Coherent Lattice Oscillations Research Articles

Orbitally driven insulator-metal transition in CuIr2S4 : Temperature-dependent transient reflectivity study

Ultrafast transient reflectivity across the unusual three-dimensional Peierls-like insulator-metal (IM) transition in CuIr_{2}S_{4} was measured as a function of temperature. The low-temperature insulating-phase transient response is dominated by broken-symmetry-induced coherent lattice oscillations that abruptly vanish at the IM transition. The coherent mode spectra are consistent with Raman spectra reported in literature. The origin of the broken-symmetry-induced is also briefly discussed.

Light control of surface\u2013bulk coupling by terahertz vibrational coherence in a topological insulator

The demand for disorder-tolerant quantum logic and spin electronics can be met by generating and controlling dissipationless spin currents protected by topology. Dirac fermions with helical spin-locking surface transport offer a way of achieving such a goal. Yet, surface-bulk coupling can lead to strong Dirac electron scattering with bulk carriers and phonons as well as impurities, assisted by such dissipative channel, which results in “topological breakdown”. Here, we demonstrate that coherent lattice vibrations periodically driven by a single-cycle terahertz (THz) pulse can significantly suppress such dissipative channel in topological insulators. This is achieved by reducing the phase space in the bulk available for Dirac fermion scattering into during coherent lattice oscillations in Bi2Se3. This light-induced suppression manifests as a remarkable transition exclusively in surface transport, absent for bulk, above the THz electric fields for driving coherent phonons, which prolongs the surface transport lifetime. These results, together with simulations, identify the critical role of spin–orbit coupling for the “phase space contraction” mechanism that suppresses the surface-bulk coupling. Imposing vibrational quantum coherence into topological states of matter may become a universal light control principle for reinforcing the symmetry-protected helical transport.

Femtosecond electron imaging of defect-modulated phonon dynamics.

Precise manipulation and control of coherent lattice oscillations via nanostructuring and phonon-wave interference has the potential to significantly impact a broad array of technologies and research areas. Resolving the dynamics of individual phonons in defect-laden materials presents an enormous challenge, however, owing to the interdependent nanoscale and ultrafast spatiotemporal scales. Here we report direct, real-space imaging of the emergence and evolution of acoustic phonons at individual defects in crystalline WSe2 and Ge. Via bright-field imaging with an ultrafast electron microscope, we are able to image the sub-picosecond nucleation and the launch of wavefronts at step edges and resolve dispersion behaviours during propagation and scattering. We discover that the appearance of speed-of-sound (for example, 6 nm ps−1) wavefronts are influenced by spatially varying nanoscale strain fields, taking on the appearance of static bend contours during propagation. These observations provide unprecedented insight into the roles played by individual atomic and nanoscale features on acoustic-phonon dynamics.

Shape- and Symmetry-Dependent Mechanical Properties of Metallic Gold and Silver on the Nanoscale

The mechanical properties of anisotropic nanoparticles like gold nanorods (AuNRs) and silver nanorods (AgNRs) are different from those of isotropic shapes such as nanospheres. We probed the coherent lattice oscillations of nanoparticles by following the modulation of the plasmonic band frequency using ultrafast laser spectroscopy. We found that while the frequency of the longitudinal vibration mode of AgNRs is higher than that of AuNRs of similar dimensions, similarly sized gold and silver nanospheres have similar lattice vibration frequencies. Lattice vibrations calculated by finite element modeling showed good agreement with the experimental results for both AgNRs and AuNRs. The accuracy of the calculations was improved by using actual pentagonal shapes rather than cylinders that did not agree well with the experimental results. As the plasmon energy is transferred into lattice vibrations, the temperature of the nanoparticle necessarily increases as a result of this electron-phonon relaxation process. This results in a decrease in the Young's modulus that was accounted for in the calculations. Calculations showed that the tips of the nanorods are "softer" than the rest of the nanorod. Because the tips comprise a larger portion of the overall rod in the smaller rods, the smaller rods were more affected by the tip effects.

