Monitoring Electron-Photon Dressing in WSe₂

Abstract

Optical pumping of solids creates a non-equilibrium electronic structure where electrons and photons combine to form quasiparticles of dressed electronic states. The resulting shift of electronic levels is known as the optical Stark effect, visible as a red shift in the optical spectrum.Here we show that in a pump-probe setup we can uniquely define a non-equilibrium quasiparticle bandstructure that can be directly measurable with photo-electron spectroscopy. The dynamical photon-dressing (and undressing) of the many-body electronic states can be monitored by pumpprobe time and angular resolved photoelectron spectroscopy (tr-ARPES) as the photon-dressed bandstructure evolves in time depending on the pump-probe pulse overlap. The computed tr-ARPES spectrum agrees perfectly with the quasi-energy spectrum of Floquet theory at maximum overlap and goes to the the equilibrium bandstructure as the pump-probe overlap goes to zero.Additionally, we show how this time-dependent non-equilibrium quasiparticle structure can be understood to be the bandstructure underlying the optical Stark effect. The extension to spinresolved PES can be used to predict asymmetric dichroic response linked to the valley selective optical excitations in monolayer transition metal dichalcogenides (TMDs).1 arXiv:1609.03218v2 [cond-mat.mtrl-sci]

Engineering Dirac states in graphene: Coexisting type-I and type-II Floquet-Dirac fermions

“…Floquet states can be observed by irradiating a laser pulse [45,46,56], resulted from the good approximation of a long pulse to a continuous wave; this is evidenced from our simulation of angel-resolved photoelectron spectrum. Laser pulse with trapezoidal envelope having 5 cycles (~ 4 fs here) is long enough to produce well defined signatures [56], indicating that not only laser intensity is below graphene damage threshold [57], but the pulse duration is so brief that phonons cannot be excited [58]. Therefore, stable graphene would survive in such a laser irradiation, producing corresponding Floquet-Bloch states.…”

Towards properties on demand in quantum materials

“…Under periodic electromagnetic excitation with frequency Ω (panel c), electronic bands become dressed, which manifests itself as a 'Floquet' copy of the original band shifted in energy by hΩ [61][62][63][64][65] . Even though the driving field oscillates at frequency Ω, crystals effectively rectify these oscillations, yielding a quasi-static dispersion on the timescale of the excitation pulse 65,78 . These photon-dressed electronic bands can overlap with other bands (left shaded region) and hybridize (right unshaded region).…”

“…In addition, questions about how to control the steady-state occupation of Floquet bands and to mitigate heating effects remain outstanding. However, recent advancements in ultrafast experimental techniques, theoretical simulations of time-domain experiments 61,62 and the development of driving protocols for population and thermal management 63,64 are boldly moving this field forward 65 . The recent successful imaging of a Floquet band structure (Fig.…”

Floquet analysis of excitations in materials

“…The observed valley-dependent circular dichroism results from an interaction of Floquet-replica bands that opens the band gap in either of the valleys, depending on the orientation of the pump polarization. In the optical spectroscopy of [66] this is detected as a Stark shift, while [73] reports the Floquet analysis of simulated photo-electron spectra of this system. The computed angular-resolved photoelectron spectroscopy (ARPES) probabilities are shown for different pump-probe delays, figure 3(e), and directly compared to the Floquet spectrum.…”