Photonic Nanojet Research Articles

Comparative Analysis of Key Parameters of Photonic Nanojets from Axisymmetric Nonspherical Microparticles

The effect of photonic nanojets (PNJ) in the scattering near field nearby the surface of dielectric axisymmetric microparticles (hemisphere, axicon, and combined particles) under optical illumination is theoretically considered. Key PNJ parameters (length, width, and peak intensity) are calculated using the discrete-dipole approximation; the dependence of these parameters on the geometric shape of microparticles is analyzed. We show that the use of a special type of combined transparent particle, which consists of an axicon and two attached hemispheres, can produce ultralocalized light fluxes with a peak intensity which considerably exceeds the corresponding values for simple particles (hemisphere and axicon).

Collective effects in the formation of an ensemble of photonic nanojets by an ordered microassembly of dielectric microparticles

Collective effects in the formation of an ensemble of photonic nanojets by an ordered microassembly of dielectric microparticles

Inverse photonic design of functional elements that focus Bloch surface waves

Bloch surface waves (BSWs) are sustained at the interface of a suitably designed one-dimensional (1D) dielectric photonic crystal and an ambient material. The elements that control the propagation of BSWs are defined by a spatially structured device layer on top of the 1D photonic crystal that locally changes the effective index of the BSW. An example of such an element is a focusing device that squeezes an incident BSW into a tiny space. However, the ability to focus BSWs is limited since the index contrast achievable with the device layer is usually only on the order of Δn≈0.1 for practical reasons. Conventional elements, e.g., discs or triangles, which rely on a photonic nanojet to focus BSWs, operate insufficiently at such a low index contrast. To solve this problem, we utilize an inverse photonic design strategy to attain functional elements that focus BSWs efficiently into spatial domains slightly smaller than half the wavelength. Selected examples of such functional elements are fabricated. Their ability to focus BSWs is experimentally verified by measuring the field distributions with a scanning near-field optical microscope. Our focusing elements are promising ingredients for a future generation of integrated photonic devices that rely on BSWs, e.g., to carry information, or lab-on-chip devices for specific sensing applications.

Microsphere-mediated optical contrast tuning for designing imaging systems with adjustable resolution gain

Upon illumination, a dielectric microsphere (μS) can generate a photonic nanojet (PNJ), which plays a role in the super-resolution imaging of a sample placed in the μS’s immediate proximity. Recent microscopy implementations pioneered this concept but, despite the experimental characterization and theoretical modeling of the PNJ, the key physical factors that enable optimization of such imaging systems are still debated. Here, we systematically analyzed the parameters that govern the resolution increase in the case of large-diameter (>20 µm) μS-assisted incoherent microscopy by studying both the illumination and the detection light paths. We determined the enhanced-resolution zone created by the μS, in which the detection system has a net resolution gain that we calculated theoretically and subsequently confirmed experimentally. Our results quantitatively describe the resolution enhancement mediated by the optical contrast between the μS and its surrounding medium, and provide concrete means for designing μS-enhanced imaging systems for several application requirements.

Ultralong photonic nanojet formed by dielectric microtoroid structure.

A photonic nanojet (PNJ) is a highly confined light beam formed by a transparent particle under light wave illumination. Here, we propose and numerically investigate the PNJ formed by a dielectric circular toroid with micro dimensions and a homogenous refractive index. Three-dimensional finite-difference time-domain (FDTD) simulations are conducted and demonstrate that ultralong PNJs can be formed by the doughnut-like structure. Besides, microtoroid structures can allow high-index materials (n=3.5) for PNJ generation. Various PNJ properties, including the focal distance, PNJ length, full width at half-maximum, and maximum intensity, can be flexibly tuned by modifying the geometry of the proposed structure.

Reduced distortion in high-index microsphere imaging by partial immersion.