D27 コヒーレントフォノン励起面におけるパルストレインビーム照射応答の時間分解計測に関する研究(OS5 加工計測・評価(1))

Femtosecond pulse processing enables fine processing without the thermal diffusion. In this paper we generate femtosecond pulse train beam, which has several femtosecond pulse in a few picosecond, and investigate the processing result using femtosecond pulse train beam. It is considered the coherent phonon, which is the forced lattice oscillation due to the surface electron vibration in the high electric field, is excited on the surface in the case of irradiation both the femtosecond pulse train and femtosecond single pulse. In this paper, we measure for the coherent lattice oscillation using femtosecond pulse train

Transient Spin Density Wave Order Induced in the Normal State of BaFe2As2by Coherent Lattice Oscillations

We trace the ultrafast dynamics of the spin density wave gap of the pnictide system BaFe2As2 probing resonantly with broadband multi-terahertz pulses. The photoexcitation in the low-temperature ground state leads to a fast suppression of the spin order followed by a slower recovery process. Surprisingly, in the normal state, we observe periodic oscillations of the spin density wave feature at a frequency as high as 5.5 THz. Our results indicate a transient development of a macroscopic order driven by a coherent lattice oscillation and attest to a pronounced spin–phonon coupling in pnictides.

Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations

The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity1,2,3. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields2 or pressure3. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order4,5,6. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe2As2 following excitation with a femtosecond optical pulse. When starting in the low-temperature ground state, optical excitation results in a melting of the SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced when we excite the normal state above the transition temperature. Very surprisingly, the transient ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a pronounced spin–phonon coupling in pnictides that supports rapid development of a macroscopic order on small vibrational displacement even without breaking the symmetry of the crystal.

Ultrafast dynamical response of strongly correlated oxides: role of coherent optical and acoustic oscillations

We describe some general features in the transient behaviour of strongly correlated transition metal oxides, following ultrafast excitation by a femtosecond laser pulse. Our analysis is based on time-resolved reflectivity measurements on (V1−x Cr x )2O3, a prototype Mott–Hubbard material, and the manganite compound (La0.67Sr0.33)MnO3, performed over a time window of several tenths of ps. We point out the contribution of coherent lattice oscillations–both optical and acoustic–to the overall signal, and show how they can depend on the crystallographic orientation of the material with respect to the laser beam wavevector and polarisation. In particular, an acoustic wave is always found to be present in our measurements, and we show that its oscillating behaviour is superposed to the average time evolution of the material, which can be instead related in different ways to its electronic properties and thermodynamic phase. All these effects are to be taken into account in any further analysis leading to the estimation of relevant physical parameters for correlated materials, and they appear to play a significant role not only for reflectivity techniques, but also in other kinds of ultrafast pump–probe experiments.

Observation of a coherent optical phonon in the iron pnictide superconductorBa(Fe1−xCox)2As2(x=0.06and 0.08)

We report the observation of a coherent lattice oscillation in a pnictide superconductor. A coherent fully symmetric optical phonon was detected in $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ ($x=0.06$ and $x=0.08$) using time-resolved pump-probe reflectivity with 40 fs time resolution. The analysis of the phonon parameters for various excitation fluences reveals no evident difference below and above the critical temperature, suggesting that the ${A}_{1g}$ mode is not involved in the superconducting phase transition.

Novel Optical Properties and Emerging Applications of Metal Nanostructures

This paper provides a brief overview of recent research activities concerning metal nanomaterials, including their synthesis, structure, surface plasmon absorption, surface enhanced Raman scattering (SERS), electron dynamics, emerging applications, and the historical context by which to view these subjects. We emphasize coinage metals, particularly silver and gold. Silver and gold nanostructures exhibit fascinating optical properties due to their strong optical absorption in the visible as a result of the collective oscillation of conduction band electrons, known as the surface plasmon. This is the origin of many interesting physical phenomena and related applications such as surface plasmon resonance (SPR) and SERS useful in chemical and biomedical detection and analysis. SERS offers high sensitivity and molecular specificity that are attractive for sensing and imaging applications. Electron dynamics in metal nanostructures have been studied using ultrafast laser techniques to gain fundamental insight into electron−phonon interaction as well as coherent lattice oscillation in different metal nanostructures. Relevant theoretical work and models are also discussed in conjunction with the related experimental work. Synthesis and structural characterization are also discussed to make this paper self-contained and easier to follow. The paper ends with some emerging applications of optical properties of metal nanomaterials including photothermal therapy for cancer.

