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Integrated optical devices that can realize a threshold filtration of signals are in demand in photonics. In particular, they play a key role in neuromorphic chips, acting as optical neurons. A list of requirements exist for thresholders to be practically applicable in this context. A value of the threshold, in general, should be independently tunable for each neuron. A sharpness of the corresponding step function should be also dynamically variable, to allow switching between deterministic and stochastic algorithms, for example, in optical Ising machines. Nonlinear ring resonators draw the attention of researchers in this field since they potentially can provide the required type of threshold behavior. Here we suggest the switching mechanism that implements the property of resonators to provide extremely sharp (with respect to the wavelength) $\pi$ phase shifts near the critical coupling regime. Adding a variable source of losses into the resonator, a well controlled broadening of the threshold can be achieved. Sharpness of the step in this case is independent on the quality factor of resonators and corresponding width of resonances. The nonlinear switching mechanism allows to use this feature to construct efficient optical neurons, that operate at small intensities. It also allows to use conventional materials like silicon with a relatively weak Kerr nonlinearity. The obtained results potentially lead the way to fast nonlinear Kerr effect based activation functions that can operate in continuous wave regime.

We investigate an optical switching mechanism for applications in active integrated photonic circuits. The mechanism utilizes a large nonlinearity of indium-tin-oxide (ITO) in epsilon-near-zero (ENZ) regime. The effect of optically induced switching is investigated in hybrid plasmonic waveguides (HPWG) with ITO layer that are often used as a platform for various active photonic components, including optical modulators. To study the effect, nonlinear Maxwell equations are solved numerically in time and frequency domains. The all-optical mechanism of switching in HPWG is quantitatively compared with an electrical gating that is often used to manipulate hybrid modes. It is shown that the optical pumping could potentially cause more efficient switching. The influence of light intensity on HPWG properties and interplay between hybrid modes are investigated numerically in different regimes. Transmittance and phase variation are calculated as a function of light intensity for Si and Si3N4 waveguides. It is shown that the transmittance and its intensity dependence are strongly affected by the ITO thickness. A significant for applications step-like variation of phase and transmittance with intensity is detected. The possibility to control the excitation conditions for surface plasmon polaritons (SPP) in HPWG using active layer of ITO in nonlinear regime is demonstrated and discussed.

An efficient electro-optical modulation has been demonstrated here by using an edge plasmon mode specific for the hybrid plasmonic waveguide. Our approach addresses a major obstacle of the integrated microwave photonics caused by the polarization constraints of both active and passive components. In addition to sub-wavelength confinement, typical for surface plasmon polaritons, the edge plasmon modes enable exact matching of the polarization requirements for silicon based input/output grating couplers, waveguides and electro-optical modulators. A concept of the hybrid waveguide, implemented in a sandwich-like structure, implies a coupling of propagating plasmon modes with a waveguide mode. The vertically arranged sandwich includes a thin layer of epsilon-near-zero material (indium tin oxide) providing an efficient modulation at small length scales. Employed edge plasmons possess a mixed polarization state and can be excited with horizontally polarized waveguide modes. It allows the resulting modulator to work directly with efficient grating couplers and avoid using bulky and lossy polarization converters. A 3D optical model based on Maxwell equations combined with drift-diffusion semiconductor equations is developed. Numerically heavy computations involving the optimization of materials and geometry have been performed. Effective modes, stationary state field distribution, an extinction coefficient, optical losses and charge transport properties are computed and analyzed. In addition to the polarization matching, the advantages of the proposed model include the compact planar geometry of the silicon waveguide, reduced active electric resistance R and a relatively simple design, attractive for experimental realization.

The scattering and absorption suppression of the silica microspheres is experimentally achieved by semicontinuous gold shell synthesized on the silica microspheres. These fractal gold shells on the silica spheres are shown to have significant extinction in the visible and mid-infrared spectral range. It is found that the silica scattering peak in the visible and the relative contribution of the vibrational stretching band in the infrared gradually decreases with increasing of the gold coverage.

We demonstrate a thermally tunable optical metamaterial with negative permeability working in the visible range. By covering coupled metallic nanostrips with aligned nematic liquid crystals (NLCs), the magnetic response wavelength of the metamaterial is effectively tuned through control of the ambient temperature, changing the refractive index of LC via phase transitions. By increasing the ambient temperature from 20 degree to 50 degree, the magnetic response wavelength shifts from 650nm to 632nm. Numerical simulations confirm our tests and match the experimental observations well.

A well-established, silver fishnet design has been further miniaturized to function as a negative-index material at the shortest wavelength to date. By studying the transmittance, reflectance, and corresponding numerical simulations of the sample, we report in this paper a negative refractive index of -0.25 at the yellow-light wavelength of 580 nm.

