Coherent Phonon Oscillations Research Articles

Enhancement of coherent longitudinal optical phonon oscillations in aGaAs∕AlAsmultiple quantum well due to intersubband energy tuning under an electric field

Enhancement of coherent longitudinal optical phonon oscillations in a<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>GaAs</mml:mi><mml:mo>∕</mml:mo><mml:mi>AlAs</mml:mi></mml:mrow></mml:math>multiple quantum well due to intersubband energy tuning under an electric field

Characteristics of coupled mode of excitonic quantum beat and coherent longitudinal optical phonon in GaAs/AlAs multiple quantum wells

We have investigated the coupled mode of excitonic quantum beats and coherent GaAs-like longitudinal optical (LO) phonons in GaAs/AlAs multiple quantum wells with the splitting energy of heavy- and light-hole excitons close to the LO phonon energy. The time-domain signals show the coherent oscillation of the coupled mode between the quantum beat and the LO phonon with a fast dephasing time and the long-lived coherent GaAs-like LO phonon oscillation. The dependence of the coupled-mode frequency on splitting energy exhibits an anticrossing behavior. We demonstrate that the amplitude of the long-lived coherent LO phonon is manipulated by controlling the coupled mode with a double-pulse excitation technique.

Numerical Simulation of THz Radiation by Coherent LO Phonons in GaAs p–i–n Diodes under High Electric Fields

A numerical simulation of terahertz (THz) radiation by coherent longitudinal optical (LO) phonons in GaAs p–i–n diodes under high electric fields is presented, with emphasis on the important role of a velocity overshoot of free carriers in coherent phonon generation. The dynamics of non-equilibrium photoexcited carriers is simulated by adopting a self-consistent ensemble Monte Carlo method. The explicit comparison of THz signals between calculations and experiments is carried out by taking into account the electronic and ionic accelerations. It is shown that the coherent phonon oscillations emitting the THz radiation are excited resonantly by synchronization between the proper lattice oscillation and ultrafast transients of electronic polarization within the initial period of lattice vibration. The appearance of saturation in THz amplitude due to the coherent phonon above 150 kV/cm as well as a significant phase shift of the oscillatory THz signal is predicted.

Coherent Vibrational Oscillation in Gold Prismatic Monolayer Periodic Nanoparticle Arrays

We studied the ultrafast laser-induced coherent phonon oscillation in prismatic shaped gold nanoparticles assembled in monolayer periodic arrays by using the nanosphere lithographic technique. The amplitude and phase of the oscillation observed by ultrafast pump−probe transient spectroscopy is monitored as the wavelength of the dipolar surface plasmon absorption decreases. At a certain wavelength, the oscillation could not be observed. As the monitoring wavelength decreases further, the sign of the amplitude changes. From the wavelength at which the oscillation is not detected, the dependence of the absorption maxima on the size of the nanoparticles, the changes in the nanoparticle size are estimated during its oscillation. This large change in the size of the prismatic nanoparticle compared to the small change reported previously for the nanosphere oscillations is discussed.

Emission of electromagnetic radiation by coherent vibrational wavesin stimulated Raman scattering

Electromagnetic radiation is emitted by the vibrational and collective modes of an opaque solid as the result of impulsive stimulated Raman scattering. Raman scattering of near-infrared femtosecond laser pulses produces coherent longitudinal optical phonon and plasmon oscillations in the semiconductor InSb. These oscillations radiate into free space at THz frequencies and are directly detected. The THz spectra exhibit features consistent with Raman selection rules including interference of allowed and forbidden Raman scattering.

Calculation of THz radiation due to coherent polar-phonon oscillations in p–i–n diode structure at high electric field

We present a calculation of terahertz (THz) radiation by coherent optical phonons in GaAs p–i–n diode structure under high electric fields. The ultrafast dynamics of photoexcited carriers is simulated by adopting a self-consistent ensemble Monte Carlo method. It is demonstrated that the coherent phonon oscillations emitting the THz radiation are excited resonantly by synchronization between the proper lattice oscillation and ultrafast transient of electronic polarization originating from a drift overshoot within the initial period of lattice vibration.

Spectrally filtered time domain study of coherent phonons in semimetals

We have investigated the spectrally resolved coherent phononoscillation in semimetals of Sb and Bi. The oscillation initial phase isπ-shifted for the high- and low-frequency components of the probe pulse, indicating that notonly the integrated intensity, but also the spectral content of the probe, are a function oftime delay. This means that the coherent phonon, corresponding to nuclear motion, is awavepacket localized in different regions of the phase space. Furthermore, the spectralfiltering results in the appearance of the signal for negative delay time with thesimultaneous disappearance of the coherent artefact. We conjecture that all these findingsare the indication of the non-classical nature of coherent phonons that has beenearlier inferred from squeezing and revivals of the lattice mode in semimetals.

