Jianping Song

Observation of enhancement and suppression in four-wave mixing processes

We report our observations of enhancement and suppression between two competing four-wave mixing (FWM) processes. The results show the evolution of the dressed effects (from pure enhancement into pure suppression) in the degenerate FWM processes. Moreover, due to induced atomic coherence in the system, there exist different interplays between these two FWM processes via different detuning parameters. In addition, the power dependences of enhancement and suppression are studied. Theoretical calculations are carried out, which are in good agreement with the experimental observations.

Nonlinear Talbot effect of rogue waves

Akhmediev and Kuznetsov-Ma breathers are rogue wave solutions of the nonlinear Schrödinger equation (NLSE). Talbot effect (TE) is an image recurrence phenomenon in the diffraction of light waves. We report the nonlinear TE of rogue waves in a cubic medium. It is different from the linear TE, in that the wave propagates in a NL medium and is an eigenmode of NLSE. Periodic rogue waves impinging on a NL medium exhibit recurrent behavior, but only at the TE length and at the half-TE length with a π-phase shift; the fractional TE is absent. The NL TE is the result of the NL interference of the lobes of rogue wave breathers. This interaction is related to the transverse period and intensity of breathers, in that the bigger the period and the higher the intensity, the shorter the TE length.

Enhanced self-Kerr nonlinearity in spontaneous emission

We propose a new scheme for achieving giant enhancement of the self-Kerr nonlinearity in a four-level Y-type atom with two highest nearly degenerate lying levels. We have found that, owing to the quantum interference effect that originates from spontaneous emission from two closely lying levels, large Kerr nonlinearity can be achieved under appropriate conditions. In particular, in addition to the amplitudes, the phases of the applied fields also affect the Kerr nonlinearity. We attribute the enhancement of Kerr nonlinearity mainly to the presence of some extra atomic coherence terms induced by the spontaneously generated coherence. We present a physical understanding of our numerical results using analytical explanation.

Polarization spectroscopy of dressed four-wave mixing in a three-level atomic system

Polarization properties of pure four-wave mixing (FWM) and dressed-FWM processes in a two-level system and a cascade three-level atomic system are theoretically and experimentally investigated. The relative intensities and polarization characteristics of the FWM signals in different laser polarization configurations and different level systems are experimentally investigated and compared. Also, the results are theoretically explained by different transition paths combinations. In the dressed-FWM processes, we study the dependence of dressing effect on the incident fields polarization. The FWM signal generated by a linearly polarized pumping field is suppressed more by the dressing field than the one generated by a circularly polarized pumping field. However, an opposite effect was observed when the probe fields polarization is changed. The multidressing mechanisms are used to explain these effects. In addition, the interference and polarization dependence of the coexisting FWM signals in the same atomic system are discussed.

Three-field noise correlation via third-order nonlinear optical processes.

We report experimental observation of three-field noise correlation between two coexisting four-wave mixing (FWM) signals and the probe signal in a coherently-prepared rubidium vapor system. The two FWM signals in a double-Λ type quasi-four-level atomic system are obtained in different directions via their respective phase-matching conditions and can be detected individually. These signals and the probe beam are found to be strongly correlated or anticorrelated with each other.

Controlling enhancement and suppression of four-wave mixing via polarized light

We show that the four-wave mixing (FWM) processes in a multi-Zeeman level atomic system can be enhanced and suppressed by changing the polarization of one of the pump beams. Different polarization states of the pump beams will act on different transition pathways among the multi-Zeeman levels with different transition strengths, which affect the FWM efficiencies. An additional dress field applied to the adjacent transition can cause energy level splitting and therefore control the enhancement and suppression of the FWM processes in the system. The experimental results are in good agreement with our theoretical calculations.

Interplay among multidressed four-wave mixing processes

We report our experimental studies of interplays between four-wave mixing (FWM) processes in multidressed two- and three-level atomic systems. The Autler–Townes splitting of the FWM signal within the electromagnetically induced transparency window has been experimentally observed. Under the same experimental conditions in different energy-level systems, the dressing effects strongly depend on the dipole moments of the transitions, which can provide an easy and qualitative way to determine the orders of magnitude of the effective dipole moments for different transitions by comparing the FWM suppressing effects.

Four-Photon Imaging with Thermal Light

In a near-field four-photon correlation measurement, ghost imaging with classical incoherent light is investigated. By applying the Klyshko advanced-wave picture, we consider the properties of four-photon spatial correlation and find that the fourth-order spatial correlation function can be decomposed into multiple lower-order correlation functions. On the basis of the spatial correlation properties, a proof-of-principle four-photon ghost imaging is proposed, and the effect of each part in a fourth-order correlation function on imaging is also analyzed. In addition, the similarities and differences among ghost imaging by fourth-, second-, and third-order correlations are also discussed. It is shown that the contrast and visibility of fourth-order correlated imaging are improved significantly, while the resolution is unchanged. Such studies can be very useful in better understanding multi photon interference and multi-channel correlation imaging.

Coexisting polarized four-wave mixing processes in a two-level atomic system

We investigate the polarization dependence of eight coexisting four-wave mixing (FWM) signals in a two-level atomic system. The intensities and polarization states of coexisting FWM signals are modulated by the polarization configurations and frequency detunings of the incident fields. The suppression and enhancement due to the dressing effects present different polarization dependences. Both the mutual-dressing effect and the self-dressing effect are considered to explain the observed phenomena.

Measurement of ac-Stark shift by a two-photon dressing process via four-wave mixing

By setting laser beams in two-photon Doppler-free configurations for both cascade and Λ-type subsystems in a four-level inverted-Y system, a four-wave mixing (FWM) process is studied and optimized within the Doppler profile. The two-photon ac-Stark effect in the FWM signal is observed, which can be used to determine the energy level shift of the atom due to dressing fields.