Thermal tweezers for nano-manipulation and trapping of interacting atoms or nanoparticles on crystalline surfaces.

Thermal tweezers is an all-optical nanofabrication technique where surface thermophoresis due to holographically induced strong temperature modulation is used for parallel manipulation and trapping of adatoms and adparticles with nanoscale resolution. This paper conducts the detailed numerical analysis of thermal tweezers in the presence of significant interaction between the adparticles on the surface. In particular, we demonstrate that the considered inter-particle interactions result in a significant enhancement of the surface thermophoretic effect and substantially increases modulation of particle concentration on the surface. In addition, we predict the possibility to achieve adparticle confinement on the surface to strongly sub-wavelength regions ∼12 times smaller than the vacuum wavelength of the optical radiation. A numerical approach to surface diffusion of interacting nanoparticles and adatoms is developed, the Monte Carlo interaction method, and its applicability conditions and limitations are discussed. The obtained results will be important for better understanding of the fundamental aspects of surface thermophoresis, as well as the development of new approaches to nano-patterning of surfaces for engineering their optical, electronic, chemical, and mechanical properties by means of the directed self-assembly of nanoparticles and adatoms.

Ehrenfest model for condensation and evaporation processes in degrading aggregates with multiple bonds.

We present an explicit theory of the degradation and thermal fragmentation kinetics of polymerlike systems and aggregates with multiple bonds in the presence of stochastic evaporation and condensation (restoration) of bonds. The analysis is conducted on the basis of the determination of the first passage time to state zero (fragmented state) in the Ehrenfest diffusion model in continuous time. The main approximations of the developed theory include the assumption that multiple bonds in any link between the primary elements in the aggregate do not interact with each other and that the coagulation rate after thermal fragmentation of the aggregates is negligible (which gives the absorbing zero state in the Ehrenfest model). In particular, it is demonstrated that even small condensation rates (of approximately 10 times smaller than the rates of bond evaporation) may have a significant effect on typical evolution times for the degrading aggregates and can result in a strong accumulation of nanoaggregates in the intermediate fragmentation modes. The simple asymptotic (predominantly exponential) behavior of the obtained solution at large evolution times is analyzed and discussed. The results will be important for the investigation of the degradation kinetics of a variety of polymerlike systems with multiple bonds, including self-arranged structures, polymer networks, different types of nanoclusters and their thermal fragmentation, etc.

Resonant coupling between bulk waves and guided modes in a dielectric slab with a thick holographic grating.

What we believe to be a new type of resonant coupling of an incident bulk wave into guided modes of a slab with a thick holographic grating is shown to occur in the presence of strong frequency detunings of the Bragg condition. This happens through the reflection of the strongly noneigen +1 diffracted order with the slab-grating boundaries, the resultant reflected waves forming a guided slab mode. Rigorous coupled-wave analysis is used for the numerical analysis of the predicted resonant effects. Possible applications include enhanced options for the design of multiplexing and demultiplexing systems, optical signal-processing devices, optical sensors, and measurement techniques.

Plasmonic subwavelength waveguides: next to zero losses at sharp bends.

We demonstrate that approximately 100% transmission of a strongly localized channel plasmon polariton can be achieved through a sharp 90 degrees bend in a subwavelength waveguide in the form of a triangular groove on a metal surface--a feature that has previously been demonstrated only for photonic crystal waveguides, which do not provide subwavelength localization. Conditions for minimum reflection and radiative losses at the bend are investigated numerically by the finite-difference time-domain algorithm. Dissipation in the structure is demonstrated to be sufficiently low to ensure significant propagation distances (a number of wavelengths) of the localized plasmon in each of the arms of the bend.

Channel plasmon-polariton in a triangular groove on a metal surface.

One-dimensional localized plasmons (channel polaritons) guided by a triangular groove on a metal substrate are investigated numerically by means of a finite-difference time-domain algorithm. Dispersion, existence conditions, and dissipation of these waves are analyzed. In particular, it is demonstrated that the localization of the predicted plasmons in acute grooves may be substantially stronger than what is allowed by the diffraction limit. As a result, the predicted waves may be significant for the development of new subwavelength waveguides and interconnectors for nano-optics and photonics.

Extremely asymmetrical scattering of optical waves in nonuniform periodic Bragg arrays.

The extremely asymmetrical scattering (EAS) of bulk and guided electromagnetic waves in nonuniform periodic Bragg arrays with steplike variations of the grating amplitude is analyzed theoretically by means of a recently developed approach based on allowance for the diffractional divergence of the scattered wave. Arrays of finite and infinite widths are investigated. It is shown that, for thin nonuniform arrays, EAS has the same pattern as for uniform arrays with mean grating amplitude. On the contrary, for wide nonuniform arrays, the scattered wave amplitudes are well determined by local values of the grating amplitude. In this case, the energy of the scattered wave is shown to concentrate mainly in regions with smaller grating amplitude. The sensitivity of EAS to small imperfections of periodic arrays is investigated theoretically. The physical explanation of the observed effects is based on the diffractional divergence of the scattered wave.

Anomalous absorption of TM electromagnetic waves by an ultrathin layer: optical analog of liquid friction.

A strong maximum of absorption (as much as 100%) of bulk TM electromagnetic waves by an ultrathin film with imaginary dielectric permittivity is shown to exist at some optimal film thickness. This typical thickness is usually much smaller than the wavelength and the wave penetration depth in the material of the film. The absorptivity maximum increases and the typical thickness decreases with increasing dielectric permittivity of the layer. An optical analog of linear (liquid) friction is discussed. A frictional contact approximation for TM electromagnetic waves is analyzed, and relevant boundary conditions with an optical coefficient of friction are derived.

Extremely asymmetrical scattering of slab modes in periodic Bragg arrays.

The steady-state extremely asymmetrical scattering of electromagnetic modes in a slab with a periodically corrugated boundary is analyzed theoretically. A new approach, based on allowance for the diffractional divergence of a scattered wave, is used with the approximation of slowly varying amplitudes and a Fourier analysis. The structure of the incident and scattered waves inside and outside the array is determined. The amplitudes of the scattered waves are found to be much larger than the amplitude of the incident wave. The typical time of relaxation to steady-state scattering is found to depend on the distance from the array boundary through which the incident wave enters the array. Conditions of applicability of the results obtained are also presented.

Planar optical waveguides coupled by means of Bragg scattering.

A new analytic approach to the analysis of grating-assisted couplers is proposed and used for description of the noncollinear coupling of two slab waveguides with arbitrary mode polarizations (TE and TM), propagation directions, and phase velocities. This approach is based on the boundary-perturbation theory, the method of successive approximations, and the energy-conservation law, and does not use any overlap integrals. The specific case in which the converted mode propagates parallel to a periodic array boundary (extremely asymmetric coupling) is considered by means of an original simple analytic approach that allows for the diffractional divergence of the converted wave. Applicability conditions of the results obtained are derived in both the cases of conventional and extremely asymmetrical coupling. Comparison with the previous methods is carried out for the collinear coupling.