Multi-channel broadband nonvolatile programmable modal switch.

Mode-division multiplexing (MDM) in chip-scale photonics is paramount to sustain data capacity growth and reduce power consumption. However, its scalability hinges on developing efficient and dynamic modal switches. Existing active modal switches suffer from substantial static power consumption, large footprints, and narrow bandwidth. Here, we present, for the first time, to the best of our knowledge, a novel multiport, broadband, non-volatile, and programmable modal switch designed for on-chip MDM systems. Our design leverages the unique properties of integrating nanoscale phase-change materials (PCM) within a silicon photonic architecture. This enables independent manipulation of spatial modes, allowing for dynamic, non-volatile, and selective routing to six distinct output ports. Crucially, our switch outperforms current dynamic modal switches by offering non-volatile, energy-efficient multiport functionality and excels in performance metrics. Our switch exhibits exceptional broadband operating bandwidth exceeding 70 nm, with low loss (< 1 dB), and a high extinction ratio (> 10 dB). Our framework provides a step forward in chip-scale MDM, paving the way for future green and scalable data centers and high-performance computers.

Introduction to the special feature: amplify Black voices in optics and photonics.

This multi-journal special issue highlights the work of Black scientists and engineers in optics and photonics to accomplish the goal of engaging the entire optics and photonics community and bring awareness to the quality of their research and contributions to the field.

Near-unity uniformity and efficiency broadband meta-beam-splitter/combiner.

Subwavelength planar structured interfaces, also known as metasurfaces, are ultra-thin optical elements modulating the amplitude, phase, and polarization of incident light using nanostructures called meta-atoms. The optical properties of such metasurfaces can be controlled across wavelengths by selecting geometries and materials of the meta-atoms. Given recent technological developments in optical device miniaturization, components for beam splitting and beam combining are sought for use within these devices as two quintessential components of every optical setup. However, realizing such devices using metasurfaces typically leads to poor uniformity of diffraction orders and narrow-band operation. Using a modified version of particle swarm optimization, we propose and numerically demonstrate a broadband, reciprocal metasurface beam combiner/splitter with uniformity > 97% and diffraction efficiency > 90% in the continuous band from λ=1525 nm to λ=1575 nm. The proposed approach significantly extends the current state of the art of metasurfaces design in terms of uniformity, bandwidth, and efficiency, and opens the door for devices requiring high power or near-unit uniformity.

Plasmonic loss-mitigating broadband adiabatic polarizing beam splitter.

The intriguing analogy between quantum physics and optics has inspired the design of unconventional integrated photonics devices. In this paper, we numerically demonstrate a broadband integrated polarization beam splitter (PBS) by implementing the stimulated Raman adiabatic passage (STIRAP) technique in a three-waveguide plasmonic system. Our proposed PBS exhibits >250 nm transverse-magnetic (TM) bandwidth with <-40 dB extinction and >150 nm transverse-electric (TE) bandwidth with <-20 dB extinction, covering the entire S-, C-, and L-bands and part of the E-band. Moreover, near-lossless light transfer is achieved in our system despite the incorporation of a plasmonic hybrid waveguide because of the unique loss mitigating feature of the STIRAP scheme. Through this approach, various broadband integrated devices that were previously impossible can be realized, which will allow innovation in integrated optics.

Tunable polarization mode conversion using thin-film lithium niobate ridge waveguide.

Lithium niobate on insulator (LNOI) waveguides, as an emerging technology, have proven to offer a promising platform for integrated optics, due to their strong optical confinement comparable to silicon on insulator (SOI) waveguides, while possessing the versatile properties of lithium niobate, such as high electro-optic coefficients. In this paper, we show that mode hybridization, a phenomenon widely found in vertically asymmetric waveguides, can be efficiently modulated in an LNOI ridge waveguide by electro-optic effect, leading to a polarization mode converter with 97% efficiency. Moreover, the proposed device does not require tapering or periodic poling, thereby greatly simplifying the fabrication process. It can also be actively switched by external fields. Such a platform facilitates technological progress of photonics circuits and sensors.

Local field enhancement using a photonic-plasmonic nanostructure.

Over the last few years, optical nanoantennas are continuously attracting interest owing to their ability to efficiently confine, localize resonance, and significantly enhanced electromagnetic fields at a subwavelength scale. However, such strong confinement can be further enhanced by using an appropriate combination of optical nanoantennas and Slanted Bound states in the continuum cavities. Here, we propose to synergistically bridge the plasmonic nanoantennas and high optical quality-factor cavities to numerically demonstrate six orders of magnitude local intensity enhancement without critical coupling conditions. The proposed hybrid system paves a new way for applications requiring highly confined fields such as optical trapping, optical sensing, nonlinear optics, quantum optics, etc.

Diffraction-limited broadband optical meta-power-limiter.

