Euglenoids are a group of predominantly free-living unicellular microorganisms that mostly live in freshwater bodies but can also be found in marine and brackish waters. These organisms have a characteristic that distinguishes them form the other protists: they are covered by a surface pellicle formed by S-shaped overlapping bands which resemble a diffraction grating. These microorganisms have developed numerous protection mechanisms intended to avoid or reduce the damage produced by UV radiation, such as the production of pigments and the repair mechanisms in hours of darkness and during daylight. In a recent paper we have investigated the role played by the pellicle of Euglenoids in the protection of the cell against UV radiation, by means of an electromagnetic approach based on the approximation of the pellicle profile by a one-dimensional diffraction grating. This simplified model allowed us to confirm that under certain incidence conditions, the corrugation of the pellicle helps increase the UV reflection, and consequently, diminish the UV radiation that enters the cell. In order to analyze the electromagnetic response of the whole cell, we extend two different approaches to calculate the reflected response: a simulation method especially developed to deal with complex biological structures that permits the introduction of the scattering object via an electron microscopy image, and the integral method, which has been widely used to compute the electromagnetic response of finite structures. Numerical results of near and far fields are shown.
In a recent paper we have investigated, from an electromagnetic point of view, the role played by the pellicle of Euglenoids –unicellular aquatic organisms– in the protection of the cell against UV radiation.14 By modelling the pellicle as a diffraction grating, we computed the electromagnetic response of different species that exhibit different behaviors against UV radiation. In this previous study, the pellicle profile was approximated by a sinusoidal grating. However, it has been observed in the transversal cut images that the profiles are not exactly sinusoidal, and also vary from sample to sample. Since the electromagnetic response depends on the geometry of the grating, reflectance calculations that take into account a more accurate representation of the actual profile could provide more insight into this problem. In this paper we investigate the electromagnetic response of the pellicle of Euglenoids for different grating profiles. The diffraction problem is solved by using the Chandezon method, which has demonstrated a successful performance for deep gratings of arbitrary profiles. We analyze the influence of the shape, depth and period of the grating on the UV reflectance. We show that the pellicle characteristics are critical parameters to increase the reflectance, thus reducing the penetration of the UV radiation within the cell and therefore, minimizing the damage and increasing the survival of these organisms.
Natural photonic structures exhibit remarkable color effects such as metallic appearance and iridescence. A rigorous study of the electromagnetic response of such complex structures requires to accurately determine some of their relevant optical parameters, e.g. the dielectric constants of the materials involved. In a recent work, we have shown that heuristic optimization strategies are suitable tools for the retrieval of the complex refractive index of the materials comprising natural multilayer systems such as the Coleoptera’s cuticle. Moreover, the numerical results obtained illustrate the great potential of this kind of algorithms not only for the study of natural photonic structures, but also for the design of biomimetic photonic devices for lightning, sensing or anti-counterfeiting applications. In a first stage, we assumed that the materials which comprise the layers are characterized by isotropic non-dispersive dielectric permittivities. However, it is well known that the cuticle of many Coleoptera exhibit anisotropy in their constituent materials, and also dispersion has been reported. In this contribution we improve our previous approach in order to have a more realistic and useful computational tool for the retrieval of the relevant parameters of biological structures. For this, we include, within the inversion algorithm, a dispersion model to describe the frequency-dependent dielectric permittivity of the layers’ materials. Also, in order to guarantee the uniqueness of the solution and the convergence to the global optimum, we simultaneously include in the fitness function the information of several angles of incidence, as well as that of the p- and s-polarization states.
Euglenoids are unicellular aquatic organisms. These microalgae show a typical surface structure that distinguishes
them from the other protists. Most cells are naked and bounded by a plasma membrane surrounded by a pellicle
formed by overlapping bands. It is well known that all terrestrial and aquatic organisms are exposed to UV-A
and UV-B radiation. This radiation is potentially harmful to life and since it can penetrate up to 12 meters in the
water, it can reduce survival, growth and production of phytoplankton. However, the organisms have developed
numerous protection mechanisms intended to reduce such damage, such as the production of pigments and other
repair mechanisms. However, the possible protection that could provide the first barriers before entering into
the cell has not been explored yet. In this paper we investigate, from an electromagnetic point of view, the role
played by the pellicle of euglenoids in the protection of the cell against UV radiation. To do so, we investigate
the electromagnetic response of different species that exhibit different behaviors against the UV radiation. We
solve the diffraction problem by using the Chandezon Method and obtain the reflectance of the pellicle for the
UV wavelengths. The results show that the corrugated pellicle could contribute to increase the reflectance, thus
reducing the penetration of the UV radiation within the cell and therefore, minimizing the damage and increasing
the survival of these organisms.
