Basic Optical Processes Research Articles

Extraordinary spin-orbit interaction in the plasmonic lens with negative index material.

Spin-orbit interactions are inherent in many basic optical processes in anisotropic and inhomogeneous materials, under tight focusing or strong scattering, and have attracted enormous attention and research efforts. Since the spin-orbit interactions depend on the materials where they occur, the study of the effects of materials on the spin-orbit interactions could play an important role in understanding and utilizing many novel optical phenomena. Here, we investigate the effect of negative-index material on the spin-orbit interactions in a plasmonic lens structure in the form of a circular slot in silver film. Numerical simulations are employed to study the influence of the negative-index material on the plasmonic vortex formation and the plasmonic focusing in the structure under circularly polarized excitations bearing different orbital angular momentum. We reveal that the presence of negative-index material leaves the plasmonic vortex field distribution and the corresponding topological charge unaltered during the spin-to-orbital angular momentum conversion, whereas reverses the rotation direction of in-plane energy flux of the plasmonic vortex and shifts the surface plasmon polariton focus position to the opposite direction compared to the case without negative-index material. This work will help further the understanding of the regulation of optical spin-orbital interactions by material properties and design optical devices with novel functions.

Spin-orbit interaction of a light beam under normal incidence at a sharp interface and its enhancement

The spin-orbit interaction (SOI) of light refers to the mutual conversion and coupling between the spin angular momentum and orbital angular momentum. It is a fundamental effect in optics, and has been widely found in many basic optical processes, such as reflection, refraction, scattering, focusing, and imaging. So it plays an important role in the fields of optics, nanophotonics, and plasmonics, and has great potential applications in precision measurement and detection, information storage and processing, particle manipulation, and various functional photonic devices. Recently, it has been found that a circularly polarized light beam normally passing through an isotropic sharp interface can undergo an SOI process, that is, part of the incident beam experiences a spin-flip and acquires a spin-dependent vortex phase with a topological charge of <inline-formula><tex-math id="Z-20200115110609-1">\begin{document}$ \pm2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191218_Z-20200115110609-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191218_Z-20200115110609-1.png"/></alternatives></inline-formula>. However, the physical origin of this phase and the role of the interface played in the SOI process are still unclear at present. In this work, a Fresnel Jones matrix is first established to describe the relationship between the incident beam and the transmitted beam, based on which we unveil that the vortex phase is in fact a spin-redirection Berry geometric phase, originating from the topological structure of the beam itself. The properties of the interface affect the conversion efficiency of the SOI. This kind of SOI is very similar to that in the azimuthal Pancharatnam-Berry phase elements. The difference lies in the fact that the Pancharatnam-Berry phase originates from the external anisotropy of the composite material. Generally, the efficiency of this SOI is extremely low, which limits its applications. The existing method of enhancing this SOI employs an isotropic epsilon-near-zero slab, whose maximum efficiency can reach only about 20%. Since the anisotropic medium (such as birefringent uniaxial crystals) has more degrees of freedom, we further point out that the weak SOI can be greatly enhanced by an optically thin uniaxial slab whose optical axis is parallel to the normal direction of the interface. And under certain conditions, the conversion efficiency can reach 100%. Our study not only establishes a simple and convenient full-wave theory for this SOI, but also reveals the relevant underlying physics, and further provides a possible scheme to significantly enhance the SOI.

A Design for an Internet Router with a Digital Optical Data Plane

This paper presents a complete design for an optical Internet router based on the component steps required for Internet protocol (IP) packet forwarding. Implementations of hop count decrement and header matching are integrated with a simulation-based approach to variable-length packet traffic merging that avoids recirculation, demonstrating an approach for an all-optical data plane. A method for IPv4 checksum computation is introduced, and this and previously designed components are extended from binary to higher-density (multiple bits per symbol) encodings. The implications of this design are considered, including the potential for chip-level and system integration, as well as the requirements of basic optical processing components.

Energy Transfer of Biexcitons in a Single Semiconductor Nanocrystal.

Photoluminescence (PL) decay dynamics of multiexcitons in semiconductor nanocrystals (NCs) are dominated by the nonradiative Auger effect, making it difficult to explore their basic optical processes such as radiative recombination and energy transfer (ET). Here we constructed a single-particle ET system by attaching several acceptor dyes to the surface of a donor NC to study the ET of biexcitons at a single-NC level. By comparing the single-exciton and biexciton PL lifetimes of the same donor NC before and after the acceptor dyes were bleached, their respective ET lifetimes could be reliably extracted without the Auger influence. From statistical measurements on a large number of single ET particles, the average ET rate ratio between biexcitons and single excitons was estimated to be larger than four, and the same scaling rule could be naturally extended to their radiative rates.

Spin–orbit interactions of light

Light carries spin and orbital angular momentum. These dynamical properties are determined by the polarization and spatial degrees of freedom of light. Modern nano-optics, photonics, and plasmonics, tend to explore subwavelength scales and additional degrees of freedom of structured, i.e., spatially-inhomogeneous, optical fields. In such fields, spin and orbital properties become strongly coupled with each other. We overview the fundamental origins and important applications of the main spin-orbit interaction phenomena in optics. These include: spin-Hall effects in inhomogeneous media and at optical interfaces, spin-dependent effects in nonparaxial (focused or scattered) fields, spin-controlled shaping of light using anisotropic structured interfaces (metasurfaces), as well as robust spin-directional coupling via evanescent near fields. We show that spin-orbit interactions are inherent in all basic optical processes, and they play a crucial role at subwavelength scales and structures in modern optics.

