All-optical multi-wavelength-channel ReLU activation function.

Optical neural networks (ONNs) are custom optical circuits promising a breakthrough in low-power, parallelized, and high-speed hardware, for the growing demands of artificial intelligence applications. All-optical implementation of ONNs has proven burdensome chiefly due to the lack of optical devices that can emulate the neurons' non-linear activation function, thus forcing hybrid optical-electronic implementations. Moreover, ONNs suffer from a large footprint in comparison to their electronic (CMOS-based) counterparts.

Investigating non-reciprocity in time-periodic media using a perturbative approach.

Lorentz famous theorem leads to clear reciprocity conditions for linear, time-invariant media based on their constitutive parameters. By contrast, reciprocity conditions for linear time-varying media are not fully explored. In this paper, we investigate whether, and how a structure containing a time-periodic medium can be truly identified as reciprocal or not.

Parametric amplification and instability in time-periodic dielectric slabs.

We study the phenomenon of parametric amplification in the context of time-periodic dielectric slabs. These structures show particular promise inasmuch as they are capable of very large amplifications when illuminated by an electromagnetic wave of half the modulation frequency. Successive studies have corroborated this finding but none have yet been able to ascertain the nature of amplification in such devices.

Self-induced backaction in optical waveguides.

In this paper, we study the backaction effect on the force exerted upon Rayleigh particles in guided structures. We show that the backaction becomes stronger as the group velocity of the guided modes is decreased, which is not unexpected since the fall of group velocity increases the interaction time between the particle and the electromagnetic field. Interestingly, the sign of the group velocity affects the pushing and pulling nature of the exerted electromagnetic force.

Frequency conversion in time-varying graphene microribbon arrays.

We investigate the possibility of frequency conversion in time-varying metasurfaces, composed of graphene microribbon arrays (GMRAs) with time-periodic modulation of their conductivity. We present a quasi-static model for the interaction of light with a temporally modulated metasurface, as well as an accurate analytical treatment of the problem of time-varying GMRAs. Results coming from numerical simulations are also available.

Optical isolation enabled by two time-modulated point perturbations in a ring resonator.

In this paper we achieve non-reciprocity in a silicon optical ring resonator, by introducing two small time-modulated perturbations into the ring. Isolators are designed using this time-perturbed ring, side-coupled to waveguides. The underlying operation of the time-modulated ring and isolator is analyzed using Temporal Coupled Mode Theory (TCMT).

Power evolution along phase-sensitive parametric amplifiers: an experimental survey.

We propose and experimentally demonstrate a method based on Brillouin optical time-domain analysis to measure the longitudinal signal power distribution along phase-sensitive fiber-optical parametric amplifiers (PS-FOPAs). Experimental results show that the amplification of a PS-FOPA could go through different longitudinal profiles and yet finish with the same overall gain. This behavior is in sheer contrast with theoretical expectations, according to which longitudinal gain distribution should follow certain profiles determined by the initial relative phase difference but can never end up in the same overall gain.

Assessment of luminescent downshifting layers for the improvement of light-harvesting efficiency in dye-sensitized solar cells.

Luminescence downshifting (LDS) of light can be a practical photon management technique to compensate the narrow absorption band of high-extinction-coefficient dyes in dye-sensitized solar cells (DSSCs). Herein, an optical analysis on the loss mechanisms in a reflective LDS (R-LDS)/DSSC configuration is reported. For squaraine dye (550-700 nm absorption band) and CaAlSiN3 :Eu(2+) LDS material (550-700 nm emission band), the major loss channels are found to be non-unity luminescence quantum efficiency (QE) and electrolyte absorption.

Improved resolution three-dimensional integral imaging using optimized irregular lens-array structure.

A rigorous approach is proposed to improve the resolution of integral imaging (InI) by finding the appropriate form of irregularity in the arrangement of the InI lenslets. The improvement of the resolution is achieved through redistribution of the sampling points in a uniform manner. The optimization process for finding the optimum pattern of the lens-array irregularity is carried out by minimizing a cost function, whose mathematical closed-form expression is provided.

Determination of complex modes in photonic crystal waveguides using the phase variation in characteristic coefficients.

An efficient frequency-domain method, the phase variation monitoring (PVM) method, is proposed to determine the electromagnetic eigenmodes in two-dimensional photonic crystal waveguides. The proposed method is based on monitoring the reflection and transmission coefficients of incident plane waves. It is successfully applied to an illustrative line-defect photonic crystal waveguide and proved to be capable of calculating the in-plane leakage through the finite-size photonic crystal surrounding the line-defect.

