Creation of a longitudinally polarized photonic nanojet via an engineered microsphere

Tailoring aberration-free photonic nanojets through the illumination of dielectric cylinders using cylindrical vector beams

This study explores the manipulation of photonic nanojets (PNJs) via axial illumination of cylindrical dielectric particles with cylindrical vector beams (CVBs). The edge diffraction effect of cylindrical particles is harnessed to achieve the near-field focusing of CVBs, minimizing the spherical aberration's impact on the quality of the PNJ. By discussing how beam width, refractive index, and particle length affect PNJs under radially polarized incidence, a simple and effective approach is demonstrated to generate rod-like PNJs with uniform transmission distances and super-diffraction-limited PNJs with pure longitudinal polarization. Azimuthal polarization, on the other hand, generates tube-like PNJs. These PNJs maintain their performance across scale. Combining edge diffraction with CVBs offers innovative PNJ modulation schemes, paving the way for potential applications in particle trapping, super-resolution imaging, photo-lithography, and advancing mesotronics and related fields.

Phase-only steerable photonic nanojets

We demonstrate numerically the feasibility of axial and angular control of the position of a photonic nanojet (PNJ) by lossless phase-only modulation of a fixed Gaussian beam illuminating a fixed 2D circular homogeneous dielectric micro-lens. We furthermore demonstrate that our phase-only modality can be used to calibrate and improve the confinement of PNJ generation.

Simulation and experimental observations of axial position control of a photonic nanojet by a dielectric cube with a metal screen

In this Letter, we report on a numerical study, fabrication, and experimental observations of photonic nanojet (PNJ) shaping by control of a tangential electric field component. Here the PNJs are generated by a single mesoscale micro-cube that is fabricated from polydimethylsiloxane, deposited on a silicon substrate and placed on thick metal screen at illuminating wavelengths of 405, 532, and 671 nm. It is shown that the length, focal length, and width of the PNJ can be significantly reduced in the presence of the metal masks along the side faces of the micro-cube. Experimental measurements of the PNJ imaging are performed by a scanning optical microscope with laser sources. Our experimental results are in reasonable agreement with simulation predictions of the finite-difference time-domain method. Due to the appearance of the metal masks, the PNJ focal length decreases 1.5 times, the PNJ decay length decreases 1.7 times, and the PNJ resolution increases 1.2 times. Such PNJs possess great potential in complex manipulation, including integrated plasmonic circuits, biosensing, and optical tweezers.

Photonic Nanojet Modulation Achieved by a Spider-Silk-Based Metal–Dielectric Dome Microlens

The photonic nanojet is a non-resonance focusing phenomenon with high intensity and narrow spot that can serve as a powerful biosensor for in vivo detection of red blood cells, micro-organisms, and tumor cells in blood. In this study, we first demonstrated photonic nanojet modulation by utilizing a spider-silk-based metal–dielectric dome microlens. A cellar spider was employed in extracting the silk fiber, which possesses a liquid-collecting ability to form a dielectric dome microlens. The metal casing on the surface of the dielectric dome was coated by using a glancing angle deposition technique. Due to the nature of surface plasmon polaritons, the characteristics of photonic nanojets are strongly modulated by different metal casings. Numerical and experimental results showed that the intensity of the photonic nanojet was increased by a factor of three for the gold-coated dome microlens due to surface plasmon resonance. The spider-silk-based metal-dielectric dome microlens could be used to scan a biological target for large-area imaging with a conventional optical microscope.

High-quality longitudinally polarized photonic nanojet created by a microdisk

The Letter reports the generation of a high-quality longitudinally polarized photonic nanojet by illuminating a dielectric microdisk with a focused radially polarized light. High-quality longitudinally polarized beams can be generated using a microdisk with a wide range of refractive indices. By optimizing the shape and refractive index of the microdisk, the radial component can be effectively suppressed, and a maximum beam quality of 90% and a field enhancement factor of up to two orders can be achieved with a sub-diffraction-limited spot size. Moreover, the focusing performance of the microdisk is observed to be stable within a wide range of focusing numerical aperture values of the incident light.

