22 research outputs found
Resonant mode coupling approximation for calculation of optical spectra of photonic crystal slabs. Part II
We propose further development of the resonant mode coupling approximation
for the calculation of optical spectra of stacked periodic nanostructures in
terms of the scattering matrix. We previously showed that given the resonant
input and output vectors as well as background scattering matrices of two
subsystems, one can easily calculate those for the combined system comprising
two subsystems. It allows us to write a resonant approximation for the combined
system and speed up calculation significantly for typical calculation problems.
The main drawback of this approach is that the background matrix in such
approximation was considered constant which is not always sufficient if the
energy range of interest is relatively wide. The aim of this article is to
solve this problem by utilizing more complicated approximations for the
background matrices. In particular, we show that consideration of
energy-dependent correction terms for the background matrices remarkably
reduces the resonant energies' calculation error. Here we first consider a
linear approximation, and although it is not suitable for large energy ranges,
it is used as a base for a piecewise-linear approximation which allows one to
keep the approximation error negligibly small with only a few sample points.
Moreover, interpolation of the background matrices allows one to apply resonant
mode coupling approximation in almost arbitrary large energy ranges. We also
consider approximation of background matrices by an arbitrary matrix function
and propose a technique to derive the resonant poles in this case. The methods
described here could be considered as an alternative approach for calculation
of optical spectra stacked systems.Comment: 11 pages, 6 figure
Twist-tunable moir\'e optical resonances
Multilayer stacks of twisted optical metasurfaces are considered as a
prospective platform for chiral nanophotonic devices. Such structures are
primarily used for the realization of circularly polarized light sources,
artificial optical rotation, and circular dichroism. At the same time, the
behavior of their hybrid photonic modes is strongly affected by the
moir\'e-pattern of superimposed periodic constituents. In this work, we show
that moir\'e-periodicity in bilayer dielectric photonic crystal slabs leads to
an arise of unlimitedly narrow optical resonances, which are very sensitive to
the relative twist and gap width between the sublayers. We demonstrate the
structure providing twist-tuning of the hybrid mode wavelength in the range of
300--600~nm with quality factor varying from~~up~to~
correspondingly. The obtained results pave the wave for the utilization of
moir\'e-assisted effects in multilayer photonic crystal slabs
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Raman Signal Enhancement Tunable by Gold-Covered Porous Silicon Films with Different Morphology
The ease of fabrication, large surface area, tunable pore size and morphology as well surface modification capabilities of a porous silicon (PSi) layer make it widely used for sensoric applications. The pore size of a PSi layer can be an important parameter when used as a matrix for creating surface-enhanced Raman scattering (SERS) surfaces. Here, we evaluated the SERS activity of PSi with pores ranging in size from meso to macro, the surface of which was coated with gold nanoparticles (Au NPs). We found that different pore diameters in the PSi layers provide different morphology of the gold coating, from an almost monolayer to 50 nm distance between nanoparticles. Methylene blue (MB) and 4-mercaptopyridine (4-MPy) were used to describe the SERS activity of obtained Au/PSi surfaces. The best Raman signal enhancement was shown when the internal diameter of torus-shaped Au NPs is around 35 nm. To understand the role of plasmonic resonances in the observed SERS spectrum, we performed electromagnetic simulations of Raman scattering intensity as a function of the internal diameter. The results of these simulations are consistent with the obtained experimental dat
Chiral light in twisted Fabry-P\'erot cavities
Fundamental studies of the interaction of chiral light with chiral matter are
important for the development of techniques that allow handedness-selective
optical detection of chiral organic molecules. One approach to achieve this
goal is the creation of a Fabry-P\'erot cavity that supports eigenmodes with a
desired electromagnetic handedness, which interacts differently with left and
right molecular enantiomers. In this paper, we theoretically study chiral
Fabry-P\'erot cavities with mirrors comprising one-dimensional photonic crystal
slabs made of van der Waals AsS, a material with one of the highest
known in-plane anisotropy. By utilizing the anisotropy degree of freedom
provided by AsS, we design Fabry-P\'erot cavities with constitutional
and configurational geometrical chiralities. We demonstrate that in cavities
with constitutional chirality, electromagnetic modes of left or right
handedness exist due to the chirality of both mirrors, often referred to as
handedness preserving mirrors in the literature. At the same time, cavities
with configurational chirality support modes of both handednesses due to chiral
morphology of the entire structure, set by the twist angle between the optical
axes of the upper and lower non-chiral anisotropic mirrors. The developed
chiral Fabry-P\'erot cavities can be tuned to the technologically available
distance between the mirrors by properly twisting them, making such systems a
prospective platform for the coupling of chiral light with chiral matter.Comment: 33 pages, 9 figure
The Size Ellect on the Infrared Spectra of Condensed Media Under Conditions of ID, 2D and 3D Dielectric Confinement
A general expression for the dielectric loss spectrum of an absorbing composite medium was obtained from a Maxwell-Garnett general equation. This expression was simplified for the cases of one, two and three dimensional dielectric confinement in both ordered and disordered thin layers, rods (wires) and spheres of absorbing medium which are considered as mesoparticles or mesoscopic molecules. This theoretical approach was verified experimentally using high purity organic liquids with strong absorption bands in the infrared range. Three organic liquids, namely benzene, chloroform and carbon disulphide, were measured in various dielectric confinement configurations using Fourier Transform Infrared (FTIR) spectroscopy with a Grazing Angle attachment GATRTM. A significant shift of the resonant absorption band of liquid mesoparticles was observed for various dielectric confinement geometries which is in good agreement with theoretical predictions. Possible applications of this work include investigations of industrial smoke, toxic aerosols and liquid droplets