A planar ultraviolet objective lens for optical axis free imaging nanolithography by employing optical negative refraction.

Lithography is one of the most key technologies for integrated circuit (IC) manufacturing and micro/nano-functional device fabrication, while the imaging objective lens plays one important role. Due to the curved surface of the conventional objective lens, the imaging field of view is limited and the objective lens system is complex. In this paper, a planar objective lens based on the optical negative refraction principle is demonstrated for achieving optical axis free and long depth of focus imaging nanolithography. Through employing a hyperbolic metamaterial composed of silver/titanium dioxide multilayers, plasmonic waveguide modes could be generated in multilayers, which results in optical negative refraction and then flat imaging at ultraviolet wavelength. The corresponding imaging characteristics are investigated in simulation and experiment. At the I-line wavelength of 365 nm, the highest imaging resolution of 165 nm could be realized in the 100 nm photoresist layer under the working gap of 100 nm between the objective lens and substrate. Moreover, this planar objective lens has good ability for cross-scale and two-dimensional imaging lithography, and is similar to a conventional projection objective lens. It is believed that this kind of planar objective lens will provide a promising avenue for low-cost nanofabrication scenarios in the near future.

Enlarging focal depth using epsilon-near-zero metamaterial for plasmonic lithography.

Plasmonic lithography can utilize evanescent waves to produce subdiffraction patterns. However, the high loss and shallow depth of patterns severely obstruct its application in practice. In this work, a large focal depth is achieved for deep subwavelength lithography. It is accomplished by employing radially polarized light to excite surface plasmons on a concentric annular grating and combining designed epsilon-near-zero metamaterial to select a high spatial frequency mode, which can shape an evanescent Bessel beam in a photoresist (PR). Moreover, the intensity distribution of the subdiffraction beam can be further enhanced and uniformized by adding reflective layers. It is shown that a needle-like beam with a focal depth of over 500 nm (1.23λ) is formed in the PR layer, and the full width at half maximum of the beam is widened from only 80 nm (0.2λ) to 94 nm (0.23λ). The analyses indicate that this design is applicable for direct writing lithography to produce super-resolution patterns with small feature size, high aspect ratio, and strong field intensity.

Plasmonic nano-tweezer based on square nanoplate tetramers.

The research fields of trapping nanoparticles have experienced a huge development in recent years, which mainly benefits from the unique field enhancement in plasmonic nanomaterials. Since the large field enhancement originates from the excited localized surface plasmon at the metal surface, exploring novel metal nanostructures with high trapping efficiency is always the main goal in this field. In this work, the plasmonic trapping of nanoparticles based on the gold periodic square tetramers (PST) was investigated through full-wave simulations using the finite-difference time-domain (FDTD) method. The electric field and surface charge distributions on the surface of PST indicate that both the trapping position and efficiency are influenced by orientations of the square nanoplates. The maximum electromagnetic enhancement is achieved when all square nanoplates rotate 45° along the z axis. Therefore, the gradient force and trapping potential of this PST with optimal orientation were further studied, and the results indicate that a dielectric nanoparticle of 15 nm radius can be stably captured. Furthermore, the calculation results show that the plasmonic trapping with this PST exhibits strong polarization dependence. It is easy to change the trapping position and the field intensity by tuning the polarization of the incident wave. Our work enables a deeper understanding of this kind of plasmonic trapping and could have potential applications in biomedical research and life science.

Optimization-free approach for generating sub-diffraction quasi-non-diffracting beams.

Sub-diffraction quasi-non-diffracting beams with sub-wavelength transverse size are attractive for applications such as optical nano-manipulation, optical nano-fabrication, optical high-density storage, and optical super-resolution microscopy. In this paper, we proposed an optimization-free design approach and demonstrated the possibility of generating sub-diffraction quasi-non-diffracting beams with sub-wavelength size for different polarizations by a binary-phase Fresnel planar lens. More importantly, the optimization-free method significantly simplifies the design procedure and the generation of sub-diffracting quasi-non-diffracting beams. Utilizing the concept of normalized angular spectrum compression, for wavelength λ0 = 632.8 nm, a binary-phase Fresnel planar lens was designed and fabricated. The experimental results show that the sub-diffraction transverse size and the non-diffracting propagation distances are 0.40λ0-0.54λ0 and 90λ0, 0.43λ0-0.54λ0 and 73λ0, and 0.34λ0-0.41λ0 and 80λ0 for the generated quasi-non-diffracting beams with circular, longitudinal, and azimuthal polarizations, respectively.

Generating a three-dimensional hollow spot with sub-diffraction transverse size by a focused cylindrical vector wave.

A three-dimensional (3D) hollow spot is of great interest for a wide variety of applications such as microscopy, lithography, data storage, optical manipulation, and optical manufacturing. Based on conventional high-numerical-aperture objective lenses, various methods have been proposed for the generation of 3D hollow spots for different polarizations. However, conventional optics are bulky, costly, and difficult to integrate. More importantly, they are diffraction-limited in nature. Owing to their unique properties of small size, light weight, and ease of integration, planar lenses have become attractive as components in the development of novel optical devices. Utilizing the concept of super-oscillation, planar lenses have already shown great potential in the generation of sub-diffraction, or even of super-oscillatory features, in propagating optical waves. In this paper, we propose a binary-phase planar lens with an ultra-long focal length (300λ) for the generation of a 3D hollow spot with a cylindrical vector wave. In addition, we experimentally demonstrate the formation of such a hollow spot with a sub-diffraction transverse size of 0.546λ (smaller than the diffraction limit of 0.5λ/NA, where NA denotes the lens numerical aperture) and a longitudinal size of 1.585λ. The ratio of central minimum intensity to the central ring peak intensity is less than 3.7%. Such a planar lens provides a promising way to achieve tight 3D optical confinement for different uses that might find applications in super-resolution microscopy, nano-lithography, high-density data storage, nano-particle optical manipulation, and nano-optical manufacturing.