An optical vortex carrying orbital angular momentum (OAM) with a fractional topological charge (TC) is a good complement to a traditional integer optical vortex and has many potential applications. However, the fractional optical vortex (FOV), which breaks the orthogonality of OAM, makes the precision measurement of the FOV complicated and difficult when using a conventional method. Thus, we propose an accurate and effective FOV detection method by using a two-dimensional multifocal array consisting of different integer vortices. Theoretically, FOV beams could be decomposed into a Fourier series of integer optical vortex beams, and the average OAM of the FOV can be calculated via the energy of these integer optical vortices, which can be accurately measured by a commercial photoelectric detector. For improving the measurement accuracy, a compensation iteration algorithm is employed to correct the detected average OAM of the FOV. Experimental results show that the error of average OAM detection is less than 0.025. This alternative FOV TC detection method is sufficient for a wide range of real-time requirements in optical manipulation, imaging, and quantum optics.

Based on an integrated array of refractory titanium nitride (TiN), a metasurface perfect absorber (MPA) in the visible-to-near infrared (NIR) band is reported. The systematic and detailed simulation study of the absorption of the MPA is performed with the finite-different time-domain (FDTD) method. Tailoring the structure, the MPA realizes as high an average as 99.6% broadband absorption, ranging from 400 nm to 1500 nm. The broadband perfect absorption can be attributed to localized surface plasmonic resonance (LSPR), excited by the continuous diameter evolution from the apex to the base of the nanocone, and the gap plasmons excited among the nanocones, as well as in the spacer layer at longer wavelengths. Particularly, the coupling of the resonances is essentially behind the broadening of the absorption spectrum. We also evaluated the electric field intensity and polarization-dependence of the nanocone MPA to offer further physical insight into light trapping capability. The MPA shows about 90% average absorption even at an oblique incidence up to 50°, which improves the acceptance capability of light-harvesting system applications. This unique design with the TiN nanocone array/aluminium oxide (Al₂O₃)/TiN structure shows potential in imminent applications in light trapping and thermophotovoltaics.

A broadband metamaterial absorber (MA) composed of hexagonal-arranged single-sized titanium nitride (TiN) nano-disk array and monolayer molybdenum disulfide (MoS2) is studied using finite-difference time-domain (FDTD) simulations. The structure of TiN nano-disk array/dielectric silica (SiO2)/aluminum (Al) is adopted in our design. By optimizing the dimension parameters of the structure, an average absorption of 96.1% is achieved from 400 to 850 nm. In addition, by inserting a monolayer MoS2 which has high absorption at the short wavelength side underneath the TiN nano-disk array, an average absorption of 98.1% over the entire visible regime from 400 to 850 nm was achieved, with a peak absorption near 100% and absorption over 99% from 475 to 772 nm. Moreover, the absorber presented in this paper is polarization insensitive. This compact and unique design with TiN nano-disk/monolayer MoS2/ SiO2/Al structure may have great potential for applications in photovoltaics and light trapping.

In this study, a structure to realize a switchover between two different responses of electromagnetically induced transparency (EIT) was designed and implemented by simulation. Taking advantage of the anisotropy in the structure and the coupling between the radiative and dark elements, a metasurface switch with modulation depth of over 85% between orthogonal polarization incident light illuminations was demonstrated. The key mode switchover between the “on” and “off” states was achieved by electrically changing the dressing light polarization with a liquid crystals layer pre-aligned with a mature technology, without changing the incident light and an expected and reversible transition from an EIT-like spectrum to a strong spectral dip was observed. The modulation in the EIT switch fabricated with the proposed straightforward approach is a promising tool to control the groping velocity delay.

Recently, orbital angular momentum (OAM) has been adopted to measure the shape of static objects and the translation motion of moving objects in optical remote sensing. Most of these studies rely on measuring the intensity variation of OAM beams. However, the OAM intensity does not change with the rotation of the spinning object, but its phase changes. The phase variation is proved to be proportional to the object’s angular velocity. Since a rotating object will cause the OAM phase dependent on time, the OAM phase needs to be measured instantaneously, to support the OAM‐based angular velocity measurement. In this work, we report a scheme to measure the angular velocity of a spinning object using a photonic OAM phase spectrum. A phase‐to‐intensity strategy is implemented to enable the real‐time multi‐OAM phase measurement, in which the phase can be determined with the intensities of four focal spots in a two‐dimensional array generated by a phase‐only spatial light modulator. The experimental results show that the average error of the measured angular velocity could be under 2.45% by detecting the phase of two OAM modes. This OAM‐based angular velocity detection method provides a complementary approach to characterize the rotational Doppler effect, especially for slow angular motion.