Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A05: Solids in Strong Laser FieldsInvited Session
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Sponsoring Units: DCMP DAMOP Chair: Tony Heinz, Stanford University Room: LACC 152 |
Monday, March 5, 2018 8:00AM - 8:36AM |
A05.00001: Theory of HHG in solids: band structure, orientation dependence, and time profiles Invited Speaker: Mette Gaarde Over the past several years we have explored the theory of high harmonic generation (HHG) in solids, using a momentum space model. We have shown that the HHG process can be described as a three-step process that involves tunneling from the valence to the conduction band(s), acceleration on one or multiple conduction bands, and radiation via coherence with the valence band. We have also shown that in a given system this dynamics can be described using a multi-level system that originates as the Gamma-point band structure of that system. We will discuss how this points to a close connection between the band structure and a number of properties of the harmonic radiation such as: (i) the cutoff energy and yield of the often multiple plateaus that can be observed in the harmonic spectrum, (ii) the dependence of the harmonic yield on the relative orientation of the crystal and the laser polarization, and (iii) the sub-cycle time structure of the harmonic radiation. We will discuss the comparison of our predictions to several recent experimental results. |
Monday, March 5, 2018 8:36AM - 9:12AM |
A05.00002: High-order sideband generation in semiconductors: colliding quasiparticles and probing Berry curvature. Invited Speaker: Mark Sherwin The direct measurement of Berry phases is still a great challenge in condensed matter systems. The bottleneck has been the ability to adiabatically drive a quasiparticle coherently across a large portion of the Brillouin zone in a solid where the scattering is strong and complicated. We break through this bottleneck and show that high-order sideband generation (HSG) in semiconductors is intimately affected by Berry phases. Electron-hole recollisions and HSG occur when a near-band gap laser beam excites a semiconductor that is driven by sufficiently strong terahertz (THz)-frequency electric fields[1-3]. I will discuss recent experimental and theoretical studies of HSG from GaAs/AlGaAs quantum wells[4]. The observed HSG spectra contain sidebands up to the 90th order. The highest-order sidebands are associated with electron-hole pairs driven coherently across roughly 10% of the Brillouin zone. A surprising dynamical birefringence is observed: high-order sidebands are usually stronger when the exciting near-infrared (NIR) and the THz electric fields are polarized perpendicular than parallel; and the sidebands exhibit significant ellipticity that increases with increasing sideband order, despite nearly linearly-polarized excitation and driving fields. We explain dynamical birefringence by generalizing the three-step model for high order harmonic generation. The hole accumulates Berry phases due to variation of its internal state as the quasi-momentum changes under the THz field. Dynamical birefringence arises from quantum interference between time-reversed pairs of electron-hole recollision pathways. |
Monday, March 5, 2018 9:12AM - 9:48AM |
A05.00003: Many-body theory of quasiparticles in strong laser fields Invited Speaker: Mackillo Kira Few cycle, extreme nonlinear excitations of semiconductors can change electron energy by more than 1eV within a unit cell – without damaging the sample. They can excite and move electronic coherences and quasiparticles between conduction and valence bands much faster than relevant scattering processes, introducing lightwave electronics as the next step for quantum technology. This scenario is highly nonperturbative, and the related optical, quantum-optical, and many-body effects can be systematically described with a first-principles cluster-expansion approach [Semiconductor Quantum Optics, (Cambridge University Press, 2012)]. I will present how this theory quantitatively explains measured high-harmonic (HH) emission [Nat. Photon. 8, 119 (2014)] as well as harmonic sideband (HSB) generation [Nature 533, 225 (2016)] around an optical resonance. |
Monday, March 5, 2018 9:48AM - 10:24AM |
A05.00004: First Experiments on Solid-state HHG Invited Speaker: Shambhu Ghimire We reported the first observation of high-order harmonics from solid materials in 2011 [1]. These observations, at the time, were surprising because until then high-order harmonic generation (HHG) was mainly studied and utilized in isolated atoms and molecules, and the underlying mechanism was understood using a three step re-collision model. HHG from bulk ZnO crystals also showed a plateau feature much like in gas phase HHG, with the high-energy cutoff extending to around 25th order when the applied peak field was ~0.6 V/Å. However, the detail analysis of the measured spectrum including the observed linear scaling of high-energy cutoff with the laser field (quadratic scaling is expected from re-collision model) indicated strongly that the microscopic mechanism in the solid-state is fundamentally different. Since then there has been a significant interest in exploring the detail mechanism, and in exploiting the potential of the solid-state. Our recent results are observation of multiple plateaus [2], strongly anisotropic high harmonic response in MgO [3], observation of high-harmonics from atomically thin isolated monolayer MoS2 [4], strong carrier-envelope phase dependence in MgO [5], and observation of high-order harmonics in amorphous solids [6]. Implications of these results include probing atomic-scale structure (real-space picture) and driven dynamics involving entire Brillouin zone (momentum-space picture). |
Monday, March 5, 2018 10:24AM - 11:00AM |
A05.00005: Atoms and solids in strong laser fields. Invited Speaker: Paul Corkum High harmonics from ionizing gases irradiated by intense infrared light were discovered in 1989, but it took 22 years before similar phenomenon were discovered in solids. Electric field driven motion of the ionizing electron wave packet is the physical mechanism for high harmonics and, in gases, the re-collision between the wave packet and its parent ion is the most important process. The question arises ``is the mechanism similar for harmonics created by electrons and holes in transparent solids. To answer this question we measure the spectral phase of the harmonics by perturbing the wave packet motion with an interrogating pulse that adds a spectral or spatial marker. We apply a second harmonic beam parallel to the fundamental with the weak even harmonics that arise from the joint fields as the observable. We find that for ZnO and Si, the harmonics have a spectral phase characteristic of a generalized re-collision (the dipole moment requires two bands). In contrast, for SiO$_{\mathrm{2}}$ all harmonics all have the same spectral phase, indicating the single band origin of the harmonics. Solids are important for technology. Solids can be shaped, doped, coated or baised. We will show that high harmonics can be controlled by any of these methods while the harmonics reveal even subtle details about the solid. |
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