Sameh Ferjani

Random lasing and weak localization of light in dye-doped nematic liquid crystals

The first observation of random laser action in a partially ordered, optically anisotropic nematic liquid crystal with long-range dielectric tensor fluctuations is reported. Above a given pump power the fluorescence curve collapses and the typical narrowing and explosion effect leads to discrete sharp peaks. The unexpected surviving of interference effects in recurrent multiple scattering provide the required optical feedback for lasing in nematics. Coherent backscattering of light waves in orientationally ordered nematic liquid crystals manifests a weak localization of light which strongly supports diffusive laser action in presence of gain medium. Intensity fluctuations of the speckle-like emission pattern indicate the typical spatio-temporal randomness of diffusive laser emission. A comparison of the laser action is reported for systems with different order degree: fully disordered semiconductor powders, self-ordered cholesterics and partially ordered nematic liquid crystals.

Composite holographic gratings containing light-responsive liquid crystals for visible bichromatic switching.

Photosensitive liquid crystals (PLCs) are promising materials that can combine a high-refractive-index modulation, typical of LCs, with high photosensitivity, due to the presence of photochromic molecules such as azobenzene and derivates. [1] These can undergo a reversible isomerization from a thermodynamically stable trans to a cis conformation when acted on by UV or visible light; a reverse transition can be obtained upon heating or irradiation with a different (longer wavelength) visible light. Thanks to these materials andprocesses, thepossibility toconfine and stabilize photosensitive materials has been considered in the past [2] and the perspective of producing fast photonic devices of scientific and technological interest looks highly realistic. There is, however, a serious problem represented by instability far from room temperature. [2] One way to reduce this effect has been found in using composite polymeric materials: [3] Holographic diffraction gratings, which exploit LC composite materials (HPDLCs), [4] are low-cost components with a wide range of possible photonic and optoelectronics applications. Therefore, the possibility of fabricating an optically controllable photonic device has been investigated by combining the photosensitivity of azobenzene materials with the optical properties of HPDLCs. [5] Devices of this kind can exhibit, however, an intrinsic drawback when the droplet size of the LC component inside the polymeric matrix is comparable with the wavelength of the diffracted light, thus yielding a strong light scattering. [6] Recently, a good improvement has been obtained by using a new kind of holographic diffraction grating with better optical qualities, which consists of polymer slices alternated to films of regularly aligned nematic LC (NLC). Depending on the particular procedure utilized for its fabrication, this structure is called POLIPHEM [7] orPOLICRYPS [8] andisobtainedby UV-curingofa mixture of monomer and NLC. In a recent attempt [9] to fabricate

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°).

Carbon nanotube-induced chirality in an achiral liquid crystal

A small quantity of carbon nanotubes was dispersed in an achiral liquid crystal (LC), and the mixture was found to exhibit a weak degree of chirality. The induced chirality in the LC was probed by means of the electroclinic effect in the LC’s smectic-A phase, which showed significant pretransitional behavior on approaching the smectic-A–smectic-C transition temperature from above. The results suggest that there is a net chirality associated with the carbon nanotubes, which is transmitted into the LC.

Random lasing in freely suspended dye-doped nematic liquid crystals.

Random lasing in fully disordered systems having organic and inorganic nature has been the subject of extensive studies since the beginning of the past decade. The interest mainly emerges from the unexpected role played by disorder in the laser action. The disorder was considered detrimental for the optical feedback in cavity laser, until it was demonstrated that multiple-scattering materials including a gain medium act as random laser. Here, a completely new approach is reported, where freely suspended complex fluid films doped with fluorescent molecules under optical excitation generate narrowband lasing peaks. The constellation of localized modes is selected by properly choosing the gain profile. The idea to have laser action in absence of mirrors and boundaries realizes an unparalleled tunable and moldable laser source.

Thermal behavior of random lasing in dye doped nematic liquid crystals

The role of the thermally modulated order parameter in the diffusive laser action observed in dye doped nematic liquid crystals was investigated. Above a given pump energy a randomly distributed series of bright tiny spots appear, giving rise to a strongly fluctuating emission pattern. The spectral analysis reveals discrete sharp peaks (about 0.5nm) slightly blueshifted with respect to the highest efficiency region of the gain medium. A comparative study was performed in systems having different sizes and confining geometries, corroborating the idea that the random lasing observed in dye doped nematic phase is fluctuation driven through a recurrent multiple scattering process.

Thermo-recurrent nematic random laser

This experimental work is aimed to investigate the thermal behavior of random laser action in dye doped nematic liquid crystals. The study evidenced an important temperature dependence of the random lasing characteristics in the nematic phase and in close proximity of the nematicisotropic (N-I) phase transition. A lowering of the laser emission intensity as the temperature increases is strictly related to the shift of the lasing threshold as function of the temperature even though the pump energy is kept fixed. The optical losses increasing owing to the thermal fluctuation enhanced scattering drive the input-output smoother behavior until the system stops to lase, because below threshold. The unexpected reoccurrence of random lasing at higher temperature, in proximity of N-I transition is found to be related to a different scattering mechanism, the micro-droplets nucleation and critical opalescence.

Soft periodic microstructures containing liquid crystals

An empty polymeric structure has been realized by combining a high precision level optical holographic setup and a selective microfluidic etching process. The distinctive features of the realized periodic microstructure enabled aligning several kinds of liquid crystal (LC) compounds, without the need of any kind of surface chemistry or functionalization. In particular, it has been possible to exploit light sensitive LCs for the fabrication of all-optical devices, cholesteric and ferroelectric LCs for ultrafast electro-optical switches, and a common LC for a two-dimensional periodic structure with high anisotropy. All-optical and electro-optical experiments, performed for investigating the samples in terms of switching voltages and response times, confirm good performances of the realized devices.

Band edge and defect modes lasing due to confinement of helixed liquid crystals in cylindrical microcavities

Peculiar light emission properties have been observed in cylindrical microcavity hosting dye-doped helixed liquid crystals, which behaves as a fiber-like multidirectional distributed feedback laser. Experimental studies performed for this level of confinement show that laser action is exhibited both axially and radially, indicating a self-organized three-dimensional blue phase-like configuration. Thermal wavelength tunability was observed for both orientations emphasizing two different linear behaviors. The distributed feedback mechanism and the Q factor of the mirrorless resonant cavity result enhanced for axial stimulated emission because of the significant increase in the number of helical periods. In addition, long-lived spectrally narrow defect modes appear within the photonic band gap owing to optical phase jumps which take place in local structural defects.

LASER ACTION IN DYE DOPED LIQUID CRYSTALS: FROM PERIODIC STRUCTURES TO RANDOM MEDIA

The birefringence and natural ability to form periodic structures make cholesteric liquid crystalline (CLC — chiral nematics) materials particularly attractive as 1D photonic band gap systems. If a CLC is doped with dye fluorescent molecules, in such a way that the maximum peak of fluorescence matches one of the edges of the selective stop band, laser action is expected at that spectral position. By confining the helical super-structure of chiral liquid crystals in polymeric micro-cavity channels, a tunable microcavity laser array was achieved. In multiple scattering systems, the propagation of the light waves is quite different, as optical scattering may induce a phase transition in the photon transport behavior. Beyond a critical scattering level, the system makes a transition into a strongly localized state and light transmission is inhibited. This effect can be used as a photon trapping mechanism to obtain laser action in the presence of a gain medium. Random lasing modes come from interference effects which survive in disordered systems and open a particular chapter in the study of the interplay between localization and amplification. Here, experiments performed on systems having different order degree and confinement are presented and possible technological implications are discussed.