Zang Wei-Ping

Influence of nonlinear absorption on Z-scan measurements of nonlinear refraction

A simple division of close-aperture Z-scan curve by open-aperture Z-scan is conveniently used to obtain the nonlinear refractive index. It usually causes an error, which even reaches up to over 50% for Z-scan measurements with a pinhole or a medium with a high nonlinear absorption. Here the influence of nonlinear absorption on the determination of nonlinear refraction by Z-scan is analysed. We suggest that the error can be reduced greatly by a simple analysis of the symmetric features (symmetric method) of Z-scan curves from the closed-aperture Z-scan curve. As an example, experiments were carried out on CS2 solution of C60 derivative, symmetric method agrees well with exact simulation.

Highly Directional Emission from a Subwavelength Slit in Metal-Dielectric Layered Films

The directional light emission from a single subwavelength slit surrounded by periodic grooves in layered films consisting of Ag and transparent dielectric is analysed numerically by the finite difference time domain method. The results show that the transmission through this structure is strongly confined by the modulation of the dielectric film with grooves on the output side. The role of evanescent waves in this phenomenon is discussed. It is the re-diffraction of the evanescent waves (that are generated by the diffraction of the subwavelength slit) caused by the grooves on the dielectric film that leads to the directional transmission. Some suggestions are given to obtain beaming light with high transmittance.

Coordinate Transformation for Fast Simulation of Z-Scan Measurements

We apply a new method consisting of a coordinate transformation and two-step scheme to simulate numerically Z-scan characteristics of a nonlinear medium. A comparison of numerical results obtained by this new method with those by the conventional numerical method is made. The results show that the new scheme yields a large, orders-of-magnitude range improvement in computational speed and accuracy for the analysis of the closed-aperture and open-aperture Z-scan characteristics, compared with the conventional numerical method.

Non-Paraxial Split-Step Semi-Vectorial Finite-Difference Method for Three-Dimensional Wide-Angle Beam Propagation

A wide-angle, split-step finite-difference method with the classical local one-dimensional scheme is presented to analyze the 3-D semi-vectorial wave equation. The method requires only matrix multiplication for beam propagation. To validate the effectiveness, numerical results for the eigen-mode propagation in tilted step-index channel waveguides are studied, and results show that the method has high accuracy and numerical efficiency.

A Novel Three-Dimensional Wide-Angle Beam Propagation Method Based on Split-Step Fast Fourier Transform

A novel three-dimensional wide-angle beam propagation method based on the split-step fast Fourier transform is developed. The formulation is based on the three-dimensional Helmholtz wave equation. Each propagation step is performed by utilizing both the FFT and split-step scheme. The solution of Helmholtz wave equation does not make the slowly varying envelope and one-way propagation approximations. To validate the efficiency and accuracy, numerical results for a propagation beam in a tilted step-index optical waveguide are compared with other beam propagation algorithms.

Effect of coupling between linear absorption and nonlinear absorption on Z-scan measurements

The contribution of multiphoton absorption to open-aperture Z-scan measurements is analysed by coordinate transformation. The approximate closed-form solution is obtained, which is suitable for application in higher-optical power regime. As an example, the coupling between linear absorption and two-photon absorption is discussed in detail.

Large Enhancement of Optical Nonlinearities of New Organophosphorus Fullerene Derivative

Optical nonlinearities of new organophosphorus fullerene derivative were determined by the Z-scan method with a pulsed Q-switch Nd:YAG laser at 532 nm. The experimental results demonstrated that the derivative has much larger excited-states nonlinear absorption and nonlinear refraction than C60. A five-level model was utilized to fit the experimental data, and a good agreement is reached. Some parameters such as excited-state absorption cross and refraction cross were obtained. To our knowledge, the excited-state cross section of new organophosphorus fullerene derivative and its effective ratio to the ground-state cross section are the largest values among the fullerene derivatives reported to date.

Theoretical Analysis on Self-Defocusing Mode-Locking in a Ring Resonator

A theoretical analysis of passive mode-locking in a thermal-stable ring cavity containing a Kerr medium and an aperture is presented. The analysis complying the beam propagation theory shows that the power dependence of the cavity loss is extremely sensitive to the relative location of the Kerr medium and the aperture, and is different for the counterpropagating modes, so the unidirectional operation is expected. The combination of Kerr medium with an aperture acts as a position feedback, which is similar to the effect of a saturable absorber.