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Tunable mode converter is a key component of channel switching and routing for optical communication system by adopting mode-division multiplexing. Traditional mode converter hardly implements high-order mode conversion and dynamic tunability simultaneously. In this study, we design a tunable mode converter filled with liquid crystal, which can convert fundamental mode into multiple high-order modes (TE0, TE1, and TE2) with a good performance and low intrinsic loss. For this multiple-objective task, we propose an inverse design framework based on the adjoint method. To experimentally prove our design, a tunable mode converter filled with air or water and a mode demultiplexer are fabricated to implement dynamic routing. The experimental results agree well with the simulation and reveal the crosstalk only around −7 dB. With its performance and efficiency, our proposed design flow can be a powerful tool for multifunction device design.

Generating 2D diffraction orders with uniform or tailored intensity distribution is highly desired for various applications including depth perception, parallel laser fabrication and optical tweezer. However, previous strategies lack the abilities to tailor multiple parameters of output light in different diffraction orders simultaneously. While such ability plays an important role in achieving various different functionalities parallelly. Here, we demonstrate a method for encoding arbitrary phase profiles to different diffraction orders with controllable polarization states by applying double-phase method into elaborately designed metasurface. Sixteen independent holograms that generated by GS algorithm are successfully encoded into 4 × 4 uniformly distributed diffraction orders. Hence, the predefined holographic images can be observed at the Fourier plane. Meanwhile, the corresponding polarization states of different orders are manipulated according to their Fourier coefficients. For verifying the polarization state of each holographic image, we calculate the Stokes parameter of each order from measured intensity distributions in the experiment. The proposed method provides an effective way to tailor multiple properties of output diffraction orders. Meanwhile, it may promote the realization of achieving various functionalities parallelly such as spectral-polarization imaging or phase-polarization detection and enhance the capabilities of optical communication systems.

In a wide range of applications such as healthcare treatment, environmental monitoring, food processing and storage, and semiconductor chip manufacturing, relative humidity (RH) sensing is required. However, traditional fiber-optic humidity sensors face the challenges of miniaturization and indirectly obtaining humidity values. Here, we propose and demonstrate an optical barcode technique by cooperating with RH meta-tip, which can predict the humidity values directly. Such RH meta-tip is composed of fiber-optic sensor based on surface plasmon resonance (SPR) effect and graphene oxide film as humidity sensitizer. While SPR sensor is composed of multimode fiber (MMF) integrated with metallic metasurface. Dynamic time warping (DTW) algorithm is used to obtain the warp path distance (WPD) sequence between the measured reflection spectrum and the spectra of the precalibrated database. The distance sequence is transformed into a pseudo-color barcode, and the humidity value is corresponded to the lowest distance, which can be read by human eyes. The RH measurement depends on the collective changes of the reflection spectrum rather than tracking a single specific resonance peak/dip. This work can open up new doors to the development of a humidity sensor with direct RH recognition by human eyes.

New kinds of dispersion elements are required for the minimization of the spectrometers. Metasurfaces offer new methods for a novel type of spectrometers due to their ultra-thin property and great ability to manipulate the electromagnetic field. Here, we propose and demonstrate a spectral modulated metasurface as a miniaturized dispersion element that possesses parabolic phase profile. Different wavelengths of the incident light can be dispersed to different spatial positions due to the accumulation of the dynamic phase varies with the wavelengths from metasurface. Detailed theoretical spectrum dispersion ability is analyzed and experimental demonstration is achieved. The polarization conversion efficiency is high, which is promising to be used in practical applications. Such metasurface provides a new and simple way to design dispersion devices and has the potential to be used in spectrometers, variable filters, spectrum tomography, etc.

In mathematics, general functions can be decomposed into a linear combination of basis functions. This principle can be used for creating an infinite number of distinct geometric patterns based on a finite number of basis patterns. Here, we propose a Dammann vortex metasurface (DVM) for optically generating an array of diverse, diffraction-multiplexed vortex patterns, based on three custom-defined basis patterns. The proposed DVM, with its capability of quantitatively correlating phase and intensity distribution in different diffraction orders, opens up doors for various applications including orbital angular momentum encryptions and quantum entanglement.

