Random Nanostructures Research Articles

Polarization dependent near-field speckle of random gold films

The near-field speckle of random gold nanostructures is experimentally and theoretically investigated. The topography and the intensity distribution, under polarized illumination, are obtained with an aperture scanning near-field optical microscope. The sharpness of the experimental images entitles us to perform a fine statistical analysis based on the autocorrelation function (ACF). The intensity ACF clearly exhibits an anisotropic behavior and subwavelength oscillations, depending on the incident polarization. A comparison with a numerical simulation exhibits the influence of the surface topography on the anisotropic near-field speckle.

Sixfold self-assembled hierarchical structures synthesized by direct annealing of Zn microtips

A facile route and the related mechanism for forming sixfold core–shell hierarchical structure with well-defined anisotropic three-dimensional arrays have been described. The regularity and symmetry of hierarchical structures prepared by annealing Zn microtips are superior to the random nanostructure arrays formed in general vapor system reported in the literature. Owing to the distinct stress distribution on the topography of microtips during annealing, the mechanism for growing branched zinc oxide (ZnO) nanowhiskers is related to the relaxation of stress. In addition, the self-assembled hierarchical structures with naturally good contact results in a lowered energy barrier between Zn metal and ZnO semiconductor, which in turn gives a much better emission property.

Periodic ordering of random surface nanostructures induced by femtosecond laser pulses on metals

In this paper, we performed a detailed study of the formation of femtosecond laser-induced periodic surface structures (LIPSSs) on platinum and gold at near-damage threshold fluences. We find a unique type of LIPSS entirely covered with nanostructures. A distinctive feature of the nanostructure-covered LIPSS is that its period is appreciably less than that of the regular LIPSS. We show that the reduced period is caused by an increase of the real part of the effective refractive index of the air-metal interface when nanostructures develop and affect the propagation of surface plasmons.

Readout contrast beyond diffraction limit by a slab of random nanostructures

A simplified Fourier optics approach is applied to study how near-field optical disks retrieve evanescent signals through random nanostructure. The statistical properties of the random nanostructures are used to realize the general behavior of near-field optical disks. The mechanism of its super-resolution capability and an analytical expression of the readout contrast of near-field optical disks with random apertures are derived. The resolution of near-field optical disk is determined by the size of the random nanostructure.

Optical Properties of ZnO Nanostructures

We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

Modeling of nonlinear microscopy of localized field enhancements in random metal nanostructures

Nonlinear microscopy of localized field enhancements in random metal nanostructures with a tightly focused laser beam scanning over a sample surface is modeled by making use of analytic representations of the Green dyadic in the near- and far-field regions, with the latter being approximated by the part describing the scattering via excitation of surface plasmon polaritons. The developed approach is applied to scanning second-harmonic (SH) microscopy of small gold spheres placed randomly on a gold surface. We calculate self-consistent fundamental harmonic (FH) and SH field distributions at the illuminated sample surface and, thereby, FH and SH images for different polarization configurations of the illuminating and detected fields. The simulated images bear close resemblance to the images obtained experimentally, exhibiting similar sensitivity to the wavelength and polarization, as well as sensitivity to the scattering configuration. We verify directly our conjecture that very bright spots in the SH images occur due to the spatial overlap of properly polarized FH and SH eigenmodes. Applications and further improvements of the developed model are discussed.

Strong Polarized Enhanced Raman Scattering via Optical Tunneling through Random Parallel Nanostructures in Au Thin Films

Random parallel nanostructures (ridges and channels) were created by scratching gold thin films deposited on glass slides. Atomic force microscope (AFM) images showed that the width of the substructures within the scratches were of the order of a few hundred nanometers. These nanometric gold features can then support localized surface plasmon resonances in the direction perpendicular to the propagation of the scratches. This surface plasmon excitation led to a remarkable dependence of the intensity of the surface-enhanced resonance Raman scattering (SERRS) on the polarization direction of the incident light relative to the orientation of the scratch. The maximum SERRS intensities for oxazine 720 (a common laser dye) adsorbed on these nanostructures were obtained when the polarization of the light field was perpendicular to the direction of the substructures. The SERRS intensities followed a squared dependence on the polarization direction of the incident field.

