Light Confinement Research Articles

A novel flexible optical fiber magnetic field sensor based on integrated Fe3O4 nanoparticles

We propose a new flexible optical fiber magnetic field sensor (FOMS) based on the combination of Michelson interferometer and the PDMS polymer-based optical fiber. The flexible optical fiber magnetic field sensor is made of stretchable elastomer. It uses a stepped refractive index core-cladding structure to achieve effective light confinement, thus enabling stable transmission of light in the waveguide. When the magnitude of the external magnetic field is changed, the micro bending of the sensing region of the optical fiber causes its equivalent refractive index to change, which leads to a drift in the interference spectrum. It is shown that the magnetic field sensitivity of this optical fiber magnetic field sensor reaches −62.8 pm/mT for wavelength detection in the measurement range of 0–200 mT. The FOMS has the characteristics of high sensitivity, good linearity, good temperature response capability, sensitive response, and resistance to electromagnetic interference. In the future, the sensor will have a broad application prospect in medical, geography, aerospace and other fields.

Efficient Poling‐Free Wavelength Conversion in Thin Film Lithium Niobate Harnessing Bound States in the Continuum

AbstractOver the past two decades, there has been a surge of interest in photonic integrated circuits (PICs) to enable low‐cost industrial mass manufacture of miniaturized optical systems while simultaneously improving performance and energy efficiency following the trajectory of electronics and large‐scale integration. An essential characteristic of any PIC lies in the ability of waveguides to efficiently confine light. The phenomenon of bound states in the continuum (BIC) shows promise in achieving confinement of light for modes that otherwise would suffer from high leakage losses, which can occur in many PIC platforms. In this contribution, the usefulness of operating near a BIC to achieve efficient second harmonic generation of ≈300% W−1 cm−2 in Silicon Nitride (SiN) loaded lithium niobate on insulator (LNOI) waveguides without needing to either pole or etch the lithium niobate is experimentally demonstrated. The guided fundamental mode is phase matched at the telecom wavelength to a higher order mode at the second harmonic wavelength that is engineered as a bound state in the continuum to mitigate leakage losses. This demonstrates the promise of using BIC in weakly confining waveguides for nonlinear optical applications all while avoiding the poling process by engineering waveguide geometry.

The Onset of Lasing in Semiconductor Nanolasers

AbstractNanolasers based on emerging dielectric cavities with deep sub‐wavelength confinement of light offer a large light‐matter coupling rate and near‐unity spontaneous emission factor, . These features call for reconsidering the standard approach to identifying the lasing threshold. Here, a new threshold definition is suggested, taking into account the recycling process of photons when is large. This threshold reduces to the classical balance between gain and loss in the limit of macroscopic lasers, but qualitative as well as quantitative differences emerge for nanolasers. In particular, this new threshold identifies the onset of a transition regime, where the quantum statistics of the emitted light evolve into the Poissonian statistics of a coherent state. It is shown that the threshold with photon recycling consistently marks the onset of the change in the second‐order intensity correlation, , toward coherent laser light, irrespective of the laser size and down to the case of a single emitter. In contrast, other threshold definitions may well predict lasing in light‐emitting diodes. An overview of different threshold definitions proposed in the literature is provided and their predictions are compared when going from macroscopic to microscopic lasers.

High performance of UV photodetectors by integration of plasmonic Ag nanoparticles on GaN

High-performance ultraviolet photodetectors have attracted much attention due to their various potential applications from household to universal. On basis of this, in this work, a high – performance and photoresponse enhancement UV photodetector based on Ag NPs/GaN heterojunction was demonstrated. By incorporating of Ag nanoparticles, light confinement determined by Ag nanostructure offers an effective approach to break through the limitation of the light-mater interaction within the photoactive layers of photodetector. Various size and elemental composition of Ag NPs are fabricated on GaN lead to significant enhancement of photocurrent in UV photodetector. Consequently, an outstanding photoresponsivity of 89 A/W, external quantum efficiency of 2.8 × 103% at low bias voltage of 0.3 V were achieved. The improve photo-response of the Ag NPs-decorated GaN-based photodetector is attributed to the simultaneous effect of strong plasmon absorption, increased injection of hot electrons into the conduction band of GaN. This study will open up the direction to fabricate the high-performance UV photodetector.

