Coherent propulsion with negative-mass fields in a photonic lattice.

In this Letter, we demonstrate the first, to the best of our knowledge, coherent propulsion with negative-mass fields in an optical analog. We observe a self-accelerating state, driven by a nonlinear coherent interaction of its two components that are experiencing diffractions of opposite signs in a photonic lattice, which is analogous to the interaction of two objects with opposite mass signs. Surprisingly, the coherent propulsion is highly immune to the initial phase of the two components, which is in sharp contrast with the behavior encountered in traditional coherent wave interactions.

Spontaneous diametric-drive acceleration initiated by a single beam in a photonic lattice.

We demonstrate that a single Gaussian-like beam can self-bend during nonlinear propagation in a uniform photonic lattice. The two components of the beam experiencing normal and anomalous diffractions spontaneously separate and form a pair in a diametric-drive acceleration due to nonlinear action. Such a diametric drive generally describes a jointly accelerating behavior of two beams analogous to positive- and negative-mass objects.

Optical amplification of Airy beams by photorefractive two-wave mixing.

We propose and demonstrate nonlinear amplifications of self-accelerating Airy beams by two-wave mixing in photorefractive crystals both numerically and experimentally. By employing a broad Gaussian beam as the pump beam, we show that weak signal Airy beams can be significantly amplified under both diffusion and drift mechanisms. It is revealed that not only higher optical gains but also faster response time can be achieved in the presence of an external electric field, where the drift mechanism dominates.

Observation of spatial optical diametric drive acceleration in photonic lattices.

We experimentally and theoretically demonstrate a spatial diametric drive acceleration of two mutually incoherent optical beams in 1D optical lattices under a self-defocusing nonlinearity. The two beams, exciting the modes at the top/bottom edges of the first Bloch band and hence experiencing normal/anomalous diffraction, can bind together and bend in the same direction during nonlinear propagation, analogous to the interplay between two objects with opposite signs of mass that breaks Newton's third law. Their spatial spectrum changes associated with the acceleration are analyzed for different lattice modulations.

Image signal transmission with Airy beams.

We propose and demonstrate an approach for image signal transmission based on self-accelerating Airy beams. The spatial information is encoded in the Fourier space through a 4-f telescope system, which can circumvent obstacles to realize a self-bending signal transmission. Furthermore, the information can be retrieved from the Airy beams after propagation through a disordered scattering medium.

THz band-stop filter using metamaterials surfaced on LiNbO(3) sub-wavelength slab waveguide.

We designed and implemented periodic bar arrays metamaterials to select appropriate frequencies of terahertz (THz) waves propagating in a LiNbO(3) sub-wavelength waveguide. The spatial and temporal electric field profiles of the THz waves were recorded using a time-resolved phase-contrast imaging system. The metamaterials can operate as a band-stop filter to realize blocking back THz waves in a band range of 0.

Dynamical deformed Airy beams with arbitrary angles between two wings.

We study both numerically and experimentally the acceleration and propagation dynamics of 2D Airy beams with arbitrary initial angles between their "two wings." Our results show that the acceleration of these generalized 2D Airy beams strongly depends on the initial angles and cannot be simply described by the vector superposition principle (except for the normal case of a 90° angle). However, as a result of the "Hyperbolic umbilic" catastrophe (a two-layer caustic), the main lobes of these 2D Airy beams still propagate along parabolic trajectories even though they become highly deformed.

Generation of linear and nonlinear propagation of three-Airy beams.

We report the first experimental demonstration of the so-called three-Airy beams. Such beams represent a two-dimensional field that is a product (rather than simple superposition) of three Airy beams. Our experiments show that, in contrast to conventional Airy beams, this new family of Airy beams can be realized even without the use of truncation by finite apertures.

Reshaping the trajectory and spectrum of nonlinear Airy beams.

We demonstrate theoretically and experimentally that a finite Airy beam changes its trajectory while maintaining its acceleration in nonlinear photorefractive media. During this process, the spatial spectrum reshapes dramatically, leading to negative (or positive) spectral defects on the initial spectral distribution under a self-focusing (or defocusing) nonlinearity.

Tunable self-shifting Bloch modes in anisotropic hexagonal photonic lattices.

We study controllable self-shifting Bloch modes in anisotropic hexagonal photonic lattices. The shifting results from a deformed band structure due to deformation of the index distribution in each unit cell. By reconfiguration of the index profile of the unit cell, the direction in which the Bloch modes move can be controlled.

Acceleration control of Airy beams with optically induced refractive-index gradient.

We demonstrate both experimentally and theoretically controlled acceleration of one- and two-dimensional Airy beams in optically induced refractive-index potentials. Enhancement as well as reduction of beam acceleration are realized by changing the index gradient, while the beam shape is maintained during propagation through the linear optical potential. Our results of active acceleration manipulation in graded media are pertinent to Airy-type beam propagation in various environments.

Anomalous interactions of spatial gap solitons in optically induced photonic lattices.

We demonstrate coherent interactions between spatial gap solitons in optically induced photonic lattices. Because of the "staggered" phase structures, two in-phase (out-of-phase) bright gap solitons can repel (attract) each other at close proximity, in contrast to soliton interaction in homogeneous media. A reversal of energy transfer direction and a transition between attractive and repulsive interaction forces can be obtained solely by changing the initial soliton separation relative to the lattice spacing.

