Atsushi Syouji

Superfocusing modes of surface plasmon polaritons in conical geometry based on the quasi-separation of variables approach

Analytic solutions to the superfocusing modes of surface plasmon polaritons in a conical geometry are theoretically studied using an ingenious method called the quasi-separation of variables. This method can be used to look for fundamental solutions to the wave equation for a field that must satisfy boundary conditions at all points on the continuous surface of tapered geometries. The set of differential equations exclusively separated from the wave equation can be consistently solved in combination with perturbation methods. This paper presents the zeroth-order perturbation solution of conical superfocusing modes with azimuthal symmetry and graphically represents them in electric field-line patterns.

Intense terahertz radiation from longitudinal optical phonons in GaAs∕AlAs multiple quantum wells

We report the generation of intense terahertz (THz) electromagnetic waves from GaAs∕AlAs multiple quantum wells (MQWs) using ultrashort laser pulses. Pronounced THz waves from the coherent GaAs-like longitudinal optical (LO) phonons with a frequency of 8.8 THz were observed in the time region over 6 ps, which indicates the long dephasing time. The power of the THz waves from the coherent GaAs-like LO phonons in the GaAs∕AlAs MQW was enhanced to be about 100 times larger than that of those from coherent LO phonons in an epitaxial-growth GaAs film. We discuss the origin of the enhancement of the THz wave from coherent GaAs-like LO phonon in the MQW.

Evaluation of a terahertz wave spectrum and construction of a terahertz wave-sensing system using a Yb-doped fiber laser

We experimentally evaluated a cadmium-telluride (CdTe) crystal as the nonlinear crystal for generation and detection of terahertz (THz) radiation by using a femtosecond-pulse laser. Numerical simulations based on the nonlinear Maxwell equations, including third-order optical dispersion within the spectral width of an optical pulse and absorption in the THz frequency region, well reproduce both the amplitude and phase spectra of the emitted THz wave. Excitation wavelength dependences of the THz radiation from the CdTe crystal indicate that the phase-matched condition is satisfied at a wavelength of 1000 nm. We also experimentally demonstrated a THz generation and detection system using a 1045 nm Yb-doped ultrashort-pulse fiber laser, and we verified that CdTe is one of the most suitable nonlinear crystals for a Yb-doped fiber laser.

Characterization of terahertz electromagnetic waves from coherent longitudinal optical phonons in GaAs/AlAs multiple quantum wells

The characteristics of terahertz (THz) electromagnetic waves emitted from coherent GaAs-like longitudinal optical (LO) phonons in GaAs/AlAs multiple quantum wells were investigated. It was found that the amplitude of the THz wave from the coherent LO phonon is increased linearly with increasing number of periods of the quantum well. This result demonstrates that the THz wave originates from the superposition of the oscillatory polarization induced by the coherent GaAs-like LO phonon in each quantum-well layer. The amplitude of the THz wave is proportional to the excitation-power density below about 0.4 μJ∕cm2, and its radiation power reaches about 1 μW at the excitation-power density of 2 μJ∕cm2. Moreover, the amplitude of the THz wave shows a sinusoidal change with the polarization angle of the pump pulse. These results indicate that the coherent LO phonon emitting the THz wave is generated through the combination of the following two mechanisms: impulsive stimulated Raman scattering and instantaneous sc...

THz Wave Generation and Detection System using ∼1000 nm Yb-doped Fiber Laser

The THz generation and detection system using ∼1000 nm Yb-doped ultrashort pulse fiber laser has been constructed. We compared three binary zinc blende semiconductors CdTe, GaAs, and ZnTe as the nonlinear optical crystals for THz radiation. The phase matching condition of THz generation is satisfied in CdTe at the pumping wavelength of ∼1000 nm. At the condition, the intensity of THz wave becomes very high and its spectrum spreads widely. This behavior agrees with our simulation based on phase-matching condition.

