Low-loss Metasurfaces Research Articles

Low-loss metasurfaces based on discretized meta-atoms

Metasurfaces are established tools for manipulating light and enhancing light-matter interactions. However, the loss of conventional meta-atoms usually limits the performance potential of metasurfaces. In this study, we propose a class of metasurfaces based on discretized meta-atoms able to mitigate the radiative and intrinsic losses. By discretizing meta-atoms, we reduce the loss of metal metasurfaces to levels comparable to dielectric metasurfaces in the short-wavelength infrared region at the surface lattice resonance mode. Furthermore, we propose a coupling model to explain the observed reduction in loss in full agreement with the results obtained from finite-element method. We also reproduce this phenomenon using dielectric metasurface at electric and magnetic resonances in the visible region. Our finding offers valuable insights for the design and application of metasurfaces, while also providing theoretical implications for other resonance fields beyond metasurfaces.

Heating-free, room-temperature operation of a radiofrequency-to-light signal transducer with a membrane oscillator and a built-in metasurface mirror

We present an electro-mechano-optical radiofrequency (rf)-to-light signal transducer robust against laser heating and thus operational at room temperature. A metal-free, low-loss metasurface mirror and an aluminum electrode made separately on a Si3N4 membrane oscillator comprise a chain of electro-mechanical and opto-mechanical systems, mediating electrical and optical signals through the (2,2)-mode characteristic oscillation. We demonstrate up-conversion of rf signals at 175.2 MHz by 6 orders of magnitude in frequency to an optical regime with the transfer efficiency of 2.3 × 10−9, also showing stable operation due to reduced laser heating of the mirror.

Lossless Complex-Valued Optical-Field Control with Compound Metaoptics

Forming a desired optical field distribution from a given source requires precise spatial control of a field's amplitude and phase. Low-loss metasurfaces that allow extreme phase and polarization control of optical fields have been demonstrated over the past few years. However, metasurfaces that provide amplitude control have remained lossy, utilizing mechanisms such as reflection, absorption, or polarization loss to control amplitude. Here, we describe the amplitude and phase manipulation of optical fields without loss, by using two lossless phase-only metasurfaces separated by a distance. We first demonstrate a combined beam-former and splitter optical component using this approach. Next, we show a high-quality computer-generated three-dimensional hologram. The proposed metaoptic platform combines the advantage of lossless, complex-valued field control with a physically small thickness. This approach could lead to low-profile, three-dimensional holographic displays, compact optical components, and high precision optical tweezers for micro-particle manipulation.

Recent Progress on Ultrathin Metalenses for Flat Optics.

SummaryAs technology advances, electrical devices such as smartphones have become more and more compact, leading to a demand for the continuous miniaturization of optical components. Metalenses, ultrathin flat optical elements composed of metasurfaces consisting of arrays of subwavelength optical antennas, provide a method of meeting those requirements. Moreover, metalenses have many other distinctive advantages including aberration correction, active tunability, and semi-transparency, compared to their conventional refractive and diffractive counterparts. Therefore, over the last decade, great effort has been focused on developing metalenses to investigate and broaden the capabilities of metalenses for integration into future applications. Here, we discuss recent progress on metalenses including their basic design principles and notable characteristics such as aberration correction, tunability, and multifunctionality.

Optical field concentrator with low absorption metasurfaces based on planar silicon nanoantennas on silica

Plasmonic metamaterials open a pathway to a multitude of different applications, from ultrasensitive sensors to merging the packaging density of electronics and the speed of photonics in a single all-optical device. A severe limitation to their wider use is high absorption due to the mandatory presence of free electron-containing conductive parts which are lossy by definition. In this contribution we consider an alternative solution in the form of ultrathin silicon films deposited on a silica substrate. The films are patterned as arrays of elongated rhombuses acting as bowtie nanoantennas. We consider the effect of sharp tips and the proximity effect on the electromagnetic field concentration in such low-loss metasurfaces. Our structures have the advantage of very low absorption losses with an order of magnitude field enhancement and the virtue of full compatibility with the standard planar technologies. This makes them convenient for various practical applications which integrate high field concentration with e.g. waveguiding properties, for instance microreactors, labs-on-a-chip, photocatalytic systems and various other Micro-Opto-Electro-Mechanical System (MOEMS) devices integrating optical, microfluidic and other functionalities.