Persistence and breakdown of Airy beams driven by an initial nonlinearity.

We study the behavior of Airy beams propagating from a nonlinear medium to a linear medium. We show that an Airy beam initially driven by a self-defocusing nonlinearity experiences anomalous diffraction and can maintain its shape in subsequent propagation, but its intensity pattern and acceleration cannot persist when driven by a self-focusing nonlinearity. The unusual behavior of Airy beams is examined from their energy flow as well as the Brillouin zone spectrum of self-induced chirped photonic lattices.

Optimal control of the ballistic motion of Airy beams.

We demonstrate the projectile motion of two-dimensional truncated Airy beams in a general ballistic trajectory with controllable range and height. We show that the peak beam intensity can be delivered to any desired location along the trajectory as well as repositioned to a given target after displacement due to propagation through disordered or turbulent media.

Observation of bandgap guidance of optical vortices in a tunable negative defect.

We experimentally demonstrate linear bandgap guidance of optical vortices as high-gap defect modes (DMs) in two-dimensional induced photonic lattices. We show that donut-shaped vortex beams can be guided in a tunable negative (lower-index) defect, provided that the defect strength is set at an appropriate level. Such vortex DMs have fine features in the "tails" associated with the lattice anisotropy and can be considered as a superposition of dipole DMs.

Self-trapping of optical vortices at the surface of an induced semi-infinite photonic lattice.

We demonstrate self-trapping of singly-charged vortices at the surface of an optically induced two-dimensional photonic lattice. Under appropriate conditions of self-focusing nonlinearity, a singly-charged vortex beam can self-trap into a stable semi-infinite gap surface vortex soliton through a four-site excitation. However, a single-site excitation leads to a quasi-localized state in the first photonic gap, and our theoretical analysis illustrates that such a bandgap surface vortex soliton is always unstable.

Tuning of Bloch modes, diffraction, and refraction by two-dimensional lattice reconfiguration.

We demonstrate controlled excitation of Bloch modes and manipulation of diffraction and refraction in optically induced two-dimensional photonic lattices. Solely by adjusting the bias condition, the lattice structures can be reconfigured at ease, enabling the observation of transition between Bloch modes associated with different high-symmetry points of a photonic band, and interplay between normal and anomalous diffraction as well as positive and negative refraction under identical excitation condition.

Saddle solitons: a balance between bi-diffraction and hybrid nonlinearity.

We demonstrate self-trapping of light by simultaneously compensating normal and anomalous (saddle-shaped) diffractions with self-focusing and self-defocusing hybrid nonlinearity in optically induced ionic-type photonic lattices. Innovative two-dimensional gap solitons, named "saddle solitons," are established, whose phase and spectrum characteristics are different from all previously observed spatial solitons.

Orientation-dependent excitations of lattice soliton trains with hybrid nonlinearity.

We demonstrate selective excitation of soliton trains residing in different gaps or within the same Bloch band of a new type of photonic lattice merely by changing the orientation of an input probe beam. A self-focusing and -defocusing hybrid nonlinearity as established in a nonconventionally biased photorefractive crystal leads to controlled soliton transitions from different band edges or subband edges, in good agreement with our theoretical analysis.

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.

Elliptical discrete solitons supported by enhanced photorefractive anisotropy.

We demonstrate elliptical discrete solitons in an optically induced two-dimensional photonic lattice. The ellipticity of the discrete soliton results from enhanced photorefractive anisotropy and nonlocality under a nonconventional bias condition. We show that the ellipticity and orientation of the discrete solitons can be altered by changing the direction of the lattice beam and/or the bias field relative to the crystalline c-axis.

Optically induced transition between discrete and gap solitons in a nonconventionally biased photorefractive crystal.

We show that optically induced photonic lattices in a nonconventionally biased photorefractive crystal can support the formation of discrete and gap solitons owing to a mechanism that differs from the conventional screening effect. Both the bias direction and the lattice orientation can dramatically influence the nonlinear beam-propagation dynamics. We demonstrate a transition from self-focusing to -defocusing and from discrete to gap solitons solely by adjusting the optical-beam orientation.

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.

Nonlinear spectrum reshaping and gap-soliton-train trapping in optically induced photonic structures.

We report the first theoretical prediction and experimental demonstration of gap soliton trains in a self-defocusing photonic lattice. Without a priori spectral or phase engineering, a stripe beam whose spatial power spectrum lies only in one transverse direction evolves into a gap soliton train with power spectrum growing also in the orthogonal direction due to nonlinear transport and spectrum reshaping. Our results suggest that, in nonlinear k-space evolution, energy can transfer not only between regions of normal and anomalous diffraction, but also from initially excited regions to initially unexcited regions.

Elliptical solitons in nonconventionally biased photorefractive crystals.

We theoretically predict and experimentally observe the two-dimensional (2-D) bright solitons in a nonconventionally biased strontium barium niobate (SBN) crystal. A theory describing light propagating in an SBN crystal with a bias field along an arbitrary direction is formulated. Then the existence of 2-D bright solitons in such a crystal is numerically verified.