Surface Plasmon Effects on the Far-Field Signals of AgOx-Type Super Resolution Near-Field Structure

A super-resolution near-field structure (Super-RENS) of AgOx type could perform the task of high-density near-field optical recording in a more feasible way. To further explore the optical resolution and controllability of super-RENS disks, the near-field and far-field optical properties of the AgOx-type super-RENS embedded with random silver nanoparticles and nano cavities of different densities were studied using finite-difference time-domain simulations. The random nanostructures yielded super-resolution capability, but the surface plasmon effects generated by the random metallic nanoparticles significantly enhanced the far-field signals. A simplified Fourier optical theory was proposed to understand the relationship between the enhanced near field of random nanostructures and the super-resolution capability in the far field.

Microscopy of localized second-harmonic enhancement in random metal nanostructures

Using second-harmonic (SH) scanning optical microscopy in reflection we study the effect of localized SH enhancement in random metal nanostructures. With a tightly focused tunable (750--830 nm) laser beam, we obtain SH images of a 55-nm-thick gold film surface covered with randomly distributed 70-nm-high gold bumps showing small $(\ensuremath{\sim}0.7\ensuremath{\mu}\mathrm{m})$ and very bright $(\ensuremath{\sim}{10}^{3}$ times the background) spots. Wavelength and polarization dependencies of both positions and intensities of these SH bright spots as well as their statistics are investigated and compared for two areas having different nominal densities of scatterers, i.e., 25 and $50\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}2}.$ For relatively large signals, it is found that, the probability density function of the SH signal follows the power-law dependence with the index being in the range of 2.5--3 for both values of the scattering density. We observe that, for incident laser powers in the range 3--40 mW, the SH bright spots exhibit, as expected, quadratic dependencies of the maximum signal on the incident power. However, for higher power levels, the interrogating laser beam causes nonlocal modifications of SH images, i.e., some SH bright spots disappear and others emerge even outside of the area that was exposed to a high power radiation. We relate the observed feature to surface plasmon polariton contribution in the process of multiple scattering, resulting in the formation of resonant eigenmodes (at fundamental and SH frequency) whose excitation in turn gives rise to the SH bright spots.

Direct observation of localized second-harmonic enhancement in random metal nanostructures.

Second harmonic (SH) scanning optical microscopy in reflection is used to image the gold film surface covered with randomly placed scatterers. SH images obtained with a tightly focused tunable (750-830 nm) laser beam show small (approximately 0.7 microm) and very bright (approximately 10(3) times the background) spots, whose locations depend on the wavelength and polarization of light. Comparing SH and fundamental harmonic (FH) images, we conclude that the localized SH enhancement occurs due to the overlap of FH and SH eigenmodes. The probability density function of the SH signal is found to follow the power-law dependence.

Nonlinear magneto-optics in magnetic nanoparticles

In this paper, the results of our recent studies of the magnetization-induced nonlinear optical effects in magnetic nanoparticles are surveyed. There are three aspects of nonlinear magneto-optics of nanoparticles, which attract attention in this review being interdisciplinary in terms of magnetism, low-dimensionality, nonlinear and coherent optics. The first is the fact that the nonlinear magneto-optical Kerr effect (NOMOKE), i.e. magnetization-induced second-harmonic generation (MSHG), exists in magnetic nanogranular films simultaneously with giant magnetoresistance effect (GMR). Moreover, the NOMOKE contrast attains maximum at the same content of magnetic component in magnetic nanogranular films as the maximum of GMR coefficient. Up to now nonlinear magneto-optics was concentrated on the regular magnetic systems and as a consequence described a coherent nonlinear response. On the contrary, in recent studies an attention is turned to the random magnetic nanostructures and, in particular, to the random two-dimensional arrays of magnetic nanoparticles fabricated by Self-Assembly techniques. Thus, an incoherent analog of MSHG, which can be called: magnetization-induced hyper-Rayleigh scattering, is observed. And finally a general issue of intrinsic weakness of magnetization-induced nonlinear optical effects governed by the fine structure parameter is discussed in terms of internal homodyne effect as one of the mechanisms of an enhancement of the magnetic effects in nonlinear optics.

Near-field optical microscopy of fractal structures

Using a photon scanning tunnelling microscope combined with a shear-force feedback system, we image both topographical and near-field optical images (at the wavelengths of 633 and 594 nm) of silver colloid fractals. Near-field optical imaging is calibrated with a standing evanescent wave pattern. Near-field optical images exhibit spatially localized (within 150-250 nm) intensity enhancement (by up to 20 times) in the form of round bright spots, whose positions and brightness are found to be sensitive to the light wavelength, polarization and angle of incidence. The observed phenomenon is related to the localization of resonant dipolar eigenmodes in random surface nanostructures.