Jeong-Geun Yun

Broadband ultrathin circular polarizer at visible and near-infrared wavelengths using a non-resonant characteristic in helically stacked nano-gratings

Modern imaging and spectroscopy systems require to implement diverse functionalities with thin thickness and wide wavelength ranges. In order to meet this demand, polarization-resolved imaging has been widely investigated with integrated circular polarizers. However, the circular polarizers which operate at the entire visible wavelengths and have a thickness of several tens of nanometers have not been developed yet. Here, a circular polarizer, operating at the entire visible wavelength range, is demonstrated using helically stacked aluminum nano-grating layers. High extinction ratio and broad operation bandwidth are simultaneously achieved by using non-resonant anisotropic characteristics of the nano-grating. It is theoretically verified that the averaged extinction ratio becomes up to 8 over the entire visible wavelength range while having a thickness of 390 nm. Also, the feasibility of the proposed structure and circular polarization selectivity at the visible wavelength range are experimentally verified. It is expected that the proposed structure will lead to extreme miniaturization of a circular polarizer and contribute greatly to the development of mobile/wearable imaging systems such as virtual reality and augmented reality displays.

Active directional switching of surface plasmon polaritons using a phase transition material

Active switching of near-field directivity, which is an essential functionality for compact integrated photonics and small optoelectronic elements, has been challenging due to small modulation depth and complicated fabrication methods for devices including active optical materials. Here, we theoretically and experimentally realize a nanoscale active directional switching of surface plasmon polaritons (SPPs) using a phase transition material for the first time. The SPP switching device with noticeable distinction is demonstrated based on the phase transition of vanadium dioxide (VO2) at the telecom wavelength. As the insulator-to-metal phase transition (IMT) of VO2 induces the large change of VO2 permittivity at telecom wavelengths, the plasmonic response of a nanoantenna made of VO2 can be largely tuned by external thermal stimuli. The VO2-insulator-metal (VIM) nanoantenna and its periodic array, the VIM metagrating, are suggested as optical switches. The directional power distinction ratio is designed to change from 8.13:1 to 1:10.56 by the IMT and it is experimentally verified that the ratio changes from 3.725:1 to 1:3.132 as the VIM metagratings are heated up to 90 °C. With an electro-thermally controllable configuration and an optimized resonant design, we expect potential applications of the active switching mechanism for integrable active plasmonic elements and reconfigurable imaging.

Numerical study on refractive index sensor based on hybrid-plasmonic mode

We propose a highly sensitive hybrid-plasmonic sensor based on thin-gold nanoslit arrays. The transmission characteristics of gold nanoslit arrays are analyzed as changing the thickness of gold layer. The surface plasmon polariton mode excited on the sensing medium, which is sensitive to refractive index change of the sensing medium, is strengthened by reducing the thickness of the gold layer. A design rule is suggested that steeper dispersion curve of the surface plasmon polariton mode leads to higher sensitivity. For the dispersion engineering, hybrid-plasmonic structure, which consists of thin-gold nanoslit arrays, sensing region and high refractive index dielectric space is introduced. The proposed sensor structure with period of 700 nm shows the improved sensitivity up to 1080 nm/RIU (refractive index unit), and the surface sensitivity is extremely enhanced.

Two-dimensional array of plasmonic V-shaped grooves for sensing applications

We numerically analyzed the refractive index sensing performance of the two-dimensional array of plasmonic V-shaped grooves. The structural parameters are optimized to show a sharp and large reflectance dip with desired sensing region (n=1.33). Acquiring the sensitivity of the environmental refractive index as 400 nm/RIU in the visible region, FWHM of the dip is ∼5 nm. It shows that the proposed structure has extreme value of quality factor and good extinction ratio. The localized mode has the hot spot at the bottom of the grooves so that localized sensing based on magnetic field enhancement is possible. Moreover, the localized mode is dependent on the tapered angle of the grooves, not the opening ratio. The performance of the dual V-shaped grooves is also discussed. The array of the closely located grooves has nearly identical reflectance spectra but a moderate amount of dip shift exists. As well as obtaining refractive index sensing by this configuration, magnetic field hot spot generation by coherent excitation can be applied to highly localized sensing and enhancing nonlinear processes.