Alexander A. Iskandar

Bistable optical response of a nanoparticle heterodimer: mechanism, phase diagram, and switching time.

We conduct a theoretical study of the bistable optical response of a nanoparticle heterodimer comprised of a closely spaced semiconductor quantum dot and a metal nanoparticle. The bistable nature of the response results from the interplay between the quantum dots optical nonlinearity and its self-action (feedback) originating from the presence of the metal nanoparticle. The feedback is governed by a complex valued coupling parameter G = G(R) + iG(I). We calculate the bistability phase diagram within the systems parameter space: spanned by G(R), G(I), and Δ, the latter being the detuning between the driving frequency and the transition frequency of the quantum dot. Additionally, switching times from the lower stable branch to the upper one (and vice versa) are calculated as a function of the intensity of the driving field. The conditions for bistability to occur can be realized, for example, for a heterodimer comprised of a closely spaced CdSe (or CdSe/ZnSe) quantum dot and a gold nanosphere.

Single-Frequency Refractive Index Sensor Based on a Finite One-Dimensional Photonic Crystals with Two Defects

A numerical analysis by means of transfer matrix method has been performed on finite one-dimensional photonic crystals consisting of two-layer repeated cells and two non-identical defect cells for the normal incident transverse electric (TE) wave. The study reveals a remarkable new feature showing that the variation of a photonic pass-band transmittance can be induced by varying the refractive index of one of the defect layer at practically the same peak frequency, which offers the potential application for single frequency sensing. The result further demonstrates the flexibility of tailoring the system parameters for application in the desired range of refractive index at the required sensitivity. It is also shown that the photonic pass-band (PPB) peak transmittance is generally less than unity in the index range considered, except for the case with the grating segment lengths (M,N,L) satisfying the condition N=M+L. This peculiar feature is explained qualitatively in this work.

Performance changes of a grated waveguide at resonance wavelengths next to its band-edges due to modified edge sections

An efficient numerical scheme developed on the basis of Green’s function method is applied to the investigation of structural effects on the performance of planar grated waveguide at the first resonance wavelengths next to the band-edges. Restricting ourselves to the transverse-electric waves, this study is focused on the effects induced by variations of the grating cell number and the depths of its four outer grooves on both sides. The different patterns of groove depth gradation or apodization considered in this study are all characterized by decreasing depth toward the ends while retaining the longitudinal grating symmetry. The effects of the modifications are expressed in terms of changes in the modal transmittance, reflectance, and out-of-plane scattering loss as well as the group velocity and resonant field enhancement. The most favorable result characterized by 15% transmittance enhancement and 85% loss reduction is achieved for the case with the most gradual changes in the groove depth. It is further shown that, for the investigated range of parameters, both the group velocity and field enhancement can best be improved by increasing the length of the uniform grating, without introducing any modification.

OPTICAL SENSING AND SWITCHING DEVICE BASED ON A FINITE DEEP NONLINEAR BRAGG GRATING WITH A MIRROR

We investigate the detailed transition of the dark to antidark soliton-like states in a system of finite deep nonlinear Bragg grating equipped with a movable metallic mirror and illuminated by a continuous laser source. As reported previously, the transition can be induced mechanically by moving the mirror as well as optically by changing the light source intensity.

Numerical study of anomalous TE-polarized light scattering by metallic nanowires using realistic data

We report the result of a numerical study of surface plasmon induced anomalous behaviors in TE light scattering by single silver and gold nanowires of radius a. Going beyond the restricted case of nondissipative and nondispersive scatterers often reported previously, the current numerical calculation is performed directly on the basis of Mies general formula, adopting the refractive index data of Johnson and Christy. Our result does not show the appearance of well resolved and multipole resonances in Qsca plotted against q(= 2πa/λ), for certain wire radii. It does show however, the growing contributions of the higher order modes as a increases. A series of closely-placed but well separated resonance curves nevertheless show up for varying wire radii within the range of small q, exhibiting systematic changes indicative of the size effects on the scattered waves. The further deduced Qsca(λ) spectra display the distinct resonance curves for different wire radii showing peculiar mix of monotonous and nonmonotonous variations of the resonance peak and spectral width with increasing a, as a result of complicated competitions among the growing contributions of the higher order modes. Finally, while the silver and gold scatterers appear to exhibit qualitatively similar behaviors, they differ largely in details due to the significantly different indices of refraction and dispersive properties.