Transmission line model for extraction of transmission characteristics in photonic crystal waveguides with stubs: optical filter design.

A simple and efficient transmission line model is proposed here to study how the transmission characteristics of photonic crystal waveguides are tailored by introduction of stubs patterned in the photonic crystal lattice. It is shown that band-pass and band-stop optical filters can be easily designed and optimized when stubs of appropriate length are brought in. Since the lengths of the designed stubs are not necessarily integer multiples of the photonic crystal lattice constant, a geometric shift in a portion of the photonic crystal structure is shown to be essential.

Three-dimensional resolvability in an integral imaging system.

The concept of three-dimensional (3D) resolvability of an integral imaging system is thoroughly investigated in this research. The general concept of 3D resolution fails to describe the 3D discrimination completely. Then the concepts of the depth-resolution plane and lateral-resolution plane are introduced to show the difference between the conventional 3D spatial resolution and the newly introduced 3D resolvability.

Optimization of the lens-array structure for performance improvement of integral imaging.

The seemingly inherent deficiencies of integral imaging systems-in particular, the depth of field limitation-are, in this Letter, partly resolved by using an irregular lens array, where each lens is either rotated or displaced from its original position in the conventional flat lens array. It is shown that having an array of lenses in the integral imaging system has some sort of redundancy that could be exploited to improve the quality of the image formation. The needed rotation or displacement of constituent lenses in the array is found by using a meticulous optimization algorithm, which tries to evenly distribute the optical rays emanating from each of the lenses to form the final image.

Analytical expression of giant Goos-Hänchen shift in terms of proper and improper modes in waveguide structures with arbitrary refractive index profile.

We analytically relate the giant Goos-Hänchen shift, observed at the interface of a high refractive index prism and a waveguide structure with an arbitrary refractive index profile, to the spatial resonance phenomenon. The proximity effect of the high refractive index prism on modal properties of the waveguide is discussed, and the observed shift is expressed in terms of proper and improper electromagnetic modes supported by the waveguide with no prism. The transversely increasing improper modes are shown playing an increasingly important role as the high refractive index prism comes closer to the waveguide.

Optimum waist of localized basis functions in truncated series employed in some optical applications.

The waist parameter is a particularly important factor for functional expansion in terms of localized orthogonal basis functions. We present a systematic approach to evaluate an asymptotic trend for the optimum waist parameter in truncated orthogonal localized bases satisfying several general conditions. This asymptotic behavior is fully introduced and verified for Hermite-Gauss and Laguerre-Gauss bases.

Transmission-line model to design matching stage for light coupling into two-dimensional photonic crystals.

The transmission-line analogy of the planar electromagnetic reflection problem is exploited to obtain a transmission-line model that can be used to design effective, robust, and wideband interference-based matching stages. The proposed model based on a new definition for a scalar impedance is obtained by using the reflection coefficient of the zeroth-order diffracted plane wave outside the photonic crystal. It is shown to be accurate for in-band applications, where the normalized frequency is low enough to ensure that the zeroth-order diffracted plane wave is the most important factor in determining the overall reflection.

Artifact-free analysis of highly conducting binary gratings by using the Legendre polynomial expansion method.

Analysis of highly conducting binary gratings in TM polarization has been problematic as the Fourier factorization fails and thus unwanted numerical artifacts appear. The Legendre polynomial expansion method (LPEM) is employed here, and the erroneous harsh variations attributed to the violation of the inverse rule validity in applying the Fourier factorization are filtered out. In this fashion, stable and artifact-free numerical results are obtained.

Geometrical approach in physical understanding of the Goos-Haenchen shift in one- and two-dimensional periodic structures.

The Goos-Haenchen shift of a totally reflected beam at the planar interface of two dielectric media, as if the incident beam is reflected from beneath the interface between the incident and transmitted media, has been geometrically associated with the penetration of the incident photons in the less-dense forbidden transmission region. This geometrical approach is here generalized to analytically calculate the Goos-Haenchen shift in one- and two-dimensional periodic structures. Several numerical examples are presented, and the obtained results are successfully tested against the well-known Artman's formula.

Longitudinal Legendre polynomial expansion of electromagnetic fields for analysis of arbitrary-shaped gratings.

The Legendre polynomial expansion method (LPEM), which has been successfully applied to homogenous and longitudinally inhomogeneous gratings [J. Opt. Soc.

Study of the numerical artifacts in differential analysis of highly conducting gratings.

Li's Fourier factorization rule [J. Opt. Soc.