Dielectric Metasurface as a Platform for Spatial Mode Conversion in Nanoscale Waveguides

Compact mode converter on SOI based on a polygonal subwavelength grating structure

In this Letter, we design and experimentally demonstrate compact mode converters with a lightning-like and arrow-like polygonal subwavelength grating (SWG) structure on a silicon-on-insulator (SOI) platform, which can convert the TE0 mode to the TE1 and TE2 modes, respectively. The footprints of the proposed TE0-1 and TE0-2 mode converters are only 4.44 × 1.3 and 5.89 × 1.8 µm2, respectively. The experimental results show the mode converters have a low insertion loss (<1 dB) and a broad bandwidth (>50 nm). The measured cross talks of the TE0-1 and TE0-2 mode converters are -7.2 dB and -10.3 dB, respectively. In addition, the proposed mode converters with the SWG structure have the advantage in fabrication, since only a one-step full-etching process is required.

Design of a bidirectional TM01(TE01)-LP01 mode converter with a metasurface-on-fiber

Mode conversion is crucial for coupling a light source to a desired waveguide. While traditional mode converters such as fiber Bragg gratings and long-period fiber gratings exhibit high transmission and conversion efficiency, the mode conversion of two orthogonal polarizations remains challenging. Here, we present a bidirectional metasurface mode converter that can convert the transverse electric (TE)01 or transverse magnetic (TM)01 mode to the fundamental mode (LP01) with orthogonal polarization, and vice versa. The mode converter is located on a facet of a few-mode fiber and connected to a single mode fiber. Through simulations, we find that 99.9% of the TM01 or TE01 mode is converted into the x- or y-polarized LP01 mode, and that 99.96% of the x- or y-polarized LP01 mode is converted to the TM01 or TE01 mode. Furthermore, we expect a high transmission of over 84.5% for all mode conversions, up to 88.7% for TE01 to y-polarized LP01 conversion.

Ultra-compact on-chip meta-waveguide phase modulator based on split ring magnetic resonance

With the development of photonic integration technology, meta-waveguides have become a new research hotspot. They have broken through the theoretical diffraction limit by virtue of the strong electromagnetic manipulation ability of the metasurface and the strong electromagnetic field limitation and guidance ability of the waveguide. However, the reported meta-waveguides lack research on dynamic modulation. Therefore, we analyze the modulation effect of the metasurface on the optical field in the waveguide and design an ultra-compact on-chip meta-waveguide phase modulator using split ring magnetic resonance. It has a very short modulation length of only 3.65 µm, wide modulation bandwidth of 116.8 GHz, and low energy consumption of 263.49 fJ/bit. By optimizing the structure, the energy consumption can be further reduced to 90.69 fJ/bit. Meta-waveguides provide a promising method for the design of integrated photonic devices.

Metasurfaces integrated with a single-mode waveguide array for off-chip wavefront shaping

Integration of metasurfaces and SOI (silicon-on-insulator) chips can leverage the advantages of both metamaterials and silicon photonics, enabling novel light shaping functionalities in planar, compact devices that are compatible with CMOS (complementary metal-oxide-semiconductor) production. To facilitate light extraction from a two-dimensional metasurface vertically into free space, the established approach is to use a wide waveguide. However, the multi-modal feature of such wide waveguides can render the device vulnerable to mode distortion. Here, we propose a different approach, where an array of narrow, single-mode waveguides is used instead of a wide, multi-mode waveguide. This approach tolerates nano-scatterers with a relatively high scattering efficiency, for example Si nanopillars that are in direct contact with the waveguides. Two example devices are designed and numerically studied as demonstrations: the first being a beam deflector that deflects light into the same direction regardless of the direction of input light, and the second being a light-focusing metalens. This work shows a straightforward approach of metasurface-SOI chip integration, which could be useful for emerging applications such as metalens arrays and neural probes that require off-chip light shaping from relatively small metasurfaces.

