A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2024.240075, discusses racemic dielectric metasurfaces for multi-dimensional optical manipulation.
Chirality is one of the fundamental phenomena widely present in nature, describing the geometric features of object structure or shape. From the distribution of galaxies in the universe to the structure of DNA, they all exhibit chirality. A pair of chiral isomers are mirror images of each other and cannot completely overlap in space - such as our hands. Light fields with chiral features such as circularly polarized states or helical wavefronts will undergo singular interactions with chiral substances, such as circular dichroism (CD) and optical activity (OA), known as chiral optical responses. Chiral metamaterials or metasurfaces as important products for simulating natural chiral substances, have customizable frequency bands and sizes for their chiral optical responses, and have achieved novel physical phenomena such as negative refraction and chiral phase, providing a new approach for chiral optical manipulation and even the design of new functional electromagnetic devices.
A racemic compound refers to a mixture (or compound) containing equal amounts of enantiomers, with zero overall optical rotation and no apparent chirality. It has the characteristic of "hidden" chirality and show important applications such as complex functional drugs. However, can an equal number of isomer resonance units also form a "composite" metamaterial or metasurface like molecules ? The novel optical phenomena and light field manipulation functions that may be triggered by this also need to be studied.
In response to the above issues, a joint research group from Chengdu University of Information Technology and Tianjin University has verified the possibility of racemic metasurfaces in the terahertz band, achieved the separation of near-field and far-field electromagnetic chirality, and demonstrated multidimensional terahertz wave manipulation based on the new platform.
In order to design the racemic hybrid structure, the research group first optimized the parameters to obtain an all-silicon "Z"-shaped chiral meta-atom with C2 symmetry, then analyzed its circularly polarized spectral and near-field distributions. At the same time, the transmission spectrum of the sample was measured using a polarization-resolved terahertz time-domain spectroscopy system. The results indicate that the structure possesses both efficient transmission CD and stable chiral Pancharatnam-Berry phase.
On this basis, the research group cross combined a pair of isomeric meta-atoms with mirror symmetry and analyzed the far-field racemic characteristics and near-field chirality of the composite structure through simulations, verifying its function of "hidden" chirality. The significance of racemic design lies not only in "racemization", but also in further improvement to achieve novel applications. The team introduced Pancharatnam-Berry phase between isomeric meta-atoms and controlled the polarization direction of transmitted terahertz waves by using equal amplitude stacking between orthogonal circularly polarized components, thus achieving customized OA. Based on a brief theoretical analysis, it is known that the polarization rotation angle of the transmitted wave is exactly equal to the relative rotation angle between isomers, which provides a convenient way for polarization manipulation of terahertz waves.
Not only that, but the group also introduced Pancharatnam-Berry phase between the "super-units", aiming to simultaneously design the polarization state and wavefront shape of the transmitted wave. A focused phase profile with 30×30 pixels (60×60 chiral meta-atoms) is designed, in which every hybrid structure composed of two pairs of isomers. Finally, the team prepared the above-mentioned metasurface samples and measured the electric field complex amplitude of the transmitted beam using focal plane terahertz imaging system and probe-based terahertz time-domain spectroscopy system, further verifying the function of the device.
This work achieved effective separation of near-field and far-field terahertz chirality through combination of isomeric chiral meta-atoms, and integrated control of polarization and wavefront for transmission terahertz wave was achieved by introducing two levels of Pancharatnam Berry phases between meta-atoms and hybrid structures. It has application prospects in chiral substance detection, terahertz imaging, and other fields. Moreover, the design concept can also be extended to other electromagnetic bands.
Keywords: racemic / dielectric metasurfaces / terahertz waves / chirality