In recent years, the need for a high-power laser has been of great interest for different applications, including direct-laser processing, light detection, medicine, and lighting. However, high-power lasers with high intensities give rise to fundamental problems for optical detectors and imaging systems with low threshold damage, which still need reliable solutions. Here we report and numerically demonstrate a hybrid system that synergistically combines a broadband OPL with a transmittance difference between on-state (70°C) and off-state (25°C) about 62.5%, and a diffraction-limited broadband metalens from 1534 to 1664 nm. Such a metalens power limiter could be used in any system requiring an intermediate focal plane in the optical path to the detector from damage by exposure to high-intensity lasers.

Octave bandwidth photonic fishnet-achromatic-metalens.

Planar structured interfaces, also known as metasurfaces, are continuously attracting interest owing to their ability to manipulate fundamental attributes of light, including angular momentum, phase, or polarization. However, chromatic aberration, limiting broadband operation, has remained a challenge for metasurfaces-based optical components and imagers. The limitation stems from the intrinsic dispersion of existing materials and design principles. Here we report and experimentally demonstrate polarization-independent fishnet-achromatic-metalenses with measured average efficiencies over 70% in the continuous band from the visible (640 nm) to the infrared (1200 nm). Results of the scalable platform are enabling for applications requiring broad bandwidth and high efficiency including energy harvesting, virtual reality and information processing devices, or medical imaging.

Broadband and linear polarization metasurface carpet cloak in the visible.

In the past few years, carpet cloaking has attracted interest because of its feasibility at optical frequencies and potential in stealth technologies. Metasurfaces have been proposed as a method to engineer ultra-thin carpet cloaking surfaces due to their abilities to manipulate wavefronts, polarization, and phase at subwavelength scale. However, achieving broadband carpet cloaking with a significant bandwidth is one of the key remaining challenges for metasurface designs. To date, broadband carpet cloaking based on metasurfaces has not been achieved, and operation has been limited to discrete wavelengths. Here, we propose and numerically demonstrate a novel metasurface design for broadband carpet cloaking with linear polarization at visible wavelengths from 650 nm to 800 nm. Our proposed method is a promising approach for broadband structured interfaces.

Planar dielectric cylindrical lens at 800 nm and the role of fabrication imperfections.

Conventional optical components have been proposed to realize high-quality line focusing with uniform intensity distribution such as cylindrical lenses, segmented wedge-arrays, or a combination of prisms and spherical mirrors. Numerous factors such as the manufacturing tolerances or the need for precise alignment of conventional lenses cause wave front aberrations that impact the performance of optical systems. These aforementioned limitations affect the uniformity of the intensity distribution and the intercept factor of lenses. Here, we experimentally demonstrate an integrable planar dielectric cylindrical lens made of titanium dioxide for uniform line focusing and discuss the sensitivity of its performance to fabrication imperfections originating from non-ideal geometrical parameters. The lens has a numerical aperture of 0.247, an intercept factor of 0.85, and an efficiency of 79% at 800 nm.

Integration of Nanomaterials into Three-Dimensional Vertical Architectures.

A novel layer-by-layer three-dimensional (3D) architecture allowing one to expand device fabrication in the vertical direction and integrating functional nanomaterials is presented by emulating civil engineering. The architecture uses SU-8 pillars as structural columns, which support multiple horizontal suspended thin films. The films then serve as platforms for the integration of nanomaterials and nanodevices. Multiple graphene layers suspended across SU-8 pillars with precise control on their vertical spacing are demonstrated. In addition to graphene, silicon nitride films that offer high strength yield and thickness control are also presented. Metallic microstructures, plasmonic nanostructures, semiconducting quantum dots, and monolayer graphene on the suspended films are achieved to prove the capability of integrating functional nanomaterials. This work provides the potential to integrate highly compact micro/nanoscale devices at different vertical levels with high surface density, which allows for more capabilities and functionalities in a single device.

Local phase method for designing and optimizing metasurface devices.

Metasurfaces have attracted significant attention due to their novel designs for flat optics. However, the approach usually used to engineer metasurface devices assumes that neighboring elements are identical, by extracting the phase information from simulations with periodic boundaries, or that near-field coupling between particles is negligible, by extracting the phase from single particle simulations. This is not the case most of the time and the approach thus prevents the optimization of devices that operate away from their optimum. Here, we propose a versatile numerical method to obtain the phase of each element within the metasurface (meta-atoms) while accounting for near-field coupling. Quantifying the phase error of each element of the metasurfaces with the proposed local phase method paves the way to the design of highly efficient metasurface devices including, but not limited to, deflectors, high numerical aperture metasurface concentrators, lenses, cloaks, and modulators.

Nonreciprocal lasing in topological cavities of arbitrary geometries.

Resonant cavities are essential building blocks governing many wave-based phenomena, but their geometry and reciprocity fundamentally limit the integration of optical devices. We report, at telecommunication wavelengths, geometry-independent and integrated nonreciprocal topological cavities that couple stimulated emission from one-way photonic edge states to a selected waveguide output with an isolation ratio in excess of 10 decibels. Nonreciprocity originates from unidirectional edge states at the boundary between photonic structures with distinct topological invariants. Our experimental demonstration of lasing from topological cavities provides the opportunity to develop complex topological circuitry of arbitrary geometries for the integrated and robust generation and transport of photons in classical and quantum regimes.

