Oblique Incidence Organic Molecular Beam Deposition and Nonlinear Optical Properties of Organic Thin Films with a Stable In-Plane Directional Order

Abstract

Organic thin films with non-centrosymmetric ordering of dipoles are very attractive as waveguides for nonlinear optic and electro-optic applications. [1,2] High electric field poling or self-assembly has been widely used to align dipolar molecules perpendicular to the film surface. [2] However, only a few examples of in-plane alignment of dipolar molecules have been described, including epitaxy on a lattice-matched organic substrate surface, [3,4] in-plane poling, [5] and Langmuir±Blodgett film deposition. [6,7] Based on a new type of nonlinear optical material, we have demonstrated, to our knowledge for the first time, that the dipolar molecules in an organic thin film can be in-situ aligned in any desired direction within the film plane by organic molecular beam deposition (OMBD) at oblique incidence. [8] This one-component, relatively fast (5 nm min ±1 ), OMBD based, and easily controlled thin film growth technique provides a new tool for the production of organic thin films with an in-plane directional order for second-order nonlinear optics (NLO) and electro-optics.Growth of organic thin films by OMBD (Fig. 1A) in ultrahigh vacuum (UHV) has many advantages over solution-based techniques, [9,10] such as greatly reduced contamination in the UHV environment, in-situ growth monitoring, high density of chromophores, and reasonably high growth rate. In addition, mask-defined microstructures such as strip waveguides, and integrated hetero-layer structures, like light emitting diode (LED) devices, [8,11] can be fabricated by OMBD. Despite these advantages, OMBD is still much less developed as compared to solution-based techniques for the preparation of second-order NLO films. The main obstacle lies within the materials.

Stereochemical Effects in Supramolecular Self-Assembly at Surfaces: 1-D versus 2-D Enantiomorphic Ordering for PVBA and PEBA on Ag(111)

“…Specifically, we employed the two molecular building blocks 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA) and 4-[(pyrid-4-yl-ethynyl)]-benzoic acid (PEBA) designed for nonlinear optics applications. [25][26][27][28] The structure formulas given in Figure 1 show that the two molecules are similar. Both species are rigid and planar.…”

The Appeal of Small Molecules for Practical Nonlinear Optics

“…Some molecular glasses are also characterized by some intrinsic noncentrosymmetric order when vapor deposited, [81–85] but were not developed for second‐order nonlinear optics or electro‐optics. Other examples of relevant systems are mixtures of nonlinear optical molecules and liquid‐crystal‐like molecules, which can show a second‐order nonlinear optical activity when poled by an electric field, [86] or self‐assembled films with an orientational order governed by directional hydrogen bonding [87,88] …”

Supramolecular Ordered Organic Thin Films for Nonlinear Optical and Optoelectronic Applications

“…This was shown to result in a non-centrosymmetric thin film with the preferential order in the direction of the molecular beam. [25][26][27] Figure 1 shows the configuration of the deposition apparatus. As a base material, one sublimes amorphous supramolecular polymers of small molecules that are linked in a head-to-tail fashion by strong hydrogen bonding.…”

“…[25] Figure 6 shows scanning tunneling microscopy (STM) images of submonolayer PVBA grown on Ag (111), [44,45] where it is observed that the molecules self-assembled into ordered, hydrogen-bonded chains.…”