From Zero to Mega-Gauss Fields: Comprehensive Magnetophotonic Spectroscopy of Graphene Dirac Cones
We investigate the infrared magneto-optical response of n-doped epitaxial graphene on 4H-SiC in ultrahigh magnetic fields up to 560~T, utilizing single-turn coil and electromagnetic flux compression techniques. The measured absorption spectra are anomalously broad, strongly deviating from conventional cyclotron resonance. Angle-resolved photoemission spectroscopy (ARPES) reveals a distorted Dirac dispersion featuring a ``camel-back" structure with an energy gap of $E_g \sim 0.2$~eV. Using the band parameters extracted via a generalized bilayer graphene model, we construct a Landau level (LL) fan chart that dictates a critical level crossing between the $N=0^+$ and $N=0^-$ states near 160--200~T. At this threshold, the optical transition mechanism undergoes a dramatic shift from an electron-dominated collective mode to a cooperative electron-hole collective excitation. Furthermore, the extreme-field absorption spectra under thulium fiber laser excitation ($\hbar\omega_0 = 0.636$~eV)---culminating in a massive resonance near 400~T with a shoulder at 200~T---are excellently reproduced by a collective Alfv\'en wave model. This analysis also evidences a magnetic-field-induced enhancement of the sublattice potential asymmetry parameter (from 0.10~eV to 0.12~eV). This directly signifies that the macroscopic electron-hole band asymmetry is further amplified by the applied magnetic field. Ultimately, the field-induced energy inversion generates a strongly interacting, fully compensated electron-hole plasma. The resonant excitation of Alfv\'en waves in this regime demonstrates that a pristine tabletop analog to the relativistic electron-positron plasmas found in astrophysical extremes can be elegantly realized within a 2D graphene system.