Recently, the Nanophotonics team of Future Optics International Laboratory led by Academician Songlin Zhuang made a breakthrough in localized superchiral light field.
Chirality describes the structural property of a three-dimensional object, whose mirror image cannot be superimposed on its original image. This is a common feature for many biochemical molecules, such as amino acids and sugars. In biochemical and pharmaceutical industries, it is important to distinguish and separate the molecules with opposite chiralities (i.e., enantiomers) due to their different toxicities.
Due to the chirality of circularly polarized light, circular dichroism (CD) measurements have been widely used for the analysis of molecular chirality. However, the molecular structure size is in the nanometer range, while the wavelength of light is in the hundreds of nanometers range, resulting in a mismatch in scale. This leads to the traditional CD measurement techniques typically analyzing the average chirality of a large number of molecules in a specific region, and cannot achieve high spatial resolution measurements of the sample. In 2010, A.E. Cohen et al. from Harvard University proposed the use of optical fields with higher chirality than circularly polarized light (i.e., superchiral optical fields) to significantly enhance the chirality signal. However, there have been no reports on the realization of highly localized superchiral optical fields.
The Nanophotonics team at the Future Optics International Laboratory has discovered a highly localized superchiral optical field generation method using the coupling of photon orbital angular momentum and spin angular momentum in a strong focusing system. By appropriate optical polarization modulation, it was found that under circularly polarized light illumination, a highly localized superchiral optical field can be generated. The research shows that at the focal point of the focusing lens, a 11.9-fold enhancement of optical chirality compared to circularly polarized light can be achieved within a scale of N/25 (one twenty-fifth of the wavelength of light). By scanning chiral samples using this highly localized superchiral optical field, molecular chirality and its spatial distribution information can be obtained, surpassing the spatial resolution of conventional optical systems. Importantly, the proposed method is compatible with traditional circular dichroism (CD) measurement techniques. This technology provides a new approach for label-free molecular imaging and has broad applications in fields such as biology and medicine. The research findings were published in the prestigious journal Physical Review Letters (H. Hu, Q. Gan, and Q. Zhan, Phys. Rev. Lett, 122, 223901 (2019)). This work received strong support from the College of Optoelectronics, the Office of Science and Technology, and the Office of Personnel.