High Q Hybrid Mie-Plasmonic Resonances in van der Waals Nanoantennas on Gold Substrate.

Dielectric nanoresonators have been shown to circumvent the heavy optical losses associated with plasmonic devices; however, they suffer from less confined resonances. By constructing a hybrid system of both dielectric and metallic materials, one can retain low losses, while achieving stronger mode confinement. Here, we use a high refractive index multilayer transition-metal dichalcogenide WS2 exfoliated on gold to fabricate and optically characterize a hybrid nanoantenna-on-gold system. We experimentally observe a hybridization of Mie resonances, Fabry-Perot modes, and surface plasmon-polaritons launched from the nanoantennas into the substrate. We measure the experimental quality factors of hybridized Mie-plasmonic (MP) modes to be up to 33 times that of standard Mie resonances in the nanoantennas on silica. We then tune the nanoantenna geometries to observe signatures of a supercavity mode with a further increased Q factor of over 260 in experiment. We show that this quasi-bound state in the continuum results from strong coupling between a Mie resonance and Fabry-Perot-plasmonic mode in the vicinity of the higher-order anapole condition. We further simulate WS2 nanoantennas on gold with a 5 nm thick hBN spacer in between. By placing a dipole within this spacer, we calculate the overall light extraction enhancement of over 107, resulting from the strong, subwavelength confinement of the incident light, a Purcell factor of over 700, and high directivity of the emitted light of up to 50%. We thus show that multilayer TMDs can be used to realize simple-to-fabricate, hybrid dielectric-on-metal nanophotonic devices granting access to high-Q, strongly confined, MP resonances, along with a large enhancement for emitters in the TMD-gold gap.

Microcavity Complex Lasers: from Order to Disorder

AbstractMicrostructures, characterized by gain, nonlinearity, internal scattering, and boundary effects, offer an exceptional platform for exploring complex optical phenomena such as random lasing, chaos, and multidimensional speckles. Specifically, complex lasers generated within microcavities and optical fibers, where strong light confinement and scattering play diverse roles, have become a significant branch of laser research. Recently, the rapid advancement of materials, micro‐nano technologies, and artificial intelligence has introduced new opportunities and challenges for the generation, control, and application of complex lasers. This review systematically examines various types of microcavity complex lasers from the perspective of microcavity structures with different degrees of disorder. It primarily focuses on the historical development, characteristics, regulation, and applications of disordered microcavity lasers and concludes with a discussion on the future trends in the development of microcavity complex lasers.

Study on mode properties of GaAs-based hybrid plasmonic terahertz waveguides

Terahertz (THz) fields are increasingly being used to address the critical challenges associated with achieving high data rates and rapid communication. In this study, a hybrid plasmonic THz waveguide is designed and analysed operating in the 2.5–3.5 THz frequency range. The waveguide is constructed using gallium arsenide as the high-refractive-index core, surrounded by aluminium arsenide and silver placed on a high-density polyethylene (HDPE) substrate. Graphene is strategically positioned between the HDPE layers to enhance light confinement. The mode properties of the proposed waveguide are simulated with Comol Multiphysics using the finite-element method and show unique characteristics. Observation of the simulated results at 2.5–3.5 THz reveals a high effective refractive index of 3.79, a maximum effective mode area of 1.88 mm2, a high birefringence of 0.2, a low dispersion of 0.10 ps THz−1 cm−1, a high mode field diameter of 15.8 mm, a high beat length of 123 mm and a low confinement loss of 1.79 × 10−9 mm−1. These features make the proposed waveguide suitable for applications in photonic integrated circuits for THz communications.

Tunable wave localization at the Dirac frequency in a metallic photonic crystal cavity.

