Photonic Nanojet Research Articles

Functional dielectric microstructure for photonic nanojet generation in reflection mode

We first have performed direct experimental imaging of extended photonic nanojet generated in reflection mode by an array of aluminum oxide hemispheres. It was found that a primary photonic nanojet generated in transmission mode acts as an additional illumination source for the microparticle and interferes with the incident laser light forming a reflective photonic nanojet. The electromagnetic field distribution obtained experimentally is in good correlation with the numerically calculated one. We have developed an easy-to-implement technique for hemispherical dielectric microstructure formation and made a functional array of aluminum oxide microparticles for various optical applications. The obtained array was successfully used for photoluminescence enhancement and provides a new platform for various optical sensing measurements as well as high-resolution optical imaging.

Coated High-Refractive-Index Barium Titanate Glass Microspheres for Optically Trapped Microsphere Super-Resolution Microscopy: A Simulation Study

As water is normally used as the immersion medium in optically trapped microsphere microscopy, the high-refractive-index barium titanate glass (BTG) microsphere shows a better imaging performance than the low-index polystyrene (PS) or melamine formaldehyde (MF) microsphere, but it is difficult to be trapped by single-beam optical trapping due to its overly high refractive index. In this study, coated BTG microspheres with a PS coating have been computationally explored for the combination of optical trapping with microsphere-assisted microscopy. The PS coating thickness affects both the optical trapping efficiency and photonic nanojet (PNJ) property of the coated BTG sphere. Compared to the uncoated BTG sphere, the coated BTG sphere with a proper PS coating thickness has a highly improved trapping efficiency which enables single-beam optical trapping, and a better PNJ with a higher optical intensity Imax and a narrower full width at half maximum (FWHM) corresponding to better imaging performance. These coated BTG spheres also have an advantage in trapping efficiency and imaging performance over conventional PS and MF spheres. The coated BTG microsphere is highly desirable for optically trapped microsphere super-resolution microscopy and potentially beneficial to other research areas, such as nanoparticle detection.

Highly Focused Fluorescence Emission Generated by a Cylinder on Sharp Convex Gold Groove

To satisfy the requirement for analyzing low concentration analyst in microfluidic bio-detection, it is extremely crucial to enhance the sensitivity of fluorescence bio-sensing. The emission efficiency of fluorescent nanoparticles (FNPs) is low and the fluorescence emission is dispersed, which brings a challenge to the collection of emission signal and the improvement of bio-sensing sensitivity. Directional fluorescence emission enhancement is crucial to solve this problem. In this work, we propose a hybrid structure consisting of a dielectric cylinder on a sharp convex gold groove to achieve highly focused fluorescence emission in both near field and far field. Due to scattering, constructive interference, guided resonance modes (cavity modes), surface plasmon mode and photonic nanojet effect, the fluorescence excitation and far-field directional fluorescence emission are enhanced obviously. In addition, the directional fluorescence enhancement is obvious for random positions of the FNPs in the designed structures and arbitrary volume of bio-solution. Such advantages are fantastic for sensitive biochemical detection and analysis.

Tunable Near‐ to Far‐Infrared Optical Breakdown in Nonlinear Interactions of Ultrashort Laser Pulses with Water Microdroplets in Ambient Air

Intense laser pulse propagation results in excitation and partial or complete ionization of aerosols and water microdroplets in the atmosphere. While the problems of light scattering by particles and laser‐induced breakdown in bulk water have been investigated in depth, nonlinear interaction of ultrashort laser pulses with microdroplets has been barely covered for a wide range of droplet distributions and wavelengths from visible to long‐wave infrared, relevant for laser breakdown spectroscopy, optical communications, and biomedical applications. Herein, detailed investigation of transient optical response and ionization processes is performed by coupling full‐vector nonlinear Maxwell and plasma kinetics equations in water and in ambient air. Such a self‐consistent approach unravels a variety of nonlinear processes occurring during laser‐droplet interactions such as extreme electron plasma confinement, nanofocusing and nanoshadowing inside water droplets, as well as plasma shielding and restriction on the maximum intensities attained in photonic nanojet and laser‐produced free carriers in the surrounding air. The numerical results provide and explain dependencies of the damage threshold on laser wavelengths and droplet sizes in a reasonable agreement with available experimental results.

