Dielectric Microspheres Research Articles

{Dielectric microsphere sizing using fluorescence microscopy of whispering gallery mode resonances from adsorbed dyes

{Dielectric microsphere sizing using fluorescence microscopy of whispering gallery mode resonances from adsorbed dyes

Generation of conventional and reflective photonic nanojets and improving intensity enhancement with their superposition

Herein, we report the theoretical investigation on photonic nanojets (PNJs) of substrate-supported single-spherical dielectric microparticles. Conventional and reflective PNJs (CPNJs and RPNJs) are observed in the case of metal and dielectric substrates. The dependence of the maximum electric field intensity enhancement (ηmax) of the CPNJs and RPNJs on the nanogap between the metal substrate and dielectric microsphere, the metal substrate's refractive indices, and the incident light's wavelength is studied. More importantly, the spatial separation between the CPNJs and RPNJs is found to be strongly dependent upon the angle of incidence (θ). A significant improvement in the EFIE is observed for the grazing incidence upon the superposition of CPNJ and RPNJ. The theoretical investigation is also performed by replacing the metal substrate with a dielectric substrate, and the results obtained are reported here for comparison. Finally, this investigation is extended for the dielectric microsphere placed on a thin metal film deposited on a dielectric substrate and studied the role of θ on the characteristic parameters of the CPNJs and RPNJs.

A new mechanical engineering strategy based on microsphere probe and its application in transition metal dichalcogenides

Mechanical engineering of 2D materials allows continuous and reversible modulation of their electronic and photonic properties. Although photoluminescence (PL) measurement is an effective way to monitor the effects of mechanical forces on 2D semiconductors, there is currently a lack of techniques to enhance PL signals during stress application. This study presents an innovative mechanical engineering approach that integrates a dielectric microsphere as an atomic force microscopy (AFM) probe into a Raman-AFM system. Force–distance curve tests and COMSOL simulations were performed to analyze and estimate the compressive stress exerted by the microsphere. Importantly, the PL signals of transition metal dichalcogenides subjected to microsphere probe's force were enhanced and reveal distinct mechanical responses depending on the substrate rigidity: compressive pressures for rigid substrates and tensile strains for flexible ones. Notably, this strategy not only amplifies spectral signals in real time but also achieves fine stress modulation in the precise targeted material region, demonstrating its superiority in sensitive mechanical engineering applications. Our work offers a new avenue for the deliberate design of mechanical strains in 2D materials, which is crucial for optimizing the performance of related devices.

Plasmonic structurally colored surfaces with metal film over microsphere lattices

Structural coloring through plasmonic nanostructures involves the manipulation of light interaction with nanostructured metallic surfaces to selectively reflect specific spectral ranges within the visible spectrum. Unlike conventional pigments that absorb light based on their chemical properties, plasmonic nanostructures achieve color through their physical configuration. This study investigates the potential of generating plasmonic structural colors by utilizing self-assembled colloidal microsphere lattices that are coated with thin metal films. Finite-Difference Time-Domain (FDTD) simulations are employed on realistic models to analyze the optical reflectance of dielectric microsphere lattices ranging in sizes from 300 to 700 nm, coated with varying thicknesses (20–200 nm) of different metals (Al, Ag, Au). The reflectance spectra are then translated into specific colors using a software algorithm, with the color representations plotted on CIE1931 diagrams. Furthermore, to validate the theoretical findings, two-dimensional polystyrene microsphere arrays coated with Au and Ag films are prepared, and their reflectance spectra and color images are examined. The resulting plasmonic colors can be adjusted by altering the sphere size, the type of metal used, and the thickness of the coating, demonstrating their potential for diverse color-based applications.

Self-Reporting Microsensors Inspired by Noctiluca Scintillans for Small-Defect Positioning and Electrical-Stress Visualization in Polymers.

Small defects induce concentrated electrical stress in dielectric polymers, leading to premature failure of materials. Existing sensing methods fail to effectively visualize these defects owing to the invisible-energy state of the electric field. Thus, it is necessary to establish a nondestructive method for the real-time detection of small defects in dielectric polymers. In this study, a self-reporting microsensor (SRM) inspired by Noctiluca scintillans is designed to endow materials with the ability of self-detection for defects and electrical stress. The SRM leverages the energy of a nearby electric field to emit measurable fluorescence, enabling defect localization and diagnosis as well as electrical-stress visualization. A controllable dielectric microsphere is constructed to achieve an adjustable electroluminescence threshold for the SRM, thereby increasing its detection accuracy while decreasing the electroluminescence threshold. The potential degradation in the polymer performance owing to SRM implantation is addressed by assembling long molecular chains on the SRM surface to spontaneously generate an interpenetrating network. Results of finite element analyses and experiments demonstrate that the SRM can effectively realize nondestructive visualization and positioning of small defects and concentrated electrical stress in polymers, positioning it as a promising sensing method for monitoring the electric field and charge distribution in materials.

