Qi Guo

Nonlocality-controlled interaction of spatial solitons in nematic liquid crystals

The authors demonstrate experimentally that interaction between nonlocal solitons in nematic liquid crystals (NLCs) can be controlled by the degree of nonlocality. For a given beam width, the degree of nonlocality can be modulated by changing the pretilt angle θ0 of NLC molecules through bias voltage V. As V increases (so does θ0), the degree of nonlocality decreases. When the degree of nonlocality is below a critical value, the solitons behave in the way like their local counterpart, i.e., in-phase solitons attract while out-of-phase solitons repulse each other. Such a voltage-controlled interaction between the solitons can be readily implemented in experiments.

Analytical vectorial structure of radially polarized light beams.

Starting from the vector angular spectrum of the electromagnetic beam, the analytical vectorial structure of the radially polarized beams (RPBs) is presented. The energy flux distributions of the RPBs are demonstrated. The physical pictures of the RPBs are well illustrated from the vectorial structure. This particular electromagnetic field is entirely transverse magnetic, and on axis it only has a longitudinal (z) electric-field component (i.e., no transverse electric field and no magnetic field at all on axis).

Hermite-Gaussian breathers and solitons in strongly nonlocal nonlinear media

Based on the Snyder-Mitchell model in the Cartesian coordinate system, exact analytical Hermite-Gaussian (HG) solutions are obtained in strongly nonlocal nonlinear media. The comparisons of analytical solutions with numerical simulations of the nonlocal nonlinear Schrodinger equation show that the analytical HG solutions are in good agreement with the numerical simulations in the case of strong nonlocality. Furthermore, we demonstrate that HG functions can be expressed as a linear superposition of individual Gaussian functions with a π phase difference under the appropriate conditions.

Propagation of radially polarized elegant light beams

A radially polarized elegant beam (RPEB) in which the argument of the Laguerre part is complex is introduced and studied. The nonparaxial and paraxial propagation properties of the RPEB are demonstrated analytically and numerically in free space in terms of the vectorial Rayleigh-Sommerfeld formulas. The electric field distribution of the RPEB preserves the radial polarization at any propagation position. For the lowest-order RPEB and the lowest-order radially polarized beam cases, the general expressions for the nonparaxial propagation field are identical in free space. The exact on-axis expression of the RPEB has been provided in closed-form terms for an arbitrary transverse beam size.

All-optical diodes based on photonic crystal molecules consisting of nonlinear defect pairs

We investigate the unidirectional transmission behavior of photonic crystal (PC) molecules consisting of nonlinear defect pairs by use of the coupled mode theory and the finite-difference time-domain technique. As compared with the all-optical diodes based on single asymmetrically confined PC defects (or PC atoms), a significant enhancement in the transmission contrast is achieved. A qualitative comparison of the dynamical response to external excitation is carried out for nonlinear PC atoms and molecules and the physical origins responsible for the enhancement of transmission contrast are clarified. In addition, the tolerance of the resulting optical diodes against the fabrication error is discussed. Furthermore, it is revealed that the figure of merit, which is defined as the product of the threshold transmission and the transmission contrast, can be greatly improved by dropping the use of air defects. Some abnormal and intriguing transmission behaviors are observed in the PC molecules without using air...

Elegant Hermite–Laguerre–Gaussian beams

A novel family of paraxial laser beams called elegant Hermite-Laguerre-Gaussian beams (EHLGBs) are presented. The EHLGBs, a unity of elegant Hermite-Gaussian beams (EHGBs) and elegant Laguerre-Gaussian beams (ELGBs), constitute the exact and continuous transition modes between EHGBs and ELGBs when an additional parameter continuously changes. The virtual source for generation of the EHLGBs is identified. From the spectral representation of the elegant Hermite-Lauguerre-Gaussian waves, we derive the first three orders of nonparaxial corrections for the corresponding paraxial EHLGBs.

Design of highly efficient optical diodes based on the dynamics of nonlinear photonic crystal molecules

We investigate the unidirectional transmission behavior of photonic crystal (PC) molecules consisting of defect pairs with Kerr nonlinearity and focus on how to enhance the transmission contrast and maximum transmission of the resulting optical diodes. Theoretical analyses in combination with the numerical simulations based on the finite-difference time-domain technique are employed to evaluate the designed optical diodes. It is found that by intentionally and properly misaligning the resonant frequencies of the constitutional PC atoms, the transmission contrast as well as the maximum transmission of the nonlinear PC molecules can be significantly improved. The figure of merit that characterizes the performance of optical diodes can be enhanced by a factor of 5 as compared with the optical diodes constructed by single asymmetrically confined PC atoms. In addition, the optimum performance of the optical diodes can be achieved only when the operating frequency is properly chosen.

All-optical switching using controlled formation of large volume three-dimensional optical matter

We demonstrated the creation of large volume three-dimensional optical matter by optically trapping polystyrene spheres in a capillary and the resulting switching operation. The formation of optical matter was confirmed by examining the diffraction pattern of the trap region. Optical switching with an extinction ratio as large as ∼20dB was realized. From the dynamics of the optical matter, it was found that the transition from a disordered state to an ordered one appeared to be quite fast while the recovery of the system to the disordered state took a much longer time.

Self-induced Anderson localization and optical limiting in photonic crystal coupled cavity waveguides with Kerr nonlinearity

The transmission behavior of photonic crystal coupled cavity waveguides (CCWs) with Kerr nonlinearity is investigated by numerical simulations based on the finite-difference time-domain technique. The authors find that a nearly ideal optical limiter can be realized by use of a nonlinear CCW. In addition, it is revealed that Anderson localization [Phys. Rev. 109, 1492 (1958)] of the extended states in the impurity band instead of the shift of the impurity band is responsible for the observed optical limiting. Therefore, nonlinear CCWs offer a convenient platform for studying Anderson localization of electromagnetic waves in a controlled fashion and will find potential applications in optical limiting and switching.

Self-induced transparency in colloidal liquids by Z-scan-based optical trapping

We demonstrated a transition from a disordered to an ordered state in a colloidal liquid by utilizing Z-scan-based optical trapping. The Z-scan process plays a role of gradually and continuously narrowing and deepening the optical potential well. When the trapping power was increased above a certain level, a self-induced transparency occurs, leading to a significant enhancement in transmission. The dynamic transition was confirmed by monitoring the diffraction pattern of the trapping region.