Proc. SPIE. 7782, The Nature of Light: Light in Nature III
KEYWORDS: Refractive index, Image processing, Reflectivity, Scanning electron microscopy, Transmission electron microscopy, Ocean optics, Multilayer interference, Electromagnetism, Systems modeling, RGB color model
Photonic microstructures in nature, specifically in endemic species of Coleoptera from Argentina and the south
of Chile have been identified, analyzed and modeled. These natural systems produce partial photonic bandgaps
(PBGs) as a result of the high periodicity of the microstructures found in some parts of their bodies. With
the aid of scanning (SEM) and transmission (TEM) electron microscopy we have identified that the elytron
(modified forewing of a beetle that encases the thin hind wings used in flight) of these insects shows a periodic
structure which originates diffractive phenomena resulting in extraordinary physical effects such as iridescent or
metallic colors. We measured the reflectance spectrum and obtained the chromaticity diagrams of the samples
with an Ocean Optics 4000 spectrophotometer. The geometrical parameters of the structure were obtained by
processing the SEM images with the ImageJ software, to introduce them in our electromagnetic model. In all
cases, a satisfactory agreement between the measurements and the numerical results was obtained. This permits
us to explain the mechanism of color production in those specimens. The study of structural colors in the
natural world can inspire the development of artificial devices with particular applications in technology, such
as intelligent sensors and new kinds of filters.
The diffraction problem of a plane wave impinging on a grating formed by nested cavities is presented. The nested cavities are formed by layers of perfectly conducting sheets that describe rectangular profiles. The problem is solved by means of the modal method, for s and p polarization modes. The electromagnetic response of the grating is analyzed, paying particular attention to the generation of resonances within the structure. The dependence of the resonances on the geometrical parameters of the grating is studied, and results of far and near field are shown. The results were checked and found in good agreement with those available in the literature for certain limit cases.
We study the phase resonances that appear in infinite conducting gratings comprising a finite number of grooves in each period (compound gratings), when illuminated by a p-polarized plane wave. In particular, we investigate a surface that separates a lossy conductor from a dielectric isotropic medium. The resonances appear when a particular distribution of the phase of the electromagnetic field inside the cavities takes place, and are identified as peaks in the specularly reflected efficiency. These resonances are accompanied by an intensification of the internal field. The diffraction problem is solved by using the modal method. We use the surface impedance boundary condition, which has been proven to be reliable for metals with high conductivity, and simplifies the numerical treatment.
In this paper we solve the homogeneous problem of an almost closed cavity in a ground plane, where the shape of the cavity is described by a multivalued function. To solve this proem we find numerically the complex depths of the cavity for which the determinant of the scattering matrix vanish. These zeros correspond to the resonant frequencies of the cavity; the real part represents the depth at which the resonance takes place, and the imagery part acknowledges for the quality of the resonances. We consider the excitation of the two lowest eigenmodes of each cavity and show that the complex resonant depths coincide with the anomalies present in the diffraction reasons of an infinite gratin formed by this kind of cavities.
We study the scattering of electromagnetic waves at lossy metallic rough surfaces with bivalued cavities. In this particular kind of scatterer two resonant mechanism of very different nature can manifest themselves to produce highly selective reflectivity responses: i) surface plasmons and ii) resonant microcavities. To separate the contributions from each resonant mechanism, we compare the results obtained for two different problems: the diffraction problem for infinitely periodic grating with cavity like grooves and the homogeneous problem for a plane surface with a single cavity.
We give numerical evidence of a new kind of resonances that appear in infinite perfectly conducting gratings comprising a finite number of grooves in each period, when illuminated by a normally incident p-polarized plane wave. The resonances appear when a particular distribution of the phase of the electromagnetic field inside the cavities take place, and are identified as sharp peaks in the specularly reflected efficiency. These resonances are accompanied by a significant intensification of the interior field. The cavities considered are rectangular and the diffraction problem is solved for s and p polarization by using the modal method.
We study the scattering of light from almost closed cylindrical cavities ruled on a highly conducting flat surface. We consider the case where the shape of each cavity can be described by an arbitrary multivalued function of the coordinates. By using the multilayer modal method, which combines the multilayer approximation with the R-matrix propagation algorithm, we investigate the electromagnetic response of this kind of scatterer when it is illuminated by s or p polarized plane waves. Our results show that for both polarizations this system exhibits a resonant behavior manifested by sharp variations in the curves of scattered intensity versus wavelength which are associated with strong intensifications of the near field. The wavelengths at which the resonance occur are intimately connected with the eigenmodes of a single cavity.