This study investigates the propagation dynamics of higher-order optical vortices carrying cross-phase. The cross-phase gives rise to two main physical effects. First, it induces the splitting of higher-order vortices. Under linear propagation, a higher-order vortex splits into an array of elementary vortices, each with unit topological charge. For a small cross-phase coefficient, the split vortices retain the same handedness as the original vortex; for a large cross-phase coefficient, however, they exhibit opposite handedness. Second, the cross-phase can cause global rotation of the optical field, with the rotation direction reversing at a specific propagation distance. When nonlinear self-focusing is taken into account, higher-order vortices with cross-phase undergo periodic splitting and recombination. By tuning the beam power, one can controllably switch between the split and combined states. Moreover, by measuring the power variation associated with the state switching, the nonlocal response parameter of the nonlinear material can be quantitatively determined in experiments.

We study mode localization in a quasi-one-dimensional disordered array of titanium dioxide waveguides above a silver film embedded inside a fused silica matrix. Disorder is introduced by randomly offsetting waveguides from their periodic positions by up to 20% of the array period. Using a perturbative coupled mode theory with nearest-neighbor coupling, we study how mode localization changes with increasing disorder and array size. Our results show mode localization, as measured by both the average effective mode area and variance for all modes, intensifies with increasing disorder, especially for the fundamental modes with tighter plasmonic confinement. Remarkably, we find the average variance scales with the array size, resulting in a universal dependence of normalized variance on disorder for arrays of any size. These findings may have implications for designing photonic devices and developing efficient photodetectors.

A kind of controllable chaotic self-trapped patterns is introduced in nonlocal nonlinear system with sine-oscillatory response. The optical patterns are generated when experiencing the defocusing nonlinearity, and accompanied by the beam splitting. Both the generating distance, at which the chaotic self-trapped pattern forms, and the pattern width can be handily controlled. The generating distance depends closely on the power of optical beams, and the pattern width relates to the characteristic length of nonlinear responses. By increasing the characteristic length, the transition from the self-trapping-like propagation to the chaotic self-trapped propagation for optical beams is realized. Furthermore, the chaotic self-trapped pattern can also induce stable optical waveguides, in which optical beams can be guided and evolve as the soliton states. It also has been shown that the spatial chirp has almost no impact on the generation of the chaotic self-trapped patterns.

By introducing anisotropy into nonlinear propagations, off-axis vortex beams exhibit significantly different characteristics compared to the isotropic case. The orbital angular momentum (OAM) is non-conservative and can periodically change between positive and negative values. Accordingly, the rotation of phase singularity can transit between clockwise and counterclockwise directions. Furthermore, the phase singularity can move to infinity when the OAM approaches zero. By using the Ehrenfest theorem, the motion of the beam center is obtained. Its trajectory can be circular and parabolic or follow other complex shapes, depending closely on the anisotropy of the nonlinearity. The rotational velocity of the beam center can be modulated by the nonlinearity anisotropy and can far exceed the initial value during its propagation. These results may find potential applications in beam shaping and optical manipulation.

We propose and demonstrate the effectual generation and control of nonparaxial self-accelerating beams by using UV-resin pendant droplets. We show that the geometrical shape of the hanging droplets formed as a result of the interplay between surface tension and gravity offers a natural curvature enabling the generation of nonparaxial self-accelerating beams. By simply adjusting the tilt angle of the surface where the droplets reside, a passing light beam is set to propagate along different curved trajectories, bending into large angles with non-diffracting features superior to a conventional Airy beam. Such self-accelerating beams are directly traced experimentally through the scattered light in yeast-cell suspensions, along with extensive ray tracing and numerical simulations. Furthermore, by modifying the shape of uncured pendant resin droplets in real time, we showcase the dynamical trajectory control of the self-accelerating beams. Our scheme and experimental method may be adopted for droplet-based shaping of other waves such as microfluidic jets and surface acoustic waves.

We study light localization in two-dimensional disordered arrays of hybrid plasmonic waveguides. Coupled mode theory and finite element simulations are used to investigate the mutual effects of surface plasmons and disorder on mode localization.

