Single-cycle THz pulses with amplitudes exceeding 1 MV/cm generated by optical rectification in LiNbO3

Using the tilted-pulse-intensity-front scheme, we generate single-cycle terahertz (THz) pulses by optical rectification of femtosecond laser pulses in LiNbO3. In the THz generation setup, the condition that the image of the grating coincides with the tilted-optical-pulse front is fulfilled to obtain optimal THz beam characteristics and pump-to-THz conversion efficiency. The designed focusing geometry enables tight focus of the collimated THz beam with a spot size close to the diffraction limit, and the maximum THz electric field of 1.2 MV/cm is obtained

Nonlinear magnetization dynamics of antiferromagnetic spin resonance induced by intense terahertz magnetic field

We report on the nonlinear magnetization dynamics of a HoFeO3 crystal induced by a strong terahertz magnetic field resonantly enhanced with a split ring resonator and measured with magneto-optical Kerr effect microscopy. The terahertz magnetic field induces a large change (~40%) in the spontaneous magnetization. The frequency of the antiferromagnetic resonance decreases in proportion to the square of the magnetization change. A modified Landau-Lifshitz-Gilbert equation with a phenomenological nonlinear damping term quantitatively reproduced the nonlinear dynamics

Exciton energy transfer between the inner and outer tubes in double-walled carbon nanotubes

We have studied photoluminescence (PL) spectrum and dynamics of the inner nanotubes in double-walled carbon nanotubes (DWNTs) and compared PL properties between DWNTs and single-walled carbon nanotubes (SWNTs). The PL peak energies of the inner tubes are redshifted from those of SWNTs with the same chiral indices. This PL redshift is enhanced with an increase in the inner tube diameter. The PL lifetime of DWNTs increases with a decrease in the inner tube diameter. The diameter dependence of PL dynamics is explained by exciton energy transfer between the inner and outer tubes through Förster-type dipole-dipole interaction

Polarization anomaly in high harmonics in the crossover region between perturbative and extreme nonlinearity in GaAs

We investigate the characteristics of high harmonics (HHs) unique to the nonperturbative nonlinear regime. We show that the polarization state of HHs generated from GaAs changes drastically across the crossover from the weak-field perturbative regime to the strong-field extreme nonlinear regime, while the linearly polarized infrared excitation field (Eexc) is fixed to a particular crystal direction. The dependence on the Eexc-field strength reveals that multiple emission processes with different nonlinear orders and temporal phases contribute to each order HH, and the interference among them plays a pivotal role. This interference manifests itself as a unique phenomenon: a large HH ellipticity emerges in the course of crossover, despite the fact that GaAs hosts no magnetization or linear birefringence. These results demonstrate that not only the material's symmetry but also the ultrafast nonlinear dynamics largely affects the HH polarization, and hence, HH polarization and its Eexc-field dependence provide a useful experimental tool to probe ultrafast coherent dynamics in light-driven solid-state materials

High-order harmonic generation in graphene: Nonlinear coupling of intraband and interband transitions

We investigate high-order harmonic generation (HHG) in graphene with a quantum master equation approach. The simulations reproduce the observed enhancement in HHG in graphene under elliptically polarized light [N. Yoshikawa et al., Science 356, 736 (2017)]. On the basis of a microscopic decomposition of the emitted high-order harmonics, we find that the enhancement in HHG originates from an intricate nonlinear coupling between the intraband and interband transitions that are respectively induced by perpendicular electric field components of the elliptically polarized light. Furthermore, we reveal that contributions from different excitation channels destructively interfere with each other. This finding suggests a path to potentially enhance the HHG by blocking a part of the channels and canceling the destructive interference through band-gap or chemical potential manipulation

Exciton localization of single-walled carbon nanotubes revealed by femtosecond excitation correlation spectroscopy

Photoluminescence (PL) dynamics in single-walled carbon nanotubes (SWNTs) has been studied by the femtosecond excitation correlation method with a 150 fs time resolution. The SWNT samples were synthesized by different methods and suspended in gelatin films or D2O solutions. The PL dynamics of SWNTs depends on the local environment surrounding the SWNTs rather than the synthesis methods. The very weak temperature dependence of PL and the environment-dependent PL reveal that the PL relaxation process is dominated by the interplay between free excitons and weakly localized excitons

Sub-Cycle Optical Response Caused by Dressed State with Phase-Locked Wavefunctions

The coherent interaction of light with matter imprints the phase information of the light field on the wavefunction of the photon-dressed electronic state. Driving electric field, together with a stable phase that is associated with the optical probe pulses, enables the role of the dressed state in the optical response to be investigated. We observed optical absorption strengths modulated on a sub-cycle timescale in a GaAs quantum well in the presence of a multi-cycle terahertz driving pulse using a near-infrared probe pulse. The measurements were in good agreement with the analytical formula that accounts for the optical susceptibilities caused by the dressed state of excitons, which indicates that the output probe intensity was coherently reshaped by the excitonic sideband emissions

Impact ionization dynamics in silicon by MV/cm THz fields

We investigate the dynamics of the impact ionization (IMI) process in silicon in extremely high fields in the MV/cm range and at low initial carrier concentrations; conditions that are not accessible with conventional transport measurements. We use ultrafast measurements with high-intensity terahertz pulses to show that IMI is significantly more efficient at lower than at higher initial carrier densities. Specifically, in the case of silicon with an intrinsic carrier concentration (~1010 cm−3), the carrier multiplication process can generate more than 108 electrons from just a single free electron. The photoexcited carrier density dependence of the IMI rate shows that with decreasing initial carrier density the rate increases and approaches the fundamental Okuto limit imposed by energy conservation