Molecular Engineering for Enhanced Second-Order Nonlinear Response in Spontaneously-Oriented Evaporated Organic Films

TL;DR

This paper demonstrates that molecular engineering, specifically intramolecular bridge-locking, significantly enhances the second-order nonlinear response in spontaneously oriented organic thin films, offering a scalable, poling-free approach for integrated photonics.

Contribution

The study introduces a molecular design strategy that improves both hyperpolarizability and orientation, leading to doubled nonlinear response in organic films for photonic applications.

Findings

Achieved twofold increase in second-order nonlinear susceptibility at 1550 nm.

Identified molecular orientation as the key factor for enhanced nonlinear response.

Validated design approach with experimental and theoretical analyses.

Abstract

Materials with large second-order nonlinearities are crucial for next-generation integrated photonics. Spontaneously oriented organic thin films prepared by physical vapor deposition offer a promising poling-free and scalable approach. This study investigates molecular engineering strategies to enhance the second-order nonlinear response of derivatives based on the donor-acceptor molecule 2-(4'-diphenylaminobiphenyl-4-yl)quinoxaline-6,7-dicarbonitrile (TPA-QCN). Four derivatives incorporating modifications designed to increase molecular hyperpolarizability ($\beta$) or promote favorable orientation were synthesized and characterized. The most successful modification, intramolecular bridge-locking, simultaneously increases hyperpolarizability and enhances spontaneous orientation by reducing detrimental electrostatic interactions during deposition. It leads to a significant enhancement of the second-order nonlinear response, achieving off-resonance $\chi^{(2)}_{31} \approx 16$ pm V$^{-1}$ and $\chi^{(2)}_{33} \approx 18$ pm V$^{-1}$ at 1550 nm, a twofold improvement over the parent TPA-QCN. Analysis combining nonlinear optical measurements, surface potential measurement, optical anisotropy, and density functional theory calculations indicates that improved molecular orientation, rather than increased $\beta$ alone, is the primary driver for the enhanced performance in the leading derivatives. These findings demonstrate the effectiveness of targeting molecular orientation via structural design and position spontaneously oriented organic films as compelling poling-free candidates for integrated nonlinear photonic applications where the increased electrode-induced optical losses, fabrication complexity and footprint are a critical limitation.