We study the coupling between a nanoscopic dielectric slot waveguide and a metallic-tip-enhanced dipole quantum emitter and search for optimal emission and guiding of photons along a titanium dioxide photonic waveguide. Finite element calculations are used to characterize the mode properties of the waveguide over a range of parameters and predict the efficiency of emission guidance along the waveguide. We show how the effective quantum yield is dependent on the waveguide parameters as well as the dipole position and orientation and show that the integrated system can reach a maximum quantum yield of more than 30% along each direction of the waveguide. By placing the dipole emitter near the apex of a silver nanotip situated above or inside the slot waveguide to increase the overall emission rate, we also enable the capability of controlling the emitter–waveguide​ coupling in situ. We numerically compute the excitation and radiation fields and find that despite a reduction in the quantum yield, a 35-fold increase in the overall emission rate can be obtained due to very large field enhancement factors of up to 4000 at the location of the dipole. By using alternative materials for the tip, this tip-enhanced-waveguide approach has the potential of further increasing the guided photon emission rate. With the possibility of controlling emitter–waveguide coupling in situ, this tip-enhanced emitter configuration provides an alternative approach for the realization of an efficient single-photon source.

Food dyes are widely used in scientific research apart from the food colorant industry. In this work, we investigate nonlinear propagation of optical beams in food dye solutions of different colors, experimentally observing tunable nonlinearity and self-collimating effects of optical beams during propagation. We discuss possible mechanisms leading to optical nonlinearity by numerical simulation and theoretical analysis of the optical forces, and show that the nonlinearity in such solutions is mainly attributed to the thermo-optic effects. In addition, with appropriately mixed food dye solutions, we observe that nonlinear response arises at otherwise inactive wavelengths, leading to coupling of two self-collimated beams of different wavelengths. These results illustrate the possibility of employing low-cost food dye solutions for optical limiting and switching applications.

We demonstrate generation and dynamical control of self-accelerating beams simply using tilted UV-resin pendant-droplets. The curved trajectories of such large-angle self-bending beams are directly observed through scattering of yeast-cell suspensions, in excellent agreement with simulations.

Fluorescence Correlation Spectroscopy (FCS) measures the concentrations and dynamics of fluorescent particles by analyzing the correlation in the concentration fluctuation of particles diffusing in a solution. Due to its diffraction-limited sample volume, the method is only applicable for concentrations that are typically in the nM range. However, biological processes under physiological conditions often involve molecules in the μM concentration range and require sample volumes well below the established limit of about one femtoliter in traditional FCS.

We study the coupling of a quantum emitter to a nanoscopic dielectric slot waveguide using numerical simulations and have found a unidirectional quantum yield of 25% can be achieved with a realistic design.

Fluorescence Correlation Spectroscopy (FCS) is a method of investigating concentration fluctuations of fluorescent particles typically in the nM range as a result of its femtoliter-sized sample volume. However, biological processes on cell membranes that involve molecules in the μM concentration range require sample volumes well below the conventional FCS limit as well as nanoscale confinement in the longitudinal direction. In this study, we show that an effective measurement volume down to the zeptoliter range can be achieved via the introduction of a nanowire waveguide, resulting in an illumination spot of about 50 nm in lateral dimensions and a longitudinal confinement of around 20 nm just above the waveguide exit surface. Using illumination profiles obtained from finite element method simulations of dielectric nanowaveguides, we perform Monte Carlo simulations of fluorescence fluctuations for two scenarios of fluorophore movement: fluorophores freely diffusing in the three-dimensional (3D) space above the nanowaveguide and fluorophores moving in a two-dimensional (2D) membrane situated directly above the nanowaveguide exit surface. We have developed analytical functions to fit the simulation results and found that an effective illumination size of about 150 zl and 4 × 10−3 µm2 can be obtained for the 3D and 2D scenarios, respectively. Given the flat surface geometry and the deep-subwavelength confinement of its illumination spot, this nanowaveguide-illuminated fluorescence correlation spectroscopy technique may be well suited for studying the concentration and dynamics of densely distributed protein molecules on cell membranes.

Controlled orientation and alignment of rod‐shaped plasmonic nanoparticles are of great interest for many applications. Herein, it is demonstrated that the nonlinear optical response of gold nanorod suspensions is dynamically controlled by electric field‐induced orientation. Merely by switching incident light polarization, the longitudinal and transverse surface plasmon resonance (SPR) absorption peaks are modulated with opposite trends, and the resulting optical nonlinearity is revealed from self‐trapping of plasmonic resonant solitons. Moreover, even with a very low concentration of fluorescent molecules, a significant increase in the fluorescent signal is observed with a transmittance‐type volume detection scheme. Such an enhancement is attributed to a combined action of optical force‐induced nonlinearity and electric field‐induced nanorod orientation, as explained by the theoretical analyses. Herein, new possibilities for engineering nonlinear plasmonic soft matter and detecting low‐concentration (yet a large total number of moleculesas needed) fluorescent samples are brought out.

