Understanding Of Light Propagation Research Articles

Reprogrammable plasmonic topological insulators with ultrafast control

Topological photonics has revolutionized our understanding of light propagation, providing a robust way to manipulate light. So far, most of studies in this field are focused on designing a static photonic structure. Developing a dynamic photonic topological platform to switch multiple topological functionalities at ultrafast speed is still a great challenge. Here we theoretically propose and experimentally demonstrate a reprogrammable plasmonic topological insulator, where the topological propagation route can be dynamically changed at nanosecond-level switching time, leading to an experimental demonstration of ultrafast multi-channel optical analog-digital converter. Due to the innovative use of electric switches to implement the programmability of plasmonic topological insulator, each unit cell can be encoded by dynamically controlling its digital plasmonic states while keeping its geometry and material parameters unchanged. Our reprogrammable topological plasmonic platform is fabricated by the printed circuit board technology, making it much more compatible with integrated photoelectric systems. Furthermore, due to its flexible programmability, many photonic topological functionalities can be integrated into this versatile topological platform.

Changes in temperature inside an optomechanical model of the human eye during emulated transscleral cyclophotocoagulation.

Currently, many diseases of the eye are treated by laser surgery. An understanding of light propagation and the heating of eye tissue during laser exposure is crucial to improving the outcome of these procedures. Here, we present the development of physical and computational models of the human eye by combining optical light propagation and thermal characteristics. For the physical model, all parts of the eye, including cornea, lens, ciliary body, sclera, aqueous and vitreous humors, and iris, were fabricated using a 3D printed holder and modified polydimethylsiloxane. We also present a computational model based on finite element analysis that allows for a direct comparison between the simulation and experimental measurements. These models provide an opportunity to directly assess the rise in temperature in all eye tissues. The simulated and physical models showed good agreement for the transmission of light at varying incident angles. The heating of optical components was investigated in the retina and the ciliary body during simulated laser surgery. Temperature increases of 45.3°C and 30.6°C in the retina and ciliary bodies, respectively, were found in the physical model after 1 minute of exposure to 186 mW of 850 nm laser radiation. This compared to 29.8°C and 33.9°C increases seen under the same conditions in the simulation model with human eye parameters and 48.1°C and 28.7°C for physical model parameters. These results and these models are very promising for further investigation of the impact of laser surgery.

New insight into light propagation and light–matter interactions with applications to experimental observations

The paper provides a new understanding of light propagation and light–matter interactions by examining the physical implications of group velocity, electromagnetic (EM) power flow, Poynting theorem, energy conservation law, and Fermat's principle. A criterion is set up to identify the justification of the group velocity definition, and a modified definition is proposed to remove the flaws that the classical definition has. It is reasonably argued that energy conservation law and Fermat's principle are physical postulates independent of Maxwell equations. A “superluminal power flow” is constructed to show that Poynting theorem cannot uniquely define the EM power flow if the energy conservation law or Fermat's principle is not taken into account. As an application, associated basic concepts in textbooks and experimental observations reported in recent research works are also reviewed, including: why the traditional formulation of Fermat's principle has a limited application; how the Fermat's principle is formulated for a plane wave; why the Fermat's principle is consistent with Maxwell EM theory; what the significant difference is between Poynting theorem and energy conservation law; why Poynting vector as EM power flow may break energy conservation law and Fermat's principle in an anisotropic medium; why the physical explanations for “spatially structured” photons in Giovannini-coworkers experiments are not consistent with the principle of relativity; why the traditionally-argued invariance of information velocity contradicts Maxwell equations; and why the superluminal light pulse propagation claimed in Wang-Kuzmich-Dogariu experiments voilates Einstein causality.

Optical Tomography

The number of applications using optical tomography has significantly increased over the past decade. A literature research providing this term as keyword gives 26 hits for 1990, 719 for 2000, and 9,202 for 2010. With an increasing number of applications, the number of different imaging modalities is also increasing. This review summarizes recent developments in tomographic methods for scattering and nonscattering samples. These two different cases of optical tomography are typically represented by biomedical imaging and atmospheric tomography, representing high- and low-scattering samples, respectively. An essential prerequisite for tomographic analyses is an understanding of light propagation in different media, which allows for the development of specific reconstruction algorithms for the different tomographic tasks.

Instrumental developments and recent experiments in near-field optical microscopy

Recent advances in the understanding of light propagation in small dimensions as well as in instrumentation make scanning near-field optical microscopy (SNOM) a very promising tool for studying optical phenomena on a nanometer scale. In this talk, we will demonstrate experiments carried out with the recently developed tunneling near-field optical microscope. We found superior image contrast, as compared with images taken with conventional aperture SNOM, along with the high resolution commonly achieved with fiber probes. This work was motivated by the theoretical investigations presented in Dr. Pohl's talk. We will further describe two recently built instruments. The first is a scanning tunneling optical microscope combined with a scanning force microscope. The second instrument is an aperture-type SNOM mounted on the sample stage of a conventional inverted optical microscope. Of particular interest to us is imaging with polarization contrast. One of the goals is to study liquid-crystal films which have been micro-patterned with the help of a force microscope tip. These samples are promising as waveguides and potential electro-optical devices. Additionally, they represent very convenient test samples for polarization SNOM.

Propagation of Light through Human Dental Enamel and Dentine

Techniques based on transillumination of teeth with visible light will be a valuable aid in caries diagnosis, if a higher sensitivity than that of the present Foti method is achieved. Therefore, a better understanding of light propagation through teeth is required, and hence it is useful to investigate the propagation of light through sound dental material. In this study the intensities emanating from the surfaces of enamel and dentine bars were measured when these bars were illuminated using a fibre rod transporting the light from a HeNe laser (lambda = 633 nm) as a light source. From the measured intensities, the radiant fluxes emanating from the surfaces were calculated. To account for a directional dependence of these fluxes, optical anisotropy in dental material was investigated by comparing the transmitted light intensity in a direction perpendicular and parallel to the approximal surface of the tooth from which the sample was cut. The mean ratio of the transmitted intensities in perpendicular and parallel direction was 0.86 +/- 0.06 for enamel and 2.88 +/- 0.43 for dentine. In addition, for enamel the asymmetry parameter, g, was estimated. The averaged value was g = 0.68 +/- 0.09. It was concluded that for dentine the optical anisotropy as measured supports the idea that tubules are the predominant cause of scattering in dentine. For enamel the results indicate that the hydroxyapatite crystals contribute significantly to scattering and that the influence of the prism structure on the light propagation is small.

Guest Editorial - Ocean Optics

Photo-optical technology has exceptionally varied uses in the sea ranging from imaging to communications and remote sensing. The optical region has many inherent advantages which will insure its continued application in ocean-related tasks. Optical systems, however, are limited to relatively short water paths due to the strong absorption and scattering of even the clearest water. Better understanding of light propagation in a scattering/absorbing medium and the development of techniques to extend operating range are among our most important tasks.