Realization of noncontact confocal optical microsphere imaging microscope

A novel noncontact confocal optical microsphere imaging microscope (COMIM) is proposed to achieve high contrast optical nano-imaging at a high scanning speed. The developed setup obtains images with significantly higher contrast when compared with conventional bright-field microsphere nano-imaging, as well as with a commercial confocal laser scanning microscope (CLSM). With the image-by-image scanning scheme, COMIM takes only 11% time to complete a three-dimensional (3D) scanning image than using a commercial CLSM, while still offers an outstanding resolving power for the nano-features on the sample.

Formation of polarization-dependent optical vortex beams via an engineered microsphere

In this work, we propose a method that can realize switchable spatial arrangement of the donut-shaped focusing beams through an engineered microsphere, tuned by changing the polarization state of an incident light. In this polarization-dependent light manipulation, the engineered microsphere is designed and fabricated by focused ion beam (FIB). We simulate and experimentally characterize the focus performance of the engineered microsphere. Under the light incidence with radial polarization, multiple focused donut beams are formed along optical axis. By adding an additional linear polarizer with a rotatable relative angle, the pair of donut beams can be re-arranged in the same transverse plane and stay close to each other. Experimental results and numerical simulation are in good agreement. Such tunable polarization-dependent micro-optics can be used for multiplane imaging applications.

Recent advances in focused ion beam nanofabrication for nanostructures and devices: fundamentals and applications

The past few decades have witnessed growing research interest in developing powerful nanofabrication technologies for three-dimensional (3D) structures and devices to achieve nano-scale and nano-precision manufacturing. Among the various fabrication techniques, focused ion beam (FIB) nanofabrication has been established as a well-suited and promising technique in nearly all fields of nanotechnology for the fabrication of 3D nanostructures and devices because of increasing demands from industry and research. In this article, a series of FIB nanofabrication factors related to the fabrication of 3D nanostructures and devices, including mechanisms, instruments, processes, and typical applications of FIB nanofabrication, are systematically summarized and analyzed in detail. Additionally, current challenges and future development trends of FIB nanofabrication in this field are also given. This work intends to provide guidance for practitioners, researchers, or engineers who wish to learn more about the FIB nanofabrication technology that is driving the revolution in 3D nanostructures and devices.

Numerical analysis of tiny-focal-spot generation by focusing linearly, circularly, and radially polarized beams through a micro/nanoparticle

Obtaining a tiny focal spot is desired for super resolution. We do a vectorial numerical analysis of the linearly, circularly, and radidally polarized electromagnetic fields being focused through a dielectric micro/nanoparticle of size comparable to the wavelength. We find tiny focal spots (up to ∼0.05 λ²) can be obtained behind micro/nanoparticles of various shapes, e.g. spherical, disk-shaped, and cuboid micro/nanoparticles. Furthermore, we also investigate the influence of the misalignment of a real lens system on the tiny focal spots. We find that tiny focal spots can still be generated even though they are distorted due to the misalignment.

Far-field polarization signatures of surface optical nonlinearity in noncentrosymmetric semiconductors

We analyse possibilities to quantitatively evaluate the surface second-order optical nonlinearity in noncentrosymmetric materials based on polarization-resolved analysis of far-field radiation patterns of second-harmonic generation. We analytically demonstrate that for plane-wave illumination the contribution to the second-harmonic signal from the surface of a nonlinear medium exhibits different polarization properties and angular dependencies compared to the contribution from the bulk. In view of this, we optimize the illumination geometry in order to enable the most efficient separation and comparison of both nonlinearities. Furthermore, we consider the illumination of an AlGaAs slab by a tightly-focused linearly-polarized Gaussian beam as an alternative measurement geometry. It is found that the reliable separation of the surface nonlinearity contribution as well as a wide range of detectable values can be achieved with this geometry as well.