As an inherent characteristic of light, polarization plays important roles in information storage, display and even encryption. Metasurfaces, composed of specifically designed subwavelength units in a two-dimensional plane, offer a great convenience for polarization manipulation, yet improving their integrability and broadband fidelity remain significant challenges. Here, based on the combination of various subwavelength cross-nanofins (CNs), a new type of metasurface for multichannel hybrid polarization distribution in near-field is proposed. Sub-wavelength CN units with various waveplate (WP) functionalities, such as frequency-division multiplexing WP, half-WP and quarter-WP are implemented with high efficiency in broadband. High-resolution grayscale image encryption, multi-image storage and rapid polarization detection are demonstrated by encoding the WP pixels into single, double and four channels, respectively. All these applications possess good fidelity in an ultrabroad wavelength band from 1.2 to 1.9 µm, and the high degree of integrability, easy fabrication and multifunction make the CN-shaped WP pixels a promising candidate in optical device miniaturization, quantum applications and imaging technologies.

Metasurfaces have been widely studied for arbitrary manipulation of the amplitude, phase and polarization of a field at the sub-wavelength scale. However, realizing a high efficiency metasurface with simultaneous and independent control of the amplitude and phase in visible remains a challenge. In this work, an ultrathin single-cell dielectric metasurface which can modulate arbitrary complex amplitude in transmission mode is proposed. The amplitude is controlled by adjusting the dipoles and quadrupoles by tuning the geometric size, while the phase is manipulated based on the Pancharatnam-Berry phase by rotating the meta-atom. Complex amplitude fields for generating holographic images and structure light are utilized to verify the reliability of the proposed structure. It has been experimentally demonstrated that the quality of holographic image of complex-amplitude hologram encoded on the proposed metasurface is better than that of phase-only holograms and verified by simulation that complex structure light can be generated by the proposed structure. Our work expands the superior limits of various applications, including arbitrary beam shaping, 3D biological imaging, optical computing, and optics-on-chip devices.

The interactions of photonic spin angular momentum and orbital angular momentum, i.e., the spin-orbit coupling in focused beams, evanescent waves or artificial photonic structures, have attracted intensive investigations for the unusual fundamental phenomena in physics and potential applications in optical and quantum systems. It is of fundamental importance to enhance performance of spin-orbit coupling in optics. Here, we demonstrate a titanium dioxide (TiO2)–based all-dielectric metasurface exhibiting a high efficient control of photonic spin Hall effect (PSHE) in a transmissive configuration. This metasurface can achieve high-efficiency symmetric spin-dependent trajectory propagation due to the spin-dependent Pancharatnam-Berry phase. The as-formed metadevices with high-aspect-ratio TiO2 nanofins are able to realize (86%, measured at 514 nm) and broadband PSHEs in visible regime. Our results provide useful insights on high-efficiency metasurfaces with versatile functionalities in visible regime.

Metasurfaces are planar devices containing delicately designed nanoantenna or resonator arrays that allow for beam shaping, super resolution imaging, and holography. Hybrid metasurface – by integrating with tunable materials such as two-dimensional materials and phase change materials (PCMs) – provides a potential platform for active modulation of wavefronts. Specifically, PCMs can flexibly switch between crystalline and amorphous states with nonvolatile property under external stimuli and provide a large refractive permittivity contrast. Using metasurfaces based on PCM to manipulate wavefronts may provide new opportunities for switchable functionalities. Here, we propose two types of metasurface devices based on whole PCM films to realize switchable holography and simultaneous phase and interference encryption. This feature can be used to encrypt information in a switched state and store camouflage information in the other state by simply applying external thermal stimuli to the entire metasurface. This method can be applied in areas such as beam shaping, optical encryption, and anti-counterfeiting.

The photonic spin Hall effect has attracted considerable research interest due to its potential applications in spin-controlled nanophotonic devices. However, realization of the asymmetrical photonic spin Hall effect with a single optical element is still a challenge due to the conjugation of the Pancharatnam–Berry phase, which reduces the flexibility in various applications. Here, we demonstrate an asymmetrical spin-dependent beam splitter based on a single-layer dielectric metasurface exhibiting strong and controllable optical response. The metasurface consists of an array of dielectric nanofins, where both varying rotation angles and feature sizes of the unit cells are utilized to create high-efficiency dielectric metasurfaces, which enables to break the conjugated characteristic of phase gradient. Thanks to the superiority of the phase modulation ability, when the fabricated metasurface is under normal incidence with a wavelength of 1550 nm, the left-handed circular polarization (LCP) light exhibits an anomalous refraction angle of 28.9°, while the right-handed circular polarization (RCP) light transmits directly. The method we proposed can be used for the flexible manipulation of spin photons and has potentials in high efficiency metasurfaces with versatile functionalities, especially with metasurfaces in a compact space.

