Deep sub-wavelength nanofocusing of UV-visible light by hyperbolic metamaterials

Hyperlens for capturing sub-diffraction nanoscale single molecule dynamics

Hyperlenses offer an appealing opportunity to unlock bioimaging beyond the diffraction limit with conventional optics. Mapping hidden nanoscale spatiotemporal heterogeneities of lipid interactions in live cell membrane structures has been accessible only using optical super-resolution techniques. Here, we employ a spherical gold/silicon multilayered hyperlens that enables sub-diffraction fluorescence correlation spectroscopy at 635 nm excitation wavelength. The proposed hyperlens enables nanoscale focusing of a Gaussian diffraction-limited beam below 40 nm. Despite the pronounced propagation losses, we quantify energy localization in the hyperlens inner surface to determine fluorescence correlation spectroscopy (FCS) feasibility depending on hyperlens resolution and sub-diffraction field of view. We simulate the diffusion FCS correlation function and demonstrate the reduction of diffusion time of fluorescent molecules up to nearly 2 orders of magnitude as compared to free space excitation. We show that the hyperlens can effectively distinguish nanoscale transient trapping sites in simulated 2D lipid diffusion in cell membranes. Altogether, versatile and fabricable hyperlens platforms display pertinent applicability for the enhanced spatiotemporal resolution to reveal nanoscale biological dynamics of single molecules.

Electrically tunable graphene-based multi-band terahertz metamaterial filters

In this study, we have designed an electrically tunable multi-band terahertz (THz) metamaterial filter based on graphene and multiple-square-loop structures. The structure contains multiple metal square loops, and these loops with different sizes correspond to different THz frequencies, achieving our expected efficacy of a multiband wave filter. Furthermore, by sweeping external voltages, we could change graphene's Fermi levels, and thus the high-sensitivity THz filter's capability from single-band to multi-band filtering can be modulated. We expect that this study of a hybrid THz wave filter would be promising for the development of selecting channels in THz and 6 G communications.

Tunable Planar Focusing Based on Hyperbolic Phonon Polaritons in α-MoO3

Manipulation of the propagation and energy-transport characteristics of subwavelength infrared (IR) light fields is critical for the application of nanophotonic devices in photocatalysis, biosensing, and thermal management. In this context, metamaterials are useful composite materials, although traditional metal-based structures are constrained by their weak mid-IR response, while their associated capabilities for optical propagation and focusing are limited by the size of attainable artificial optical structures and the poor performance of the available active means of control. Herein, a tunable planar focusing device operating in the mid-IR region is reported by exploiting highly oriented in-plane hyperbolic phonon polaritons in α-MoO₃ . Specifically, an unprecedented change of effective focal length of polariton waves from 0.7 to 7.4 μm is demonstrated by the following three different means of control: the dimension of the device, the employed light frequency, and engineering of phonon-plasmon hybridization. The high confinement characteristics of phonon polaritons in α-MoO₃ permit the focal length and focal spot size to be reduced to 1/15 and 1/33 of the incident wavelength, respectively. In particular, the anisotropic phonon polaritons supported in α-MoO₃ are combined with tunable surface-plasmon polaritons in graphene to realize in situ and dynamical control of the focusing performance, thus paving the way for phonon-polariton-based planar nanophotonic applications.

Mechanism of emitters coupled with a polymer-based hyperbolic metamaterial

We study a polymer-based hyperbolic metamaterial (HMM) structure composed of three Au-polymer bilayers with a hyperbolic dispersion relation. Using an effective refractive index retrieval algorithm, we obtain the effective permittivity of the experimentally fabricated polymer-based structure. In particular, the unique polymer-based HMM shows the existence of high-k modes that propagate in the metal-dielectric multilayered structure due to the excitation of bulk plasmon-polaritonic modes. Moreover, we compare the experimental luminescence and fluorescence lifetime results of the multilayered Au and a dye-doped polymer (PMMA) to investigate the dynamics of three different emitters, each incorporated within the unique polymer-based HMM structure. With emitters closer to the epsilon-near-zero region of the HMM, we observed a relatively high shortening of the average lifetime as compared to other emitters either close or far from the epsilon-near-zero region. This served as evidence of coupling between the emitters and the HMM as well as confirmed the increase in the non-radiative recombination rate of the different emitters. We also show that the metallic losses of a passive polymer-based HMM can be greatly compensated by a gain material with an emission wavelength close to the epsilon-near-zero region of the HMM. These results demonstrate the unique potential of an active polymer-based hyperbolic metamaterial in loss compensation, quantum applications, and sub-wavelength imaging techniques.

