Photonic Structures Research Articles

Critical Design and Operating Parameters of Active Waveguide Bragg Gratings for Laser Performance.

Active waveguide Bragg gratings (AWBGs) are promising photonic structures that combine the very efficient reflective properties of a Bragg grating with the power amplification character of rare earths. This combination may lead to a potential monolithic laser under the proper conditions. However, the photonic response of these structures highly depends on the grating design and operating parameters, so modeling its response for their laser performance is a must. In this work, a numerical method is employed to calculate the optical power propagation along an Er/Yb-codoped integrated AWBG. A complete database of the AWBG response as a function of the most relevant operational parameters is obtained. As a result, the critical values of each design and operating parameter to achieve its laser performance have been determined, which represents very useful information for further experimental design, optimization, and fabrication of these photonic structures.

A single nanophotonic platform for producing circularly polarized white light from non-chiral emitters

Direct manipulation of light spin-angular momentum is desired in optoelectronic applications such as, displays, telecommunications, or imaging. Generating polarized light from luminophores avoids using optical components that cause brightness losses and hamper on-chip integration of light sources. Endowing chirality to achiral emitters for direct generation of polarized light benefits from existing materials and can be achieved by chiral nanophotonics. However, most chiral nanostructures operate in narrow wavelength ranges and involve nanofabrication processes incompatible with high-throughput production. Here, a single nanophotonic architecture is designed to sustain chiroptical resonances along the visible spectrum. This platform, fabricated with scalable soft-nanoimprint lithography transfers its chirality to conventional emitters (CdSe/CdS nanoplatelets, CdSe/CdS quantum dots, CsPbBr3, CsPbI3 perovskite nanocrystals and F8BT) placed atop, achieving a high dissymmetry emission factor (glum > 1). The dynamics study suggests enhanced out-coupling efficiency for one helicity by the photonic structure. Finally, a white light-emitting blend containing different emitters shows simultaneous dissymmetric emission values along the visible spectrum with this chiral nanophotonic platform.

Retraction Note: Two-ways chip to chip communications through 2-dimensional photonic structures via photonic integrated circuit

Retraction Note: Two-ways chip to chip communications through 2-dimensional photonic structures via photonic integrated circuit

Programmable nonlinear optical neuromorphic computing with bare 2D material MoS2

Nonlinear optical responses in two-dimensional (2D) materials can build free-space optical neuromorphic computing systems. Ensuring the high performance and the tunability of the system is essential to encode diverse functions. However, common strategies, including the integration of external electrode arrays or photonic structures with 2D materials, and barely patterned 2D materials, exhibit a contradiction between performance and tunability. Because the unique band dispersions of 2D materials can provide hidden paths to boost nonlinear responses independently, here we introduced a new free-space optical computing concept within a bare molybdenum disulfide array. This system can preserve high modulation performance with fast speed, low energy consumption, and high signal-to-noise ratio. Due to the freedom from the restrictions of fixed photonic structures, the tunability is also enhanced through the synergistic encodings of the 2D cells and the excitation pulses. The computing mechanism of transition from two-photon absorption to synergistic excited states absorption intrinsically improved the modulation capability of nonlinear optical responses, revealed from the relative transmittance modulated by a pump-probe-control strategy. Optical artificial neural network (ANN) and digital processing were demonstrated, revealing the feasibility of the free-space optical computing based on bare 2D materials toward neuromorphic applications.

Photonic Aptasensor Based on the Smart Cholesteric Liquid Crystal Network Structure for Cylindrospermopsin Detection.

