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

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

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

Generation of wavelength-tunable few-cycle pulses in the mid-infrared at repetition rates up to 10 kHz

We demonstrate a compact and tunable mid-infrared light source that provides carrier-envelope-phase (CEP)-locked pulses at repetition rates from 500 Hz to 10 kHz. The seed pulses were generated by intra-pulse difference frequency mixing of the output of an Yb:KGW regenerative amplifier that had been spectrally broadened by continuum generation using multiple plates. Then, a two-stage optical parametric amplifier was used to obtain output energies of about 100 µJ/pulse for center wavelengths between 2.8 and 3.5 µm. Owing to the intense pulse energies, it was possible to compress the multi-cycle pulses down to two-cycle pulses using YAG and Si plates

Enhancing the Hot-Phonon Bottleneck Effect in a Metal Halide Perovskite by Terahertz Phonon Excitation

ハライドペロブスカイト半導体においてテラヘルツ励起によるホットキャリアの長寿命化を実現 --太陽電池材料のフォノン操作による高効率化への新たな指針--. 京都大学プレスリリース. 2021-02-19.We investigate the impact of phonon excitations on the photoexcited carrier dynamics in a lead-halide perovskite CH3NH3PbI3, which hosts unique low-energy phonons that can be directly excited by terahertz pulses. Our time-resolved photoluminescence measurements reveal that strong terahertz excitation prolongs the cooling time of hot carriers, providing direct evidence for the hot-phonon bottleneck effect. In contrast to the previous studies where phonons are treated as a passive heat bath, our results demonstrate that phonon excitation can significantly perturb the carrier relaxation dynamics in halide perovskites through the coupling between transverse- and longitudinal-optical phonons

Excitonic enhancement of optical nonlinearities in perovskite CH₃NH₃PbCl₃ single crystals

Metal halide perovskites have emerged as versatile photonic device materials because of their outstanding band structure and excellent optical properties. Here, we determined the excitation wavelength dependences of the two-photon absorption coefficient and the Kerr-effect-induced nonlinear refractive index of CH₃NH₃PbCl₃ perovskite single crystals by means of the Z-scan method. From theoretical analysis, we found that the electron-hole interaction, so-called exciton effect, significantly enhances the nonlinear optical responses even for the interband transitions. This interaction explains the universal relation between the exciton reduced mass and the bandgap for lead halide perovskites

Strong spin-orbit coupling inducing Autler-Townes effect in lead halide perovskite nanocrystals

ペロブスカイトナノ粒子において近赤外光による大きな超高速光変調を室温で実現 --光通信帯における新たな超高速光スイッチング技術の開発に期待--. 京都大学プレスリリース. 2021-05-24.Manipulation of excitons via coherent light-matter interaction is a promising approach for quantum state engineering and ultrafast optical modulation. Various excitation pathways in the excitonic multilevel systems provide controllability more efficient than that in the two-level system. However, these control schemes have been restricted to limited control-light wavelengths and cryogenic temperatures. Here, we report that lead halide perovskites can lift these restrictions owing to their multiband structure induced by strong spin-orbit coupling. Using CsPbBr₃ perovskite nanocrystals, we observe an anomalous enhancement of the exciton energy shift at room temperature with increasing control-light wavelength from the visible to near-infrared region. The enhancement occurs because the interconduction band transitions between spin-orbit split states have large dipole moments and induce a crossover from the two-level optical Stark effect to the three-level Autler-Townes effect. Our finding establishes a basis for efficient coherent optical manipulation of excitons utilizing energy states with large spin-orbit splitting

Ultrastrong coupling between THz phonons and photons caused by an enhanced vacuum electric field

テラヘルツ周波数帯の真空量子揺らぎと格子振動の超強結合状態の実現 --量子光を駆動力とする新規物性制御への挑戦--. 京都大学プレスリリース. 2021-07-30.Ultrastrong coupling (USC) between phonons and vacuum photons can result in fascinating quantum phenomena, though it is difficult to achieve due to the small dipole moments of phonons in solids. Here, we investigate the vacuum Rabi splitting by coupling phonons in perovskite CH₃NH₃PbI₃ films with photons in split ring resonators. As the gap size of the resonator decreases, the coupling strength η increases due to the enhanced vacuum field in the gap, reaching the USC regime (η∼0.24) at a gap size of 100 nm. Our results show that nanoresonators are an excellent platform for studies of vacuum-dressed phonon properties