Strong Coupling in Semiconductor Hyperbolic Metamaterials

Semiconductor-based layered hyperbolic metamaterials (HMMs) house high-wavevector volume plasmon polariton (VPP) modes in the infrared spectral range. VPP modes have successfully been exploited in the weak-coupling regime through the enhanced Purcell effect. In this paper, we experimentally demonstrate strong coupling between the VPP modes in a semiconductor HMM and the intersubband transition of epitaxially embedded quantum wells. We observe clear anticrossings in the dispersion curves for the zeroth-, first-, second-, and third-order VPP modes, resulting in upper and lower polariton branches for each mode. This demonstration sets the stage for the creation of novel infrared optoelectronic structures combining HMMs with embedded epitaxial emitter or detector structures.

Mechanically stretchable metamaterial with tunable mid-infrared optical properties

Over the past decade, tremendous efforts have been devoted to the design of metamaterials with ultrahigh absorption. These perfect absorbers can realize the annihilation of incident electromagnetic waves by eliminating reflection and transmission of microwaves, infrared, visible, and ultraviolet. However, the optical properties are usually unchanged due to a rigid structure. In this work, we propose a mechanically stretchable metamaterial composed of polydimethylsiloxane and gold with tunable optical properties in the mid-infrared region. A large variation of absorptances with different gold filling ratios is demonstrated as well as the corresponding electric field distributions. Under moderate uniaxial and biaxial tensions, the proposed two-dimensional grating structure has achieved a dynamic tuning of infrared thermal properties, including a sharp reflectance-absorptance switch. This mechanically stretchable metamaterial can serve different optical and sensing functions due to its facile tunability.

Angular selection of transmitted light and enhanced spontaneous emission in grating-coupled hyperbolic metamaterials

We propose dielectric grating-coupled hyperbolic metamaterials as a functional device that shows angular selection of transmitted light and enhanced radiative emission rate. We numerically demonstrate that the surface plasmon polaritons in the hyperbolic metamaterials can be effectively outcoupled to the surrounding space by using gratings and facilitate control of the light transmission in the visible frequency. We confirm that the high density of states and the effect of outcoupled plasmonic modes of the proposed structure lead to the increase of Purcell factor and radiative emission. This work will provide multifunctionalities in sensing and imaging systems that use hyperbolic metamaterials.

Planar cascaded triangular hyperlens structures

Planar-ports hyperlens structures made of two or four cascaded triangular cuts of planar periodic structures are presented. The hyperlenses are capable of converting electromagnetic evanescent fields to propagating waves, featuring subwavelength resolution and image magnification. One of the two proposed structures features parallel input and output ports. The proposed structures improve the phase and amplitude unbalances, magnification, and resolution of previous planar hyperlenses. Compared to several previous cylindrical hyperlens structures, the proposed structures show competitive or better features. One of the best designs of the proposed structures offers a magnification of 3.86, a resolution of 45 nm, $\lambda /{8}$λ/8 at the free space wavelength of 365 nm, optical modulation, a measure of image contrast of 0.286, and amplitude unbalance, a measure of image quality of 0.08, the smallest among all previous structures. Detailed comparative data of performance are provided. Although the magnification of the proposed planar structures is somewhat smaller than some of the previous cylindrical ones, the performance of the proposed hyperlenses is better or competitive with respect to resolution and image quality.

Deep plasmonic direct writing lithography with ENZ metamaterials and nanoantenna

In this article, we demonstrate a specially designed resonant metamaterial with epsilon-near-zero (ENZ) and nanoantenna to enhance the exposure depth in plasmonic direct writing lithography more than 10 times. The ENZ metamaterial composed of a Ag/Si₃N₄ multilayer thin film, converts the evanescent field generated by the bowtie aperture nanoantenna to propagating waves with low divergence and high collimation. Deep sub-diffraction limited resolution of less than 65 nm (λ/7) with exposure depth greater than 100 nm is achieved. This work brings plasmonic direct writing lithography one step closer to practical applications.

Realization of broadband negative refraction in visible range using vertically stacked hyperbolic metamaterials

Negative refraction has generated much interest recently with its unprecedented optical phenomenon. However, a broadband negative refraction has been challenging because they mainly involve optical resonances. This paper reports the realization of broadband negative refraction in the visible spectrum by using vertically-stacked metal-dielectric multilayer structures. Such structure exploits the characteristics of the constituent metal and dielectric materials, and does not require resonance to achieve negative refraction. Broadband negative refraction (wavelength 270–1300 nm) is numerically demonstrated. Compared to conventional horizontally-stacked multilayer structures, the vertically-stacked multilayer structure has a broader range of working wavelength in the visible range, with higher transmittance. We also report a variety of material combinations with broad working wavelength. The broadband negative refraction metamaterial provides an effective way to manipulate light and may have applications in super-resolution imaging, and invisibility cloaks.