Hybridized metamaterial platform for nano-scale sensing.

Plasmonic/metamaterial sensors are being investigated for their high sensitivity, fast response time, and high accuracy. We propose, characterize and experimentally realize subwavelength bilayer metamaterial sensors operating in the near-infrared domain. We measure the figure-of-merit (FOM) and the bulk sensitivity (S) of the two fundamental hybridized modes and demonstrate both numerically and experimentally that the magnetic dipolar mode, degenerate with the electric quadrupolar mode, has higher sensitivity to a variation of the refractive index compared to the electric dipolar mode. In addition, the hybridized system exhibits a four fold increase in the FOM compared to a standard dipolar plasmonic system.

From parabolic-trough to metasurface-concentrator: assessing focusing in the wave-optics limit.

Metasurfaces are promising tools toward novel designs for flat optics applications. As such, their quality and tolerance to fabrication imperfections need to be evaluated with specific tools. However, most such tools rely on the geometrical optics approximation and are not straightforwardly applicable to metasurfaces. In this Letter, we introduce and evaluate for metasurfaces parameters such as intercept factor and slope error usually defined for solar concentrators in the realm of ray-optics. After proposing definitions valid in physical optics, we put forward an approach to calculate them. As examples, we design three different concentrators based on three specific unit cells and assess them numerically. The concept allows for comparison of the efficiency of the metasurfaces and their sensitivities to fabrication imperfections and will be critical for practical systems implementation.

Tunable Bloch surface waves in anisotropic photonic crystals based on lithium niobate thin films.

We present an original type of one-dimensional photonic crystal that includes one anisotropic layer made of a lithium niobate thin film. We demonstrate the versatility of such a device sustaining different Bloch surface waves (BSWs), depending on the orientation of the incident wave. By varying the orientation of the illumination of the multilayer, we measured an angle variation of 7° between the BSWs corresponding to the extraordinary and the ordinary index of the lithium niobate thin film. The potential of such a platform opens the way to novel tunable and active planar optics based on the electro- and thermo-optical properties of lithium niobate.

Guided resonances on lithium niobate for extremely small electric field detection investigated by accurate sensitivity analysis.

We present a theoretical study of guided resonances (GR) on a thin film lithium niobate rectangular lattice photonic crystal by band diagram calculations and 3D Finite Difference Time Domain (FDTD) transmission investigations which cover a broad range of parameters. A photonic crystal with an active zone as small as 13µm×13µm×0.7µm can be easily designed to obtain a resonance Q value in the order of 1000. These resonances are then employed in electric field (E-field) sensing applications exploiting the electro optic (EO) effect of lithium niobate. A local field factor that is calculated locally for each FDTD cell is proposed to accurately estimate the sensitivity of GR based E-field sensor. The local field factor allows well agreement between simulations and reported experimental data therefore providing a valuable method in optimizing the GR structure to obtain high sensitivities. When these resonances are associated with sub-picometer optical spectrum analyzer and high field enhancement antenna design, an E-field probe with a sensitivity of 50 µV/m could be achieved. The results of our simulations could be also exploited in other EO based applications such as EEG (Electroencephalography) or ECG (Electrocardiography) probe and E-field frequency detector with an 'invisible' probe to the field being detected etc.

Fano resonance-based highly sensitive, compact temperature sensor on thin film lithium niobate.

In this Letter, we report a Fano resonance-based highly sensitive and compact temperature sensor fabricated on thin film lithium niobate (TFLN) Suzuki phase lattice (SPL) photonic crystal. The experimental sensitivity is estimated to be 0.77 nm/°C with a photonic crystal size of only 25 µm × 24 µm. This sensitivity is 38 times larger than the intrinsic one of lithium niobate which is 0.02 nm/°C. The demonstrated sharp and high extinction ratio characteristics of the Fano lineshape resonance could be an excellent candidate in developing a high sensitivity temperature sensor, electric field sensor, etc.

Low-loss LiNbO(3) tapered-ridge waveguides made by optical-grade dicing.

We report on low-loss vertical tapers for efficient coupling between confined LiNbO3 optical ridge waveguides and Single Mode Fibers. 3D-Pseudo-Spectral-Time-Domain calculations and Optical-Coherence-Tomography-based methods are advantageously used for the numerical and experimental study of the tapers. The tapered-section is done simultaneously with the ridge waveguide by means of a circular precision dicing saw, so that the fabrication procedure is achieved in only two steps. The total insertion losses through a 1.6 cm long ridge waveguide are measured to be improved by 3 dB in presence of the taper. These tapered-ridge waveguides open the way to the low-cost production of low-loss phase modulators or resonators.