Giovanni Carbone

Short pitch cholesteric electro-optical device based on periodic polymer structures

The helical flexoelectro-optic effect produces a submillisecond, temperature-independent in-plane rotation of the optical axis and is potentially interesting for the display industry. The main drawback is that it relies on a texture, the uniform lying helix (ULH), which is intrinsically unstable. We present a method based on the use of periodic polymeric microchannels to create highly ordered and stable ULH structures. Electro-optic measurements performed on a test device show a large contrast ratio between bright and dark states (better then 100:1), fast switching (200 μs), and large optical rotation (>30°).

Analysis of intraocular lens surface adhesiveness by atomic force microscopy

PURPOSE: To analyze intraocular lens (IOL) optic surface adhesiveness using atomic force microscopy (AFM). SETTING: LiCryL Laboratory, University of Calabria, Rende, Italy. METHODS: The surface adhesive properties of poly(methyl methacrylate) (PMMA), silicone, hydrophilic acrylic, and hydrophobic acrylic IOLs were evaluated by AFM. Analysis was performed at room temperature (21°C) in a liquid environment using the force‐versus‐distance mode of a commercial instrument (NanoScope III). Measurements were acquired with rectangular silicon cantilevers of a nominal elastic constant of 10 Newton/m. The nominal value of the tips radius of curvature was 1 μm, and the scanning speed during the acquisitions ranged from 10 to 400 nm/s. RESULTS: The adhesion force measurements showed different characteristics for the various types of IOLs (P<.001, analysis of variance). The hydrophobic acrylic IOL had the largest mean adhesive force (283.75 nanoNewton [nN] ± 0.14 [SD]) followed by the hydrophilic acrylic (84.76 ± 0.94 nN), PMMA (45.77 ± 0.47 nN), and silicone (2.10 ± 0.01 nN) IOLs. CONCLUSIONS: The surface properties of the biomaterials used to manufacture IOLs are important because they can influence the incidence and severity of posterior capsule opacification (PCO). Although further studies are necessary to elucidate the mechanism of PCO development and the interface interactions between the IOL and capsule, the results in this study may bolster the theory of manufacturing more‐adhesive materials to prevent PCO.

Uniform Lying Helix Alignment on Periodic Surface Relief Structure Generated via Laser Scanning Lithography

The main drawback in the exploitation of the chiral-flexo-electro-optic effect is that it relies on a texture, the Uniform Lying Helix (ULH), which is unstable when the cholesteric is sandwiched between spatially uniform aligning surfaces (UASs). It has been shown that the ULH can be promoted by periodic (horizontal/vertical) anchoring conditions or by the presence of periodic polymeric walls. Here we show that periodic surface relief structures can also promote the formation of a stable ULH texture. The surface relief structure was created by curing an ultraviolet curable material via a two-photon excitation laser-lithography process. The process allows sub-micron resolution and flexibility in creating the topographic relief.

Fullerenes surface gratings for liquid crystal alignment

We report on the formation of surface structures by photopolymerization of C60 and C70 in isotropic solutions. The structures show the same periodicity of the interference patterns used for photopolymerization and behave as diffraction gratings. Mass spectrometry confirmed that the deposited material contains polymerized fullerenes, while the structure of the deposit was investigated by atomic force microscopy. We have also shown that these periodic structures are useful for inducing mesophase orientation.

Planar degenerate substrate for micro- and nanopatterned nematic liquid-crystal cells

A micro- or nanopatterned planar-aligned cell ordinarily requires a pair of mirror-image patterned substrates that must be aligned in register to ⪡1μm. As an alternative we examine the director orientation profile of a nematic liquid crystal in a cell composed of one substrate whose easy axis is patterned on micrometer length scales λ and a second substrate treated for planar degenerate alignment using polymethyl methacrylate. For cell thickness <λ, the experimentally measured director profile in the bulk corresponds approximately to that imposed at the micropatterned substrate. The results are compared with theoretical calculations. This method provides an excellent alternative to the use of a pair of substrates aligned in close register.

Corneal light backscattering after transepithelial corneal crosslinking using iontophoresis in donor human corneal tissue.

