Spontaneous Periodic Structures Research Articles

Kinetic Monte Carlo simulations of ion-induced ripple formation: Dependence on flux, temperature, and defect concentration in the linear regime

Low-energy ion bombardment produces spontaneous periodic structures (sputter ripples) on many surfaces. Continuum theories describe the pattern formation in terms of ion-surface interactions and surface relaxation kinetics, but many features of these models (such as defect concentration) are unknown or difficult to determine. In this work, we present results of kinetic Monte Carlo simulations that model surface evolution using discrete atomistic versions of the physical processes included in the continuum theories. From simulations over a range of parameters, we obtain the dependence of the ripple growth rate, wavelength, and velocity on the ion flux and temperature. The results are discussed in terms of the thermally dependent concentration and diffusivity of ion-induced surface defects. We find that in the early stages of ripple formation the simulation results are surprisingly well described by the predictions of the continuum theory, in spite of simplifying approximations used in the continuum model.

Small-angle scattering during formation of periodic structures in light-sensitive films subjected to laser radiation

Patterns of small-angle scattering that appear in thin AgCl films containing granular silver upon the formation of spontaneous periodic structures by p-polarized and circularly polarized laser beams (He-Ne laser, P=8 mW, λ = 632.8 nm) are studied. It is found that, at angles of incidence exceeding 4° regardless of the type of polarization, the diffraction of the waveguide modes excited by the dominant C gratings from neighboring microgratings with the vectors K ≠ K c dominantly contributes to the small-angle scattering. In the case of circularly polarized light, the pattern of small-angle scattering becomes more complicated as a result of the formation of S − gratings and related secondary regular gratings.

Spontaneous periodic structures induced in thin AgCl-Ag films by the continuum in the case of optical rotation of the plane of polarization

The formation of spontaneous periodic structures in thin AgCl-Ag films subject to the continuum (λ≈450–700 nm) in the case of rotation of the plane of polarization with the help of crystalline quartz is studied. The periodic structures are formed due to the excitation of waveguide TE0 modes in spontaneous Rayleigh scattering of radiation with a wavelength λ incident on the film. The structure periods are smaller than λ and vary in proportion with λ. After special treatment of the irradiated sample (fixing and deposition of an Al layer), periodic structures form a thin reflection hologram capable of reproducing the direction of linear polarization, the angle of optical rotation, and the color of the spectrum recorded. The study of diffraction patterns and small-angle scattering from small portions of a hologram, which were exposed to light with various wavelengths in the recording process, with the help of a probing UV beam (λp=337 nm), made it possible to reveal special features of the manifestation of Rayleigh and Wood anomalies associated with the domain structure of PS.

Influence of photoinduced gyrotropy on the formation of spontaneous periodic structures in light-sensitive AgCl–Ag thin films

The temporal evolution of spontaneous periodic structures was studied under successive action of circularly and linearly polarized laser radiation in light-sensitive waveguide AgCl–Ag films. The essential role of positive feedback in the growth of periodic structures is established by measuring the differential diffraction efficiency. An asymmetry in the azimuthal distribution of diffraction efficiency is found due to gyrotropy induced by circularly polarized radiation. The existence of induced gyrotropy in the AgCl–Ag films was determined directly by measuring circular dichroism and optical rotatory dispersion. It is established that TE 0 mode scattering on anisotropic Ag particles leads to amplification of the gyrotropy influence on spontaneous grating growth at initial stages of irradiation by the linearly polarized laser beam.

Photoinduced effects in light-sensitive films

A review of the authors' experimental studies of photoinduced effects in thin light-sensitive films since 1976 is presented. In the first part of the review, the results of the optical and electron microscopic study of the Weigert effect in thin silver halide films containing granular Ag are observed. It is shown that the Weigert effect in these systems is connected with chains consisting of spherical silver granules oriented by a polarized laser beam. The second part gives the results of experimental studies of spontaneous periodic structures induced in thin light-sensitive films by a single polarized beam on scattered waveguide modes. The third part concerns the study of spontaneous gratings induced by an off-normal incident laser beam and their self-organization.

Temperature gradients effects in low-power laser annealing of ion implanted?-Ge

The influence of temperature gradients in laser annealing ofα-Ge, obtained below the melting threshold, was tested by irradiating the samples with a linear fringe pattern from a pulsed ruby laser. The peak temperature reached on the specimen surface was calculated to be well below the melting threshold of the material. Temperature gradients are observed to enhance the crystallization process in the material. Spontaneous periodic structures, or ripples, having a period equal to the laser wavelength, are frequently observed in areas corresponding to minima of the fringe pattern.