Organic crystals exhibit well-ordered molecule arrangements and highly symmetric polygonal morphology, making them suitable for a wide range of optical applications like resonators and sensors. Organic chemistry has allowed fine-tuning of emission bands in the visible spectral region by modifying the molecular structure and associated electronic energetics in the transient and steady states. Lately, research efforts have focused on the precise control of crystal morphology to enable further exploitation and improvement of the optical properties of organic crystals.
Generally, organic crystals demonstrated in previous studies represent unique and emergent materials in optics. However, these organic crystals are highly susceptible to external bending stress, limiting their practical applications in devices requiring mechanical assembling or those involving mechanical oscillations and related impacts during operation. The recent pioneering synthesis of mechanically flexible organic crystals (FOCs) has opened the door for many possibilities in this line. For instance, mechanically flexible and elastic organic crystals capable of working as waveguides after bending have been used to fabricate photonic integrated circuits.
Due to the recent research progress, optical resonators made of FOCs have been sought after. Unfortunately, the fabrication of single-crystal optical resonators capable of maintaining the optical performance after mechanical bending remains a big engineering challenge. This can be attributed to various factors, such as the optical loss during light propagation and reflection at the end facets. Since light leakage and confinement in these resonators are orientationally anisotropic in most cases, improving the light confinement efficiency of FOC is one way of addressing this challenge. This may require accurate optimization of the detection and excitation positions.
On this account, a team of researchers from the University of Tsukuba: Mr. Shuai Zhao, Dr. Hiroshi Yamagishi, Dr. Osamu Oki, Mr. Yuta Ihara, Dr. Naoki Ichiji, Dr. Atsushi Kubo and Professor Yohei Yamamoto in collaboration with Professor Shotaro Hayashi from Kochi University of Technology developed a FOC that works as a tunable optical resonator, and whose resonance wavelength is a function of the mechanical bending angle. The rod-shaped single-crystalline grains (FOCCOPV) were synthesized from the fluorescent π-conjugated molecule and cyano-substituted oligo(phenylenevinylene)s (COPV) via slow precipitation. Their work is currently published in the journal, Advanced Optical Materials.
The researchers demonstrated that the resulting crystal (FOCCOPV) worked as an efficient optical waveguide with a loss coefficient of 0.249 dB µm–1 along its longitudinal direction. Unlike most previously reported FOC waveguides, this crystal confined light along lateral direction, keeping the cross-sectional resonance intact even after mechanical bending. The peak spacing and position and the resonance wavelength were tunable in the shorter wavelength direction through mechanical bending. This was plausibly attributed to the Poisson effect characterized by the contraction of the geometric dimensions due to strain, alongside the changes in the refractive index. Furthermore, the mechanical bending and related spectral shift were repeatable for many cycles without optical and mechanical properties deterioration.
In summary, the authors reported the successful synthesis of a highly luminescent, mechanically elastic and bendable rod-shaped crystal FOCCOPV. It showed excellent optical waveguiding and Fabry-Pérot resonance properties along the longitudinal and lateral directions, respectively. The demonstrated organic crystal resonators are remarkably mechanically flexible and optically tunable and significantly contributed to enhancing the optical functionality and mechanical robustness of various devices. In a statement to Advances in Engineering, Professor Yohei Yamamoto specifically pointed out that the new findings provide an effective path that would promote the development and practical application of FOCs-based optical resonators and integrated devices.
Zhao, S., Yamagishi, H., Oki, O., Ihara, Y., Ichiji, N., & Kubo, A. et al. (2021). Mechanically Flexible and Optically Tunable Organic Crystal Resonator. Advanced Optical Materials, 10(2), 2101808.