Dielectric Microspheres Research Articles

Photonic Jets and Single‐Photon Emitters

Photonic jets (PJs) obtained by illuminating a dielectric microsphere have recently shown to provide an efficient and cost‐effective means of laser‐writing and localizing single‐photon emitters with sub‐diffraction precision. The fabrication technique relies on the photoinduced formation of GaAsN quantum dots (QDs) that are self‐aligned to the microsphere, which in turn enhances the collection efficiency of their emission. Similarly, the angular magnification introduced by a microsphere placed on top of two close emitters allows to detect and resolve their separation below the diffraction limit by analyzing their angular emission pattern in momentum space. Along with a brief review of the two methods, a systematic numerical study on the formation and properties of PJs to streamline the optimization of the fabrication process is presented.

Hysteresis and balance of backaction force on dielectric particles photothermally mediated by photonic nanojet

Abstract Reversible control over the microparticle motion using light excites interesting applications in optofluidics, microswimmers, artificial optical matter, and biomedical engineering. The dielectric microspheres swim towards the near infrared pulsed laser in response to the backaction force mediated by photonic nanojet. Hereby, we report that the backaction force exhibits hysteretic behaviour owing to the distinguishable responses of the temperature rise inside the nanojet and the temperature rise of the liquid ensemble. Accordingly, the magnitude of backaction force at the same laser power varies between power increase and decrease stages. In order to develop multidimensional manipulation tool, we studied the possibility of using lasers with different spatiotemporal profiles to mediate the backaction force, and developed the counterpropagating beam scheme for reversible control of the particle motion directions. We further harness the hysteresis to reverse the direction of backaction force on dielectric particles in presence of a constant force from a counter-propagating beam with broadband supercontinuum spectrum. In contrast to the microsphere caught in the single beam gradient trap, the microsphere encounters augmented Brownian motion at higher balanced power level. The microsphere would eventually escape from the common region of the paired beams, enabling high throughput morphology analysis for cancer cell classification, biopsy, and diagnosis.

Role of Symmetry Breaking in Observing Strong Molecule-Cavity Coupling Using Dielectric Microspheres.

The emergence of dielectric open optical cavities has opened a new research avenue in nanophotonics. In particular, dielectric microspheres support a rich set of cavity modes with varying spectral characteristics, making them an ideal platform to study molecule–cavity interactions. The symmetry of the structure plays a critical role in the outcoupling of these modes and, hence, the perceived molecule–cavity coupling strength. Here, we experimentally and theoretically study molecule–cavity coupling mediated by the Mie scattering modes of a dielectric microsphere placed on a glass substrate and excited with far-field illumination, from which we collect scattering signatures both in the air and glass sides. Glass-side collection reveals clear signatures of strong molecule–cavity coupling (coupling strength 2g = 74 meV), in contrast to the air-side scattering signal. Rigorous electromagnetic modeling allows us to understand molecule–cavity coupling and unravel the role played by the spatial mode profile in the observed coupling strength.

Structural engineering and compositional manipulation for high-efficiency electromagnetic microwave absorption

Structural engineering and component modulation are effective strategies for designing superior electromagnetic wave (EMW) absorbing materials. The challenge of understanding the structure-component-property relationship and developing efficient microwave absorbers, however, remains a huge challenge. In this work, we demonstrate a microemulsion phase transition approach to the design of different structures of dielectric microwave absorbing materials. Three different structures of MoO2/C (MC) microspheres, solid, yolk-shell, and hollow can be prepared by regulating the ratio of the two phases of water and oil. The MoO2/MoS2/C (MSC) trilayer dielectric microspheres with Schottky barriers were prepared by a subsequent vulcanization process in which component modulation allowed the construction of multi-interfaces and vacancy engineering. The analysis reveals that the sensitivity of the different structures to component modulation varies considerably, mainly in the number of vacancies generated by the sulfidation process. This difference is determined by both the free space of the microspheres and the number of effective media. Such a phenomenon provides insight into the interplay between structure and components in attenuating EMW. Of these, with the 50% sulfated MoO2/MoS2/C yolk-shell microspheres (YSMSC) exhibiting a reflection loss (RL) of −80.73 dB at a thin thickness of 1.7 mm; the fully sulfated MoS2/C hollow microspheres (HMSC) achieve an effective absorption bandwidth (EAB) of 7.04 GHz at 2.2 mm. Our study contributes to an in-depth understanding of the structure-component-property relationship of EMW absorbers and provides a reference for tuning the product structure using microemulsion phase conversion methods.

