Session N7: Strong Field Phenomena in Soft and Condensed Matter

Anisotropy in high-harmonics from bulk and 2D crystals

We present our experimental results on observation of anisotropic high-order harmonics from bulk and atomically thin 2D crystals. For linear polarization, the rotation of 100 cut bulk MgO crystal around its normal produces a strong 4-fold distribution consistent to its cubic crystal structure [1]. The ellipticity dependence is also strongly anisotropic and depends on the orientation of crystallographic axis with respect to the major axis of laser polarization [1]. We use real-space electron trajectory analysis to investigate the underlying electron dynamics. We also find that much of the anisotropy originates in the crystal structure of solids such as in MoS2, we measure 6-fold distribution dictated by its hexagonal crystal structure [2]. Finally, single layer MoS2 produces additional set of even order harmonics because of the lack of reflection symmetry [2]. The understanding of microscopic origin of anisotropy could lead to an all-optical method suitable for probing the distribution of valance charge density in bulk and 2D crystalline solids. References: [1]\underline { Y. You }\underline {\textit{et al.}}\underline {, Nature physics, DOI: 10.1038/nphys3955, (2016)} [2]\underline { Liu}\underline {\textit{ et al}}\underline {., Nature Physics, DOI: 10.1038/nphys3946, (2016}   

High-order sideband generation: colliding quasiparticles, probing Berry curvature, and generating tunable frequency combs in semiconductors

High-order sideband generation (HSG) is a recollision phenomenon that is closely related to high-order harmonic generation (HHG). A weak laser, tuned near the band gap of a semiconductor, resonantly injects pairs of quasiparticles (electrons and holes) while the semiconductor is driven by a strong terahertz-frequency electric field. The terahertz field accelerates the photo-injected electrons and holes first away from, then back towards each other. If the electrons and holes recombine, a high-order sideband is emitted. The sidebands form a frequency comb with teeth spaced by twice the terahertz frequency, and anchored by the frequency of the laser that generates the electron-hole pairs. Recently, we have observed HSG spectra containing sidebands up to 90th order and spanning over 12 percent of the laser wavelength, which is near 800 nm. As the electrons and holes are accelerated through the band structure of GaAs quantum wells, the holes experience significant Berry curvature, which leaves its imprint on the polarizations of the high-order sidebands.   

HHG in solids: dynamics of multilevel adiabatic states spanning the band structure

We investigate high harmonic generation in a solid, modeled as a multilevel system dressed by a strong infrared laser field [1]. We show that the cutoff energies and relative strengths of the multiple plateaus that emerge in the harmonic spectrum can be understood both qualitatively and quantitatively by considering the dynamics of the laser-dressed system. Such a model was recently used to interpret the multiple plateaus exhibited in harmonic spectra generated by solid argon and krypton [2]. We show that when the multilevel system originates from the Bloch states at the gamma$-$point of the band structure, the laser-dressed states (which are equivalent to the so-called Houston states [3]) map out the band structure away from the gamma$-$point as the laser field increases. This means that the cutoff energy of a given plateau can never exceed the maximum band gap between the valence band (VB) and the conduction band (CB) responsible for that plateau, thereby extending the cutoff limitation proposed in [4] to a multiband system. Finally, we discuss how this understanding leads to a semiclassical three-step picture in momentum space that describes the HHG process in a solid. In this picture, the delocalized electron first tunnels from the VB to the CB at the zero of the vector potential and then is accelerated on the CB as the vector potential increases and decreases through an optical half-cycle. The coherence between the VB and the CB populations leads to the emission of XUV radiation, with photon energies corresponding to the instantaneous energy difference between the VB and the CB. This means that each energy below the cutoff energy is emitted twice in each laser half-cycle. [1] M. Wu \textit{et al}. PRA \textbf{94}, 063403 (2016). [2] G. Ndabashimiye \textit{et al.}, Nature \textbf{534}, 520 (2016). [3] J. B. Krieger and G. J. Iafrate, PRB \textbf{33}, 5494 (1986). [4] G. Vampa \textit{et al.}, PRL 113, 073901 (2014).   

Route to Coherent Electronics

Laser-driven generation of coherent radiation in bulk solids extending up to the extreme ultraviolet part of the spectrum [1] has recently open up completely new possibilities for study of electronic phenomena which lie beyond the scope of standard condensed phase physics spectroscopies. I will present how previous [2] and new tools of attosecond metrology [3][4] can now allow us to gain detailed insight into the fundamental microscopic processes responsible for the EUV emission in solids. We will show that this emission is in reality a macroscopic probe of nanoscale intraband coherent electric currents [5] the frequency of which is extending into multiPetahertz range. On the basis of these findings, I will try to persuade you that we are now entering the realm of coherent electronics. A regime in which electronic circuitry can be conceived on the atomic level and where electronic properties of materials can be accessed [5] and controlled on attosecond time scales.\\ \\$[1]$ Luu T.T. et al., Nature 521,498 (2015),[2] Goulielmakis E. et al., Science 305, 1267 (2004) [3] Wirth A. et al., Science 334, 195 (2011). [4] Hassan M. Th et al., Nature 530, 66 ( 2016) , [5]Garg et .al., Nature 538, 359 (2016).