Short and elongated photonic nanojets emerged from single solid/hollow core-shell microparticles illuminated by focused Gaussian beams and plane wave

For the first time, herein, we report the theoretical investigation on short and elongated photonic nanojets (PNJs) generated by single solid/hollow core-shell microparticles illuminated with focused Gaussian beams (FGBs). This investigation is carried out using developed analytical equations based on Bromwich formalism. The effect of the beam waist, thickness of shell, size and refractive index of the microparticle on the characteristic parameters of the PNJs such as maximum electric field enhancement, spatial separation between the PNJs, and length of the PNJs is studied in detail. The electric field enhancement values of the PNJs upon FGB illumination are compared to those obtained from the plane wave illumination.

Microconical silicon mid-IR concentrators: spectral, angular and polarization response.

It is widely discussed in the literature that a problem of reduction of thermal noise of mid-wave and long-wave infrared (MWIR and LWIR) cameras and focal plane arrays (FPAs) can be solved by using light-concentrating structures. The idea is to reduce the area and, consequently, the thermal noise of photodetectors, while still providing a good collection of photons on photodetector mesas that can help to increase the operating temperature of FPAs. It is shown that this approach can be realized using microconical Si light concentrators with (111) oriented sidewalls, which can be mass-produced by anisotropic wet etching of Si (100) wafers. The design is performed by numerical modeling in a mesoscale regime when the microcones are sufficiently large (several MWIR wavelengths) to resonantly trap photons, but still too small to apply geometrical optics or other simplified approaches. Three methods of integration Si microcone arrays with the focal plane arrays are proposed and studied: (i) inverted microcones fabricated in a Si slab, which can be heterogeneously integrated with the front illuminated FPA photodetectors made from high quantum efficiency materials to provide resonant power enhancement factors (PEF) up to 10 with angle-of-view (AOV) up to 10°; (ii) inverted microcones, which can be monolithically integrated with metal-Si Schottky barrier photodetectors to provide resonant PEFs up to 25 and AOVs up to 30° for both polarizations of incident plane waves; and iii) regular microcones, which can be monolithically integrated with near-surface photodetectors to provide a non-resonant power concentration on compact photodetectors with large AOVs. It is demonstrated that inverted microcones allow the realization of multispectral imaging with ∼100 nm bands and large AOVs for both polarizations. In contrast, the regular microcones operate similar to single-pass optical components (such as dielectric microspheres), producing sharply focused photonic nanojets.

Experimental demonstration of a tunable photonic hook by a partially illuminated dielectric microcylinder.

In this Letter, we report the experimental observations of a tunable curved photonic nanojet (photonic hook) generated by a 5 µm polydimethylsiloxane microcylinder deposited on a silicon substrate and illuminated by 405 nm laser beam. A moveable opaque aluminum-mask is mounted in front of the microcylinder implementing partial illumination and imparting spatial curvature to the photonic nanojet. Experimental results of main parameters (tilt angle, width, and intensity) of emerging photonic hooks exhibit close agreement with numerical predictions of the near-field optical structures. The experimentally measured full widths at half-maximum of photonic hooks are 0.48λ, 0.56λ, and 0.76λ for tilt angles of θ=0∘, 5.7°, and 20.1°, respectively. Photonic hooks possess great potential in complex manipulation such as super-resolution imaging, surface fabrication, and optomechanical manipulation along curved trajectories.

Twin photonic hooks generated from two adjacent dielectric cylinders

Photonic hooks (PHs) are a new class of curved light beams, they have been theoretically predicted and experimentally observed. The generation of photonic hook (PH) is related to the symmetry-broken of micro-particles under a plane wave illumination. Recently, twin photonic hooks (t-PHs) can be produced utilizing twin-ellipse micro-cylinder. In this paper, we investigate two adjacent parallel cylinders illuminated by a single TE-polarization plane wave for the t-PHs generation using the finite element method. We find that the full width at half-maximum (FWHM) of the t-PHs is less than the diffraction limit (λ/2), but larger than the FWHM of the photonic nanojet (PNJ) formed by a single cylinder. The shape profiles of the t-PHs can be modulated by changing the spacing length and the diameter of the two cylinders. As the spacing length increasing, the curvature degrees of center lines of the t-PHs become smaller. The curvature degree is fluctuant as the cylindrical diameter changing. This work will provide novel applications in optical trapping, nanolithography and integrated optics.

Reflective photonic nanojets generated from cylindrical concave micro-mirrors

Reflective photonic nanojets (r-PNJs) produced by cylindrical concave micro-mirrors are numerically investigated. Simulation shows that the full-width at half-maximum (FWHM) of the r-PNJ is associated with the angle of the cylindrical concave mirror. It is found that the FWHM of the r-PNJ can achieve 0.37 of the wavelength, when the mirror with the angle θ = 130° is put in air. For the concave mirror immersed in water with θ = 100°, the FWHM of the r-PNJ can reach about 0.3λ. Two symmetric vortexes of Poynting vectors are close to the r-PNJ, which leads to the narrowed r-PNJ. Through combining a dielectric micro-cylinder with the concave mirror immersed in water, the waist of the r-PNJ can achieve 0.27 of the incident wavelength.

