Multifocal metalens with a controllable intensity ratio

Axially multifocal metalens for 3D volumetric photoacoustic imaging of neuromelanin in live brain organoid

Optical resolution photoacoustic imaging of uneven samples without z-scanning is transformative for the fast analysis and diagnosis of diseases. However, current approaches to elongate the depth of field (DOF) typically imply cumbersome postprocessing procedures, bulky optical element ensembles, or substantial excitation beam side lobes. Metasurface technology allows for the phase modulation of light and the miniaturization of imaging systems to wavelength-size thickness. Here, we propose a metalens composed of submicrometer-thick titanium oxide nanopillars, which generates an elongated beam of diffraction-limited diameter with an aspect ratio of 286 and a uniform intensity throughout the DOF. The metalens enhances visualization of phantom samples with tilted surfaces compared to conventional lenses. Moreover, the volumetric imaging of neuromelanin is facilitated for depths of up to 500 micrometers within the human midbrain and forebrain organoids that are 3D biological models of human brain regions. This approach provides a miniaturized platform for neurodegenerative disease diagnosis and drug discovery.

Tight focusing of fractional-order topological charge vector beams by cascading metamaterials and metalens

Vector beams have attracted widespread attention because of their unique optical properties; in particular, their combination with tight focusing can produce many interesting phenomena. The rise of 3D printing technology provides more possibilities for exploration. In this work, a cascading method involving a metamaterial and a metalens is used to generate a tightly focused field of vector beams in the terahertz band, which is prepared via 3D printing. As a proof-of-concept demonstration, a series of metamaterial modules capable of generating states of different orbital angular momentum are proposed by cascading with a metalens. The experimental results are in good agreement with the simulation results, fully verifying the feasibility of the scheme. The proposed design and fabrication strategy provides a new idea for the tight focusing of terahertz vector beams.

Bifocal lenses with adjustable intensities enabled by bilayer liquid crystal structures

In this paper, we propose bifocal lenses based on bilayer structures composed of a liquid crystal (LC) cell and LC polymer, and the relative intensity of two foci can be adjusted arbitrarily through applying an external voltage. Two LC layers have different light modulation functions: when circularly polarized light passes through the first layer, part of the outgoing light is converted with PB phase modulation and another part is not converted; followed by the second layer, PB modulation of these two parts would be simultaneously realized but with opposite signs; thus the transmitted left- and right-handed circularly polarized (LCP and RCP) light can be independently controlled. As proof-of-concept examples, longitudinal and transverse bifocal lenses are designed to split an incident LCP light into two convergent beams with orthogonal helicity, and the position of the two foci can be flexibly arranged. Benefitting from the electrically controlled polarization conversion efficiency (PCE) of the LC cell, the relative intensity of the two foci can be adjusted arbitrarily. Experimental results agree well with theoretical calculations. Besides, a broadband polarization and an edge imaging system based on the proposed bifocal LC lenses have also been demonstrated. This paper presents a simple method to design a functional multilayer LC device and the proposed bifocal lenses may have potentials in the optical interconnection, biological imaging, and optical computing.

Metasurface-based circular polarizer with a controllable phase and its application in holographic imaging

A diatomic circular polarizer based on a single-layered metasurface is proposed. This metasurface circular polarizer carries the controllable phase besides the desired circular dichroism, which is different from the existing circular polarizers. The diatoms contain two nanoholes equivalent to half-wave plates with a specified cross angle and a fixed phase difference. The alternative circular polarization transmission of this circular polarizer depends on the relative angular position of diatoms, and the controllable phase of this circular polarizer can be adjusted through rotating nanoholes. The generation of the optical vortex and holographic imaging verifies the polarization and phase manipulation of the diatomic circular polarizer. The numerical simulations and the experimental measurement give the powerful verification. Simple design, compact structure, and poly-functionality enable the wide applications of circular polarizer in integrated and polarized optics.

