Dielectric Metasurfaces Research Articles

Dielectric Metasurface for Synchronously Spiral Phase Contrast and Bright-Field Imaging.

Spiral phase contrast imaging and bright-field imaging are two widely used modes in microscopy, providing distinct morphological information about objects. However, conventional microscopes are always unable to operate with these two modes at the same time and need additional optical elements to switch between them. Here, we present a microscopy setup that incorporates a dielectric metasurface capable of achieving spiral phase contrast imaging and bright-field imaging synchronously. The metasurface not only can focus the light for diffraction-limited imaging but also can perform a two-dimensional spatial differentiation operation by imparting an orbital angular momentum to the incident light field. This allows two spatially separated images to be simultaneously obtained, one containing high-frequency edge information and the other showing the entirety of the object. Combined with the advantages of planar architecture and ultrathin thickness of the metasurface, this approach is expected to provide support in the fields of microscopy, biomedicine, and materials science.

Carbon quantum initiators enabled direct laser writing: A technique for fabrication of dielectric, all-carbon chiral metasurfaces

Chiral metasurfaces exhibit extraordinary capabilities of steering polarization of electromagnetic waves; and hence offering enormous potential in development of ultrathin components for advanced photonic technologies such as quantum computing and communication, imaging, optical memories and logics. These structures are realized by periodically arranged subwavelength resolved artificial elements (metaelements). Metaelements are mainly metallic, dielectric or metal-dielectric based and their fabrication is a daunting task due to the associated complex fabrication methodologies involving serial, time-consuming processes and sophisticated instrumentation, hence limiting the growth, and application of these metasurfaces to its true potential. To mitigate these issues, we exploited the single-step fabrication capabilities of two-photon polymerization based lithography to generate a range of all-carbon-based dielectric chiral metasurfaces in a carbon-polymer nanocomposite resin. Additionally, theoretical responses of these structures are also calculated. These structures derive their responses from structural chirality. With the advantages of single-step fabrication of any arbitrary structure with sub-wavelength resolved features, along with the capability of rapid prototyping in the proposed material system; this work holds a great potential towards generation of all-carbon metamaterials and 3D metasurfaces for several advanced photonic and optical technologies.

Design and Tuning of Substrate-Fabricated Dielectric Metasurfaces Supporting Quasi-Trapped Modes in the Infrared Range

Design and Tuning of Substrate-Fabricated Dielectric Metasurfaces Supporting Quasi-Trapped Modes in the Infrared Range

Cylindrical vector beams demultiplexing communication based on a vectorial diffractive optical element

Abstract Cylindrical vector beams with polarization singularities, transmission stability and turbulence resilient, are orthogonally structured light beams providing new degrees of freedom for multiplexing optical communications. The demultiplexing of the CVBs with high efficiency and low crosstalk is of vital importance for the practical applications. Here, we propose a lens-less CVB sorting approach with a set of dielectric metasurface devices. The metasurface is composed of elliptical silicon nanopillars, which are capable of vector field steering. By performing mode transformations on both left-handed and right-handed polarization components of the CVBs, cylindrical vector beams can be demultiplexed with high efficiency and reduced crosstalk. Furthermore, by adjusting the phase response of the vectorial diffractive element into a set of Pancharatnam–Berry (PB) phase planes, we experimentally demonstrate 11 CVBs sorting with a set of P–B phase liquid crystal devices. The proposed device may benefit the CVB-based mode multiplexing communications in future.

Polarization-directed growth of spiral nanostructures by laser direct writing with vector beams

Chirality is pivotal in nature which attracts wide research interests from all disciplines and creating chiral matter is one of the central themes for chemists and material scientists. Despite of significant efforts, a simple, cost-effective and general method that can produce different kinds of chiral metamaterials with high regularity and tailorability is still demanding but greatly missing. Here, we introduce polarization-directed growth of spiral nanostructures via vector beams, which is simple, tailorable and generally applicable to both plasmonic and dielectric materials. The self-aligned near field enhances the photochemical growth along the polarization, which is crucial for the oriented growth. The obtained plasmonic chiral nanostructures present prominent optical activity with a g-factor up to 0.4, which can be tuned by adjusting the spirality of the vector beams. These spiral plasmonic nanostructures can be used for the sensing of different chiral enantiomers. The dielectric chiral metasurfaces can also be formed in arrays of sub-mm scale, which exhibit a g-factor over 0.1. However, photoluminescence of chiral cadmium sulfide presents a very weak luminescence g-factor with the excitation of linearly polarized light. A number of applications can be envisioned with these chiral nanostructures such as chiral sensing, chiral separation and chiral information storage.

