Accessing the optical nonlinearity of metals with metal- dielectric photonic bandgap structures

Optically induced metal-to-dielectric transition in Epsilon-Near-Zero metamaterials

Epsilon-Near-Zero materials exhibit a transition in the real part of the dielectric permittivity from positive to negative value as a function of wavelength. Here we study metal-dielectric layered metamaterials in the homogenised regime (each layer has strongly subwavelength thickness) with zero real part of the permittivity in the near-infrared region. By optically pumping the metamaterial we experimentally show that close to the Epsilon-Near-Zero (ENZ) wavelength the permittivity exhibits a marked transition from metallic (negative permittivity) to dielectric (positive permittivity) as a function of the optical power. Remarkably, this transition is linear as a function of pump power and occurs on time scales of the order of the 100 fs pump pulse that need not be tuned to a specific wavelength. The linearity of the permittivity increase allows us to express the response of the metamaterial in terms of a standard third order optical nonlinearity: this shows a clear inversion of the roles of the real and imaginary parts in crossing the ENZ wavelength, further supporting an optically induced change in the physical behaviour of the metamaterial.

Optical Realization of Double-Continuum Fano Interference and Coherent Control in Plasmonic Metasurfaces

Classical realization of a ubiquitous quantum mechanical phenomenon of double-continuum Fano interference using metasurfaces is experimentally demonstrated by engineering the near-field interaction between two bright and one dark plasmonic modes. The competition between the bright modes, one of them effectively suppressing the Fano interference for the orthogonal light polarization, is discovered. Coherent control of optical energy concentration and light absorption by the ellipticity of the incident light is theoretically predicted.

Transparency and stability of Ag-based metal-dielectric multilayers

We fabricated and tested periodic metal (Ag)-dielectric (SiO2 or TiO2) multilayers with transparency bands in the visible range. For samples with Ag-TiO2 interfaces, the optical properties exhibited relatively poor predictability, likely due to oxidation of the Ag layers. Ag/SiO2-based multilayers were found to be more predictable and stable, but the relatively low refractive index of SiO2 limits their inherent transparency and pass-band bandwidth. We show that termination of the multilayer with a single high-index layer reduces the admittance mismatch with the ambient media, and thus improves the properties of the transparency band.

Plasmonic grating as a nonlinear converter-coupler

The paper introduces a wavelength converter composed of a metallic finite 2-dimensional particle grating on top of an optical waveguide. The particles sustain plasmonic resonances which will result in the near-field enhancement and therefore, high conversion efficiency. Due to near-field interaction of the grating field with the propagating modes of the waveguide, the generated third harmonic wave is phase-matched to a propagating mode of the waveguide, while the fundamental frequency component is not coupled into the output waveguide of the structure. The performance of this structure is numerically investigated using a full-wave transmission line method for the linear analysis and a three-dimensional finite-difference time-domain method for the nonlinear analysis.

Corrugated metallodielectric superlattices via release-rollup assembly

A 'release-rollup' assembly (RRA) technique is described that yields corrugated metallodielectric superlattices. Bilayers of polymer/Au cast onto diffraction gratings are released and rolled into multilayers with registration of the stacked corrugations across mm-scales. Optical imaging reveals Moiré fringes with reflection spectra that track the bilayer thickness due to mis-stacking. Angular-resolved spectra show spectrally-modulated diffraction opposite to that of the metallic stop-bands, but which agrees with a simple model. This scalable fabrication strategy is thus widely exploitable for laterally patterned metamaterials and optical superlattices.

Plasmon-resonance-induced enhancement of the reflection band in a one-dimensional metal nanocomposite photonic crystal

We report the realization of a one-dimensional photonic crystal consisting of alternating layers of metal nanocomposite and polymer layers. The structure shows a large change in the width of the reflection band due to the interplay between the plasmon resonance of the metal nanoparticle and the Bloch modes of the photonic crystal. The width of the reflection band is found to increase by 200% when the photonic band edge approaches the plasmon resonance of the silver nanoparticles.

Harmonic generation in metallic, GaAs-filled nanocavities in the enhanced transmission regime at visible and UV wavelengths

We have conducted a theoretical study of harmonic generation from a silver grating having slits filled with GaAs. By working in the enhanced transmission regime, and by exploiting phase-locking between the pump and its harmonics, we guarantee strong field localization and enhanced harmonic generation under conditions of high absorption at visible and UV wavelengths. Silver is treated using the hydrodynamic model, which includes Coulomb and Lorentz forces, convection, electron gas pressure, plus bulk χ(3) contributions. For GaAs we use nonlinear Lorentz oscillators, with characteristic χ(2) and χ(3) and nonlinear sources that arise from symmetry breaking and Lorentz forces. We find that: (i) electron pressure in the metal contributes to linear and nonlinear processes by shifting/reshaping the band structure; (ii) TE- and TM-polarized harmonics can be generated efficiently; (iii) the χ(2) tensor of GaAs couples TE- and TM-polarized harmonics that create phase-locked pump photons having polarization orthogonal compared to incident pump photons; (iv) Fabry-Perot resonances yield more efficient harmonic generation compared to plasmonic transmission peaks, where most of the light propagates along external metal surfaces with little penetration inside its volume. We predict conversion efficiencies that range from 10(-6) for second harmonic generation to 10(-3) for the third harmonic signal, when pump power is 2 GW/cm2.