Ultra-compact and ultra-broadband arbitrary-order silicon photonic multi-mode converter designed by an intelligent algorithm

Multi-mode converters, which can achieve spatial mode conversion in multimode waveguide, play a key role in multi-mode photonics and mode-division multiplexing (MDM). However, rapid design of high-performance mode converters with ultra-compact footprint and ultra-broadband operation bandwidth is still a challenge. In this work, through combining adaptive genetic algorithm (AGA) and finite element simulations, we present an intelligent inverse design algorithm and successfully designed a set of arbitrary-order mode converters with low excess losses (ELs) and low crosstalk (CT). At the communication wavelength of 1550 nm, the footprint of designed TE_0-n (n = 1, 2, 3, 4) and TE_2-n (n = 0, 1, 3, 4) mode converters are only 1.8 × 2.2 µm². The maximum and minimum conversion efficiency (CE) is 94.5% and 64.2%, and the maximum and minimum ELs/CT are 1.92/-10.9 dB and 0.24/-20 dB, respectively. Theoretically, the smallest bandwidth for simultaneously achieving ELs ≤ 3 dB and CT ≤ -10 dB exceeds 70 nm, which can be as large as 400 nm for the case of low-order mode conversion. Moreover, the mode converter in conjunction with a waveguide bend allows for mode-conversion in ultra-sharp waveguide bends, significantly increasing the density of on-chip photonic integration. This work provides a general platform for the realization of mode converters and has good prospect in application of multimode silicon photonics and MDM.

Metamaterial-enabled arbitrary on-chip spatial mode manipulation

On-chip spatial mode operation, represented as mode-division multiplexing (MDM), can support high-capacity data communications and promise superior performance in various systems and numerous applications from optical sensing to nonlinear and quantum optics. However, the scalability of state-of-the-art mode manipulation techniques is significantly hindered not only by the particular mode-order-oriented design strategy but also by the inherent limitations of possibly achievable mode orders. Recently, metamaterials capable of providing subwavelength-scale control of optical wavefronts have emerged as an attractive alternative to manipulate guided modes with compact footprints and broadband functionalities. Herein, we propose a universal yet efficient design framework based on the topological metamaterial building block (BB), enabling the excitation of arbitrary high-order spatial modes in silicon waveguides. By simply programming the layout of multiple fully etched dielectric metamaterial perturbations with predefined mathematical formulas, arbitrary high-order mode conversion and mode exchange can be simultaneously realized with uniform and competitive performance. The extraordinary scalability of the metamaterial BB frame is experimentally benchmarked by a record high-order mode operator up to the twentieth. As a proof of conceptual application, an 8-mode MDM data transmission of 28-GBaud 16-QAM optical signals is also verified with an aggregate data rate of 813 Gb/s (7% FEC). This user-friendly metamaterial BB concept marks a quintessential breakthrough for comprehensive manipulation of spatial light on-chip by breaking the long-standing shackles on the scalability, which may open up fascinating opportunities for complex photonic functionalities previously inaccessible.

Optical meta-waveguides for integrated photonics and beyond

The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous meta-waveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems, by enhancing light-matter interaction strength to drastically boost device performance, or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities. We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits, biomedical sensing, artificial intelligence and beyond.

Topology design of digital metamaterials for ultra-compact integrated photonic devices based on mode manipulation

Precise manipulation of mode order in silicon waveguides plays a fundamental role in the on-chip all-optical interconnections and is still a tough task in design when the functional region is confined to a subwavelength footprint. In this paper, digital metamaterials consisting of silicon and air pixels are topologically designed by an efficient method combining 2D finite element method for optical simulations, density method for material description and method of moving asymptotes for optimization. Only around 150 iterations are required for searching satisfactory solutions. Six high-quality and efficient conversions between four TE-polarized modes are achieved in a functional region with footprint 0.645λ ² (center wavelength λ = 1550 nm). Based on asymmetric mode conversion, a reciprocal optical diode with high contrast ratio is further obtained with the optimization starting from TE0-to-TE1 mode converter. Moreover, we successfully design a 1 × 2 demultiplexer with footprint 1.0λ ² and demonstrate a simple mode division multiplexing system with satisfactory performances. Finally, by changing the refractive index to an equivalent value, quasi-3D designs are obtained and the functionalities are validated in 3D simulations for both free-standing and SOI configurations.