Super-Resolution Imaging with Direct Laser Writing-Printed Microstructures.

Dielectric microstructures coupled with a conventional optical microscope have been proven to be a successful way to achieve super-resolution imaging. However, a limitation of such super-resolution imaging is the microstructure fabrication ability, which generally provides natural structures (such as spherical, hemispherical, superhemispherical microlenses, and so on). Meanwhile, the influences of microstructures with complex shapes on the super-resolved imaging still remain unknown. In this paper, direct laser writing (DLW) lithography is used to produce a series of complex microstructures, which are capable of achieving super-resolution imaging in the optical far-field region. Cylinder, truncated cone, hemisphere, and protruding hemisphere microstructures are successfully fabricated by this 3D printing technology, allowing us to resolve features as small as 100 nm under classical microscopy. Moreover, different microstructures lead to different photonic nanojet (PNJ) illuminations and collection efficiencies, resulting in a critical role in super-resolved imaging. The microstructures with spherical surfaces can easily collect the light scattered by the object and convert the high-spatial-frequency evanescent waves into propagating waves.

Peculiarities of the formation of photonic nanojets by a matrix of dielectric microtoroids

We report the results of theoretical modelling of photonic nanojets (PNJs) formed by laser radiation scattering on a single-layer ordered assembly of dielectric microtoroids placed on the surface of a transparent matrix (silicone film). Using the method of computational electrodynamics (FDTD), the main PNJ parameters (length, width, peak intensity) are analysed under the conditions of mutual influence of the light fields of neighbouring microparticles. It is shown that the main factor affecting the PNJ characteristics under study is the spatial configuration of the radiation-scattering particle, namely, the internal diameter of its cross section. It is found that for certain configurations of toroid placement in a cluster, a PNJ ensemble is implemented with parameters significantly better than those for a single toroid.

Photonic Nanojets and Whispering Gallery Modes in Smooth and Corrugated Micro-Cylinders under Point-Source Illumination

We numerically investigate the generation of photonic nanojets (PNJs) and the excitation of whispering gallery modes (WGMs) supported by both smooth and corrugated dielectric micro-cylinders under point-source illumination. Results show that the location of the point-source defines the location and properties of PNJs, whereas stability of WGMs exists in smooth micro-cylinders but vanishes in corrugated ones. It is shown that the location of the point-source acts as an additional degree of freedom for controlling the characteristics of the generated PNJs for both smooth and corrugated dielectric micro-cylinders. Furthermore, the influence of the point-source location on the stability of the excited WGMs was diminished for the smooth micro-cylinders, while being fully pronounced for their corrugated counterparts.

Specular-reflection photonic nanojet: physical basis and optical trapping application

A specular-reflection photonic nanojet (s-PNJ) is a specific type of optical near-field subwavelength spatial localization originated from the constructive interference of direct and backward propagated optical waves focused by a transparent dielectric microparticle located near a flat reflecting mirror. The unique property of s-PNJ is reported for maintaining its spatial localization and high intensity when using microparticles with high refractive index contrast when a regular photonic nanojet is not formed. The physical principles of obtaining subwavelength optical focus in the specular-reflection mode of a PNJ are numerically studied and a comparative analysis of jet parameters obtained by the traditional schemes without and with reflection is carried out. Based on the s-PNJ, the physical concept of an optical tweezer integrated into the microfluidic device is proposed provided by the calculations of optical trapping forces of the trial gold nanosphere. Importantly, such an optical trap shows twice as high stability to Brownian motion of the captured nano-bead as compared to the conventional nanojet-based traps and can be relatively easy implemented.

Overcoming refractive index limit of mesoscale light focusing by means of specular-reflection photonic nanojet.

The physical origin of subwavelength photonic nanojet in specular-reflection mode (s-PNJ) is theoretically considered. This specific type of photonic nanojet (PNJ) emerges upon double focusing of a plane optical wave by a transparent dielectric microparticle located near a flat mirror. For the first time, to the best of our knowledge, we report a unique property of s-PNJ for increasing its focal length and intensity using circular-shaped microparticles with a refractive index ratio exceeding the known limiting value that fundamentally distinguishes s-PNJ from regular PNJ behavior. This may drastically increase the trapping potential of PNJ-based optical tweezers.

