Numerical study of the plasmonic slab lens for improving direct-write nano lithography.

Plasmonic direct-write lithography (PDWL) provides a potential tool for the fabrication and manufacturing at the nano scale due to its high-resolution and low-cost. However, the shallow exposure depth hinders its practical application. Here, we incorporate the plasmonic slab lenses (PSLs) into PDWL to amplify and compensate evanescent waves, leading to improved light intensity, depth, resolution and better tolerance to the air gap beyond the near field optical lithography. Two typical plasmonic probes with different nanostructure and localized plasmonic resonance mechanisms are designed and fabricated as representatives, the local intensity enhancement of which mainly depend on the oscillations of transverse and longitudinal electric field components, respectively. Optimizations considering the PSL structure, material and the illuminating wavelength are performed to amplify different field components and figure out the best lithography configuration. Simulation results indicate that Ag-Ag cavity PSL and 355 nm illumination is the best combination for the lithography with bowknot aperture probe, while the semi-ring probe exhibits better performance under the condition of Ag-Al cavity PSL and 405 nm illumination. The semi-ring probe in combination with a plasmonic cavity, for instance, is demonstrated to enhance the light intensity by 4 times at the bottom layer of the photoresist compared to that without PSL and realize a lithography resolution of 23 nm. Our scheme is believed to boost the application of PDWL as a high-resolution and low-cost nanofabrication technology, and it may even serve as an alternative for the high-cost scanning method, such as focused ion beam and electron beam lithography.

Excitation and manipulation of both magnetic and electric surface plasmons.

Surface plasmons (SPs) is the cornerstone in terahertz (THz) near-field photonics, which play crucial roles in the miniaturization and integration of functional devices. The excitation and manipulation of SPs, however, is currently restricted to electric SPs paradigm, while magnetic SPs receive less attention despite the importance of magnetic light-matter interactions. Here, a scheme is proposed to simultaneously convert the propagating waves in free space into magnetic and electric SPs using a single ultracompact device. First, a plasmonic structure composed of connected slit rings is designed and demonstrated to support both electric and magnetic SPs, which is ascribed to the two distinct eigenmodes of oscillating electrons and vortex currents, respectively. Second, with the assistance of an anisotropic and gradient metasurface, orthogonal linear polarized components of incident THz beams are coupled into different electric and magnetic SP channels with little crosstalk. Furthermore, by encoding two distinct polarization-dependent phase profile into the metasurface, it is shown that the resulting meta-device can individually tailor the wavefronts of magnetic and electric SPs, thus simultaneously engineering magnetic and electric near-field distributions. This work can pave the road to realize bi-channel and on-chip devices, and inspire more integrated functionalities especially related to near-field manipulations of magnetic SPs.

Pure toroidal dipole in a single dielectric disk.

The toroidal dipole is a peculiar electromagnetic excitation and has attracted increasing interests because of unusual radiation characteristics. However, the realization of toroidal moment requires complicated structure and are often disturbed by the conventional electric and magnetic multipoles. In this paper, we explore the electromagnetic properties of a simple dielectric disk illuminated by a focused radially polarized beam and demonstrate a pure toroidal dipolar response. A comprehensive approach is proposed to suppress other undesirable electromagnetic multipolar resonances step by step. The disk with optimized geometry is employed to construct an all-dielectric electric mirror dominated by toroidal dipolar resonance. And two kinds of anapole modes with total suppression of far-field radiation are investigated, which proves electric and magnetic non-radiating sources, respectively. Besides, by simultaneously introducing the asymmetry in both structure and incidence, a transformation from Mie-type mode to trapped mode is observed. Our study provides an opportunity to realize a unique pure toroidal dipole and may boost the relevant light-matter interaction.

Tunable anisotropic parameters realized by metal-dielectric hybrid meta-atoms.

Rational design of the structure enables metamaterials to go beyond the ingredients and achieve unprecedented material properties. However, the realization of complicated and anisotropic electromagnetic parameters relies on the elaborate design of building blocks, and the mutual coupling between the anisotropic responses makes precise control of material parameters even more difficult. Here, we propose a metal-dielectric hybrid metamaterial, not only realizing the decoupling between anisotropic electromagnetic responses, but also establishing a one-to-one correspondence between independent geometric dimensions and anisotropic parameter components. Moreover, a tuning theoretical paradigm applied to an anisotropic and resonant system is further suggested, which proves that the operating frequency of this hybrid metamaterial can be easily adjusted by changing external fields. As prototypes, two typical and tunable microwave meta-devices, a transformation-optics cloak and a frequency splitter, are constructed with Ba-Sm-La-Ti ferroelectric ceramic and flexible printed circuit board, which successfully demonstrate our proposed design theory. This work provides a simple strategy for the design and fabrication of tunable anisotropic metamaterials, and boost the development of meta-devices toward practical application.

Energy Band Attraction Effect in Non-Hermitian Systems.

The energy band attraction (EBA) caused by the nonorthogonal eigenvectors is a unique phenomenon in the non-Hermitian (NH) system. However, restricted by the required tight-binding approximation and meticulously engineered complex potentials, such an effect has never been experimentally demonstrated before. Here by a suitable design of all-dielectric Mie resonators in a parallel-plate transmission line, we for the first time verify the photonic analog of the EBA effects both theoretically and experimentally. The evolution of the EBA effect in a two-level NH system from gapped bands to gapless bands to flat bands is observed by precisely tuning the loss of the Mie resonators. The transmission spectra can be theoretically connected to the eigenvalues and eigenvectors of the NH Hamiltonian. Furthermore we extend our methods to a graphenelike two-dimensional NH system. Our works show a metamaterial approach toward NH topological photonics and foster a deeper understanding of band theory in open systems.

Forty-Nanometer Plasmonic Lithography Resolution with Two-Stage Bowtie Lens.

Optical imaging and photolithography hold the promise of extensive applications in the branch of nano-electronics, metrology, and the intricate domain of single-molecule biology. Nonetheless, the phenomenon of light diffraction imposes a foundational constraint upon optical resolution, thus presenting a significant barrier to the downscaling aspirations of nanoscale fabrication. The strategic utilization of surface plasmons has emerged as an avenue to overcome this diffraction-limit problem, leveraging their inherent wavelengths. In this study, we designed a pioneering and two-staged resolution, by adeptly compressing optical energy at profound sub-wavelength dimensions, achieved through the combination of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). By synergistically combining this plasmonic lens with parallel patterning technology, this economic framework not only improves the throughput capabilities of prevalent photolithography but also serves as an innovative pathway towards the next generation of semiconductor fabrication.