Metalenses as miniature flat lenses exhibit a substantial potential in replacing traditional optical component. Although the metalenses have been intensively explored, their functions are limited by poor active ability, narrow operating band and small depth of field (DOF). Here, we show a dielectric metalens consisting of TiO2 nanofins array with ultrahigh aspect ratio to realize active multiband varifocal function. Regulating the orbital angular momentum (OAM) by the phase assignment covering the 2π range, its focal lengths can be switched from 5 mm to 35 mm. This active optical multiplexing uses the physical properties of OAM channels to selectively address and decode the vortex beams. The multiband capability and large DOFs with conversion efficiency of 49% for this metalens are validated for both 532 nm and 633 nm, and the incidence wavelength can further change the focal lengths. This non-mechanical tunable metalens demonstrates the possibility of active varifocal metalenses.

Airy beams are widely used in various optical devices and optical experiments owing to their unique characteristics such as self-acceleration, self-recovery, and non-diffraction. Here we designed and demonstrated a metasurface capable of encoding two phase distributions independently in dual circular polarization channels. We experimentally observed the generated Airy beam arrays loaded on the metasurface in the real and K spaces. Compared with the traditional method, such method provides a more efficient solution to generate large capacity Airy beam arrays with switchable working modes in the broadband spectrum. The results may pave the way for the integration and miniaturization of micro-nano devices and provide a platform for information processing, particle manipulation, space–time optical wave packets, and Airy lasers.

Compared with the traditional holographic technology, metasurface holography is a promising technology due to the large field angle and high spatial resolution. Thanks to the precise control of phase, amplitude, polarization and so on, metasurface holography provides a flexible platform for light modulation, optical encryption and so on. Besides, the process of image reconstruction by single pixel imaging is similar to a form of encoding and decoding, which is realized by calculating the correlation between a series of modulation patterns and their corresponding intensity signals. In this work, an optical encryption scheme is proposed based on spatial multiplexing metasurface, which depends on the combination of holographic technology and single pixel imaging technology. In the encryption scheme, the image transmitted by single pixel imaging based on metasurface is used as the addressing key of holography. Besides, illuminating different positions of the metasurface can generate different holographic reconstructed images, and there is 50% information overlapped between adjacent sub-holograms. This work makes use of the spatial multiplexing property of metasurface, which can complete different functions, paving the way for the application in the field of optical imaging encryption and information security.

Phase retrieval is a generic calculating phase method through intensity images and prior conditions. Usually, the image recording system requires stable mechanical movement and accurate alignment between different phase-shift images, which greatly limits the imaging speed and stability, resulting in mismatch or low phase retrieval quality. In this work, we propose a single-shot polarization-sensitive phase retrieval developed from triple transport of the intensity method with anisotropic metasurface. By applying gradient phase along the interface, three images with diffractive phase differences are projected on the recording plane. The three images are formed with known diffraction distances due to the conjugate Pancharatnam−Berry phase modulation of metasurface. The recorded images are calculated and post-processed using transport of intensity equation technique to obtain the phase of object. As a proof-of-principle, we demonstrate the phase imaging results through measuring phase-only objects with single-shot capture. Compared to traditional transport-of-intensity equation optical systems, our method requires no mechanical or system tuning, and polarization or electric controller switching. Thus, it can acquire three differently focused images with single-shot. This significantly improves the time efficiency of the experiments. Such compact metadevice with simplified manipulation and easy post-processing for phase retrieval can be applied to various applications, such as optical microscopy, fluid measurement, and on-ship integration.