Topologically protected optical pulling force on synthetic particles through photonic nanojet

Abstract A dielectric microsphere concentrates light into a photonic nanojet (PNJ), and swims towards the near-infrared laser in response to the nanojet-mediated force. In contrast, a Janus particle with an opaque metal layer was thought to be impossible to concentrate light into a stable nanojet. However, the Janus particle may experience optical torque owing to the inhomogeneous composition on both sides even in linearly polarized non-resonant light. Herein, we report on topologically protected PNJ produced by a synthetic Janus particle, and observed the backaction force on the Janus particle. Due to symmetry, the counter-propagating beams can both form PNJ on the respective opposite sides, and pull Janus particles towards respective sources. Furthermore, we unveil that the hysteresis on backaction force with respect to the injection power also exists on synthetic Janus particle compared with their dielectric counterparts. Additionally, the magnitude of the backaction force varies between power increase and decrease stages even with the same laser power. We anticipate that the observation offers great possibilities to pull irregular particles by concentrating light with the particle, and such scheme may be applied for parallel particle manipulation and classification.

Shock Wave Detection for In-Process Depth Measurement in Laser Ablation Using a Photonic Nanojet

Three-dimensional micro- and submicrometer-scale structures exhibit unique functions that cannot be obtained with bulk materials. To create such three-dimensional microstructures with high precision and efficiency, we proposed laser ablation using a photonic nanojet. A photonic nanojet is an optical beam with both a small beam diameter and a large depth of focus, which is obtained by irradiating a dielectric microsphere using a laser beam. In this study, we proposed an in-process depth measurement method to improve the machining accuracy of laser ablation using a photonic nanojet. We focused on the propagation characteristics of the shock waves generated during laser ablation. Shock waves were generated at the deepest point of the machining area and reached the microspheres as the pressure decayed, showing that different machining depths exerted different pressures on the microspheres. The microspheres were displaced by the pressure of the shock wave, and the amount of displacement depended on the pressure. Therefore, microspheres can be used as probes for shock wave detection, and the machining depth can be determined by measuring the displacement of microspheres during photonic nanojet machining. In this study, the displacement of a microsphere was measured simultaneously during photonic nanojet machining using a confocal optical system. From the obtained microsphere vibration data, the effect of the shock wave pressure was extracted, and the displacement of the microsphere due to the shock wave was obtained. When the hole depth varied from 155 to 1121 nm, the displacement of the microspheres varied from 0.58 to 0.03 µm. The experimental results show that the displacement of the microspheres vibrated by the shock wave decreased as the machining depth increased. This was due to an increase in the shock wave propagation distance and a decrease in the pressure of the shock wave as the machining depth increased. In conclusion, in-process depth measurements are possible in laser ablation using a photonic nanojet with a microsphere as a probe to detect shock waves.

Simple high-resolution 3D microscopy by a dielectric microsphere: a proof of concept.

We present a simple high-resolution approach for 3D and quantitative phase imaging (QPI). Our method makes the most of a glass microsphere (MS) for microscopy and a glass plate for lateral shearing self-referencing interferometry. The single MS serves all the functions of a microscope objective (MO) in digital holographic microscopy (DHM) while offering the advantages of compactness, lightness, and affordability. A proof-of-concept experiment is performed on a standard diffraction grating, and various effective parameters on the imaging performance are investigated. The results are validated by atomic force microscopy and Mirau-DHM, and 3D morphometric information of the sample under inspection is obtained. The technique is then applied for 3D quantitative measurement and visualization of a human red blood cell, proving the principle of our easy-to-implement and vibration-immune arrangement for high-contrast label-free QPI of biological samples, and its utility in cell morphology, identification, and classification.

Enhanced brightness of quantum emitters via in situ coupling to the dielectric microsphere

Achieving higher brightness of a single-photon emitter (SPE) is central for advanced applications from quantum information processing to quantum sensing. However, most approaches of integrating quantum emitters and photonic resonators require accurately localizing the emitter into a photonic structure, which is a challenge. Here, we report using dielectric microspheres for laser focusing to create SPEs in hexagonal boron nitride and in situ enhance the emission via photonic coupling between the SPE and the dielectric microspheres. The photoluminescence intensity is increased by 2.4-fold, achieving a high brightness SPE with a saturation intensity of up to 19.6 Mcounts s−1. This approach provides a feasible way to generate a high-performance SPE while simultaneously enabling precise coupling of the quantum light source and optical resonators.

