Research Team led by Professor Ok Kang-min Publishes Paper in Prestigious International Journal JACS
A research team led by Professor Ok Kang-min of the Department of Chemistry at Sogang University has successfully developed a new design strategy for inorganic optical crystals that achieves ultrahigh birefringence through the “coaxial alignment” of linear functional units.
This study is regarded as the first to systematically demonstrate that the coaxiality of linear functional units is a key structural factor for maximizing optical anisotropy, moving beyond the concept of “coplanarity,” which has been the core criterion in previous research on birefringent crystals. In particular, by presenting a pure inorganic crystal system that simultaneously exhibits ultraviolet (UV) transparency and ultrahigh birefringence, the study has established a new paradigm for the design of materials for next-generation optical devices.
The study was conducted under the leadership of the Center for Noncentrosymmetric Materials (CNCSM) at Sogang university, with Professor Ok Kang-min serving as the corresponding author. Postdoctoral researchers Jin Congcong and Li Yang—both Sogang PhD graduates currently affiliated with West Anhui University in China—participated as co-first authors. It was published in the Journal of the American Chemical Society (JACS, IF = 15.7), the most prestigious international journal in the field of chemistry.
Birefringence is a phenomenon in which the refractive index varies depending on the polarization direction when light passes through a material, and it is known as a key physical property in advanced optical technologies such as lasers, optical communications, polarization imaging, precision sensors, and optical memory. In particular, with the rapid miniaturization and high integration of optical devices in recent years, the development of optical crystals that are transparent in the ultraviolet region while possessing high birefringence (Δn ≥ 0.5) has emerged as a critical challenge.
However, most existing high-performance birefringent crystals were concentrated in organic or organic-inorganic hybrid materials, and it was difficult to simultaneously achieve these properties in pure inorganic crystals with excellent chemical stability and mechanical strength. Furthermore, existing design strategies primarily relied on the coplanarity of planar π-conjugated molecules, and new design principles based on linear functional units had not yet been established.
Accordingly, the research team systematically synthesized four UV-transparent inorganic crystals—NH4Cl·HgCl2(1), HgCl(SCN)(2), Hg(SCN)2(3), and NH4Hg(SCN)3(4)—using a strategy that involved the stepwise substitution of chloride (Cl‒) with linear thiocyanate anions (SCN‒).
In particular, Hg(SCN)2(3) exhibited an experimental birefringence value of Δn = 0.720 at 546 nm (calculated value: Δn = 0.754), demonstrating the world’s highest level of optical anisotropy among currently reported pure inorganic UV birefringent crystals. Furthermore, by simultaneously maintaining a wide bandgap of 3.53 eV, the team succeeded in achieving both high birefringence and UV transmittance.
Through precise structural analysis and REDA (Response Electron Distribution Anisotropy) theoretical calculations, the research team determined that ultra-high birefringence cannot be achieved simply by increasing the density of functional units, and that the degree to which linear functional units are aligned along the same axis (coaxiality) plays a decisive role.
Interestingly, in the case of NH4Hg(SCN)3(4), although the functional unit density was very high and the planarity was excellent, a decrease in birefringence was observed because the linear modules were not arranged along the same axis. This is an important finding demonstrating that linear functional unit-based birefringent systems cannot be explained by the conventional “coplanarity-driven design principle” alone.
Through this study, the research team proposed a new structure-property correlation that extends from the conventional “coplanarity-driven design” to “coaxiality-driven design.” In particular, by proposing the concept of “molecular-level dimensionality reduction”—which simplifies functional units from a planar structure to a linear structure—along with the resulting mechanism for high-density integration of functional units, the team has presented a new direction for achieving ultra-high birefringence.
Professor Ok Kang-min explained, “This study is the first to systematically demonstrate how the spatial arrangement of linear functional units governs birefringence,” adding, “In particular, by proposing coaxiality as a new structural factor, we have established a new theoretical and experimental foundation for the design of next-generation ultraviolet optical crystals.” He further emphasized, “The developed material possesses both high optical anisotropy and UV transmission properties while maintaining the excellent stability characteristic of pure inorganic crystals, making it highly likely to be applied in various advanced optical fields such as next-generation laser optical devices, polarization control devices, optical communications, and UV optical systems.”