Solid-state cholesteric liquid crystals (CLCsolid) with one-dimensional photonic structure offer a promising platform for constructing chemical and biological optical sensors, owing to their facile fabrication, signal readout, and sensitive and selective responsiveness to target analytes. In this study, we designed a CLCsolid photonic structure intertwined with an interpenetrating polymeric network (IPN) immobilized with a cylindrospermopsin aptamer (CY9) for the selective detection of the cylindrospermopsin toxin (CYT) in water. Upon exposure to CYT, it induced a blue shift in the color of the IPNCY9 biosensor chip. This shift occurred because the CY9 aptamer selectively bound to the CYT, reducing the polarity of the IPN hydrogel, leading to water release and shrinkage of the photonic structure. The IPNCY9 biosensor chips demonstrated the ability to detect CYT within a linear range of 4.2-120 nM, with a limit of detection of 2.55 nM. This innovative biosensor chip not only provides a new strategy for designing targeted toxin biosensors by immobilizing different receptors but also exhibits significant potential for use in portable kits for remote areas.

Defect engineering in organic semiconductor based metal-dielectric photonic crystals

We investigate the band structure of metal-dielectric photonic crystals comprising stacked organic semiconductor microcavities with silver metal mirrors incorporating crystal defects: individual unit cells with aperiodic dimensionality. Both transfer matrix simulation and experimental verification are performed to investigate the impact on the photonic band structure as a single cavity is varied in size. The resulting mid-gap defect states are shown to hybridize with a photonic band at certain resonant dimensions. The resonance of the defect cavity affects the transmittance of light through the device, disrupting or enhancing the coupling between otherwise resonant cavities. We outline potential applications for defect engineering of these devices through controlled manipulation of the transmission spectrum.

(Invited) Engineering Anodic Porous Alumina-Based Photonic Structures and Prospective Applications

Nanoporous anodic alumina (NAA) has become a popular material because of cost-competitive fabrication processes and fully scalable process compatible with conventional micro- and nanofabrication approaches. NAA is a nanostructured material that under specific conditions, their porous structure presents a self-ordering defined by a close-packed hexagonal array of well-defined cylindrical nanopores. The outstanding set of properties of NAA such as self-organized nanoporous structure, straight cylindrical nanopores of high aspect ratio, optical properties, chemical resistance and thermal stability and intrinsic photoluminescence demonstrate its versatility and potential [1]NAA is produced by electrochemical etching of aluminum, and their geometric characteristics such as pore diameter and length and separation distance between pores can be precisely tuned by the anodization conditions (voltage and time of anodization, temperature, and electrolyte) and by post-anodization treatments (etching and annealing). The highly effective surface area (hundreds of m2/cm3) makes of NAA an interesting platform for sensing and loading and releasing of active agents. Recently, different anodization approaches have been proposed to create new structures and pore geometries such as cone-like, funnel-like, modulated, tip-like, etc. [2-4].NAA has been used to fabricate photonic structures with the aim to control, enhance or inhibit the propagation of light. Photonic properties can be obtained by engineering the pore shape. For instance, the application of periodic variations of current or voltage during the anodization is transferred to the material as the periodic variation of the pore diameter and consequently, it is possible design, 3D structures and photonic structures with stop bands tunable within the UV-VIS-NIR range [5]. The fabrication of new photonic structures with single or multiple narrow photonic stopbands at different spectral positions, remains a challenge. These photonic structures can be obtained by using non-conventional pulse anodization for example, averaging the sum of multiple sinusoidal waves, Gaussian-like pulse or combining a gold coating layer on top of a NAA−photonic structure to create a hybrid metal−dielectric structure (Tamm plasmon resonance) [6-8]In this work, will show recent advances in the design and fabrication of nanoporous anodic alumina and their photonic and optic applications such as optical encoding tags, optical sensing, photonics and photovoltaics. We will analyze different technological parameters and its effect on the structure and future applications. Acknowledgements This work was supported by the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN/FEDER) PDI2021-128342OB-I00, by the Agency for Management of University and Research Grants (AGAUR) 2021-SGR-00739 and by the Catalan Institution for Research and Advanced Studies (ICREA) under the ICREA Academia Award 2021.

