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Excited carrier dynamics in plasmonic nanostructures determines many important optical properties such as nonlinear optical response and photocatalytic activity. Here it is shown that mesoscopic plasmonic covellite nanocrystals with low free‐carrier concentration exhibit a much faster carrier relaxation than in traditional plasmonic materials. A nonequilibrium hot‐carrier population thermalizes within first 20 fs after photoexcitation. A decreased thermalization time in nanocrystals compared to a bulk covellite is consistent with the reduced Coulomb screening in ultrathin films. The subsequent relaxation of thermalized, equilibrium electron gas is faster than in traditional plasmonic metals due to the lower carrier concentration and agrees well with that in a bulk covellite showing no evidence of quantum confinement or hot‐hole trapping at the surface states. The excitation of coherent optical phonon modes in a covellite is also demonstrated, revealing coherent lattice dynamics in plasmonic materials, which until now was mainly limited to dielectrics, semiconductors, and semimetals. These findings show advantages of this new mesoscopic plasmonic material for active control of optical processes. [ABSTRACT FROM AUTHOR]

Hybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the Ag vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead–iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales. [ABSTRACT FROM AUTHOR]

We theoretically investigate the behavior of coherent optical phonons in an electron–phonon coupled system at moderate laser intensity: below GW/cm 2. We calculate the coherent phonons from impulsive stimulated Raman scattering (ISRS) and impulsive absorption (IA) mechanisms with a nonperturbative model. The generation efficiency of coherent phonons in single-pulse excitation exhibits gradual nonlinear saturation with increasing optical intensity in a nonperturbative regime. The ISRS mechanism exhibits substantial saturation at relatively low pulse intensity, whereas the IA mechanism remains roughly linear. We further examine the influence of double-pulse excitation on quantum path interference. The ISRS mechanism exhibits a pronounced saturation effect near 0 fs of pump–pump delay, whereas the IA process exhibits negligible saturation behavior. These results reproduce well experimental data obtained by double-pulse excitation and support our proposal that ISRS is the primary quantum path for the observed coherent phonons in gallium arsenide. • Theoretical study of the influence of pulse intensity on coherent optical phonons. • Coherent phonons in ISRS exhibit nonlinear saturation with optical pulse intensity. • Double-pulse excitation in GaAs proves definitely coherent phonon mechanism is ISRS. [ABSTRACT FROM AUTHOR]

The generation of coherent phonons is theoretically studied using a thermal equilibrium phonon distribution as the initial condition. The time evolution of the coherent phonon amplitude and phonon population is calculated for 4.5-THz phonons with initial phonon distributions at 10 and 300 K. It is determined that the coherent-phonon amplitudes of state-selected phonons are dependent on the initial phonon distribution. The coherent-phonon amplitudes, when considering all phonon states instead of selecting certain phonon levels, are almost the same for both initial distributions. The variance of the coherent-phonon amplitudes is found to oscillate with twice the phonon frequency, and its oscillation amplitude depends on the initial phonon distribution. • Theoretical study of the influence of initial phonon states for coherent phonons. • Selected-state coherent phonons were not proportional to ratio of initial states. • Variance of coherent phonons oscillated with twice the phonon frequency. [ABSTRACT FROM AUTHOR]

The phononic interference of optical phonons in diamond is studied via transient transmittance change measurements using the double pump–probe protocol with polarized pump pulses. With orthogonal-polarized pulses, the phonon amplitude is enhanced/suppressed if the second pump pulse is irradiated by a half-integer/integer multiple of the phonon period. This is contrary to the usual phononic interference (enhancement at integer multiples of the phonon period) observed when using parallel-polarized pulses. These observed results are well explained using a quantum-mechanical model in calculations involving the Raman tensor of the diamond lattice structure. The amplitude of the optical phonons generated via the off-diagonal term of Raman tensor is proportional to the product of the x - and y − components of the optical electric field. The phase of the phonon oscillations is dependent on the angle between the crystal orientation and the polarization of the pump pulse. • Optical phonons in diamond were coherently controlled using polarized optical pulses. • Phononic interferences under orthogonal and parallel polarized pulses were reversed. • Detected phononic interferences were well explained with the quantum model. [ABSTRACT FROM AUTHOR]

Excitonic effects underpin the fascinating optoelectronic properties of 2D perovskites that are highly favorable for photovoltaics and light-emitting devices. Analogous to switching in transistors, manipulating these excitonic properties in 2D perovskites using coherent phonons could unlock new applications. Presently, a detailed understanding of this underlying mechanism remains modest. Herein, the origins of the carrier-phonon coupling in 2D perovskites using transient absorption (TA) spectroscopy are explicated. The exciton fine structure is modulated by coherent optical phonons dominated by the vibrational motion of the PbI 6 octahedra via deformation potential. Originating from impulsive stimulated Raman scattering, these coherent vibrations manifest as oscillations in the TA spectrum comprising of the generation and detection processes of coherent phonons. This two-step process leads to a unique pump- and probe-energy dependence of the phonon modulation determined by the imaginary part of the refractive index and its derivative, respectively. The phonon frequency and lattice displacement of the inorganic octahedra are highly dependent on the organic cation. This study injects fresh insights into the exciton-phonon coupling of 2D perovskites relevant for emergent optoelectronics development., (© 2021 Wiley-VCH GmbH.)