Wideband metasurfaces usually have increased structure complexity as compared to their narrowband counterparts, by integrating multiple in-plane or cascaded resonant structures. We propose a dielectric grating structure as a quarter-wave plate with alleviated structure complexity and wide bandwidth at terahertz frequencies, by simply enlarging the period from subwavelength to superwavelength scale. From the modal theory, the superwavelength grating supports more than one waveguide modes. The superposition of the fundamental mode and higher-order mode with diverse dispersion characteristics is used to engineer the spectral response and to achieve a wideband linear-to-circular polarization conversion, which is validated by experimental study.
Graphene has attracted widespread attention in dynamic optoelectronic devices due to its tunable electrical and optical properties. But different modulation capabilities of the graphene-based designs at different frequencies are less studied. We study the electrical tunability of transmissive metalenses based on graphene when working at three frequencies 0.3, 1.25, and 2 THz, respectively. The constitutive meta-atoms are composed of graphene patches and metallic gratings for efficient phase shift in the orthogonal polarization. Although the conductivity of graphene is tunable at all the frequencies, responses of meta-atoms show weak and strong dependence on the Fermi level at the low and high frequencies, respectively. Therefore, the focal length of the metalens is not electrically tunable at 0.3 THz. In contrast, the metalenses designed at 1.25 and 2 THz show electrically adjustable focal lengths, and the tuning range of the focal length increases with frequency. The research here provides clear guidance for the design of graphene metalenses with different electrical tunabilities for a variety of application scenarios.
The orientation of the dipole moment, which is one of important parameters in single-molecule fluorescence
spectroscopy, is determined by highly focused generalized cylindrical vector beams combined with a confocal far-field
microscope. A generalized cylindrical vector beam can be decomposed into a linear superposition of radially polarized
and azimuthally polarized components and the ratio of the radial and longitudinal components of the focal electric field
can be adjusted by changing the polarization direction. As compared to the case of a radially polarized incident beam,
numerical results demonstrate that fluorescence intensity difference among single molecules with different orientations
can be effectively overcome for a particular polarization direction of the generalized cylindrical vector beam, which may
increases the signal-to-noise ratio of the practical experiments and then ease the difficulty in orientation determination of
single molecules.
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