With the development of micro/nano fabrication technology, metasurface holography has emerged as a revolutionary technology for the manipulation of light with excellent performance. However, for applications of full-Stokes polarization encryption and time sequence holographic display, multiplexing strategies of metasurfaces with large bandwidths and simple operations still need to be developed. As one of the most popular schemes of multiplexing, polarization multiplexed metasurfaces have shown flexible recording abilities for both free-space beam and surface waves. Here, by using a dielectric metasurface equipped with double phase holograms, we have achieved flexible polarization multiplexed transformations from one full-Stokes space to another. The vectorial hologram is optimized by a hybrid genetic algorithm and digitalized with subwavelength modulated units. Based on a quantitative map and remarkable information capacity, time sequence holographic display and complex optical encryption are experimentally demonstrated by changing input/output polarization channels in real time. We believe our method will facilitate applications in smart compact devices of dynamic display, dynamic optical manipulation, optical encryption, anticounterfeiting, etc.

Metallic high aspect ratio (HAR) nano-architectures provide new opportunities for a series of plasmonic devices due to their additional controllable degrees in height space compared to 2D patterns, but there is no efficient way that suitable for the rapid fabrication of large area HAR structures limited by the processing ability of traditional methods. Here in this work, we have developed a templated electrochemical deposition (ECD) method to fabricate various HAR metallic nano-structures for diversified plasmonic devices. The templated ECD method is based on the ECD filling of the nanopores that are fabricated by electron beam lithography. With this templated ECD method, numbers of HAR architectures including nanorods, nanofins and even mushroom-like structures, which have a line width as small as 100 nm and the aspect ratio up to 10:1, are established over a large scale. What is more, by simultaneously considering the designed layout and edge effect, sub 10 nm nanogap arrays are prepared, whose aspect ratio reaches 100:1 and the gap width reduces to 5 nm. Due to the extreme light confinement ability brought from Fabry–Perot resonance, the HAR nanogaps can be treated as a surface enhanced Raman scattering (SERS) substrate. Finite domain time difference simulation shows that fan-like 10 nm nanogap with a height of 700 nm has the largest light enhancement factor (EF). The configuration optimized nanogap is capable for the sensing of rhodamine 6G with a 10−9 M concentration. And the SERS EF of the nanogap is calculated to be 4 × 106, indicating the ultrasensitive molecular detection ability of the HAR nanogap. The templated ECD method not only brings a new chance for the construction of HAR metallic 3D structures, but also opens up a new horizon for the design of a series of plasmonic devices.

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.

Metasurface holography is becoming a universal platform that has made a considerable impact on nanophotonics and information optics, due to its advantage of large capacity and multiple functionalities. Here, we propose a correlated triple amplitude and phase holographic encryption based on an all-dielectric metasurface. We develop an optimized holographic algorithm to obtain quantitatively correlated triple holograms, which can encrypt information in multiple wavelength and polarization channels. We apply the “static” and “dynamic” pixels in our design, respectively. Two kinds of isotropic square nanofins are selected, one functioning as a transmitter and the other functioning as a blocker counterintuitively at both working wavelengths, while another anisotropic rectangle nanofin can transmit or block light in co-polarization selectively, mimicking “dynamic” amplitude switches. Meanwhile, such “dynamic” nanofins can simultaneously function as a phase modulator in cross-polarization only at the transmission wavelength. That is, through smart design, different dielectric meta-atoms functioning as spectral filters as well as phase contributors can compositely achieve triple hybrid amplitude and phase holograms. Such strategy promises to be applied in compact large-capacity information storage, colorful holographic displays, optical encryption, multifunctional imaging devices, and so on.

Nanodevices have higher requirements for nanofabrication in tuning the size, shape and spatial arrangement of nanostructures and their assemblies in nanoscale, however, which are often beyond the reach of conventional lithography or self-assembly techniques. In view of the above, we develop atomic layer assembled nanofabrication based on soft-templates to break through the limitations of traditional rigid-templates, having very well scalability and powerful fabrication capability for multiple solid or hollow nanostructures. Versatile soft-templates can be freely patterned at the nanoscale by mature lithographic processes, along which a precisely controlled atomic layer deposition can assemble high-aspect-ratio nanostructures with a flexible tailoring of the size, shape and spatial array, and then a dry etching process removes soft scaffolds and leaves freestanding nanostructures over large-area, rigid or soft substrates. To highlight the potentials of this fabrication strategy, the high-performance optical metasurface and ultra-sensitive H2 gas sensor are demonstrated. This approach endows the conventional lithography and assembly techniques with new powerful functionalities and more scalability in nanofabrication, providing a simply promising route to generating complex multiple nanostructures, towards a broad application in modern nanodevices.