Ikufumi Katayama

Ultrabroadband terahertz generation using 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate single crystals

Ultrabroadband terahertz generation up to 200 THz has been demonstrated using a 5 fs Ti:sapphire laser and a 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) crystal. The high-frequency components beyond 100 THz are much stronger than those generated using conventional electro-optic crystals such as GaSe. A simple simulation of the difference frequency generation in the DAST crystal by considering the refractive index dispersion can reproduce the broadband generation of the terahertz wave and its chirp dependence. Because the generated terahertz wave is coherent and has a broad bandwidth, it can serve as a suitable light source for ultrabroadband terahertz time-domain spectroscopy of a material.

Single-shot measurement of a terahertz electric-field waveform using a reflective echelon mirror

Single-shot measurements of terahertz (THz) electric-field waveforms are demonstrated using a reflective echelon mirror, which produces multiple probe pulses with different time-delays. The polarization rotation of the probe pulses, due to the electro-optic effect induced by the THz electric field generated from grating-coupled LiNbO3, was imaged onto a two-dimensional complementary metal-oxide-semiconductor camera. A waveform with a weak peak field strength of 0.6 kV/cm was obtained with a good signal-to-noise ratio, demonstrating precise single-shot detection of the THz electric field waveform.

Ultrafast Dynamics of Surface-Enhanced Raman Scattering Due to Au Nanostructures

Ultrafast dynamics of surface-enhanced Raman scattering (SERS) was investigated at cleaved graphite surfaces bearing deposited gold (Au) nanostructures (∼10 nm in diameter) by using sensitive pump-probe reflectivity spectroscopy with ultrashort (7.5 fs) laser pulses. We observed enhancement of phonon amplitudes (C═C stretching modes) in the femtosecond time domain, considered to be due to the enhanced electromagnetic (EM) field around the Au nanostructures. Finite-difference time-domain (FDTD) calculations confirmed the EM enhancement. The enhancement causes drastic increase of coherent D-mode (40 THz) phonon amplitude and nanostructure-dependent changes in the amplitude and dephasing time of coherent G-mode (47 THz) phonons. This methodology should be suitable to study the basic mechanism of SERS and may also find application in nanofabrication.

Ultrafast crystalline-to-amorphous phase transition in Ge2Sb2Te5 chalcogenide alloy thin film using single-shot imaging spectroscopy

We have observed an irreversible ultrafast crystalline-to-amorphous phase transition in Ge2Sb2Te5 chalcogenide alloy thin film using broadband single-shot imaging spectroscopy. The absorbance change that accompanied the ultrafast amorphization was measured via single-shot detection even for laser fluences above the critical value, where a permanent amorphized mark was formed. The observed rise time to reach the amorphization was found to be ∼130–200 fs, which was in good agreement with the half period of the A1 phonon frequency in the octahedral GeTe6 structure. This result strongly suggests that the ultrafast amorphization can be attributed to the rearrangement of Ge atoms from an octahedral structure to a tetrahedral structure. Finally, based on the dependence of the absorbance change on the laser fluence, the stability of the photoinduced amorphous phase is discussed.

Hardening of the ferroelectric soft mode in SrTiO3 thin films

We have studied the origin of soft-mode hardening in SrTiO3 thin films using broadband terahertz time-domain spectroscopy. We measured the dielectric dispersions in the terahertz region of as-deposited, O2 annealed, and high-temperature annealed SrTiO3 thin films deposited on MgO and La0.3Sr0.7Al0.65Ta0.35O3 substrates. The results show that the ferroelectric soft mode softens dramatically by the high-temperature annealing. We also measure x-ray diffractions and atomic force microscope images and conclude that the hardening of the ferroelectric soft mode in the thin films is determined by the grain size of each crystalline domain which is enlarged by the high-temperature annealing due to remelting.

