Temperature dependence of longitudinal optical phonon lifetimes in GaP

The decay of coherently excited longitudinal optical phonons in GaP has been observed through time delayed coherent anti-Stokes Raman scattering measurements with picosecond light pulses. The temperature dependence of the decay is compared to that observed through Raman linewidth measurements and to that predicted theoretically for spontaneous three-phonon decay processes.     

The creation of supra-thermal densities of high-energy phonons in He II

We report the new phenomenon that high-energy phonons can be created from low-energy phonons. This arises because the dynamics of phonons in propagating pulses are quite different to those in isotropic phonon distributions. A pulse of low-energy phonons rapidly thermalises by three-phonon processes. On a much longer time scale four-phonon processes occur within this phonon cloud which create high-energy (10 K) phonons that cannot spontaneously decay. These phonons have a lower velocity and so are lost from the back of phonon cloud; their deficit is restored continuously by four-phonon processes. These now isolated high-energy phonons are very stable and propagate ballistically behind the low-energy phonons, so giving the two pulses which are detected in experiments. For long pulses the high-energy phonons may also decay within the cloud, however the available low-energy phonons for scattering are confined to a narrow-angle cone, so the decay probability is very low because the four phonon process requires large angle scattering. A supra-thermal density of these high-energy phonons is predicted.     

Femtosecond reflectivity study of coherent phonons in doubly photoexcited bismuth

Ultrafast optical excitation generates coherent A_1g optical phonons in bismuth via the mechanism of displacive excitation of coherent phonons (DECP). Here, femtosecond pump-probe reflectivity spectroscopy examines the dynamics of coherent phonons in doubly photoexcited bismuth. Two successive pump pulses are employed to generate optic phonons and thus to investigate the phonon dynamics including bond softening and dephasing with variable pump fluence and inter-pump separation times. Combined with thermal analysis, it distinguishes thermal and nonthermal effects of photoexcitation on the phonon dynamics. It also reveals that photocarriers relax via electron-hole recombination and diffusion out of the optical penetration depth on ultrafast timescales of 10–20?ps.     

On the effect of phonon decay on electronic transitions in ionic crystal semiconductors

Generally electronic processes in semiconductors are accompanied by phonon excitations. These excitations themselves influence electronic transitions. On the other hand excited phonons decay by interaction with other impurities in the crystal, which act as a heat-bath. The resulting competition between phonon exciting electronic processes and phonon decay is described by Pauli's master equation. By expansion of its solutions into phonon decay solutions the problem can be separated into different decay equations for phonons and electrons as proposed by Stumpf. Assuming linear phonon-heat-bath coupling the phonon decay equation can be solved exactly by a generating function technique. The appropriate phonon decay frequencies are calculated for a simple heat-bath model.     

The role of acoustic phonons for Rabi oscillations in semiconductor quantum dots

The damping of Rabi oscillations in quantum dots as well as the renormalization of the carrier-light coupling, due to the interaction with longitudinal acoustic phonons are studied as a function of temperature and laser pulse parameters. Numerical results are obtained by using a correlation expansion within the density matrix theory. The observed features like a non-monotonous dependence of the damping on the pulse duration are characteristic for the strongly non-Markovian nature of the phonon coupling in these systems. The results can be well interpreted on the level of a perturbation expansion in the carrier-phonon interaction. PACS 78.67.Hc; 63.20.Kr; 03.65.Yz     

Chirality-induced dynamic kohn anomalies in graphene

We develop a theory for the renormalization of the phonon energy dispersion in graphene due to the combined effects of both Coulomb and electron-phonon (e-ph) interactions. We obtain the renormalized phonon energy spectrum by an exact analytic derivation of the phonon self-energy, finding three distinct Kohn anomalies (KAs) at the phonon wave vector q=omega/v, 2k_{F}+/-omega/v for LO phonons and one at q=omega/v for TO phonons. The presence of these new KAs in graphene, in contrast to the usual KA q=2k_{F} in ordinary metals, originates from the dynamical screening of e-ph interaction (with a concomitant breakdown of the Born-Oppenheimer approximation) and the peculiar chirality of the graphene e-ph coupling.     