Nonlinear optical properties of induced transmission filters

The nonlinear optical (NLO) properties of induced transmission filters (ITFs) based on Ag are experimentally determined using white light continuum pump-probe measurements. The experimental results are supported using simulations based on the matrix transfer method. The magnitude of the NLO response is shown to be 30 times that of an isolated Ag film of comparable thickness. The impacts of design variations on the linear and NLO response are simulated. It is shown that the design can be modified to enhance the NLO response of an ITF by a factor of 2 or more over a perfectly matched ITF structure.

Highly efficient all-optical diode action based on light-tunneling heterostructures

We theoretically investigate the feasibility of constructing compact and highly efficient all-optical diodes (AODs) based on light tunneling mechanism in heterostructures. Due to light tunneling behaviors in heterostructures with one-dimensional photonic crystals (1D PC) and lossy metallic film, not only very large nonlinear permittivity of metal can be utilized sufficiently but also the structures with strongly nonreciprocal electric field distributions can be constructed. Finally we design a composite structure consisting of 1D PC-metal heterostructures to achieve the optimal unidirectional light transmission with 0.984 transmission contrasts, 42% transmission and 0.93 GW/cm(2) operating light power at working wavelength 557.2nm.

Modeling of Z-scan characteristics for one-dimensional nonlinear photonic bandgap materials

We propose a Z-scan theory for one-dimensional nonlinear photonic bandgap materials. The Z-scan characteristics for this material are analyzed. Results show that the Z-scan curves for photonic bandgap materials with nonlinear refraction are similar to those of uniform materials exhibiting both nonlinear refraction and nonlinear absorption simultaneously. Effects of nonlinear absorption on reflected and transmitted Z-scan results are also discussed.

Tunable time response of the nonlinearity of nanocomposites by doping semiconductor quantum dots

We report an approach to achieving a tunable time response of the nonlinearity of nano-Ag:polymer nanocomposites. The response time of the nonlinearity of the nanocomposite made of Ag nanoparticles dispersed in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] matrix can be tuned by adjusting the doping concentration of CdTe(0.13)S(0.87) quantum dots. An ultrafast response time of 14.5 ps is achieved for a doping concentration of 27%. Moreover, the third-order nonlinear susceptibility, achieving the order of 10(-5) esu, is one order of magnitude larger than that of the undoped nanocomposite. An ultrafast and low-power photonic crystal all-optical switching is also realized based on the photonic bandgap shift.

Optical nonlinearity enhancement in heterostructures with thick metallic film and truncated photonic crystals

It is hard to use bulk metals with high nonlinear susceptibilities since they are nearly opaque. Metal-dielectric photonic crystals can be transparent, but the electric field at each metal layer is still low. We theoretically studied the nonlinear response of heterostructures composed of truncated all-dielectric photonic crystals and thick metallic films. Because of the localized interface modes, both transmittance and the electric field in the metal are enhanced greatly. Compared to metal-dielectric photonic crystals, the critical intensity of threshold for bistability in the heterostructures with the same thickness of metal can be reduced by nearly 2 orders of magnitude.

Subwavelength discrete solitons in nonlinear metamaterials

We present the first study of subwavelength discrete solitons in nonlinear metamaterials: nanoscaled periodic structures consisting of metal and nonlinear dielectric slabs. The solitons supported by such media result from a balance between tunneling of surface plasmon modes and nonlinear self-trapping. The dynamics in such systems, arising from the threefold interplay between periodicity, nonlinearity, and surface plasmon polaritons, is substantially different from that in conventional nonlinear dielectric waveguide arrays. We expect these phenomena to inspire fundamental studies as well as potential applications of nonlinear metamaterials, particularly in subwavelength nonlinear optics.

Steplike transmission of light through a metal-dielectric multilayer structure due to an intensity-dependent sign of the effective dielectric constant

We numerically study light propagation through a specially designed nonlinear nanoscale metal-dielectric multilayer structure with a linear effective dielectric constant just below zero. The calculated dependence of the output intensity on the input intensity shows a steplike behavior. It rests upon an intensity-dependent change of the effective dielectric constant from negative (low-transmission state) to positive (high-transmission state) values, corresponding to a transition of the optical properties from metalliclike to dielectriclike. The study of the transient behavior of the structure demonstrates a switching time of around 1 ps.

