Second harmonic generation in anisotropic stratified media: A generalization of the Berreman method and its application to photonic materials

We have developed a numerical method for calculating the second harmonic generation (SHG) generated by an anisotropic material whose optical properties present an arbitrary modulation in one dimension. The method is based on the Berreman 4x4 matrix formalism, which is generalized to include nonlinear optical phenomena. It can be used under oblique incidences of the input beam, and is valid even when the SHG frequency is close to photonic bands, where the usual slowly-varying-amplitude approximation breaks down. As an example of application, we have studied the SHG performance of ferroelectric and helielectric nematic liquid crystals. The latter present a helicoidal structure that can be distorted under electric field. In the different tests of the method we have analyzed the conditions for the most efficient SHG, and compared with previous results in the case there were any. The obtained results indicate that the present procedure may contribute to improve the structural design and enlarge the variety of nonlinear optical materials for their application in optical devices.Comment: 14 pages, 6 figure

Liquid crystalline and nonlinear optical properties of bent-shaped compounds derived from 3,49-biphenylene.

9 pages, 6 figures, 3 tables.The synthesis of different bent-core molecules derived from 3,49-biphenylene bearing azo, azoxy, imine or ester linkages in their lateral structures is reported. Structure–activity relationships for their liquid crystalline behaviour are discussed. SmCP, USmCP and Colob mesophases are found depending on the type and number of these connecting units. The sequence ester#azoxy . imine . azo can be proposed for the mesophase range, with significant differences observed in terms of mesopahase stabilization. SHG studies on these compounds give nonlinear coefficients in the range of 1–8 pm V21. The molecular origin for these values is analyzed semi- uantitatively. It was concluded that the SHG performance of bent-core mesogens in general can still be increased substantially. An approach to improve the properties of these materials is briefly outlined.This work was supported by projects MAT2003-07806-C02 and MAT2006-13571-C02 (CICYT-FEDER) from Spain-EU and by the DGA (Spain) and University of the Basque Country (Project. Nu 9/UPV 00060.310-13562/2001). I. P. and I. A. also thank DGA and the Ministry of Education of Spain, respectively, for grants.Peer reviewe

A Columnar Liquid Crystal with Permanent Polar Order

The self-assembly of axial dipolar subphthalocyanine molecules in the presence of electric fields leads to uniaxially oriented columnar liquid crystalline materials that exhibit permanent polarizationFunding from MINECO (MAT2012-38538-CO3-01, 02 and CTQ2012- 31914), MICINN and MEC (F.P.U. fellowship (JG), CTQ-2011- 24187/BQU, CTQ2011-23659 and Consolider-Ingenio Nanociencia Molecular, CDS2007-00010), Comunidad de Madrid (MADRISOLAR-2, S2009/PPQ/1533), European Research Council (StG-279548), Aragon government, E04, Basque Country Government GI/IT-449-10, Generalitat Valenciana (PROMETEO/ 2012/053), and FEDER is acknowledge

Surfaces roughness effects on the transmission of Gaussian beams by anisotropic parallel plates

Influence of the plate surfaces roughness in precise ellipsometry experiments is studied. The realistic case of a Gaussian laser beam crossing a uniaxial platelet is considered. Expression for the transmittance is determined using the first order perturbation theory. In this frame, it is shown that interference takes place between the specular transmitted beam and the scattered field. This effect is due to the angular distribution of the Gaussian beam and is of first order in the roughness over wavelength ratio. As an application, a numerical simulation of the effects of quartz roughness surfaces at normal incidence is provided. The interference term is found to be strongly connected to the random nature of the surface roughness.Comment: 18 pages, Journal of Physics D: Applied Physics, volume 36, issue 21, pages 2697 - 270

Ferroelectric chiral nematic liquid crystals: New photonic materials with multiple bandgaps controllable by low electric fields

We have carried out a spectroscopic study on a prototype ferroelectric nematic material (RM7343) doped with a non-polar chiral compound. The mixture presents two chiral nematic phases, one conventional (N*) and the other polar (NF*), whose behaviours under electric fields E are totally different. On the one hand, the N* phase shows a single reflection band if E is not very high, while fields perpendicular to the helix of a few V/mm are already sufficient to produce multiple bands in the NF* phase. On the other hand, the pitch of the NF* phase grows notably on increasing fields, however remaining practically constant in the N* phase. Both effects have been explained in terms of the different type of interaction with E in each phase (ferroelectric in NF* and dielectric in N*). We argue that the NF* phase behaves as a photonic material with multiple gaps tunable by small fields, which presents important potentialities for applications. A study of the bandgaps has been carried out through the analysis of the dispersion relation of the optical eigenmodes in the NF* phase under field. Finally, an example of the potentialities of the NF* phase within the field of nonlinear optics is briefly presented.Comment: 22 pages, 7 figure

The ferroelectric nematic phase: an optimum liquid crystal candidate for nonlinear optics

Materials that exhibit high nonlinear optical (NLO) susceptibilities are considered as promising candidates for a wide range of photonic and electronic applications. Here, we argue that the ferroelectric nematic (NF) materials have sufficient potentialities to become materials for the next-generation of NLO devices. We have carried out a study of the efficiency of optical second-harmonic generation in a prototype NF material, finding a nonlinear susceptibility of 5.6 pm.V−1 in the transparent regime, one of the highest ever reported in ferroelectric liquid crystals. Given the fact that the studied molecule was not specifically designed for NLO applications, we conclude there is still margin to obtain NF materials with enhanced properties that should allow their practical use.This work was supported by Euskal Herriko Unibertsitatea Unibertsitatea [Project GIU18/146], the MICIU/AEI/FEDER, EU funds [Project PGC2018-093761-B-C31] and the Gobierno de Aragón-FSE [Project E47_20R].Peer reviewe

Achiral bent-core liquid crystals with azo and azoxy linkages: structural and nonlinear optical properties and photoisomerization

10 pages, 13 figures, 1 scheme.We present a structural study of the mesophases of three achiral bent-core compounds with azo and azoxy bonds. The study is based on textures observation and small-angle X-ray diffraction. It is found that the materials exhibit columnar and tilted smectic mesophases. Optical second-harmonic generation measurements are performed on aligned samples of one of the compounds, and the whole second-order susceptibility tensor is determined. Finally, some simple aspects of the photoisomerization of the materials are examined.I.A. and I.P. thank the Ministry of Education of Spain and the Aragón Government (DGA), respectively, for grants. This work was supported by the CICYTFEDER of Spain-EU (Project No. MAT2003-07806-C02), the University of the Basque Country (Project No. 9/UPV 00060.310-13562/2001), and the Arago´n Government.Peer reviewe