In this study, the two-dimensional (2D) triangular lattice metallic photonic crystals (PCs) in visible and infrared bands have been utilized to achieve light confinement at the Dirac frequency. Distinct from the traditional bandgap or total internal reflection cavity modes, the unique photonic localization mechanism leads to an unusual algebraic decay of state and a unique frequency located beyond any bandgaps. This investigation delves into the band structure analysis of 2D metallic PCs, specifically focusing on their distinctive features, such as photonic bandgaps and Dirac cones. The plane wave expansion (PWE) method, enhanced with a linearization technique, is employed for band structure calculations, considering both the frequency-dependent dielectric properties and the intrinsic lossy nature of metallic materials described by the Drude model. The study provides a comprehensive derivation of the PWE equations for metallic PCs and investigates their band characteristics under both TM and TE polarizations. Focusing on TM modes in triangular lattice metallic PCs, it reveals zero density of states (DOS) at K points of the Brillouin corner and the existence of Dirac cones with linearly dispersion and linearly vanishing DOS. The study extends to exploring localized modes at Dirac frequencies, employing a relativistic quantum mechanics approach analogous to graphene's charge carriers. Theoretical predictions are corroborated by numerical simulations, and the potential for tunable Dirac localized modes is highlighted. This research not only deepens the understanding of Dirac properties in graphene-like systems but also lays the groundwork for further exploration of the practical quasi-2D devices, which will provide assistance in the integration of micro- and nano- devices, especially in applications requiring long-range coupling, given the critical importance of optical cavities in contemporary optical technologies.

Nano-optomechanical fiber-tip sensing

Nano-optomechanical sensors exploit light confinement at the nanoscale to enable very precise measurements of displacement, force, acceleration, and mass. Their application is hampered by the complex optical set-ups or packaging schemes required to couple light to and from the nano-optomechanical resonator. In this work, we present a fiber-coupled nano-optomechanical sensor that requires no coupling optics. This is achieved by directly placing a nano-optomechanical structure, a double membrane photonic crystal (DM-PhC), on the facet of a fiber, using a simple and scalable wafer-to-fiber transfer method. The device is probed in reflection and has a resonance at telecom wavelengths with a relatively broad spectral width of 3–10 nm, which is advantageous for a simple read-out and achieves a displacement imprecision of 10fm/Hz\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10\\,{{\\rm{fm}}}/{\\sqrt{{\\rm{Hz}}}}$$\\end{document}. Using resonant driving and a ringdown measurement, we can induce and monitor mechanical oscillations with an nm-scale amplitude via the fiber, which allows for tracking the mechanical resonant frequency and the mechanical linewidth with imprecisions of 79 and 12 Hz, respectively, at integration times of 4.5 s. We further demonstrate the application of this fiber-tip sensor to the measurement of pressure, using the effect of collisional damping on the mechanical linewidth, leading to the imprecision of 9×10−4mbar\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$9\ imes {10}^{-4}\\,{\\rm{mbar}}$$\\end{document} with an integration time of 290 s. This combination of optomechanics and fiber-tip sensing may open the way to a new generation of fiber sensors with unprecedented functionality, ultrasmall footprint, and low-cost readout.

High‐Speed and Low‐Power 2 × 2 Thermo‐Optic Switch Based on Dual Silicon Topological Nanobeam Cavities

AbstractA 2 × 2 thermo‐optic (TO) switch using dual topological photonic crystal nanobeam (PCN) cavities on the silicon‐on‐insulator (SOI) platform is proposed and experimentally demonstrated. A Fano resonance is observed due to the interference between the topological interface state of the 1D topological PCN cavity and the Fabry‐Perot (F‐P) cavity mode formed between the two facets of the finitely long nanobeam waveguide. Thanks to the sharp rising edge of the spectral response of the Fano resonance and the high confinement of light in the topological PCN cavities, a 2 × 2 TO switch is realized with short switching time and low power consumption. The measured switching power is only 1.55 mW, and the rising time and the falling time are 3 and 5.6 µs, respectively in the on‐off switching experiments. To the best of the knowledge, this is the first time that a dual topological PCN structure is utilized to realize a high‐speed and low‐power TO switch, revealing the possibility of designing high‐performance reconfigurable optical devices and networks using topological photonics.

