Excitation Of Acoustic Phonons Research Articles

Coherent superposition of photon- and phonon-assisted tunneling in coupled quantum dots

We report on electron transport through an artificial molecule formed by two tunnel coupled quantum dots, which are laterally confined in a two-dimensional electron system of an ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ heterostructure. Coherent molecular states in the coupled dots are probed by photon-assisted tunneling (PAT). Above 10 GHz, we observe clear PAT as a result of the resonance between the microwave photons and the molecular states. Below 8 GHz, a pronounced superposition of phonon- and photon-assisted tunneling is observed. Coherent superposition of molecular states persists under excitation of acoustic phonons.

Coherent Acoustic Phonons in Semiconductor Superlattices

We investigate the excitation of coherent acoustic phonons in GaAs/AlAs superlattices with femtosecond laser pulses. Zone-folded acoustic modes of different order are observed in the time-domain. The excitation of the modes is based on an impulsive stimulated Raman process. The coherent phonon amplitudes can be traced over more than 100 ps which allows us to distinguish propagation and decay of the phonons. The corresponding linewidths of the time-domain data are below <0.2 cm—1 in the frequency domain. This is a new approach for high resolution spectroscopy of low energy phonons in semiconductors.

Coherent Acoustic Phonons in Semiconductor Superlattices

We investigate the excitation of coherent acoustic phonons in GaAs/AlAs superlattices with femtosecond laser pulses. Zone-folded acoustic modes of different order are observed in the time-domain. The excitation of the modes is based on an impulsive stimulated Raman process. The coherent phonon amplitudes can be traced over more than 100 ps which allows us to distinguish propagation and decay of the phonons. The corresponding linewidths of the time-domain data are below <0.2 cm—1 in the frequency domain. This is a new approach for high resolution spectroscopy of low energy phonons in semiconductors.

High Q merit factor, low frequency acoustic resonant modes from a Pt{100} single crystal

A spectroscopic technique, acoustic wave resonance spectroscopy (AWRS), has been developed to study the influence of long wavelength acoustic phonon excitations on surface chemical processes. Results are presented which demonstrate the sensitivity of AWRS to the state and composition of a Pt{100} single crystal sample, using an ultrahigh frequency and amplitude resolution acoustic spectrometer, a vector analyzer. A novel part of the ultrahigh vacuum compatible excitation system is a retractable piezo holder allowing the lithium niobate piezo transducer to be moved in situ into the bonding position against the side of the sample. Very sharp acoustic resonant modes with a Q merit factor of 1×104 appear only when the sample is clean. Carbon contamination reduces drastically both the intensity and the density of the peaks.

Interaction of Hopping Electrons with Acoustic Phonon and Soliton Excitation in an Inharmonic Linear Chains with Thermal Vibration

With the existence of thermal vibration effect taking into account, interactions of hopping electrons with lattice acoustic phonon and soliton excitation in inharmonic linear chains are discussed. A new nonlinear equation of probability amplitude for electron motion is obtained. The new results of soliton solution in the form of hyperelliptic integral are obtained as well. It is proved that there exists soliton excitation of kink type in addition to that of bell type found in general.

Characterization of a-Si:H/a-Ge:H superlattices by Raman scattering.

We have investigated the vibrational spectra of a-Si:H/a-Ge:H superlattices by Raman scattering. For high frequencies (&gt;100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) the intensity of the Raman spectrum is proportional to the density of vibrational states and shows contributions from the Si---Si and Ge---Ge bonds within the superlattice layers and from the Si---Ge bonds in the interface region. From the relative intensities of these modes we determine the individual layer thicknesses and the width of the interface. These results are discussed in terms of the growth conditions. For low frequencies (&lt;80 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) the Raman spectra show peaks corresponding to the excitation of folded-zone longitudinal acoustic phonons. The dispersion relation for these phonons is determined by measuring the peak positions as a function of laser energy and of the superlattice period. The result is well described by a continuum elastic model. The linewidth of these peaks increases with phonon frequency, indicating that the dispersion relation acquires a non-negligible imaginary component due to the loss of phase coherence of the acoustic vibrations.

Raman scattering study of folded acoustic phonons in GaAs/InxGa1−xAs strained-layer superlattices

The excitation of zone-folded longitudinal acoustic phonons in GaAs/InxGa1−xAs strained-layer superlattices is observed by Raman scattering. The Raman data agree well with theory and are consistent with high indium concentrations achieved in the fabrication of these metastable superlattices of good crystalline quality.

Observation of folded-zone acoustical phonons by Raman scattering in amorphous Si-SiNx superlattices.

The excitation of folded-zone longitudinal acoustic phonons in a-Si:H\char21{}a-${\mathrm{SiN}}_{\mathrm{x}}$:H periodic multilayer structures is observed by Raman scattering. The energies of the two lowest folded phonon branches is followed by varying the lattice period and the laser frequency. The data are quantitatively explained by a description of the phonons in the elastic continuum limit.

Anisotropy effects and optical excitation of acoustic phonons in n-i-p-i doping superlattices

The effect of impurity charges associated with a microstructured one-dimensional periodic n- and p-doping sequence in an otherwise homogeneous semiconductor on its lattice dynamical properties is studied. The Coulomb interaction does not remove the degeneracy of the back-folded acoustic phonons at the “mini-Brillouin zone” boundaries, introduced by the superstructure. Some of these phonon branches are, however, slightly affected at the Γ-point. The most striking result is the possibility to excite by electromagnetic waves, longitudinal and transverse acoustic phonos whose wavevectors are odd multiples of the smallest reciprocal superlattice vector.

Coherent Brillouin Spectroscopy

We have established the viability of coherent Raman techniques in the study of acoustic phonon excitations. Using the Raman-induced Kerr effect scheme, we have obtained the Brillouin spectrum of CS2. The nonlinear polarization in this case is so large that the effects of Stokes gain and anti-Stokes loss must be included in the theoretical interpretation of the spectrum.

Low-temperature phonon, magnon and exciton effects on the polarization of BaXF4

Studies of the polarization deviation ΔP and of the dynamic pyroelectric response (I/cρ) (dP/dT) at low temperatures of the family of barium-transition metal-fluorides BaXF4, for X = Mg, Zn, Mn, Ni and Co are reported. The various effects of acoustic and optic phonon excitation, 2D and 3D antiferromagnetic ordering in the Ni and Co salts. Frenkel excitons in the Mn salt and the subtle structural transition in the Mn salt are demonstrated.

Non-equilibrium heating-up of thermal phonons in a dielectric by microwave field

The investigation of the noise characteristics of a microwave parametric amplifier with ferroelectric nonlinear elements has shown that the noise temperature of such elements under the high microwave pumping was much higher than the equilibrium temperature of the material. The dependence of the noise temperature of the ferroelectric element, a SrTiO3 film, on the frequency and the pumping voltage has been obtained at a temperature of 78°K. The parametric mechanism of thermal acoustic phonon excitation was proposed for the explanation of the observed phenomena.