The resolution of conventional optical lens systems is always hampered by the diffraction limit. Recent developments in artificial metamaterials provide new avenues to build hyperlenses and metalenses that are able to image beyond the diffraction limit. Hyperlenses project super-resolution information to the far field through a magnification mechanism, whereas metalenses not only super-resolve subwavelength details but also enable optical Fourier transforms. Recently, there have been numerous designs for hyperlenses and metalenses, bringing fresh theoretical and experimental advances, though future directions and challenges remain to be overcome.

/pdf/hyperlenses-and-metalenses-for-far-field-super-resolution-3h8v1zh6s8.pdf

Hyperlenses and metalenses for far-field super-resolution imaging

Surface plasmons (SPs) are electromagnetic excitations existing at the interface between a metal and a dielectric material. Control and manipulation of light based on SPs at the nanometer scale offers significant advantages in nanophotonic devices with very small elements, since the peculiar properties of SPs can be tailored by construction of nanostructures with various interfaces between metals and dielectric materials. Recent progress in nanostructures for SPs is reviewed. Resonance frequencies or wavelengths of SPs can be tuned by design of metal nanostructures, such as nanoparticles, nanorods, nanowires, nanosheets, and nanodisks. Moreover, SP resonance modes can also be tuned by control of the shapes and sizes of nanostructures, where the resonance modes include longitudinal and transversal resonances, dipolar and multipolar resonances, and Fano resonances. Based on SP coupling for metal nanostructures, metal–semiconductor nanostructures, metal–dielectric nanostructures, and metal–polymer nanostructures, propagating and guiding of SP can be achieved through the metal nanostructures and the hybrid structures. Additionally, metal nanostructures exhibit remarkable field enhancement effects (e.g., local near-field enhancement, and optical transmission enhancement) due to SP coupling. Furthermore, SP nanostructures perform unique focusing and imaging characteristics at the nanometer scale beyond the diffraction limit. Tailoring SPs by control of the nanostructures is expected to be used for design and development of high-performance optical components and circuits, which offer both potential and challenges for new generations of nanophotonic devices.

Nanostructures for surface plasmons

Subwavelength imaging by metamaterials and extended work to pursue total transmission has been successfully demonstrated with electromagnetic and acoustic waves very recently. However, no elastic counterpart has been reported because earlier attempts suffer from considerable loss. Here, for the first time, we realize an elastic hyperbolic metamaterial lens and experimentally show total transmission subwavelength imaging with measured wave field inside the metamaterial lens. The main idea is to compensate for the decreased impedance in the perforated elastic metamaterial by utilizing extreme stiffness, which has not been independently actualized in a continuum elastic medium so far. The fabricated elastic lens is capable of directly transferring subwavelength information from the input to the output boundary. In the experiment, this intriguing phenomenon is confirmed by scanning the elastic structures inside the lens with laser scanning vibrometer. The proposed elastic metamaterial lens will bring forth significant guidelines for ultrasonic imaging techniques.

/pdf/extreme-stiffness-hyperbolic-elastic-metamaterial-for-total-k2o7n8x4j9.pdf

Extreme stiffness hyperbolic elastic metamaterial for total transmission subwavelength imaging

Dyadic Green Functions

We optimize the effective skin-depth and resolution of Ag-TiO2, Ag-SrTiO3, and Ag-GaP multilayers for imaging with sub-wavelength resolution. In terms of transmission and resolution, the optimized multilayers outperform simple designs based on combined use of effective medium theory, impedance matching and Fabry–Perot resonances. For instance, an optimized Ag-GaP multilayer consisting of only 17 layers, operating at the wavelength of 490 nm and having a total thickness equal to one wavelength, combines 78% intensity transmission with a resolution of 60 nm. It is also shown that use of the effective medium theory leads to sub-optimal multilayer designs with respect to the trade-off between the skin depth and resolution already when the period of the structure is on the order of 40 nm or larger.

Optimized low-loss multilayers for imaging with sub-wavelength resolution in the visible wavelength range

Metamaterials with isotropic property have been shown to possess novel optical properties such as a negative refractive index that can be used to design a superlens. Recently, it was shown that metamaterials with anisotropic property can translate the high-frequency wave vector k values from evanescence to propagating. However, electromagnetic waves traveling in single-layer anisotropic metamaterial produce diverging waves of different spatial frequency. In this work, it is shown that, using bilayer metamaterials that have complementary anisotropic property, the diverging waves are recombined to produce a subwavelength image, i.e., a superlens device can be designed. The simulation further shows that the design can be achieved using a metal/oxide multilayer, and a resolution of 30 nm can be easily obtained in the optical frequency range.

Superlens from complementary anisotropic metamaterials

In this work, we proposed a resonant half-cylindrical far field superlens by combining the strong hyperbolic dispersion relationship with the coupled Fabry–Perot cavities. The half-cylindrical far field superlens consisting of the resonant and nonresonant Ag/GaP multilayer are simulated and compared under red light illumination. The calculated results reveal that the resonant far field superlens with coupled Fabry–Perot cavities has much higher transmission efficiency than that of the nonresonant far field superlens and maintains a high resolution.

F. Y. Wang

Papers

Superlens from complementary anisotropic metamaterials

Half-cylindrical far field superlens with coupled Fabry-Perot cavities