Resonance effects on coherent phonon generation in lead phthalocyanine crystalline films

We report the first observation of coherent phonons in crystalline lead phthalocyanine (PbPc) films grown on a (0001) sapphire substrate by using a pump–probe technique. Coherent phonon oscillations corresponding to intermolecular (lattice) vibrations in the PbPc film are observed in the frequency region from 1 to 5 THz. It is found that the intensities of coherent phonons are resonantly enhanced at the exciton energies of the Q band observed in the absorption spectrum.

Photon control of phonons in mixed crystal quantum dots

Coherent phonon oscillations in solids can be excited impulsively by a single femtosecond laser pulse whose duration is shorter than a phonon period. In the impulsive stimulated Raman scattering (ISRS) experiment, scattering of probe is monitored as a function of time with respect to pump to generate time domain spectra of coherent phonons. In this paper, we present one such study of CdSe 0.68Te 0.32 ( d∼80 A) quantum dots in glass matrix, i.e semiconductor-doped glass (SDG) RG780 from Schott, USA and the experiment was performed at Prof. Merlin's laboratory at the University of Michigan, USA. Here, we present first report of selectively driving only CdSe-like modes in these mixed crystal quantum dots using photon control with two pump beams.

Coherent phonon oscillations excited by second-order Raman scattering in CdZnTe multiple quantum wells and cubic ZnTe

We report observation of strong phonon-pair oscillations in CdZnTe multiple quantum wells and cubic ZnTe(001). Using femtosecond laser pulses with photon energies far below the bandgap, the coherent phonon-pair oscillations are generated via the impulsive stimulated Raman scattering of the second order. Polarization- and phase-sensitive measurements allow us to resolve both relative Raman polarizabilities of various symmetry modes and their initial phases, simultaneously.

Coherent phonons, nanoseismology and THz radiation in InGaN/GaN heterostructures

The ultrafast optical photoexcitation of hot electrons and holes in semiconductors by femtosecond laser pulses can trigger coherent phonon oscillations. We discuss the huge coherent acoustic phonons which have been generated in InGaN/GaN heterostructures and epilayers and how they might be used in imaging of surfaces and interfaces in nanostructures. We also discuss the THz radiation emitted from these phonons.

Coherent optical phonon oscillations in cubic ZnSe

We report the observation of coherent optical phonon oscillations in cubic bulk ZnSe(001). With a photon energy far below the band gap, the generation mechanism of the coherent longitudinal optical phonon mode is revealed to be the impulsive stimulated Raman scattering. Dephasing of the coherent longitudinal optical phonon modes by electron-phonon interaction and anharmonic processes is studied by investigating excitation intensity and temperature dependence of the dephasing rates.

Coherent optical phonon mode oscillations in wurtzite ZnO excited by femtosecond pulses

We report on coherent optical phonon oscillations in wurtzite ZnO. The high- and low-frequency E2 modes are excited by the impulsive stimulated Raman scattering, and detected through the electro-optic effect. The dephasing times are measured to be 1.75 and 29.2 ps for the high- and the low-E2 modes, respectively. The dependency of the phonon amplitude on the spectral width of the femtosecond pulses is shown.

Spectral analysis of high-harmonic coherent acoustic phonons in piezoelectric semiconductor multiple quantum wells

Multiple harmonic components have been observed in the Fourier spectra of the coherent acoustic phonon oscillations in InGaN/GaN multiple quantum wells (MQW's). The dependence of the relative magnitudes of the first two Fourier components on the sample geometry was analyzed by conducting time-resolved pump-probe measurements on MQW samples with different well-to-barrier ratios. The results were also compared with the recently proposed loaded-string model [Phys. Rev. B 64, 235316 (2001)], and an overall agreement is obtained which validates the picture of string oscillation with inhomogeneous loadings. We also demonstrated manipulation of the spectral components using the coherent optical control technique. Our demonstration can be regarded as a first step to the realization of wave-form synthesis of nanoacoustic waves.

