A. V. Akimov

Coherent magnetization precession in ferromagnetic (Ga,Mn)As induced by picosecond acoustic pulses.

We show that the magnetization of a thin ferromagnetic (Ga,Mn)As layer can be modulated by picosecond acoustic pulses. In this approach a picosecond strain pulse injected into the structure induces a tilt of the magnetization vector M, followed by the precession of M around its equilibrium orientation. This effect can be understood in terms of changes in magnetocrystalline anisotropy induced by the pulse. A model where only one anisotropy constant is affected by the strain pulse provides a good description of the observed time-dependent response.

Energy relaxation by hot electrons in n-GaN epilayers

The energy relaxation rate for hot electrons in n-type GaN epilayers has been measured over the temperature range 1.5–300 K. Several samples grown by molecular-beam epitaxy and having different electron concentrations have been studied. At low electron temperatures (Te<20 K), the energy relaxation is via acoustic phonon emission. The magnitude and temperature dependence of the energy relaxation are found to be in good agreement with theoretical calculations using appropriate values of the deformation potential and piezoelectric coupling constants and ignoring screening. For Te⩾70 K, the dominant mechanism of energy loss is optic phonon emission. For the several samples studied, consistent values of the optic phonon energy and electron-optic phonon relaxation time, 90±4 meV and 5–10 fs, respectively, are measured. The energy agrees well with values obtained by other methods and the relaxation time is consistent with theoretical calculations of the Frohlich interaction and indicate that hot phonon effects a...

Subpicosecond shifting of the photonic band gap in a three-dimensional photonic crystal

We demonstrate spectral shifting of the photonic band gap in a three-dimensional photonic crystal within a time of less than 350fs. Single 120fs high-power optical pulses are capable to induce the transition from the semiconductor to the metallic phase of VO2 in the pores of our artificial silica opal. The phase transition produces a substantial decrease of the real part of the effective refractive index of the photonic crystal and shifts the spectral position of the photonic band gap.

Dynamics of a vertical cavity quantum cascade phonon laser structure

Driven primarily by scientific curiosity, but also by the potential applications of intense sources of coherent sound, researchers have targeted the phonon laser (saser) since the invention of the optical laser over 50 years ago. Here we fabricate a vertical cavity structure designed to operate as a saser oscillator device at a frequency of 325 GHz. It is based on a semiconductor superlattice gain medium, inside a multimode cavity between two acoustic Bragg reflectors. We measure the acoustic output of the device as a function of time after applying electrical pumping. The emission builds in intensity reaching a steady state on a timescale of order 0.1 μs. We show that the results are consistent with a model of the dynamics of a saser cavity exactly analogous to the models used for describing laser dynamics. We also obtain estimates for the gain coefficient, steady-state acoustic power output and efficiency of the device.

Coherent elastic waves in a one-dimensional polymer hypersonic crystal

Using the methods of picosecond acoustics, we inject high amplitude hypersonic wavepackets into a polymer superlattice and optically detect the propagating coherent elastic waves. The spectrum of the optically detected signal shows the elastic modes typical for folded phonon dispersion curves. The experimental results and related modeling show the feasibility of using polymer one-dimensional hypersonic crystals as acoustic devices in the gigahertz frequency range.

Ultrafast band-gap shift induced by a strain pulse in semiconductor heterostructures.

The conventional piezospectroscopic effect is extended to picosecond time scales by using ultrashort strain pulses injected into semiconductor heterostructures. The strain pulses with durations of approximately 10 ps are generated in a metal transducer film by intense femtosecond laser pulses. They propagate coherently in the GaAs/(Al,Ga)As heterostructure over a distance of 100 microm and shift the band gaps by several meV as detected optically for quantum well exciton resonances by pump-probe techniques and time-resolved photoluminescence.

Picosecond inverse magnetostriction in galfenol thin films

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.

Lasing from active optomechanical resonators

Planar microcavities with distributed Bragg reflectors (DBRs) host, besides confined optical modes, also mechanical resonances due to stop bands in the phonon dispersion relation of the DBRs. These resonances have frequencies in the 10- to 100-GHz range, depending on the resonator’s optical wavelength, with quality factors exceeding 1,000. The interaction of photons and phonons in such optomechanical systems can be drastically enhanced, opening a new route towards the manipulation of light. Here we implemented active semiconducting layers into the microcavity to obtain a vertical-cavity surface-emitting laser (VCSEL). Thereby, three resonant excitations—photons, phonons and electrons—can interact strongly with each other providing modulation of the VCSEL laser emission: a picosecond strain pulse injected into the VCSEL excites long-living mechanical resonances therein. As a result, modulation of the lasing intensity at frequencies up to 40 GHz is observed. From these findings, prospective applications of active optomechanical resonators integrated into nanophotonic circuits may emerge.

Molecular beam epitaxy as a method for the growth of freestanding zinc-blende (cubic) GaN layers and substrates

The authors have studied the growth of bulk, freestanding zinc-blende (cubic) GaN layers by plasma-assisted molecular beam epitaxy (PA-MBE). They have established that the best structural properties of freestanding zinc-blende GaN can be achieved with initiation under Ga-rich conditions but without Ga droplet formation. It is difficult to initiate the growth of zinc-blende GaN, but it is even more difficult to sustain the growth of the pure zinc-blende polytype in thick layers without any wurtzite inclusions. In order to grow high quality freestanding cubic GaN layers, it is necessary to maintain the same growth conditions for about 1week. The best quality zinc-blende phase GaN was achieved in the first 10μm of the GaN layers. The authors have produced zinc-blende GaN substrates from our thick bulk GaN layers and they used the side previously attached to the GaAs substrate as the episide of these zinc-blende GaN substrates. They have demonstrated the scalability of the process by growing zinc-blende GaN l...

Excitation of spin waves in ferromagnetic (Ga,Mn)As layers by picosecond strain pulses

We report the excitation of spin waves in ferromagnetic semiconductor (Ga,Mn)As films by picosecond strain pulses. The strain pulse with a broad acoustic spectrum excites a number of magnon modes, which contribute to the precession of magnetization. The spectrum of the excited spin waves shows two well-resolved peaks with intensities dependent on the applied magnetic field. For a certain range of magnetic fields only the low-frequency spin wave is detected. We present the theoretical analysis and compare it with the experimental results, addressing the spatial overlap of the magnon and phonon eigenfunctions. Depending on the boundary conditions and the spectrum of the spin waves the spatial matching of the spin wave and resonance phonon eigenfunctions may provide high excitation efficiency for only one magnon mode, while other modes are not excited.