Light-Driven Extremely Nonlinear Bulk Photogalvanic Currents

Abstract

We report on the generation of bulk photocurrents in materials driven by non-resonant bi-chromatic fields that are circularly polarized and co-rotating. The non-linear photocurrents have a fully controllable directionality and amplitude without requiring carrier-envelope-phase stabilization or few-cycle pulses, and are generated with photon energies much smaller than the band gap (reducing heating in the photo-conversion process). We demonstrate with ab-initio calculations that the photocurrent generation mechanism is universal and arises in gaped materials (Si, diamond, MgO, hBN), in semi-metals (graphene), and in two-and three-dimensional systems. Photocurrents are shown to rely on sublaser-cycle asymmetries in the nonlinear response that build-up coherently from cycle-to-cycle as the conduction band is populated. Importantly, the photocurrents are always transverse to the major axis of the co-circular lasers regardless of the material's structure and orientation (analogously to a Hall current), which we find originates from a generalized time-reversal symmetry in the driven system. At high laser powers (~10 13 W/cm 2 ) this symmetry can be spontaneously broken by vast electronic excitations, which is accompanied by an onset of carrier-envelope-phase sensitivity and ultrafast many-body effects. Our results are directly applicable for efficient light-driven control of electronics, and for enhancing sub-band-gap bulk photovoltaic effects.Light-driven dynamics in solids with femtosecond and sub-femtosecond resolution have recently attracted considerable attention. Light-matter interactions can result in novel effects that originate from ultrafast dynamics including high harmonic generation (HHG) [1], and the creation of new states of matter [2][3][4][5][6][7][8][9][10]. The ability to control the electron motion in real-space, momentum-space and time, can give rise to unprecedented control over observable properties such as light emission [11][12][13][14][15][16][17][18][19] and magnetic fields [20]. One main avenue of research here is the generation and characterization of light-driven bulk electric currents in the absence of external bias. In materials with broken inversion symmetry, second-order nonlinear effects lead to shift currents through the bulk photovoltaic effect [21,22]. The driving force for carrier separation in the shift current mechanism is the coherent evolution of electron and hole wavefunctions, such that above-bandgap photovoltages can surpass the Shockley-Queisser limit [23]. Photocurrents can also arise in inversion-symmetric materials (where they are standardly forbidden) via mixing of bi-chromatic carrier waves with frequencies ω and 2ω [20,[24][25][26][27][28]. Here the photon energies are resonant with 2 nd -and 3 rdorder perturbative transitions that interfere, and the inversion symmetry is effectively broken by the twocolor field (making the effect highly sensitive to the two-color relative phase). The resonant and perturbative nature of these effects precludes access to ultrafast dyn...

Light-field control of real and virtual charge carriers

“…A phenomenological dephasing is incorporated in the simulations shown in Fig. 3, in agreement with previous results on ultrafast carrier dynamics (see Methods for details) [26,28,29,47].…”

Electronic Coherence and Coherent Dephasing in the Optical Control of Electrons in Graphene

“…Figure a shows schematically the experimental setup to measure a coherently injected residual current using two ultrashort laser pulses, one oscillating at frequency ω (≈800 nm, red pulse) and one at 2ω (≈400 nm, blue pulse). In particular, light of low temporal symmetry can generate net currents in solids, even in the absence of a bias voltage or spatial asymmetries in the material. ,− ,,− In the perturbative limit, such light-induced symmetry breaking arises by the interference of an even- and odd-order pathway from a given initial to a final momentum state, as schematically shown in Figure b for one- versus two-photon absorption. − ,, By changing the relative phase via the temporal delay between the two colors, the quantum interference and thus the net current amplitude and direction can be controlled. Intriguingly, this interference process persists beyond the perturbative regime and expands to the strong-field regime.…”

“…While the current amplitude, which is the result of interference, is only sensitive to the overlap of the two populations, a possible initial population-dependent decoherence time might break the symmetry . Time-dependent density functional theory simulations accounting for the critical role of decoherence and correlation effects are required, which are beyond the scope of this work. , To determine the lower bound of the electronic coherence time, we take the mean of both values to obtain 22 ± 4 fs.…”

Controlling Valley-Polarisation in Graphene via Tailored Light Pulses

“…Furthermore, a simple recipe to read out the valley polarisation, using high-harmonic generation (HHG) driven by an additional 3ω pulse is proposed [21]. In recent years, high-harmonic spectroscopy became a powerful method to probe various aspects of electron dynamics in solids [22][23][24][25]. Also, the idea of ω − 2ω bi-circular fields driven HHG is extended from atomic systems [26,27] to solids [28].…”