Role of surface damping on hybrid modes and electric field enhancement distribution of ultrathin metal-coated dielectric microspheres illuminated by non-resonant light

Herein, we report a theoretical investigation on the characteristic parameters of hybrid plasmonic–photonic modes (or simply hybrid modes) and electric field enhancement (EFE) distribution of ultrathin Au- and Ag-coated dielectric microspheres illuminated by non-resonant light obtained with and without consideration of the size-dependent damping (SDD) or surface damping in ultrathin metal nanoshells. From the obtained scattering and absorption spectra, the role of SDD on characteristic parameters such as wavelength locations, full width at half maximum, and EFE of the hybrid modes is investigated for the first time, to our knowledge. In addition, the effect of metal shell thickness (t) on the hybrid modes is studied. The EFE distribution inside/outside the ultrathin metal-coated dielectric microspheres illuminated by non-resonant light is investigated. More importantly, the sensitivity of the EFE distribution of bare and metal-coated microspheres is explored by slightly varying the resonant and non-resonant wavelengths.

Surface roughness influence on photonic nanojet parameters of dielectric microspheres

All naturally found and technologically fabricated solid microparticles possess surface roughness. Upon optical wave scattering from such particles, in addition to its geometric shape, the surface relief becomes an important morphological factor determining the optical properties of the scatterer. We present results of the numerical 3D-simulations of focusing an optical wave with a dielectric microsphere with randomly distributed surface roughness. We address different cases of azimuthally symmetric and asymmetric distortions of the particle surface. We show that the key parameters of the near-field focal region (intensity, longitudinal and transverse dimensions) referred to as a photonic nanojet (PNJ) are sensitive to changes in the microsphere surface texture. Two important PNJ parameters, the peak intensity and the longitudinal length, are subject to more prominent changes. The influence of the optical contrast (relative refractive index) of the microsphere on PNJ parameters is investigated in detail. The possibility of reducing the influence of surface roughness on the near-field focusing strength by microsphere watering (water-uptake) is demonstrated.

Near-Field Picometer Vibration Detection Based on Dielectric Microsphere Probe

We propose a near-field picometre vibration detection method based on a dielectric microsphere probe. Using an optical fiber combined with a dielectric microsphere to form a near-field probe. When parallel light is injected into the dielectric microsphere, interaction occurs between the surface of the microsphere and the focused beam in the near-field, at the front end of the dielectric microsphere will generate a highly localized electromagnetic beam, i.e., a photonic nanojet(PNJ). The nanojet of the probe in the near-field can enhance the scattered light of the object under the test surface. We used a differential structure that was formed by two optical fiber probes to detect the picometre vibration in the near-field. From the experimental results, we know that the method can detect the picometre vibration signal, while the minimum vibration signal of 20 pm can be detected. The method has the potential to apply in cellular diagnostics, single-molecule detection, and in situ measurements.

Microsphere‐Aided Super‐Resolution Scanning Spectral and Photocurrent Microscopy for Optoelectronic Devices

AbstractDielectric microspheres naturally possess unique optical properties by which the light's focus and confinement can be manipulated on a microscale. Combining microspheres and optical or Raman microscopy, super‐resolution imaging beyond the diffraction limit and enhancement of Raman signals are demonstrated to provide abundant spectroscopic information on materials. However, microsphere‐aided super‐resolution scanning photocurrent imaging remains challenging to date. Here, based on the photonic nanojet mechanism, a super‐resolution photocurrent and spectral microscopy equipped with a scanning tip with silica dielectric microspheres are presented. With such microsphere‐aided microscopy, order of magnitude enhancements for single‐point Raman and photoluminescence signals can be achieved, and the spatial resolutions for photocurrent and spectral mapping surpass the best resolution of the original confocal system restricted by the diffraction limit. This versatile system enables correlative super‐resolution spectral and photocurrent imaging, serving as a reliable tool for comprehensively understanding and uncovering the optoelectronic properties of materials.

Miniaturized microsphere-assisted microscopy

Microsphere-assisted microscopy is a sub-diffraction-limited imaging technique, which uses dielectric microspheres to collect the near-field information of an object. In this work, the bulk of the classical microsphere-based system is reduced by the simple introduction of a ball lens. The miniaturized imaging system has been designed and vertically assembled. Performance of the proposed dual-sphere system is quantified through numerical simulations and experiments. A theoretical impulse response with a full width at half maximum smaller than half of the wavelength is reached in air. As a proof of concept, 500-nm-period standard gratings and gold nanoparticles are observed without a classical objective, validating a sub-wavelength resolution with a high imaging quality and low aberrations over the field of view.

Size-dependent optical forces on dielectric microspheres in hollow core photonic crystal fibers: erratum.

The beam shape coefficients for cylindrical vector modes are of great importance for other researchers to reproduce our results, however they were accidentally reported incorrectly in our recently published manuscript [Opt. Express30(14), 24407 (2022)10.1364/OE.458674]. This erratum reports the correct form for the two expressions. Two typographical errors in auxiliary equations are also reported and two labels in particle time of flight probability density function plots are fixed.

