Build-up and dephasing of Floquet-Bloch bands on subcycle timescales.

Strong light fields have created opportunities to tailor novel functionalities of solids. Floquet-Bloch states can form under periodic driving of electrons and enable exotic quantum phases. On subcycle timescales, lightwaves can simultaneously drive intraband currents and interband transitions, which enable high-harmonic generation and pave the way towards ultrafast electronics.

Tunable non-integer high-harmonic generation in a topological insulator.

When intense lightwaves accelerate electrons through a solid, the emerging high-order harmonic (HH) radiation offers key insights into the material. Sub-optical-cycle dynamics-such as dynamical Bloch oscillations, quasiparticle collisions, valley pseudospin switching and heating of Dirac gases-leave fingerprints in the HH spectra of conventional solids. Topologically non-trivial matter with invariants that are robust against imperfections has been predicted to support unconventional HH generation.

Super-resolution lightwave tomography of electronic bands in quantum materials.

Searching for quantum functionalities requires access to the electronic structure, constituting the foundation of exquisite spin-valley-electronic, topological, and many-body effects. All-optical band-structure reconstruction could directly connect electronic structure with the coveted quantum phenomena if strong lightwaves transported localized electrons within preselected bands. Here, we demonstrate that harmonic sideband (HSB) generation in monolayer tungsten diselenide creates distinct electronic interference combs in momentum space.

Temporal and spectral fingerprints of ultrafast all-coherent spin switching.

Future information technology demands ever-faster, low-loss quantum control. Intense light fields have facilitated milestones along this way, including the induction of novel states of matter, ballistic acceleration of electrons and coherent flipping of the valley pseudospin. These dynamics leave unique 'fingerprints', such as characteristic bandgaps or high-order harmonic radiation.

Subcycle observation of lightwave-driven Dirac currents in a topological surface band.

Harnessing the carrier wave of light as an alternating-current bias may enable electronics at optical clock rates. Lightwave-driven currents have been assumed to be essential for high-harmonic generation in solids, charge transport in nanostructures, attosecond-streaking experiments and atomic-resolution ultrafast microscopy. However, in conventional semiconductors and dielectrics, the finite effective mass and ultrafast scattering of electrons limit their ballistic excursion and velocity.