Dynamical resonance quench and Fano interference in spontaneous Raman scattering from quasiparticle and collective excitations

Abstract

Time-resolved spontaneous Raman spectroscopy serves as a probe for incoherent quasiparticle and collective excitation dynamics, and allows to distinguish symmetry changes across a photoinduced phase transition through the inelastic light scattering selection rules. Largely unexplored is the role of the Raman resonance enhancement in the time-domain, and the transient interaction between scattering from quasiparticles and collective excitations, with the latter interaction leading to a Fano interference.In this work, we report on the observation of a phonon Raman tensor quench and Fano interference after strong photoexcitation of an intrinsic semiconductor. We observed a dynamic phonon scattering rate asymmetry and spectral asymmetry through simultaneous detection of both the anti-Stokes and Stokes response. The asymmetric phonon scattering rate is ascribed to the combined effect of the transient phonon population and the reduction of the phonon Raman tensor resulting from the photoexcited hole population. This same hole population results in a strong enhancement of the Fano spectral asymmetry.A quantum-mechanical description of the nature and transport properties of the various quasiparticles and collective excitations in semiconductors is integral to characterizing the physical properties of a wide array of technologically relevant materials. 1,2 Hot carrier and phonon relaxation dynamics in semiconductors, 3,4 for example, plays a central role in our understanding of carrier transport and energy dissipation in electronic and optoelectronic devices. As such, an accurate interpretation of the quasiparticle and collective excitation interactions and dynamics is vital not only from a fundamental perspective, but also to enable new applications.Over the past four decades, a range of ultrafast spectroscopic probes have provided us insight into carrier and phonon relaxation dynamics and their respective timescales in semiconductors. 4,5 These measurements often rely on the coherent excitation of phonons, of which the dynamics is consecutively probed in the modulation of the refractive index. 6-8 A more direct view of phonon dynamics is provided by time-resolved inelastic scattering techniques. 9-12 This has the advantage over coherent excitation techniques that one can follow the true time evolution of a photoexcited material through its incoherent response, rather than the time evolution of coherent excitations.Time-resolved spontaneous Raman scattering is a reasonably well-established probe for incohorent phonon population dynamics in semiconductors, 11,13-18 and vibrational dynamics in molecules. 11,[19][20][21][22][23][24][25] More recently the technique has been applied to study phonon dynamics in a range of carbon allotropes. 11,26-32 Through detailed balance of the anti-Stokes I AS to Stokes I S scattering intensity ratio the (phonon) population number n can be directly quantified by I AS /I S ∝ n/(n+1). This relation stems from the fluctuation dissipation theorem, 33 which on ultrafast timescales ( ps) may los...

Unraveling the Excitonic Transition and Associated Dynamics in Confined Long Linear Carbon Chains with Time‐Resolved Resonance Raman Scattering

“…A few representative relaxation dynamic curves are presented in Figure a, in which all curves are normalized to the pre‐zero Raman signal and to the pump excitation photon density. The maximum reduced signal amplitude (around 1 ps) is converted into the ground state bleaching ratio by taking the changes of Raman response into account as described in previous reports, [ 16e,25 ] and is presented in Figure 4b. More decay curves at specific excitation energies are presented in Figure S5 in the Supporting Information.…”

“…Time-resolved Raman scattering spectroscopy: Details of the time-resolved resonance Raman scattering spectroscopy method have been described elsewhere. [21,26] Briefly, an integrated ultrafast laser system (Light Conversion PHAROS) with two outputs of the fundamental 1030 14 nm pulses (compressed 300 fs and chirped 150 ps) pumps two optical parametric amplifiers (Light Conversion) to generate wavelength tunable laser pulses for selective excitation (~300 fs) and narrow-bandwidth laser pulses for Raman probing (~1.5 ps), respectively. The widely tunable pump pulse allows to cover a photon energy range from the near ultraviolet to the near infrared.…”