The Borrmann effect is known as an increase of the X-rays transmission of a perfect crystal in the Laue diffraction scheme when the Bragg diffraction condition are satisfied. Following the trend of the transfer of the X-ray phenomena into the optical spectral range, we experimentally observed and studied the optical analogue of the Borrmann effect for the case of one-dimensional photonic crystals (PhC). For the experiments we made the samples of PhCs based on porous fused silica, which reveal periodical modulation of the refractive index and light absorption. We show that in such structures the Borrmann effect reveals itself as increasing transmission when light propagates through a PhC at the Bragg angle of incidence. Pronounced differences of the Borrmann effect are observed for the PhC structures with light losses concentrated in high or low refractive index layers. The spectral features of the effect are analyzed both experimentally and theoretically.
Organic microstructures attract much attention due to their unique properties originating from the design of their shape and optical parameters. In this work we discuss the linear, second- and third-order nonlinear optical effects in arrays and in individual organic microstructures composed by self-assembling technique and formed randomly on top of a solid substrate. The structures under study consist of micro-spheres, -hemispheres or -frustums made of red laser dye and reveal an intense fluorescence (FL) in the visible spectral range. Importantly, that due to a high value of the refractive index and confined geometry, such micro-structures support the excitation of whispering gallery modes (WGM), which brings about strong and spectrally-selected light localization. We show that an amplification of the nonlinear optical effects is observed for these structures as compared to a homogeneous dye film of similar composition. The obtained data are in agreement with the results of the FDTD calculations performed for the structures of different dimensions. Perspectives of application of such type of organic nonlinear microresonators in optical devices are discussed.