Coherent Lattice Oscillations in Solids and Their Optical Control

Revealing a distortive polar order buried in the Fermi sea

Polar metals are challenging to identify spectroscopically because the fingerprints of electric polarization are often obscured by the presence of screening charges. Here, we unravel unambiguous signatures of a distortive polar order buried in the Fermi sea by probing the nonlinear optical response of materials driven by tailored terahertz fields. We apply this strategy to investigate the topological crystalline insulator Pb1-xSnxTe, tracking its soft phonon mode in the time domain and observing the occurrence of inversion symmetry breaking as a function of temperature. By combining measurements across the material's phase diagram with ab initio calculations, we demonstrate the generality of our approach. These results highlight the potential of terahertz driving fields to reveal polar orders coexisting with itinerant electrons, thus opening additional avenues for material discovery.

Optically Induced Coherent Phonons in Bismuth Oxyiodide (BiOI) Nanoplatelets

Bismuth oxyiodide (BiOI) is a promising material for photocatalysis combining intriguing optical and structural properties. We show that excitation by a femtosecond laser pulse creates coherent phonons inducing a time-variant oscillating modulation of the optical density. We find that the two underlying frequencies originate from lattice vibrations along the [001] crystallographic axis, the stacking direction of oppositely charged layers in BiOI. This is consistent with a subpicosecond charge separation driven by a built-in dipolar field. This partially screens the field, launching coherent phonons. Further, we determine the two major dephasing mechanisms that lead to the loss of vibronic coherence: (i) the anharmonic decay of an optical phonon into two acoustic phonons and (ii) phonon-carrier scattering. Our results provide a direct demonstration of the presence of an electric field in BiOI along the [001] axis and show its role in efficient charge separation that is crucial for photocatalytic applications of BiOI.

Femtosecond control of phonon dynamics near a magnetic order critical point

The spin-phonon interaction in spin density wave (SDW) systems often determines the free energy landscape that drives the evolution of the system. When a passing energy flux, such as photoexcitation, drives a crystalline system far from equilibrium, the resulting lattice displacement generates transient vibrational states. Manipulating intermediate vibrational states in the vicinity of the critical point, where the SDW order parameter changes dramatically, would then allow dynamical control over functional properties. Here we combine double photoexcitation with an X-ray free-electron laser (XFEL) probe to control and detect the lifetime and magnitude of the intermediate vibrational state near the critical point of the SDW in chromium. We apply Landau theory to identify the mechanism of control as a repeated partial quench and sub picosecond recovery of the SDW. Our results showcase the capabilities to influence and monitor quantum states by combining multiple optical photoexcitations with an XFEL probe. They open new avenues for manipulating and researching the behaviour of photoexcited states in charge and spin order systems near the critical point.

Precise control of vibrational states coupled to electronic degrees of freedom could enable control over charge or magnetic order in a material. Here, the authors use a double-pulse photoexcitation combined with an X-ray probe to control vibrational states near the critical point of spin density wave in Cr films.

Coherent acoustic phonon dynamics in chiral copolymers

Coherent phonon oscillations in the UV-Vis transient absorption and circular dichroism response of two chiral polyfluorene-based copolymer thin films are investigated. A slow oscillation in the hundred picosecond regime indicates the propagation of a longitudinal acoustic phonon with a frequency in the gigahertz range through cholesteric films of PFPh and PFBT, which allow for the optical determination of the longitudinal sound velocity in these polymers, with values of (2550 ± 140) and (2490 ± 150) m s^-1, respectively. The oscillation is induced by a strain wave, resulting in a pressure-induced periodic shift of the electronic absorption bands, as extracted from a Fourier analysis of the transient spectra. The acoustic phonon oscillation is also clearly detected in the transient circular dichroism (TrCD) response of PFPh, indicating a transient pressure-induced shift of the CD spectrum and possibly also phonon-induced chirality changes via pitch length modulation of the cholesteric helical polymer stack.

Pronounced exciton and coherent phonon dynamics in BiI3

Long-lived coherent oscillations in transient absorption signals of BiI₃ suggest strong electron–phonon coupling in this material for photovoltaics.

Phonon mode of TiO2 coupled with the electron transfer from N3 dye

Low frequency vibrational spectra of submonolayer N3 dye (Ru(4,4(')-dicarboxy-2,2(')-bipyridine)2(NCS)2) adsorbed on TiO2 (110) were reported by using fourth-order coherent Raman spectroscopy, which is interface-sensitive vibrational spectroscopy. Most of the peaks observed in the experiment were at the same frequency as that of Raman and infrared spectra of the dye and TiO2. Two interfacial modes at 640 and 100 cm(-1) and one resonantly enhanced phonon at 146 cm(-1) appeared in addition to the pure TiO2 and N3 spectra. Adsorption of N3 dye on TiO2 contributed to the enhancement of 100 and 146 cm(-1) mode. The results not only reported interfacial low-frequency vibrations of TiO2 (110) with N3 dye adsorption but also suggested the coupling between the surface vibrations of TiO2 and charge transfer between N3 dye and TiO2 on the surface.

Theory of coherent phonons in carbon nanotubes and graphene nanoribbons

We survey our recent theoretical studies on the generation and detection of coherent radial breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial breathing like mode (RBLM) phonons in graphene nanoribbons. We present a microscopic theory for the electronic states, phonon modes, optical matrix elements and electron-phonon interaction matrix elements that allows us to calculate the coherent phonon spectrum. An extended tight-binding (ETB) model has been used for the electronic structure and a valence force field (VFF) model has been used for the phonon modes. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on the photoexcited carrier density. We discuss the dependence of the coherent phonon spectrum on the nanotube chirality and type, and also on the graphene nanoribbon mod number and class (armchair versus zigzag). We compare these results with a simpler effective mass theory where reasonable agreement with the main features of the coherent phonon spectrum is found. In particular, the effective mass theory helps us to understand the initial phase of the coherent phonon oscillations for a given nanotube chirality and type. We compare these results to two different experiments for nanotubes: (i) micelle suspended tubes and (ii) aligned nanotube films. In the case of graphene nanoribbons, there are no experimental observations to date. We also discuss, based on the evaluation of the electron-phonon interaction matrix elements, the initial phase of the coherent phonon amplitude and its dependence on the chirality and type. Finally, we discuss previously unpublished results for coherent phonon amplitudes in zigzag nanoribbons obtained using an effective mass theory.