Twin photonic hooks generated from two coherent illuminations of a micro-cylinder

Recently, twin photonic nanojets (t-PNJs) generated from two coherent illuminations of a single micro-cylinder have been emerging as a new research field in nanophotonics. Herein, we investigate the t-PNJs generation using a single micro-cylinder illuminated by two opposite polarized plane waves. The shape profiles of the t-PNJs are related to the angles of the incident waves, while twin photonic hooks (t-PHs) can be generated at large incident angles. The properties of the t-PHs are also investigated. Then, we use Gaussian beams to describe the physical mechanism of the t-PHs generation. Finally, the asymmetry t-PHs produced by the position modification of the cylinder are presented. Our work presents a new approach to generate t-PHs. The t-PHs will provide novel applications in nanolithography and optical trapping.

Microsphere-lens coupler with 100 nm lateral resolution accuracy in visible light.

We propose a microsphere-lens coupler that can successfully constrain the beam with near-field details to achieve ∼100nm lateral resolution accuracy. In theory, we use the finite-difference time-domain method to analyze the properties of the "photonic nanojet." The optimal focusing ability scope can be obtained by adjusting the focusing parameters, such as refractive index, paraxial optical intensity attenuation ratio, sizes of microspheres, and so forth. In experiment, the "non-brush scrubbing" cleaning technology is adopted to optimize the experimental results. Aiming to self-assemble properties of dielectric spheres, we introduce the deagglomeration method to produce homogeneous liquid. Meanwhile, by tiling a 5 µm-diameter microsphere-lens on a specimen, a traditional optical microscope can realize 100 nm lateral superresolution microimaging, which lays a certain foundation for further development of superresolution microimaging.

Optimal photonic nanojet beam shaping by mesoscale dielectric dome lens

In this work, we thoroughly investigate the shape, size, and location of the photonic nanojets (PNJs) generated from the illuminated dome lens. The silk fiber is directly extracted from the cellar spider and used to form the dome lens by its liquid-collecting ability. The solidified dielectric dome lenses with different dimensions are obtained by using ultraviolet curing. Numerical and experimental results show that the long PNJs are strongly modulated by the dimension of the dome lens. The optimal PNJ beam shaping is achieved by using a mesoscale dielectric dome lens. The PNJ with a long focal length and a narrow waist could be used to scan over a target for large-area imaging. The silk fiber with a dome lens is especially useful for bio-photonic applications by combining its biocompatibility and flexibility.

Enhanced high-quality super-resolution imaging in air using microsphere lens groups.

Most microsphere-assisted super-resolution imaging experiments require a high-refractive-index microsphere to be immersed in a liquid to improve the super-resolution. However, samples are inevitably polluted by residuals in the liquid. This Letter presents a novel (to the best of our knowledge) method employing a microsphere lens group (MLG) that can easily achieve high-quality super-resolution imaging in air. The performance of this method is at par or better than that of the high-refractive-index microspheres immersed in liquid. In addition, the MLG generates a real image that is closely related to the photonic nanojet position of the microsphere super-lens. This imaging method is beneficial in microsphere imaging applications where liquids are impractical.

Double clad tubular anti-resonant hollow core fiber for nonlinear microendoscopy

We report the fabrication and characterization of the first double clad tubular anti-resonant hollow core fiber. It allows to deliver ultrashort pulses without temporal nor spectral distortions in the 700-1000 nm wavelength range and to efficiently collect scattered light in a high numerical aperture double clad. The output fiber mode is shaped with a silica microsphere generating a photonic nanojet, making it well suitable for nonlinear microendoscopy application. Additionally, we provide an open access software allowing to find optimal drawing parameters for the fabrication of tubular hollow core fibers.

Large-Scale Fabrication of Photonic Nanojet Array via Template-Assisted Self-Assembly.

A large-scale homogenized photonic nanojet array with defined pattern and spacing facilitates practical applications in super-resolution imaging, subwavelength-resolution nanopatterning, nano objects trapping and detection technology. In this paper, we present the fabrication of a large-scale photonic nanojet array via the template-assisted self-assembly (TASA) approach. Templates of two-dimensional (2D) large-scale microwell array with defined pattern and spacing are fabricated. Melamine microspheres with excellent size uniformity are utilized to pattern on the template. It is found that microwells can be filled at a yield up to 95%. These arrayed microspheres on the template serve as microlenses and can be excited to generate large-scale photonic nanojets. The uniformly-sized melamine spheres are beneficial for the generation of a homogenized photonic nanojet array. The intensity of the photonic nanojets in water is as high as ~2 fold the background light signal. Our work shows a simple, robust, and fast means for the fabrication of a large-scale homogenized photonic nanojet array.