Sound-transparent anisotropic media for backscattering-immune wave manipulation

The scattering behavior of the anisotropic acoustic medium is analyzed to reveal the possibility of routing acoustic signals through the anisotropic layers with no backscattering loss. The sound-transparent effect of such medium is achieved by independently modulating the anisotropic effective acoustic parameters in a specific order, and experimentally observed in a bending waveguide by arranging the subwavelength structures in the bending part according to transformation acoustics. With the properly designed filling structures, the original distorted acoustic field in the bending waveguide is restored as if the wave travels along a straight path. The transmitted acoustic signal is maintained nearly the same as the incident modulated Gaussian pulse. The proposed schemes and the supporting results could be instructive for further acoustic manipulations such as wave steering, cloaking and beam splitting.

Theoretical fabrication of subwavelength structures by surface plasmon interference based on complementary grating

This paper presents a surface plasmon interference lithography technique based on the complementary grating, which comprises silicon gratings and complementary aluminum grating masks, for fabricating subwavelength structures. In this theoretical study, the optimal parameters of the complementary grating structure were determined using the reflectance spectrum. The optical field distributions of one- and two-dimensional subwavelength structures were obtained using the finite-difference time-domain method and rotation-related formulas. The results of numerical evaluations show that a one-dimensional periodic structure with a half-pitch resolution of 60.5 nm (approximately [Formula: see text]/6.7) can be fabricated. In addition, subwavelength structures can be diversified using different rotation methods to expose the photolithography samples, such as square dot arrays and quasi-hexagonal closely packed structures. The proposed method combines surface plasmon interference with sample rotation, thereby enabling fabrication of abundant subwavelength structures.

Metasurface Photodetectors

Photodetectors are the essential building blocks of a wide range of optical systems. Typical photodetectors only convert the intensity of light electrical output signals, leaving other electromagnetic parameters, such as the frequencies, phases, and polarization states unresolved. Metasurfaces are arrays of subwavelength structures that can manipulate the amplitude, phase, frequency, and polarization state of light. When combined with photodetectors, metasurfaces can enhance the light-matter interaction at the pixel level and also enable the detector pixels to resolve more electromagnetic parameters. In this paper, we review recent research efforts in merging metasurfaces with photodetectors towards improved detection performances and advanced detection schemes. The impacts of merging metasurfaces with photodetectors, on the architecture of optical systems, and potential applications are also discussed.

Recent advances of wide-angle metalenses: principle, design, and applications

Optical imaging systems, like microscopes, cameras, and telescopes, continue to expand the scope of human observation of the world. As one of the key indicators of imaging systems, the field-of-view (FOV) is often limited by coma aberration. Expanding it generally relies on a combination of complex lenses, leading to a bulky and cumbersome system. Recently, the emergency of meta-optics provides an alternative to constructing compact and lightweight large-FOV metalens through elaborated phase modulation within a flat surface, showing great potential in surveillance, unmanned vehicles, onboard planes or satellites, medical science, and other new applications. In this article, we review recent advances of wide-angle metalenses, including operation principles, design strategies, and application demos. Firstly, basic principles of wide-angle imaging using a single metalens are interpreted. Secondly, some advanced methods for designing subwavelength structures with high angle robustness and high efficiency are discussed. Thirdly, some representative functional devices and applications are surveyed. Finally, we conclude with an outlook on future potentials and challenges that need to be overcome.

Vertical Injection and Wideband Grating Coupler Based on Asymmetric Grating Trenches

A Silicon-on-insulator (SOI) perfectly vertical fibre-to-chip grating coupler is proposed and designed based on engineered subwavelength structures. The high directionality of the coupler is achieved by implementing step gratings to realize asymmetric diffraction and by applying effective index variation with auxiliary ultra-subwavelength gratings. The proposed structure is numerically analysed by using two-dimensional Finite Difference Time Domain (2D FDTD) method and achieves 76% (-1.19 dB) coupling efficiency and 39 nm 1-dB bandwidth.

Development and characterization of a superresolution ultrasonic transducer

Highly sensitive ultrasound probes are needed to expand the capabilities of biomedical ultrasound and industrial non-destructive testing (NDT). Pursuing better imaging quality, while keeping fabrication costs low, is an important trend in the current development of ultrasound imaging systems. In this paper, we report the development and characterization of an ultrasonic transducer that (super)focuses ultrasonic waves beyond the so-called diffraction limit, i.e., the beamwaist is roughly narrower than one wavelength. The transducer comprises an additive manufactured case with a circular flat piezoelectric actuator fixed at the bottom and a core-shell lens (with a stainless steel core and a polymer shell) placed at the probe's conical tip. The core-shell lens is responsible to superfocusing effect of ultrasonic waves. Operating at approximately 3 MHz, the transverse and axial resolution for C- and B-scan images are, respectively, 0.65λ and 3λ/2, with the wavelength being λ = 0.5 mm. Whereas the system depth-of-field is 6.3λ. To demonstrate the transducer capability to resolve subwavelength structures, we successfully obtain images of a copper wire forming a Y-intersection, whose branches a diameter similar to human hair (0.15 mm ≈ 0.3λ). Our results represent a solid step toward the development of ultrasonic superresolution transducer applied for biomedical imaging and shallow NDT of materials.