We report measurements of acoustic phonon emission from a weakly coupled AlAs/GaAs superlattice (SL) under vertical electron transport. The phonons were detected using superconducting bolometers. A peak (resonance) was observed in emission parallel to the SL growth axis when the electrical energy drop per SL period matched the energy of the first SL mini-Brillouin zone-center phonon mode. This peak was mirrored by an increase of the differential conductance of the SL. These results are evidence for stimulated emission of terahertz phonons as previously predicted theoretically and suggest that such a SL may form the basis of a SASER (sound amplification by stimulated emission of radiation) device.

Coherent high-amplitude precession of the magnetization and spin waves with frequencies up to 40 GHz are generated by injecting picosecond compressive and shear acoustic pulses into nanometer-sized galfenol (Fe81Ga19) films. The magnetization modulation is due to the picosecond inverse magnetostrictive effect. The oscillations of the magnetization measured by magneto-optical Kerr rotation last for several nanoseconds, and the maximum modulation of the in-plane effective magnetic field is as high as 40 mT. These results in combination with a comprehensive theoretical analysis show that galfenol films possess excellent properties for ultrafast magnetization control based on the picosecond inverse magnetostrictive effect.

. (2016) Ultrafast changes of magnetic anisotropy driven by laser-generated coherent and noncoherent phonons in metallic films. Physical Review B, 93 (21 A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. ABSTRACTUltrafast optical excitation of a metal ferromagnetic film results in a modification of the magnetocrystalline anisotropy and induces the magnetization precession. We consider two main contributions to these processes: an effect of non-coherent phonons, which modifies the temperature dependent parameters of the magneto-crystalline anisotropy; and coherent phonons in the form of a strain contributing via inverse magnetostriction. Contrary to the earlier experiments with high symmetry ferromagnetic structures, where these mechanisms could not be separated, we study the magnetization response to femtosecond optical pulses in the low-symmetry magnetostrictive Galfenol film so that it is possible to separate the coherent and non-coherent phonon contributions. By choosing certain experimental geometry and external magnetic field, we can distinguish the contribution from a specific mechanism. Theoretical analysis and numerical calculations are used to support the experimental observations and proposed model.

A theoretical model of the coherent precession of magnetization excited by a picosecond acoustic pulse in a ferromagnetic semiconductor layer of (Ga,Mn)As is developed. The short strain pulse injected into the ferromagnetic layer modifies the magnetocrystalline anisotropy resulting in a tilt of the equilibrium orientation of magnetization and subsequent magnetization precession. We derive a quantitative model of this effect using the Landau-Lifshitz equation for the magnetization that is precessing in the time-dependent effective magnetic field. After developing the general formalism, we then provide a numerical analysis for a certain structure and two typical experimental geometries in which an external magnetic field is applied either along the hard or the easy magnetization axis. As a result we identify three main factors, which determine the precession amplitude: the magnetocrystalline anisotropy of the ferromagnetic layer, its thickness, and the strain pulse parameters.

Based on the symmetry properties of the graphene lattice, we derive the effective Hamiltonian of graphene under spatially nonuniform acoustic and optical strains. Comparison with the published results of the first-principles calculations allows us to determine the values of some Hamiltonian parameters, and suggests the validity of the derived Hamiltonian for acoustical strain up to 10%. The results are generalized for the case of graphene with broken plane reflection symmetry, which corresponds, for example, to the case of graphene placed on a substrate. Here, essential modifications to the Hamiltonian give rise, in particular, to the gap opening in the spectrum in the presence of the out-of-plane component of optical strain, which is shown to be due to the lifting of the sublattice symmetry. The developed effective Hamiltonian can be used as a convenient tool for analysis of a variety of strain-related effects, including electron-phonon interaction or pseudo-magnetic fields induced by the nonuniform strain.

Quasi-longitudinal and quasi-transverse picosecond strain pulses injected into a ferromagnetic (311) (Ga,Mn)As film induce dynamical shear strain in the film, thereby modulating the magnetic anisotropy and inducing resonant precession of the magnetization at a frequency ~10 GHz. The modulation of the out-of-plane magnetization component by the quasitransverse strain reaches amplitudes as large as 10% of the equilibrium magnetization. Our theoretical analysis is in good agreement with the observed results, thus providing a strategy for ultrafast magnetization control in ferromagnetic films by strain pulses.

We demonstrate a variety of precessional responses of the magnetization to ultrafast optical excitation in nanolayers of Galfenol (Fe,Ga), which is a ferromagnetic material with large saturation magnetization and enhanced magnetostriction. The particular properties of Galfenol, including cubic magnetic anisotropy and weak damping, allow us to detect up to 6 magnon modes in a 120nm layer, and a single mode with effective damping α ef f = 0.005 and frequency up to 100 GHz in a 4nm layer. This is the highest frequency observed to date in time-resolved experiments with metallic ferromagnets. We predict that detection of magnetisation precession approaching THz frequencies should be possible with Galfenol nanolayers.

We consider an electron-acoustic phonon coupling mechanism associated with the dependence of crystal dielectric permittivity on the strain (the so-called Pekar mechanism) in nanostructures characterized by strong confining electric fields. The efficiency of Pekar coupling is a function of both the absolute value and the spatial distribution of the electric field. It is demonstrated that this mechanism exhibits a phonon wavevector dependence similar to that of piezoelectricity and must be taken into account for electron transport calculations in an extended field distribution.In particular, we analyze the role of Pekar coupling in energy relaxation in silicon inversion layers.Comparison with the recent experimental results is provided to illustrate its potential significance.