Single-shot time-frequency imaging spectroscopy using an echelon mirror

We demonstrate single-shot time-frequency imaging spectroscopy with an echelon mirror for measuring ultrashort laser pulses as well as ultrafast responses of materials using the same optical setup. The echelon mirror produces a spatially encoded time delay for the probe pulse whereby both the probe and pump pulses are focused on samples with small spot size. Using the optical Kerr gate apparatus, we successfully mapped the time-frequency images of ultrashort laser pulses and subsequently evaluated the chirp characteristics with the phase-retrieval procedure on a single-shot basis. By simply replacing the Kerr medium with samples, we could also visualize the phonon-polariton oscillations in ferroelectric LiNbO3.

Surface metallic states in ultrathin Bi(001) films studied with terahertz time-domain spectroscopy

Dynamical response of surface metallic states in single crystalline ultrathin Bi(001) films on Si(111) 7 × 7 surface was investigated at a spectral range of 0.1–12 THz by broadband terahertz time-domain spectroscopy. The observed transmittance increased with a decrease in the thickness, without showing a gap structure. The measured complex dielectric dispersion was analyzed using a Drude model, and the plasma frequency (ωp) and damping constant (γ) were found to be inversely proportional to the thickness. The results strongly indicate the existence of surface metallic states, whose carrier density and damping constant are estimated to be 3.08 × 1019 cm−3 and 4.83 × 102 THz, respectively.

Ultrafast time-resolved electron diffraction revealing the nonthermal dynamics of near-UV photoexcitation-induced amorphization in Ge2Sb2Te5.

Because of their robust switching capability, chalcogenide glass materials have been used for a wide range of applications, including optical storages devices. These phase transitions are achieved by laser irradiation via thermal processes. Recent studies have suggested the potential of nonthermal phase transitions in the chalcogenide glass material Ge2Sb2Te5 triggered by ultrashort optical pulses; however, a detailed understanding of the amorphization and damage mechanisms governed by nonthermal processes is still lacking. Here we performed ultrafast time-resolved electron diffraction and single-shot optical pump-probe measurements followed by femtosecond near-ultraviolet pulse irradiation to study the structural dynamics of polycrystalline Ge2Sb2Te5. The experimental results present a nonthermal crystal-to-amorphous phase transition of Ge2Sb2Te5 initiated by the displacements of Ge atoms. Above the fluence threshold, we found that the permanent amorphization caused by multi-displacement effects is accompanied by a partial hexagonal crystallization.

Terahertz-Field-Induced Nonlinear Electron Delocalization in Au Nanostructures

Improved control over the electromagnetic properties of metal nanostructures is indispensable for the development of next-generation integrated nanocircuits and plasmonic devices. The use of terahertz (THz)-field-induced nonlinearity is a promising approach to controlling local electromagnetic properties. Here, we demonstrate how intense THz electric fields can be used to modulate electron delocalization in percolated gold (Au) nanostructures on a picosecond time scale. We prepared both isolated and percolated Au nanostructures deposited on high resistivity Si(100) substrates. With increasing the applied THz electric fields, large opacity in the THz transmission spectra takes place in the percolated nanostructures; the maximum THz-field-induced transmittance difference, 50% more, is reached just above the percolation threshold thickness. Fitting the experimental data to a Drude-Smith model, we found furthermore that the localization parameter and the damping constant strongly depend on the applied THz-field strength. These results show that ultrafast nonlinear electron delocalization proceeds via strong electric field of THz pulses; the intense THz electric field modulates the backscattering rate of localized electrons and induces electron tunneling between Au nanostructures across the narrow insulating bridges without any material breakdown.

High-Frequency Coherent Phonons in Graphene on Silicon

High-frequency coherent vibrations have been investigated in graphene on silicon substrates using pump-probe anisotropic reflectivity spectroscopy with 7.5 fs laser pulses. The coherent phonons of both the intralayer C=C stretching mode (G-mode) and the disorder-induced mode (D-mode) are clearly observed even in monolayer graphenes. The G-mode shifts to higher frequencies as the number of graphene layers decreases, whereas the D-mode does not exhibit such hardening. Moreover, the D-mode softens upon photoexcitation and then shifts to higher frequencies during the relaxation process. The different frequency shift behaviors in the G- and D-modes are discussed.