Yi-Fan Chen

Negative refraction at optical frequencies in nonmagnetic two-component molecular media.

The sign of the refractive index of any medium is soley determined by the requirement that the propagation of an electromagnetic wave obeys Einstein causality. Our analysis shows that this requirement predicts that the real part of the refractive index may be negative in an isotropic medium even if the electric permittivity and the magnetic permeability are both positive. Such a system may be a route to negative index media at optical frequencies. We also demonstrate that the refractive index may be positive in left-handed media that contain two molecular species where one is in its excited state.

Criteria for the experimental observation of multidimensional optical solitons in saturable media.

Criteria for experimental observation of multidimensional optical solitons in media with saturable refractive nonlinearities are developed. The criteria are applied to actual material parameters (characterizing the cubic self-focusing and quintic self-defocusing nonlinearities, two-photon loss, and optical-damage threshold) for various glasses. This way, we identify operation windows for soliton formation in these glasses. It is found that two-photon absorption sets stringent limits on the windows. We conclude that, while a well-defined window of parameters exists for two-dimensional solitons (spatial or spatiotemporal), for their three-dimensional spatiotemporal counterparts such a window does not exist, due to the nonlinear loss in glasses.

Induced group-velocity dispersion in phase-mismatched second-harmonic generation

We show theoretically and experimentally that, in phase-mismatched second-harmonic generation, an effective group-velocity dispersion is induced at the second-harmonic frequency. Experimental results agree quite well with the theoretical predictions. Although large phase mismatch is required to induce appreciable group-velocity dispersion in birefringent nonlinear crystals, similar effects can be observed in quasi-phase-matched structures at much lower phase mismatch. Some implications of the induced dispersion for pulse propagation in quadratic media are discussed.

Sign of the refractive index in a gain medium with negative permittivity and permeability

We show how the sign of the refractive index in any medium may be derived using a rigorous analysis based on Einstein causality. In particular, we consider left-handed materials, i.e., media that have negative permittivities and permeabilities at the frequency of interest. We find that the consideration of gain in such media can give rise to a positive refractive index.

Negative refractive index in a right-handed medium

Our theoretical analysis shows that the refractive index may be negative in a homogeneous isotropic non-magnetic medium with positive electric permittivity. This provides a possible route to negative refraction at optical frequencies.