Benjamin G. Penn

Synthesis and characterization of various Schiff bases for non-linear optical applications

A series of salicylidenes have been synthesized to investigate non-linear optical properties. Attempts have been made to correlate second harmonic generation properties of these Schiff base compounds with ring substitution patterns, as well as the nature of the ring substituents. The result of double substitutions to the aniline moiety is also discussed. It is reported that meta substitution in the aniline moiety gives comparable values for second harmonic generation with respect to urea.

Perovskites: transforming photovoltaics, a mini-review

Abstract. The recent power-packed advent of perovskite solar cells is transforming photovoltaics (PV) with their superior efficiencies, ease of fabrication, and cost. This perovskite solar cell further boasts of many unexplored features that can further enhance its PV properties and lead to it being branded as a successful commercial product. This article provides a detailed insight of the organometal halide based perovskite structure, its unique stoichiometric design, and its underlying principles for PV applications. The compatibility of various PV layers and its fabrication methods is also discussed.

Systematic study of polymorphism in crystalline non-linear optical materials

Abstract Crystalline polymorphism is quite common for non-linear optical (NLO) compounds. During our studies of NLO materials we found several compounds that crystallize in two or more forms. We present a short review of our recent data on the crystalline polymorphism of NLO compounds. Among the compounds studied, we found examples of concomitant polymorphism, induced conformational polymorphism, and organic polytypism. For several compounds, X-ray data are discussed along with computational data on molecular and crystal structure energy.

Optical limiting behavior of octa-decyloxy metallo-phthalocyanines

One of the key issues involved in the development of passive optical power limiters is the search for appropriate materials that show effective reverse saturable absorption. Metallo-phthalocyanines seem to be good candidates for such applications because of their high optical nonlinearity and their unique electronic absorption characteristics. A series of 1,4,8,11,15,18,22,25-octa-decyloxy metallo-phthalocyanines containing palladium, nickel, cobalt, copper, and zinc as central metal atoms were characterized for their nonlinear absorptive properties to evaluate their suitability to function as reverse saturable absorbers. Nonlinear transmission measurements were analyzed in terms of a five-state model and a simple model based on the effective excited-state absorption cross sections without ascribing their origin to the states involved. Optical limiting thresholds were also estimated and compared with the absorption cross sections.

One-pot polymorphism of nonlinear optical materials. First example of organic polytypes

Two nonlinear optical materials 1,1-dicyano-4-(4-dimethylaminophenyl)-1,3-butadiene (I) and 3-methoxydicyanovinylbenzene (II) have been investigated with regard to crystal growth, polymorphism, structure characterization and physical properties. It was found that both compounds form one-pot (concomitant) polymorphs, that might also be described as organic polytypes. Both polymorphs of compounds I and II form identical molecular layers with molecules located in the layer plane. In the two phases of compound I the layers superposition and their sequence differs significantly. This circumstance most probably determines the different color of the two crystal modifications. In the two crystal phases of compound II molecular layers and their superposition types are almost identical. The only difference between the phases is the sequence of the layer superposition. Single crystal and powder X-ray diffraction techniques, powder test for second harmonic generation, UV spectroscopy, and computational methods were used for characterization of these compounds.

Crystal growth and optical properties of 4-aminobenzophenone crystals for NLO applications

Single crystals of 4-aminobenzophenone, a nonlinear optical (NLO) material have been grown successfully by a novel solution growth as well as modified Bridgman-Stockbarger techniques. The transmission spectrum (420-1400 nm) and SHG efficiencies were determined. 4-Aminobenzophenone crystals gave a lower limit on the second harmonic generation (SHG) efficiency of 10%. Laser damage threshold measurements conducted at the indicated wavelengths gave values of 1-2 GW/cm 2 .

