Phase-locked and phase-unlocked multicolor solitons in a fiber laser.

Multicolor solitons are nonlinear pulses composed of two or more solitons centered at different frequencies, propagating with the same group velocity. In the time domain, multicolor solitons consist of an envelope multiplying a more rapidly varying fringe pattern that results from the interference of these frequency components. Here, we report the observation in a fiber laser of a novel, to the best of our knowledge, type of dynamics in which different frequency components still have the same group velocity but have different propagation constants. This causes the relative phases between the constituent spectral components to change upon propagation, corresponding to the fringes moving under the envelope. This leads to small periodic energy variations that we directly measure. Our experimental results are in good agreement with realistic numerical simulations based on an iterative cavity map.

Rational design of an integrated directional coupler for wideband operation.

We consider a design procedure for directional couplers for which the coupling length is approximately wavelength-independent over a wide bandwidth. We show analytically that two coupled planar waveguides exhibit a maximum in the coupling strength, which ensures both wideband transmission and minimal device footprint. This acts as a starting point for mapping out the relevant part of phase space. This analysis is then generalized to the fully three-dimensional geometry of rib waveguides using an effective medium approximation. This forms an excellent starting point for fully numerical calculations and leads to designs with unprecedented bandwidths and compactness.

Optics and Photonics in Sydney: introduction to the focus issue.

This focus issue provides an overview of current applied optics research activities in the Sydney region in Australia, illustrating the breadth and depth of the research carried out in the region. Below we first give an overview of some of the history of optics research in Sydney and then brief descriptions of the 10 papers in the issue.

Microwave analogy of Förster resonance energy transfer and effect of finite antenna length.

The near-field interaction between quantum emitters, governed by Förster resonance energy transfer (FRET), plays a pivotal role in nanoscale energy transfer mechanisms. However, FRET measurements in the optical regime are challenging as they require nanoscale control of the position and orientation of the emitters. To overcome these challenges, microwave measurements were proposed for enhanced spatial resolution and precise orientation control. However, unlike in optical systems for which the dipole can be taken to be infinitesimal in size, the finite size of microwave antennas can affect energy transfer measurements, especially at short distances. This highlights the necessity to consider the finite antenna length to obtain accurate results. In this study, we advance the understanding of dipole-dipole energy transfer in the microwave regime by developing an analytical model that explicitly considers finite antennas. Unlike previous works, our model calculates the mutual impedance of finite-length thin-wire dipole antennas without assuming a uniform current distribution. We validate our analytical model through experiments investigating energy transfer between antennas placed adjacent to a perfect electric conductor mirror. This allows us to provide clear guidelines for designing microwave experiments, distinguishing conditions where finite-size effects can be neglected and where they must be taken into account. Our study not only contributes to the fundamental physics of energy transfer but also opens avenues for microwave antenna impedance-based measurements to complement optical FRET experiments and quantitatively explore dipole-dipole energy transfer in a wider range of conditions.

Modulation instability with high-order dispersion: fundamental limitations of pattern formation.

We theoretically and numerically investigate modulation instability in the presence of even, high-order dispersion, focusing on general trends rather than on specific results for a particular dispersion order. We show that high-order dispersion leads to increasingly poor phase matching between the three central waves (i.e. the pump and the ±1 sidebands) and the higher sideband orders, inhibiting in effect four-wave mixing frequency generation. For sufficiently large dispersion orders, the problem in effect can reduce to a three-wave system. Our predictions are in excellent agreement with numerical simulations and show that high-order dispersion imposes a fundamental limit on modulation instability dynamics.

Experimental observation of linear pulses affected by high-order dispersion.

We experimentally study the linear propagation of optical pulses affected by high-order dispersion. We use a programmable spectral pulse-shaper that applies a phase that equals that which would result from dispersive propagation. The temporal intensity profiles of the pulses are characterized using phase-resolved measurements. Our results are in very good agreement with previous numerical and theoretical results, confirming that for high dispersion orders m the central part of the pulses follow the same evolution, with m only determining the rate of evolution.

Plasmonic Sensors beyond the Phase Matching Condition: A Simplified Approach.

The conventional approach to optimising plasmonic sensors is typically based entirely on ensuring phase matching between the excitation wave and the surface plasmon supported by the metallic structure. However, this leads to suboptimal performance, even in the simplest sensor configuration based on the Otto geometry. We present a simplified coupled mode theory approach for evaluating and optimizing the sensing properties of plasmonic waveguide refractive index sensors. It only requires the calculation of propagation constants, without the need for calculating mode overlap integrals. We apply our method by evaluating the wavelength-, device length- and refractive index-dependent transmission spectra for an example silicon-on-insulator-based sensor of finite length. This reveals all salient spectral features which are consistent with full-field finite element calculations. This work provides a rapid and convenient framework for designing dielectric-plasmonic sensor prototypes-its applicability to the case of fibre plasmonic sensors is also discussed.