Wavelength selective beam-steering in a dual-mode multi-layer plasmonic laser

Due to its improved localization and confinement of light in single or multiple wavelength modes, nanolasers based on plasmonic crystals have grown in popularity in recent years. However, the lasing modes are not spatially separated, making applying different modes to different applications difficult. This work demonstrates an effective technique for spatially separating the two modes of a merged lattice metal nanohole array-based dual-mode plasmonic laser. A flat dielectric metasurface-based beam-splitter that exploits phase gradient profiles on the interfaces has been added to the laser to separate the modes into distinct spatial beams. The proposed structure successfully separates two modes by ∼23°, and the separation can be raised to ∼63° by tuning structural parameters such as the radius of the nanocylinders and the number of supercell rows. In addition, multiple beams can be generated, allowing for manual beam steering. This approach has a high emission output with a narrow linewidth, clarity, and a substantial degree of future tunability potential. The proposed integrated structure will provide a novel means of device miniaturization and may also serve advanced optical applications such as optical communication, quantum optics, interferometry, spectroscopy, and light detection and ranging (LiDAR).

Enhanced self-collimation effect by low rotational symmetry in hexagonal lattice photonic crystals

In this study, we present the design of a photonic crystal (PC) structure with a hexagonal lattice, where adjustments to the PC unit cell symmetry reveal an all-angle self-collimation (SC) effect. By optimizing opto-geometric parameters, such as the rotational angle of auxiliary rods and adjacent distances, we analyze the SC property in detail, leveraging group velocity dispersion (GVD) and third-order dispersion (TOD) characteristics. We also investigate the relationship between symmetry properties and their influence on dispersion characteristics. Through symmetry manipulation, we gain a comprehensive understanding of the underlying mechanisms governing light collimation and confinement in the proposed configurations. The PC structure with a C1 symmetry group exhibits all-angle SC effect within the range of a/λ = 0.652 and a/λ = 0.668 normalized frequencies, with a bandwidth of Δω/ω c =2.4% Further breaking the symmetry, transforming from C1 to C2 group symmetry enhances the SC bandwidth to Δω/ω c =6.5% and reveals the perfect linear equi-frequency contours (EFC) at two different frequency bands: all angle SC between a/λ = 0.616 and a/λ = 0.344 normalized frequencies in the 4th transverse magnetic (TM) band and between a/λ = 0.712 and a/λ = 0.760 in the 5th TM band. Here, GVD and TOD values of the TM 4th band vary between 7.3 (a/2πc2)–254.3 (a/2πc2) and 449.2 (a2/4π 2c3)–1.3×105 (a2/4π 2c3), respectively. Also, GVD and TOD values of the TM 5th band vary between 182.5 (a/2πc2)–71.3 (a/2πc2) and −24380(a2/4π 2c3)–−9619 (a2/4π 2c3) values, respectively. Additionally, we propose a composite/hybrid PC structure resembling C2 group symmetry, where two auxiliary rods are replaced by rectangular photonic wires with the same refractive index and width equal to the diameter of auxiliary rods. This hybrid structure exhibits an all-angle SC effect with an operating bandwidth of Δω/ω c =11.7%, which displays near-zero GVD and TOD performance and offers enhanced robustness against potential fabrication precision issues.

Helical hollow channel waveguide in YAG fabricated by femtosecond laser enhanced wet etching.

Three-dimensional optical waveguides with hollow channels have many advantages, such as strong mode confinement and excellent dispersion control ability. Femtosecond laser enhanced wet etching is widely used to fabricate hollow channel waveguides in transparent dielectric materials. We propose a method for fabricating hollow channel waveguides in YAG using femtosecond laser enhanced wet etching with a simpler fabrication process and shorter etching time compared with the previous work. After 90 h of etching, a series of helical hollow channel waveguides with a length of 5 mm and a radius of 32 µm were successfully fabricated. At a pitch of 3 µm, the waveguide exhibited a loss (including coupling loss and transmission loss) as low as 0.68 dB at 1030 nm. The helical hollow channel waveguide also exhibited exceptional isotropic light confinement capability and remarkable supercontinuum-generating properties. Moreover, helical hollow channel waveguides with a radius of 2 µm were successfully fabricated. According to simulations, waveguides of such size can effectively control dispersion. Our work presents, to our knowledge, a novel approach to fabricating hollow channel waveguides with arbitrary lengths using femtosecond laser-enhanced wet etching.

