Optimal vector phase-matching conditions in biaxial crystalline materials determined by the extreme surfaces method: the case of orthorhombic crystals.

The optimal geometries of vector phase matching are determined for the cases of second harmonic generation, sum frequency generation, and difference frequency generation in a number of orthorhombic nonlinear optical crystals: KTP, KTA, KB5, K N b O 3, LBO, CBO, and LRB4. The extreme surface method was used to define the wave vector directions of highest possible generation efficiency. As shown, in a significant number of cases vector phase matching ensures higher efficiencies than the scalar one.

Nanoarhitectonics of Inorganic-Organic Silica-Benzil Composites: Synthesis, Nanocrystal Morphology and Micro-Raman Analysis.

The synthesis of nanosized organic benzil (C6H5CO)2 crystals within the mesoporous SiO2 host matrix was investigated via X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and ab initio lattice dynamics analysis. Combining these methods, we have proved that the main structural properties of benzil nanocrystals embedded into SiO2 host membranes with pore diameters of 6.0, 7.8, 9.4, and 13.0 nm are preserved compared to a bulk benzil crystal. Space confinement has an insignificant impact on the lattice vibrational properties of benzil crystals implanted into the host matrices. The ab initio lattice dynamics calculation of the phonon spectrum in the Brillouin zone center shows the mechanical and dynamical stability of benzil lattice, revealing very low optical frequency of 11 cm-1 at point Γ.

Second harmonic generation on crystalline organic nanoclusters under extreme nanoconfinement in functionalized silica-benzil composites.

We demonstrate a series of organic-inorganic nanocomposite materials combining the mesoporous silica (PS) and benzil (BZL) nanocrystals embedded into its nanochannels (6.0-13.0 nm in diameter) by capillary crystallization. One aims to design novel, efficient nonlinear optical composite materials in which inactive amorphous host PS-matrix provides a tubular scaffold structure, whereas nonlinear optical functionality results from specific properties of the deposited guest BZL-nanocrystals. A considerable contraction of the BZL melt during its crystallization inside the silica nanochannels results in a formation of the texture consisting of (221)- and (003)-oriented BZL nanoclusters (22 nm in length), separated by voids. Specificity of the textural morphology similarly to the spatial confinement significantly influences the nonlinear optical features of composite PS:BZL materials being explored in the second harmonic generation (SHG) experiment. The light polarization anisotropy of the SHG response appears to be considerably reduced at channel diameters larger than 7 nm apparently due to the multiple scattering and depolarization of the light on randomly distributed and crystallographically oriented BZL-nanoclusters. The normalized SHG response decreases nonlinearly by more than one order of magnitude as the channel diameter decreases from 13.0 to 6.0 nm and vanishes when spatial cylindrical confinement approaches the sizes of a few molecular layers suggesting that the embedded BZL clusters indeed are not uniformly crystalline but are characterized by more complex morphology consisting of a disordered SHG-inactive amorphous shell, covering the channel wall, and SHG-active crystalline core. Understanding and controlling of the textural morphology in inorganic-organic nanocrystalline composites as well as its relationships with nonlinear optical properties can lead to the development of novel efficient nonlinear optical materials for the light energy conversion with prospective optoelectronic and photonic applications.

Enhanced nonlinearities in hybrid structure based on nanoporous membrane and a metallohelicate with promising application in nanophotonics and NLO.

The main purposes of this work are designing new hybrid structures based on alumina nanoporous membranes with specific metallosupramolecular structure as well as studies of their usefulness in nonlinear optics (NLO). The NLO studies of the hybrid material is performed on the basis of two methods: the first by the Maker fringe technique, where the second harmonic generation (SHG) signal is recorded by rotating the sample; and the second by SHG imaging microscopy, where the SHG signal is collected point by point on the sample surface. The enhanced SHG signals were obtained without the use of the corona poling method needed during the experiment on thin films in our previous works and clearly shows the efficiency of hybrid materials based on nanoporous membranes as promising materials in devices developed based on NLO.

Correlation between Nonlinear Optical Effects and Structural Features of Aurone-Based Methacrylic Polymeric Thin Films.

In this study, new photonics architectures and aurone-based methacrylic polymers were designed and synthesized for their optical and nonlinear optical properties. The studied polymeric thin films were deposited by spin coating method. SHG and THG effects were measured via Maker fringe technique in transmission mode and determined using theoretical models. Investigations involved the theoretical quantum chemical calculation of dipole moments, frontier molecular orbital HOMO and LUMO energies, and first (ß) and second (γ) hyperpolarizabilities. We determined the impact of the substitution in the para position of the phenyl ring and at the dipole moment of the chromophore on the nonlinear optical properties of the investigated polymers. The presented theoretical and experimental studies provide important information with respect to the design of methacrylic-based polymeric thin film devices and supplement existing knowledge with respect to their nonlinear behaviour.

