Nonlinear Optical Waveguides Research Articles

Conservation laws and Darboux transformation for the coupled cubic–quintic nonlinear Schrödinger equations with variable coefficients in nonlinear optics

In this paper, by Darboux transformation and symbolic computation we investigate the coupled cubic–quintic nonlinear Schrodinger equations with variable coefficients, which come from twin-core nonlinear optical fibers and waveguides, describing the effects of quintic nonlinearity on the ultrashort optical pulse propagation in the non-Kerr media. Lax pair of the equations is obtained, and the corresponding Darboux transformation is constructed. One-soliton solutions are derived; some physical quantities such as the amplitude, velocity, width, initial phases, and energy are, respectively, analyzed; and finally an infinite number of conservation laws are also derived. These results might be of some value for the ultrashort optical pulse propagation in the non-Kerr media.

Actively coupled optical waveguides

We consider light propagation through a pair of nonlinear optical waveguides with absorption, placed in a medium with power gain. The active medium boosts the in-phase component of the overlapping evanescent fields of the guides, while the nonlinearity of the guides couples it to the damped out-of-phase component creating a feedback loop. As a result, the structure exhibits stable stationary and oscillatory regimes in a wide range of gain-loss ratios. We show that the pair of actively-coupled ($\mathcal{AC}$) waveguides can act as a stationary or integrate-and-fire comparator sensitive to tiny differences in their input powers.

Double pulse generation in a highly nonlinear GaAs optical waveguide

This paper presents a simulation and investigation about the nonlinear interplay between an optical pulse and GaAs waveguide. The results obtained show that the nonlinear processes including self-phase modulation, 2-photon absorption, and free carrier related effects will have significant influences on the temporal shape and frequency spectrum of a propagating pulse in the GaAs waveguide with the result that 2-photon absorption can extend the pulse duration that can be again compressed by the free carrier absorption. The outcome spectrum is also asymmetrical due to the free carrier related effects, which takes on a complicated oscillation structure resulting from the interference. In addition, an interesting phenomenon is that the input pulse will eventually evolve into a double pulse along the GaAs waveguide by means of judiciously adjusting the intensity, waveform, and time duration of the input pulse, i.e. when the time duration of the input pulse is larger than the carrier lifetime, the input pulse with high enough intensity can develop into a double pulse at the end of the waveguide.

Nonlinear optical waveguides fabricated in Mg-doped LiNbO3 by swift heavy ion irradiation: anomalous photorefractive damage behavior

Using swift heavy fluorine ion irradiation, we have successfully fabricated optical waveguides in Mg-doped LiNbO3 substrates. A systematic characterization of these structures has been carried out including refractive index profiles, propagation losses, nonlinear coefficients, and, specially, photorefractive optical damage. Step-like refractive index profiles with Δne ≈ 0.1 and Δno ≈ 0.2, propagation losses lower than 0.5 dB/cm and high nonlinear optical coefficients similar to those of the substrate have been obtained. Unexpectedly, the photorefractive damage is only moderately reduced with regard to the one presented in congruent LiNbO3 waveguides. Specifically, light intensity damage thresholds Ith are only a factor 2 higher at RT and a factor 4 at 90 °C with regard to undoped waveguides. At this latter temperature, a remarkably high Ith = 30.000 W/cm2 is reached. A final discussion on the observed anomalous optical damage behavior induced by swift heavy ion irradiation is also included.

Femtosecond laser writing over silver nanoparticles system embedded in silica using nonlinear microscopy

We present results for the induction and monitoring of structural modification of a composite consisting of elongated silver nanoparticles films embedded in silica using ultrafast femtosecond laser irradiation and second harmonic generation imaging, respectively. Waveguide-like patterns are written and characterized under a laser scanning nonlinear microscope system by simply changing the laser fluence in the sample; switching in this way between two different physical processes occurring only within the composite film: second harmonic generation and laser induced nanoparticles removal. A study of the nanoparticles damage process as a function of the laser energy, polarization and scan velocity is also presented and discussed. The use of the non-irradiated zone between two written channels is proposed as a potential linear and nonlinear optical waveguide.

Nonlinear Optical Bioinspired Peptide Nanostructures

A nonlinear optical effect of a second harmonic generation (SHG) was first observed in quartz and then found in many inorganic materials that have an asymmetric crystalline structure. Second‐order nonlinear responses were also found in organic and biomaterials and are exploited for fundamental studies of helical and chiral biological molecules, such as proteins and amyloid fibrils. Another class of biomaterials is man‐made bioinspired nanostructures, which are composed of chemically synthesized biomolecules and are self‐assembled into supramolecular nanofibrils. Most of these materials have an asymmetric crystalline structure and possess ferroelectric and related phenomena. Here, the SHG effect is studied in bio‐organic peptide nanostructures of different morphologies and symmetries. These nanostructures are self‐assembled in different solvents from peptide precursors with a variable number of phenylalanine (F) amino acid units. A pronounced SHG response is detected in FFF‐nanobelts, FF‐nanotubes, and FFF‐nanospheres. SHG and Raman spectroscopy studies during phase transformation in FF‐nanotubes allow the definition of the intermolecular bonds responsible for SHG. Using two‐photon optical microscopy, orientational molecular ordering in aligned peptide supramolecular structures is found by adapting a generic model developed earlier for diverse biological protein fibrils. Efficient optical frequency conversion from NIR to green and blue light is demonstrated, as well as an effect of nonlinear optical waveguiding. These results suggest these bioinspired nanostructures as promising for a new generation of nonlinear optical nanomaterias, which can be integrated into nanophotonic devices.