Using Photonic Glasses as Colored Covers for Solar Energy Harvesting

AbstractThe increasing demand for renewable energy is promoting technologies that integrate solar energy harvesting materials with the human living environment, such as building‐integrated photovoltaics. This places requirements on developing colored covers with a trade‐off between efficiency and aesthetics, providing a new stage for the large‐scale application of structural color technologies. This study investigates the theoretic feasibility of employing the photonic glass, a random packing of monodisperse dielectric microspheres, as the colored cover for solar energy harvesting. Based on numerous optical simulations, the color and average solar transmissivity of the photonic glasses with varying parameters are evaluated. Results show that using non‐absorbing microspheres with relatively high refractive index, about 3 µm thick photonic glasses can enable colors with lightness over 50 while keeping average solar transmissivity over 80%. Due to the short‐range structural correlation, it is demonstrated that photonic glasses can generate purple, blue, cyan, light green, and gray colors, but cannot help with yellow and red hues. Finally, the effects of several enhancement methods are clarified, and possible ways for expanding the color range are demonstrated. These results provide a comprehensive guide to the practical implementations of structural color using photonic glasses, particularly in the colorization of solar energy materials.

Unraveling Dipolar Regime and Kerker Conditions in Mid‐Index Mesoscale Dielectric Materials

AbstractNanophotonic phenomena, such as zero optical back scattering, nonradiating anapole states, etc. are related to the excitation of single dipolar modes—hence so far, they have only been observed within a few relatively high‐index dielectric materials (refractive index, n > 3.5) in the nanoscale regime at the optical frequencies. Here, dipolar regime is unraveled, close‐to‐zero backscattering is demonstrated, and optical anapoles are excited in mid‐index dielectric spheres (titanium di‐oxide, TiO2; n ≈ 2.6) at the mesoscale regime (particle diameter, d ≈ incident wavelength, λ) under illumination with tightly focused Gaussian beams (TFGBs). Successive scattering minima associated with dipolar excitation are observed satisfying the first Kerker condition in the scattering spectra of single TiO2 spheres with diameters in the micrometer range. Moreover, at specific wavelengths, the electric and magnetic dipolar scattering amplitudes of the dielectric microspheres simultaneously go close‐to‐zero, leading to the excitation of hybrid optical anapoles with a total scattering intensity ≈ 5 times weaker for TFGB illumination with numerical aperture, NA ≈ 0.95 compared to NA ≈ 0.1. The result pushes the boundary of the observation of nanophotonic phenomena to a new regime with regards to type and size of the materials.

Coupled Optical Resonances in a Dielectric Microsphere: Physical Concept of a Miniature Optical Pressure Sensor

Coupled Optical Resonances in a Dielectric Microsphere: Physical Concept of a Miniature Optical Pressure Sensor

Contactless and robust dielectric microspheres-assisted surface-enhanced Raman scattering sensitivity improvement for anthrax biomarker detection.

This report presents a contactless and robust dielectric microspheres (DMs)-assisted surface enhanced Raman scattering (SERS) enhancement method to improve SERS detection sensitivity detection sensitivity. DMs that could focus and collect light were embedded within the polydimethylsiloxane (PDMS) film to avoid direct contact with the analytical solution and improve detection reliability. The as prepared DMs embedded PDMS DMs PD MS film was integrated with a microfluidic technique to enhance the SERS signal of a liquid substrate. Detection in microfluidic systems can reduce reagent consumption, shorten assay time, and avoid evaporation of the colloid substrate solution. The robustness and potential influencing factors of DMs PDMS film assisted SERS enhancement (DERS) were evaluated using 4-aminothiophenol (4-ATP) as the Raman probe. The sensing performance of the proposed method toward dipicolinic acid (DPA) was evaluated, and an evident signal intensification was obtained. Remarkably, the DMs PDMS film can also be implemented on solid substrates. A proof-of-concept experiment was performed by covering the DMs PDMS film directly over an AgNPs@Si solid substrate wherein a 5.7-fold sensitivity improvement was achieved.

Surface optical sensitivity enhanced by a single dielectric microsphere.

Single dielectric microspheres can manipulate light focusing and collection to enhance optical interaction with surfaces. To demonstrate this principle, we experimentally investigate the enhancement of the Raman signal collected by a single dielectric microsphere, with a radius much larger than the exciting laser spot size, residing on the sample surface. The absolute microsphere-assisted Raman signal from a single graphene layer measured in air is more than a factor of two higher than that obtained with a high numerical aperture objective. Results from Mie's theory are used to benchmark numerical simulations and an analytical model to describe the isolated microsphere focusing properties. The analytical model and the numerical simulations justify the Raman signal enhancement measured in the microsphere-assisted Raman spectroscopy experiments.