We experimentally demonstrate that pincushion distortion is obvious when using a BaTiO3 glass (BTG) microsphere fully immersed in ethanol or SU-8 2002 resist to image a Blu-ray disk with sub-diffraction features. The distortion is related to the layer thickness of the immersing medium. For a BTG microsphere partially immersed in the SU-8 resist where the SU-8 thickness is around 4/5 the diameter of the microsphere, its distortion decreases dramatically, but it can still clearly resolve the Blu-ray disk. For such a partially immersed microsphere, the calculated position of the photonic nanojet is outside the microsphere and close to the object, indicating the microsphere has a super-resolution imaging property, and the distortion simulated by ZEMAX is decreased.

First experimental observation of array of photonic jets from saw-tooth phase diffraction grating

Modern materials science and technologies based on the advanced lithographies, nanophotonics and electronics use the effect of super-focusing of light based on photonic nanojet (PNJ) array produced by close packing of spherical microparticles. To control the key parameters of photonic jets array the phase grating may be applied. For the first time the experimental direct imaging of the spatial and amplitude features of PNJ ensemble formation near the shadow surface of a saw-tooth phase diffraction grating is presented here for 3 different wavelengths: 405, 532, and 761 nm. It is shown that the PNJs array from such phase diffraction gratings, in contrast to microparticle self-assemblies, produces an array of PNJs with controlled characteristics: the length, width, and focal length of PNJ. They are changed in proportion to the incident wavelength. For the designed wavelength of , the length of PNJ is , the beam waist is and the focal length is equal to .

Non-contact optical super-resolution imaging study based on height-variable microsphere

Super-resolution images can be achieved by coupling the microscale dielectric microsphere with white-light optical microscopy. This paper presented a method for adjusting the distance between microsphere and sample to study the imaging ability of the microsphere. By this method, the nanofeatures on the Blu-ray disc surface could be observed. The super-resolution images when microspheres at different heights were compared to analyse the changing trend of imaging ability with distance. The optical simulation results illustrated that super-resolution imaging ability was related to the position of the photonic nanojet. As the photonic nanojet was inside the sample, the super-resolution image could be achieved even if the microspheres didn’t touch the sample. This work has great significance for manipulating microsphere to non-contact image.

Analysis of dielectric circular cylinder light spot narrowing by whispering gallery modes and influence of material absorption

We present results of numerical analysis of the diffraction of a plane monochromatic TE-wave on an ideal homogeneous dielectric cylinder with several resonant wavelength scale radii. Two subsequent near-surface maxima of intensity (two focuses) generated at the cylinder output were found on the optical axis. The first subwavelength focus is formed by one of the whispering gallery mode lobes. Its intensity is 50 times the incident light intensity and its full width at the half maximum of the intensity is equal to 0.155 of the incident wavelength. The second focus is two times less in intensity. Its focal spot known as a photonic nanojet is stretched toward the optical axis. The second focus is formed at a distance about the wavelength from the cylinder surface. Its width is equal to 0.44 of the wavelength and its length is two wavelengths. The influence of material absorption on the light focusing is also examined by numerical simulation.

Side-lobes-controlled photonic nanojet with a horizontal graded-index microcylinder

A photonic nanojet generated by the transparent dielectric microparticle (microsphere or microcylinder) is a sub-wavelength focused beam. The properties of the photonic nanojet have been modified by changing the refractive index and structure of the micropaticles. In this Letter, a super-narrow photonic nanojet with a full width at half-maximum waist of approximately 116.6nm (λ/4.3, with 500nm excitation wavelength) is obtained by a horizontal graded-index microcylinder, which is divided by multilayers parallel to the direction of light propagation. The method for side lobes controlling by the waves' superposition from the modified graded refractive index and that of the center layer is proposed and discussed. Also, a structure with the composition of different height plates approaching the desired circular section is suggested for decreasing the difficulties in fabrication, and generates a similar photonic nanojet.