Structure and morphology of phthalocyanine films grown in electrical fields by vapor deposition

Phthalocyanine (Pc) films have been synthesized by vapor deposition on quartz substrates, some of which were coated with a very thin gold film before depositing the Pc films. Electrical fields up to 6200 V/cm between a mech electrode and the substrate are introduced during film growth. These films have been characterized by X-ray diffraction and scanning electron microscopy. The molecular orientations and surface morphology of Pc films were changed under the electrical fields. The surface of these films grown without an electrical field shows whisk-like morphology. When films are deposited under an electrical field, a dense film with a flat surface is obtained.

Dielectric study of dynamics of organic glasses

The dynamics of organic compounds 2-cyclo-octylamino-5-nitropyridine (COANP), (S)-2-N--(methylbenzylamino)-5-nitropyridine (MBANP), 2-(N-prolinol)-5-nitropyridine (PNP), and N-(4-nitrophenyl)-(L)-prolinol (NPP) were studied by dielectric relaxation spectroscopy in the frequency range of 10 Hz-2 MHz and differential scanning calorimetry (DSC). The dielectric and DSC studies showed that COANP, MBANP and PNP underwent glass transition. However, NPP crystallized so rapidly upon cooling that the glass state could not be observed. It was found that the crystalline process of COANP did not slow the structure relaxation of COANP glass. The relaxation times fitted well to the empirical Vogel-Fulcher equation = exp[Ea/kb(T-TVF)]. The activation energies Ea and the Vogel-Fulcher temperature TVF were 54.5 meV and 239 K for COANP, 86.2 meV and 249 K for MBANP and 84.9 meV and 245 K for PNP, respectively. The crystalline temperatures of COANP and MBANP were given as 300 K and 330 K, respectively. An anomalous behaviour of the dielectric permittivity of PNP glass was observed.

Photoinduced Optical and Electrical High-Voltage Pulsations and Pattern Formation in Photorefractive Crystals

We review our results on conversion of CW laser Ar-ion beam power into pulsating multi-channel outputs: optical, electric and piezoelectric with simultaneous dynamic pattern formation. We show, that electrical pulsations generate high-voltage signals that may be used as a driving voltage for the conventional electro-optical modulator. We have also demonstrated the possibility of synchronization of two optical pulsators, through regulated optical coupling in a photorefractive LiNbO3 crystal. Spatial distribution of scattered light is self-organized in different patterns (hexagonal and cross-type).

Buoyancy-driven heat transfer during application of a thermal gradient for the study of vapor deposition at low pressure using an ideal gas

Abstract A mathematical model has been developed to determine heat transfer during vapor deposition of source materials under a variety of orientations relative to gravitational accelerations. The model demonstrates that convection can occur at total pressures as low as 10−2 mm Hg. Through numerical computation, using physical material parameters of air, a series of time steps demonstrates the development of flow and temperature profiles during the course of vapor deposition. These computations show that in unit gravity vapor deposition occurs by transport through a fairly complicated circulating flow pattern when applying heat to the bottom of the vessel with parallel orientation with respect to the gravity vector. The model material parameters for air predict the effect of kinematic viscosity to be of the same order as thermal diffusivity, which is the case for Prandtl number ∼ 1 fluids. Qualitative agreement between experiment and the model indicates that 6-(2-methyl-4-nitroanilino)-2,4-hexadiyn-1-ol (DAMNA) at these pressures indeed approximates an ideal gas at the experiment temperatures, and may validate the use of air physical constants. It is apparent that complicated nonuniform temperature distribution in the vapor could dramatically affect the homogeneity, orientation, and quality of deposited films. The experimental test is a qualitative comparison of film thickness using ultraviolet-visible spectroscopy on films generated in appropriately oriented vapor deposition cells. In the case where heating of the reaction vessel occurs from the top, deposition of vapor does not normally occur by convection due to a stable stratified medium. When vapor deposition occurs in vessels heated at the bottom, but oriented relative to the gravity vector between these two extremes, horizontal thermal gradients induce a complex flow pattern. In the plane parallel to the tilt axis, the flow pattern is symmetrical and opposite in direction from that where the vessel is positioned vertically. The ground-based experiments are sufficient preliminary tests of theory and should be of significant interest regarding vapor deposited films in microgravity.