Wavelength-tunable focusing via a Fresnel zone microsphere

In this Letter, a novel, to the best of our knowledge, structural configuration on a transparent microsphere is proposed to engineer the focusing light field. By patterning a hybrid diffractive Fresnel zone plate structure on a partially milled microsphere using a focused ion beam, wavelength-dependent switching between mono-focal and multi-focal functionalities can be achieved. Generation of on-axis tri-foci and mono-focus light fields under high numerical-aperture (${\rm NA}\gt {0.67}$NA>0.67) conditions at two working wavelengths (405 nm and 808 nm) have been demonstrated both numerically and experimentally.

Experimental verification of twin photonic nanojets from a dielectric microcylinder

In this Letter, the direct generation of twin photonic nanojets (PNJs) through two coherent illuminations of a microcylinder is investigated theoretically and experimentally. The dielectric microcylinder (polydimethylsiloxane) with 5 μm diameter and 6 μm height is employed to generate symmetric twin PNJs. The finite-difference time-domain calculation is used to simulate the electric field distributions inside and outside the microcylinder. The scanning optical microscope system is performed for experimental verification of twin photonic nanojets. In both theory and in practice, the intensity null of electric field creates two separate PNJs. Compared to a single PNJ, twin PNJs have a smaller subwavelength waist and more complex intensity distribution. The focal distance, interval, and full width at half-maximum of twin PNJs are a function of the offset angle. The twin PNJs will provide novel applications in nanolithography, optical trapping, biophotonic sensing, and therapy.

Flexible Photonic Nanojet Formed by Cylindrical Graded-Index Lens

Photonic nanojets formed in the vicinity of the cylindrical graded-index lens with different types of index grading are numerically investigated based on the finite-difference time-domain method. The cylindrical lens with 1600 nm diameter is assembled by eighty-seven hexagonally arranged close-contact nanofibers with 160 nm diameter. Simulation and analysis results show that it is possible to engineer and elongate the photonic nanojet. Using differently graded-index nanofibers as building elements to compose this lens, the latitudinal and longitudinal sizes of the produced photonic nanojet can be flexibly adjusted. At an incident wavelength of 532 nm, the cylindrical lens with index grading = 2 can generate a photonic nanojet with a waist about 173 nm (0.32 wavelength). This lens could potentially contribute to the development of a novel device for breaking the diffraction limit in the field of optical nano-scope and bio-photonics.

Planar Diffractive Lenses: Fundamentals, Functionalities, and Applications

Traditional objective lenses in modern microscopy, based on the refraction of light, are restricted by the Rayleigh diffraction limit. The existing methods to overcome this limit can be categorized into near-field (e.g., scanning near-field optical microscopy, superlens, microsphere lens) and far-field (e.g., stimulated emission depletion microscopy, photoactivated localization microscopy, stochastic optical reconstruction microscopy) approaches. However, they either operate in the challenging near-field mode or there is the need to label samples in biology. Recently, through manipulation of the diffraction of light with binary masks or gradient metasurfaces, some miniaturized and planar lenses have been reported with intriguing functionalities such as ultrahigh numerical aperture, large depth of focus, and subdiffraction-limit focusing in far-field, which provides a viable solution for the label-free superresolution imaging. Here, the recent advances in planar diffractive lenses (PDLs) are reviewed from a united theoretical account on diffraction-based focusing optics, and the underlying physics of nanofocusing via constructive or destructive interference is revealed. Various approaches of realizing PDLs are introduced in terms of their unique performances and interpreted by using optical aberration theory. Furthermore, a detailed tutorial about applying these planar lenses in nanoimaging is provided, followed by an outlook regarding future development toward practical applications.

Orbital angular momentum generation via a spiral phase microsphere

Vortex beam carrying orbital angular momentum (OAM) attracts much attention in many research fields for its special phase and intensity distributions. In this Letter, a novel design called the spiral phase microsphere (SPMS) is proposed for the first time, to the best of our knowledge, which can convert incident plane wave light into the focused vortex beam that carries OAM with different topological charges l=±1 and ±2. The vortex beam generation is verified by a self-interfered modification of the SPMS. The generation of the vortex beams by the SPMS irradiated by a single-wavelength incident light is studied using the CST MICROWAVE STUDIO simulation. The SPMS provides a new approach to achieve high-efficiency and high-integrated photonic applications related with OAM.