Assembly method of biological lens array based on maskless lithography

<p indent="0mm">In the field of micro and nano optics, transparent dielectric microspheres and living biological cells have attracted more and more attention due to their optical imaging capabilities. However, it is difficult to integrate them directly onto optical systems because of their size and bioactivity limitations. Although multi-physics field methods based on optical tweezers and acoustic tweezers can manipulate microspheres and cells for imaging, they tend to have specific requirements for imaging samples. Here, we proposed a method for the preparation of microsphere/cellular lens array modules based on maskless lithography. Natural sedimentation principle is utilized to form microlens arrays by assembling SiO₂ microspheres and MCF-7 cells on the hydrogel microporous module. The experimental and simulation results show that the SiO₂ microsphere array embedded in the hydrogel still maintains its super-resolution imaging capability, and the MCF-7 cells can achieve the image magnification. As a result of the arrayed imaging, a larger viewing field can be obtained than that of individual microspheres or cells. The modular imaging method proposed in this paper is expected to be applied in the field of biophotonic devices and <italic>in vivo</italic> optical imaging.

Simulation of enhanced optical trapping in a perforated dielectric microsphere amplified by resonant energy backflow

Optical energy flow inside a dielectric microsphere exposed to an optical wave is usually codirected with its wave vector. At the same time, if the optical field in a microparticle is in resonance with a high-quality spatial eigenmode, referred to as the whispering-gallery mode (WGM), at least two regions of reverse energy flow emerge in the illuminated and shadow particle hemispheres. These areas are of considerable practical interest due to their enhanced optical trapping potential provided they should be previously cleared from particle material. In this paper, we consider a perforated microsphere with an air-filled pinhole fabricated along the particle diameter and theoretically analyze the conditions for WGMs excitation. A pinhole isolates the energy backflow regions of WGM and multiple enhances the optical pull-in force that transforms a perforated microsphere into an efficient optical tweezer for trapping various nanoobjects.

Synthetic hybrid particles to improve the down-conversion efficiency of quantum dots via simultaneously induced scattering and plasmonic effects

Quantum dots (QDs) are promising color-conversion materials for various display applications owing to their high color purity, wavelength tunability, and high quantum yield. However, the self-quenching of QDs and internal reflection in the QD film reduce the external quantum efficiency, thereby hindering the applicability of the QD film. In this study, we propose a strategy to enhance the QD photoluminescence (PL) using hybrid particles, in which both scattering particles and plasmonic nanoparticles (NPs) are integrated. Using a facile synthetic route, we fabricated dielectric SiO2 microspheres decorated with plasmonic Ag NPs, and QDs were subsequently attached to the microsphere surface. The synthesized hybrid particles were embedded in a polymer matrix as down-conversion films. The proposed QD-embedded film exhibited up to a threefold improvement in the QDPL compared to the reference film. Along with the prominent down-conversion characteristics, the multifunctionality of our synthesized hybrid particles, such as flexibility, color tunability, and patternability, paves the way for wearable devices in a variety of practical applications.

High-Efficiency, Mass-Producible, and Colored Solar Photovoltaics Enabled by Self-Assembled Photonic Glass.

Building-integrated photovoltaics is a crucial technology for developing zero-energy buildings and sustainable cities, while great efforts are required to make photovoltaic (PV) panels aesthetically pleasing. This places an urgent demand on PV colorization technology that has a low impact on power conversion efficiency (PCE) and is simultaneously mass-producible at a low cost. To address this challenge, this study contributes a colorization strategy for solar PVs based on short-range correlated dielectric microspheres, i.e., photonic glass. Through theoretical studies, first we demonstrate that the photonic glass self-assembled by high-index microspheres could enable both colored solar cells and modules, with easily variable colors and negligible parasitic absorption. By a fast spray coating process of colloidal monodisperse ZnS microspheres, we show the photonic glass layer could be easily deposited on silicon solar cells, enabling them to have structural colors. Through varying microsphere sizes, solar cells with different colors are achieved, showing low PCE loss compared to normal black cells. These colored solar cells are also encapsulated with a general lamination process to produce PV modules with various colors and patterns at a stunning PCE approaching 21%. Moreover, the long-term stability is subsequently verified by aging tests including an outdoor exposure for 10 days and a damp-heat test for 1000 h, and the mass producibility is demonstrated by presenting a colored PV panel with an output power over 108 W. These results confirm photonic glass as a promising strategy for colored PVs possessing high efficiency and practical applicability.