Circular-target-style bifocal zoom metalens

Optical zoom plays an important role in realizing high-quality image magnification, especially in photography, telescopes, microscopes, etc. Compared to traditional bulky zoom lenses, the high versatility and flexibility of metalens design provide opportunities for modern electronic and photonic systems with demands for miniature and lightweight optical zoom. Here, we propose an ultra-thin, lightweight and compact bifocal zoom metalens, which consists of a conventional circular sub-aperture and a sparse annular sub-aperture with different focal lengths. The imaging resolutions of such single zoom metalens with 164 lp/mm and 117 lp/mm at magnifications of 1× and 2× have been numerically and experimentally demonstrated, respectively. Furthermore, clear zoom images of a dragonfly wing pattern have been also achieved using this zoom metalens, showing its distinctive aspect in biological imaging. Our results provide an approach for potential applications in integrated optical systems, miniaturized imaging devices, and wearable devices.

Scattered beam control of encoded metasurface based on near-field coupling effects of elements

The coupling effect between the element structures of the traditional Huygens metasurface is easy to cause the efficiency of the designed functional devices to be reduced. In order to eliminate or reduce the coupling effect between the element structures, a border-type Huygens metasurface element structure is proposed. In order to confirm that the bounding element structure can significantly reduce the coupling effect, the near-field distribution and far-field properties of two Huygens metasurfaces with and without bounding are compared. Through comparative analysis, we find that the bounding Huygens element structure can significantly reduce the coupling effect between the element structures, and the far-field scattering angle is more consistent with the theoretical calculation value. In order to realize the free regulation of the far-field scattering angle of THz waves, we introduce the Fourier convolution principle in digital signal processing, and operate the element sequence of Huygens metasurface on the addition principle to realize the free regulation of scattered beams. In addition, we performed functional addition operations on the bounding and unbounding coding sequences. The bounding code structure can accurately achieve the synthesis of functions.

Tight focusing field of cylindrical vector beams based on cascaded low-refractive index metamaterials

Using low-refractive-index metamaterials, we design a transmission-type radial-angular cylindrical vector beam generator. A high numerical aperture lens is constructed using an asymmetric meta-grating structure. The metamaterial vector beam generator and the meta-grating lens are physically cascaded to obtain the tight focusing characteristics of the vector light field. The vector beam generator module and the meta-lens module are prepared by 3D printing technology, and the near-field test has been carried out on the samples in the terahertz band. Using the physical cascading method, two modules are cascaded to construct a vector beam tight focusing device, and the focusing electric field distribution test has been carried out. The use of 3D printing technology for sample preparation further reduces the manufacturing difficulty and production cost, and ensures the realization of its design function on the basis of miniaturization and light weight, which provides the possibility for the research of tight focusing field regulation in the terahertz band.

Dual-band achromatic metalens-assisted grating couplers for wavelength demultiplexing

The design of grating couplers (GCs) that can (de)multiplex and couple arbitrarily defined spatial light into photonic devices is crucial for miniaturized integrated chips. However, traditional GCs have a limited optical bandwidth due to their wavelength's dependency on the coupling angle. In this paper, we propose a device that addresses this limitation by combining a dual-broadband achromatic metalens (ML) with two focusing GCs. By controlling the frequency dispersion, the waveguide-mode-based ML achieves excellent dual-broadband achromatic convergence and separates broadband spatial light into opposing directions at normal incidence. The focused and separated light field matches the grating diffractive mode field and is then coupled into two waveguides by the GCs. This ML-assisted GCs device exhibits a good broadband property with -3 dB bandwidths of 80 nm at 1.31 µm (CE ∼ -6 dB) and 85 nm at 1.51 µm (CE ∼ -5 dB), which almost covers the entire designed working bands, representing an improvement over traditional spatial light-GC coupling. This device can be integrated into optical transceivers and dual-band photodetectors to enhance the bandwidth of wavelength (de)multiplexing.

Broadband stealth devices based on encoded metamaterials

Based on the generalized Snell's law, the relationship between the phase gradient of the metasurface and the incident frequency is demonstrated, and the principle of the achromatic metasurface is developed. By adjusting the phase gradient and linear dispersion simultaneously, the function of achromatic aberration is realized, and the influence of chromatic aberration on the metasurface is reduced. We propose a metasurface stealth device with achromatic multilayer frame metasurfaces with beam deflection, steering, and collection functions so that the incident electromagnetic beam is transmitted around the stealth object without scattering. In the range of 0.45-0.9 THz, the stealth function can be achieved. We have shown that the achromatic principle, design method, and stealth structure provide a guide for achieving transmissive cloaking.

