Light-Enhanced Electron-Phonon Coupling in Photoexcited Graphene

Electron-phonon coupling (EPC) is a fundamental interaction governing energy and charge transport in materials. Understanding and controlling EPC is crucial for various applications, especially in the context of ultrafast electronics and optoelectronics. Graphene, a two-dimensional material with unique electronic properties, has emerged as a promising platform for exploring EPC. Previous studies have investigated EPC in graphene, but a comprehensive understanding of its dynamics, particularly under non-equilibrium conditions induced by photoexcitation, remains elusive. This research addresses the lack of understanding of EPC dynamics under photoexcitation by exploring the interplay between photocarriers and phonons using advanced computational techniques and comparing these findings with experimental data. The primary goal is to unveil the mechanism behind the light-enhanced EPC and to provide a method for tracking EPC dynamics in real-time. This knowledge is vital for the development of graphene-based devices and for expanding the understanding of EPC in various other quantum materials.

The literature on EPC in graphene is extensive, covering both theoretical and experimental investigations. Early studies using Raman spectroscopy revealed the existence of Kohn anomalies and anomalous phonon softening due to EPC [refs 35, 36, 73-82]. Theoretical work, utilizing Green's function theory and other approaches, has explored the energy relaxation of hot Dirac fermions [refs 9, 83, 84]. Experimental measurements using various techniques, including time-resolved ARPES and ultrafast optical spectroscopy, have provided insights into carrier dynamics and energy dissipation in graphene [refs 39-51]. However, previous work largely focuses on equilibrium conditions or provides limited information on the real-time dynamics of EPC under photoexcitation. The current study attempts to address these gaps by combining advanced TDDFT simulations with experimental findings to provide a more complete picture of EPC in photoexcited graphene.

The authors employed time-dependent ab initio calculations using the TDAP package implemented in SIESTA. This package is specifically designed to describe photoexcitation dynamics in solids [refs 5, 100, 101]. Numerical atomic orbitals with double zeta polarization were used as the basis set, with a k-point sampling of 144x144x1 in the Brillouin zone. Electron-nuclear interactions were described by norm-conserving pseudopotentials within the local-density approximation (LDA). A Gaussian laser pulse with a wavelength of 800 nm (ħω = 1.55 eV) was used to simulate photoexcitation. The pulse parameters, such as peak location (t0 = 20 fs) and full width at half maximum (FWHM = 7 fs), were chosen to mimic experimental conditions. Phonon dynamics were incorporated by introducing a weak initial stretch of the carbon-carbon bond. The coherent E2g phonon (ħωph ≈ 0.2 eV), being the dominant optical phonon in graphene [refs 70, 102], was specifically considered. The energy transport rate P(t) was calculated to characterize the energy transfer between photocarriers and phonons. The EPC strength λ was extracted from P(t) using a generic expression derived by Tse et al. [refs 9, 83, 84], which relates macroscopic energy transport to microscopic EPC parameters. The authors also analyzed the carrier resolved EPC by examining the population distribution of excited carriers and their contributions to λ at different energy ranges and doping concentrations. This approach allowed for the separation of contributions from low-energy carriers excited by phonons and high-energy carriers excited by photons. Real-time tracking of EPC dynamics was achieved by analyzing the ultrafast carrier relaxation process.

The study's key findings include: 1. **Significant Light-Enhanced EPC:** Laser illumination dramatically enhances the EPC strength in graphene, approximately threefold compared to the ground state. This enhancement is strongly dependent on the non-equilibrium distribution of photoexcited carriers, highlighting the impact of light on carrier-phonon interactions. 2. **Doping Concentration Dependence:** Both the energy transport rate and EPC strength show a non-monotonic dependence on doping concentration, exhibiting a logarithmic singularity near the half-phonon frequency. This behavior is attributed to phonon-induced interband transitions and Pauli blocking effects. 3. **Mechanism of Light-Enhanced EPC:** Photoexcitation creates a non-thermal distribution of photocarriers, predominantly populating high-energy levels. These non-equilibrium carriers undergo phonon-assisted intraband transitions (PAITs), providing additional EPC channels. PAITs, arising from electronic broadening and scattering between different k points, account for approximately 80% of the enhanced EPC, highlighting their crucial role in light-enhanced EPC. 4. **Real-time Tracking of EPC:** The authors successfully tracked the ultrafast EPC dynamics during carrier relaxation, demonstrating that the EPC strength is strongly dependent on the instantaneous carrier distribution. The time evolution of the EPC strength shows excellent agreement with results from experimental measurements using time-resolved ARPES [refs 42, 44], demonstrating the accuracy and reliability of the theoretical approach. 5. **Parameter Dependence:** The EPC strength is also shown to be proportional to the distribution of photocarriers and is further investigated under variations of pump fluence, pulse width and wavelength. The findings provide direct evidence that the photocarrier distribution is pivotal in determining the light-induced EPC enhancement.

The findings address the research question by demonstrating the significant enhancement of EPC in graphene under photoexcitation and elucidating the underlying mechanism. The significant threefold increase in EPC strength under illumination is a substantial result, implying the potential for manipulating EPC through optical control. The non-monotonic dependence of EPC on doping concentration adds to the understanding of how charge carrier density affects carrier-phonon interactions. The identification of PAITs as the dominant contribution to light-enhanced EPC provides a crucial insight into the microscopic mechanisms involved. The successful real-time tracking of EPC dynamics further solidifies the understanding of how photocarrier distribution determines the strength of EPC. These findings are relevant to the broader field of ultrafast optoelectronics and materials science by showing how light can be used to control EPC, leading to novel device functionalities. This research could influence the design of next-generation optoelectronic devices based on graphene and similar 2D materials.

This study provides a comprehensive understanding of the light-enhanced electron-phonon coupling in photoexcited graphene. The significant enhancement of EPC under laser illumination, its dependence on doping concentration and the detailed explanation of the microscopic mechanism via PAITs of nonequilibrium photocarriers are significant contributions. Real-time tracking of EPC dynamics showcases the potential for controlling EPC using optical techniques. Future research could focus on exploring similar phenomena in other 2D materials, examining the effect of different laser parameters and investigating potential applications of light-modulated EPC in novel devices.

The study primarily focuses on the E2g phonon mode in graphene, and further investigation may be needed to determine the role of other phonon modes in light-enhanced EPC. The theoretical calculations employ the LDA, which might not fully capture the effects of electron correlation. The study's focus is on the near-equilibrium region after photoexcitation. While the findings are consistent with short-timescale experimental data, further experimental validation might be needed for longer timescales. The effects of other carrier relaxation processes, such as electron-electron scattering, which compete with the EPC, are not explicitly modeled in detail. These limitations provide potential avenues for future research.