Photorealistic full-color nanopainting enabled by low-loss metasurface.

We realize a dielectric metasurface that enables full-color generation and ultrasmooth brightness variation. The reproduced artwork "Girl with a Pearl Earring" features photorealistic color representation and stereoscopic image impression, mimicking the texture of an oil-painting.

Dark-State-Based Low-Loss Metasurfaces with Simultaneous Electric and Magnetic Resonant Response

The realization of metamaterials or metasurfaces with simultaneous electric and magnetic response and low loss is generally very challenging at optical frequencies. Traditional approaches using nanoresonators made of noble metals, while suitable for the microwave and terahertz regimes, fail at frequencies above the near-infrared, due to prohibitive high dissipative losses and the breakdown of scaling resulting from the electron mass contribution (kinetic inductance) to the effective reactance of these plasmonic meta-atoms. The alternative route based on Mie resonances of high-index dielectric particles normally leads to structure sizes that tend to break the effective-medium approximation. Here, we propose a subwavelength dark-state-based metasurface, which enables configurable simultaneous electric and magnetic responses with low loss. Proof-of-concept metasurface samples, specifically designed around telecommunication wavelengths (i.e., λ ≈ 1.5 μm), were fabricated and investigated experimentally to val...

All-Dielectric Chiral Metasurfaces Based on Crossed-Bowtie Nanoantennas.

Circular dichroism spectroscopy is a technique used to discriminate molecular chirality, which is essential in fields like biology, chemistry, or pharmacology where different chiral agents often show different biological activities. Nevertheless, due to the inherently weak molecular-chiroptical activity, this technique is limited to high concentrations or large analyte volumes. Finding novel ways to enhance the circular dichroism would boost the performance of these techniques. So far, the enhancement of light–matter interaction mediated by plasmons is the most common way to develop chiral plasmonic structures with extraordinarily strong chiroptical responses. However, absorptive losses of metals at optical frequencies has hindered its practical use in many scenarios. In this work, we propose an all-dielectric low-loss chiral metasurface with unit cells built by high-refractive-index crossed-bowtie nanoantennas. These unit cells, built of silicon, strongly increase the chiroptical effect through the simultaneous interaction of their electric and magnetic modes, which in contrast to other recent proposals shows at the same time a high concentration of the electric field in its gap that leads to the presence of hotspots. The proposed structure exhibits a circular dichroism spectra up to 3-fold higher than that of previous proposals that use complex plasmonic or hybrid nanostructures, making it a clear alternative to develop low-loss metasurfaces with potential applications in chiral target sensing/biosensing. For completeness, single triangular shaped and symmetric (achiral) bowtie nanostructures were also studied as possible candidates for a detection up to the single-molecule level due the lack of a circular dichroism background of the nanostructures themselves.

Metasurface-Integrated Photonic Platform for Versatile Free-Space Beam Projection with Polarization Control

Densely integrated photonic circuits enable scalable, complex processing of optical signals, including modulation, multiplexing, wavelength conversion, and detection. Directly interfacing such integrated circuits to free-space optical modes will enable novel optical functions, such as chip-scale sensing, interchip free-space interconnect and cooling, trapping, and interrogation of atoms. However, doing this within the limits of planar batch fabrication requires new approaches for bridging the large mode scale mismatch. Here, by integrating a dielectric metasurface with an extreme photonic mode converter, we create a versatile nanophotonic platform for efficient coupling to arbitrary-defined free-space radiation of 780 nm wavelength with well-controlled spatially-dependent polarization, phase, and intensity. Without leaving the chip, the high index photonic mode is converted first to a ≈ 200 μm wide, precisely collimated, linearly-polarized Gaussian beam, which is then modified by a planar, integrated, low-loss metasurface. We demonstrate high numerical aperture, diffraction limited focusing to an ≈ 473 nm spot at an ≈ 75 μm working distance, and combine it with simultaneous conversion from linear to elliptical polarization. All device components are lithographically defined and can be batch fabricated, facilitating future chip-scale low-cost hybrid photonic systems for bio-sensing, nonlinear signal processing and atomic quantum sensing, frequency references and memory.

