Resonant excitation and manipulation of high-index dielectric nanostructures (such as Silicon, Germanium) provide great opportunities for engineering novel optical phenomena and applications. Here, we report selective excitation and enhancement of multipolar resonances, and non-radiating optical anapoles in silicon nanospheres using cylindrical vector beams (CVBs). Our approach can be used as a spectroscopy tool to enhance and identify multipolar resonances as well as a straightforward alternate route to excite electrodynamic anapoles at the optical frequencies.
Nonradiating anapoles are superposition of internal modes that can act as an energy reservoir by reducing the far-field scattering. We report experimental excitation of the electrodynamic anapole mode in isotropic silicon nanospheres at the optical frequencies using radially polarized beam illumination. The superposition of equal and out-of-phase amplitudes of the Cartesian electric and toroidal dipoles produces by a pronounced dip in the scattering spectra with the scattering intensity almost reaching zero – a signature of anapole excitation. The total scattering intensity associated with the anapole excitation is found to be more than 10 times weaker, and the internal energy is found to be 6 times greater for illumination with radially vs. linearly polarized beams. Our approach provides a simple, straightforward alternative path to realize electrodynamic anapole mode at the optical frequencies.
In this work, we present a colloidal Mie resonators consisting of highly crystalline Si nanospheres [1-4]. Such colloidal Mie resonators acting as nanoantennas and building blocks for colloidal metamaterials have advantages in solution-based fabrication of thin film optical devices.
Pavel Kashkarov, Olga Shalygina, Denis Zhigunov, Dmitri Sapun, Sergei Teterukov, Victor Timoshenko, Johannes Heitmann, Michael Schmidt, Margit Zacharias, Kenji Imakita, Minoru Fujii, Shinji Hayashi
Photoluminescence properties of Er-doped nanocrystalline Si/ Si02 structures have been investigated under strong optical excitation. The energy of optical excitation of Si nanocrystals was shown to be almost completely transferred to Er3+ ions in surrounding Si02. It was found that at high pump intensity the energy transfer process competes successfully with nonradiative Auger-recombination in Si nanocrystals. At high excitation level the population inversion of Er3+ ions was achieved and a decrease of the decay time of the photoluminescence at 1.5 μm was observed. Possible mechanisms of the shortening of the Er3+ ion lifetime are discussed.
We report on a medium exhibiting extremely efficient light scattering properties: a liquid network formed in a porous matrix. Liquid fragments confined in the solid matrix result in a random fluctuation of the dielectric function and act as scattering objects for photons. The optical scattering efficiency is defined by the filling factor of the liquid in the pores and its dielectric constant. The spectral dependence of the scattering length of photons indicates that the phenomenon is governed by a Mie-type scattering mechanism. The degree of the dielectric disorder of the medium, i.e. the level of opacity is tunable by the ambient vapor pressure of the dielectric substance. In the strongest scattering regime the scattering length of photons is found to be in the micrometer range. By incorporation of dye molecules in the voids of the porous layer a system exhibiting optical gain is realized. In the multiple scattering regime the optical path of diffusively propagating photons is enhanced and light amplification through stimulated emission occurs: a strong intensity enhancement of the dye emission accompanied by significant spectral narrowing is observed above the excitation threshold for a layer being in the opalescence state.
We report on a strong intrinsic optical anisotropy of silicon induced by its dielectric nanopatterning. As a result, an in-plane birefringence for nanostructured (110) Si surfaces is found to be 105 times stronger than that observed in bulk silicon crystals. A difference in the main values of the anisotropic refractive index exceeds that one of any natural birefringent crystals. The anisotropy parameters are found to be strongly dependent on the typical size of the silicon nanowires assembling the layers. The value of birefringence is dependent also on the dielectric surrounding of silicon nanoparticles assembling these layers. We show that stacks of layers having alternative refractive indices act as a distributed Bragg reflectors or optical microcavities. Dichroic reflection/transmission behavior of these structures sensitive to the polarization of the incident linearly polarized light is demonstrated. These findings open the possibility of an application of optical devices based on birefringent silicon layers in a wide spectral range.
We report on a strong intrinsic optical anisotropy of silicon induced by dielectric nanopatterning. As a result, in-plane birefringence of anisotropically nanostructured (110) oriented Si is found to be 105 times larger than that observed in bulk silicon. The difference of the main values of the anisotropic refractive index (Δn) exceeds that of any natural birefringent crystal. Δn depends strongly on the typical size of the silicon nanowires assembling the layers and the dielectric constant of the medium surrounding these silicon nanoparticles. We show that dielectric stacks of anisotropically nanostructured Si can act as a dichroic distributed Bragg reflectors or optical microcavities. The reflection/transmission behavior of these structures is sensitive to the polarization of the incident linearly polarized light. These findings open the possibility of an application of optical devices based on birefringent silicon layers in a wide field.
Incorporation of Si nanocrystal into Er-doped glasses strongly enhances the IR luminescence of Er3+. The enhancement is believed to be due to the energy transfer from nc-Si. However, the mechanism of the interaction between nc-Si and Er3+ has not been fully understood. In this work, we have studied the interaction between nc-Si and Er3+ by photoluminescence (PL) spectroscopy and PL decay dynamics. In order to tune the luminescence energy of nc-Si to the energy separations between the discrete electronic sates of Er3+, the size of nc-Si was changed in a wide range; the PL energy of nc-Si was changed from 1.2 to 1.5 eV. At low temperatures, periodic features were observed in the PL spectra of nc-Si. The observation of the features is the first spectroscopic evidence that indicates the strong coupling between nc-Si and Er3+. Furthermore, size dependence of the energy transfer rate was estimated from the delay time of the Er3+. The effects of quantum confinement of excitons in nc-Si on the high PL efficiency of Er3+ are discussed.
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