image: CPL-sensitive near field modes in achiral structures. a. Schematic of the CPL-sensitive near field modes. b. Hot carriers driven by near-field modes. c. Simulation results of near-field modes under circularly polarized light. d. Observation of near-field modes through photoemission electron microscopy.
Credit: by Zhang, G., Lyu, X., Qin, Y. et al.
Circularly polarized light (CPL) plays an important role in various contemporary applications such as chiral molecule distinguishing, remote sensing, quantum optics, and spintronics. Traditional methods for CPL detection require bulky optical components to convert them into linearly polarized light, which hinders device miniaturization and fails to meet the evolving demands of on-chip applications. In recent years, researchers have explored three main approaches for miniaturized CPL detectors: chiral semiconductors with differential absorption for left- and right-hand CPL, novel photogalvanic effects that directly respond to photon spins, and artificial nanostructures with ultrahigh circular dichroism. While these approaches have shown promising results, photodetectors based on chiral semiconductors or novel photogalvanic effects often suffer from poor discrimination capabilities, and quasi-3D artificial structures with intrinsic chirality require complex fabrication processes.
In a new paper published in Light: Science & Applications, the extreme optics research team from Frontiers Science Center for Nano-Optoelectronics, School of Physics, Peking University, China, along with collaborators, developed a broadband CPL photodetector with an ultrahigh discrimination ratio. This ratio, also known as the asymmetry factor, measures a device's ability to distinguish between left- and right-hand CPL. It is defined as the absolute difference in photoresponses to the two types of CPL, relative to their average value. The device operates across the entire visible spectrum and achieves a maximum discrimination ratio of ~107. Additionally, it can accurately detect small changes in light ellipticity.
Contrary to traditional approaches, this work uses an achiral structure to probe chiral light, leveraging CPL-sensitive near-field modes within the achiral design. When excited by CPL light, the intensity distribution on either side of the symmetry plane of the achiral structure differs, and the near-field modes generated by left- and right-handed CPL are opposite. This phenomenon is thought to result from the interference between x- and y-polarized light excitation modes with different phases. Through photoemission electron microscopy, scientists observed this unique near-field pattern and used it to design a CPL detector.
To create the CPL detector, the researchers used a simple achiral structure: V-shaped grooves. By optimizing the geometry of the structure, they ensured that left-handed and right-handed CPL light excited different light field modes, concentrating energy on opposite arms of the groove. This led to different temperature rises on each side, driving carriers in the photothermoelectric material (tellurium) to diffuse toward the cooler side. Electrodes placed on both sides of the V-groove collected these hot carriers, enabling a circular polarization-sensitive photoelectric response. The device consists of multiple rows of these "electrode-V-groove-electrode" configurations to minimize substrate photovoltages caused by processing errors and contact asymmetry. Since the circular polarization-sensitive near-field mode exists in a broadband range, and the near-field modes excited by left-handed light and right-handed light can drive photovoltages in opposite directions; the device has an extremely wide operating bandwidth and excellent circular polarization discrimination ratio.
This circular polarization detection method, based on achiral dielectric structures, is versatile and can be applied across various optoelectronic materials, geometric designs, photoresponse mechanisms, and wavelengths. It offers a new approach for compact CPL detection.
Article Title
High discrimination ratio, broadband circularly polarized light photodetector using dielectric achiral nanostructures