A Photonic Nanojet as Tunable and Polarization‐Sensitive Optical Tweezers

AbstractThe ability to manipulate small objects with focused laser beams has opened a venue for investigating dynamical phenomena relevant to both fundamental and applied sciences. However, manipulating nano‐sized objects requires subwavelength field localization, provided by auxiliary nano‐ and microstructures. Particularly, dielectric microparticles can be used to confine light to an intense beam with a subwavelength waist, called a photonic nanojet (PNJ), which can provide sufficient field gradients for trapping nano‐objects. Herein, the scheme for wavelength‐tunable and nanoscale‐precise optical trapping is elaborated, and the possibility of lateral nanoparticle movement using the PNJ's side lobes is shown for the first time. In addition, the possibility of subwavelength positioning using polarization switching is shown. The estimated stability with respect to Brownian motion is higher compared to conventional optical trapping schemes.

Near field focusing by edge diffraction.

Spherical microparticles have the ability for nonresonant focusing of light in the near field zone, forming nanojet (NJ) beams. Arbitrary-shaped microstructures, with wavelength-scale dimensions, may offer similar functionality with lower fabrication complexity. The focusing properties are ruled by the edge diffraction phenomenon. The diffraction of light on the edge of a dielectric microstructure forms a tilted focused beam whose deviation angle depends on the index ratio between the structure material and host medium. The beam geometry and field intensity enhancement can be tuned by varying the curvature of the edge line. Interference of edge diffracted waves from different segments of the edge line creates a condensed beam in the near field zone, the photonic nanojet.

Systematic study and comparison of photonic nanojets produced by dielectric microparticles in 2D- and 3D-spatial configurations

We present the systematic study of key characteristics (field intensity enhancement, spatial extents) of the 2D- and 3D-photonic nanojets (PNJs) produced by geometrically-regular micron-sized dielectric particles illuminated by a plane laser wave. By means of the finite-difference time-domain calculations, we highlight the differences and similarities between PNJs in these two spatial configurations for curved- (sphere, circular cylinder) and rectangle-shaped scatterers (cube, square bar). Our findings can be useful, for example, for the design of particle-based high-resolution imaging because the spatial resolution by such systems might be further controlled by the optimization of refractive index contrast and geometrical shape of the particle-lens.

Microparticle manipulation using femtosecond photonic nanojet-assisted laser cavitation.

We report the effect of laser cavitation in water initiated by femtosecond pulses confined into subwavelength volume of photonic nanojet of spherical microparticles. The effect of nanoscale optical breakdown was employed for controllable and nondestructive micromanipulation of silica microspheres. We combine this technique with optical trapping for cyclic particle movements and estimate a peak velocity and an acceleration acquired by microspheres propelled by nanojet cavitation. Our study provides a strategy for nondestructive optical micromanipulation, cavitation-assisted drug delivery, and laser energy transduction in microdevices.

Enhancement of Raman scattering signal using photonic nanojet of portable and reusable single microstructures

AbstractWe report the enhancement of Raman scattering signal for the first time using photonic nanojet of a lollipop‐shaped microstructure (LMS) fabricated with the help of a nanosecond CO2 laser. The LMS consists of a dielectric microsphere attached to a long stem/tapered fiber. The enhancement was achieved by focusing the incident laser light on sample surface through the microsphere of LMS. The experimentally observed enhancement was found to be comparable with that obtained from commercial microspheres that are not attached to any stems. The value of enhancement was found to increase with the microsphere diameter. The dielectric stems/tapered fibers alone of the LMS are found not efficient for enhancing the Raman scattering signals. In contrast to the commercial microspheres, the fabricated LMSs are easy to handle, portable, and reusable. Also, unlike commercial microspheres, these LMSs allow the enhancement of Raman scattering without modifying the surface of substrates containing sample. Hence, these LMSs are extremely useful in the case of photonic nanojet‐mediated surface‐enhanced Raman scattering and fluorescence techniques for enhancing the Raman scattering and fluorescence signals of single/few molecules, respectively.

Ultra-narrow photonic nanojets through a glass cuboid embedded in a dielectric cylinder.

