Vapor and liquid optical monitoring with sculptured Bragg microcavities

Abstract

Sculptured porous Bragg Microcavities (BMs) formed by the successive stacking of columnar SiO2 and TiO2 thin films with a zig-zag columnar microstructure are prepared by glancing angle deposition. These BMs act as wavelength dependent optical retarders. This optical behavior is attributed to a self-structuration of the stacked layers involving the lateral association of nanocolumns in the direction perpendicular to the main flux of particles during the multilayer film growth, as observed by Focused Ion Beam Scanning Electron Microscopy. The retardance of these optically active BMs can be modulated by dynamic infiltration of their open porosity with vapors, liquids or solutions with different refractive indices. The tunable birefringence of these nanostructured photonic systems have been successfully simulated with a simple model that assumes that each layer within the BMs stack has uniaxial birefringence. The sculptured BM have been incorporated as microfluidic chips for optical transduction for label free vapor and liquid 2 sensing. Several examples of the detection performance of these chips, working either in reflection or transmission configuration, for the optical monitoring of vapor and liquids of different refractive index and aqueous solutions of glucose flowing through the microfluidic chips are described.

Liquid switchable radial polarization converters made of sculptured thin films

“…The observed effect was reversible and the LB magnitude recovered its initial value after empting the pores (e.g., drying the BM). Similar optofluidic effects have been reported for vapor and liquid sensing using sculptured slanted or zigzag BM [7]. Reversibility of this phenomenon is illustrated in Video V2 included in the supporting information showing that qBM sample recovers its performance as radial polarization converter after liquid removal (drying in our case).…”

“…By this procedure, fancy anisotropic architectures formed by columnar nanostructures with slanted, zig-zag, S-shape or helix arrangements can be achieved varying the orientation of the incoming flux of material with respect to the sample surface (e.g., through the azimuthal rotation of sample during deposition) [1]. Optical birefringence can be obtained with a bundled arrangement of nanocolumns that depicts a thin film microstructural anisotropy that can be tailored by controlling the polar and azimuthal deposition angles [5][6][7]. These outstanding optical properties have prompted the incorporation of this type of thin films into optically active devices such as retarders [8], tunable circular polarizers [9] lasers with tuned polarization emission [10], mirrors for circular polarized light [11][12], microfluidics liquid detection [6,13], or vapor sensing [7,14].…”

Nanostructural Analysis of Porous Oblique Angle Deposited (OAD) Multilayer Systems by Grazing‐Incidence Small‐Angle X‐Ray Scattering

“…[10] In the present work, we want to further characterize ordered two-oxide-layered systems formed by the alternant stacking of porous SiO2 and TiO2 thin films deposited by PV-OAD which in previous works were utilized as Bragg photonic structures. [11] This type of 1D photonic structures is formed by the staking of various equally thick layers of two materials with different refraction index and incorporate a central thicker layer of the lower refraction index material that act as optical defect. The transmission spectrum of this structure is characterized by an optical gap and a resonant peak in the center.…”

Environmentally Tight TiO2–SiO2 Porous 1D‐Photonic Structures

“…[9a,b,11,15] For the purpose of the present work looking for the stability of passive optical elements, it is of relevance that the desorption isotherm branch does not completely converge to the initial position of the adsorption branch, indicating that, once reached saturation, a certain amount of water remains irreversibly condensed in the pores, even after pumping at ambient temperature. Such a dependence on environmental conditions would preclude the use of these devices for outdoor static applications (except for the already mentioned use as photonic sensors) where the photonic structures might undergo a large number of uncontrolled heating and water exposure cycles. Similar curves to those in Figure c were reported for BMs prepared by e‐beam deposition at glancing angles or for nanoparticle multilayers .…”

“…This geometrical configuration, either by e‐beam evaporation or magnetron sputtering (MS), induces the growth of nanocolumnar films with porosities that may reach up to 50–60% of the film volume . Highly porous OAD films and multilayers have been utilized in the pass for the fabrication of antireflection layers for passive optical applications,[7,11a,b] photocatalytic layers,[5,10a] humidity and other vapor sensors[10a,b,15] or optofluidic devices. [9c,d,16] Herein, we firstly prove that the optical behavior of MS‐OAD multilayers in the form of Bragg reflectors (BR) or Bragg microcavities (BM) can be tailored by adjusting the porosity of the TiO 2 and SiO 2 thin films (e.g., making the TiO 2 compact and the SiO 2 highly porous, as proposed in previous works [7,12c]).…”