Negative refraction and sub-wavelength focusing in the visible range using transparent metallo-dielectric stacks

We numerically demonstrate negative refraction of the Poynting vector and sub-wavelength focusing in the visible part of the spectrum using a transparent multilayer, metallo-dielectric photonic band gap structure. Our results reveal that in the wavelength regime of interest evanescent waves are not transmitted by the structure, and that the main underlying physical mechanisms for sub-wavelength focusing are resonance tunneling, field localization, and propagation effects. These structures offer several advantages: tunability and high transmittance (50% or better) across the visible and near IR ranges; large object-image distances, with image planes located beyond the range where the evanescent waves have decayed. From a practical point of view, our findings point to a simpler way to fabricate a material that exhibits negative refraction and maintains high transparency across a broad wavelength range. Transparent metallo-dielectric stacks also provide an opportunity to expand the exploration of wave propagation phenomena in metals, both in the linear and nonlinear regimes.

Subdiffraction focusing of scanning beams by a negative-refraction layer combined with a nonlinear layer

We evaluate the possibility to focus scanning light beams below the diffraction limit by using the combination of a nonlinear material with a Kerr-type nonlinearity or two-photon absorption to create seed evanescent components of the beam and a negative-refraction material to enhance them. Superfocusing to spots with a FWHM in the range of 0.2 lambda is theoretically predicted both in the context of the effective-medium theory and by the direct numerical solution of Maxwell equations for an inhomogeneous pho-tonic crystal. The evolution of the transverse spectrum and the dependence of superfocusing on the parameters of the negative-refraction material are also studied. We show that the use of a Kerr-type nonlinear layer for the creation of seed evanescent components yields focused spots with a higher intensity compared with those obtained by the application of a saturable absorber.

Accessing quadratic nonlinearities of metals through metallodielectric photonic-band-gap structures

We study second harmonic generation in a metallodielectric photonic-band-gap structure made of alternating layers of silver and a generic, dispersive, linear, dielectric material. We find that under ideal conditions the conversion efficiency can be more than two orders of magnitude greater than the maximum conversion efficiency achievable in a single layer of silver. We interpret this enhancement in terms of the simultaneous availability of phase matching conditions over the structure and good field penetration into the metal layers. We also give a realistic example of a nine-period, Si3/N4Ag stack, where the backward conversion efficiency is enhanced by a factor of 50 compared to a single layer of silver.

Nonlinear pulse propagation in one-dimensional metal-dielectric multilayer stacks: ultrawide bandwidth optical limiting

We numerically study the nonlinear optical properties of metal-dielectric photonic band gap structures in the pulsed regime. We exploit the high chi3 of copper metal to induce nonlinear effects such as broadband optical limiting, self-phase modulation, and unusual spectral narrowing of high intensity pulses. We show that in a single pass through a typical, chirped multilayer stack nonlinear transmittance and peak powers can be reduced by nearly two orders of magnitude compared to low light intensity levels across the entire visible range. Chirping dielectric layer thickness dramatically improves the linear transmittance through the stack and achieves large fields inside the copper to access the large nonlinearity. At the same time, the linear properties of the stack block most of the remaining electromagnetic spectrum.

Engineering the structure-induced enhanced absorption in three-dimensional metallic photonic crystals

Order-of-magnitude enhancement of light absorption can take place near the photonic band gaps in three-dimensional layer-by-layer metallic photonic crystals in the mid-infrared wavelength regime where a conventional bulk metallic material shows weak absorption. In this paper we investigate the dependence of this enhanced absorption on several structural parameters of the crystal by means of a plane-wave-based transfer-matrix method in combination with an analytic model expansion approach. We find that when the metallic layers are brought out of touch, the magnitude of the absorption peaks grows rather than decays, in contrast with the conventional wisdom that the connectivity of metallic structure can enhance the absorption. Besides, the increase in the metallic layer separation distance will result in redshifts of absorption peak. The position of the absorption peak can also be engineered conveniently by simply changing the refractive index of the dielectric material filling the open domain between metal walls. Due to the approximate scalability of Maxwell's equations, the absorption peak shows a nearly linear dependence in position on this refractive index.

Enhanced nonlinear optical response of one-dimensional metal-dielectric photonic crystals

We describe a new type of artificial nonlinear optical material composed of a one-dimensional metal-dielectric photonic crystal. Because of the resonant nature of multiple Bragg reflections, the transmission within the transmission band can be quite large, even though the transmission through the same total thickness of bulk metal would be very small. This procedure allows light to penetrate into the highly nonlinear metallic layers, leading to a large nonlinear optical response. We present experimental results for a Cu/SiO(2) crystal which displays a strongly enhanced nonlinear optical response (up to 12X) in transmission.

Near-field localization of ultrashort optical pulses in transverse 1-D periodic nanostructures

We present a transverse 1-D periodic nanostructure which exhibits lateral internal Thorneld localization for normally incident ultrashort pulses, and which may be applied to the enhancement of nonlinear optical phenomena. The peak intensity of an optical pulse propagating in the nanostructure is approximately 12 times that of an identical incident pulse propagating in a bulk material of the same refractive index. For second harmonic generation, an overall enhancement factor of approximately 10.8 is predicted. Modeling of pulse propagation is performed using Fourier spectrum decomposition and Rigorous Coupled-Wave Analysis (RCWA).