Beyond Equilibrium Raman Thermometry: Fermi-Level Control of the Anti-Stokes Response in Graphene

Abstract

Non-equilibrium electron and phonon populations govern heat generation and dissipation in nanoscale devices, yet their direct characterization remains challenging. Anti-Stokes Raman scattering provides a sensitive probe of phonon populations and local temperatures, although its interpretation becomes more complex when electronic and phononic populations are driven out of equilibrium. Here, we investigate the gate-dependent anti-Stokes Raman response of monolayer graphene and find that the anti-Stokes response deviates from that expected from an equilibrium phonon population. Using a spatially localized microdroplet spectroelectrochemical platform, dielectric back-gates, and chemical doping, we observe a strong and reversible suppression of the G-mode anti-Stokes intensity as the Fermi level is tuned away from the Dirac point, while the Stokes intensity remains essentially unchanged. By contrast, the G-mode frequency and linewidth of both the Stokes and anti-Stokes branches evolve with doping as expected from the established Kohn-anomaly picture. The anti-Stokes/Stokes intensity ratio decreases by several-fold over the measured range and displays a pronounced, gate-dependent power response with maximum sensitivity at charge neutrality. Comparison with previous studies of non-equilibrium carrier dynamics in doped graphene suggests an important role of Fermi-level-dependent electronic phase space, which can influence both hot-carrier-driven phonon populations and electronically mediated Stokes-anti-Stokes scattering. Our findings establish anti-Stokes Raman scattering as a sensitive probe of electron-phonon coupling and non-equilibrium phonon dynamics in graphene.