Systematic Design of Printable Metasurfaces: Validation Through Reverse-Offset Printed Millimeter-Wave Absorbers

In this paper, we present a systematic methodology for realizing desired sheet impedances of printable metasurfaces. This methodology allows independent control of the sheet reactance (capacitance and series inductance) and its resistance, even if the conductor properties as well as the dielectric substrate thickness and permittivity are fixed due to manufacturing process restrictions. The derived analytical formulas allow us to easily find the physical dimensions of conductive patterns, which implement the required surface impedance. Numerical verification of the method shows excellent agreement with the analytical predictions, allowing the design of an arbitrary impedance without any optimization process. The method can be applied for designing lossy and low-loss metasurfaces, which can be used for absorption and wavefront manipulation of electromagnetic waves. As a representative example, the design of thin perfect absorbers has been approached using the developed method. The results demonstrate that the methodology adapts various material sheet resistivity, opening new possibilities for the design of printable metasurfaces, where the sheet resistivity of conductor strongly depends on the specific printing method. Finally, an experimental validation of absorbers designed for millimeter waves and printed using reverse-offset techniques is presented. To the best of our knowledge, this is the first time when reverse-offset printing has been used to provide well-working devices for short millimeter waves.

Nanoimprinted perovskite metasurface for enhanced photoluminescence.

Recently, solution-processed hybrid halide perovskite has emerged as promising materials for advanced optoelectronic devices such as photovoltaics, photodetectors, light emitting diodes and lasers. In the mean time, all-dielectric metasurfaces with high-index materials have attracted attention due to their low-loss and high-efficient optical resonances. Because of its tunable by composition band gap in the visible frequencies, organolead halide perovskite could serve as a powerful platform for realizing high-index, low-loss metasurfaces. However, direct patterning of perovskite by lithography-based technique is not feasible due to material instability under moisture. Here we report novel organolead halide perovskite metasurfaces created by the cost-effective thermal nanoimprint technology. The nanoimprinted perovskite metasurface showed improved surface morphology and enhanced optical absorption properties. Significantly enhanced optical emission with an eight-fold enhancement in photoluminescence (PL) intensity was observed under room temperature. Temperature-dependent PL of perovskite nanograting metasurface was also investigated. Based on our results, we believe that thermal nanoimprint is a simple and cost-effective technique to fabricate perovskite-based metasurfaces, which could have broad impact on optoelectronic and photonic applications.

A Low-loss Metasurface Antireflection Coating on Dispersive Surface Plasmon Structure.

Over the years, there has been increasing interest in the integration of metal hole array (MHA) with optoelectronic devices, as a result of enhanced coupling of incident light into the active layer of devices via surface plasmon polariton (SPP) resonances. However, not all incident light contributes to the SPP resonances due to significant reflection loss at the interface between incident medium and MHA. Conventional thin-film antireflection (AR) coating typically does not work well due to non-existing material satisfying the AR condition with strong dispersion of MHA’s effective impedances. We demonstrate a single-layer metasurface AR coating that completely eliminates the refection and significantly increases the transmission at the SPP resonances. Operating at off-resonance wavelengths, the metasurface exhibits extremely low loss and does not show resonant coupling with the MHA layer. The SPP resonance wavelengths of MHA layer are unaffected whereas the surface wave is significantly increased, thereby paving the way for improved performance of optoelectronic devices. With an improved retrieval method, the metasurface is proved to exhibit a high effective permittivity () and extremely low loss (tan δ ~ 0.005). A classical thin-film AR coating mechanism is identified through analytical derivations and numerical simulations.