(Invited) Designing Solution-Processed Photonic Light- and Heat-Management Structures for Solution-Processable and Printable Organic Optoelectronic Devices

An ever-increasing interest in the development and application of innovative optical and optoelectronic devices places greater emphasis for the advancement of new smart and functional materials that are readily processable. Significant progress has already been realized in the fields of organic light-emitting diodes (OLEDs) and photovoltaic cells (OPVs) through development of novel semiconducting materials. Here we discuss developments and advancements in materials design towards photonic structures that aid and improve light management in organic and inorganic/organic hybrid devices, with focus on solar cells. Specifically, we present a novel inorganic/organic hybrid material that is produced in a one-pot synthesis and is solution processable, and can manipulated to exhibit a refractive index of 1.5 to up to 2.05. The material is of very low optical loss and, thus, can be readily integrated into efficient photonic structures. We cover systems targeted for use in light in-coupling structures, anti-reflection coatings, and beyond. Extension to architectures for heat management, important for a broad range of photovoltaic device platforms, including inorganic, inorganic/organic hybrid and organic devices, will also be presented.

Nanocellulose-Based Chiroptical Sensors for Detecting Circularly Polarized Light

Natural materials exhibit fascinating optical properties that manipulate light across multiple length scales through hierarchical photonic structures, forming a promising platform for advanced optical materials. However, developing photonic structures that are both environmentally sustainable and capable of maintaining superior optical and structural properties for use in electronic devices remains a significant challenge.In this work, we present chiroptical nanocellulose papers as flexible circularly polarized light (CPL) photodetectors. Optoelectrically activated 2D semiconducting channels are seamlessly integrated onto the nanocellulose papers, enabling high polarization-sensitive photoresponsivity that detects CPL handedness, facilitated by selective light propagation with substantial asymmetry. These flexible devices also demonstrate excellent mechanical durability while retaining their structural and chiroptical properties. We will discuss polarization-dependent optoelectronic behavior in the context of CPL sensing, with applications in next-generation bio-photonic devices.

(Invited) Advancing Quantitative, Contactless, Non-Destructive Characterization of Photonic and Plasmonic Devices

Non-destructive, contactless, and fast optical spectroscopy and imaging strategies provide great advantages for the modelling and optimization of thin films and nanostructured materials in photonic and optoelectronic applications. In particular, accurate, high-precision determination of the dispersion properties of sub-wavelength materials remains challenging and overlooked despite its massively demonstrated need and benefits to the semiconductor industry. Over the years, our group has used these strategies to study self-assembled nanolasers, hybrid photonic-plasmonic sensors, and photovoltaic devices. To adapt to these challenges we develop a modelling strategy based on Finite-Difference Time-Domain (FDTD) due to its flexibility and generality that makes it suitable to model non-linear and photo-thermal effects. The advantages of FDTD include i) the ability to obtain a wide spectral response from a single time-domain simulation and ii) its capability to model non-periodic structures. Significantly, we demonstrated that quantitative FDTD modelling can achieve sub-monolayer precision thus matching the capabilities of spectroscopic ellipsometry (SE). In turn, SE stands out from other optical techniques because, as a self-referenced and phase sensitive strategy, its demonstrated precision and sensitivity and has been demonstrated in multiple spectral ranges and operation conditions. We will present and discuss the synergic use of the FDTD-SE strategy as a quantitative tool for the characterization of photonic and plasmonic subwavelength structures of interest in optoelectronic, sensing, and energy applications.We gratefully acknowledge the financial support from the RGC (Projects CityU - 11210218, 11219919, 11215121 and 11310122) and ITC (Project ITS/461/18) of HKSAR, China.

(Invited) Self-Assembled Si Color Centers: Confinement to the Nanoscale Via Ultra-Low Temperature Molecular Beam Epitaxy