Scaling and cascading compact metamaterial photonic waveguide filter blocks

In this work, we reported the design and fabrication of a compact and scalable metamaterial longpass filter with an ultrasmall footprint of 5.1µm×5.1µm. In the stopband, light transmission can be blocked and reflected with ∼25dB attenuation. In the passband, light can pass through with a low insertion loss around -0.28dB. The transition band can be redshifted or blueshifted by scaling up or down the filter block; i.e., scaling down 1% can produce a transition band blueshift of 11.4 nm. The power roll-off can be enhanced by cascading multiple filter blocks, i.e., 1.34 dB/nm by cascading three filters. These results demonstrate the great potential of the metamaterial-based waveguide devices for scalable photonic filtering applications.

Theoretical analysis of mode conversion by refractive-index perturbation based on a single tilted slot on a silicon waveguide

We propose a compact mode converter operating at the mid-infrared wavelength of 3.4 µm, comprising an etched parallelogram slot filled with silicon nitride on a silicon-on-calcium fluoride platform. The tilted slot introduces transverse and longitudinal index perturbations on the waveguide eigenmodes, achieving mode conversion in the propagation direction. Differing from previous reports using massive parameter sweep, we provide analytical formulas to determine geometry parameters by considering the modified phase-matching condition and the profiles of coupling coefficient of coupled-mode theory. Rigorous 3D numerical examples demonstrate the transverse electric (TE)₀-to-TE₁, TE₀-to-TE₂, TE₀-to-TE₃, and TE₀-to-TE₄ converters to achieve conversion efficiencies (inter-modal crosstalk [CT] values) of >92.7% (<-27 dB), >91.7% (<-16 dB), >88.2% (<-13 dB), and >75.8% (<-10 dB), respectively, with a total transmitted power of >93%. Converter device lengths range from 16.84 to 24.61 µm for TE₀-to-TE₁ to TE₀-to-TE₄, respectively. Over a broadband wavelength of 100 nm, the conversion efficiency, power transmission, and maximum inter-modal CT are almost >80%, >90%, and <-10 dB, respectively. Also, the fabrication tolerance of the proposed structure is addressed. The proposed model can not only realize arbitrary mode-order conversion but extend to other wavelength bands. To validate the feasibility of our model, the numerical results of our device operating at the wavelength of 1.55 µm are also offered and compared with those of other reports. The proposed idea may pave a new approach to designing mode converters with arbitrary geometries.

Crosstalk reduction of integrated optical waveguides with nonuniform subwavelength silicon strips

Suppression of the crosstalk between adjacent waveguides is important yet challenging in the development of compact and dense photonic integrated circuits (PICs). During the past few years, a few of excellent approaches have been proposed to achieve this goal. Here, we propose a novel strategy by introducing nonuniform subwavelength strips between adjacent waveguides. In order to determine the widths and positions of nonuniform subwavelength strips, the particle swarm optimization (PSO) algorithm is utilized. Numerical results demonstrate that the coupling length between adjacent waveguides is increased by three (five) orders of magnitude in comparison with the case of uniform (no) subwavelength strips. Our method greatly reduces crosstalk and is expected to achieve a highly compact integrated density of PICs.

Ultra-compact hybrid plasmonic mode convertor based on unidirectional eigenmode expansion

An ultra-compact hybrid plasmonic mode convertor is demonstrated based on a hybrid plasmonic slot waveguide structure. Benefiting from the unidirectional eigenmode expansion approach, a mode-interference-based ${{\rm TE}_{00}}$TE₀₀-to-${{\rm TM}_{01}}$TM₀₁ mode convertor is realized for the first time, to the best of our knowledge, with an ultra-compact footprint of only ${2}.{33} \times {7}\,\,\unicode{x00B5} {{\rm m}^2}$2.33×7µm². At the wavelength of 1550 nm, the insertion loss is below 2.34 dB, and the extinction ratio is 25.6 dB with mode conversion purity as high as 94.6%. The extinction ratio is over 15.5 dB for the entire C-band with a bandwidth of extinction ratio above 10 dB larger than 110 nm. The transmissivity of the crosstalk ${{\rm TE}_{10}}$TE₁₀ and ${{\rm TE}_{02}}$TE₀₂ at 1550 nm is $ - {16.1}$-16.1 and $ - {22.7}\,\,{\rm dB}$-22.7dB, respectively.