Generation of the Bessel Beam of Longitudinally Varied Polarization with Dielectric Metasurfaces

AbstractThe beam with longitudinally varied polarization provides a new dimension for polarization applications, such as material processing, longitudinal depth detection, optical communications, etc. A few different methods have so far been investigated, including spatial light modulators and diffractive optical elements. However, these methods have complex optical path, large volume, which are difficult to integrate. Here, a birefringent dielectric metasurface is demonstrated, which is capable of generating Bessel beam whose polarization state continuously rotates along the propagation direction. According to the design, when a linearly polarized light illuminates on the metasurface, the outgoing light becomes linearly polarized zero‐order Bessel beam, whose polarization angle changes as the propagation distance increases. Experimental measurements are successfully carried out. Such scheme may open new doors for potential applications in depth perception, microscopic detection, and so on.

Nonlinear Chiral Metasurfaces Based on the Optical Kerr Effect

AbstractChiral nanostructures have immense potential for complete polarization control of light. Given the intrinsic 3D structure of circularly polarized light, nanoarchitectures with structural chirality in 3D space are usually required to achieve “full” chiroptical response. On the other hand, due to the lack of available active materials at optical wavelengths, examples of chiral photonic devices that can be tuned in an active manner remain rare. Here, based on the large Kerr nonlinearities of silicon, the nonlinear chiroptical response from a planar Si metasurface supporting high quality (Q)‐factor guided mode resonances (GMRs) at near‐infrared wavelengths, are numerically demonstrated. In the linear regime, the optical anisotropy of the C2‐symmetric structure associated with the observed GMRs leads to a pair of chiral resonance modes, enabling pronounced cross‐polarization transmission circular dichroism and a strong handedness‐dependent field enhancement effect. Owing to the nonlinear Kerr effect, such Si metasurfaces are seen to exhibit prominent chiral‐selective optical nonlinearity. Importantly, a pronounced co‐polarization transmission circular dichroism is observed, suggesting that the circularly polarized waves in the nonlinear regime possess asymmetric Jones matrices. Dielectric chiral metasurfaces with Kerr nonlinearities can be utilized to facilitate polarization‐state modulators with simple planar structures.

Mid-infrared silicon metasurfaces for near-field enhancement of molecular fingerprints.

Mid-infrared dielectric metasurfaces are promising fundamental building blocks for integrated sensing with high sensitivity, compositional selectivity, and low loss. We have designed and fabricated a silicon metasurface with resonance properties in the 4∼5 µm mid-infrared region and a volume enhancement of up to 9 times. Benchmark FTIR characterizations of solutions of tungsten hexacarbonyl molecules showed a detection limit of 1 mg/mL without the usage of surface enrichment treatment. We further rationalize the detection limit of the molecules-nanostructure open interface with volume field enhancement analysis. Our results show that mid-infrared silicon metasurfaces may be a suitable platform for potential integration with microfluidic for in vivo detection.

Tunable dielectric BIC metasurface for high resolution optical filters

The dielectric metasurface has become a powerful tool for compact optical components with various wavefront controlling functionalities accompanied by negligible losses at the corresponding working frequencies. In this work, we propose a tunable all-dielectric metasurface as an optical filter with high resolution covering different optical communication bands, where tunability is realized by a combination of changing the incident angle and modulating the refractive index of an optical phase changing material (OPCM). When the incident angle varies, our optical filter based on a two-dimensional bound state in continuums (BIC) metasurface can achieve sequential, extremely sharp resonances. In addition, the resonance peaks could be further shifted to a different frequency band by the refractive index change of OPCM via pulsed laser heating. The proposed scheme can offer optical filters with high spectral resolution and large tunable working wavelength range, which greatly benefits from the topological property of BIC and large modulation depth of OPCM.

Theoretical Study on All-Dielectric Elliptic Cross Metasurface Sensor Governed by Bound States in the Continuum

The appearance of all-dielectric micro-nano photonic devices constructed from high refractive index dielectric materials offers a low-loss platform for the manipulation of electromagnetic waves. The manipulation of electromagnetic waves by all-dielectric metasurfaces reveals unprecedented potential, such as focusing electromagnetic waves and generating structured light. Recent advances in dielectric metasurfaces are associated with bound states in the continuum, which can be described as non-radiative eigen modes above the light cone supported by metasurfaces. Here, we propose an all-dielectric metasurface composed of elliptic cross pillars arranged periodically and verify that the displacement distance of a single elliptic pillar can control the strength of the light-matter interaction. Specifically, when the elliptic cross pillar is C4 symmetric, the quality factor of the metasurface at the Γ point is infinite, also called the bound states in the continuum. Once the C4 symmetry is broken by moving a single elliptic pillar, the corresponding metasurface engenders mode leakage; however, the large quality factor still exists, which is called the quasi-bound states in the continuum. Then, it is verified by simulation that the designed metasurface is sensitive to the refractive index change of the surrounding medium, indicating that it can be applied for refractive index sensing. Moreover, combined with the specific frequency and the refractive index variation of the medium around the metasurface, the information encryption transmission can be realized effectively. Therefore, we envisage that the designed all-dielectric elliptic cross metasurface can promote the development of miniaturized photon sensors and information encoders due to its sensitivity.

