Chengpu Liu

Origin of Unexpected Low Energy Structure in Photoelectron Spectra Induced by Midinfrared Strong Laser Fields

Using a semiclassical model which incorporates tunneling and Coulomb field effects, the origin of the low-energy structure (LES) in the above-threshold ionization spectrum observed in recent experiments [Blaga, Nature Phys. 5, 335 (2009); Quan, Phys. Rev. Lett. 103, 093001 (2009).] is identified. We show that the LES arises due to an interplay between multiple forward scattering of an ionized electron and the electron momentum disturbance by the Coulomb field immediately after the ionization. The multiple forward scattering is mainly responsible for the appearance of LES, while the initial disturbance mainly determines the position of the LES peaks. The scaling laws for the LES parameters, such as the contrast ratio and the maximal energy, versus the laser intensity and wavelength are deduced.

Effects of spontaneous emission-induced coherence on population inversion in a ladder-type atomic system

Spontaneous emission can create coherence in a ladder-type three-level atom with equispaced levels, subject to the condition that the atomic dipole moments are nonorthogonal. We study the effects of this kind of coherence on the steady-state population inversion in the atomic system. We show that the population inversion can be greatly enhanced on one of the optical transitions due to the spontaneous emission-induced coherence. Furthermore, we find, within suitable parameters regions, that such coherence can also lead to unexpected population inversion on both of the optical transitions.

Atom localization via interference of dark resonances

A scheme of atom localization based on the interference of resonance of double-dark states is proposed, in which the atom interacts with a classical standing-wave field. It is found that the localization property is significantly improved due to the interaction of double-dark resonances. It is realized that the atom is localized just at the nodes of the standing-wave field with higher precision. Moreover, an improvement by a factor of 2 in the detecting probability of a single atom within the subwavelength domain can be achieved by adjusting the probe-field detuning. This scheme shows more advantages than other schemes of atom localization.

Near dipole-dipole effects on the propagation of few-cycle pulse in a dense two-level medium

The propagation behaviors, which include the carrier-envelope phase, the area evolution and the solitary pulse number of few-cycle pulses in a dense two-level medium, are investigated based on full-wave Maxwell-Bloch equations by taking Lorentz local field correction (LFC) into account. Several novel features are found: the difference of the carrier-envelope phase between the cases with and without LFC can go up to pi at some location; although the area of ultrashort solitary pulses is lager than 2pi, the area of the effective Rabi frequency, which equals to that the Rabi frequency pluses the product of the strength of the near dipole-dipole (NDD) interaction and the polarization, is consistent with the standard area theorem and keeps 2pi; the large area pulse penetrating into the medium produces several solitary pulses as usual, but the number of solitary pulses changes at certain condition.

Coulomb focusing in above-threshold ionization in elliptically polarized midinfrared strong laser fields

The role of Coulomb focusing in above-threshold ionization in an elliptically polarized mid-infrared strong laser field is investigated within a semiclassical model incorporating tunneling and Coulomb field effects. It is shown that Coulomb focusing up to moderate ellipticity values is dominated by multiple forward scattering of the ionized electron by the atomic core that creates a characteristic low-energy structure in the photoelectron spectrum and is responsible for the peculiar energy scaling of the ionization normalized yield along the major polarization axis. At higher ellipticities, the electron continuum dynamics is disturbed by the Coulomb field effect mostly at the exit of the ionization tunnel. Due to the latter, the normalized yield is found to be enhanced, with the enhancement factor being sharply pronounced at intermediate ellipticities.

Flexible metamaterial narrow-band-pass filter based on magnetic resonance coupling between ultra-thin bilayer frequency selective surfaces

A novel flexible metamaterial narrow-band-pass filter is designed and proved to be reliable by both numerical simulations and experimental measurements. The unit cell of the designed structure consists of circle ring resonators on top of a thin dielectric layer backed by a metallic mesh. The investigations on the distribution of the surface current and magnetic field as well as the analysis of the equivalent circuit model reveal that the magnetic resonance response between layers induced by the reverse surface current contributes to the high quality factor band-pass property. Importantly, it is a flexible design with a tunable resonance frequency by just changing the radius of the circle rings and can also be easily extended to have the multi-band-pass property. Moreover, this simplified structure with low duty cycle and ultra-thin thickness is also a symmetric design which is insensitive to the polarization and incident angles. Therefore, such a metamaterial narrow-band-pass filter is of great importance in the practical applications such as filtering and radar stealth, and especially for the conformal structure applications in the infrared and optical window area.

Doppler-enhanced gain in an open ladder inversionless lasing system

Abstract An analysis is made of the effect of Doppler broadening on the gain in an open ladder inversionless lasing system. It is shown that for co-propagating probe and driving fields, the gain does not monotonously decrease or increase with increasing Doppler width, and at a suitable Doppler width one can obtain a maximum value much larger than that without Doppler broadening. For counterpropagating probe and driving fields, when the Doppler width is large enough, gain oscillation occurs, and the oscillation amplitude and region increase with increasing Doppler width. This conclusion is very different from that obtained in the corresponding closed system.

Wavelength and intensity dependence of multiple forward scattering of electrons at above-threshold ionization in mid-infrared strong laser fields

The contribution of multiple forward scattering in Coulomb focusing of low-energy photoelectrons at above-threshold ionization in mid-infrared laser fields is investigated. It is shown that the high-order forward scattering can have a nonperturbative effect in Coulomb focusing. The effective number of rescattering events is defined and is shown to depend weakly on laser intensity and wavelength. Nevertheless, the relative contribution of forward scattering in Coulomb focusing and the Coulomb focusing in total decrease with increasing laser intensity and wavelength.

Simplified Transparent Conductive Oxides-Based UltraBroadband Absorber Design

Broadband electromagnetic wave absorbers are of practical importance in many fields. Current absorbers with multilayered configurations at high frequencies suffer from fabrication challenges and design complexity. In this study, we realize a broadband, wide-angle, polarization-independent, perfect infrared absorber with conventional pyramid configuration but only employing one single transparent conductive oxide material without using multilayered design. A transmission line model is introduced to rein the complex frequency responses. Full-wave simulations give consistent results with theoretical predictions. This simplified configuration might relieve the fabrication and design difficulties, exhibiting great potentials in the applications of energy harvesting, IR detection, and imaging systems.

Equivalent-nanocircuit-theory-based design to infrared broad band-stop filters

We theoretically introduced a design paradigm and tool by extending the circuit functionalities from radio frequency to near infrared domain, and a broad band-stop filter, is successfully demonstrated by cascading triple layers of nano-square arrays. The feasibility is confirmed by its consistency with the rigorous FDTD calculation. Moreover, such a third-order Butterworth filter is not only insensitive to the incident angle and but also to input lights polarization. The new paradigm forms a theoretical foundation for designing optical devices and also enriches the classic circuit operations at the optical frequency region.