Topological oscillated edge states in trimer lattices.

We investigate a 1D trimer optical lattice model. Two kinds of topological oscillating optical transmission phenomena at edges are shown. The exact and the approximate solutions of the system's edge states are obtained with and without the inversion symmetry for this system respectively. Based on the solutions, the existence and the periods of the oscillations can be controlled arbitrarily. Moreover, in a system without inversion symmetry, controlling the incident beam can eliminate both types of oscillations, resulting in a more stable edge state compared to the one with inversion symmetry. This prompts us to reconsider topological systems under symmetry protection.

Engineering of energy band and its impact on the light transmission in a non-reciprocal Hermitian hourglass lattice.

We study a quasi-one-dimensional non-reciprocal Hermitian hourglass photonic lattice that can accomplish multiple functions. Under the effect of non-reciprocal coupling, this lattice can produce an energy isolation effect, two kinds of flatbands, and energy band inversion. The excitation and propagation of a single energy band and multiple energy bands can be realized; in the flatband condition, the system has compact localized states, and the flatbands can be excited by a straightforward method. Our findings advance the theory of energy band regulation in artificial photonic lattices.

Light localization in defective periodic photonic moiré-like lattices.

Photonic moiré-like lattices, a readily accessible platform for realizing the spatial localization of light, attract intensive attention due to their unique flatband characteristics. In this paper, a periodic moiré-like lattice with embedded defects is proposed theoretically, and the linear propagation of the probe beam in such a system is investigated intensively. The results show that the positions of defects in periodic moiré-like lattices depend on the sublattice rotation angle. Further studies show that the localization of light could be improved by adjusting the apodization function of defects. In addition, the experimental observation of the moiré-like lattice with apodized defects also confirms the theoretical analysis. Our study enriches the physical connotation of photonic moiré lattices and guides the design of novel photonic crystal fibers.

Imaginary coupling induced Dirac points and group velocity control in the non-reciprocal Hermitian lattice.

We propose a mechanism to achieve the group velocity control of bifurcation light via an imaginary coupling effect in the non-reciprocal lattice. The physical model is composed of two-layer photonic lattices with non-reciprocal coupling in each unit cell, which can support a real energy spectrum with a pair of Dirac points due to the hermicity. Furthermore, we show that the systems experience topological phase transition at the Dirac points, allowing the existence of topological edge states on the left or right boundaries of respective lattice layers. By adjusting the imaginary coupling and the wave number, the group velocity of the light wave can be manipulated, and bifurcation light transmission can be achieved both at the Dirac points and the condition without the group velocity dispersion. Our work might guide the design of photonic directional couplers with group velocity control functions.

Hybridized nanolayer modified Ω-shaped fiber-optic synergistically enhances localized surface plasma resonance for ultrasensitive cytosensor and efficient photothermal therapy.

Inadequate sensitivity and side-effect are the main challenges to develop cytosensors combining with therapeutic potential simultaneously for cancer diagnosis and treatment. Herein, localized surface plasma resonance (LSPR) based on hybridized nanolayer modified Ω-shaped fiber-optic (HN/Ω-FO) was developed to integrate cytosensor and plasmonic photothermal treatment (PPT). On one hand, hybridized nanolayers improve the coverage of nanoparticles and refractive index sensitivity (RIS). Moreover, the hybridized nanoploymers of gold nanorods/gold nanoparticles (AuNRs/AuNPs) also result in intense enhancement in electronic field intensity (I). On the other hand, Ω-shaped fiber-optic (Ω-FO) led to strong bending loss in its bending part. To be specific, a majority of light escaped from fiber will interact with HN. Thus, HN/Ω-FO synergistically enhances the plasmonic, which achieved the goal of ultrasensitive cytosensor and highly-efficient plasmonic photothermal treatment (PPT). The proposed cytosensor exhibits ultrasensitivity for detection of cancer cells with a low limit of detection down to 2.6 cells/mL was realized just in 30 min. HN/Ω-FO-based LSPR exhibits unique characteristics of highly efficient, localized, and geometry-dependent heat distribution, which makes it suitable for PPT to only kill the cancer cells specifically on the surface or surrounding fiber-optic (FO) surface. Thus, HN/Ω-FO provides a new approach to couple cytosensor with PPT, indicating its great potential in clinical diagnosis and treatment.