Gigahertz Optical Modulation Resulting from Coherent Lattice Oscillations Induced by Femtosecond Laser Pumping of 2D Photonic Crystals of Gold‐Capped Polystyrene Microspheres

A gigahertz all-optical modulation technique, based on a mechanism in which the modulation of the transmitted light is caused by the coherent oscillation of the phonon modes of gold caps on periodic polystyrene sphere monolayer arrays, is demonstrated. The modulation frequency can be tuned by changing the size of the polystyrene spheres.

Photothermally excited coherent lattice phonon oscillations in plasmonic nanoparticles

The photothermal property of (Ag and Au) plasmonic nanoparticles has brought about many important discoveries and applications in nanoscience and nanotechnology. In this review, we briefly summarize a photothermal effect, the coherent phonon oscillation, of plasmonic nanoparticles irradiated with ultrafast laser pulses of low power density. The coherent phonon oscillation is created in the nanoparticle by the ultrafast impulsive photothermal heating. The effects of size, shape, thickness, and interparticle interaction on the period of coherent phonon oscillations are discussed. The detection of the coherent lattice oscillation of metallic nanoparticles provides a powerful tool to characterize the mechanical and structural properties of nanostructures.

Ultrafast time-resolved spectroscopy of the spin-Peierls compound CuGeO 3

Coherent phonons and electronic response of the spin-Peierls compound CuGeO 3 are investigated at room temperature using time-resolved transmission measurement. Four A g modes are observed strongly in the E ∥ b polarization, consistent with the Raman spectra. Coherent lattice oscillation with the highest frequency of 860 cm −1 is kicked on with an initial phase of −(0.5±0.2) π radian and decays with a time constant of about 0.9 ps, indicating the ground-state phonon is induced by the process of impulsive stimulated Raman scattering (ISRS). The decay-time constants are rather long compared to the values expected from the line width of the stationary Raman spectrum. In addition, instantaneous photo-induced absorption is observed in the E ∥ b polarization at delay-time zero, whose temporal width is close to that of the cross-correlation between pump and probe, attributed to the two-photon absorption of the pump and the probe photons. The intensity of the negative peak-like response at delay-time zero increases in the E ∥ c polarization, and is ascribed to a cascade excitation via the Zhang–Rice (ZR)-like exciton. This absorption decays rapidly within 100 fs, indicating that the ZR excitons experience extremely fast relaxation after photoexcitation.

The Effect of Plasmon Field on the Coherent Lattice Phonon Oscillation in Electron-Beam Fabricated Gold Nanoparticle Pairs

By using electron beam lithography, we fabricated pairs of gold nanoparticles with varying interparticle separation. Double-beam femtosecond transient absorption spectroscopy was used to determine the coherent lattice oscillation frequency as a function of the interparticle separation in the presence of the plasmon field excited by the monitoring probe light. We found that the fractional shift in the coherent lattice phonon oscillation frequency follows an exponential decay with respect to the interparticle gap scaled by the disc diameter with the same decay constant as that previously observed for the fractional shift in the surface plasmon electronic oscillation resonance frequency. This strongly suggests that it is the near-field coupling between the particles that shifts both the coherent electronic oscillation (plasmon) frequency and the coherent lattice oscillation (phonon) frequency. The similar trend in the effect of interparticle coupling on the plasmon frequency and the phonon frequency is essentially a reflection of the universal scaling behavior of the distance decay of the interparticle plasmonic near-field. It is shown that the observed decrease in the lattice oscillation frequency with decrease in the interparticle distance is the result of a reduction in the effective free electron density within each nanoparticle pair partner as a result of the polarizing perturbation of the plasmonic field of the other nanoparticle in the pair.

