- [Show abstract] [Hide abstract] ABSTRACT: The dynamics of photoexcited electrons in various excited d-states was investigated in a transition metal oxide MnO by tunable optical pump-terahertz probe measurements. Photoexcited electrons in the lowest excited d-state showed the longest relaxation time among the three excited d-states that are accessible in MnO at room temperature. The relaxation rate in the lowest excited d-state showed a drastic increase below the Neel temperature TN = 120 K in MnO. We conclude that this increase is caused by the appearance of a decay channel related to magnetic-excitation-assisted photoluminescence from self-trapped exciton(STE) states. The opening of relaxation channels to the STE states in an antiferromagnetic phase suggests that it may be possible to control photocarrier lifetime by magnetic order in transition metal oxides.
- [Show abstract] [Hide abstract] ABSTRACT: We examined the photoluminescence (PL) in single-crystal MnO(111) and observed a new strong emission band with a high-energy-side edge of 1.905 eV below 20 K. The emission bands with high-energy-side edges of 1.875 eV and 1.815 eV were also observed between 20 K and 30 K, and above 40 K, respectively. The temperature dependence of the intensity of each PL band showed step-like decreases at different critical temperatures, which suggests that the emission spectrum consist of contributions from three independent excited states. The fine structures of emission sidebands in the PL spectrum were interpreted in terms of phonons and magnons coupled with three types of self-trapped exciton (STE) states. Based on the Stokes shifts observed in our experiments and the thermal activation energies estimated from the temperature dependence of the PL intensity, we propose a configuration coordinate diagram for STE states and explain the mechanism of radiative and nonradiative relaxations in the magnetic-excitation-assisted PL process from the three types of STE states.
- [Show abstract] [Hide abstract] ABSTRACT: Optical pump-THz probe (OPTP) measurements allow us to investigate the dynamic response of photoconductivity in materials by observing the transient change of THz transmittance after optical excitation. In this study, the THz response of d -electrons in various photoexcited states in MnO was investigated by pump-energy-tunable OPTP measurements to elucidate the photoexcited d -electron dynamics in a transition metal oxide. At room temperature, photoexcited d -electrons induced by a d -d transition to the lowest excited d -state showed the longest relaxation time. The relaxation time decreased drastically below the antiferromagnetic transition temperature of 120 K. This decrease was accompanied by the emergence of magnetic-excitation-assisted photoluminescence from a self-trapped exciton (STE) state. This suggests that photoexcited d -electrons relax to the STE state below the transition temperature. These findings shed new light on the photoexcited d -electron dynamics, contrasting with the behavior of photoexcited carriers generated in the upper excited d -states that have been shown in a previous study to relax into trap states through carrier scattering.
- [Show abstract] [Hide abstract] ABSTRACT: Terahertz pulses trapped as surface waves on a wire waveguide can be flexibly transmitted and focused to sub-wavelength dimensions by using, for example, a tapered tip. This is particularly useful for applications that require high-field pulses. However, the generation of strong terahertz surface waves on a wire waveguide remains a challenge. Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10(18) W/cm(2) is characterized by femtosecond electron deflectometry. From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave. The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.
- [Show abstract] [Hide abstract] ABSTRACT: We review recent progress in polarization-sensitive time-domain spectroscopy at THz frequencies (THz-TDS). Developments in spectroscopic polarimetry and ellipsometry and various polarization components targeting THz frequencies are introduced. Polarization-sensitive measurements are suitable for studying the anisotropic responses of materials induced by factors such as structure, stress, and magnetic fields. Examples of observations of anisotropic characteristics of materials such as birefringence, optical activity, and magneto-optic effects are also presented.
- [Show abstract] [Hide abstract] ABSTRACT: Generation of a wide-range and stable THz waves from a photoconductive antenna excited by a multimode semiconductor chaotic oscillation laser with an optical delayed feedback using an external mirror is investigated. A high bias voltage is also used to generate wide range THz wave. The THz wave near to 1 THz is obtained using this system.
- [Show abstract] [Hide abstract] ABSTRACT: Terahertz (THz) radiation from antiferromagnetic (AFM) MnO excited by femtosecond laser pulses has been investigated. The radiated THz pulse consists of a single-cycle broadband pulse and an oscillating component with a frequency of 0.83 THz at 10 K. The oscillations are magnetic dipole radiation from optically excited AFM magnons. We found that the peak amplitude of the broadband pulse from MnO increases as the temperature decreases and is significantly enhanced below AFM order temperature. The origin of the broadband pulse radiation observed below AFM order temperature can be attributed to second-order nonlinear optical effects depending on the AFM order.
- [Show abstract] [Hide abstract] ABSTRACT: Upon driving ultrafast electron current on a metal wire by irradiating a femtosecond laser pulse with an intensity of 1018 W/cm2, we obtained an intense half-cycle THz surface wave with electric field strength exceeding 1 MV/cm.