Single photon emission in the telecom wavelength range from isolated silicon (Si) color centers (CCs) was recently demonstrated [1,2], renewing the interest in this well-known defect class [3], originating from radiation damage in Si. It can be envisioned that single SiCCs lead to scalable deterministic group-IV-based quantum light sources [4]. Moreover, recent results for the carbon (C) based G- and T-centers outline their promising spin properties [5,6] for realizing spin-photon interfaces on a scalable Si quantum photonics platform.However, a severe obstacle remains towards their high-yield photonic integration, which is the standard SiCC fabrication process that includes single- or two-step ion implantation [1,7]. The resulting ion implantation profiles are broad and lead to a decisive lack of control over the vertical emitter position. This drawback significantly degrades the coupling efficiency to photonic structures such as resonators or waveguides. Additionally, no two SiCC emitters will be located vertically at the same position below the substrate surface, and determining the depth of isolated emitters is difficult, especially when using non-destructive methods.This contribution presents a completely different, all-epitaxial approach for fabricating variable SiCCs, departing entirely from ion implantation [8]. The presented fabrication process enables us to restrict the formation of SiCCs to a specific epilayer and control their vertical position in a structure even with sub-nm precision [8]. They self-assemble in situ during ultra-low temperature (ULT) growth [9-11] of thin Si and Si:C layers deposited at TG=200°C via molecular beam epitaxy (MBE). While the ULT growth just allows for the lattice disorder required for their formation, exceptionally pristine chamber conditions ensure a high optical quality of the SiCCs and allow fully crystalline Si overgrowth at higher TG. To verify the vertical position control, we compare the photoluminescence signal of the created CCs for a systematic variation of TG of active layer and cap vs. undoped references. The emitter density can be conveniently controlled via C doping concentration and Si:C layer thickness. Furthermore, we show the first results for electrically-pumped self-assembled SiCCs by integrating the Si:C nanolayers into ULT-grown p-i-n light-emitting diodes.

Tantalum pentoxide on a fused quartz substrate platform for advanced photonic integrated circuits

Integrated photonics is a growing field in optics and microelectronics. In particular, tantalum pentoxide (Ta2O5) is a promising material for advancing integrated photonic circuits. Ta2O5 exhibits favorable characteristics, such as a high refractive index, wide transparency window, and low autofluorescence. Therefore, this study develops low-loss Ta2O5 waveguide-based microring resonators optimized for telecom band operations on fused quartz substrates. The experiments demonstrated the excellent optical properties of Ta2O5 for fabricating high-performance photonic structures. Moreover, we explored integrating diamond-inverted nanocones with Ta2O5 waveguides for single-photon emission. The findings provide insights into using Ta2O5 to develop single-photon emitters integrated into photonic circuits.

Non-Hermitian ideal Weyl photonic metamaterials and polarization-momentum resolved ultrahigh absorption.

In the past decade, there has been a significant surge of interest in investigating non-Hermitian Hamiltonians, particularly in photonics. The eigenvalues of general non-Hermitian Hamiltonians are complex and possess unique topological features such as exceptional degeneracy. The introduction of non-Hermitian perturbations into Weyl semimetals can transform Weyl points into exceptional rings characterized by multiple topological invariants. However, the ideal realization of Weyl rings within practical three-dimensional structures has remained a significant challenge. In this work, we extend artificial photonic metamaterial structures that can transform ideal Weyl points into non-Hermitian exceptional rings. We show the associated intriguing polarization-momentum ultrahigh absorption, which enables what we believe to be a new device application in non-Hermitian photonics. Our study not only proposes the practical model for ideal non-Hermitian photonic Weyl exceptional rings but also opens the gate of non-Hermitian scattering characterization.

Bioinspired Microhinged Actuators for Active Mechanism-Based Metamaterials.

Mechanism-based metamaterials, comprising rigid elements interconnected by flexible hinges, possess the potential to develop intelligent micromachines with programmable motility and morphology. However, the absence of efficient microactuators has constrained the ability to achieve multimodal locomotion and active shape-morphing behaviors at the micro and nanoscale. In this study, inspiration from the flight mechanisms of tiny insects is drawn to develop a biomimetic microhinged actuator by integrating compliant mechanisms with soft hydrogel muscle. A Pseudo-Rigid-Body mechanical model is introduced to analyze structural deformation, demonstrating that this hydrogel-based microactuator can undergo significant folding while maintaining high structural stiffness. Furthermore, multiple microhinged actuators are combined to facilitate folding in multiple degrees of freedom and arbitrary directions. Fabricated by a multi-step four-dimensional (4D) direct laser writing technique, the microhinged actuators are integrated into 2D and 3D metamaterials enabling programable shape morphing. Additionally, micro-kirigami with photonic structures is demonstrated to show the pattern transforming actuated by the microhinges. This bioinspired design approach opens new avenues for the development of active mechanism-based metamaterials capable of intricate shape-morphing behaviors.

