First-principles study of excitonic effects in Raman intensities

Abstract

The ab initio prediction of Raman intensities for bulk solids usually relies on the hypothesis that the frequency of the incident laser light is much smaller than the band gap. However, when the photon frequency is a sizeable fraction of the energy gap, or higher, resonance effects appear. In the case of silicon, when excitonic effects are neglected, the response of the solid to light increases by nearly three orders of magnitude in the range of frequencies between the static limit and the gap. When excitonic effects are taken into account, an additional tenfold increase in the intensity is observed. We include these effects using a finite-difference scheme applied on the dielectric function obtained by solving the Bethe-Salpeter equation. Our results for the Raman susceptibility of silicon show stronger agreement with experimental data compared with previous theoretical studies. For the sampling of the Brillouin zone, a double-grid technique is proposed, resulting in a significant reduction in computational effort.Comment: 10 pages, 10 figures, accepted by Physical Review B for publication (http://prb.aps.org/accepted/04072OdeQ2b16026a41081b31f5872037b42287ba

Strongly Coupled Coherent Phonons in Single-Layer MoS2

“…As for the detection process, the data reveal a resonant behavior and show that the amplitude of the A ′ 1 oscillations has a maximum at the C exciton, decreasing at higher energy. The continuous line in Fig.7b is the calculated amplitude of the Raman tensor α(ω) for the A ′ dielectric susceptibility χ with respect to the nuclear coordinates (α = ∂χ/∂R) and it is computed in the framework of finite-differences of the dielectric function [66], also including excitonic effects [57]. Similar to the CP amplitude, α has a resonant profile peaked around the C exciton.…”

Strong exciton regulation of Raman scattering in monolayer MoS2

“…The resonant Raman intensity of silicon amplified by excitonic effects (compared with the independent quasiparticle case) in Ref. [19] is presumably attributed to this mechanism, given the abundance of parallel bands in silicon [51]. As a consequence of the general validity of the three-band dominance demonstrated here, resonant Raman measurements can directly probe how excitons undergo inter -state scattering by phonons, which affects exciton population dynamics and lifetimes [52].…”

“…Despite the rich experimental data on Raman measurements of 2D solids, the role of excitons on Raman spectra is rarely modeled at a first-principles level beyond calculating shifted resonance energies, because of the high computational cost of many-body perturbation theory calculations and the sparsity of implementations that consolidate electron-phonon and many-body phenomena. One recent important theoretical advance [19] (implemented in [20,21]) used finite differences through solution of the Bethe-Salpeter equation (BSE) on the quasiparticle (GW) band structure, but employed a quasistatic Placzek approximation that is only valid in the non-resonant regime. Here we follow the generalized approach of Ref.…”

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

“…48 In addition to this, excitonic effects are thought to play a role as well. 49 The pump laser pulse optically induces a hole density in the valence band, as evidenced by the observed changes in the hole continuum scattering (Fig. 4d).…”

“…46 The optically induced charge density could lead to changes in the excitonic effects on the Raman resonance discussed in Ref. 49, which would lead to a strongly probe pulse energy dependent change in χ 2 . However, our experiments show that the optically induced changes in the Raman spectra do not depend on probe energy within our experimental accuracy.…”