摘要:近年來對光子晶體的研究開啟了一種新的光和物質交互作用之可能性,更發現了生物體中也具有許多光子晶體結構。若要研究生物光子晶體,非線性光學可說是最佳的工具。除了晶體結構特性能夠透過高階非線性張量完整的求得外,還具有高解析度切片能力、可深層觀測及不需染色等良好特性。包括我們在內的幾個國際團隊都曾發表過利用二倍頻來量測奈米尺度之生物分子結構。
目前所有文獻中關於生物光子晶體非線性光學現象的研究均是以傳統晶體光學理論來解釋。但是由巨分子組成之生物光子晶體中的結構遠較由簡單晶格組成之傳統晶體複雜許多,因此生物光子晶體整體的結構對稱性極有可能和單一巨分子的結構對稱性有所差異。這點在以膠原蛋白為樣本的初步實驗中已被證實。而光學二倍頻已知其特性和材料的對稱性有極大的關係,故此計畫中,我們提出一個利用二倍頻技術來探討生物光子晶體對稱性的研究。希望藉此瞭解雷射光場和分子材料交互作用時,究竟是單一分子的組成還是材料的整體結構對稱性影響較大。並期望能因此讓光學倍頻技術成為具有分子解析能力的生醫影像工具。
Abstract: Photonic crystal can alter the fundamental process of light interaction in the material and has made a major impact to both physics and engineering researches. During the last decade, many photonic crystal structures in biology have been found. To understand how light interacts within a biophotonic crystal structure, the study of crystal symmetry is essential. However, linear optical properties such as absorption are not adequate to fully resolve the symmetry characteristics of matter. Nonlinear optical interactions, on the other hand, are described with higher order tensors with multiple elements, which encrypt full material symmetry properties. We are among the first few groups who studied the nonlinear photonic response in the biophotonic crystal structures. Recently, we have demonstrated that efficient second-order nonlinear scattering can be obtained in various biophotonic crystals, such as collagen fibers, muscle fibers, cellulose fibers, microtubules, and starch granules, etc. Such nonlinear scattering has been adopted as a novel microscopic imaging modality for biomedical researches, providing advantages of intrinsic optical sectioning capability, deep penetration ability, and contrast specificity. Though optical imaging resolution is limited by diffraction, the coherent nature of nonlinear optical response provides an attractive possibility to probe molecular symmetry inside these crystallized structures. By analyzing the nonlinear scattering polarization dependency, the polypeptide chain inclination angle of a single collagen triple helix was obtained. In addition, the nonlinear scattering radiation pattern was found to be highly sensitive to collagen fibril thickness, and such property has been applied to determine fibril thickness down to several tens of nanometers.
All analyses of nonlinear scattering in biophotonic crystals in literature to date are based on conventional nonlinear optics theory, which is deduced from perfect crystallized structures. Based on our preliminary studies, we propose an innovative idea that the symmetry of biophotonic crystals exhibits a fundamental difference from conventional crystals: hierarchy. Conventional crystals are composed of lattices and the crystal symmetry is the same from a single lattice to bulk materials. However, biophotonic crystals are composed of macromolecules, and the symmetry of the crystals may well be diverse from that of constituent molecules. Optical nonlinear scattering provides a simple probe to explore such molecular and structural symmetry compared to other methods such as X-ray diffraction. Moreover, optical methods offer potentials for deep observation of biological specimens in vivo.
Taking collagen fibril as an example, our preliminary study has shown that a fibril with thickness less than 25 nm exhibits trigonal symmetry, which is the predicted symmetry for a single triple helix molecule. However, when the fibril grows thicker, the packing orientation of each collagen molecule becomes more randomized and cylindrical symmetry turns out to be the dominant mechanism in determining nonlinear scattering properties. The main purpose of this proposal is to open up a new research area in nonlinear optics as well as in soft matter physics by expanding our preliminary results to other structural proteins in biological tissues. Through investigating such fundamental laser-molecule interactions, new applications on bio-tissue examination are highly expected.