Design of a compact silicon-based TM-polarized mode-order converter based on shallowly etched structures

A mode-order converter is an indispensable component in some typical on-chip mode-division-multiplexing (MDM) systems. Here, we propose a compact and high-performance TM-polarized mode-order converter using shallowly etched structures on the silicon waveguide. This device consists of a rhombus etching part at the waveguide center and two same triangle etching parts on both sides of the waveguide along the propagation direction to achieve the mode conversion from the input ${{\rm TM}_0}$TM₀ to the output ${{\rm TM}_2}$TM₂ mode, where both the effective waveguide structure and the corresponding effective index distribution can be changed. By optimizing the dimensions of the rhombus and triangle etching parts as well as the relative position between them, we have realized the efficient ${{\rm TM}_0}$TM₀ to ${{\rm TM}_2}$TM₂ mode conversion in a conversion length of only 6 µm, which distinguishes from mainly reported TE-polarized mode-order converters, and the obtained mode conversion efficiency (CE), cross talk (CT), and insertion loss (IL) are $\sim{94}\% $∼94%, $ \lt - {15}\;{\rm dB}$<-15dB, and $\sim{0.5}\;{\rm dB}$∼0.5dB, respectively, at the wavelength of 1.55 µm. Meanwhile, the allowable bandwidth can be extended to 128 nm by keeping ${\rm CE} \gt {94}\% $CE>94%, where the mode CT and IL are $ \lt - {15}\;{\rm dB}$<-15dB and ${\rm IL} \lt {0.7}\;{\rm dB}$IL<0.7dB, respectively, in the same wavelength range. Also, the device fabrication processes and tolerance analyses have been done. We hope this device could be beneficial for the capacity improvement of the on-chip MDM systems.

Sub-wavelength grating assisted mode order converter on the SOI substrate

We have designed three mode order converters using the sub-wavelength grating on the silicon-on-insulator substrate. The proposed mode order converters can separately realize the mode order conversion from the fundamental transverse electric mode to the first-order transverse electric mode (TE₀-to-TE₁), the second-order transverse electric mode (TE₀-to-TE₂) and the third-order transverse electric mode (TE₀-to-TE₃) with compact device sizes and good performances. The simulation results show that the mode order conversion efficiencies of TE₀-to-TE₁, TE₀-to-TE₂ and TE₀-to-TE₃ are larger than 94.4%, 95.7% and 83.7% in the wavelength ranging from 1.5 µm to 1.6 µm, the corresponding device length are 8.72 µm, 4.98 µm and 14.54 µm. In addition, the mode order converter can be fabricated with only once etching which will be advantage to the device fabrication.

Spatial eigenmodes conversion with metasurfaces engraved in silicon ridge waveguides

We explore the discrete nature of waveguide modes and the effective medium concept to achieve an ultra-compact highly efficient mode conversion device in a high-index platform such as a silicon waveguide. The proposed device is based on a co-directional coupler that has a periodic variation in its refractive index along the propagation direction. The transverse variation of the index profile is calculated based on the interference pattern between the modes of interest. We show that mode conversion can be realized with dielectric metasurfaces engraved in the silicon waveguide. We derive the equation for effective index and show proof-of-concept numerical results of the device performance. We obtain conversion efficiencies of 95.4% between the TE₀-TE₁ modes over 8.91 μm interaction distance and 96.4% between the TE₀-TE₂ over 6.32 μm. The resulting coupling coefficient changes as a function of the interaction distance in a sinusoidal manner, which is crucial for constructive energy transfer from one mode to another. Such mode coupling devices have the potential for application in dispersion compensations, wavelength division multiplexing systems, and sensing.