Multi-Function Reflective Vector Light Fields Generated by All-Dielectric Encoding Metasurface

Traditional optics usually studies the uniform polarization state of light. Compared with uniform vector beams, non-uniform vector beams have more polarization information. Most of the research on generating cylindrical vector beams using metasurfaces focuses on generating transmitted beams using the geometric phase. However, the geometric phase requires the incident light to be circularly polarized, which limits the design freedom. Here, an all-dielectric reflective metasurface is designed to generate different output light according to the different polarization states of the incident light. By combining the two encoding arrangements of the dynamic phase and the geometric phase, the output light is a radial vector beam when the linearly polarized light is incident along the x-direction. Under the incidence of linearly polarized light along the y-direction, the generated output light is an azimuthal vector beam. Under the incidence of left-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of -1. Under the incidence of right-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of +1. The proposed reflective metasurface has potential applications in generating vector beams with high integration.

Largest aperture metalens of high numerical aperture and polarization independence for long-wavelength infrared imaging

Because of unique superiorities of planar optical devices based on metalens on manipulating amplitude, phase, polarization, wavelength of incident light, metalenses have great prospects to replace traditional catadioptric optical components, especially in imaging and optoelectronic integration. However, the research of metalens has focused on visible or near-infrared wavelength in the past few years and little attention was paid to the long-wavelength infrared metalens. Here, we demonstrate the largest aperture, high numerical aperture, and polarization-independent metalens operating at long-wavelength infrared. The metalens has a numerical aperture of 0.45 at the center wavelength of10 µm. The aperture of the metalens is 80 mmwhich is much larger than the existing level we know. It has high-resolution imaging ability with focusing incident light down to a spot as small as ∼1.04λ. Ambient light imaging experiments are carried out to show the performance of the metalens. In addition, metalens is flimsy, large-scale and low-cost, which provides an effective solution for the development of ultra-lightweight and compact optical devices for LWIR technology.

Manipulation force analysis of nanoparticles with ultra-high numerical aperture metalens

Metalens optical tweezers technology has several advantages for manipulating micro-nano particles and high integration. Here, we used particle swarm optimization (PSO) to design a novel metalens tweezer, which can get 3-dimensional trapping of particles. The numerical aperture (NA) of the metalens can reach 0.97 and the average focusing efficiency is 44%. Subsequently, we analyzed the optical force characteristics of SiO₂ particles with a radius of 350 nm at the focal point of the achromatic metalens. We found the average maximum force of SiO₂ particles in the x-direction and z-direction to be 0.88 pN and 0.72 pN, respectively. Compared with the dispersive metalens, it is beneficial in maintaining the constant of optical force, the motion state of trapped particles, and the stability of the trapping position.

Improvement of beam shaping on a metasurface by eliminating the interaction between coding units

The interaction between subwavelength elements must be considered when constructing a metasurface. Generally, the interaction between cell structures is ignored when metasurface optoelectronic devices are designed, which results in a significant decrease in the design performance and efficiency of the overall metasurface structure. To reduce or further eliminate the interaction between cell structures, we propose a cell structure with borders to construct coded metasurface sequences. At the same time, we design a common frameless cell structure to construct a traditional coding metasurface. By numerical simulation of the near-field distribution and far-field scattering characteristics of these two types of coded metasurface sequences, we find that the element structure with a medium frame can attenuate the interaction between adjacent encoded particles. In the process of transmission on the encoding metasurface with a frame, different encoded particles can independently express their transmission phase and are not affected by adjacent structures, thus realizing a low coupling coding metasurface.

Transmission terahertz power beam splitter based on a single-layer metal metasurface

A periodic metasurface composed of a single layer of copper structure is proposed. The general transmission power beam splitter is composed of a multilayer structure, which is difficult to fabricate. The proposed single-layer terahertz wave power beam splitter contains only a single-layer circular hole cell structure, and it can control the transmission angle by controlling the arrangement mode of the coding cells. At the same time, we can control the transmission angle and the transmitted energy distribution of each beam based on different incident angles. A simple monolayer round-hole metasurface was prepared and its transmission characteristics were analyzed based on a terahertz time domain spectrometer. Compared with traditional splitter devices, our coding metasurface beam splitters with a single layer have the potential to promote the development of integrated optical systems.