THz radiation from CdTe crystal by differential frequency generation under phase-matching conditions at 1000nm system

THz radiation from II-VI semiconductor, <110>CdTe has observed under NIR laser pulse. According to the frequency dependence of radiation intensity, ~1000nm wavelength is suitable for effective THz generation and detection. Introduction In resent years, progress of ultrashort pulse laser technique bring the development of the coherent terahertz (THz) wave detection based on a free-space Ti:Sapphire laser system. For further enhancement of the THz application, it is necessary to combine with compact, stable, and low-cost solid lasers. Especially Yb-doped fiber laser (~1045nm) is attractive light source in the point of high conversion efficiency of excitation energy. A non-resonant electro-optical (EO) process is familiar method for THz generation with optical rectification because the optimization of nonlinear crystal is well established even in the long wavelength region. It is shown that ZnTe is most suitable EO crystal using 800nm laser system [1] because coherent length in ZnTe enhanced at the wavelength. A longer than 800nm excitation wavelength region, the optical rectification of other binary zinc-blende semiconductors, CdTe, InP, and GaAs were measured [2] and response of GaP as an EO field sensor was measured [3]. On the contrary, it is suggests that <110>CdTe and <110>GaP are the most suitable for EO crystal in the binary zinc-blende semiconductors for emitter and detector because the coherent length of them are enhanced at excitation wavelength of ~1000nm [4]. In this report, we compare terahertz electric wave generated by <110> CdTe crystal using several laser pulse wavelength for the first time. Experimental Setup The light source is Ti:S base regenerative amplifier (regen) laser (800nm wavelength, 200kHz repetition rate, 50 fs pulse duration, and 900 mW output power). Output beam from regen laser is divided into two beams for pumping optical parametric amplifier (OPA) laser and sampling terahertz waveform. Excitation pulse with the power of fixed 5mW from OPA laser is focused on the 1mm-thick <110> CdTe crystal as an emitter. The pulse duration of OPA system is changed as wavelength. The shorter we change the wavelength of OPA, the longer the auto correlation duration becomes from 90fs to 120fs. In order to collect and focus the emitted terahertz pulse, two off-axis parabolic mirrors are used. The waveform is measured by EO detection method, using with 1mm-thick <110> ZnTe crystal. The sampling beam is also focused onto the same spot of the EO crystal and the intensity of terahertz field was measured as birefringence of the sampling beam modulated by the electric field using a quarter waveplate, a Wolston prism, and two balanced photo-diode detectors. A 343Hz optical chopping and lock-in detection were used in order to improve the sensitivity. Experimental Results In the nonlinear process, longer interaction length and higher nonlinear coefficient are important parameter to get higher efficiency. We estimate coherent length from a phase-matching condition. Considering the dispersion at an optical frequency, phase matching condition is satisfied when the phase of the THz wave travels at the group velocity of the optical pulse. Corresponding coherent length is described by where nTHz is THz refractive index and ng is the group index of the optical beam. Figure 1 shows the generated THz wave intensity from 1mm thickness CdTe excited several wavelength. The intensity is integrated from 0 to 4THz of fourier amplitude spectrum calculated from the waveform. Here, the waveform are detected by an EO crystal of ZnTe and 800nm sampling pulse. The peak position of this figure is located around 1000nm and the peak is very high intensity. JThC3-P3

Intense terahertz radiation from coherent optical phonons confined in a GaAs/AlAs multiple quantum well

Coherent phonons in semiconductor superlattices and multiple quantum wells (MQW’s) have been extensively studied. However, to our knowledge, the terahertz (THz) radiation from the coherent phonons in MQW’s has not been reported. In the present work, we report on the intense THz radiation from the coherent GaAs-like longitudinal optical (LO) phonon confined in the GaAs-well layer of a GaAs/AlAs MQW. It is found that the THz wave from the coherent LO phonon in the MQW has a considerably large amplitude and a long dephasing time, compared with that in an epitaxial-growth GaAs film.

Phase-Matching Condition of Terahertz Generation of CdTe with Near-Infrared Pulse Excitation

The enhancement and spectral change of terahertz wave generation were observed under the phase-matching condition of 1mm-thick 110 CdTe wafer and NIR laser pulse excitation. For terahertz generation and tuning, the phase-matching condition is important.