Dark solitons under higher-order dispersion.

We show theoretically that stable dark solitons can exist in the presence of pure quartic dispersion, and also in the presence of both quadratic and quartic dispersive effects, displaying a much greater variety of possible solutions and dynamics than for pure quadratic dispersion. The interplay of the two dispersion orders may lead to oscillatory non-vanishing tails, which enables the possibility of bound, potentially stable, multi-soliton states. Dark soliton-like states that connect to low-amplitude oscillations are also shown to be possible. Dynamical evolution results corroborate the stability picture obtained, and possible avenues for dark soliton generation are explored.

Omnidirectional field enhancements drive giant nonlinearities in epsilon-near-zero waveguides.

Bulk materials with a relative electric permittivity ε close to zero exhibit giant Kerr nonlinearities. However, harnessing this response in guided-wave geometries is not straightforward, due to the extreme and counterintuitive properties of such epsilon-near-zero materials. Here we investigate, through rigorous calculations of the nonlinear coefficient, how the remarkable nonlinear properties of such materials can be exploited in several structures, including bulk films, plasmonic nanowires, and metal nanoapertures. We find the largest nonlinear response when the modal area and group velocity are simultaneously minimized, leading to omnidirectional field enhancement. This insight will be key for understanding nonlinear nanophotonic systems with extreme nonlinearities and points to new design paradigms.

Establishing the nonlinear coefficient for extremely lossy waveguides.

The nonlinear coefficient γ is central to the study of cubically nonlinear optical guided-wave structures. It is well understood for lossless waveguides, but less so for lossy systems. A number of methods for calculating γ in lossy systems have been proposed, each resulting in different expressions. Here we identify the most accurate and practical expression for γ. We do so by applying the different expressions γ to air-gold surface plasmon polariton modes in the interband region of gold and compare with a fully numerical iterative method. We thus resolve the outstanding issue of which expression for the nonlinear coefficient to use for lossy waveguides, enabling new insights into the nonlinear response of such systems.

Self-similar propagation of optical pulses in fibers with positive quartic dispersion.

We study the propagation of ultrashort pulses in optical fiber with gain and positive (or normal) quartic dispersion by self-similarity analysis of the modified nonlinear Schrödinger equation. We find an exact asymptotic solution, corresponding to a triangle-like T4/3 intensity profile, with a T1/3 chirp, which is confirmed by numerical simulations. This solution follows different amplitude and width scaling compared to the conventional case with quadratic dispersion. We also suggest, and numerically investigate, a fiber laser consisting of components with positive quartic dispersion that emits quartic self-similar pulses.

Modular nonlinear hybrid plasmonic circuit.

Photonic integrated circuits (PICs) are revolutionizing nanotechnology, with far-reaching applications in telecommunications, molecular sensing, and quantum information. PIC designs rely on mature nanofabrication processes and readily available and optimised photonic components (gratings, splitters, couplers). Hybrid plasmonic elements can enhance PIC functionality (e.g., wavelength-scale polarization rotation, nanoscale optical volumes, and enhanced nonlinearities), but most PIC-compatible designs use single plasmonic elements, with more complex circuits typically requiring ab initio designs. Here we demonstrate a modular approach to post-processes off-the-shelf silicon-on-insulator (SOI) waveguides into hybrid plasmonic integrated circuits. These consist of a plasmonic rotator and a nanofocusser, which generate the second harmonic frequency of the incoming light. We characterize each component's performance on the SOI waveguide, experimentally demonstrating intensity enhancements of more than 200 in an inferred mode area of 100 nm2, at a pump wavelength of 1320 nm. This modular approach to plasmonic circuitry makes the applications of this technology more practical.

Simple model for orthogonal and angled coupling in dielectric-plasmonic waveguides.

We numerically and analytically study orthogonal and angled coupling schemes between a dielectric slab waveguide and a plasmonic slot waveguide for a large range of geometric and material parameters. We obtain high orthogonal coupling transmission efficiencies (up to 78% for 2D calculations, and 54% for 3D calculations) over a wide range of refractive indices, and provide simple analytic arguments that explain the underlying trends. The insights obtained point to angled couplers with even higher coupling efficiencies (up to 86% in 2D, and 61% in 3D). We find that angled plasmonic coupling is well suited for large dielectric waveguides at the phase matching angle. These results suggest new capabilities for efficient dielectric-plasmonic interconnects that can be applied to a wide variety of material combinations and geometries.

Stationary and dynamical properties of pure-quartic solitons.

