Chapter 1 - Dynamic Photonic Materials Based on Liquid Crystals

Liquid Crystals (LCs), combining optical nonlinearity and self-organizing properties with fluidity and being responsive to a wide variety of stimuli, have reached a key point in their development for photonic applications, for the realization of devices that can be dynamically reconfigurable, widely tunable and ultra-fast controlled. In this chapter, we overview recent advances in obtaining alignment of LCs to be used for photonics applications; in particular, we report on our recent efforts on developing a new generation of LC devices based on isotropic polymeric materials. We have realized an empty polymeric template by etching a periodic liquid crystalline composite material, called POLICRYPS (acronym of POlymer LIquid CRYstal Polymer Slices), which is a nano/micro-composite holographic grating made of slices of almost pure polymer alternated to films of well aligned Nematic Liquid Crystal (NLC). The distinctive features of the realized periodic microstructure enabled aligning several kinds of self-organizing materials, without the need of any kind of surface chemistry or functionalization.

Optical nonlinear properties and all optical switching in a synthesized liquid crystal

A new and efficient proposal for all-optical tunable devices and systems using electromagnetically induced transparency (EIT) is proposed. For this purpose a slab doped with quantum dots for realization of three-level atomic system is considered. Density matrix formulation for evaluation of the proposed structure is used. The reflection and transmission coefficients of the considered slab are calculated and the related amplitude and phase quantities studied versus parameters of the structure. We show that some nanometer tuning with application of the control field is obtained. So, the proposed idea can open a new realization method of all-optical tunable devices and systems towards all-optical systems.

In this paper, we have discussed the spatial dependence of linear and nonlinear optical properties of infrared laser pulse in a single-layer graphene system. We have proposed two scenarios for adjusting the linear and nonlinear optical properties of the medium. In the first assumption, the graphene system interacts by an elliptical polarized optical vortex light and we adjust the linear and nonlinear properties via ellipticity and vorticity parameters. We found that the enhanced nonlinear coefficient was obtained with reduced linear absorption. In the second assumption, we assumed that two composite optical vortex lights interact with graphene layer. In this case the linear and nonlinear optical properties can be controlled via orbital angular momentum (OAM) of the applied lights. Here, we have shown that by adjusting the azimuthal angle of the composite vortex light, the spatial control of linear and nonlinear properties is possible. We found that in some regions of space the probe absorption vanishes and enhanced the nonlinear coefficient accompanied with optical transparency. Our results may be useful applications in future in all-optical system devices in nanostructures.

<p>Nonlinear optical materials with optimum properties are essential in continuous development of photonic and electro-optical devices, used in optical communications, networking, optical computation for signal processing, and data storage equipments. The changing trend is to use organic materials/dyes that exhibit exceptional nonlinear optical properties instead of conventional materials which have comparatively low nonlinear properties. These organic materials/dyes are easy to prepare in solution or solid form. The resulting organic material<br />\nhas a low dielectric constant, eliminating the need for poling while maintaining the refractive<br />\nindex. However, these organic materials have few of the drawbacks inherent in the<br />\nprocessing of comparable inorganic materials like of intense light induced degradation or<br />\nbleaching and aggregation at higher dye concentration. In order to overcome these drawbacks and for effective use of highly nonlinear dyes, the dye molecules are doped in polymer matrix. This idea of dye-doped polymer material matrix may increase the concentration of absorptive or fluorescence centers as well as the opto-chemical and opto-physical stability. In this paper, we have discussed the strategic advantages of dye-doped polymer nonlinear materials in comparison with organic and inorganic nonlinear materials for optical phase<br />\nconjugation and all optical switches in future photonics technology. This was done by studying the linear optical properties and nonlinear optical Phase Conjugation properties of two azo dye-doped polymer films by considering organic dyes disperse orange (DO-25) and disperse yellow (DY-7) doped in a polymer matrix Polymethyl methacrylate methacrylic acid (PMMA-MA). The nonlinear optical phase conjugation properties are studied using Degenerate Four Wave Mixing set-up using 532 nm wavelength CW laser beam. The effect of dye concentration, intensity of backward, forward pump, and inter beam angle between probe and forward pump beam on phase conjugation reflectivity are also studied and compared.</p>

Here, we present the investigations of photo-isomerization behavior and the nonlinear optical properties of azobenzene derivative LB films. The few-layer LB films of AOB-t4 and BNB-t4 exhibit positive nonlinear refraction and two-photon absorption properties as revealed by picosecond Z-scan. The increased conjugation by introducing an oxadiazole group improves the photo-isomerization rate and the nonlinear optical properties, due to a weaker intermolecular interaction and the formation of J-aggregates within AOB-t4 LB film. The third-order susceptibility of cis-AOB-t4 9-layer LB film reaches 1.866 × 10-9 esu and the two-photon absorption coefficient is on the order of 10-8 m/W. Interestingly, the 15-layer AOB-t4 LB film shows negative nonlinear refraction and saturable absorption. Taken together, we have demonstrated the switchable nonlinear optical absorption and refraction properties of AOB-t4 LB film with changing film thickness, which is of significance for nonlinear optics and photonics applications.