- [Show abstract] [Hide abstract] ABSTRACT: Generation of a wide-range and stable THz waves from a photoconductive antenna excited by a multimode semiconductor chaotic oscillation laser with an optical delayed feedback using an external mirror is investigated. A high bias voltage is also used to generate wide range THz wave. Near to 1THz THz wave is obtained using this system.
- [Show abstract] [Hide abstract] ABSTRACT: Terahertz (THz) radiation from atomic clusters illuminated by intense femtosecond laser pulses is investigated. By studying the angular distribution, polarization properties and energy dependence of THz waves, we aim to obtain a proper understanding of the mechanism of THz generation. The properties of THz waves measured in this study differ from those predicted by previously proposed mechanisms. To interpret these properties qualitatively, we propose that the radiation is generated by time-varying quadrupoles, which are produced by the ponderomotive force of the laser pulse.
- [Show abstract] [Hide abstract] ABSTRACT: Generation of a wide-range THz wave from a photoconductive antenna excited by a multimode semiconductor chaotic oscillation laser with an optical delayed feedback using an external mirror is investigated. An optical fiber is also used for easy alignment. The properties of the generated THz wave are compared with those of generated THz wave by using a CW steady state laser.
- [Show abstract] [Hide abstract] ABSTRACT: We performed optical-pump terahertz-probe measurements on antiferromagnetic MnO. The transient decrease in transmission at the peak of THz single-cycle pulses suggests that photoexcited carriers are produced in MnO. Below the Neel temperature, magnon scattering mainly contributes the relaxation of photoexcited carriers.
- [Show abstract] [Hide abstract] ABSTRACT: We investigated terahertz (THz) radiation from twinned NiO(110) single crystals excited by optical laser pulses with various polarization states. We found that the polarity of THz radiation from optically excited coherent antiferromagnetic (AFM) magnons can be controlled by switching the helicity of circular polarization and by rotating the direction of linear polarization of the laser pulses. The dependence of THz radiation on the polarization of the laser pulses suggests that AFM magnons are excited by the inverse Faraday effect in a twinned crystal with linear magnetic birefringence.
- [Show abstract] [Hide abstract] ABSTRACT: Terahertz (THz) radiation from air plasma produced by linearly polarized intense femtosecond laser pulses was investigated. The laser energy dependence, directionality, and polarization properties of THz waves, measured in the present experiment, differed from those in previous reports and can be explained by parametric decay of laser light to R-waves in the presence of a spontaneous magnetic field.
- [Show abstract] [Hide abstract] ABSTRACT: The generation of a stable and wide range THz wave using a chaotic oscillation in a multimode semiconductor laser with an optical delayed feedback by the external mirror is investigated. fiber coupler is also used for easy optical alignment. In this paper, the spectrum of laser chaos is compared with that of the CW steady state oscillation. THz spectrum is limited to 35.2GHz and a THz signal (a current of photoconductive antenna) is not stable using a CW steady state laser. Wide range spectrum of THz wave above 35.2GHz is obtained and a THz signal is also stronger and stable in the case of using a chaotic laser compared with that of using a CW steady state laser.
- [Show abstract] [Hide abstract] ABSTRACT: Terahertz (THz) emission from argon cluster plasma, generated by intense femtosecond laser pulses in the energy range of 10-70 mJ, has been investigated. THz polarization, energy dependence, and angular distribution were measured to provide an initial discussion on the mechanisms of THz emission. THz pulses of much higher energy were generated from argon clusters than from argon gas, which indicates that plasma produced from atomic clusters holds considerable promise as an intense THz source. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3672814]
- [Show abstract] [Hide abstract] ABSTRACT: We present a frequency-domain electron spin resonance (ESR) measurement system using terahertz time-domain spectroscopy. A crossed polarizer technique is utilized to increase the sensitivity in detecting weak ESR signals of paramagnets caused by magnetic dipole transitions between magnetic sublevels. We demonstrate the measurements of ESR signal of paramagnetic copper(II) sulfate pentahydrate with uniaxial anisotropy of the g-factor under magnetic fields up to 10 T. The lineshape of the obtained ESR signals agrees well with the theoretical predictions for a powder sample with the uniaxial anisotropy.
- [Show abstract] [Hide abstract] ABSTRACT: We have observed directional elliptically polarized terahertz (THz) waves emitted from air plasma produced by circularly polarized femtosecond laser pulses. The spatial distribution of the THz waves shows that the radiation is strongly directed forward with a peak around the laser propagation direction. Measured THz power shows a square dependence on laser energy. We consider the parametric decay of laser light to R-waves in plasma in the presence of a spontaneous magnetic field as a possible explanation for the polarization, power dependence, and direction of the THz beam.
Kioto, Kyōto, Japan
- Department of Physics II
Suika, Ōsaka, Japan
- • Institute of Laser Engineering
- • Department of Applied Physics