Acceleration and collision of particles has been a key strategy for exploring the texture of matter. Strong light waves can control and recollide electronic wavepackets, generating high-harmonic radiation that encodes the structure and dynamics of atoms and molecules and lays the foundations of attosecond science. The recent discovery of high-harmonic generation in bulk solids combines the idea of ultrafast acceleration with complex condensed matter systems, and provides hope for compact solid-state attosecond sources and electronics at optical frequencies. Yet the underlying quantum motion has not so far been observable in real time. Here we study high-harmonic generation in a bulk solid directly in the time domain, and reveal a new kind of strong-field excitation in the crystal. Unlike established atomic sources, our solid emits high-harmonic radiation as a sequence of subcycle bursts that coincide temporally with the field crests of one polarity of the driving terahertz waveform. We show that these features are characteristic of a non-perturbative quantum interference process that involves electrons from multiple valence bands. These results identify key mechanisms for future solid-state attosecond sources and next-generation light-wave electronics. The new quantum interference process justifies the hope for all-optical band-structure reconstruction and lays the foundation for possible quantum logic operations at optical clock rates.

We demonstrate a compact source of energetic and phase-locked multi-terahertz pulses at a repetition rate of 190 kHz. Difference frequency mixing of the fundamental output of an Yb:KGW amplifier with the idler of an optical parametric amplifier in GaSe and LiGaS2 crystals yields a passively phase-locked train of waveforms tunable between 12 and 42 THz. The shortest multi-terahertz pulses contain 1.8 oscillation cycles within the intensity FWHM. Pulse energies of up to 0.16 µJ and peak electric fields of 13 MV/cm are achieved. Electro-optic sampling reveals a phase stability better than 0.1 π over multiple hours combined with free CEP tunability. The scalable scheme opens the door to strong-field terahertz optics at unprecedented repetition rates. OCIS codes: (140.3070) Infrared and far-infrared lasers; (190.4970) Parametric oscillators and amplifiers; (320.7100) Ultrafast measurements; (320.7100) Ultrafast nonlinear optics; (120.5050) Phase measurementUltrashort pulses in the terahertz (THz) and mid-infrared region of the electromagnetic spectrum have attracted tremendous interest in the past few years as resonant probes of low-energy elementary excitations in condensed matter [1,2]. The combination of CEP-stable pulses with ultrabroadband electro-optic sampling [3][4][5][6][7][8] has allowed for studies of electronic and structural dynamics of molecules and solids, on time scales faster than a single cycle of the carrier wave [1,2]. The recent advent of high-power sources [9][10][11][12][13] has prompted an ongoing revolution of ultrabroadband THz nonlinear optics and resonant THz quantum control of condensed matter [14][15][16][17][18][19][20]. In particular, when the ponderomotive energy exceeds the fundamental bandgap of semiconductors or dielectrics, the carrier wave acts like an AC bias field that can accelerate and recollide quasiparticles [15,16]. It can drive dynamical Bloch oscillations and highharmonic generation [17], or induce tunneling of electrons out of sharp metal tips [18] or through the tunneling junction of a scanning tunneling microscope (STM) [19,20]. In the multi-THz range, non-perturbative dynamics of this nature, often dubbed 'lightwave electronics', have occurred for field amplitudes typically above 10 MV/cm.Optical rectification, i.e. difference frequency generation (DFG) within the broad spectrum of a single femtosecond near-infrared (NIR) pulse, gives rise to passively phaselocked THz pulses [3][4][5][6][7][8]21]. While this concept warrants a particularly stable carrier-envelope phase (CEP), its observed low quantum efficiency has made it a popular choice for the generation of probe pulses [1,2]. Difference frequency mixing between the signal waves of two optical parametric amplifiers driven by the same pump laser, in contrast, has generated CEP-stable few-and single-cycle multi-THz pulses with field amplitudes in excess of 10 MV/cm or even above 100 MV/cm [9][10][11]22]. An innovative in-line scheme of two-color parametric amplification in a single OPA has further improved the long...

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The ultrafast scattering dynamics of intersubband polaritons in dispersive cavities embedding GaAs=AlGaAs quantum wells are studied directly within their band structure using a noncollinear pump-probe geometry with phase-stable midinfrared pulses. Selective excitation of the lower polariton at a frequency of ∼25 THz and at a finite in-plane momentum k k leads to the emergence of a narrowband maximum in the probe reflectivity at k k ¼ 0. A quantum mechanical model identifies the underlying microscopic process as stimulated coherent polariton-polariton scattering. These results mark an important milestone toward quantum control and bosonic lasing in custom-tailored polaritonic systems in the mid and far infrared.

We demonstrate ultrabroadband electro-optic detection of multi-THz transients using mechanically exfoliated flakes of gallium selenide of a thickness of less than 10 µm, contacted to a diamond substrate by van-der-Waals bonding. While the low crystal thickness allows for extremely broadband phase matching, the excellent optical contact with the index-matched substrate suppresses multiple optical reflections. The high quality of our structure makes our scheme suitable for the undistorted and artifact-free observation of electromagnetic waveforms covering the entire THz spectral range up to the near-infrared regime without the need for correction for the electro-optic response function. With the current revolution of chemically inert quasi-two-dimensional layered materials, we anticipate that exfoliated van-der-Waals materials on index-matched substrates will open new flexible ways of ultrabroadband electro-optic detection at unprecedented frequencies.