Wave Optical Phenomena Research Articles

Simulation of light scattering in large, disordered nanostructures using a periodic T-matrix method

To model and design light propagation in disordered optical nanostructures and materials, any applicable simulation technique has to cope with enormous computational challenges in a bearable time frame. To circumvent these, the introduction of an artificial periodicity to the disordered particle structure allows to rely on computational techniques that exploit periodic boundary conditions. Choosing a rather large periodicity promises to preserve randomness in form of a close-range disorder but can introduce false interferences. So far, it remains open how the artificial periodicity has to be chosen to minimize its detrimental influence. Here, we combine the superposition T-matrix scheme with an Ewald sum formulation to account for light scattering in periodic particle arrangements that contain hundreds to thousands of individual scatterers per unit cell. Simulations reveal that the periodicity’s influence cannot be minimized by simply choosing one single period much longer than the excitation wavelength. The excitation of lattice induced resonances prevents so. However, choosing a periodicity that does not sustain such detrimental features allows for reliable predictions. With that, the presented approach is suitable to derive spectral information about wave-optical phenomena in large, random particle arrangements with a spatial extend beyond those accessible with other full-wave solvers.

Biobeam-Multiplexed wave-optical simulations of light-sheet microscopy.

Sample-induced image-degradation remains an intricate wave-optical problem in light-sheet microscopy. Here we present biobeam, an open-source software package that enables simulation of operational light-sheet microscopes by combining data from 105–106 multiplexed and GPU-accelerated point-spread-function calculations. The wave-optical nature of these simulations leads to the faithful reproduction of spatially varying aberrations, diffraction artifacts, geometric image distortions, adaptive optics, and emergent wave-optical phenomena, and renders image-formation in light-sheet microscopy computationally tractable.

Very low energy scanning electron microscopy in nanotechnology

The group of low energy electron microscopy at ISI AS CR in Brno has developed a methodology for very low energy scanning electron microscopy at high image resolution by means of an immersion electrostatic lens (the cathode lens) inserted between the illumination column of a conventional scanning electron microscope and the sample. In this way the microscope resolution can be preserved down to a landing energy of the electrons one or even fractions of an electronvolt. In the range of less than several tens of electronvolts the image signal generation processes include contrast mechanisms not met at higher energies, which respond to important features of the 3D inner potential of the target and visualise its local crystallinity as well as the electronic structure. The electron wavelength comparable with interatomic distances allows observation of various wave–optical phenomena in imaging. In addition, the cathode lens assembly secures acquisition of electrons backscattered from the sample at large angles with respect to the surface normal, which are abandoned in standard microscopes although they provide enhanced crystallinity information and surface sensitivity even at medium electron energies. The imaging method is described and illustrated with selected application examples.

Light absorption and emission in nanowire array solar cells

Inorganic nanowires are under intense research for large scale solar power generation intended to ultimately contribute a substantial fraction to the overall power mix. Their unique feature is to allow different pathways for the light absorption and carrier transport. In this publication we investigate the properties of a nanowire array acting as a photonic device governed by wave-optical phenomena. We solve the Maxwell equations and calculate the light absorption efficiency for the AM1.5d spectrum and give recommendations on the design. Due to concentration of the incident sunlight at a microscopic level the absorptivity of nanowire solar cells can exceed the absorptivity of an equal amount of material used in thin-film devices. We compute the local density of photon states to assess the effect of emission enhancement, which influences the radiative lifetime of excess carriers. This allows us to compute the efficiency limit within the framework of detailed balance. The efficiency is highly sensitive with respect to the diameter and distance of the nanowires. Designs featuring nanowires below a certain diameter will intrinsically feature low short-circuit current that cannot be compensated even by increasing the nanowire density. Optimum efficiency is not achieved in densely packed arrays, in fact spacing the nanowires further apart (simultaneously decreasing the material use) can even improve efficiency in certain scenarios. We observe absorption enhancement reducing the material use. In terms of carrier generation per material use, nanowire devices can outperform thin-film devices by far.

Time-dependent FEL wavefront propagation calculations: Fourier optics approach

Numerical examples of time-dependent FEL wavefront propagation calculation and coherence analysis are presented. The calculations are performed using the Synchrotron Radiation Workshop (SRW)—wave-optics computer code optimized for synchrotron radiation, and the 3D FEL simulation code GENESIS 1.3. In the examples described, the electric field obtained at the exit of an FEL undulator after running GENESIS in time-dependent mode is transformed from time to frequency domain, and then propagated, using Fourier-optics methods implemented in SRW, through interferometer-type optical schemes. Time-averaged intensity patterns, obtained in image planes of the interferometers as a result of these simulations, allow for straightforward coherence characterization by visibility of interference fringes. The presented examples show that Fourier optics-based wavefront propagation method, as implemented in the SRW, can be effectively used for detailed analysis of characteristics of FEL radiation in time and in frequency domains. Besides, due to its high accuracy and CPU-efficiency, this method is very well suited for optimization of optical beamlines for fourth generation synchrotron radiation sources, for which accurate treatment of wave-optical phenomena, aimed to preserve radiation coherence and time structure, is of paramount importance.

Wave optics of the central spot in planetary occultations

THE detection of a bright central spot1 during the occultation of e Geminorum by Mars2 has demonstrated that an exponential planetary atmosphere can act as a lens with somewhat peculiar properties. In the course of a general study of certain wave-optical phenomena in planetary occultations, we have carried out an investigation into the diffraction nature of the central spot, which has not been previously considered in occultation theory. Some of the results are presented in this letter, along with implications for possible future experiments.