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

Optical forces on microspheres inside hollow core photonic crystal fibers (HC-PCFs) are often predicted using a ray optics model, which constrains its validity based on wavelength and microsphere sizes. Here, we introduce a rigorous treatment of the electromagnetic forces based on the Lorenz-Mie theory, which involves analytical determination of beam shape coefficients for the optical modes of a HC-PCF. The method is more practicable than numerical approaches and, in contrast with ray optics models, it is not limited by system size parameters. Time of flight measurements of microspheres flying through the HC-PCF lead to results consistent with the Lorenz-Mie predictions.

Numerical investigation of the resonance modes for the dielectric optical micro-spheres by using radial basis function method

In this paper, we present a numerical meshless method, which is based on the radial basis functions (RBF), to solve the morphology-dependent resonance (MDR) modes of a dielectric micro-sphere. To check the accuracy of the considered RBF method, we calculated the transverse electric (TE) and transverse magnetic (TM) modes of a perfect sphere that has an analytical characteristic equation. Besides, we employed the traditional mesh-dependent finite difference (FD) method to study the same problem. When we compared the results obtained by the three methods, we observe that the RBF method’s results are in very good agreement with the analytical and FD method’s results. We conclude that the presented meshless method may also be applicable to prolate and oblate spheroidal geometries, which do not have an analytical equation to calculate the modes.

Flexible microsphere‐coupled surface‐enhanced Raman spectroscopy (McSERS) by dielectric microsphere cavity array with random plasmonic nanoparticles

AbstractSurface‐enhanced Raman spectroscopy (SERS) is a powerful tool for nondestructive and ultrasensitive optical trace‐detection. However, the sophisticated fabrication processes and performance degradation on flexible substrates block SERS for practical uses. Here, we report a facile flexible microsphere‐coupled SERS (McSERS) substrate composed of a dielectric microsphere cavity array (MCA) and random gold nanoparticles (AuNPs) capping on a polydimethylsiloxane (PDMS) film (MCA/AuNPs/PDMS) for giant Raman enhancement. The random distribution of AuNPs provides a hydrophilic surface against to the coffee‐ring effect for uniform localized surface plasmon resonance (LSPR) response. The MCA capped on the AuNPs boosts the Raman intensity via the multiple optical manipulation processes, in which the photonic nanojet (PNJ) confines the excitation intensity near the AuNPs, whispering‐gallery mode (WGM) facilitates the energy transfer from microsphere cavities to AuNP gaps for LSPR boosting, and directional antenna effect converts near‐field Raman signals into far‐field with a small divergence. Therefore, the Raman scattering is dramatically improved with the enhancement factor (EF) to 107 for the limit of detection (LoD) of 4‐nitrobenzenethiol (4‐NBT) molecules down to 0.1 nM, two orders of magnitude higher via MCA coupling. Moreover, the flexible McSERS substrate exhibits outstanding durability and compatibility as an ultrasensitive Raman test strip, by which the thiram concentration is detectable down to 2.42 ng/cm2 on apple peels. The present work provides a facile strategy to fabricate SERS substrates with high flexibility for optical trace‐detection in real‐world applications.

Raman scattering enhancement of dielectric microspheres on silicon nitride film

Circulating light in the total internal reflection within dielectric spheres or disks is called the whispering gallery mode (WGM), which by itself is highly sensitive to its surface and capable of detecting viruses and single atomic ions. The detection site of the sensors using WGM is created by the evanescent light from the circulating light inside spheres. Here we report anomalous Raman scattering enhancement in dielectric microspheres on a silicon nitride (SiN) film. This Raman enhancement occurs at the periphery of the spheres, and a similar ring of light was also observed under a fluorescence microscope. This is caused by the light circulating around the dielectric spheres as in the WGM. We observed anomalously enhanced Raman spectrum at the periphery of 3 μm diameter polystyrene (PS) microspheres on a SiN film using confocal laser Raman microscopy. The wavelength intensity of this enhanced Raman spectrum was accompanied by periodic changes due to interference. These features may lead to the development of high-sensitive sensors and optical devices.

Whispering-gallery modes promote enhanced optical backflow in a perforated dielectric microsphere.