Silicon nitride (Si3N4) leads to enhanced performance of silica-silver based plasmonic sensor for colorectal cancer detection under optimum radiation damping

Silicon Nitride (Si3N4) possesses large refractive index and transparent behaviour in a broad spectral range from ultraviolet to infrared. These properties of Si3N4 facilitate an efficient light-matter interaction leading to enhanced light confinement. In this work, we have investigated Si3N4-based plasmonic sensor for pathogenic colorectal tissues detection at a wavelength of 1000 nm. The sensor structure consists of fused silica prism and Ag–Si3N4 heterojunction. The figure-of-merit (FOM) of the sensor is optimized by judiciously coordinating the thickness of Ag layer (dM) and Si3N4 layer (dA) under optimum radiation damping (ORD). An optimized FOM as high as 6011 RIU−1 (at dM = 47.3 nm) has been achieved corresponding to dA = 8.6 nm under ORD. At ORD, the corresponding power loss ratio (PLR) is as high as 2.773, and the normalized electric field enhancement factor (FEF) is about 1.06. The magnitude of combined performance factor (CPF), which is the product of FOM, PLR and FEF, of the proposed sensor is as large as 17668.61 RIU−1. The proposed application of Si3N4 in plasmonic sensor in combination with ORD condition leads to substantially enhanced sensing performance and has been explored for the first time to the best of our knowledge.

Hybrid plasmonic valley-Hall topological insulators

Abstract The emerging field of photonic topological insulators offers promising platforms for high-performance optical communication, computing, and sensing. However, conventional photonic topological insulator designs typically operate within the diffraction limit due to their dielectric nature. This limitation imposes constraints on device miniaturization, reduces light–matter interaction, and decreases overall device sensitivity. Introducing a new valley-Hall hybrid plasmonic topological insulator, we overcome this limitation by exploiting the coupling of surface plasmon oscillations with the optical modes of a dielectric photonic crystal, allowing for sub-diffraction vertical confinement of light. Deep-subwavelength chiral edge states can, therefore, be generated and robustly guided along disordered Z-shaped topological boundaries with much lower propagation loss compared to purely plasmonic platforms. Such extreme manipulation of light on an integrated chip platform maximizes light–matter interaction and opens the door for truly compact and efficient optical modulators, molecular sensors, and next-generation nanophotonic and quantum devices.

Lasing emission from ZnO hierarchical spherical microcavity

We investigated lasing characteristics of ZnO hierarchical micro-spherical particles with diameters of 1–5 μm . The lasing emission consists of a small number of discrete laser peaks unlike conventional random lasing from ZnO nanopowder-assembly. Theoretical calculations based on a many-body theory reveal that the optical gain is achieved at the observed lowest lasing threshold value, 4 mJ⋅cm−2⋅pulse−1 , which corresponds to the excited carrier density ∼2.4×1025 m−3 . Because the carrier density is much higher than the Mott density, the gain origin for lasing is electron-hole plasma recombination. The lasing frequency mode shift ( ∼1.2 meV ) is due to the refractive index change induced by exciting high carrier density up to 6.1×1025 m−3 . By changing hierarchical structures via controlling growth condition and performing the annealing treatment and performing the calculation of scattering efficiency of ZnO particles, we found that the hierarchy of the micro-spherical particle plays a crucial role of the lasing feedback: the strong light scattering at the interface between outer nanoparticles with the sizes of 100–200 nm and smaller ones with the sizes of 10–60 nm consisting in the micro-spherical particle results in a light confinement. Furthermore, it has been confirmed that each discrete lasing mode shows strong gain competition with each other probably due to spatial overlap between the modes. These results suggest that both random scattering and microcavity modes contribute to the lasing oscillation.

