Tailored disorder - an advanced materials design for innovative photonic applications

Resume : Turbostratic carbon consists mainly of polyaromatic carbon with or without heteroatoms (N, H, O). Its structure is made of stacked graphene layers in parallel array but without any three dimensional order, as in graphite structure. Such disordered graphitic microstructure is produced very simply, by heating organic polymers in inert atmosphere. This work discusses about the thermal conversion of a commercially available organic polymer, Kapton® HN polyimide, and the interaction of terahertz radiation on various carbon-microstructures. The progress of carbonization was followed by a variety of analytical methods including thermogravimetry coupled with mass spectrometry, Infra-red and Raman spectroscopies. Raman spectra acquired from the carbonaceous residues exhibited the two characteristic bands, D (1358 cm-1) and G (1593 cm-1), indicating the formation of turbostratic carbon. The electrical conductivity progressively increases with the heat treatment temperature from 2×10-3 S cm-1 at 700 °C to 298 S cm-1 > 900 °C. Moreover, the heat-treated materials show variant degrees of transmission and reflection in the frequency range 220-500 GHz. The thickness of the material, as well as the structural rearrangement of carbon can cause significant deviations in how a sub-millimetric wave is transmitted, and reflected, leading to different absorptions. The maximal absorption of 47% was obtained with the carbonaceous residue obtained between 750-800 °C.

Resume : In butterflies, coloration is often achieved by nanoarchitectures generating structural color. In their case a chitinous layer forms a complex nanocomposite, the periodicity of which fits in the range of the visible light wavelength. A characteristic reflection develops providing the various structural colors of butterflies [1]. Both sides of the wings are covered by scales which may exhibit a large variety of photonic nanostructures [2]. Blue polyommatus butterflies in this study possess a short range ordered structure called pepper-pot type structure. We demonstrated on closely related species living in the same habitat the unambiguous connection between the scale nanostructure variations and the color [3]. Recently we investigated the variability of the color of P. icarus males living in the same location. We found for 100 samples that the variation in the spectral position of the reflectance maximum is smaller than +/-40 nm. We investigated for the first time a large set (2824 data points) of specimens collected over a wide geographical area: the palearctic ecozone (west to east: Western Europe to Eastern Asia; north to south: Finland to Afganistan). The spectral position of the reflectance maxima was found to have the same variation of the order of +/- 40 nm, but clear geographical particularities were evidenced. [1] LP Biró & JP Vigneron, Laser & Photonics Rev. 5 (2011) 27 [2] H. Ghiradella, Appl. Opt. 30 (1991) 3492 [3] Zs. Bálint et al. JRS Interface 73 (2012) 1745

Resume : The transmission of light through a fibre plays an important role in many optical devices and techniques such as optical communication, medical light sensing and fibre optical sensing. Microscale optical waveguides are crucial for steering and manipulating light in such devices. Organic materials are easily processed into a variety of nano and microshapes cost effectively and efficiently depending on the solution in which they are dissolved and nature of their polarity making them attractive for opto-electronics. We study self-assembled hollow microtubes formed from an organic molecule (nile red). These microtubes were seen to passively waveguide light when probed at a higher wavelength relative to their absorption band threshold. The input light directly propagated through the other end of the tube creating Fabry-Perot fringing in the observed waveguide light. Studies of microtube interactions/junctions were also performed and compared to isolated microstructures. Active waveguiding was also observed when probing within the absorption band of the microtube. The emitted light from the ends of tubes were spectrally shifted relative to the emission wavelengths of the starting material which is indicative of active waveguiding.

Resume : Optofluidic materials are of interest in a variety of device applications, particularly in bio-system-on-a-chip devices. An important aspect of optimising these materials for such devices is the ability to engineer materials properties, e.g., structure and charge, which allow the contact angle (CA) to be tuned. Using biocompatibile ferroelectric lithium niobate (LN), which is well known for various nonlinear optical applications, it is shown that the CA can be tuned via ferroelectric domain and polarization engineering and subsequent polarization directed photo-assisted deposition of metallic nanostructures. The LN templates have been patterned via electric field poling to create periodically poled (PP) LN or using a periodic proton exchange (PPE) chemical patterning process to create PPELN. PPLN samples present an optically flat surface with charge-patterned regions that can be preferentially etched to create structured, three dimensional surfaces. PPELN surfaces have patterned surface chemistry and topographical features associated with the fabrication processing, e.g., swelling during proton exchange and over-etching during masking. The chemical patterning provides the ability to tailor the electrostatic fields at the surface, which can be used to fabricate microscale arrays of metallic nanostructures. By combining patterned LN surfaces and metal deposition, it is shown that the CA of LN can be reproducibly tailored between 41.40 ± 0.05° and 106.40 ± 0.67°.

Resume : A study of bend-induced losses in a silica-based single-mode microstructured fiber G.652 has been conducted. With the use of the equivalent step-index profile method in approximation of waveguide parameters of microstructured fiber (normalized frequency and normalized transverse attenuation constant) the effect of bending on the spectral position of the fundamental mode short-wavelength leakage boundary has been analyzed. Upon measurement of spectral characteristics of attenuation in the considered fibers good accordance of numerical and experimental data has been found out. It is shown that increase of the air content in the holey cladding leads to expansion of the mentioned boundary to lower wavelengths for the value from 150 to 800 nm depending on the core size and bending conditions. A single-transverse-mode propagation is achieved on fiber length of 5-10 meters due to a substantial difference in losses of fundamental and higher-order guided modes attained by bending. Optical losses in all studied samples are less than 10 dB/km at the wavelength λ = 1550 nm.