Elsevier

Optics Communications

Volume 415, 15 May 2018, Pages 140-145
Optics Communications

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

https://doi.org/10.1016/j.optcom.2018.01.050Get rights and content

Abstract

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 λ4.5 (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 λ6.7.

Introduction

Generation of photonic nanojet (PNJ) and controlled manipulation of its characteristics has attracted considerable current research interest. Photonic nanojet, characterized by high intensity, low divergence and sub wavelength lateral confinement, was first reported by Chen et al. in 2004 for the scattering of plane wave by lossless dielectric micro-cylinder and microsphere [[1], [2]]. Generation of photonic nanojet is a non-resonant phenomenon and has been shown to be relatively insensitive to the deformation and surface corrugations in the microspheres [[3], [4]]. Due to their unique characteristics, PNJ have found many applications in areas like Raman signal enhancement [[5], [6]], single molecule spectroscopy [7], fluorescence correlation spectroscopy [8], nano photolithography [[9], [10], [11]] and nanoscopy [[12], [13]] etc. Several approaches have been proposed for manipulating the length and confinement of photonic jet as many of aforesaid applications require control over the length and/or lateral dimension of the nanojet. The results reported in literature show that the length and full width at half maxima (FWHM) of photonic nanojet depend on size and shape of micro particle, refractive index of particle and that of the surrounding medium, wavelength, polarization and beam profile of the excitation light etc. [[14], [15]]. Apart from conventional shapes like sphere and cylinder, particles of other shapes such as axicon [16], micro cuboids [[17], [18]], micro discs [19], concentric graded index microsphere or ellipsoids [[20], [21]], core–shell microspheres [[22], [23]] and truncated microspheres [[24], [25]] etc. have also been explored in various studies to investigate tuning of PNJ characteristics. More recently Eti et al. have demonstrated controlled manipulation of photonic nanojet using controlled tuning of the refractive index in liquid crystals filled micro shells [26].

Wu et al. [9] have demonstrated mask-less lithography with a feature size of 250 nm (λ1.6) using a self-assembled planar structure of silica microsphere with a 400 nm centred ultraviolet (UV) broad band light source. McLeod and Arnold [10] have shown nano-lithography using photonic nano jet generated by optically trapped submicron size polystyrene microspheres with 355 nm laser source. A resolution of 102 nm (λ3.5) and 130 nm (λ/2.7), was shown for 0.5 μm and 0.8 μm sized polystyrene microspheres respectively. The approach followed by McLeod is however difficult to implement for practical applications due to inherent instability in trapping of small micro-spheres and associated Brownian motion. Using a bigger micro-sphere ( few micrometer diameter) can therefore be more useful for controlled nano photolithography if PNJ with narrow FWHM can be generated with such microspheres.

In this paper, we present the results of our studies on the variation of FWHM and length of PNJ generated from micro-spheres having crescent shape refractive index profile (CSRP). The results show that PNJ with FWHM down to λ/4.5 can be achieved in free space with multilayer CSRP lossless dielectric microsphere of 3 μm diameter. In the presence of commonly used substrates with higher refractive index this would enables writing feature as small as λ/6.

Section snippets

Theory and simulations

A number of analytical and numerical approaches have been developed and applied to study the near as well as far field distribution of electromagnetic field in the photonic nanojet generated by microscopic dielectric objects. Analytical approach based on Mie theory and its suitable extension have been used to analyse the field distribution inside and outside the microsphere for concentric as well as eccentric inclusions [[22], [27], [28], [29]]. However, with increasing number of inclusions the

Results and discussion

Simulations were first performed for single microsphere of 3μm diameter while varying the refractive index ‘n’ of microsphere from 1.3 to 2.0. The excitation wavelength was taken as 500 nm and the surrounding refractive index was taken as 1.0. A typical intensity distribution in the photonic nanojet and the variation of FWHM as a function of refractive index of micro-sphere are shown in Fig. 1. As the refractive index of microsphere increases the FWHM of photonic nanojet generated by

Conclusion

To conclude we have shown that extremely narrow PNJ with FWHM down to λ/4.5 in free space can be generated using the microspheres with multi-layer CSRP. The multi-layer CSRP microsphere allows confining PNJ well below the dimension that can be achieved using sub-micron sized microspheres and offer significant advantage in terms of ease in handling for various applications. For the applications requiring direct contact with microsphere the substrate refractive index was also

Acknowledgements

The authors would like to acknowledge the help received from Dr. J.T. Andrews, Department of Applied Physics, SGSITS, Indore, for help with numerical simulation using COMSOL and Dr. S.K. Majumder, Head, Laser Biomedical Applications Section, RRCAT, Indore, for his support. We thank Dr. T.K. Sharma, Head, Semiconductor Materials Lab. RRCAT, for critical reading of the manuscript.

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