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Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates (reference). In this study, we investigate optical properties of an epitaxial grown LT-GaAs/Si sample, compared to a reference grown under the same substrate temperature, and with the same layer thickness. Anti-phase domains and some crystal misorientation are present in the LT-GaAs/Si. From coherent phonon spectroscopy, the intrinsic carrier densities are estimated to be ~$10^{15}$ cm$^{-3}$ for either sample. Strong plasmon damping is also observed. Carrier dynamics, measured by time-resolved THz spectroscopy at high excitation fluence, reveals markedly different responses between samples. Below saturation, both samples exhibit the desired fast response. Under optical fluences $\geq$ 54 $\mu$ J/cm$^2$, the reference LT-GaAs layer shows saturation of electron trapping states leading to non-exponential behavior, but the LT-GaAs/Si maintains a double exponential decay. The difference is attributed to the formation of As-As and Ga-Ga bonds during the heteroepitaxial growth of LT-GaAs/Si, effectively leading to a much lower density of As-related electron traps., Comment: 8 pages, supplementary info separate

Organic-inorganic layered perovskites are two-dimensional quantum wells with layers of lead-halide octahedra stacked between organic ligand barriers. The combination of their dielectric confinement and ionic sublattice results in excitonic excitations with substantial binding energies that are strongly coupled to the surrounding soft, polar lattice. However, the ligand environment in layered perovskites can significantly alter their optical properties due to the complex dynamic disorder of soft perovskite lattice. Here, we observe the dynamic disorder through phonon dephasing lifetimes initiated by ultrafast photoexcitation employing high-resolution resonant impulsive stimulated Raman spectroscopy of a variety of ligand substitutions. We demonstrate that vibrational relaxation in layered perovskite formed from flexible alkyl-amines as organic barriers is fast and relatively independent of the lattice temperature. Relaxation in aromatic amine based layered perovskite is slower, though still fast relative to pure inorganic lead bromide lattices, with a rate that is temperature dependent. Using molecular dynamics simulations, we explain the fast rates of relaxation by quantifying the large anharmonic coupling of the optical modes with the ligand layers and rationalize the temperature independence due to their amorphous packing. This work provides a molecular and time-domain depiction of the relaxation of nascent optical excitations and opens opportunities to understand how they couple to the complex layered perovskite lattice, elucidating design principles for optoelectronic devices., 7 pages, 4 figures, SI

© 2020 by the authors, The implementation of thermal barriers in thermoelectric materials improves their power conversion rates effectively. For this purpose, material boundaries are utilized and manipulated to affect phonon transmissivity. Specifically, interface intermixing and topography represents a useful but complex parameter for thermal transport modification. This study investigates epitaxial thin film multilayers, so called superlattices (SL), of TiNiSn/HfNiSn, both with pristine and purposefully deteriorated interfaces. High-resolution transmission electron microscopy and X-ray diffractometry are used to characterize their structural properties in detail. A differential 3 ω -method probes their thermal resistivity. The thermal resistivity reaches a maximum for an intermediate interface quality and decreases again for higher boundary layer intermixing. For boundaries with the lowest interface quality, the interface thermal resistance is reduced by 23% compared to a pristine SL. While an uptake of diffuse scattering likely explains the initial deterioration of thermal transport, we propose a phonon bridge interpretation for the lowered thermal resistivity of the interfaces beyond a critical intermixing. In this picture, the locally reduced acoustic contrast of the less defined boundary acts as a mediator that promotes phonon transition., The authors gratefully acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - project number 121583221, by the Graduate School of Excellence Material Science in Mainz (GSC266). E.C.A. acknowledges support from Severo Ochoa Program (MINECO, Grant SEV-2017-0706) and funding from the CERCA Programme/ Generalitat de Catalunya.

The interaction between ultrashort light pulses and non-absorbing materials is dominated by Impulsive Stimulated Raman Scattering (ISRS). The description of ISRS in the context of pump\&probe experiments is based on effective classical models describing the interaction between the phonon and pulsed electromagnetic fields. Here we report a theoretical description of ISRS where we do not make any semi-classical approximation and we treat both photonic and phononic degrees of freedom at the quantum level. The results of the quantum model are compared with semiclassical results and validated by means of spectrally resolved pump\&probe measurements on $��$-quartz., Includes supplementary material

We employed femtosecond pump probe technique to investigate the dynamics of coherent optical phonons in iron garnet. A phenomenological symmetry-based consideration reveals that oscillations of the terahertz T2g mode are excited. Selective excitation by a linearly polarized pump and detection by a circularly polarized probe confirm that impulsive stimulated Raman scattering (ISRS) is the driving force for the coherent phonons. Experimental results obtained from ISRS measurements reveal excellent agreement with spontaneous Raman spectroscopy data, analyzed by considering the symmetry of the phonon modes and corresponding excitation and detection selection rules., Comment: 20 pages, 6 figures, 3 tables

We observe anharmonic decay of the photoexcited coherent A1g phonon in bismuth to points in the Brillouin zone where conservation of momentum and energy are satisfied for three-phonon scattering. The decay of a coherent phonon can be understood as a parametric resonance process whereby the atomic displacement periodically modulates the frequency of a broad continuum of modes. This results in energy transfer through resonant squeezing of the target modes. Using ultrafast diffuse x-ray scattering, we observe build up of coherent oscillations in the target modes driven by this parametric resonance over a wide range of the Brillouin zone. We compare the extracted anharmonic coupling constant to first principles calculations for a representative decay channel., Comment: 5 pages, 6 figures

This paper reviews our recent study on a coherent optical phonon in a hexagonal YMnO3 thin film together with related optical studies in hexagonal RMnO3 (R = Y, Lu, Ho) compounds. Coherent phonons have been observed in RMnO3 compounds by pump-probe spectroscopy with subpicosecond laser pulses, whereas the observation of coherent optical phonons was reported only in LuMnO3. Recently, we succeeded in the observation of the coherent optical phonon in a YMnO3 thin film. The generation process of the coherent optical phonon is assigned to a displacive mechanism, which is identical to that in LuMnO3. The coherent optical phonon is observed in the temperature range from 10 K to room temperature, while the oscillation intensity strongly decreases as the temperature increases to the Néel temperature of ~70 K from a lower temperature range. It is interesting that the temperature dependence is largely different from that in LuMnO3. We describe that the result can be qualitatively explained by the property of an isostructural transition around the Néel temperature in RMnO3 compounds. In addition, we briefly discuss ultrafast incoherent responses of excited electronic states from the viewpoint of the excitation photon energy of laser pulses.