Overstepping the upper refractive index limit to form ultra-narrow photonic nanojets

Guoqiang Gu,Jun Song,Mengjie Zhao,Hongda Liang,Junle Qu,Yue Chen

doi:10.1038/s41598-017-05781-4

Abstract

In general, photonic nanojets (PNJs) occur only when the refractive index (Ri) difference between the microparticle and background media is less than 2. The minimum full width at half-maximum (FWHM) of the PNJ is ~130 nm (approximately one-third of the illumination wavelength λ = 400 nm) formed within the evanescent field region. This paper proposes and studies a method to overstep the Ri upper bound and generate ultra-narrow PNJs. Finite element method based numerical investigations and ray-optics theoretical analyses have realized ultra-narrow PNJs with FWHM as small as 114.7 nm (0.287 λ) obtained from an edge-cut, length-reduced and parabolic-profiled microparticle with Ri = 2.5 beyond evanescent decay length. Using simple strain or compression operations, sub-diffraction-limited PNJs can be flexibly tuned on the order of several wavelengths. Such ultra-narrow PNJs offer great prospects for optical nonlinearity enhancements of greater enhancing effect, optical nanoscopy of higher spatial resolution, optical microprobes of smaller measurement accuracy, nano/micro-sized sample detections of higher sensing sensitivity, nanoscale objects of more accurate control, advanced manufactures of smaller processing size, optical-disk storage of larger data capacity and all-optical switching of lower energy consumption.

Highlights

Focusing light onto the shadow-side surface of a plane-wave-illuminated dielectric microparticle has opened the interesting field photonic nanojets (PNJs)[1]
When light illuminates the whole boundary of the first LRI, the usual case shown in Fig. 1b, the working distance of the generated PNJ (Wd ≈ ~2.7 λ) is greater than the evanescent decay length (~λ/2π)[24], and full width at half-maximum (FWHM) ω = 0.43 λ is smaller than the diffraction limit (0.5 λ)
The FWHM beam width is limited to a certain value of ~130 nm and the main body of this minimum PNJ is embedded within the microparticle

Summary

Introduction

Focusing light onto the shadow-side surface of a plane-wave-illuminated dielectric microparticle has opened the interesting field photonic nanojets (PNJs)[1] Such a nonresonant, high-intensity light beam can propagate along a path extending beyond the evanescent field region without significant diffraction, and maintain a subwavelength full width at half-maximum (FWHM) transverse beamwidth. Many papers have studied improving and optimizing these properties to further explore and efficiently utilize PNJs. When the refractive index (Ri) of the microparticles are increased to a certain value (Ri ~ 1.75 in Fig. S1a), the tightly focused beam forms a PNJ that moves in the opposite direction of the illumination light towards the shadow-side surface of the microparticle, and FWHM (ω) narrows (from 0.43 to 0.39 λ). By exerting mechanical strain or compression, sub-diffraction-limited PNJs can be flexibly tuned within several wavelengths

Methods

Results

Conclusion