Xiaosheng Wang

Observation of optical Shockley-like surface states in photonic superlattices

We provide what we believe to be the first experimental demonstration of linear Shockley-like surface states in an optically induced semi-infinite photonic superlattice. Such surface states appear only when the induced superlattice consisting of alternating strong and weak bonds is terminated properly at the surface. Our experimental results are in good agreement with our theoretical analysis.

Observation of lower to higher bandgap transition of one-dimensional defect modes

We demonstrate one-dimensional optically-induced photonic lattices with a negative defect and observe linear bandgap guidance in such a defect. We show that a defect mode moves from the first bandgap to a higher bandgap as the lattice potential is increased. Our experimental results are in good agreement with the theoretical analysis of these effects.

OBSERVATION OF ONE- AND TWO-DIMENSIONAL DISCRETE SURFACE SPATIAL SOLITONS

The recent theoretical predictions and experimental observations of discrete surface solitons propagating along the interface between a one- or two-dimensional continuous medium and a one- or two-dimensional waveguide array are reviewed. These discrete solitons were found in second order (periodically poled lithium niobate) and third order nonlinear media, including AlGaAs, photorefractive media and glass, respectively.

Guiding light in optically induced ring lattices with a low-refractive-index core

We demonstrate a ring-shaped Bessel-like photonic lattice akin to a photonic bandgap fiber with a low-index core. While the ring lattice is optically induced in a bulk crystal with a self-defocusing nonlinearity, guidance of a probe beam propagating linearly through the core is clearly observed. The possible mechanism for such guidance is also discussed.

Beam control and multi-color routing with spatial photonic defect modes

We demonstrate tunable re-directing, blocking, and splitting of a light beam along defect channels based on spatial bandgap guidance in two-dimensional photonic lattices. We show the possibility for linear control of beam propagation and multicolor routing with specially designed junctions and surface structures embedded in otherwise uniform square lattices.

Demonstration of surface soliton arrays at the edge of a two-dimensional photonic lattice

We demonstrate surface soliton arrays at the interface between a homogeneous medium and an optically induced two-dimensional semi-infinite photonic lattice. These are nonlinear Tamm-like surface states localized in one but extended periodically in the other transverse dimension. Both in-phase and staggered out-of-phase soliton arrays are observed, and the experimental results are corroborated by numerical simulations.

Self-trapping of optical vortices in waveguide lattices with a self-defocusing nonlinearity

We demonstrate the self-trapping of single- and double-charged optical vortices in waveguide lattices induced with a self-defocusing nonlinearity. Under appropriate conditions, a donut-shaped single-charged vortex evolves into a stable discrete gap vortex soliton, but a double-charged vortex turns into a self-trapped quadrupole-like structure. Spectrum measurement and numerical analysis suggest that the gap vortex soliton does not bifurcate from the edge of the Bloch band, quite different from previously observed gap spatial solitons. Our numerical findings are in good agreement with experimental observations.

Observation of dipole-like gap solitons in self-defocusing waveguide lattices

We observe dipole-like gap solitons in two-dimensional waveguide lattices optically induced with a self-defocusing nonlinearity. Under appropriate conditions, two mutually coherent input beams excited in neighboring lattice sites evolve into a self-trapped state, whose spatial power spectrum and stability depend strongly on the initial excitation conditions. Our experimental observations are compared with numerical simulations.

Light Localization by Defects in Optically Induced Photonic Structures

In the past ten years, there has blossomed an interest in the study of collective behavior of wave propagation in periodic waveguide arrays and photonic lattices [1–3]. The unique bandgap structures of these periodic media, coupled with nonlinear effects, give rise to many types of novel soliton structures [1– 26]. On the other hand, it is well known that one of the unique and most interesting features of photonic band-gap structures is a fundamentally different way of waveguiding by defects in otherwise uniformly periodic structures. Such waveguiding has been demonstrated with an “air-hole” in photonic crystal fibers (PCF) for optical waves [27, 28], in an isolated defect in two-dimensional arrays of dielectric cylinders for microwaves [29–31], and recently in all-solid PCF with a lower-index core [32, 33]. In addition, laser emission based on photonic defect modes has been realized in a number of experiments [34–38]. In one-dimensional (1D) fabricated semiconductor waveguide arrays, previous experiments have investigated nonlinearity-induced escape from a defect state [39] and interactions of discrete solitons with structural defects [40] (see also [41]). Despite the above efforts, theoretical understanding on defect guiding was still limited, and experimental demonstrations of defect guiding was still scarce. In addition, when nonlinear effects are significant, how defect guiding is affected by nonlinearity is largely an open issue. Recently, in a series of theoretical and experimental studies, we optically induced 1D, 2D and ringlike photonic lattices with single-site negative defects in photorefractive crystals, and investigated their linear and nonlinear light guiding properties [42–48]. This work will be reviewed in this Chapter. In addition, we present the first experimental demonstration of nonlinear defect modes which undergoes nonlinear propagation through the defects. Our work not only has a direct link to technologically important systems of periodic structures such as PCF, but also brings about the possibility for studying, in an optical setting, many novel phenomena in periodic systems beyond optics such as edge dislocation, defect healing, eigenmode splitting, and nonlinear mode coupling which have been intriguing scientists for decades [49–51].

Optical Shockley-Like Surface States in Photonic Superlattices

Applications of photonic structures continue to expand not only into the optical equivalents of semiconductor properties but into quantum optics.