Highly Precise and Broadband Full-Stokes Polarimeter Based on a Deep Learning Algorithm

Recent developments in nanophotonics enable the practical realization of ultracompact full-Stokes polarimeters and imaging devices based on the well-defined polarization-sensitive structures of subwavelength size. However, the resonant feature of building blocks and the limited ability of the current processing technology greatly limit their detection accuracy and working waveband. Herein, we experimentally demonstrate a highly precise and broadband full-Stokes polarimeter based on the high-performance dielectric chiral shells (DCSs) and the deep learning (DL) algorithm. The large-area dielectric chiral metasurfaces (DCMs) fabricated by depositing Si on the self-assembled microsphere monolayer own a large transmittance, strong chirality, and anisotropy in the visible waveband, which enable the effective perception of the polarization state of light. The Stokes parameters are successfully detected by measuring several transmittances from DCSs with different rotation angles, i.e., Î, and then analyzing with the pre-established mapping relationship between the Stokes parameters Ŝ and the recorded transmittance Î based on the DL algorithm, i.e., Ŝ = f(Î). In contrast to the traditional polarimeters, the detection accuracy based on the DL approach seems insensitive to the fabrication errors. A mean square error (MSE) of smaller than 0.5% at 760.06 nm was recorded, and an averaged MSE of less than 4% for Stokes parameters over a broad waveband from 400 to 840 nm was obtained, although the optical anisotropy and chirality of DCMs own a strong dispersion feature. The small MSE in a broad waveband, together with the easily accessing DCMs, make our polarimeter as the most potential candidate for future applications, such as the color polarization imaging devices and the ultraviolent polarimeter.

Metasurface-Controlled Photonic Rashba Effect for Upconversion Photoluminescence.

We demonstrate the effect of spin-momentum locking of upconversion photoluminescence emitted from rare-earth doped nanocrystals coupled to a phase-gradient dielectric metasurface. We observe different directionalities for left and right circular polarized light and associate this experimental observation with the photonic Rashba effect realized for upconverted photoluminescence that is manifested in the spin-dependent splitting of emitted light in the momentum space.

Bound States in the Continuum in Asymmetric Dielectric Metasurfaces (Laser Photonics Rev. 17(3)/2023)

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Bicontrollable all dielectric metasurface absorber for chemical and biosensing applications

A bi-controllable all dielectric metasurface structure is designed at terahertz, which consists of an InAs micro-cylindrical array on the gold ground plane. The maximum absorptance frequency is tunable with the help of an external magnetic field and also by geometrical rotation of the metasurface as InAs is a gyrotropic material whose property depends on the magnitude and direction of an external magnetic field. Simulation results indicate that the proposed metasurface can achieve an absorptance of more than 99 % at 9.756 THz as well as it can provide a high tunability rate of around 2.55 THz T−1 in lower band and 3.33 THz T−1 in the upper band frequency spectrum with the application of the magnetic field. Additionally, the metasurface also acts as polarization insensitive when no external magnetic field is applied. Furthermore, the maximum absorptance frequency shows sensitivity towards the change in the refractive index value of the surrounding medium, thus the metasurface can be used for chemical and biosensing applications. It shows high chemical sensitivity of around 2727 GHz/RIU for the detection of different chemicals and bio sensitivity of 787 GHz/RIU for the detection of various viruses.

Metasurface-stabilized optical microcavities

Cavities concentrate light and enhance its interaction with matter. Confining to microscopic volumes is necessary for many applications but space constraints in such cavities limit the design freedom. Here we demonstrate stable optical microcavities by counteracting the phase evolution of the cavity modes using an amorphous Silicon metasurface as cavity end mirror. Careful design allows us to limit the metasurface scattering losses at telecom wavelengths to less than 2% and using a distributed Bragg reflector as metasurface substrate ensures high reflectivity. Our demonstration experimentally achieves telecom-wavelength microcavities with quality factors of up to 4600, spectral resonance linewidths below 0.4 nm, and mode volumes below 2.7{lambda }^{3}. The method introduces freedom to stabilize modes with arbitrary transverse intensity profiles and to design cavity-enhanced hologram modes. Our approach introduces the nanoscopic light control capabilities of dielectric metasurfaces to cavity electrodynamics and is industrially scalable using semiconductor manufacturing processes.

Resonance coupling between chiral quasi-BICs and achiral molecular excitons in dielectric metasurface J-aggregate heterostructures.

