Shangqing Gong

Electromagnetically induced grating in asymmetric quantum wells via Fano interference

We propose a scheme for obtaining an electromagnetically induced grating in an asymmetric semiconductor quantum well (QW) structure via Fano interference. In our structure, owing to Fano interference, the diffraction intensity of the grating, especially the first-order diffraction, can be significantly enhanced. The diffraction efficiency of the grating can be controlled efficiently by tuning the control field intensity, the interaction length, the coupling strength of tunneling, etc. This investigation may be used to develop novel photonic devices in semiconductor QW systems.

Three-dimensional atom localization in a five-level M-type atomic system

By using three mutually perpendicular standing-wave fields, we propose a scheme for three-dimensional (3D) subwavelength atom localization in a five-level M-type atomic system. Based on the electromagnetically induced transparency, position probability distribution of the atom in 3D space could be determined via measuring the probe absorption which is proportional to the filter function. It is shown that patterns of 3D atom localization depends sensitively on the coupling schemes of the standing-wave fields. When the standing-wave fields couple three different transitions, the same atom localization patterns are formed in the eight subspaces of the 3D space. While all the standing-wave fields are applied on one transition, we can realize different atom localization patterns in the eight subspaces. From the view of the xy plane, various symmetric or asymmetric atom localization patterns could be formed at different z positions by adjusting the parameters of the laser fields.

Ultranarrow linewidth and high gain of an optical cavity with enhanced self-Kerr nonlinearity in quantum dot molecules

The enhanced self-Kerr nonlinearity of quantum dot molecules may be used to realize optical cavities with an ultranarrow linewidth and high gain. The resonant tunneling induces constructive interference for the self-Kerr nonlinearity, and then a narrow gain window with large normal dispersion appears with frequency detuning. The competition between linear and nonlinear dispersion leads to strong normal dispersion of the total susceptibility, which significantly narrows the cavity linewidth; the nonlinear gain introduces the total gain effects contributing to high transmission. Simulation results show that the cavity linewidth could be narrowed by nearly 30 times and the transmission peak enhanced about 40 times compared with a linear case.

Observation of multi-electromagnetically induced transparency in V-type rubidium atoms

A detailed experimental investigation and theoretical analysis have been made in the V-type Rb atomic medium. Seven induced transparency windows, including a central double-peak-structure, have been observed experimentally when a coupling field and a probe field are applied into the ground and first excited states. By taking into account the hyperfine splitting of the excited state, our theoretical analysis gives good explanation for the observed phenomena.

Isolated sub-100 attosecond pulse generation driven by a multi-cycle chirped laser pulse and a polarization gating

We theoretically investigate the high-order harmonic generation driven by the combination of a multi-cycle nonlinear chirped laser pulse and a polarization gating. On the one hand, the x-component of the polarization gating enlarges the asymmetry of the pulse centre of the chirped laser field, which leads to the extension of the cutoff of the high-order harmonic spectrum; on the other hand, only a few cycles of the combination field centre remain linearly polarized due to the existence of the y-component of the polarization gating, which helps to generate the continuum spectrum. Therefore, in the high-order harmonic spectrum, a very broad continuum spectrum appears even in the multi-cycle regime, from which an isolated sub-100 as pulse could be obtained.

Cavity linewidth narrowing by optical pumping-assisted electromagnetically induced transparency in V-type rubidium at room temperature

A tunable high resolution frequency reference is constructed using the narrowed cavity-linewidth by the optical pumping-assisted V-type electromagnetically induced transparency (EIT). At room temperature, the optical pumping effect will increase the transparency for the V-type EIT and therefore the cavity-linewidth can be narrowed apparently. For the seven EITs observed in our previous study, cavity-linewidth narrowing is observed in all of them. More importantly, we find that the cavity-linewidth can keep at 1.2MHz in a wide frequency range of 100MHz by utilizing the central EIT. This property provides a novel way for constructing high resolution tunable frequency reference via the intracavity EIT.A detailed experimental investigation is made in the intracavity V-type electromagnetically induced transparency (EIT) of the Rb atomic medium at room temperature. The optical pumping effect will increase the transparency for the V-type EIT at room temperature and therefore the cavity linewidth can be narrowed apparently. For the seven EITs observed in our previous study, cavity linewidth narrowing is observed in all of them. More importantly, we find that the cavity linewidth can keep at 1.2 MHz over a wide frequency range of 100 MHz by utilizing the central double-peak-structure EIT. This property provides a novel way for constructing a high resolution tunable frequency reference via the intracavity EIT.

Coherent control of high-order harmonic generation by phase jump pulses

We theoretically investigate the high-order harmonic generation driven by laser pulses with a π-phase jump. The cutoff of high-order harmonic spectrum extends dramatically due to the phase jump which enlarges the asymmetry of the laser field. We find that the intensity and the coherence of the continuum can be controlled by the jump time. By selecting proper frequency of the continuum spectrum, a sub-100-attosecond pulse can be generated without any phase compensation.

Extremely narrowed and amplified gain spectrum induced by the Doppler effect

The effect of Doppler broadening on the gain spectrum is theoretically investigated in a hot N-type active Raman gain atomic system. It is found that the gain peak in the spectrum can be extremely narrowed and amplified by two orders at room temperature as compared with that in the cold atomic system. This remarkable result originates from the shift of the dressed levels and the modification to the transition probability between dressed states when the Doppler effect is considered in the hot atomic system. The enhanced subluminal and superluminal pulse propagation for the probe field in the hot atomic system has also been obtained by numerical simulation; the probe pulse undergoes no absorption and little pulse distortion.

Controllable twin laser pulse propagation and dual-optical switching in a four-level quantum dot nanostructure

We investigate control of the propagation dynamics of weak twin laser pulses and propose a dual-optical switching scheme in four-level semiconductor quantum dots of diamond configuration. It is shown that the propagation dynamics of the two probe pulses depend not only on the intensity of corresponding control field in each cascade transition path but also on the relative intensities of the two control fields. This property provides the probability for realizing all-optical switching in the quantum dots. Possible all-optical switching operations for any one of the probe fields or both probe fields simultaneously with the same or adverse switching status can be realized by modulating the corresponding control fields. In addition, the relative phase of the laser fields also influences the switching operation and can be used to realize optical switching, which is also discussed in the paper.

Pronounced enhancement of exciton Rabi oscillation for a two-photon transition based on quantum dot coupling control

We theoretically investigate how to control the Rabi oscillation of excitons of the coupling quantum dots by manipulating static electric fields. Our results show that, for a single-photon process, when direct excitons change into indirect excitons with a bias applied on the sample, the Rabi oscillation rarely alters. However, for the two-photon process, a pronounced enhancement of Rabi oscillation is observed, which can be utilized as the logic gate in quantum information.