双能态自旋-晶格声子耦合量子隧道系统的非经典能态和量子相干耗散

采用关联表象变分波函数方案, 介入三个非经典关联效应, 求解有限温度双能态自旋-晶格声子耦合量子隧道系统的非经典态, 着重研究化解由于粒子自旋-单声子相互作用引起的量子涨落导致双能态系统的退相干性量子耗散. 这三个非经典关联效应是: 1) 声子位移-粒子自旋 (σ_z)间强非绝热关联; 2) 声子压缩态效应及其伴随发生的单声子相干态-声子压缩态两过程相干效应; 3) 由关联表象导致的声子位移(U_D)与声子压缩(U_S)的表象关联非绝热修正. 结果表明: 由于引入粒子自旋-双声子相互作用, 大幅度地增强了声子场压缩态, 特别是更进一步极大幅度地增强了非经典压缩-相干态效应. 因此, 由粒子自旋-单声子相互作用产生的Debye-Walle相干弹性散射效应导致量子隧道项(-Δ₀σ_x)的强烈指数衰减及其伴随严重的量子相干损失的极大幅度的抑制, 并且自旋-晶格声子耦合量子隧道系统的非经典态能量大幅度降低. 关键词： 非经典能态 量子隧穿相干损失 自旋-双声子相互作用 压缩相干态效应     

Theoretical and experimental investigations of coherent phonon dynamics in sapphire crystal using femto- second time-resolved coherent anti-Stokes Raman scattering

We report on the theoretical and the experimental investigations of the coherent phonon dynamics in sapphire crystal using the femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) technique. The temporal chirped white-light continuum (WLC) is used for the Stokes pulse, therefore we can perform the selective excitation of the phonon modes without using a complicated laser system. The expected quantum beat phenomenon is clearly observed. The theoretical formulas consist very well with the experimental results. The dephasing times of the excited phonon modes, the wavenumber difference, and the phase shift between the simultaneously excited modes are obtained and discussed. This work opens up a way to study directly high-frequency coherent phonon dynamics in bulk crystals on a femtosecond time scale and is especially helpful for understanding the nature of coherent phonons.     

Coherent external and internal phonons in quasi-one-dimensional organic molecular crystals

We have directly time resolved coherent phonon oscillations in quasi-one-dimensional organic crystals of MePTCDI ( N-N'-dimethylperylene-3,4,9,10-dicarboximide), using femtosecond pump-probe experiments. We observe both higher-energy oscillations caused by intramolecular vibrations (internal phonons) and, for the first time in a quasi-one-dimensional organic system, lower-energy modulations which are related to coherent lattice phonons (external phonons). For internal A(g) vibrations, the coherence decay time of about 2 ps is almost independent of the mode. In contrast, the damping time of the external phonons increases strongly with decreasing energy.     

The far infrared optical constants of GaP

The FIR optical constants of GaP have been determined on either side of the Reststrahlen band by single pass transmission dispersive Fourier transform spectroscopy. These results extend the range of measured refractive index data for GaP, and reveal the dispersion in the refractive index in regions of weak absorption. Features in the spectrum are assigned as phonon combination bands with the aid of a critical point analysis based on phonon frequencies calculated using an 11-parameter rigid ion model.     

Acoustic phonon impact on the inter-Coulombic decay process in charged quantum dot pairs

ABSTRACT

Recently, highly accurate multi-configuration time-dependent Hartree electron dynamics calculations demonstrated the efficient long-range energy transfer inter-Coulombic decay (ICD) process to happen in charged semiconductor quantum dot (QD) pairs. ICD is initiated by intraband photoexcitation of one of the QDs and leads to electron emission from the other within a duration of about 150 ps. On the same time scale electronically excited states are reported to relax due to the coupling of electrons to acoustic phonons. Likewise, phonons promote ionisation. Here, the QDs' acoustic breathing mode is implemented in a frozen-phonon approach. A detailed comparison of the phonon effects on electron relaxation and emission as well as on the full ICD process is presented, which supports the previous empirical finding of ICD being the dominant decay channel in paired QDs. In addition the relative importance of phonon–phonon, phonon–electron and electron–electron interaction is analysed.     

Phonon-induced polariton superlattices

We show that the coherent interaction between microcavity polaritons and externally stimulated acoustic phonons forms a tunable polariton superlattice with a folded energy dispersion determined by the phonon population and wavelength. Under high phonon concentration, the strong confinement of the optical and excitonic polariton components in the phonon potential creates weakly coupled polariton wires with a virtually flat energy dispersion.     

Two-phonon absorption in InP and GaP

Far-infrared absorption in crystals of GaP and InP was investigated using a high-resolution Fourier-transform spectrometer. The two-phonon absorption was found to be very similar in these two materials. Van Hove singularities on the GaP spectrum were identified with the aid of calculated two-phonon sum and overtone density-of-states curves. The analysis of the InP spectrum was based on its homology to GaP. Many of the prominent features of the absorption spectra are assigned to pairs of phonons on the hexagonal face of the Brillouin zone while phonon pairs at Γ, X and L generally contribute only minor features to the spectra.     

