We theoretically study high-harmonic generation (HHG) from solids driven by intense laser pulses using a one-dimensional model periodic crystal. By numerically solving the time-dependent Schrödinger equation directly on a real-space grid, we successfully reproduce experimentally observed unique features of solid-state HHG such as the linear cutoff-energy scaling and the sudden transition from a single-to multiple-plateau structure. Based on the simulation results, we propose a simple model that incorporates vector-potential-induced intraband displacement, interband tunneling, and recombination with the valence-band hole. One key parameter is the valley-to-peak amplitude of the pulse vector potential, which determines the crystal momentum displacement during the half cycle. When the maximum peak-to-valley amplitude A peak reaches the half width π a of the Brillouin zone with a being the lattice constant, the HHG spectrum exhibits a transition from a single-to multiple-plateau structure, and even further plateaus appear at A peak = 2π a , 3π a , · · · . The multiple cutoff positions are given as functions of A peak and the second maximum A peak , in terms of the energy difference between different bands. Using our recipe, one can draw electron trajectories in the momentum space, from which one can deduce, for example, the time-frequency structure of HHG without elaborate quantum-mechanical calculations. Finally, we reveal that the cutoff positions depend on not only the intensity and wavelength of the pulse, but also its duration, in marked contrast to the gas-phase case. Our model can be viewed as a solid-state and momentum-space counterpart of the familiar three-step model, highly successful for gas-phase HHG, and provide a unified basis to understand HHG from solid-state materials and gaseous media. arXiv:1611.08033v3 [physics.optics]
We report the first all-optical production of dual Bose-Einstein condensates (BECs) of paired 6 Li (fermion) and one spin state of 7 Li (boson) at the magnetic field where the s-wave interactions between fermions are resonant. Fermions are cooled efficiently by evaporative cooling and they serve as coolant for bosons. As a result, the dual condensates can be achieved by using a simple experimental apparatus and procedures, as in the case of the all-optical production of a single BEC. We show that the all-optical method enables us to realize variety of ultracold Bose-Fermi mixtures.
We investigate the multielectron effects on high-harmonic generation from solid-state materials using the time-dependent Hartree-Fock theory. We find qualitative change in harmonic spectra, in particular, multiple-plateau formation at significantly lower laser intensities than within the independent-electron approximation. We reveal its origin in terms of interband polarization, i.e, electron-hole polarization, enabling interband excitation at remote crystal momenta via Coulomb potential.
One of the readily accessible observables in trapped cold-atom experiments is the column density, which is determined from optical depth (OD) obtained from absorption imaging and the absorption cross-section (σ abs ). Here we report on simple and accurate determination of OD for dense gases of light atoms such as lithium-6. We investigate theoretically and experimentally an appropriate condition for the probe intensity and duration to achieve good signal-to-noise ratio by considering the influences of photon recoils and photon shot noises. As a result, we have succeeded in measuring OD which reached 2.5 with a signal-to-noise ratio of 10 under spatial resolution of 1.7 µm.
On an AgNO3 crystal, an equilateral or a right-angle triangle-shaped Ag trimer was selectively fabricated through near-field photo-reduction and observed in situ by using an apertured cantilever coupled with an atomic force microscope. By using the different triangle-shaped Ag trimers, irradiation wavelength and polarization dependence of surface-enhanced Raman scattering were investigated.
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