![]() In inversion-asymmetric systems, exemplified by polar molecules, orientation-selective ionization has been demonstrated with CEP-controlled ultrashort pulses. In systems with inversion symmetry, CEP-stable pulses have been demonstrated to give rise to asymmetric photoelectron ionization, control electron localization in dissociating D 2 molecules, and excite CEP-dependent photocurrents in monolayer graphene. ![]() In the frequency domain, this control can be understood as the interference between multiphoton transitions with different parity such interference inevitably arises in few-cycle pulses with the spectral bandwidth comparable to an octave. Changes in the CEP control the balance between sub-cycle excitation processes occurring during neighboring half-cycles, inducing asymmetric response. The imbalance manifests in the nonlinear interaction and is controlled by the phase of the carrier oscillations under the envelope (the carrier envelope phase, CEP). Indeed, in nearly single-cycle driving pulses, the neighboring half-cycles of the pulse can be unbalanced even if the symmetry is present in the pulse on average, i.e. The same principle can be transferred to the ultrafast domain by using few-cycle fields with nearly-octave spectral bandwidth. The phase between the two fields then becomes the control knob of the induced response, steering the formation of parity-breaking currents in atoms, quantum wells, and bulk semiconductors. ![]() The most intuitive way of breaking the dynamic half-cycle inversion symmetry is by using phase-locked mixed-color fields with differing parity, typically ω and 2 ω. This would allow for the observation of phenomena prohibited in the case of perfect inversion symmetry, such as even harmonic generation and optical rectification. One needs to break either the symmetry between neighboring half-cycles or the inversion symmetry of the system to make the “arms” unbalanced. The medium thus acts as a perfectly balanced interferometer, where different half-cycles correspond to different arms, and the excitations generated in the successive half-cycles of the pulse interfere destructively for all even-order harmonics of the excited current. If the neighboring half-cycles of the incident pulse are perfect inverted images of each other, and the medium itself is inversion-symmetric, the generated excitation should exhibit no asymmetry. Indeed, a pulse incident on a medium excites a current whose symmetry is defined by the symmetry properties of both the field and the medium. From the applied perspective, our work suggests a novel route to ultrafast manipulation of robust memory cells.Īt a first glance, our result may appear counter-intuitive. From the fundamental perspective, our work extends the ideas of coherent control to strongly correlated systems. Thus, our scheme takes advantage of both the sub-cycle control over the asymmetry of the individual oscillations of the driving pulse and the symmetrizing many-body interactions after the pulse. The induced asymmetry is controlled by the sub-cycle structure of the pulse and is preserved during and after thermalization, leading to preferential population of the ground state with the desired polarity, long after the end of the pulse. Here we show how a short pulse with no DC component can selectively depopulate one of these two states, achieving unidirectional conversion between them, with asymmetric many-body excitations generated on the sub-cycle time-scale via highly nonlinear interaction with a tailored pump pulse. Although the potential and therefore the equations of motion preserve space inversion symmetry, the low-energy trajectories of the particle defined by them may still break it. Pictorial representation of the principle of spontaneous symmetry breaking with q and E representing generalized coordinate and energy. ![]()
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