Investigation of the properties of photonic crystal resonant cavities based on valley spin reversal

Resonators have been treated as essential elements in optics because of their capacity to store and enhance light and exhibit a wide range of applications such as semiconductor lasers and optical communication components. In this article, we reveal a new mechanism of light field confinement in an optical cavity composed of different valley photonic crystals. The electromagnetic field of light is localized because of the valley spin states contrasted between the inner and outer regions, which leads a high Q-factor and a small model volume of the resonator. Furthermore, the whispering-gallery-mode modulated vortex phase distribution is demonstrated in the proposed structure, which offers a new method for manipulating the light field. The energy spectrum as well as the light field distributions show the simultaneous appearance of both bulk and edge states. Such effect becomes pronounced or diminished when the domain wall changes, and can be explained by the location of the edge states in the shared bandgap. Our findings offer a novel mechanism of light field confinement and phase modulation, which may pave the way for a new type of topological device and provide broad applications in the areas of micro-lasers, optical communications, and other light-matter interaction systems.

A colour-encoded nanometric ruler for axial super-resolution microscopies

Recent progress has boosted the resolving power of optical microscopies to spatial dimensions well below the diffraction limit. Yet, axial super-resolution and axial single-molecule localisation typically require more complicated implementations than their lateral counterparts. Also, comparison between techniques remains a major challenge due to the absence of suitable calibration tools that would permit a metrology along the microscope's optical axis. Likewise, such test samples would be useful as well for quantifying nanometric drift, or axial fluorophore mobility, or again for quantifying the sub-wavelength light confinement in near-field microscopies. In the present work, we propose a colour-encoded nanoscopic axial ruler. We provide a multi-layered single-excitation, dual-emission test slide, which translates the axial-distance problem into a spectral measurement. Our test slide combines, on a standard microscope coverslip substrate, two flat, thin, uniform and brightly emitting fluorophore layers, separated by a nanometric transparent spacer layer having a refractive index close to a biological cell. The ensemble is sealed in an index-matched protective polymer. As a proof-of-principle, we demonstrate the light confinement resulting from evanescent-wave excitation in total internal reflection fluorescence (TIRF) microscopy. Our test sample permits, even for the non-expert user, a facile axial metrology at the sub-100-nm scale, a critical requirement for axial super-resolution, as well as near-surface imaging, spectroscopy and sensing. (201 words).

Plasmonic hotspot arrays boost second harmonic generation in thin-film lithium niobate

Focusing light down to subwavelength scales to enhance the light-matter interaction has been highly sought after, which has promoted significant researches and applications in nanophotonics. Plasmonic nanoantennae are a significant tool to achieve this goal since they can confine light into ultra-small volumes far below the diffraction limit. However, metallic materials have the property of central symmetry, resulting in weak second-order nonlinear effects. Here, we design plasmonic bowtie nanoantennae on thin-film lithium niobate (TFLN) for deep-subwavelength light confinement to boost the second-harmonic generation (SHG) in TFLN via the plasmonic hotspot enhancement. The SHG enhancement factor of about 20 times as compared to unpatterned TFLN is achieved in the experiment when resonantly excited by femtosecond laser. This work proposes a route for subwavelength nonlinear optics on the TFLN platform.

Spatial Writing of Ultrafast All-Optical Switching.

Writing spatial information on ultrafast all-optical switching is essential for constructing ultrafast processing units in photonic applications, such as optical communication and computing networks. However, most methods ignore the fabrication and imaging of controllable switching area, limiting its spatial information and the further design in ultrafast devices. Here, we propose a method to spatially write in ultrafast all-optical switching based on MAPbI3 perovskite with nanocone structure and visualize the switching effect in arbitrary designed area. Due to the light confinement effect of nanocone fabrication using a fs laser, the light is strongly absorbed by perovskite and reach saturable absorption. It leads to ultrafast broadband transmittance change with 25 fs switching time and 10% modulation depth in nanocone perovskite area. Our preparation method offers high efficiency, performance, and flexibility for the spatial writing of ultrafast all-optical switching, which is promising for developing ultrafast all-optical networks and the next generation of communication technology.