Observation of light-driven band structure via multiband high-harmonic spectroscopy

Abstract

Intense light–matter interactions have revolutionized our ability to probe and manipulate quantum systems at sub-femtosecond timescales1, opening routes to the all-optical control of electronic currents in solids at petahertz rates2–7. Such control typically requires electric-field amplitudes in the range of almost volts per angstrom, when the voltage drop across a lattice site becomes comparable to the characteristic bandgap energies. In this regime, intense light–matter interaction induces notable modifications to the electronic and optical properties8–10, dramatically modifying the crystal band structure. Yet, identifying and characterizing such modifications remain an outstanding problem. As the oscillating electric field changes within the driving field’s cycle, does the band structure follow and how can it be defined? Here we address this fundamental question, proposing all-optical spectroscopy to probe the laser-induced closing of the bandgap between adjacent conduction bands. Our work reveals the link between nonlinear light–matter interactions in strongly driven crystals and the sub-cycle modifications in their effective band structure.

Onset of Bloch oscillations in the almost-strong-field regime

“…For short laser pulses, this happens already at intensities where the contribution of intraband currents due to ionization is not yet dominant. The mechanism is different from the influence of Bloch electron motion which has been observed before in HHG 10,11 because that is related to intraband currents, which are seeded by the irreversible (or real) population. Furthermore, the observed effect is in the realm of perturbative optics, because the iterative series 20 converges and reproduces the interference structure.…”

“…For terahertz fields it was found using a numerical switch-off analysis that Bloch oscillations contribute substantially to HHG 10 . Very recently, it has been reported that HHG produced by two-color fields exhibit phase variations that can be associated with reaching the zone edges 11 . On the other hand, the decoherence dynamics of strong-field processes in solids are disputed, with many recent calculations assuming the ultrafast loss of interband coherence with few-femtosecond dephasing times 7,[12][13][14][15] .…”

Onset of Bloch oscillations in the almost-strong-field regime

“…For short laser pulses, this happens already at intensities where the contribution of intraband currents due to ionization is not yet dominant. The mechanism is different from the influence of Bloch electron motion on intraband currents (which are seeded by the irreversible or real population), which has been observed before in HHG [10,11]. The observed effect is in the realm of perturbative optics, because the iterative series [20] converges and reproduces the interference structure.…”

“…For terahertz fields it was found using a numerical switch-off analysis that Bloch oscillations contribute substantially to HHG [10]. Very recently, it has been reported that HHG produced by two-color fields exhibit phase variations that can be associated with reaching the zone edges [11]. On the other hand, the decoherence dynamics of strong-field processes in solids are disputed, with many recent calculations assuming ultrafast loss of interband coherences with few-femtosecond dephasing times [7,[12][13][14][15].…”

Attosecond Transient Interferometry

“…This scheme can be directly applied to laser-driven solids where the band structure can be transiently modified 48,49 within the optical cycle. In such systems, the multiple band structure, dressed by the laser field, defines several competing quantum paths, which are all coherently mapped onto the transient absorption spectrum.…”