Bent-core liquid crystal (LC) molecules are known to form mesophases with fascinating polar order and supramolecular chirality despite the achiral nature of the mesogens. The assembly of colloidal particles with geometrical similarity to bent-core molecular mesogens not only provides new insights into the physical behaviors of atoms or molecules but also leads to new materials with broad applications. Despite tremendous progress in colloidal synthesis and assembly, there has been a lack of colloidal model systems of bent-core molecular mesogens for LC property discovery and application development. This article describes a systematic study on the phase behaviors of colloidal analogs of bent-core LC mesogens in both experiments and simulations. We demonstrated that bent rods with controlled bending angle (α) and aspect ratio (L/D, with L and D as the length and diameter of each rod arm, respectively) can spontaneously assemble into several typical banana phases including smectic A, smectic C, synclinic tilted antiferroelectric-like smectic, and twist smectic phases, resembling bent-core LC molecules. The formation and transition of these phases were found to be strongly dependent on the geometric parameters of rods. Phase diagrams were developed to illustrate the existence and stability range of all the LC phases in α and L/D space. This work opens the door to the development of novel complex types of molecular or colloidal self-organization and new functional materials with electro-optical or nonlinear optical properties.

A simplified and sensitive experimental technique named z-scan has been used in this work, to study the optical nonlinearity and optical limiting of olive oil. Olive oil is classified as organic compounds which have a good nonlinear optical properties candidate to be used in photonic applications. A high purity sample of olive oil has been subjected to spectrophotometer to determine the transmission spectrum using UV-VIS spectrophotometer. The nonlinear optical properties represented by nonlinear refractive index and nonlinear absorption coefficient are determined by using a CW of 532 nm in two parts. The first part has been done using a closed aperture (with two different diameter 1 mm and 2 mm) placed in front of the detector to measure the nonlinear refractive index which exhibits negative refractive index (defocusing). Second part was done using an open aperture to measure the nonlinear absorption coefficient, where the samples exhibit two photon absorption behavior under the experimental conditions. Real and imaginary parts of the third-order optical nonlinearity, χ (3) were evaluated. The third-order nonlinearity of olive oil is dominated by nonlinear absorption, which leads to strong optical limiting of the laser.

Influence of temperature and hydrostatic pressure on the nonlinear optical properties in a GaAs/GaAlAs Kratzer-Fues confined quantum well

Collagen possesses a strong second order nonlinear susceptibility; when it is irradiated with intense laser light, some of the reflected and transmitted light will have twice the frequency of the incident beam, a phenomenon known as second harmonic generation (SHG). Polarization modulation of an ultra-short pulse laser beam can be used to simultaneously measure collagen fiber orientation, SHG intensity, and a parameter related to the second order non-linear susceptibility. This technique has made it possible to discriminate among patterns of fibrillar orientation in many tissues. In the present study the role that organizational complexity plays in the relationship between nonlinear optical properties and collagen structure is investigated. As a component of tissues and organs, collagen’s structure and function is inextricably intertwined with that of the many other matrix components; to what extent do these noncollagenous components affect its nonlinear properties? To answer this, we investigated SHG in two different collagenous tissues, liver and cartilage; in addition we looked at the effect of progressive pathological changes in these tissues on SHG. At the other end of the spectrum, we studied collagen organized at the minimal level of complexity necessary for SHG detection: fibrils generated from solutions containing only a single type of collagen. Data obtained from these studies suggest that collagen’s strong nonlinear susceptibility, a property no other biologically significant macromolecule shares to the same degree, may serve as more than the basis of a novel imaging device for soft tissue. Collagen’s nonlinear optical properties in conjunction with its vast capacity for self-initiated conformational change--through self-assembly, site recognition, post-translational modification, and the like -make it an attractive candidate molecule for any of several demanding engineering applications, such as nanopatterning.