Tailoring the emission polarization with metasurface-based emitters designed on a plasmonic ridge waveguide

Because of low propagation losses and flexible communication paths, inter-chip optical communications based on plasmonic emitters and receivers can overcome the obstacle of the inherent ohmic loss in metallic nanostructures. To increase the communication capacity and integration density in inter-chip optical communications, we propose to tailor the polarization states of the free-radiation fields from the emitters in both the spectral domain and spatial domain by designing the phased and polarized arrangement of subwavelength metallic nanogroove antennas on a two-dimensional plasmonic ridge waveguide. Herein, the utilization of the two-dimensional plasmonic waveguide with tight field confinements considerably decreases the crosstalk to nearby plasmonic devices in plasmonic circuits and chips. In the spectral domain, three different polarization states of the free-radiation fields from the same emitter are experimentally realized at three specific wavelengths. In the spatial domain, the different polarization states as well as the gear polarization states are experimentally demonstrated. Moreover, the separation of the adjacent polarization-tailoring plasmonic emitters is only 5% of that in dielectric emitters because of the ultra-compact size (<0.6λ2) of metasurface-based plasmonic emitters.

General silicon-on-insulator higher-order mode converter based on substrip dielectric waveguides

A general silicon mode-converter waveguide that converts a fundamental mode to any higher-order mode is proposed. Specifically, dielectric substrip waveguides are inserted in the fundamental mode propagation path so that the conversion is done directly in the same propagation waveguide, without coupling the power into another waveguide as it happens in traditional mode converters. The device has a very small footprint compared to traditional converters. A mathematical model is developed to determine the design parameters of the used dielectric material and analyze the whole performance of the proposed device. Both the effective index method (EIM) and the perturbative mode-coupled theory are used in our mathematical analysis to get exact values for both the coupling coefficient and the length of the used dielectric material, so as to ensure a maximum coupled power transfer to the higher-order mode. In addition, full vectorial 3D-FDTD simulations are performed to validate our mathematical model. Our results show good agreement between the approximate EIM method and accurate full vectorial 3D-finite-difference time-domain (FDTD) simulations in characterizing the device parameters and performance. In order to validate the design model, two mode converters are simulated, fabricated, and tested for converting a fundamental TE₀ mode into both first- and second-order (TE₁ and TE₂) modes, respectively. Good insertion losses and low crosstalks are obtained. Good agreement between simulated and fabricated results are achieved.

SiNx-Si interlayer coupler using a gradient index metamaterial

In this work, we demonstrate a metamaterial (MM)-based SiN_x-Si interlayer coupler. A gradient index (GRIN) MM was employed to achieve refractive index matching, balancing the large difference in the refractive index between Si (∼3.42) and SiN_x (∼2). In addition, a two-layer SiN_x interlayer coupler was proposed to further minimize the coupling loss and interlayer crosstalk. Finally, we demonstrated a three-dimensional (3D) SiN_x-Si interlayer coupler with the -0.6  dB insertion loss per layer propagation, with the 40 nm 1-dB-bandwidth (1530-1570 nm) and <-45  dB interlayer crosstalk. These promising results demonstrate the great potential of a GRIN MM-based SiN_x-Si interlayer coupler in 3D photonic integration.

On-chip silicon mode blocking filter employing subwavelength-grating based contra-directional coupler

Mode blocking may be required in mode-division multiplexing (MDM) systems. We demonstrate a silicon mode blocking filter using a subwavelength grating-based contra-directional coupler. The device is capable of blocking the undesired mode channel without affecting the propagation of the other modes. As a proof-of-concept experiment, two mode blocking filters are experimentally demonstrated, which can block the TE₀ mode and the TE₁ mode, respectively. Low crosstalk (≤21.0 dB) and reasonable insertion losses (≤2.3 dB) are achieved for both the TE₀-mode- and the TE₁-mode blocking filters.