Asymmetric localization and symmetric diffraction-free transmission in synthetic photonic lattice with anti-parity-time symmetry.

We study, to the best of our knowledge, the first observations of light propagation in synthetic photonic lattice with anti-parity-time symmetry by tuning the gain or loss of two coupled fiber rings alternatively and corresponding phase distribution periodically. By tuning the phase φ and the wave number Q in the lattice, asymmetric transmission of the light field can be achieved for both long and short loops when φ≠nπ/2 (n is an integer). Further investigations demonstrate that asymmetric localization of the light field in the long loop and symmetric diffraction-free transmission in two loops can both be realized by changing these two parameters. Our work provides a new method to obtain anti-parity-time symmetry in synthetic photonic lattice and paves a broad way to achieve novel optical manipulation in photonic devices.

Observation of reverse self-sweeping effect in an all-polarization-maintaining bidirectional ytterbium-doped fiber laser.

In this article, we report, to the best of our knowledge, the first observation of the reverse self-sweeping phenomenon in an all-polarization-maintaining bidirectional ytterbium-doped fiber laser. Conventional behaviors, including the dependence of sweeping range, sweeping rate and average pulse repetition rate on the pump power, can be observed in our fiber laser. Two couplers with ratio of 50/50 and 10/90 are respectively employed as the output coupler in fiber laser, which generates the reverse self-sweeping phenomenon for comparison.

Asymmetric localization induced by non-Hermitian perturbations with PT symmetry in photonic lattice.

We study both theoretically and numerically the asymmetric localization of lightwave in a three-layered photonic lattice with non-Hermitian perturbations. The results indicate that the gauge potential for photons can arise from the non-Hermitian perturbations, once the perturbations satisfy parity-time symmetry. Further study shows that the Peierls phase between adjacent waveguides has an important impact on the shapes of the band structures, which result in asymmetric localization of a lightwave in such a system when the wave number and Peierls phase satisfy k=ϕ=±π/2. This Letter provides a new way to control the light transmission and a feasible method to realize gauge potential for photons.

Black phosphorus flakes covered microfiber for Q-switched ytterbium-doped fiber laser.

We demonstrate a passively Q-switched ytterbium-doped fiber laser based on black phosphorus (BP) flakes covered microfiber. The BP saturable absorber is fabricated by sandwiching a microfiber between two pieces of polydimethylsiloxane supported BP flakes film, which is prepared by the mechanical exfoliation method. In this case the BP flakes can be well protected from the action of air and moisture. By incorporating BP flakes covered microfiber into a ytterbium-doped ring fiber laser, stable and reliable Q-switched operation at 1064 nm can be realized via interaction between few-layers BP flakes and the evanescent field of the laser. The laser allows Q-switched pulse generation with a repetition rate in the range of 26-76 kHz and a pulse duration in the range of 5.5-2.0 µs, by varying the pump power from 38 mW to 100 mW.

Two-dimensional non-reciprocal transmission in dynamically modulated photonic lattices.

We propose a method for realizing two-dimensional (2D) non-reciprocal (one-way) transmission of discretized light in a dynamically modulated optical waveguide array. By adjusting the phase of the modulation between the defect site and the most adjacent waveguides, asymmetric transmission in the same layer or between the first and third waveguide layers can be obtained. In particular, when the defect waveguide is lossless, 2D non-reciprocal transmission is realized perfectly.

Nonlinear evolution of Airy-like beams generated by modulated waveguide arrays.