Anomalous optical processes in photoluminescence from ultrasmall quantum dots of ZnO

Ensembles of alumina capped ZnO quantum dots (ZQDs) were grown using pulsed laser deposition. The ZQDs of mean radii comparable to and smaller than the pertinent excitonic Bohr radius (∼2.34 nm), called ultrasmall quantum dots, show size dependent optical absorption edges, which follow the strong confinement model. In this model the confinement energy and Coulombic interaction energy of the localized electron-hole pairs are significantly higher than their correlation energy and the optical transitions are perceived to be nonexcitonic in nature. In photoluminescence (PL) spectra of such ZQDs of mean radius of ∼2.3 nm at temperatures of 6 K and above, the primary recombinations are found to be due to the surface bound and Al donor bound electron-hole pairs. The band-edge recombination peak of the PL spectra appeared at about 70 K and above, which was found to be about 166 meV Stoke and/or thermally redshifted with respect to the experimentally observed absorption edge. Almost all of the PL spectra at different temperatures conspicuously showed the LO and 2LO phonon replicas of the primary transitions, suggesting strong coupling between the recombining charge carriers and the LO phonon, which is rather unusual for nonexcitonic recombinations. The temperature dependent PL peak positions followed the well known Varshni’s relation with fitting parameters close to that of the bulk ZnO. The peak intensity of the observed PL transitions followed the normal law of thermal quenching which could be fitted with the Arrhenius equation having activation energy of about 10 meV. Temperature dependence of full width at half maximum of the PL peaks when fitted with the Hellmann and O’Neill models did not result in a close match. However, from this fit one could estimate a value of the carrier-LO phonon coupling coefficient of ∼980 meV, which is higher than that reported earlier for the ZQDs. These observations are hitherto unfamiliar and expected to provide further insight into the basic optical processes in alumina capped ultrasmall ZQDs.

Inconsistency of basic optical processes in plasmonic nanocomposites

Plasmonic nanocomposites, comprising metallic clusters within a semiconductor matrix, exhibit a huge discrepancy between characteristic energies of luminescence, absorption, and generation of photocarriers. This phenomenon, discovered experimentally in InN/In and supported by GaAs/In data, appears because the optical processes occur in spatially different areas which undergo different influence of local plasmons. Manifestations of that in infrared emission and thermally detected absorption are considered using the formalism of electromagnetic enhancement, taking into account the statistics of cluster shapes and the transitions between parallel bands inherent for In.

Structural Colors in Nature: The Role of Regularity and Irregularity in the Structure

Coloring in nature mostly comes from the inherent colors of materials, but it sometimes has a purely physical origin, such as diffraction or interference of light. The latter, called structural color or iridescence, has long been a problem of scientific interest. Recently, structural colors have attracted great interest because their applications have been rapidly progressing in many fields related to vision, such as the paint, automobile, cosmetics, and textile industries. As the research progresses, however, it has become clear that these colors are due to the presence of surprisingly minute microstructures, which are hardly attainable even by ultramodern nanotechnology. Fundamentally, most of the structural colors originate from basic optical processes represented by thin-film interference, multilayer interference, a diffraction grating effect, photonic crystals, light scattering, and so on. However, to enhance the perception of the eyes, natural creatures have produced various designs, in the course of evolution, to fulfill simultaneously high reflectivity in a specific wavelength range and the generation of diffusive light in a wide angular range. At a glance, these two characteristics seem to contradict each other in the usual optical sense, but these seemingly conflicting requirements are realized by combining appropriate amounts of regularity and irregularity of the structure. In this Review, we first explain the fundamental optical properties underlying the structural colors, and then survey these mysteries of nature from the viewpoint of regularity and irregularity of the structure. Finally, we propose a general principle of structural colors based on structural hierarchy and show their up-to-date applications.

Time-resolved photoluminescence studies of InGaN epilayers

Time-resolved photoluminescence (PL) has been employed to study the optical transitions and their dynamic processes and to evaluate materials quality of InGaN epilayers grown by metalorganic chemical vapor deposition. Our results suggest that the PL emissions in InGaN epilayers result primarily from localized exciton recombination. The localization energies of these localized excitons have been obtained. In relatively lower quality epilayers, the localized exciton recombination lifetime τ, decreases monotonically with an increase of temperature. In high quality epilayers, τ increases with temperature at low temperatures, which is a well-known indication of radiative exciton recombination. Our results demonstrate that time-resolved PL measurements uniquely provide opportunities for the understanding of basic optical processes as well as for identifying high quality materials.

RESONANT OPTICS OF FRACTALS

Fractal objects can sustain strongly localized optical modes with high quality-factor. Intense local fields associated with the dipolar eigenmodes in fractals ultimately result in significant enhancement of a number of optical effects. In particular, such basic optical processes as Rayleigh and Raman scattering, degenerate four-wave mixing and harmonic generation, two-photon absorption and non-linear photoemission can be enhanced by several orders of magnitude in small-particle fractal composites and self-affine films.

Remote measurement of atmospheric parameters: New applications of physics with lasers

Abstract Basic optical processes, e.g. absorption, fluorescence, Raman and Rayleigh scattering, that are relevant to lidar applications are briefly described and the principle and relative merits of existing lidar (light detection and ranging) techniques reviewed. The potential and physics of high-spectral-resolution lidar techniques, which currently are in the developmental stage, are discussed in this paper.