Topological insulators are a new phase of matter, with the striking property that conduction of electrons occurs
only on the surface. In two dimensions, surface electrons in topological insulators do not scatter despite defects
and disorder, providing robustness akin to superconductors. Topological insulators are predicted to have wideranging
applications in fault-tolerant quantum computing and spintronics. Recently, large theoretical efforts were
directed towards achieving topological insulation for electromagnetic waves. One-dimensional systems with
topological edge states have been demonstrated, but these states are zero-dimensional, and therefore exhibit no
transport properties. Topological protection of microwaves has been observed using a mechanism similar to
the quantum Hall effect, by placing a gyromagnetic photonic crystal in an external magnetic field. However,
since magnetic effects are very weak at optical frequencies, realizing photonic topological insulators with scatterfree
edge states requires a fundamentally different mechanism - one that is free of magnetic fields. Recently, a
number of proposals for photonic topological transport have been put forward. Specifically, one suggested
temporally modulating a photonic crystal, thus breaking time-reversal symmetry and inducing one-way edge
states. This is in the spirit of the proposed Floquet topological insulators, where temporal variations in solidstate
systems induce topological edge states. Here, we propose and experimentally demonstrate the first external
field-free photonic topological insulator with scatter-free edge transport: a photonic lattice exhibiting topologically
protected transport of visible light on the lattice edges. Our system is composed of an array of evanescently coupled
helical waveguides arranged in a graphene-like honeycomb lattice. Paraxial diffraction of light is described by
a Schrödinger equation where the propagation coordinate acts as ‘time’. Thus the waveguides' helicity breaks zreversal
symmetry in the sense akin to Floquet Topological Insulators. This structure results in scatter-free, oneway
edge states that are topologically protected from scattering.
We present two potentially interesting new venues in all-optical signal processing. First, we demonstrate experimentally that collisions between vector (Manakov-like) solitons involve energy exchange; this feature could be explored for all-optical signal processing. Second, our detailed theoretical studies show how inserting materials that support electro-magnetically induced transparency into microcavities enables design of microcavities with extraordinarily long lifetimes, and enables all-optical signal processing at single photon power levels.
We demonstrate experimentally a fixed two-dimensional (2D) periodic waveguide array by using plane-wave interference in a photorefractive (PR) ferroelectric crystal, and then report on the experimental observation of diffraction management in the 2D "fixed" periodic waveguide arrays.
We demonstrate, for the first time, optical guidance of light beams using incoherent light. Such guidance is made possible by generating self-trapped dark beams (dark spatial solitons) inside a bulk photorefractive material using spatially incoherent light. We show that, in the 1D case, planar or Y-splitting waveguides induced by self-trapped incoherence dark stripes can guide other coherent light of a different wavelength. In the 2D case, incoherent dark solitons generated from optical vortices induce refractive- index changes akin to circular channel waveguides. These experiments introduce the possibility of controlling high- power laser beams with low-power incoherent light sources such as Light Emitting Diodes.
Since they have been predicted and observed six years ago, photorefractive spatial solitons have attracted substantial research interest. Photorefractive solitons bring about several new fundamental aspects related to solitons in general. Perhaps the single most important aspect is being the first system in which solitons were demonstrated. This has enabled the study of interactions between 2D solitons in a full 3D medium, which has revealed a fundamentally new property of interacting solitons: conversation of angular momentum when the solitons are bound to each other in a spiraling configuration. Another key property of the photorefractive nonlinearity that has had a major impact on soliton research, is its non-instantaneous nature. This has allowed us to generate a new type of 'self-trapped' light beams: incoherent solitons, which are made of partially spatially incoherent light or of temporally and spatially incoherent white light. In this review, we start from the formation mechanism of photorefractive spatial solitons, and especially focus on the bright screening solitons. We then describe the waveguides induced by these solitons, and use this understanding to explain and demonstrate soliton interactions, which are probably the most fascinating features of all solitons in nature, because they shows how a soliton is related to areal particle. Then, we describe Incoherent Solitons and end by discussing several ideas on how to utilize the photorefractive solitons for useful applications.
Steady-state dark photorefractive spatial screening solitons are observed in an odd- or even-number sequence when a laser beam containing a dark stripe generated from a phase or amplitude discontinuity in the center of the beam is launched into a biased bulk strontium barium niobate (SBN) crystal. If the initial width of the dark stripe is small, only a fundamental soliton or a Y-junction soliton is generated, corresponding to the lowest order in the odd- or even-number soliton sequence. As the initial width and the bias field are increased, we observe a progressive transition from a lower-order soliton to a sequence of higher-order solitons. We show that these dark solitons induce stable waveguides which can guide an intense beam of a different wavelength into multiple channels.
Focusing and defocusing of laser light has been observed for many years. Kerr type materials exhibit this effect but only for high intensities. We show experimental evidence that photorefractive materials can also produce dramatic focusing and defocusing. Whereas Kerr materials produce this effect for high intensities, photorefractive materials produce these effects independent of intensity indicating that this effect would be ideal for an optical limiter. We compare the characteristics of Kerr and photorefractive materials, discuss the physical models for both materials and present experimental evidence for photorefractive defocusing. Self-focusing and defocusing was observed for any incident polarization although the effect was more pronounced using extraordinary polarized light. In addition, self-focusing or defocusing could be observed depending on the direction of the applied electric field. When the applied field was in the same direction as the crystal spontaneous polarization, focusing was observed. When the applied field was opposite the material spontaneous polarization, the incident laser light was dramatically defocused.
We suggest a method for coding high resolution computer-generated volume holograms. It involves splitting the computer-generated hologram into multiple holograms, each individually recorded as a volume hologram utilizing the maximal resolution available from the spatial light modulator. Our method enables their simultaneous subsequent reconstruction. We demonstrate the recording and the reconstruction of a computer-generated volume hologram with a space bandwidth product much higher than the maximal one of the spatial light modulator used as an interface. Finally, we analyze the scheduling procedure of the multiple holographic recording process in photorefractive medium in this specific application.