We numerically study the formation of modulated waveguide generated Airy-like beams and their subsequent evolution in homogeneous medium. The results show that the Airy-like beams could be generated from narrow Gaussian beams propagating in one-dimensional transverse separation modulated unbent, cosine bent, or logarithm bent waveguide arrays, respectively. The waveguide-generated Airy-like beams maintain their characteristics when propagating without nonlinearity or under the self-defocusing nonlinearity in homogeneous medium, while the beams are distorted under the self-focusing nonlinearity. The deformation depends on the waveguide bending and the outgoing angles of the Airy-like beams. Our results provide a new way to generate and manipulate the Airy-like beam.

Light localization and nonlinear beam transmission in specular amorphous photonic lattices.

We demonstrate specular photonic "lattices" with random index variations at disordered positions of lattice sites. These amorphous lattice structures, optically induced in a bulk nonlinear crystal, remain invariant during propagation since they are constructed from random components residing on a fixed ring in momentum space. We observe linear spatial localization of a light beam when probing through different "defect" points in such specular lattices, as well as the nonlinear destruction of localized modes. In addition, we illustrate the possibility of image transmission through the disordered lattices, when a self-defocusing nonlinearity is employed.

Observation of accelerating Wannier-Stark beams in optically induced photonic lattices.

We generate optical beams analogous to the Wannier-Stark states in semiconductor superlattices and observe that the two main lobes of the WS beams self-bend (accelerate) along two opposite trajectories in a uniform one-dimensional photonic lattice. Such self-accelerating features exist only in the presence of the lattice and are not observed in a homogenous medium. Under the action of nonlinearity, however, the beam structure and acceleration cannot be preserved. Our experimental observations are in qualitative agreement with theoretical predictions.

Asymmetric light propagation in transverse separation modulated photonic lattices.

We investigate, both theoretically and numerically, the asymmetric light propagation in transverse separation modulated photonic lattices. The theoretical results show that the transmission contrast η of the structure is determined only by the coupling strengths between the chirped lattice and the two boundary uniform lattice portions. The numerical studies demonstrate that η is independent of the separation modulation function of the chirped lattice, which is in good agreement with the theory.

Light controlling in transverse separation modulated photonic lattices.

We numerically study the propagation of Gaussian beams in four different types of transverse separation modulated photonic lattices. We find the modulation obeying hyperbolic secant or rectangular functions can act as optical potentials, and the light waves can be localized or recur in the regions between such two positive potentials, respectively. While the beams decay in the regions between such two negative potentials since these structures could not support localized modes. Our results provide new ways for light controlling and manipulation in photonic lattices.

Polychromatic solitons and symmetry breaking in curved waveguide arrays.

We theoretically and experimentally study the nonlinear propagation of polychromatic light in curved waveguide arrays. We show that at moderate light powers the nonlinear self-action breaks the left-right symmetry of the polychromatic beam, resulting in the separation of different spectral components owing to the wavelength-dependent spatial shift. At high light powers a diffraction-managed polychromatic soliton is formed. These results demonstrate new possibilities for tunable demultiplexing and spatial filtering of supercontinuum light.

Observation of nonlinear surface waves in modulated waveguide arrays.

We describe theoretically and study experimentally nonlinear surface waves at the edge of a modulated waveguide array with a surface defect and a self-defocusing nonlinearity. We fabricate such structures in a LiNbO(3) crystal and demonstrate the beam switching to different output waveguides with a change of the light intensity due to nonlinear coupling between the linear surface modes supported by the array.

Linear and nonlinear discrete light propagation in weakly modulated large-area two-dimensional photonic lattice slab in LiNbO3:Fe crystal.

A weakly modulated large-area two-dimensional square photonic lattice slab was fabricated through optical induction technique in a photorefractive photovoltaic LiNbO(3):Fe crystal. Bragg-matched diffraction technique was used to characterize the square photonic lattice slab. Interestingly, linear discrete diffraction typical for waveguide arrays was observed in such a square photonic lattice slab, indicating that the lattice slab can be viewed effectively as a one-dimensional waveguide array. Furthermore, discrete soliton was demonstrated in the photonic lattice slab due to a saturable self-defocusing nonlinearity arising from the bulk photorefractive photovoltaic effect of LiNbO(3):Fe.