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Register a new userOptically-Induced Symmetry Breaking via Nonlinear Phononics
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Optical nonlinearities in solids reveal information about both the in-plane rotational and out-of-plane inversion symmetries of a crystal. In the van der Waals material hexagonal boron nitride (hBN) both these symmetries and the linear vibrational properties have led to the rich physics of mid-infrared phonon-polaritons. However, the role of strong electron-phonon nonlinearities requires further study. In this work, we investigate both theoretically and experimentally the rich interplay of phonon anharmonicity and symmetry in phonon-polariton mediated nonlinear optics. We show that large enhancements (>30x) of third-harmonic generation occur for incident femtosecond pulses that are resonant with the hBN transverse optical phonons. In addition, we predict and observe large transient sub-picosecond duration second-harmonic signals during resonant excitation, which in equilibrium is forbidden by symmetry. This surprising result indicates that instantaneous crystal inversion symmetry breaking can be optically induced and controlled via phonon interactions by both the power and polarization of the pump laser.
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Pauli blockade occurs when the excited electrons fill up the states near the conduction bands and block subsequent absorption in semiconductors, and has been widely applied in mode-locking for passively-pulsed-laser systems. In this letter, we report the first direct observation that the Pauli blockade is broken by ultrafast cooling of hot electrons in optically-pumped graphene. With femtosecond spectroscopy, we demonstrate that the time scale to excite an electron (~100 fs) is of the same order as that of the electron decay via electron-electron scattering, which allows the electron excitation interplays strongly with the cooling of hot electrons. Consequently, Pauli blockade is dismissed, leading to an unconventionally enhanced optical absorption. We suggest that this effect is a universal feature of two-dimensional layered materials, which sheds the light of ultrafast carrier dynamics in nonlinear physics and inspires the designing of new-generation of ultrafast optoelectronic devices.
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We study the possibility of switching the types of symmetry breaking bifurcation (SBB) in the cylinder shell waveguide with helical double-well potential along propagation direction. This model is described by the one-dimensional nonlinear Schr{o}dinger (NLS) equation. The symmetry- and antisymmetry-breakings can be caused by increasing the applied voltage onto the waveguide in the self-focusing and -defocusing cases, respectively. In the self-focusing case, the type of SBB can be switched from supercritical to subcritical. While in the self-defocusing case, the type of SBB can not be switched because only one type of SBB is found.
We study the effect of an applied magnetic field on the nonequilibrium transport properties of a general cubic quantum network described by a tight-binding Hamiltonian with specially designed couplings to the leads that preserve open-system symmetries. We demonstrate that the symmetry of open systems can be manipulated by the direction of the magnetic field. Starting with all the symmetries preserved in absence of a field, the anisotropic and isotropic fields systematically break the symmetries, influencing all nonequilibrium properties. For simple cubic systems, we are able to identify the steady states that comprise of pure states, bath-dependent states (nonequilibrium steady states), and also nonphysical states. As an application, we show numerically for large cubic networks that the symmetry breaking can control nonequilibrium currents and that different environmental interactions can lead to novel features which can be engineered in artificial super-lattices and cold atoms.
We experimentally demonstrate PT-symmetric optical lattices with periodical gain and loss profiles in a coherently-prepared four-level N-type atomic system. By appropriately tuning the pertinent atomic parameters, the onset of PT-symmetry breaking is observed through measuring an abrupt phase-shift jump between adjacent gain and loss waveguides. The experimental realization of such readily reconfigurable and effectively controllable PT-symmetric waveguide array structure sets a new stage for further exploiting and better understanding the peculiar physical properties of these non-Hermitian systems in atomic settings.
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