A glass cuboid, embedded inside a dielectric cylinder is studied when illuminated with a monochromatic plane wave. A photonic nanojet (PNJ) with a full-width at half-maximum (FWHM) waist of around 0.25λ0 is obtained outside the external surface of the cuboid. The influence of the parameters of a square section cuboid is studied. Three particular phenomena can be obtained and are discussed: an ultra-narrow PNJ on the external surface of the cuboid, a long photonic jet and the excitation of whispering gallery modes (WGMs). A parametric study, over the width and the height of a rectangular section cuboid, shows that these parameters can be used to control the photonic jet properties. We also study several other geometries of the insert, which shows that the key parameter is the refractive index of the inserted material. Finally, we show that by changing the incident angle we can obtain a curved photonic jet.

Effects of immersion depth on super-resolution properties of index-different microsphere-assisted nanoimaging

In related applications of microsphere-assisted super-resolution imaging in biomedical visualization and microfluidic detection, liquids are widely used as background media. For the first time, we quantitatively demonstrate that the maximum irradiances, focal lengths, and waists of photonic nanojets (PNJs) will logically vary with different immersion depths (IMDs). The experimental observations also numerically illustrate the trends of the lateral magnification and field of view (FOV) with the gradual evaporation of ethyl alcohol. This work can provide exact quantitative information for the proper selection of microspheres and IMD for the high-quality discernment of nanostructures.

Generation of highly confined photonic nanojet using crescent-shape refractive index profile in microsphere

Photonic nanojets (PNJs) owing to their sub-wavelength near-field features have found many interesting applications like nanoscopy, nano photolithography, high density optical storage, enhancement of Raman signal and single molecule spectroscopy etc. More recently, the focus of research has been on tailoring of PNJs either for better confinement and thus higher peak intensity or for elongation of nanojet for high resolution far field applications. In this paper, we show that crescent-shape refractive index profile (CSRP) of microspheres can be used to generate highly confined PNJ. By optimizing the refractive index of different layers in CSRP microsphere, we show a free space confinement down to ∼λ∕4.5 (FWHM ∼110 nm for excitation with 500 nm wavelength). Further, it was observed that the optical properties of substrates also modulate the PNJ characteristics and lead to a further improvement in the transverse confinement to∼λ∕6.7.

Super-narrow focusing and ultra-long working distance by different shapes of dielectric microlenses

The dielectric microlens, surmounts the diffraction limit, consider as a super-focusing and super-resolution imaging microlens. In this paper, the super-focusing and working distance between microlens and test sample are investigated through different shapes of dielectric microlenses. The result shows that the hollow sphere microlens, an innovative design of nanoparticle with a solid sphere including a hollow core region, is an optimal spherical microlens with super-resolution. After optimization, a hollow sphere microlens with the ratio of inner and outer radius r/R of 1:2 and the refractive index of 1.6 is designed and a super-narrow photonic nanojet with the full width at half maximum of 164 nm has been achieved under the incident wavelength of 500 nm. When the hollow sphere microlens is immersed in water or oil, it can focus the 500 nm incident light 3 μm away from the hollow sphere lower surface. Increasing the refractive index of hollow spheres, the resolution reaches 0.22 λ. With the super-focusing and ultra-long working distance, the hollow sphere microlens could be applied for detecting a test sample to acheive a super-resolution imaging and a test sample can be distinguished clearly when combined with a conventional optical microscope, which is particularly available for the test samples with the extreme matte surfaces.

Twin photonic nanojets generated from coherent illumination of microscale sphere and cylinder

Photonic nanojets, highly focused beams of light created by planar illumination of a microsphere, have been shown to produce narrow subwavelength beams over distances of several wavelengths in the near field. In this work, we investigate the generation of twin photonic nanojets through the illumination of a microsphere or cylinder from two coherent sources with relative phase shift. Under these conditions, symmetric twin nanojets separated by an intensity null can be generated. Compared to a photonic nanojet, the twin nanojets can achieve an even smaller subwavelength beam, and have the added advantage of having more complex intensity profiles that can be controlled by multiple parameters. Using both finite-difference time-domain and Mie theory models, the width, length, and intensity enhancement factor of the nanojet geometry are found to be functions of the phase, angle offsets, and particle geometry. Such twin photonic nanojets can find applications in optical trapping, manipulation, nanolithography, and enhancement of nonlinear optical properties.