DARK, ANTIDARK SOLITON-LIKE SOLUTIONS AND THEIR CONNECTION IN A FINITE DEEP NONLINEAR BRAGG GRATING WITH A MIRROR

We report the results of our study on the in-gap soliton-like solutions in a system of a uniform finite deep nonlinear Bragg grating with a mirror and continuous light source on the opposite sides of the grating. The system was shown to exhibit a new feature consisting of homoclinic and heteroclinic orbits in phase plane associated with the in-gap bright and dark/antidark solitons respectively. The multi-valued connection between the dark and antidark solitons was explicitly displayed. It was further demonstrated that a transition from dark to antidark soliton could be affected by either changing the mirror position or changing the source intensity.

Second Order Approximation for Band Gap Characterization of One-Dimensional Dielectric Omnidirectional Reflector

A derivation of approximate analytical expressions for band edges ω± of the first band gap of a multilayer periodic structure is presented for both TE and TM waves at arbitrary angles of incidence. It is found that the approximate expressions give an excellent agreement with the numerical results as verified by the band edge variation with respect to the filling fraction [..] and refractive index contrast Δn. The analytical expressions for the band edges are further employed to derive a semi numerical optimization of the relative gap width [..] with respect to the filling fraction ν. The result is again shown to be in good agreement with the numerical result.

Photonic states mixing beyond the plasmon hybridization model

A study is performed on a photonic-state mixing-pattern in an insulator-metal-insulator cylindrical silver nanoshell and its rich variations induced by changes in the geometry and dielectric media of the system, representing the combined influences of plasmon coupling strength and cavity effects. This study is performed in terms of the photonic local density of states (LDOS) calculated using the Green tensor method, in order to elucidate those combined effects. The energy profiles of LDOS inside the dielectric core are shown to exhibit consistently growing number of redshifted photonic states due to an enhanced plasmon coupling induced state mixing arising from decreased shell thickness, increased cavity size effect, and larger symmetry breaking effect induced by increased permittivity difference between the core and the background media. Further, an increase in cavity size leads to increased additional peaks that spread out toward the lower energy regime. A systematic analysis of those variations for a silver nanoshell with a fixed inner radius in vacuum background reveals a certain pattern of those growing number of redshifted states with an analytic expression for the corresponding energy downshifts, signifying a photonic state mixing scheme beyond the commonly adopted plasmon hybridization scheme. Finally, a remarkable correlation is demonstrated between the LDOS energy profiles outside the shell and the corresponding scattering efficiencies.

Enhanced energy confinement induced by metallic coating of central rod in square array photonic crystal of dielectric rods for TM light

We study the effects of a defect created by metallic coating of the central rod in a square lattice of dielectric rods for TM polarization lights. A calculation using the Plane Wave Expansion Method (PWE) in the supercell model shows that the photonic band structure and field distribution in the defect area varies with changing metallic layer thickness. The optimal energy localization is explored by varying the thickness of the metal layer. Enhancement of the energy confinement is described by the narrower spatial distribution profile of the energy with thicker metal coating. A more quantitative description, given in terms of confinement quality (CQ) defined by the normalized integrated intensity in the central rod, exhibits monotonous increase of CQ with growing metal layer thickness. The highest CQ value achieved is around 80% for a 3 × 3 supercell, which is considerably higher than the 44% optimal value achievable in the same dielectric PC structure with a defective central rod. Further calculation using the Extended Plane Wave Expansion Method for determining the imaginary part of the Bloch wave vector shows increasing with increasing metal coating thickness. The following analysis explains and corroborates the enhanced energy confinement effect.

STRUCTURE DEPENDENT VARIATIONS OF GROUP VELOCITY, ENERGY LOSS AND CONFINEMENT IN A REGULAR GRATED WAVEGUIDE

The Greens function method in the Dysons formulation has been employed for the ab initio numerical study of structural effects on the device performances in terms of the minimum group velocity, energy loss and energy confinement in a quasi two-dimensional grated waveguide device model. The structure parameters to be varied in this work consist of the number of the grating teeth (N) and the grating groove depth (g). It is found that those parameters exhibit roughly linear and mostly monotonous variations at the lower resonance. The reduction of the group velocity and enhancement of energy confinement are also shown to be effectively attained by increasing N, while leaving the loss parameter relatively unaffected. On the other hand, the calculated results for the upper resonance wavelength are shown to exhibit consistently non-monotonous responses to increasing g. Similarly distinct behaviors are also found in the relationship between the minimum group velocity (vg, min) and the energy loss (L) as well as that between vg, min and the confined energy (W) for various N and g at the upper resonances. This peculiarly different behaviors are shown to be related to the variations of the associated local density of states with respect to N and g calculated for the upper resonance.