Optical energy flow inside a dielectric microsphere is usually co-directed with the optical wave vector. At the same time, if the optical field in a microsphere is in resonance with one of the high-quality spatial eigenmodes (whispering-gallery modes- WGMs), a region of reverse energy flow emerges in the shadow hemisphere. This area is of considerable practical interest due to increased optical trapping potential. In this Letter, we consider a perforated microsphere with an air-filled pinhole fabricated along the particle diameter and numerically analyze the peculiarities of WGM excitation in a nanostructured microsphere. A pinhole isolates the energy backflow region of a resonant mode and changes a perforated microsphere into an efficient optical tweezer. For the first time, to the best of our knowledge, a multiple enhancement of the energy backflow intensity in the pinhole at a WGM resonance is revealed and we discuss the ways for its manipulation.

Proof-of-concept of a miniature pressure sensor based on coupled optical WGMs excited in a dielectric microsphere

Optical whispering-gallery mode (WGM) resonators retain increased interest as a key element in various high-resolution sensing and spectroscopic devices in modern optoelectronic, optofluidic and Lab-on-chip technologies. As a theoretical proof-of-concept, we present a new miniature pressure sensor based on coupled WGMs optically excited in a dielectric microsphere placed near a flexible reflective membrane which acts as an ambient pressure sensing element. WGMs are excited by free-space coupling of incoming optical radiation to a spherical microparticle. The distinctive feature of proposed sensor is double WGM excitation by forward and backward propagating radiation upon reflection from a membrane that causes WGMs interference in particle volume. Unlike known WGM-sensors, the proposed design utilize not the resonant frequency shift but the changes in optical intensity of resulting field established in the microsphere which carries information about the exact position of pressure-loaded membrane. The sensitivity of the proposed sensor strongly depends on the quality factor of the working resonance, as well as geometrical and mechanical parameters of flexible reflecting membrane. Important advantages of the proposed sensor are its miniature design and the absence of a mechanical contact of pressure-sensitive element with WGM resonator.

Strong Coupling of Multimolecular Species to Soft Microcavities.

Can we couple multiple molecular species to soft cavities? The answer to this question has relevance in designing open cavities for polaritonic chemistry applications. Because of the differences in adhesiveness, it is difficult to couple multiple molecular species to open cavities in a controlled and precise manner. In this Letter, we discuss the procedure to coat multiple dyes, TDBC and S2275, onto a dielectric microsphere using a layer-by-layer deposition technique so as to facilitate the multimolecule coupling. We observed the formation of a middle polariton branch due to the intermolecular mixing facilitated by the whispering gallery modes. The coupling strength, 2g, of the TDBC molecules was found to be 98 meV, while that of the S2275 molecules was 78 meV. The coupling strength was found to be greater than the cavity line width and the molecular absorption line width, showing that the system is in the strong coupling regime.

Characteristic parameters of photonic nanojets of single dielectric microspheres illuminated by focused broadband radiation

Herein, we report the theoretical investigation on the photonic nanojets (PNJs) of single dielectric microspheres illuminated by focused broadband radiation (polychromatic light) from a Halogen lamp, supercontinuum source, light-emitting diode, and Hg arc lamp. The role of incident beam waist, refractive index of the surrounding medium, and radius of the microsphere on the characteristic parameters such as the electric field intensity enhancement, effective width, and length of the PNJ is studied. Interestingly, the characteristic parameters of the PNJs of solid microspheres obtained for the above-mentioned broadband radiation sources are found close to those observed for the focused monochromatic radiation of wavelengths which are near to the central wavelengths of the sources. Moreover, the characteristic parameters of PNJs of the core–shell microspheres of different thicknesses (t) illuminated by polychromatic radiation from most commonly used sources such as Halogen and Hg arc lamps are studied. For each t value, a suitable wavelength of monochromatic radiation has been found to generate the PNJ with characteristic parameters which are close to those obtained in the case of polychromatic radiation. We believe that the analytical theory and the theoretical simulations reported here would be useful for researchers who work in the fields such as PNJ assisted photoacoustic spectroscopy, white light nanoscopy, low-coherence phase-shifting interference microscopy, and Mirau interferometry.

Far-field directional emission of fluorescence enhanced by dielectric microsphere and metallic planar nanolayers