Molecular imprinting is a technique to synthesize molecularly imprinted polymers (MIPs) with specific and selective recognition ability for a given template molecule. As a molecular recognition component, MIPs combined with highly sensitive fluorescent detection to construct molecular imprinting-based fluorescence sensors (MI-FL sensors), which have attracted much attention in the field of trace detection of environmental organic pollutants. According to the expression of fluorescence emission signal, this review introduces the various strategies for MI-FL sensors. The applications of MI-FL sensors in pesticide residues, estrogen, and antibiotic in the environment are described briefly, and the challenges and development are prospected.

Fluorescence Correlation Spectroscopy (FCS) is a method for investigating particle concentration fluctuations in small volumes that are usually defined by a focused laser beam for excitation and a confocal pinhole for collection resulting in a size of about one femtoliter. Such sampling volumes are suitable for investigating dynamics of molecules in the nM concentration range. However, biological processes on cell membranes that involve molecules in the μM concentration range require sampling volumes well below the conventional FCS limit as well as nanoscale confinement in the longitudinal direction. In this study, we demonstrate that an effective measurement volume as small as one zeptoliter can be achieved via the introduction of a nanowire waveguide resulting in an illumination spot of about 50 nanometers in lateral dimensions and a longitudinal confinement of around 20 nanometers. Using illumination profiles obtained from finite element method simulations of dielectric nanowaveguides, we perform Monte Carlo simulations of fluorescence fluctuation for fluorophores that are confined in a membrane situated directly above the nanowaveguide exit surface with a background of fluorophores freely diffusing in the solution above the membrane. We have developed an analytical model to fit the simulation results and show that the signal is dominated by fluorophores on the membrane due to the extreme confinement of illumination in the longitudinal direction. These nanowaveguide devices are being fabricated and experiments are currently under way to verify the simulation results.

We provide the first demonstration of nonlinear self-trapping of light in solutions of food coloring dyes, leading to dirt cheap soliton waveguides. The optical self-focusing nonlinearity observed in such solutions is attributed to the optical absorption force acting on dye molecules, as the thermal effect at relatively higher power results in a self-defocusing nonlinearity. We found the nonlinear response increases when the laser wavelength is switched to near the peak absorption for all food dyes tested. Furthermore, the absorption-dependent nonlinearity can be tuned by making purposive samples of different food dyes. In particular, with appropriately mixing of dye solutions, nonlinear response arises for otherwise inactive wavelengths, leading to interaction between two beams of different wavelengths and formation of a coupled soliton-like pair. At milliwatt low power levels, self-trapping achieved at one wavelength in food-dye solutions can serve as low-cost waveguides for optical beams at other wavelengths.

We perform first-principle calculations of silicon vacancies induced ferromagnetism in 6H-SiC. Our results showed that except for V-Si(0) defects, both V-Si(-) and V-Si(2-) defects can induce local magnetic moments, most of which are derived from the C 2p states of the nearest-neighbor atoms of silicon vacancies. Furthermore, We found the coupling between the V-Si(-) defects is antiferromagnetic, while the V-Si(2-) defects favor a ferromagnetic coupling. The 6H-SiC systems with V-si and N-C (substitution of carbon atom with nitrogen atom) can show the same properties of spin-polarization and magnetic coupling as the systems of V-si with attached extra electrons, as N-C introduces carriers and a higher carrier concentration favors a stable ferromagnetic behavior. Therefore, ferromagnetism in 6H-SiC is determined by both the silicon vacancy and the carrier concentration. Our theoretical calculations provide possible guidance for the design of magnetic SiC.

We experimentally demonstrate self-trapping of light, as a result of plasmonic resonant optical nonlinearity, in both aqueous and organic (toluene) suspensions of gold nanorods. The threshold power for soliton formation is greatly reduced in toluene as opposed to aqueous suspensions. It is well known that the optical gradient forces are optimized at off-resonance wavelengths at which suspended particles typically exhibit a strong positive (or negative) polarizability. However, surprisingly, as we tune the wavelength of the optical beam from a continuous-wave (CW) laser, we find that the threshold power is reduced by more than threefold at the plasmonic resonance frequency. By analyzing the optical forces and torque acting on the nanorods, we show theoretically that it is possible to align the nanorods inside a soliton waveguide channel into orthogonal orientations by using merely two different laser wavelengths. We perform a series of experiments to examine the transmission of the soliton-forming beam itself, as well as the polarization transmission spectrum of a low-power probe beam guided along the soliton channel. It is found that the expected synthetic anisotropic properties are too subtle to be clearly observed, in large part due to Brownian motion of the solvent molecules and a limited ordering region where the optical field from the self-trapped beam is strong enough to overcome thermodynamic fluctuations. The ability to achieve tunable nonlinearity and nanorod orientations in colloidal nanosuspensions with low-power CW laser beams may lead to interesting applications in all-optical switching and transparent display technologies.

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