Dynamic Kerr and Pockels electro-optics of liquid crystals in nanopores for active photonic metamaterials.

Photonic metamaterials with properties unattainable in base materials are already beginning to revolutionize optical component design. However, their exceptional characteristics are often static, as artificially engineered into the material during the fabrication process. This limits their application for in-operando adjustable optical devices and active optics in general. Here, for a hybrid material consisting of a liquid crystal-infused nanoporous solid, we demonstrate active and dynamic control of its meta-optics by applying alternating electric fields parallel to the long axes of its cylindrical pores. First-harmonic Pockels and second-harmonic Kerr birefringence responses, strongly depending on the excitation frequency and temperature, are observed in a frequency range from 50 Hz to 50 kHz. This peculiar behavior is quantitatively traced by a Landau-De Gennes free energy analysis to an order-disorder orientational transition of the rod-like mesogens and intimately related changes in the molecular mobilities and polar anchoring at the solid walls on the single-pore, meta-atomic scale. Thus, our study provides evidence that liquid crystal-infused nanopores exhibit integrated multi-physical couplings and reversible phase changes that make them particularly promising for the design of photonic metamaterials with thermo-electrically tunable birefringence in the emerging field of space-time metamaterials aiming at full spatio-temporal control of light.

General method of extreme surfaces for geometry optimization of the linear electro-optic effect on an example of LiNbO3:MgO crystals.

A general method for determining the global maximum of the linear electro-optic effect in crystalline materials based on the construction and analysis of extreme surfaces obtained as a result of the optimization procedure is proposed. The electrically induced optical path length changes for ordinary and extraordinary waves as well as the optical path difference for orthogonally polarized waves were used as the objective functions in the optimization. The objective functions were determined for units of the electric field and crystal thickness in the light pass direction. In the example of LiNbO3:MgO, it is shown that the maximal achievable given values of the optical path length change (global maxima) for ordinary and extraordinary waves are 119 pm/V and 277 pm/V, respectively. The global maximum of the optical path difference for orthogonally polarized waves is 269 pm/V (for 632.8 nm wavelength and at room temperature). These global maxima are exceeded by ∼1.5, 1.7, and 2.3 times the respective maximum values on direct cut crystals of LiNbO3:MgO and are ∼5%, 9%, or 11% larger than the global maxima for undoped LiNbO3 crystal. This ensures a possibility to increase the energy efficiency by ∼2.9 or 5.3 times in the case of using of LiNbO3:MgO crystals with optimal cuts as sensitive elements of electro-optic devices.

Electric-field-induced optical path length change in LiNbO3:MgO crystals: spatial anisotropy analysis.

In this paper we describe the methodology behind the calculation of the indicative surfaces (ISs) of the electric-field-induced optical path length change (EFIOPC) in anisotropic crystal materials accounting for the piezoelectric deformation. It is considered in detail for a particular case of 3m point group symmetry and applied to LiNbO(3) single crystals doped with 7 mol. % MgO (hereafter LiNbO(3):MgO). The contribution of the inverse piezoelectricity into EFIOPC appears to be considerable and, in many cases, modifying, for instance, the spherical coordinates of the extreme directions or even leading to the appearance of new directional maxima on relevant ISs. The ISs of EFIOPC are of considerable practical importance as they allow us to determine an optimal geometry for electro-optic coupling. The spatial anisotropic analysis of EFIOPC in LiNbO(3):MgO crystals suggests that the lowest effective driving voltage is provided by electro-optic cells representing the rectangular slabs of X/50° crystal cut. The modulation efficiency of such electro-optic cells is about 1.5 times better than ones fabricated in the usual way (i.e., as rectangular crystal slabs with the faces parallel to the principal crystallographic directions).

Piezo-optic coefficients of MgO-doped LiNbO3 crystals.

We describe an interferometric technique suitable for determination of piezo-optic coefficients (POCs) in crystals. The method considers real nonparallelism of measured samples, thereby improving the measuring precision of POCs significantly. Corresponding equations are derived for the interferometric half-wave stress method. Using this technique we have determined a complete set of POCs of pure and MgO-doped LiNbO(3) crystals. The reliability of the data has been confirmed by comparing the effective POCs expressed through the combinations of measured POCs and the effective POCs determined independently using highly precise optical birefringence measurements. Pure and MgO-doped LiNbO(3) crystals reveal nearly the same magnitudes of POCs. However, LiNbO(3):MgO exhibits about 4 times higher resistance with respect to powerful light radiation, making it more suitable for application in acousto-optic devices that deal with superpowerful laser radiation.