Yusuke Hashimoto

Ultrafast magneto-optics in ferromagnetic III–V semiconductors

We investigate various ultrafast optical processes in ferromagnetic (III,Mn)V semiconductors induced by femtosecond laser pulses. Two-colour timeresolved magneto-optical spectroscopy has been developed, which allows us to observe a rich array of dynamical phenomena. We isolate several distinct temporal regimes in spin dynamics, interpreting the fast (< 1p s) dynamics as spin heating through sp–d exchange interaction between photo-carriers and Mn ions while the ∼100 ps component is interpreted as a manifestation of spin– lattice relaxation. Charge carrier and phonon dynamics were also carefully studied, showing an ultrashort charge lifetime of photo-injected electrons (∼ 2p s) and propagating coherent acoustic phonon wavepackets with a strongly probe energy dependent oscillation period, amplitude and damping. (Some figures in this article are in colour only in the electronic version)

Propagating coherent acoustic phonon wave packets in In x Mn 1 − x As ∕ Ga Sb

We observe pronounced oscillations in the differential reflectivity of a ferromagnetic

Anisotropic Decay Dynamics of Photoexcited Aligned Carbon Nanotube Bundles

\mathrm{In}\mathrm{Mn}\mathrm{As}∕\mathrm{Ga}\mathrm{Sb}

Two pulse control of magnetization precession in ferrimagnetic GdFe films under low-power excitation

heterostructure using two-color pump-probe spectroscopy. Although originally thought to be associated with the ferromagnetism, our studies show that the oscillations instead result from changes in the position and frequency-dependent dielectric function due to the generation of coherent acoustic phonons in the ferromagnetic InMnAs layer and their subsequent propagation into the GaSb. Our theory accurately predicts the experimentally measured oscillation period and decay time as a function of probe wavelength.

Long-lived dilute photocarriers in individually-suspended single-walled carbon nanotubes

change exhibited a slow (1.0-1.5 ps) component. We attribute this slow dynamics to the relaxation of surface plasmons excited in the perpendicular direction. The VA-SWNT film used in this study was catalytically synthesized on both sides of a 0.5-mm-thick optically-polished fused quartz substrate [15, 16, 17], using the alcohol chemical vapor deposition method [18, 19]. The characteristics of this type of VA-SWNT films have been described in great detail in a previous publication [17]. The average nanotube diameter, determined by high-resolution transmission electron microscopy, was �1.9 nm, and most of SWNTs in the sample were bundles whose diameters ranged from 5 to 15 nm. The thickness of the sample investigated in the present work was approximately 1 � m in total. Linear optical properties of this sample, including its anisotropic absorption properties, have been fully characterized previously [20]. Polarization-dependent pump-probe experiments were performed using �150-fs pulses from a mode-locked Ti:Sapphire laser operating at 80 MHz. The wavelength of both pump and probe beams was 800 nm (or 1.55 eV), which corresponded to the excitation energy of the first metallic subband of the sample [20]. The pump-probe intensity ratio was �10:1, and the signal-to-noise ratio

Experimental Investigation of Controlling Light Polarization in Optical Waveguides with Magneto-optical Materials GdFe

We demonstrated two pulse control of magnetization precession of GdFe films using low-power pulse of 18 μJ/cm2. Our result suggests that the accurate control of magnetization precession should be achieved only by the low-power excitation.

Wave number modulation of spin wave via magneto-elastic coupling at the magnetic body interface

We have observed slow (> 1 ns) carrier decays in single-walled carbon nanotubes. This previously-unreported signal appears in a dilute limit and is accompanied by polarization memory, which persists as long as the photocarriers exist