We present a comprehensive study of the structural and electronic properties of ultrathin films containing graphene layers synthesized by chemical vapor deposition (CVD) based surface segregation on polycrystalline Ni foils then transferred onto insulating SiO2/Si substrates. Films of size up to several mm's have been synthesized. Structural characterizations by atomic force microscopy (AFM), scanning tunneling microscopy (STM), cross-sectional transmission electron microscopy (XTEM) and Raman spectroscopy confirm that such large scale graphitic thin films (GTF) contain both thick graphite regions and thin regions of few layer graphene. The films also contain many wrinkles, with sharply-bent tips and dislocations revealed by XTEM, yielding insights on the growth and buckling processes of the GTF. Measurements on mm-scale back-gated transistor devices fabricated from the transferred GTF show ambipolar field effect with resistance modulation ~50% and carrier mobilities reaching ~2000 cm^2/Vs. We also demonstrate quantum transport of carriers with phase coherence length over 0.2 $\mu$m from the observation of 2D weak localization in low temperature magneto-transport measurements. Our results show that despite the non-uniformity and surface roughness, such large-scale, flexible thin films can have electronic properties promising for device applications., Comment: This version (as published) contains additional data, such as cross sectional TEM images

A new class of artificially structured materials called metamaterials makes it possible to achieve electromagnetic properties that do not exist in nature. In this paper we review the recent progress made in the area of optical metamaterials. It was predicted that nanostructured metamaterials could provide us with artificial magnetic response and negative refractive index at optical frequencies. To date, optical metamagnetics have been already fabricated to demonstrate artificial magnetic response in the infrared and across the entire visible spectrum, while metamaterials showing negative refractive index, also called negative index materials (NIM), have been demonstrated in the infrared and at the border with the visible spectral range. Here we report the results of a sample that displays NIM behavior for red light at a wavelength of 710 nm. This is the shortest wavelength so far at which NIM behavior has been observed for light (excluding a device built on surface plasmon polaritons). We also discuss the fabrication challenges and the impact of fabrication limitations, specifically surface roughness of the fabricated structures on the optical properties of the metamaterials., Comment: 22 pages, 9 figures. Separate page for each figure

A family of coupled nano-strips with varying dimensions is fabricated to obtain optical magnetic responses across the whole visible spectrum, from red to blue. The proposed approach provides one with a universal building block and a general recipe for producing controllable optical magnetism for various practical implementations., Comment: 13 pages, 5 figures

This work is concerned with the experimental demonstration of a dual-band negative index metamaterial. The sample is double-negative (showing both a negative effective permeability and a negative effective permittivity) for wavelengths between 799 and 818 nm of linearly polarized light with a real part of refractive index of about -1.0 at 813 nm; the ratio -Re(n)/Im(n) is close to 1.3 at that wavelength. For an orthogonal polarization, the same sample also exhibits a negative refractive index in the visible (at 772 nm). The spectroscopic measurements of the material are in good agreement with the results obtained from a finite element electromagnetic solver for the actual geometry of the fabricated sample at both polarizations., Comment: 3 pages, 4 figures

Experimental demonstration of a negative permeability due to near-field coupling of periodic thin silver strips is presented. Two samples with different strip thicknesses are fabricated; optical measurements of the samples confirm our initial design projections by showing the real part of permeability to be about -1 for the sample with thinner strips and -0.8 for the sample with thicker strips at wavelengths of 770 nm and 720 nm., Comment: 6 pages, 3 figures

A key optical parameter characterizing the existence of negative refraction in a thin layer of a composite material is the effective refractive index of an equivalent, homogenized layer with the same physical thickness as the initial inhomogeneous composite. Measuring the complex transmission and reflection coefficients is one of the most rigorous ways to obtain this parameter. We dispute Grigorenko's statement (Optics Letters 31, 2483 (2006)) that measuring only the reflection intensity spectrum is sufficient for determining the effective refractive index. We discuss fundamental drawbacks of Grigorenko's technique of using a best-fit approximation with an a priori prescribed dispersive behavior for a given metamaterial and an 'effective optical thickness' that is smaller than the actual thickness of the sample. Our simulations do not confirm the Grigorenko paper conclusions regarding the negative refractive index and the negative permeability of the nanopillar sample in the visible spectral range., Comment: 18 pages, 7 figures

The race to engineering metamaterials comprising of a negative refractive index in the optical range has been fueled by the realization of negative index materials for GHz frequencies six years ago. Sheer miniaturization of the GHz resonant structures is one approach. Alternative designs make use of localized plasmon resonant metal nanoparticles or nanoholes in metal films. Following this approach, a negative refractive index has been realized in the optical range very recently. We review these recent results and summarize how to unambiguously retrieve the effective refractive index of thin layers from data accessible to measurements. Numerical simulations show that a composite material comprising of silver strips and a gain providing material can have a negative refractive index of -1.3 and 100% transmission, simultaneously.

An array of pairs of parallel gold nanorods is shown to have a negative refractive index n'=-0.3 at the optical communication wavelength of 1.5 micron. This effect results from the plasmon resonance in the pairs of nanorods for both the electric and magnetic components of light. The refractive index is retrieved from the direct phase and amplitude measurements for transmission and reflection, which are all in excellent agreement with our finite difference time domain simulations. The refraction critically depends on the phase of the transmitted wave, which emphasizes the importance of phase measurements in finding n'., Comment: an improved version (17 pages, 5 figures) with a new sample and additional measurements