The realization of flexible tuning and enhanced chiral responses is vital for many applications in nanophotonics. This study proposes to manipulate the collective optical responses with heterostructures consisting of chiral dielectric metasurfaces and achiral J-aggregates. Owing to the resonance coupling between the chiral quasi-bound states in the continuum (QBICs) and the achiral exciton mode, large mode splitting and anticrossing are observed in both the transmission and circular dichroism (CD) spectra, which indicates the formation of hybrid chiral eigenmodes and the realization of the strong coupling regime. Considering that the radiative and dissipative damping of the hybrid eigenmodes depends on the coherent energy exchange, the chiral resonances can be flexibly tuned by adjusting the geometry and optical constants for the heterostructure, and the CD of the three hybrid eigenmodes approach the maximum (∼1) simultaneously when the critical coupling conditions are satisfied, which can be promising for enhanced chiral light-matter interactions.

Metasurface Enabled On-Chip Generation and Manipulation of Vector Beams from Vertical Cavity Surface-Emitting Lasers.

Metasurface polarization optics that consist of 2D array of birefringent nano-antennas have proven remarkable capabilities to generate and manipulate vectorial fields with subwavelength resolution and high efficiency. Integrating this new type of metasurface with the standard vertical cavity surface-emitting laser (VCSEL) platform enables an ultracompact and powerful solution to control both phase and polarization properties of the laser on a chip, which allows to structure a VCSEL into vector beams with on-demand wavefronts. Here, this concept is demonstrated by directly generating versatile vector beams from commercially available VCSELs through on-chip integration of high-index dielectric metasurfaces. Experimentally, the versatility of the approach for the development of vectorial VCSELs are validated by implementing a variety of functionalities, including directional emission of multibeam with specified polarizations, vectorial holographic display, and vector vortex beams generations. Notably, the proposed vectorial VCSELs integrated with a single layer of beam shaping metasurface bypass the requirements of multiple cascaded optical components, and thus have the potential to promote the advancements of ultracompact, lightweight, and scalable vector beams sources, enriching and expanding the applications of VCSELs in optical communications, laser manipulation and processing, information encryption, and quantum optics.

RCS reduction using grounded multi-height multi-dielectrics metasurfaces

In this paper, the PO method with the array theory are used to formulate the RCS of a grounded multi-height dielectric surface which can be employed in the design and optimization of a metasurface consisting of dielectric tiles with different heights and permittivities. The proposed closed form relations can be used instead of full wave simulation to design an optimized dielectric grounded metasurface, properly. Finally, three different RCS reducer metasurfaces are designed and optimized with three different dielectric tiles by using the proposed analytical relations. The results prove that the proposed ground dielectric metasurface achieve more than 10 dB RCS reduction at 4.4–16.3 GHz (114.9%) frequency range. This result proves the accuracy and effectiveness of the proposed analytical method which can be used in the RCS reducer metasurfaces design.

Tunable localization of light using nested invisible metasurface cavities

Abstract An invisible cavity is an open resonant device that confines a localized field without producing any scattering outside of the device volume. By exploiting the scatter-less property of such device, it is possible to nest two invisible cavities, as the outer cavity would simply not notice the presence of the inner one, regardless of their relative position. As a result, the position of the inner cavity becomes a means to easily control the field localized inside the cavity and its quality factor. In this paper, we discuss the properties of nested invisible cavities as a simple method to achieve stronger localized fields and high tunable quality factor. Furthermore, we show that in optics, these cavities can be implemented using nanodisk-based dielectric metasurfaces that operate near their electric resonances.

Extremely high-Q quasi-BICs induced by simultaneously broken out-of- and in-plane symmetries in dielectric metasurfaces of low-index materials

Generating high-Q resonances is a difficult task to realize with low-index materials as the building blocks of metasurfaces, despite of its significance. Up to date, various schemes for obtaining strong resonances with low-index materials have been reported, while the highest record of Q-factor still remains in the order of several thousands. This work shows that quasi-bound states in the continuum (quasi-BICs) with extremely high Q-factors in the orders of 105∼107 can be obtained in the low-index metasurfaces, taking lithium niobate (LN) metasurfaces supported by silica substrate as examples. In such metasurfaces, BICs with inversion symmetric distributions evolve to quasi-BICs with extremely high Q-factors by simultaneously breaking out-of- and in-plane symmetries due to multiple symmetry protectedness. Dramatically high enhancement of mode intensity in the order of 104∼106 due to the high Q-factor enables highly efficient nonlinear frequency conversion: the results based on the self-consistent approach show that the total efficiency of second-harmonic generation reaches around 9.4% for pump intensity of 0.5 W/cm2. As the first report on extremely high-Q quasi-BICs in low-index metasurfaces, the presented results pave the way towards broad applications for ultraviolet nanophotonics.