Enhancing the performance of graphene and LCP 1x2 rectangular microstrip antenna arrays for terahertz applications using photonic band gap structures

The terahertz (THz) frequency band (0.1–10 THz) has drawn a lot of attention due to the growing demand for greater resolutions, lower latency, faster data rates, and wider bandwidth in 6 G technologies. This range provides data speeds exceeding tens of gigabits per second, large bandwidth, great spectral resolution, and non-ionizing characteristics. THz signals have potential; however they are affected by attenuation, route losses, and atmospheric conditions, necessitating the use of specialised antenna designs. This work presents a 300 GHz rectangular microstrip patch antenna with Graphene as the patch material and Liquid Crystal Polymer (LCP) as the substrate. Photonic band gap (PBG) substrates are used to incorporate cuboid and cylindrical air gaps in square and triangular lattices, hence improving performance. The highest performance is found with cylindrical air gaps in a triangular lattice PBG substrate, which has a bandwidth of 29.56 GHz, a return loss of –48.12 dB, a gain of 10.4 dBi, a directivity of 10.8 dBi, and a radiation efficiency of 91 %. These results establish the proposed antennas as highly effective for broadband and high-speed THz applications, particularly in 6 G systems like advanced sensing applications, ultra-fast device-to-device (D2D) communications, potential beam steering applications, and non-invasive imaging solutions.

Inducing Efficient and Multiwavelength Circularly Polarized Emission From Perovskite Nanocrystals Using Chiral Metasurfaces.

Chiral nano-emitters have recently received great research attention due to their technological applications and the need for a fundamental scientific understanding of the structure-property nexus of these nanoscale materials. Lead halide perovskite nanocrystals (LHP NCs) with many interesting optical properties have anticipated great promise for generating chiral emission. However, inducing high anisotropy chiral emission from achiral perovskite NCs remains challenging. Although chiral ligands have been used to induce chirality, their anisotropy factors (glum) are low [10-3 to 10-2]. Herein, the generation of high anisotropy circularly polarized photoluminescence (CPL) from LHP NCs is demonstrated using chiral metasurfaces by depositing nanocrystals on top of prefabricated resonant photonic structures (2D gammadion arrays). This scalable approach results in CPL with glum to a record high of 0.56 for perovskite NCs. Furthermore, the differences between high-index dielectric chiral metasurfaces and metallic ones are explored for inducing chiral emission. More importantly, the generation of simultaneous multi-wavelength circularly polarized light is demonstrated by combining dielectric and metallic chiral metasurfaces.

Quantum-inspired genetic algorithm for designing planar multilayer photonic structure

Quantum algorithms are emerging tools in the design of functional materials due to their powerful solution space search capability. How to balance the high price of quantum computing resources and the growing computing needs has become an urgent problem to be solved. We propose a novel optimization strategy based on an active learning scheme that combines the Quantum-inspired Genetic Algorithm (QGA) with machine learning surrogate model regression. Using Random Forests as the surrogate model circumvents the time-consuming physical modeling or experiments, thereby improving the optimization efficiency. QGA, a genetic algorithm embedded with quantum mechanics, combines the advantages of quantum computing and genetic algorithms, enabling faster and more robust convergence to the optimum. Using the design of planar multilayer photonic structures for transparent radiative cooling as a testbed, we show superiority of our algorithm over the classical genetic algorithm (CGA). Additionally, we show the precision advantage of the Random Forest (RF) model as a flexible surrogate model, which relaxes the constraints on the type of surrogate model that can be used in other quantum computing optimization algorithms (e.g., quantum annealing needs Ising model as a surrogate).