▶Paper Title: From Coplanarity to Coaxiality: A Linearity-Oriented Design Paradigm for Ultrahigh Ultraviolet Birefringent Inorganic Crystals
▶Paper Link: https://pubs.acs.org/doi/10.1021/jacs.6c04217
[SEO Keyword]
JACS, Ultrahigh Birefringence, Coaxiality-Driven Design
This study is regarded as the first to systematically demonstrate that the coaxiality of linear functional units is a key structural factor for maximizing optical anisotropy, moving beyond the concept of “coplanarity,” which has been the core criterion in previous research on birefringent crystals. In particular, by presenting a pure inorganic crystal system that simultaneously exhibits ultraviolet (UV) transparency and ultrahigh birefringence, the study has established a new paradigm for the design of materials for next-generation optical devices.
The study was conducted under the leadership of the Center for Noncentrosymmetric Materials (CNCSM) at Sogang university, with Professor Ok Kang-min serving as the corresponding author. Postdoctoral researchers Jin Congcong and Li Yang—both Sogang PhD graduates currently affiliated with West Anhui University in China—participated as co-first authors. It was published in the Journal of the American Chemical Society (JACS, IF = 15.7), the most prestigious international journal in the field of chemistry.
Birefringence is a phenomenon in which the refractive index varies depending on the polarization direction when light passes through a material, and it is known as a key physical property in advanced optical technologies such as lasers, optical communications, polarization imaging, precision sensors, and optical memory. In particular, with the rapid miniaturization and high integration of optical devices in recent years, the development of optical crystals that are transparent in the ultraviolet region while possessing high birefringence (Δn ≥ 0.5) has emerged as a critical challenge.
However, most existing high-performance birefringent crystals were concentrated in organic or organic-inorganic hybrid materials, and it was difficult to simultaneously achieve these properties in pure inorganic crystals with excellent chemical stability and mechanical strength. Furthermore, existing design strategies primarily relied on the coplanarity of planar π-conjugated molecules, and new design principles based on linear functional units had not yet been established.
Accordingly, the research team systematically synthesized four UV-transparent inorganic crystals—NH4Cl·HgCl2(1), HgCl(SCN)(2), Hg(SCN)2(3), and NH4Hg(SCN)3(4)—using a strategy that involved the stepwise substitution of chloride (Cl‒) with linear thiocyanate anions (SCN‒).
In particular, Hg(SCN)2(3) exhibited an experimental birefringence value of Δn = 0.720 at 546 nm (calculated value: Δn = 0.754), demonstrating the world’s highest level of optical anisotropy among currently reported pure inorganic UV birefringent crystals. Furthermore, by simultaneously maintaining a wide bandgap of 3.53 eV, the team succeeded in achieving both high birefringence and UV transmittance.
Through precise structural analysis and REDA (Response Electron Distribution Anisotropy) theoretical calculations, the research team determined that ultra-high birefringence cannot be achieved simply by increasing the density of functional units, and that the degree to which linear functional units are aligned along the same axis (coaxiality) plays a decisive role.
Interestingly, in the case of NH4Hg(SCN)3(4), although the functional unit density was very high and the planarity was excellent, a decrease in birefringence was observed because the linear modules were not arranged along the same axis. This is an important finding demonstrating that linear functional unit-based birefringent systems cannot be explained by the conventional “coplanarity-driven design principle” alone.
Through this study, the research team proposed a new structure-property correlation that extends from the conventional “coplanarity-driven design” to “coaxiality-driven design.” In particular, by proposing the concept of “molecular-level dimensionality reduction”—which simplifies functional units from a planar structure to a linear structure—along with the resulting mechanism for high-density integration of functional units, the team has presented a new direction for achieving ultra-high birefringence.
Professor Ok Kang-min explained, “This study is the first to systematically demonstrate how the spatial arrangement of linear functional units governs birefringence,” adding, “In particular, by proposing coaxiality as a new structural factor, we have established a new theoretical and experimental foundation for the design of next-generation ultraviolet optical crystals.” He further emphasized, “The developed material possesses both high optical anisotropy and UV transmission properties while maintaining the excellent stability characteristic of pure inorganic crystals, making it highly likely to be applied in various advanced optical fields such as next-generation laser optical devices, polarization control devices, optical communications, and UV optical systems.”
▶Paper Title: From Coplanarity to Coaxiality: A Linearity-Oriented Design Paradigm for Ultrahigh Ultraviolet Birefringent Inorganic Crystals
▶Paper Link: https://pubs.acs.org/doi/10.1021/jacs.6c04217
[SEO Keyword]
JACS, Ultrahigh Birefringence, Coaxiality-Driven Design