A phased array antenna with a broadly steerable beam based on a low-loss metasurface lens

A new concept for a gradient phase discontinuity metasurface lens integrated with a phased array antenna possessing a broadly steerable beam is presented in this paper. The metasurface lens is composed of a metallic H-shaped pattern and the metallic square split ring can achieve complete 360° transmission phase coverage at 30° phase intervals. The metasurface can refract an incident plane wave to an angle at will by varying the lattice constant. We demonstrate that the beam steering range of the phased array antenna is between 12° and 85° when the metasurface lens with a refracting electromagnetic wave is employed at 45°. Interestingly, the proposed array antenna has a much higher gain than a conventional phased array antenna at low elevation angles. It is expected that the proposed array antenna will have potential applications in wireless and satellite communications. Furthermore, the proposed array antenna is fabricated easily and is also low in cost due to its microstrip technology.

Low-Contrast Dielectric Metasurface Optics

The miniaturization of current image sensors is largely limited by the volume of the optical elements. Using a sub-wavelength patterned quasi-periodic structure, also known as a metasurface, one can build planar optical elements based on the principle of diffraction. However, it was believed that high refractive index materials are required for metasurface optics. Here, we show that one can employ the design principles of a metasurface even with low contrast materials, such as silicon nitride. We validate our theory by fabricating and experimentally characterizing several silicon nitride based lenses and vortex beam generators . The fabricated lenses achieved beam spots of less than 1 {\mu}m with numerical apertures as high as ~ 0.75. A transmission efficiency of 90% and focusing efficiency of 40% in the visible regime was observed. Our results pave the way towards building low-loss metasurface based optical elements at visible frequencies using low contrast materials.

Cramér-Rao bounds for determination of electric and magnetic susceptibilities in metasurfaces.

Accurate and robust characterization of metasurfaces and metamaterials in terms of effective parameters is critical to the design of novel metadevices. We compute the Cramér-Rao lower bounds on the variance of any estimator for both the electric and magnetic surface susceptibilities of metasurfaces. We show that retrieval of such effective properties is inherently difficult around resonances, most notably for low-loss metasurfaces. We also put forth a least-squares estimator to mitigate this difficulty for the normal components of susceptibility tensors, which are observed to be the most ill-behaved. The present work is relevant to the development of loss-compensated metasurfaces for which noise has to be closely considered for accurate and robust device characterization.

MEMS-Reconfigurable Metamaterials and Antenna Applications

This paper reviews some of our contributions to reconfigurable metamaterials, where dynamic control is enabled by microelectromechanical systems (MEMS) technology. First, we show reconfigurable composite right-/left-handed transmission lines (CRLH-TLs) having state of the art phase velocity variation and loss, thereby enabling efficient reconfigurable phase shifters and leaky-wave antennas (LWA). Second, we present very low loss metasurface designs with reconfigurable reflection properties, applicable in reflectarrays and partially reflective surface (PRS) antennas. All the presented devices have been fabricated and experimentally validated. They operate in X- and Ku-bands.

Plasmonic Nanogap Tilings: Light-Concentrating Surfaces for Low-Loss Photonic Integration

Owing to their ability to concentrate light on nanometer scales, plasmonic surface structures are ideally suited for on-chip functionalization with nonlinear or gain materials. However, achieving a high effective quantum yield across a surface requires not only strong light localization but also control over losses. Here, we report on a particular class of tunable low-loss metasurfaces featuring dense arrangements of nanometer-sized focal points on a photonic chip with an underlying waveguide channel. Guided within the plane, the photonic wave evanescently couples to the nanogaps, concentrating light in a lattice of hot-spots. In studying the energy transfer between photonic and plasmonic channels of single trimer molecules and triangular nanogap tilings in dependence on element size, we identify different regimes of operation. We show that the product of field enhancement, propagation length, and element size is close to constant in both the radiative and subwavelength regimes, opening pathways for device designs that combine high-field enhancements with large propagation lengths.