Amorphous Photonic Structure Patterns with Thin Film Interference Effects for Multilevel Anticounterfeiting.

Colloidal photonic structures with the ability to control and manipulate light propagation offer long-term color stability, low optical loss, and angle-dependent color properties, while combinations of different photonic structures across multiple scales provide an extensive color range and enhanced optical functionalities, presenting significant potential for advanced anticounterfeiting applications. However, the proper design or manufacture of such complex structures is still challenging. In this study, amorphous photonic structures (APSs) with thin film interference (TFI) effects were fabricated for multilevel anticounterfeiting. The APSs inherit the isotropic resonant scattering and render partial TFI effects, resulting in unprecedented dynamic specular and diffuse color-shifting features as the viewing or incident direction shifts. Additionally, incorporating a certain concentration of fluorescent microspheres into the colloidal ink adds a third layer of fluorescent anticounterfeiting mode to the APSs. Enabled by infiltration-assisted (IFAST) colloidal assembly technologies, the sophisticated color distributions and randomly arranged fluorescent microspheres on the microscale of APSs grant unique and inherent fingerprint features. The unique and unpredictable optical and structural characteristics of APSs provide physical unclonable functions (PUFs) to prevent replication and tampering, demonstrating their potential as optical PUF security labels for anticounterfeiting applications through artificial intelligence (AI) reading and authentication.

Programmable orientation of blue phase soft photonic crystal

Understanding the structure formation and its underlying physical mechanism is a fundamental topic in condensed matter systems, with both academic and practical implications. Soft matter is playing a remarkable role in current era of information explosion, demonstrating enormous potential in integrated functional photonics. As unique soft photonic crystals with cubic symmetries, not only liquid crystalline blue phases (BPs) have circularly polarized selective reflection and ultra-fast electro-optical response, but also their three-dimensional photonic structures increase degrees-of-freedom for multiplexed optical modulation. In the thriving field of soft-matter-based photonics, precise and programmable engineering of BP crystal orientation is of vital importance for planar optical elements, which remains a challenging task due to the complexity of the nucleation process as well as the interaction between the BP building blocks and the boundary conditions. Aiming to gain a comprehensive understanding of how to tailor the orientation of BP crystals for the photonic applications of next generation, here we discuss the solutions for uniformity improvement and orientation control of BP crystals, about which a few of examples in combination with the underlying mechanisms are explained. In addition, the remaining challenges and the efforts that are expected are also reviewed. We expect this work provides a deeper understanding of phase transitions and resulting structures in soft crystals, which may open encouraging perspectives for their applications in photonics, biosensing, interfacial, and chemical engineering.

Enhancing computational efficiency in topology-optimized mode converters via dynamic update rate strategies

In the big data era, mode division multiplexing, as a technology for extended channel capacity, demonstrates potential in enhancing parallel data processing capability. Consequently, developing a compact, high-performance mode converter through efficient design methods is an urgent requirement. However, traditional design methodologies for these converters face significant computational complexities and inefficiencies. Addressing this challenge, this paper introduces a novel topology optimization design method for mode converters employing a Dynamic Adjustment of Update Rate (DAUR). This approach markedly reduces computational overhead, accelerating the design process while ensuring high performance and compactness. As a proof-of-concept, an ultra-compact dual-mode converter was designed. The DAUR method demonstrated an 80% reduction in computational time compared to traditional methods, while maintaining a compact design (only 1.4 μm × 1.4 μm) and an insertion loss under 0.68 dB across a wavelength range of 1525 nm to 1575 nm. Meanwhile, simulated inter-mode crosstalk remained below − 24 dB across a 40 nm bandwidth. A comprehensive comparison with traditional inverse design algorithms is presented, demonstrating our method’s superior efficiency and effectiveness. Our findings suggest that DAUR not only streamlines the design process but also facilitates exploration into more complex micro-nano photonic structures with reduced resource investment.