Temperature dependence of coherent A1g and Eg phonons of bismuth

Bismuth has been a model material in the study of femtosecond dynamics of coherent lattice oscillations. The generation mechanism was first proposed to be displacive for the symmetric A1g mode, which was the only mode observed as a coherent phonon. The absence of the other Raman active mode Eg has not been fully explained, but was phenomenologically attributed to the exclusive coupling of the hot electrons at k∼0 and high symmetry phonons. In the present study, we demonstrate that both A1g and Eg modes are excited as coherent phonons at low temperature and confirm that the coherent phonons are generated via a Raman process in bismuth. We found a puzzling π∕2 difference in the initial phases of the two coherent phonons, which suggests that the initial phase cannot be a clear-cut index for the generation mechanism in absorbing media.

Theory of Photoinduced Phase Transitions

Theories of photoinduced phase transitions have developed along with the progress in experimental studies, especially concerning their nonlinear characters and transition dynamics. At an early stage, paths from photoinduced local structural distortions to global ones are explained in classical statistical models. Their dynamics are governed by transition probabilities and inevitably stochastic, but they were sufficient to describe coarse-grained time evolutions. Recently, however, a variety of dynamics including ultrafast ones are observed in different electronic states. They are explained in relevant electronic models. In particular, a coherent lattice oscillation and coherent motion of a macroscopic domain boundary need appropriate interactions among electrons and lattice displacements. Furthermore, some transitions proceed almost in one direction, which can be explained by considering relevant electronic processes. We describe the history of theories of photoinduced phase transitions and discuss a future perspective.

The Optically Detected Coherent Lattice Oscillations in Silver and Gold Monolayer Periodic Nanoprism Arrays: The Effect of Interparticle Coupling

Using femtosecond transient spectroscopy, we studied the optically detected laser-induced coherent phonon oscillation of monolayers of periodic arrays of prismatic-shaped silver and gold nanoparticles, assembled by using the technique of nanosphere lithography. In this method, the same size of polystyrene sphere and the same vacuum conditions are used. Under these circumstances, the gold nanoprisms formed are found to have sharper tips than the corresponding silver nanoprisms. For both gold and silver nanoparticles, the surface plasmon absorption maximum is found to depend linearly on size. The coherent lattice oscillation periods are also found to depend linearly on size. However, although the observed dependence for the silver nanoparticle is found to follow the calculated dependence of a single particle on size (based on a one-dimensional standing wave model), the gold nanoparticle deviates from this model, and the deviation is found to increase with the size of the nanoparticles. This deviation can be explained by considering interparticle coupling. A simple interparticle lattice oscillating dipolar coupling model of the dimer is found to qualitatively account for both the sign and the size dependence of the deviation. The absence of this deviation in the silver nanoparticle arrays is blamed on the weak interparticle coupling due to their rounded tips and the possibility of oxidation of their surfaces.

Coherent Phonons in Semimetals and Semiconductors-Effect of Irradiation-induced Defects on Relaxation Dynamics-

Coherent lattice oscillation can be excited in semimetals and semiconductors with femtosecond optical pulses. The dephasing of the coherent phonons, typically in the picosecond time scale, is dominated by the interaction with photo-excited carriers, incoherent (thermal) phonons, and defects such as impurities and vacancies. Here we present our femtosecond pump-probe study on the relaxation dynamics of the coherent phonons and the photo-excited carriers in ion-irradiated semimetals and semiconductors. The dephasing rate of the coherent optical phonon in bismuth is increased linearly with increasing ion dose due to the scattering by irradiation-induced defects. The dose dependence of the relaxation of the coherent acoustic phonon of graphite is quantitatively explained by a simple model in which a propagating wave is scattered by single vacancies, whereas that of the optical phonon by a modified mass-defect scattering model. In GaAs, the relaxation dynamics of LO phonon-plasmon coupled modes and photo-excited carriers provide quantitative information of carrier trapping due to vacancies.

Impulsive excitation of coherent phonons observed in reflection in bismuth and antimony

We report time domain observations of coherent lattice vibrations in bismuth and antimony. Phonons are impulsively generated, and detected through reflectivity modulation with 70 fs pulses of laser light at 1.98 eV. With this technique, we demonstrate that coherent lattice oscillations can be studied by reflection in opaque materials, but with selection rules which may differ from conventional impulsive stimulated Raman scattering.