Organoboron compounds are attracting immense research interest due to their wide range of applications. Particularly, low-coordinate organoboron complexes are receiving more attention due to their improbable optical and nonlinear optical properties, which makes them better candidates for medical applications. In this review, we summarize the various synthetic methods including multicomponent reactions, microwave-assisted and traditional pathways of organoboron complexes, and their optical and nonlinear properties. This review also includes the usage of organoboron complexes in various fields including biomedical applications.

In the present work we have shown the opportunity of using the process of stimulated light scattering due to recording of dynamic orientation gratings for directly measuring the characteristic orientation relaxation time of liquid crystals (LCs) and related material parameters, the elastic constants and the coefficients of orientational viscosity. The measurement consists in obtaining the gain spectra of LCs and determining the magnitude of the gain coefficient and the optimum frequency shift between interfering laser beams. We have designed a setup where the gain spectra of stimulated light scattering process in LCs are obtained within seconds. These spectra are highly specific to LCs serving as a precise and fast tool for studying LC materials and their mixtures with the purpose of optimizing their linear and nonlinear optical properties. Using this technique we have found that certain azo LC materials are faster compared to several well-known Merck's formulations for LC displays.

Four 1,8‐naphthalimide hydrazone molecules with different electron‐donating groups have been applied in the study of linear and nonlinear optical (NLO) properties. These compounds showed strong green emission in solution. Their NLO properties such as two‐photon absorption (TPA) behavior with femtosecond laser pulses ca. 800 nm and excited‐state absorption (ESA) behavior with nanosecond laser pulses at 532 nm were investigated. Compound 4 presented the largest two‐photon cross section (550 GM) among them due to two factors: the conjugated length of compound 4 is the longest and the electron‐donating ability of compound 4 is the strongest. Different from TPA behavior, compound 2 showed the best nonlinear absorption properties at 532 nm and its nonlinear absorption coefficient and third‐order nonlinear optical susceptibilities χ(3) were up to 1.41×10−10 MKS and 4.65×10−12 esu, respectively. Through the modification of the structure, the nonlinear optical properties of these compounds at different wavelengths (532 and 800 nm) were well tuned. The great broad‐band nonlinear optical properties indicate hydrazones are good candidates for organic nonlinear optical absorption materials.

Liquid crystals (LCs) are used in displays, visors, navigation systems and many more. Amongst a wide range of LCs, p-azoxyanisole (PAA) is considered to be an active LC. Focusing on different properties of this molecule, in the reported study, the theoretical identification of quantum mechanical parameters and the identification of electro-optic properties are carried out. Different functional theories such as B3LYP, M06-2X and M06L are used along with three basis sets 6-31G**, 6-311G and 6-311G**. A comparative study revealed that the M06-2X method produces higher values of band gap, ionization potential, electronegativity and electronic global hardness while M06L produces lower values and B3LYP gives intermediate values. Nonlinear optical properties of liquid crystals are evaluated. The nonlinear optical properties obtained for a PAA liquid crystal are much higher than those of urea. Due to its high nonlinear optical properties, our liquid crystal can be used in the field of telecommunication and optical interconnection. The order parameter and birefringence are calculated with variable electric field. We found out that the order parameter and birefringence increase with a gradually increasing electric field, which suggests that the PAA liquid crystal can be used for developing electro-optic and tunable metamaterial devices.

The synthesis and characterization of organic semiconductors is being pursued in three primary structure formats: single crystal, liquid crystal and organic-inorganic hybrid. The strategy here is to share common structures, synthesis methods and fabrication techniques across these formats and to utilize common characterization tools such as the time of flight technique. The single crystal efforts concentrate on aromatic and heteroaromatic compounds including simple benzene derivatives and derivatives of the acenes. The structure-property relationships due to incorporation of small substituents and heteroatoms are being examined. Crystals are grown by solution, melt or vapor transport techniques. The liquid crystal studies exploit their self-organizing properties and relative ease of sample preparation. Though calamitic systems tha deliver the largest mobilities are higher order smectics, even some unusual twist grain boundary phases are being studied. We are attempting to synthesize discotic acene derivatives with appropriate substitution patterns to render them mesogenic. The last format being examined is the hybrid organic-inorganic class. Here, layered materials of alternating organic and inorganic composition are designed and synthesized. Typical materials are conjugated aromatic compounds, usually functinalized with an amine or a pyridine and reacted with appropriate reactive metal derivatives to incorporate them into metal oxide or sulfide layers.