2Cu3O7−δ thin film loop using femtosecond laser pulses

We discovered a new type of terahertz (THz) radiation from YBa2Cu3O7-δ thin films. Ultrashort electromagnetic pulses, which are referred to as THz radiation, were emitted into free space by irradiating the superconductive films with femtosecond laser pulses. The THz radiation, with a spectrum extending from 0.1 to 3 THz, was excited in both the films applied with a magnetic field and the flux-trapped films. The results suggest that the screening current for the external magnetic field and circulating current around the trapped flux play an important role in the emission mechanism.

Novel Terahertz Radiation from Flux-Trapped YBa$_{\bf 2}$Cu$_{\bf 3}$O$_{\bf 7-}$δ Thin Films Excited by Femtosecond Laser Pulses

We observed the temperature dependence of a femtosecond time-transient photoresponse in YBa2Cu3O7-δ thin films by detecting terahertz (THz) emission. Ultrashort electromagnetic pulses which are referred to as THz emission were generated by ultrafast supercurrent modulation with femtosecond laser pulse irradiation. The transient response is discussed in relation to the quasiparticle dynamics of the nonequilibrium superconductor.

https://iopscience.iop.org/article/10.1143/JJAP.35.L1578/pdf

Terahertz Emission Study of Femtosecond Time-Transient Nonequilibrium State in Optically Excited YBa$_{\bf 2}$Cu$_{\bf 3}$O$_{{\bf 7}-\ninmbi{\delta}}$ Thin Films

We discovered a new type of terahertz (THz) radiation from YBa 2 Cu 3 O 7−δ (YBCO) thin films. Ultrashort electromagnetic pulses with their spectra extending from 0.1 to 3 THz were excited by irradiating magnetic-flux-trapped YBCO thin films with femtosecond laser pulses. The emission mechanism is discussed in relation to the trapped flux.

N. Wada

Papers

Enhanced THz radiation from YBCO thin film bow-tie antennas with hyper-hemispherical MgO lens

Control of magnetic flux in a YBa2Cu3O7−δ thin film loop using femtosecond laser pulses

Novel Terahertz Radiation from Flux-Trapped YBa$_{\bf 2}$Cu$_{\bf 3}$O$_{\bf 7-}$δ Thin Films Excited by Femtosecond Laser Pulses

Terahertz Emission Study of Femtosecond Time-Transient Nonequilibrium State in Optically Excited YBa$_{\bf 2}$Cu$_{\bf 3}$O$_{{\bf 7}-\ninmbi{\delta}}$ Thin Films

Terahertz emission properties from flux-trapped YBCO thin films