Mario Ariosto Matranga

Light Control of Cholesteric Liquid Crystals Using Azoxy-Based Host Materials

The characteristics of cholesteric liquid crystals can be controlled by light irradiation if conformationally photo-active molecules are present. Recently, control of the selective reflection band (spiral pitch) in nemato-chiral mixtures was demonstrated when photosensitive molecules, namely nematic azoxy-based compounds, were used as the host material. In this report, the investigation of light induced effects in cholesterics with azoxy-based host materials is continued to highlight the mechanisms of the response. Different non-photosensitive chiral materials were added to different azoxy-nematic liquid crystals and the pitch change caused by UV irradiation was investigated. A change in the pitch of 50–210 nm was observed depending on the exposure time and the intensity of the light. This effect is reversible: under illumination at wavelengths greater than 410 nm, the pitch shifts in the opposite direction. The dependence of the selective reflection band and the full-width-at-half-maximum of the band on the exposure time and the temperature dependence of the selective reflection band were investigated. The lowering of the phase transition temperature and narrowing of the width of the selective reflection band can be explained by a decrease in the orientational order parameter. The blue shift of the selective reflection band is due to a decrease in both the order parameter and the concentration of linear nematogenic molecules.

Wide band gap materials as a new tuning strategy for dye doped cholesteric liquid crystals laser

A new tuning strategy for mirror-less liquid crystals laser is presented. A three layer cell is prepared with two cholesteric layers sandwiching a layer containing an isotropic mixture of a photoluminescent dye. One of the chiral layers contains a wide band gap material while the second layer consists of a series of small band gap materials. Through the combination of these two layers, a set of mirrors that can selectively reflect different wavelengths is obtained. A different laser wavelength is emitted from different regions of the cell under the pumping beam irradiation.

Thermal and electrical laser tuning in liquid crystal blue phase I

Thermal and electrical tuning of laser emission from optically pumped blue phase I of dye-doped short pitch cholesteric mixtures have been achieved. Temperature changes or applied electric field to the liquid crystal cells induce structural changes in the blue phase configuration, producing a shift of the photonic band gap. The emission tunability in a structure that in addition allows multidirectional emission may herald a new age of multipurpose laser sources. Furthermore, the reversibility of the effect points out the potential applications of these soft photonic self-assembled materials.

Lasing in an intermediate twisted phase between cholesteric and smectic A phase

This work describes the observation of lasing in an intermediate chiral phase of a dye-doped cholesteric liquid crystal mixture. This intermediate phase exists between cholesteric and smectic A phase and it presents anomalous selective reflection properties. The lasing was observed at the long-wavelength edge of the photonic band gap.

Enhancing cholesteric liquid crystal laser stability by cell rotation

Stability of dye doped cholesteric liquid crystal laser emission from several minutes up to two hours and more was achieved by rotating the liquid crystal cell. Significant dependence of stability on surface treatment was observed.

Novel UV Sensor Based on a Liquid Crystalline Mixture Containing a Photoluminescent Dye

The Erythemal Response Spectrum is a scientific expression that describes the skin sensitivity to the ultraviolet radiation. A long exposure to UV radiation causes skin erythema once a threshold dose has been exceeded. We present a sensor based on a liquid crystalline mixture, containing dye, that absorbs photons at UV wavelength and emits them in the visible range of the electromagnetic spectrum. In particular the absorption spectrum of the mixture can be modulated to be similar to the Erythemal Response Spectrum. This system presents several innovative features: the absorption range of the mixture can be varied to be sensitive to different wavelengths, the luminescence intensity can be tuned, the system can be implemented on flexible devices, providing the possibility to create a dosimeter for ultraviolet radiation.

Lasing in Three Layer Systems Consisting of Cholesteric Liquid Crystals and Dye Solution

We have investigated defect mode lasing in a multilayer system consisting of a dye doped isotropic solvent sandwiched between two cholesteric liquid crystal (CLC) cells. In this case we can use dyes not soluble in liquid crystals and avoid the degradation of the CLC structure caused by the absorption of the pumping energy. When the CLC pitches of both cells are equal, the dye emission generates the typical multi-mode lasing peaks inside the photonic stop band. In CLC mixtures whose pitches were shifted one respect to the other in such a way that only the edges of band gaps of the CLC layers overlapped, we have observed single mode lasing. The divergence of the laser beam spot is considerably lower than the one observed in dye doped CLC.

Single mode lasing in multilayer sandwiched systems consisting of cholesteric liquid crystals and dye solution

We have investigated defect mode lasing when a cell containing a dye solution is sandwiched between two cholesteric liquid crystal cells. When the cholesteric pitch of both cells is equal, the dye emission generates the typical multi-mode lasing peaks inside the photonic stop band. We prepared two cholesteric mixtures whose pitches were shifted one respect to the other in such a way that only the edges of band gaps of the cholesteric liquid crystals layers overlapped. Using these mixtures we have observed single mode lasing. Moreover the divergence of the laser beam spot is considerably lower than the one observed in dye doped cholesteric liquid crystal lasers.

Paper like cholesteric interferential mirror.

A new type of flexible cholesteric liquid crystal mirror is presented. The simple and effective method for the deposition of a cholesteric mixture on a paper substrate and the particular design of the device give a homogeneous alignment of the cholesteric texture providing mirrors with an intense and uniform light reflectance. A desired polarization state for the reflected light, linear or circular, can be easily obtained varying the thickness and optical anisotropy of the polymer cover film. By using non-azobenzene based photosensitive materials a permanent array of RGB mirrors with high reflectivity can be obtained on the same device. Paper like reflective mirrors are versatile and they can find applications in reflective displays, adaptive optics, UV detectors and dosimeters, information recording, medicine and IR converters.

Frequency Tunable Lasing in a Three Layer Cholesteric Liquid Crystal Cell

Novel combinations of liquid crystalline materials for liquid crystals lasers tuning are presented. The cell is based on a three layer system, with two cholesteric films sandwiching a third layer containing an isotropic mixture of a photoluminescent dye. One of the cholesteric films has a wide band gap and the other one presents a narrow band gap, allowing the laser emission tuning over a wide spectral range, from UV to near IR. Further, using mixtures containing cholesteryl esters, a temperature control can be achieved.