Purpose To analyze the spatial distribution and time course of corneal light backscattering before and after transepithelial corneal crosslinking using iontophoresis. Setting Fondazione G.B. Bietti‐IRCCS, Rome, Italy. Design Experimental study. Methods Three donor human eyes with an intact corneal epithelium had transepithelial iontophoresis corneal crosslinking (using rapid ultraviolet‐A [UVA] irradiation), and 3 donor eyes without corneal epithelium had standard corneal crosslinking (using standard UVA irradiation). In addition, 3 donor eyes had iontophoresis and rapid corneal crosslinking after corneal deepithelialization (epi‐off iontophoresis corneal crosslinking). Scheimpflug images (Pentacam HR) of each eye globe were acquired before and immediately after administration of riboflavin 0.1% solutions and 5, 10, 30, and 120 minutes after the corneal crosslinking procedures. Corneal light backscattering was quantified across the anterior 280 &mgr;m thickness at several points from the optical center to 3.0 mm from the center. Results Light backscattering significantly increased after iontophoresis (P < .001) in specimens with and without intact epithelium. It decreased significantly after transepithelial iontophoresis corneal crosslinking and epi‐off iontophoresis corneal crosslinking (P < .001), approaching the baseline values. After standard stromal soaking with riboflavin, a large increase in corneal light backscattering was found compared with baseline measurements (P < .001) that remained unchanged up to 30 minutes after standard corneal crosslinking (P = .92). The light backscattering increase after iontophoresis in corneas with epithelium was lower than after standard soaking (P = .01). No differences were found between specimens without epithelium after iontophoresis and standard stromal soaking (P = .06). Conclusions Scheimpflug photography provided an indirect biomarker of stromal permeation of riboflavin. Iontophoresis efficiently delivered riboflavin through the epithelium. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

Electrically switchable, polarization-independent diffraction grating based on negative dielectric anisotropy liquid crystal

An atomic force microscope is used to scribe polymer-coated substrates to create an electrically controlled liquid crystal-based polarization grating. The grating is nondiffracting in the voltage-off state and diffracting in the voltage-on state. Based upon an optical phase difference of approximately π between adjacent pixels, the grating’s efficiency is independent of optical polarization and can be prepared for diffraction in either one or two dimensions.

Force spectroscopy based on temperature controlled atomic force microscope head using piezoresistive cantilevers

We have built a double temperature controlled atomic force microscope (AFM) head, based on the force detection using piezoresistive cantilevers. The temperature of the sample is controlled to better than ±2mK in a temperature interval up to 60°C. The force sensitivity is 300pN and is limited by the intrinsic low frequency noise of the piezoresistive cantilevers. By using a colloidal probe of the radius of R≈10μm, the normalized force sensitivity is F∕R≈0.02mN∕m. Although this is comparable to the sensitivity of a surface force apparatus (SFA), the piezoresistive AFM requires only ≈1μl of a fluid material, that is typically 10.000× less compared to the SFA. The capability of the force spectroscopy using a piezoresistive AFM head is illustrated by measuring the structural forces in the nematic liquid crystal.

Fast Electro-Optical Device Based on Chiral Liquid Crystals Encapsulated in Periodic Polymer Channels

The chiral flexo-electro-optic effect produces a sub-millisecond, temperature independent in-plane rotation of the optical axis and is potentially interesting for the display industry. The main drawback in the exploitation of this effect is that it relies on a texture, the Uniform Lying Helix (ULH), which is intrinsically unstable since neither planar nor homeotropic surface conditions are compatible with it. We present a method, based on the use of periodic polymeric micro-channels, to create highly ordered and stable ULH structures. We show that the periodic structure, which is created holographically, naturally aligns the cholesteric helical superstructure along the micro-channels, without requiring any elaborate ad-hoc procedure, even when the size Λ of the micro-channels is much larger than the pitch P (Λ > 20P). Electro-optic measurements performed on the test-device show a large contrast ratio between bright and dark states (better then 100:1), short switching time (200μs) and large optical rotation (>30°).

Apertureless SNOM Microscopy on a Commercial AFM

We have implemented an apertureless Scanning Near-Field Optical Microscope (SNOM) setup on a commercial Atomic Force Microscope (AFM), using a suitable optical apparatus and preserving AFM performances. This approach is promising for extending SNOM analysis to delicate samples, such as liquid or polymer films. Preliminary measurements on glass-metal nanometric steps show a lateral resolution better than 10 nm both in topography and in optical signal.