Analysis and design of a rib-like-based slot waveguide for nonlinear silicon nanophotonics

Design of efficient nonlinear optical waveguides is an essential requirement for development of silicon nanophotonics. These waveguides should have a unique capability to play the main role in realization of both passive and active optical devices. A high-performance dielectric rib-like-based slot waveguide is proposed for nonlinear silicon nanophotonics. Its slot region can be filled with Si-nc:SiO2 which exhibits a high third-order nonlinear effect. Study of numerical results shows that this new slot waveguide has a nonlinear parameter of the same order of magnitude as an equally sized silicon nanophotonics strip-based slot waveguide. The new waveguide can be fabricated easily by etching a slot in the core region of the silicon-on-insulator (SOI)-based rib-like waveguide.

Temporal mode selectivity by frequency conversion in second-order nonlinear optical waveguides

We explore theoretically the feasibility of using frequency conversion by sum- or difference-frequency generation, enabled by three-wave-mixing, for selectively multiplexing orthogonal input waveforms that overlap in time and frequency. Such a process would enable a drop device for use in a transparent optical network using temporally orthogonal waveforms to encode different channels. We model the process using coupled-mode equations appropriate for wave mixing in a uniform second-order nonlinear optical medium pumped by a strong laser pulse. We find Green functions describing the process, and employ Schmidt (singular-value) decompositions thereof to quantify its viability in functioning as a coherent waveform discriminator. We define a selectivity figure of merit in terms of the Schmidt coefficients, and use it to compare and contrast various parameter regimes via extensive numerical computations. We identify the most favorable regime (at least in the case of no pump chirp) and derive the complete analytical solution for the same. We bound the maximum achievable selectivity in this parameter space. We show that including a frequency chirp in the pump does not improve selectivity in this optimal regime. We also find an operating regime in which high-efficiency frequency conversion without temporal-shape selectivity can be achieved while preserving the shapes of a wide class of input pulses. The results are applicable to both classical and quantum frequency conversion.

Three-input majority function with nonlinear material in all-optical domain

In this paper, we have developed a nonlinear directional coupler theory to describe a novel all-optical majority function. Our scheme is composed of linear and nonlinear optical waveguides in which switching operation between them depending on the power of input light. Adjusting the refractive indices, and selecting the proper length of waveguides are simulated by use of RSoft CAD-Layout (BeamPROP) simulator.

Self-trapping threshold in disordered nonlinear photonic lattices

We investigate numerically and experimentally the influence of coupling disorder on the self-trapping dynamics in nonlinear one-dimensional optical waveguide arrays. The existence of a lower and upper bound of the effective average propagation constant allows for a generalized definition of the threshold power for the onset of soliton localization. When compared to perfectly ordered systems, this threshold is found to decrease in the presence of coupling disorder.

Transverse localization in nonlinear photonic lattices with second-order coupling

We investigate numerically the effect of long-range interaction on the transverse localization of light. To this end, nonlinear zigzag optical waveguide lattices are applied, which allows precise tuning of the second-order coupling. We find that localization is hindered by coupling between next-nearest lattice sites. Additionally, (focusing) nonlinearity facilitates localization with increasing disorder, as long as the nonlinearity is sufficiently weak. However, for strong nonlinearities, increasing disorder results in weaker localization. The threshold nonlinearity, above which this anomalous result is observed grows with increasing second-order coupling.

Temporal Characteristics of Pulsed Squeezing in a Nonlinear Optical Waveguide

We experimentally investigate the temporal characteristics of picosecond squeezed pulses generated via an optical parametric amplification in a periodically poled MgO:LiNbO3 waveguide. The noise variances for the various time delays between the squeezed pulse and a local oscillator are measured using a balanced homodyne detector. The measured delay dependence is asymmetric, and this asymmetry indicates that the phase chirping of the squeezed pulse is different from that of the local oscillator pulse. We show that the observed squeezing can be improved by using the pulse shaping of the local oscillator.

OPTICAL BISTABILITY IN ONE-DIMENSIONAL COUPLED RESONATOR NONLINEAR OPTICAL WAVEGUIDE

In this paper, the optical bistability in finite size one-dimensional coupled resonator nonlinear optical waveguide has been investigated. To calculate the bistable switching manner in five proposed structures, we use the linear and nonlinear transfer matrices with fixed point iteration method. Our results show that the threshold and domain of bistable switching answer depend on the defect region in one-dimensional coupled resonator nonlinear optical waveguides. Also, the best answer of switch takes place at the left and right edges of the photonic band gap of the structures depending on the defect regions. In fact, due to the presence of the photonic band gap in the slow wave system, the density of states of the electromagnetic fields is large at the photonic band edge and photonic defect modes and thus the group velocity is small, and the local field is enhanced. All these factors contribute to a significant decrease in the threshold of optical bistable switching devices.