We numerically solve a generalized nonlinear Schrödinger equation and find a family of pure-quartic solitons (PQSs), existing through a balance of positive Kerr nonlinearity and negative quartic dispersion. These solitons have oscillatory tails, which can be understood analytically from the properties of linear waves with quartic dispersion. By computing the linear eigenspectrum of the solitons, we show that they are stable, but that they possess a nontrivial internal mode close to the radiation continuum. We also demonstrate evolution into a PQS from Gaussian initial conditions. The energy-width scaling of PQSs differs strongly from that for conventional solitons, opening up possibilities for PQS lasers.

Stimulated Brillouin scattering in layered media: nanoscale enhancement of silicon.

We report a theoretical study of stimulated Brillouin scattering (SBS) in general anisotropic media, incorporating the effects of both acoustic strain and local rotation. We apply our general theoretical framework to compute the SBS gain for layered media with periodic length scales smaller than all optical and acoustic wavelengths, where such composites behave like homogeneous anisotropic media. We predict that a layered medium composing nanometer-thin layers of silicon and As2S3 glass has a bulk SBS gain of 1.28×10-9 W-1 m. This is more than 500 times larger than that of silicon and almost double the gain of As2S3. The enhancement is due to a combination of roto-optic, photoelastic, and artificial photoelastic contributions in the composite structure.

A theory of waveguide design for plasmonic nanolasers.

We propose a theory for the waveguide design and analysis for plasmonic nanolasers by reformulating the fundamental waveguide requirements. This theory does not rely on further optimizing previously used structures, but examines each possible design without prejudice. Our exploration of one-dimensional (i.e., layered) plasmonic nanowaveguide geometries and the subsequent extension to 2D structures not only provides a deep understanding of the characteristics of currently used designs, but also leads to superior structures with the potential to address long-standing challenges in plasmonic nanolasers. In addition, we discover analogies between the reformulated fundamental requirements for the waveguide for nanolasers and nanoscale four-wave mixing (FWM) devices. Therefore, after a slight modification, our theory can also be applied to the waveguide design for plasmonic FWM devices.

Analysis and design of fibers for pure-quartic solitons.

The recently discovered pure-quartic solitons, arising from the interaction of quartic dispersion and Kerr nonlinearity, open the door to unexplored soliton regimes and ultrafast laser science. Here, we report a general analysis of the dispersion and nonlinear properties necessary to observe pure-quartic solitons in optical platforms. We apply this analysis, in combination with numerical calculations, to the design of pure-quartic soliton supporting microstructured optical fibers. The designs presented here, which have realistic fabrication tolerances, support unperturbed pure-quartic soliton propagation providing access to an unmatched platform to study novel soliton physics.

General analytic expression and numerical approach for the Kerr nonlinear coefficient of optical waveguides.

The Kerr nonlinear coefficient γ is a key parameter that quantifies the nonlinear strength of an optical waveguide. For lossy waveguides such as plasmonic waveguides, the literature is confusing because various expressions derived by different groups have generally not been validated, and the conditions when they apply are not explicitly specified. Here we derive a rigorous and full-vectorial model, leading to both a general analytic expression and a general numerical approach for finding γ, as well as to their underlying relationship. Our results, exemplified by lossless and lossy waveguides, are consistent not only with each other, but also with the results in literature under appropriate limiting conditions. This work provides a benchmarked framework to understand and engineer nonlinear nanophotonic devices.

Stimulated Brillouin scattering enhancement in silicon inverse opal waveguides.

Silicon is an ideal material for on-chip applications, however its poor acoustic properties limit its performance for important optoacoustic applications, particularly for stimulated Brillouin scattering (SBS). We theoretically show that silicon inverse opals exhibit a strongly improved acoustic performance that enhances the bulk SBS gain coefficient by more than two orders of magnitude. We also design a waveguide that incorporates silicon inverse opals and which has SBS gain values that are comparable with chalcogenide glass waveguides. This research opens new directions for opto-acoustic applications in on-chip material systems.

Metamaterial control of stimulated Brillouin scattering.

Using full opto-acoustic numerical simulations, we demonstrate enhancement and suppression of the SBS gain in a metamaterial comprising a subwavelength cubic array of dielectric spheres suspended in a dielectric background material. We develop a general theoretical framework and present several numerical examples using technologically important materials. For As2S3 spheres in silicon, we achieve a gain enhancement of more than an order of magnitude compared to pure silicon and for GaAs spheres in silicon, full suppression is obtained. The gain for As2S3 glass can also be strongly suppressed by embedding silica spheres. The constituent terms of the gain coefficient are shown to depend in a complex way on the filling fraction. We find that electrostriction is the dominant effect behind the control of SBS in bulk media.