Dual-Wavelength Polarization-Dependent Bifocal Metalens for Achromatic Optical Imaging Based on Holographic Principle

Recently, ultrathin metalenses have attracted dramatically growing interest in optical imaging systems due to the flexible control of light at the nanoscale. In this paper, we propose a dual-wavelength achromatic metalens that will generate one or two foci according to the polarization of the incident. Based on geometric phase modulation, two unit cells are attentively selected for efficient operation at distinct wavelengths. By patterning them to two divided sections of the metalens structure plane, the dual-wavelength achromatic focusing effect with the same focal length is realized. In addition, the holographic concept is adopted for polarization-dependent bifocal generation, in which the objective wave is originated from two foci that are respectively formed by two orthogonal polarization states of circularly polarized light, namely Left-handed circularly polarized (LCP) light and Right-handed circularly polarized (RCP) light. The incident light is considered as the reference light. The achromatic focusing and polarization-dependent bifocusing are numerically verified through simulations. The proposed design opens the path for the combination of multi-wavelength imaging and chiral imaging, which may find potential applications, such as achromatic optical devices and polarization-controlled biomedical molecular imaging systems.

Highly sensitive biosensor based on an all-dielectric asymmetric ring metasurface

We propose an all-dielectric asymmetric ring-cylindrical metasurface. Based on the analysis of transmission characteristics and the calculation of electromagnetic field distribution of the metasurface with this element structure, it is found that the high Q resonance of the ultra-narrowband can be realized when the symmetry of the ring-cylindrical structure is broken. Meanwhile, it is found that the degree of asymmetry of the ring, the refractive index of the material, the radius of the ring, and the substrate have great influence on the Q value and resonant frequency of the metasurface. Our proposed metasurface structure is applied to the detection of biological molecules based on the change in refractive index of biomolecular solutions. The designed metasurface with high sensitivity to detect biomolecules with different refractive indices, the Q value can reach 365.03, and the sensitivity is increased by 90.36 GHz/RIU compared to that without substrate, while the figure of merit value is as high as 100.56, providing label-free detection of biomolecules.

Enhancement of efficiency on the Pancharatnam-Berry geometric phase metalens in the terahertz region

Traditional terahertz lenses face high thickness, low transmittance, difficult processing, and other problems that are not conducive to mass production and integration. Here, we propose a wideband all-dielectric Pancharatnam-Berry geometric phase cell structure to construct a metasurface flat lens. However, when the geometrical phase element structure rotates, the transmission efficiency of the periodic element structure obviously decreases, which will lead to the decrease of the efficiency of the designed flat lens. In order to improve the efficiency, we propose to add a layer of tapered microstructure on the flat substrate to greatly improve the transmission efficiency of the element structure, thus leading to the improvement of the efficiency of the metasurface lens. By comparing the metasurface lens with conical and planar substrates, the metasurfaces with conical structure can greatly improve the transmission efficiency at broadband and wide angle ranges.

Transmission and reflection cloaking by using a zero-refractive-index photonic crystal in the microwave region

We propose a rectangular column two-dimensional square lattice photonic crystal to realize zero refractive index. Through analysis of the energy band structure of the photonic crystal structure, the lattice constant and side length of the rectangular columns can be optimized, and the Dirac cone dispersion appears at the center of the Brillouin zone. The Dirac cone is formed by the interaction of a monopolar eigenstate and a dipolar eigenstate to form a triple accidental degenerate state. The effective medium theory is used to invert the effective electromagnetic parameters of the photonic crystal with a double zero refractive index. The zero-phase change and the focusing characteristic of the concave lens of this kind of zero-refractive-index material are verified. Importantly, we have achieved transmission and reflection cloaking with this zero-index medium. Through the analysis of the amplitude and phase distribution characteristics of the electromagnetic field, it is proved that the designed cloaking devices have obvious cloaking effect.

Performance Analysis of Metalenses Based on Three Kinds of Phase Compensation Techniques

The phase delays introduced by anisotropic nanounits include propagation phase delay, resonant phase delay and geometric phase delay. Various phase devices can be formed based on the metasurfaces consisting of anisotropic nanounits and the phase devices of the same kind function have different performances because of different working modes. In this paper, metalenses and vortex metalenses are chosen as examples to compare the optical performance of metasurface phase devices based on three kinds of phase compensation techniques. We design separately three kinds of metalenses and vortex metalenses using the cross nanoholes, L-shaped nanohole and V-shaped nanoholes and simulate numerically their intensity and phase distributions. Additionally, the results show the differences among these elements in structure complexity, polarization dependence, working efficiency and phase uniformity. The comparison for three kinds of metalenses clearly shows the merits of different phase compensation techniques and this work must be helpful for expanding the practical applications of metasurfaces.