Dynamical properties of three component Fibonacci quasicrystal

We present a real space renormalization group (RSRG) method to study the lattice dynamics of a three component Fibonacci (3CF) quasicrystal. Phonon dispersion relations corresponding to different models of this lattice are obtained. Some features of the phonon dispersion curves are compared with experiments on real quasicrystal. It is observed that the positions of the strongest Bragg peaks calculated analytically are in perfect agreement with our RSRG calculations. Received 23 October 2000 and Received in final form 11 January 2001     

Lattice dynamics in the mixed valence compounds Sm(Y)S,CeSn3 and CePd3

Renormalized phonon energies of typical mixed valence compounds such as Sm(Y)S, CeSn₃ and CePd₃ are calculated. The renormalization is caused by the additional interaction of localized 4f electrons with phonons which can lead to drastic phonon anomalies. These anomalies are found to be completely different for the various systems under consideration.     

Theory of non-Markovian dynamics in resonance fluorescence spectrum

We present a detailed theoretical study of non-Markovian dynamics in the fluorescence spectrum of a driven semiconductor quantum dot (QD), embedded in a cavity and coupled to a three-dimensional (3D) acoustic phonon reservoir. In particular, we investigate the effect of pure dephasing on one of the side-peaks of the Mollow-triplet spectrum, expressed in terms of the off-diagonal element of the reduced system operator. The QD is modeled as a two-level system with an excited state representing a single exciton, and ground state represents the absence of an exciton. Coupling to the radiative modes of the cavity is treated within usual Born–Markov approximation, whereas dot-phonon coupling is discussed within non-Markovian regime beyond Born approximation. Using an equation-of-motion technique, the dot-phonon coupling is solved exactly and found that the exact result coincides with that of obtained within Born approximation. Furthermore, a Markov approximation is carried out with respect to the phonon interaction and compared with the non-Markovian lineshape for different values of the phonon bath temperature. We have found that coupling to the phonons vanishes for a resonant pump laser. For a non-resonant pump, we have characterized the effect of dot-laser detuning and temperature of the phonon bath on the lineshape. The sideband undergoes a distinct narrowing and acquires an asymmetric shape with increasing phonon bath temperature. We have explained this behavior using a dressed-state picture of the QD levels.     

Intrinsic recombination radiation from GaP

Intrinsic recombination radiation has been detected in GaP crystals grown by vapour transport on GaAs and GaP substrates and in the form of needles, using 50 KeV electron beam excitation in the temperature range 25°K to 80°K. The three major components of the radiation are associated with the decay of free excitons with the emission of the transverse acoustic, longitudinal acoustic and transverse optic phonons which conserve momentum for transitions across the indirect energy gap. Structure has also been observed associated with the emission of two phonons, and for T > 77 >K a phonon absorption component can also be detected. In crystals grown on GaAs substrates, a no-phonon component has been observed with a threshold at the intrinsic exciton energy gap. This is thought to be associated with the presence of arsenic and there is a corresponding shift in the band gap to lower energy. close agreement is observed between the measured shapes of the emission components and those computed by the principle of detailed balance from the intrinsic edge absorption spectrum, when the broadening associated with the crystal imperfections and anomalous structure in the absorption spectrum are taken into account. The phonon energies derived from the emission spectra are in good agreement with the values determined from intrinsic edge absorption measurements.     

Coherent phonon excitation in bismuth

Ultrafast time-resolved reflectivity of a bismuth thin film evaporated on a silicon substrate is measured to investigate coherent phonons in bismuth. The reflectivity result is analyzed by a linear chirp approximation to obtain the time dependent frequencies of coherent phonons. Not only the optical modes are detected, which are generated by a combination of impulsive stimulated Raman scattering and displacive excitation of coherent phonon, acoustic phonon modes are also observed, which are emitted by the A_1g optical phonon.     

Interaction of phonons and dirac fermions on the surface of Bi2Se3: a strong Kohn anomaly

We report the first measurements of phonon dispersion curves on the (001) surface of the strong three-dimensional topological insulator Bi2Se3. The surface phonon measurements were carried out with the aid of coherent helium beam surface scattering techniques. The results reveal a prominent signature of the exotic metallic Dirac fermion quasiparticles, including a strong Kohn anomaly. The signature is manifest in a low energy isotropic convex dispersive surface phonon branch with a frequency maximum of 1.8 THz and having a V-shaped minimum at approximately 2kF that defines the Kohn anomaly. Theoretical analysis attributes this dispersive profile to the renormalization of the surface phonon excitations by the surface Dirac fermions. The contribution of the Dirac fermions to this renormalization is derived in terms of a Coulomb-type perturbation model.