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

Levitated 2D manipulation on dielectric metasurface by the tuning of polarization states

In this Letter, we have proposed a particle manipulation system based on a polarization-dependent dielectric metasurface (PDM), which enables far-field trapping and 2D arbitrary transporting. Based on flexible phase manipulation, by tuning the size and angle of meta-atoms, polarization-selective focusing in different modules of the metasurface can be realized. Then, when those regional focuses are continuously lighted in a relay way, the trapped particle at the focus could be delivered to the next one. When six different characteristic polarization states are tuned in order, the trapped particle could be transported to any adjacent hot spots so that 2D manipulation can be realized in an extended range. With the consideration of the Brownian motion, our simulation results show that the success rate of the particle transport can reach more than 96.0%, even after 20 periods when excited at the wavelength of 1064 nm with a power density of 0.15 mW/µm2. We believe that our research provides a new and promising method for particle manipulation and furthers on-chip optofluidic applications.

Optical-field-induced surface nanobumps in near-infrared laser direct cleaning of nanoparticles on silicon

The evolution of surface damage in laser direct cleaning (LDC) of nanoparticulate contamination on silicon (Si) was theoretically and experimentally investigated. Nanobumps with a volcano-like shape were found in near-infrared laser cleaning of polystyrene latex nanoparticles on Si wafers. According to the finite-difference time-domain simulation and the high-resolution surface characterization, unusual particle-induced optical field enhancement in the vicinity of the interface between Si and nanoparticles is mainly responsible for the generation of volcano-like nanobumps. This work is of fundamental significance for the understanding of the laser-particle interaction during LDC and will promote the development of nanofabrication and nanoparticle cleaning applications in optics, microelectromechanical systems, and semiconductors.

To generate a photonic nanojet outside a high refractive index microsphere illuminated by a Gaussian beam

A non-resonant, concentrated, narrow beam of light emerging from an illuminated microlens is called a photonic nanojet (PNJ). According to currently prevailing opinion, microspheres and microcylinders are only able to generate a PNJ in their exterior when their refractive index ns (or refractive index contrast) is less than 2. In this Letter we demonstrate that a PNJ can emerge from a microsphere even when ns > 2: first by employing the laws of geometrical optics for a divergent light source; then, by using ray transfer matrix analysis, a mathematical condition for the Gaussian beam (GB) outside the high ns microsphere is derived. The PNJ outside the microsphere with ns = 2.5 is simulated using Generalized Lorenz-Mie theory (GLMT), by using a front focused GB source. The simulated difference between front and back focusing on the dependence of ns is confirmed experimentally by Raman imaging. By opening the PNJ field for high refractive index materials, we believe this work will be a nucleus for new ideas in the field and enable new PNJ applications.

Photonic Nanojet Generation Using Integrated Silicon Photonic Chip with Hemispherical Structures

Photonic nanojet (PNJ) is a tightly focused diffractionless travelling beam generated by dielectric microparticles. The location of the PNJ depends on the refractive index of the material and it usually recedes to the interior of the microparticle when the refractive index is higher than 2, making high index materials unsuitable to produce useful PNJs while high index favours narrower PNJs. Here we demonstrate a design of CMOS compatible high index on-chip photonic nanojet based on silicon. The proposed design consists of a silicon hemisphere on a silicon substrate. The PNJs generated can be tuned by changing the radius and sphericity of the hemisphere. Oblate spheroids generate PNJs further away from the refracting surface and the PNJ length exceeds 17λ when the sphericity of the spheroid is 2.25 The proposed device can have potential applications in focal plane arrays, enhanced Raman spectroscopy, and optofluidic chips.