Metasurface circular polarizer based on rotational symmetric nanoholes

A circular polarizer is proposed based on a single layered metasurface. This metasurface circular polarizer is composed of L-shaped nanoholes etched on the silver film. The L-shaped nanoholes are rotational symmetric, and the special symmetric structure determines the polarization selection transmission of the metasurface. The theoretical analysis elaborates the design process of the metasurface circular polarizer. The proposed metasurface circular polarizers have good polarization selective transmittance, and more interestingly, they take on the opposite circular dichroism at different wavebands. The numerical simulations and experiment measurement confirm the circular dichroism of the proposed circular polarizers. The compact design, ultrathin thickness and available performance can expand the applications of the metasurface circular polarizers in the integrated optics.

Liquid crystal bifocal lens with adjustable intensities through polarization controls

In this Letter, transverse and longitudinal liquid crystal bifocal lenses (LCBLs) are proposed to continuously control the relative intensity of two foci through a simple polarization control. The modulation of a LCBL comes from the geometric phase control and is designed through the principle of holography, where the object wave is a light field from two foci respectively formed by the left-circular polarized (LCP) and right-circular polarized (RCP) light, and the reference wave is the incident plane wave. Constructed millimeter-scale LCBLs are verified experimentally, and the foci are precisely formed at the preset plane. Besides, the relative intensity can be easily controlled with different weights of LCP and RCP light. The proposed strategy overcomes the shortcomings of previous bifocal lenses, such as a complex design method, a long optimization time, and an unchangeable relative intensity, and it is expected to find potential applications in parallel optical processing and optical interconnections.

Spin-dependent dual-wavelength multiplexing metalens

The Pancharatnam-Berry (PB) phase is generally utilized to realize a single wavelength spin-dependent function or dual-wavelength functions but operating only in one spin state. A dual-wavelength multifunctional metasurface relying on both spins has been rarely designed due to the rather complicated degrees of freedom to be considered. In this Letter, both dynamic and PB phases are adopted, instead of a pure PB phase, to propose a multiplexing metasurface that can independently and simultaneously manipulate left- and right-handed circularly polarized incidences at dual wavelengths. It is demonstrated experimentally as well as numerically that such spin-dependent dual-wavelength metalenses can make circularly polarized incidences of different wavelengths split into and focus at multi-dimensional positions. Our work demonstrates a new avenue in designing spin-dependent dual-wavelength multifunctional optical devices.

Polarization-Independent Metasurface Lens Based on Binary Phase Fresnel Zone Plate

Based on the binary phase Fresnel zone plate (FZP), a polarization-independent metasurface lens that is able to focus incident light with any polarization state, including circular, linear, and elliptical polarizations, has been proposed and investigated. We demonstrate that the metasurface lens consisting of metal subwavelength slits can operate in a wide bandwidth in the visible range, and has a higher focusing efficiency than that of an amplitude FZP lens without phase modulation. A multi-focus FZP metasurface lens has also been designed and investigated. The proposed lens can provide potential applications in integrated nanophotonic devices without polarization limitations.

All-dielectric bifocal isotropic metalens for a single-shot hologram generation device

Planar metalenses are regarded as promising functional nanodevices because of their lightweight, nano-resolution properties, and, therefore, they can serve as versatile platforms for imaging and Fourier transforming. Here, we demonstrate a meta-device that functions as an isotropic bifocal all-dielectric Huygens' metalens to realize nanoscale real-time coaxial digital hologram generation. We design an isotropic bifocal metalens for micro/nano hologram recording, and the metalens utilizes the complete region compared to a previously reported interleaved multifocal metalens scheme. In addition, the hologram generation does not depend on complex polarization conversion, thereby improving the practical efficiency. For high-fidelity reconstruction, compressive reconstruction is utilized to remove twin-image and zero-order items and to suppress noise. Such concept would be extended to white-light achromatic meta-holography and three-dimensional micro/nano in vivo incoherent super-resolution imaging under subwavelength modulation.