Vertically integrated visible and near-infrared metasurfaces enabling an ultra-broadband and highly angle-resolved anomalous reflection

An optical device with minimized dimensions, which is capable of efficiently resolving an ultra-broad spectrum into a wide splitting angle but incurring no spectrum overlap, is of importance in advancing the development of spectroscopy. Unfortunately, this challenging task cannot be easily addressed through conventional geometrical or diffractive optical elements. Herein, we propose and demonstrate vertically integrated visible and near-infrared metasurfaces which render an ultra-broadband and highly angle-resolved anomalous reflection. The proposed metasurface capitalizes on a supercell that comprises two vertically concatenated trapezoid-shaped aluminum antennae, which are paired with a metallic ground plane via a dielectric layer. Under normal incidence, reflected light within a spectral bandwidth of 1000 nm ranging from λ = 456 nm to 1456 nm is efficiently angle-resolved to a single diffraction order with no spectrum overlap via the anomalous reflection, exhibiting an average reflection efficiency over 70% and a substantial angular splitting of 58°. In light of a supercell pitch of 1500 nm, to the best of our knowledge, the micron-scale bandwidth is the largest ever reported. It is noted that the substantially wide bandwidth has been accomplished by taking advantage of spectral selective vertical coupling effects between antennae and ground plane. In the visible regime, the upper antenna primarily renders an anomalous reflection by cooperating with the lower antenna, which in turn cooperates with the ground plane and produces phase variations leading to an anomalous reflection in the near-infrared regime. Misalignments between the two antennae have been particularly inspected to not adversely affect the anomalous reflection, thus guaranteeing enhanced structural tolerance of the proposed metasurface.

Bifunctional gap-plasmon metasurfaces for visible light: polarization-controlled unidirectional surface plasmon excitation and beam steering at normal incidence

Integration of multiple diversified functionalities into a single, planar and ultra-compact device has become an emerging research area with fascinating possibilities for realization of very dense integration and miniaturization in photonics that requires addressing formidable challenges, particularly for operation in the visible range. Here we design, fabricate and experimentally demonstrate bifunctional gap-plasmon metasurfaces for visible light, allowing for simultaneous polarization-controlled unidirectional surface plasmon polariton (SPP) excitation and beam steering at normal incidence. The designed bifunctional metasurfaces, consisting of anisotropic gap-plasmon resonator arrays, produce two different linear phase gradients along the same direction for respective linear polarizations of incident light, resulting in distinctly different functionalities realized by the same metasurface. The proof-of-concept fabricated metasurfaces exhibit efficient (>25% on average) unidirectional (extinction ratio >20 dB) SPP excitation within the wavelength range of 600-650 nm when illuminated with normally incident light polarized in the direction of the phase gradient. At the same time, broadband (580-700 nm) beam steering (30.6°-37.9°) is realized when normally incident light is polarized perpendicularly to the phase gradient direction. The bifunctional metasurfaces developed in this study can enable advanced research and applications related to other distinct functionalities for photonics integration.

Theory for Perfect Transmodal Fabry-Perot Interferometer

We establish the theory for perfect transmodal Fabry-Perot interferometers that can convert longitudinal modes solely to transverse modes and vice versa, reaching up to 100% efficiency. Two exact conditions are derived for plane mechanical waves: simultaneous constructive interferences of each of two coupled orthogonal modes, and intermodal interference at the entrance and exit sides of the interferometer with specific skew polarizations. Because the multimodal interferences and specific skew motions require unique anisotropic interferometers, they are realized by metamaterials. The observed peak patterns by the transmodal interferometers are similar to those found in the single-mode Fabry-Perot resonance, but multimodality complicates the involved mechanics. We provide their design principle and experimented with a fabricated interferometer. This theory expands the classical Fabry-Perot resonance to the realm of mode-coupled waves, having profound impact on general wave manipulation. The transmodal interferometer could sever as a device to transfer wave energy freely between dissimilar modes.

Controlling propagation and coupling of waveguide modes using phase-gradient metasurfaces

Research on two-dimensional designer optical structures, or metasurfaces, has mainly focused on controlling the wavefronts of light propagating in free space. Here, we show that gradient metasurface structures consisting of phased arrays of plasmonic or dielectric nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Based on this design principle, we experimentally demonstrate waveguide mode converters, polarization rotators and waveguide devices supporting asymmetric optical power transmission. We also demonstrate all-dielectric on-chip polarization rotators based on phased arrays of Mie resonators with negligible insertion losses. Our gradient metasurfaces can enable small-footprint, broadband and low-loss photonic integrated devices.