Bandstructure measurement in nonlinear optical waveguide arrays

We demonstrate a technique for measuring the linear bandstructure of periodic systems with quadratic nonlinearity. The phase-matching constraints of parametric optical frequency mixing are employed to probe the longitudinal wavevectors of the eigenmodes. The method is verified experimentally by measuring the diffraction relation of eigenmodes in periodically poled titanium-indiffused lithium niobate waveguide arrays with second-harmonic generation.

Efficient second harmonic conversion efficiency through one-dimensional coupled resonator poled nonlinear optical waveguide

In this paper, the second harmonic generation in finite size one-dimensional coupled resonator poled nonlinear optical waveguide has been investigated. To calculate the conversion efficiency, fundamental and second harmonic wave propagation among two proposed structures, we use the transfer matrix method. In the designed nonlinear photonic crystal structure, the linear and nonlinear optical parameters are both periodically modulated, and thus the second harmonic generation efficiency can be several orders of magnitude larger than in a conventional quasi-phase-matched nonlinear structure with the same sample length. In fact, due to the presence of a photonic band gap in a slow wave system, the density of states of the electromagnetic fields is large at the photonic band edge and photonic defect modes, and thus the group velocity is small, and the local field is enhanced. All these factors contribute to significant enhancement of the nonlinear optical interactions.

Contrast and phase closure acquisitions in photon counting regime using a frequency upconversion interferometer for high angular resolution imaging

In the context of new generation instruments for astronomical interferometer, we have developed in laboratory a new kind of optical interferometer using the frequency conversion of the star light. We investigate the sensitivity limit and reliability of our so-called upconversion interferometer when operating in photon counting mode. For this purpose, we have implemented a laboratory test bench including two three-arm interferometers dedicated to high angular resolution imaging. The first classical one works at the same wavelength as the laboratory star (infrared) and operates with high optical flux levels (used as a reference interferometer). The second one, under test, uses sum frequency generation process in a non-linear optical waveguide placed on each arm to shift the wavelength of the infrared laboratory star to the visible domain. The observables are obtained in photon counting operation, involving signal processing to recover unbiased data. The reference measurement (high flux complex visibilities in infrared) and the upconversion interferometer observables (complex visibilities obtained in photon counting regime) are in good agreement. We notice a degradation on the phase closure terms reliability for low values of the triple product (bispectrum). We estimate from the experimental results the related limiting magnitude for several configurations using an upconversion interferometer.

Theoretical Analysis of Multimodal Four-Wave Mixing in Optical Microwires

Optical fiber microwires (OFMs) are nonlinear optical waveguides that support several spatial modes. The multimodal generalized nonlinear Schrödinger equation (MM-GNLSE) is deduced taking into account the linear and nonlinear modal coupling. A detailed theoretical description of four-wave mixing (FWM) considering the modal coupling is developed. Both, the intramode and the intermode phase-matching conditions is calculated for an optical microwire in a strong guiding regime. Finally, the FWM dynamics is studied and the amplitude evolution of the pump beams, the signal and the idler are analyzed.

Exponential Attractor for Coupled Ginzburg-Landau Equations Describing Bose-Einstein Condensates and Nonlinear Optical Waveguides and Cavities

The existence of the exponential attractors for coupled Ginzburg-Landau equations describing Bose-Einstein condensates and nonlinear optical waveguides and cavities with periodic initial boundary is obtained by showing Lipschitz continuity and the squeezing property.

Analysis of long-range surface plasmon polaritons in nonlinear plasmonic waveguides using pseudospectral method

A full-vectorial pseudospectral method is reported for solving the mode characteristics of nonlinear dielectric and plasmonic waveguides. The coupled equations are formulated in terms of transverse magnetic-field components, and self-consistent solutions are obtained through an iterative procedure. The proposed scheme applies in a saturable medium with biaxial anisotropy of practical interest. The accuracy and efficiency of this scheme are demonstrated by solving the mode bistability of a nonlinear dielectric optical waveguide, analyzed by the well-known finite-element-method-based imaginary-distance beam propagation method. Furthermore, the relationship between geometry and input power is studied by analyzing the power dispersion curve of the long-range surface plasmon polariton modes of a nonlinear plasmonic waveguide.

Spinons and holons with polarized photons in a nonlinear waveguide

We show that the spin–charge separation predicted for correlated fermions in one dimension could be observed using polarized photons propagating in a nonlinear optical waveguide. Using coherent control techniques and employing a cold atom ensemble interacting with the photons, large nonlinearities in the single-photon level can be achieved. We show that the latter can allow for the simulation of a strongly interacting gas, which is made of stationary dark-state polaritons of two species and is shown to form a Luttinger liquid of effective fermions for the right regime of interactions. The system can be tuned optically to the relevant regime where the spin–charge separation is expected to occur. The characteristic features of the separation as demonstrated in different spin and charge densities and velocities can be efficiently detected via optical measurements of the emitted photons with current optical technologies.