arXiv subjects

Masahiko Tani

Publications and source records attributed to Masahiko Tani.

4 recordsLinked to original sources

arXiv2023

Terahertz emission from transient currents and coherent phonons in layered MoSe$_2$ and WSe$_2$

Terahertz (THz) time-domain emission spectroscopy was performed on layered 2H-MoSe2 and 2H-WSe2. The THz emission shows an initial cycle attributed to surge currents and is followed by oscillations attributed to coherent interlayer phonon modes. To obtain the frequencies of the interlayer vibrations, analysis of the THz emission waveforms were performed, separating the two contributions to the total waveform. Results of the fitting show several vibrational modes in the range of 5.87 to 32.75 cm-1 for the samples, attributed to infrared-active interlayer shear and breathing modes. This study demonstrates that THz emission spectroscopy provides a means of observing these low frequency vibrational modes in layered materials.

cond-mat.mtrl-sci↗

arXiv2021

Effect of heteroepitaxial growth on LT-GaAs: ultrafast optical properties

Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates (reference). In this study, we investigate optical properties of an epitaxial grown LT-GaAs/Si sample, compared to a reference grown under the same substrate temperature, and with the same layer thickness. Anti-phase domains and some crystal misorientation are present in the LT-GaAs/Si. From coherent phonon spectroscopy, the intrinsic carrier densities are estimated to be ~$10^{15}$ cm$^{-3}$ for either sample. Strong plasmon damping is also observed. Carrier dynamics, measured by time-resolved THz spectroscopy at high excitation fluence, reveals markedly different responses between samples. Below saturation, both samples exhibit the desired fast response. Under optical fluences $\geq$ 54 $μ$ J/cm$^2$, the reference LT-GaAs layer shows saturation of electron trapping states leading to non-exponential behavior, but the LT-GaAs/Si maintains a double exponential decay. The difference is attributed to the formation of As-As and Ga-Ga bonds during the heteroepitaxial growth of LT-GaAs/Si, effectively leading to a much lower density of As-related electron traps.

cond-mat.mtrl-sci↗

arXiv2018

Efficient Terahertz Generation Using Fe/Pt Spintronic Emitters Pumped at Different Wavelengths

Recent studies in spintronics have highlighted ultrathin magnetic metallic multilayers as a novel and very promising class of broadband terahertz radiation sources. Such spintronic multilayers consist of ferromagnetic (FM) and non-magnetic (NM) thin films. When triggered by ultrafast laser pulses, they generate pulsed THz radiation due to the inverse spin-Hall effect, a mechanism that converts optically driven spin currents from the magnetized FM layer into transient transverse charge currents in the NM layer, resulting in THz emission. As THz emitters, FM/NM multilayers have been intensively investigated so far only at 800-nm excitation wavelength using femtosecond Ti:sapphire lasers. In this work, we demonstrate that an optimized spintronic bilayer structure of 2-nm Fe and 3-nm Pt grown on 500 μm MgO substrate is just as effective as a THz radiation source when excited either at λ = 800 nm or at λ = 1550 nm by ultrafast laser pulses from a fs fiber laser (pulse width close to 100 fs, repetition rate around 100 MHz). Even with low incident power levels, the Fe/Pt spintronic emitter exhibits efficient generation of THz radiation at both excitation wavelengths. The efficient THz emitter operation at 1550 nm facilitates the integration of such spintronic emitters in THz systems driven by relatively low cost and compact fs fiber lasers without the need for frequency conversion.

physics.app-ph↗

arXiv2003

Attempt to Generate Narrow Linewidth, CW Terahertz Radiation by Using Optical Frequency Comb

We report on an attempt to generate highly stable continuous terahertz (THz) wave by using optical frequency comb (OFC). About 10-nm wide OFC has been generated through a deep phase modulation of a 852 nm laser line in lithium niobate crystal cavity. The multiple optical modes (side bands) of the OFC, which are equally separated from each other by the modulation frequency (=6 GHz) are taken as the frequency reference. When another semiconductor laser is frequency locked, the stability of the difference frequency between the master laser and the second laser is improved on the same order of the RF modulator. An ultra-narrow line and tunable THz radiation source can be achieved by photomixing of this stable difference-frequency optical beat in a photoconductive antenna.

physics.optics↗