Excitation of evanescent acoustic waves by ultrafast laser action on crystal surface (abstract)

Following the advances in femtosecond laser technology, significant progress was achieved during the last 10 years in excitation of terahertz coherent lattice vibrations [W. A. Kütt, W. Albrecht, and H. Kurz, IEEE J. Quantum Electron. 28, 2434 (1992)]; [R. Merlin, Solid State Communications, 102, 207 (1997)]. Coherent terahertz phonons were excited in various type of crystals (in particular, in semiconductors and superconductors). However, until recently all-optical excitation and detection were reported on optical phonons only [W. A. Kütt, W. Albrecht, and H. Kurz, IEEE J. Quantum Electron. 28, 2434 (1992)]; [R. Merlin, Solid State Communications, 102, 207 (1997)]. Last year, ultrafast impulsive excitation of coherent acoustic phonon oscillations at frequencies around 4 THz was achieved [M. D. Cummings and A. Y. Elezzabi, Appl. Phys. Lett. 79, 770 (2001)]. We present here the results of theoretical analysis of acoustic signals excited by laser-induced mechanical stresses, which have spectral components around and above the cutoff frequency for propagating acoustic waves. For the analysis of both excitation process and of photo-excited wave propagation, discrete periodic structure of material should be taken into account. We are using for this purpose the simplest chain model of crystal. Analytical solutions, which reveal the role of strong acoustic velocity dispersion and the role of forbidden frequency band existence (where only evanescent modes are excited), are derived. Frequency spectra and temporal profiles of mechanical displacement are determined both at front (laser-irradiated) surface and at rear surface of photo-excited thin film. The contribution of the excited evanescent modes to the motion of the front surface is found. It is expected that evanescent modes might be very sensitive to possible surface and subsurface defects, and because of this they might be useful in nondestructive photoacoustic testing of surfaces and interfaces.

Coherent optical phonon oscillations in bulk GaN excited by far below the band gap photons.

We report on generation of coherent optical phonon oscillations in 150 microm thick bulk GaN. With photon energy far below the band gap, the generation mechanisms of coherent phonon modes of A1(LO), high- and low-frequency E2 are revealed to be the impulsive stimulated Raman scattering. We find that one among the two degenerate E2 modes is selectively detected with a proper choice of probe polarization. Dephasing times range from 1.5 to 70 ps for different modes, and phonon-three-photon absorbed carrier interactions are compared between the A1(LO) and the E2 mode.

Free space radiation of coherent, coupled plasmon–phonon modes from InAs

The coupled plasmon–phonon system in bulk InAs is studied by ultrafast THz spectroscopy. Both upper- and lower-hybrid modes associated with coherent LO phonon and bulk plasma oscillations are identified in the radiated signals. The contribution of coherent phonons strongly depends on their spectral proximity to the plasmons.

Generation of coherent acoustic phonons in strained GaN thin films

Coherent acoustic phonon oscillations were generated and studied in strained GaN thin films. Inside the bulk GaN film, the longitudinal interference of an ultraviolet femtosecond pump pulse created periodic carrier distribution that screened out the strain-induced piezoelectric field and initiated the coherent longitudinal acoustic phonon oscillations corresponding to the carrier periods. The created coherent phonon oscillation modulated the piezoelectric field thus modified the absorption property of the GaN thin film through Franz–Keldysh effect. This time-dependent absorption modulation was reflected in the transmission variation of the followed probe pulses, resulting a long decay time ∼300 ps for the initiated coherent phonon oscillations.

Coherent phonons in quasi-one-dimensional organic crystals

Coherent phonon oscillations in quasi-1D organic crystals of MePTCDI (N-N′-dimethylperylene-3,4,9,10-dicarboximide) are directly time-resolved by femtosecond pump–probe experiments. We observe both oscillations caused by intra-molecular vibrations (internal phonons) and, for the first time in a quasi-1D organic system, modulations which are related to coherent lattice phonons (external phonons). From the observed phonon coherence of a few picoseconds, we assign the observed phonon oscillations to the electronic ground state potential.

Ultrafast impulsive excitation of coherent longitudinal acoustic phonon oscillations in highly photoexcited InSb

We report on the direct time-resolved measurement of high-frequency coherent longitudinal acoustic (LA) phonon oscillations in InSb. Two LA-phonon modes are impulsively generated in a highly photoexcited InSb sample and detected through reflectivity modulation with a temporal resolution of 15 fs. The measured frequencies, νLA(L1)=4.34 THz and νLA(X3)=3.75 THz, are in excellent agreement with those obtained through time-independent techniques. A frequency redshift in the LA(L1)-phonon frequency, as the energy fluence is increased beyond 10% of the damage threshold fluence, is attributed to an anisotropic impulsive softening of the InSb lattice.