Recent Advances in Tunable and Reconfigurable Metamaterials

Metamaterials are composed of nanostructures, called artificial atoms, which can give metamaterials extraordinary properties that cannot be found in natural materials. The nanostructures themselves and their arrangements determine the metamaterials’ properties. However, a conventional metamaterial has fixed properties in general, which limit their use. Thus, real-world applications of metamaterials require the development of tunability. This paper reviews studies that realized tunable and reconfigurable metamaterials that are categorized by the mechanisms that cause the change: inducing temperature changes, illuminating light, inducing mechanical deformation, and applying electromagnetic fields. We then provide the advantages and disadvantages of each mechanism and explain the results or effects of tuning. We also introduce studies that overcome the disadvantages or strengthen the advantages of each classified tunable metamaterial.

Demonstration of a Hyperlens-integrated Microscope and Super-resolution Imaging

The use of super-resolution imaging to overcome the diffraction limit of conventional microscopy has attracted the interest of researchers in biology and nanotechnology. Although near-field scanning microscopy and superlenses have improved the resolution in the near-field region, far-field imaging in real-time remains a significant challenge. Recently, the hyperlens, which magnifies and converts evanescent waves into propagating waves, has emerged as a novel approach to far-field imaging. Here, we report the fabrication of a spherical hyperlens composed of alternating silver (Ag) and titanium oxide (TiO2) thin layers. Unlike a conventional cylindrical hyperlens, the spherical hyperlens allows for two-dimensional magnification. Thus, incorporation into conventional microscopy is straightforward. A new optical system integrated with the hyperlens is proposed, allowing for a sub-wavelength image to be obtained in the far-field region in real time. In this study, the fabrication and imaging setup methods are explained in detail. This work also describes the accessibility and possibility of the hyperlens, as well as practical applications of real-time imaging in living cells, which can lead to a revolution in biology and nanotechnology.

Selective dual-band metamaterial perfect absorber for infrared stealth technology

We propose a dual-band metamaterial perfect absorber with a metal-insulator-metal structure (MIM) for use in infrared (IR) stealth technology. We designed the MIM structure to have surface plasmon polariton (SPP) and magnetic polariton (MP) resonance peaks at 1.54 μm and 6.2 μm, respectively. One peak suppresses the scattering signals used by laser-guided missiles, and the other matches the atmospheric absorption band, thereby enabling the suppression of long-wavelength IR (LWIR) and mid-wavelength IR (MWIR) signals from objects as they propagate through the air. We analysed the spectral properties of the resonance peaks by comparing the wavelength of the MP peak calculated using the finite-difference time-domain method with that obtained by utilizing an inductor-capacitor circuit model. We evaluated the dependence of the performance of the dual-band metamaterial perfect absorber on the incident angle of light at the surface. The proposed absorber was able to reduce the scattering of 1.54 μm IR laser light by more than 90% and suppress the MWIR and LWIR signatures by more than 92%, as well as maintain MWIR and LWIR signal reduction rates greater than 90% across a wide temperature range from room temperature to 500 °C.

Demonstration of nanoimprinted hyperlens array for high-throughput sub-diffraction imaging

Overcoming the resolution limit of conventional optics is regarded as the most important issue in optical imaging science and technology. Although hyperlenses, super-resolution imaging devices based on highly anisotropic dispersion relations that allow the access of high-wavevector components, have recently achieved far-field sub-diffraction imaging in real-time, the previously demonstrated devices have suffered from the extreme difficulties of both the fabrication process and the non-artificial objects placement. This results in restrictions on the practical applications of the hyperlens devices. While implementing large-scale hyperlens arrays in conventional microscopy is desirable to solve such issues, it has not been feasible to fabricate such large-scale hyperlens array with the previously used nanofabrication methods. Here, we suggest a scalable and reliable fabrication process of a large-scale hyperlens device based on direct pattern transfer techniques. We fabricate a 5 cm × 5 cm size hyperlenses array and experimentally demonstrate that it can resolve sub-diffraction features down to 160 nm under 410 nm wavelength visible light. The array-based hyperlens device will provide a simple solution for much more practical far-field and real-time super-resolution imaging which can be widely used in optics, biology, medical science, nanotechnology and other closely related interdisciplinary fields.