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.

Unusual photocarrier and coherent phonon dynamics behaviors of layered PdSe_(2)unveiled by ultrafast spectroscopy of the edge surface

Layered materials exhibit different electronic and phonon properties along in-plane and out-of-plane directions;existing studies focus on their in-plane behaviors,and the influence of such anisotropies on the dynamics of photocarriers and phonons is unknown.Here,we fabricate layered PdSe_(2)crystals with flat edge surfaces and compare the time-resolved ultrafast spectroscopies on their basal and edge surfaces.Pronounced differences in the transient reflection spectroscopies reveal the inconsistent photocarrier and phonon dynamics behaviors on the two surfaces:the slow hot carrier relaxation process is accelerated and the thermoelasticity-induced longitudinal coherent acoustic phonon oscillation completely vanishes on the edge surface,as compared with the basal surface.Theoretical analysis reveals that the inconsistent hot carrier dynamics originate from the anisotropic properties of low-energy phonons in PdSe_(2),and the absence of phonon oscillation on the edge surface results from the wavevector-limited sensitivity of acoustic B_(1u)mode.Moreover,polarization-dependent spectroscopies indicate the diverse optical anisotropies beyond the in-plane of PdSe_(2).This work provides a new method to explore unique physical properties and modulate the optical anisotropy of layered materials.

Ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi;Te;thermoelectric films

The ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi;Te;thermoelectric films are studied through transient differential transmission spectroscopy.An ultralow frequency coherent optical phonon at 0.16 THz emerges,especially in ultrathin films,and it is ascribed to interlayer breathing modes.It can divide the ultrathin films into two groups which have outof-phase vibration along the normal of a film plane,causing a destructive interference between in-plane propagating thermal waves in the two groups of quintuple layers,and thus possibly reducing the thermal conductivity of the ultrathin films.The excitation power dependence of ultrafast dynamics reveals carrier-carrier scattering dominating thermalization,which provides a microscopic understanding of the reported high electrical conductivity and anomalously high power factor of ultrathin Bi;Te;films at room temperature.

Heat conduction of multilayer nanostructures with consideration of coherent and incoherent phonon transport

We report a theoretical investigation of coherent-to-incoherent heat conduction in multilayer nanostructures.In the coherent regime where the phonon motion is quasi-harmonic,the elastic continuum model gives accurate cross-plane thermal conductivity predictions of upper limits and demonstrates that the coherent transport is the result of the interplay between intrinsic wave effects.As the temperature or system size increases,the phonon dephasing scattering results in the deviation of thermal conductivity from the coherent-limit calculation.By further introducing the incoherence of phonons,we reproduce the classical minimum thermal conductivity,indicating the feasibility of extending the pure wave model into the wave-particle crossing regime.

Ultrafast optical probe of coherent acoustic phonons in Co2MnAl Heusler film

In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co2MnAl films which are epitaxially grown on Ga As substrates are observed and investigated as a function of film thickness, probe wavelength,external magnetic field and temperature. Our results suggest that the oscillation response at 24.5 GHz results from the coherent phonon generation in Co2MnAl film and can be explained by a propagating strain pulse model. From the probe wavelength dependent oscillation frequency, a sound velocity of（3.85±0.1）×10-3m/s at 800 nm for the epitaxial Co2MnAl film is determined at room temperature. The detected coherent acoustic phonon generation in Co2MnAl reported in this work provides a valuable reference for exploring the high-speed magnetization manipulation via magnetoelastic coupling for future spintronic devices based on Heusler alloy films.

Coherent Acoustic Phonon and Its Chirping in Dirac Semimetal Cd_3As_2

Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping（i.e.,variation of the phonon frequency）is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.

Resonant enhancement of magnetic damping driven by coherent acoustic phonons in thin Co_(2)FeAl film epitaxied on GaAs

The magnetic dynamics of a thin Co_(2)FeAl film epitaxially grown on GaAs substrate was investigated using the timeresolved magneto-optical Kerr measurement under an out-of-plane external field.The intrinsic magnetic damping constant,which should do not vary with the external magnetic field,exhibits an abnormal huge increase when the precession frequency is tuned to be resonant with that of the coherent longitudinal acoustic phonon in the Co_(2)FeAl/GaAs heterostructure.The experimental finding is suggested to result from the strong coherent energy transfer from spins to acoustic phonons via magnetoelastic effect under a resonant coupling condition,which leads to a huge energy dissipation of spins and a greatly enhanced magnetic damping in Co_(2)FeAl.Our experimental findings provide an experimental evidence of spin pumping-like effect driven by propagating acoustic phonons via magnetoelastic effect,suggesting an alternative approach to the possible long-range spin manipulation via coherent acoustic waves.

Large bandgap oscillations in two-dimensional Dion–Jacobson phase perovskites caused by coherent longitudinal acoustic phonons [Invited]

Two-dimensional Dion–Jacobson(D-J)phase perovskites are prospective photovoltaic and optoelectronic materials.To study their mechanical properties and carrier-lattice interactions,we conduct femtosecond spectroscopic experiments on the films of a D-J perovskite.After optical excitation,a∼33 meV bandgap oscillation is observed in the film by transient absorption spectroscopy.With the help of transient reflection methods,we reveal that the oscillation originates from the transport of coherent longitudinal acoustic phonons through the film.Large bandgap oscillation indicates a strong coupling between carriers and lattice,and significant bandgap modulation by strains in D-J perovskites.

Thermal conductivity of carbon nanotube superlattices:Comparative study with defective carbon nanotubes

We use molecular dynamics simulation to calculate the thermal conductivities of（5, 5） carbon nanotube superlattices（CNTSLs） and defective carbon nanotubes（DCNTs）, where CNTSLs and DCNTs have the same size. It is found that the thermal conductivity of DCNT is lower than that of CNTSL at the same concentration of Stone–Wales（SW） defects. We perform the analysis of heat current autocorrelation functions and observe the phonon coherent resonance in CNTSLs, but do not observe the same effect in DCNTs. The phonon vibrational eigen-mode analysis reveals that all modes of phonons are strongly localized by SW defects. The degree of localization of CNTSLs is lower than that of DCNTs, because the phonon coherent resonance results in the phonon tunneling effect in the longitudinal phonon mode. The results are helpful in understanding and tuning the thermal conductivity of carbon nanotubes by defect engineering.