Controlling the emission characteristics of fluorescent substances and increasing the intensity of fluorescence emission are crucial for fluorescence detecting technology in single-molecule detection, biomedicine, and sensing applications. Since fluorescence emission is isotropic in nature, the collected fluorescence is only accounted for a small fraction of the total emitted fluorescence. In this paper, a composite structure composed of dielectric microsphere and metallic planar nanolayers is proposed to enhance the fluorescence far-field directional emission intensity and improve the fluorescence collection efficiency. The excitation process and the emission process of quantum dots (QDs) located between the dielectric microspheres and the gold layer are investigated by the finite difference time domain (FDTD) method. In the emission process, the emission of QDs in a homogeneous medium is isotropic. Therefore, we usually select several special polarizations in theoretical analysis state for research. In this paper, we first study the effect of the structure on the fluorescence emission enhancement of QDs when the QDs are in the &lt;i&gt;x-&lt;/i&gt;, &lt;i&gt;y-&lt;/i&gt;, and&lt;i&gt; z-&lt;/i&gt;polarization state. Some results can be obtained as shown below. When the radiation direction of the QDs is perpendicular to the microsphere plane layered structure, the structure is coupled with the emitted fluorescence, thereby realizing the directional enhancement of the emitted fluorescence of the QDs, and the obvious fluorescence enhancement is obtained in the &lt;i&gt;x-&lt;/i&gt; and &lt;i&gt;y-&lt;/i&gt;polarization state. Therefore, in the research, we choose and investigate the dipole light source of &lt;i&gt;x&lt;/i&gt;-polarization state. We mainly study the influence of microsphere radius, refractive index, and QDs position on the fluorescence directional enhancement. The QDs as a fluorescent material are coated in polymethyl methacrylate (PMMA) to control the distance from the gold layer to tune the fluorescence enhancement. The structure is based on the synergistic effect among plasmon coupling, whispering gallery mode and photonic nanojet, which enhances the far-field fluorescence of QDs by a factor of 230, and the fluorescence collection efficiency is as high as 70%. Comparing with the enhanced fluorescence of the dielectric microspheres and the gold sphere dimer composite structure, the distance between the gold sphere dimers is not easy to control, and the QDs should be placed at specific positions between the gold spheres. The structure we propose is more convenient to implement. In this paper, not only the emission enhancement process of QDs is studied in detail, but also the excitation process of QDs is investigated. Our proposed dielectric microsphere metal planar nanolayered structure can enhance the excitation of QDs in most areas, proving that our designed structure can effectively realize the excitation enhancement of QDs. The above results have very important applications in the fluorescence biological detection, imaging, and light-emitting devices.

Coupled optical resonances in a dielectric microsphere: physical concept of miniature optical pressure sensor

Coupled optical resonances in a dielectric microsphere: physical concept of miniature optical pressure sensor

Unveiling the Coupling of Single Metallic Nanoparticles to Whispering-Gallery Microcavities.

Whispering-gallery mode resonators host multiple trapped narrow-band circulating optical resonances that find applications in quantum electrodynamics, optomechanics, and sensing. However, the spherical symmetry and low field leakage of dielectric microspheres make it difficult to probe their high-quality optical modes using far-field radiation. Even so, local field enhancement from metallic nanoparticles (MNPs) coupled to the resonators can interface the optical far field and the bounded cavity modes. In this work, we study the interaction between whispering-gallery modes and MNP surface plasmons with nanometric spatial resolution by using electron-beam spectroscopy with a scanning transmission electron microscope. We show that gallery modes are induced over a selective spectral range of the nanoparticle plasmons, and additionally, their polarization can be controlled by the induced dipole moment of the MNP. Our study demonstrates a viable mechanism to effectively excite high-quality-factor whispering-gallery modes and holds potential for applications in optical sensing and light manipulation.

Scalable aqueous processing-based radiative cooling coatings for heat dissipation applications

Passive daytime radiative cooling (PDRC) is an electricity-free cooling technology that has received great interest for its’ environmental benefits. Current organic polymer PDRC materials often require harmful organic solvents for dispersion and inorganic dielectric PDRC materials require costly vacuum processing or have poor flexibility and water-repellence. In addition, most PDRC applications focus on refrigeration at sub-ambient temperatures using a selective emittance spectrum. Here a scalable aqueous-based processing method utilizing commercially available hollow dielectric microspheres is reported to create dielectric PDRC coatings with broadband emittance spectra for the heat dissipation of outdoor devices used above-ambient temperatures. A high solar reflectance and thermal infrared emittance of 0.93 at a thickness of 500 μm can be achieved due to the multi-interface reflectivity between the dielectric material and air in the hollow stacked microspheres. The max temperature (∼ 46 °C) of the outdoor device at a heat density of 5000 W m−2 can drop 20 °C at a solar intensity of ∼ 925 W m−2 compared to an Al metal wall (∼ 66 °C), which is only 4 °C higher than the ambient temperature of 42 °C. More importantly, the aqueous paint does not require any volatile organic compounds and has great stability, water repellence, and excellent flexibility.