The evolution of hot spot and nanojet by engineering the local modes of microcylinder

The ability of mesoscale dielectric particles (microspheres or microcylinders) to localize the optical wave below sub-wavelength volumes has attracted tremendous attentions in recent years. Photonic nanojet is a high-intensity narrow focus beam which can be generated from microparticles illuminated with a plane wave. PNJ has the characteristics of high-intensity super-resolution focusing and easy assembly [1], which has broad applications in Microsphere-assisted Microscopy [2], [3], [4], [5], Fluorescence Sensing [6], [7], Photolithography [8]. The mechanism of PNJ’s formation, the characteristic of microspheres with different structures and their key parameters (such as full width at half maximum (FWHM), peak intensity, effective length (L)) have already been extensively discussed in resonant or non-resonant conditions [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. And optical phenomena in Janus particles also attracts great attentions [20], [21], [22].

In addition, hot-spot engineering also has the ability to localize optical waves to sub-wavelength scale, and the waist is below the diffraction limit. Hot spots of metal nanoparticles are created by the optical resonant excitation of localized surface plasmons. Due to the characteristics of great enhancement of electromagnetic field intensity, it is widely used in the field of surface-enhanced Raman spectroscopy (SERS) [23]. In order to obtain high-resolution and single-molecule-level detection, some research has used nanojet to excite hot spots of SERS metal nanoparticles [24], [25]. Recent reports indicate that dielectric mesoscale particles can generate hot spots with the size of approximately λ/5 [26], [27], [28]. Hot spots in super-enhancement focusing [29] and magnetic field intensity enhancement [21] has been discussed. However, the focus of hot spots is localized inside the particles, relevant applications (super-resolution imaging or single-molecule detection) cannot directly utilize its best optical performance part. And the construction of hot spots which is beyond the diffraction limit and manufacturing-friendly is still under discussion.

In this paper, we propose a method to generate the hot spot by utilizing a high-refractive index cladding and modulating local modes at the exit end of the microcylinder. The hot spot with FWHM of 66.70 nm (λ/8.47) on the surface is achieved. The focal position of the hot spot can be adjusted and even be extended outward gradually by modulating the height of the local structures. Within a certain range, FWHM changes regularly with local structures. Field localization can be shifted from hot spot to nanojet by changing the refractive index and structural parameters. Based on this method, we introduce a cubic array chip to decrease difficulty of fabrication. Also, we can manipulate parameters of the local structure to control the number and position of the hot spot.

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