Photonic Nanojets

Abstract

This paper reviews the substantial body of literature emerging since 2004 concerning photonic nanojets. The photonic nanojet is a narrow, high-intensity, non-evanescent light beam that can propagate over a distance longer than the wavelength λ after emerging from the shadow-side surface of an illuminated lossless dielectric microcylinder or microsphere of diameter larger than λ. The nanojet’s minimum beamwidth can be smaller than the classical diffraction limit, in fact as small as ~λ/3 for microspheres. It is a nonresonant phenomenon appearing for a wide range of diameters of the microcylinder or microsphere if the refractive index contrast relative to the background is less than about 2:1. Importantly, inserting within a nanojet a nanoparticle of diameter dν perturbs the far-field backscattered power of the illuminated microsphere by an amount that varies as dν3 for a fixed λ. This perturbation is much slower than the dν6 dependence of Rayleigh scattering for the same nanoparticle, if isolated. This leads to a situation where, for example, the measured far-field backscattered power of a 3-μm diameter microsphere could double if a 30-nm diameter nanoparticle were inserted into the nanojet emerging from the microsphere, despite the nanoparticle having only 1/10,000th the cross-section area of the microsphere. In effect, the nanojet serves to project the presence of the nanoparticle to the far field. These properties combine to afford potentially important applications of photonic nanojets for detecting and manipulating nanoscale objects, subdiffraction-resolution nanopatterning and nanolithography, low-loss waveguiding, and ultrahigh-density optical storage.

Gouy phase anomaly in photonic nanojets

“…Moreover, the spatial distance between these phase singularities, which correspond to roots of the intensity, are much larger separated than the full-width at half-maximum ͑FWHM͒; a measure usually used to estimate the field localization in space. 17 In measurements and simulations which consider the numerical aperture of the imaging objective this FWHM was 0.38 m, in simulations which do not consider the imaging process but take the field directly behind the sphere it even was 0.25 m; which corresponds to a localization of light in space in the order of 0.39.…”

“…15 Such photonic nanojets generated a heated debate since their creation is considered as a scattering phenomenon and requires a rigorous treatment. 16,17 Their fascination is attached to the promise to beat the diffraction limit at the expenses of being bound to the near-and intermediate-field region of the dielectric sphere. If photonic nanojets would be fully understood, their ability to steer the light in subwavelength domains could be exploited in many applications.…”

Tailoring the Properties of Photonic Nanojets by Changing the Material and Geometry of the Concentrator

“…There are different ways to couple light from free-space/optical fiber into integrated circuits by using, for example, grating couplers [22] or micro-objects (micro-cylinders) to create the so-called photonic nanojets [23]. Nanojets are subwavelength tightly focused electromagnetic beams that emerge from a micro-object when it is illuminated by a plane wave -the so-called nanojets have been observed at optical [24] and microwave [25] frequencies.…”

Photonic Nanojet Array for Fast Detection of Single Nanoparticles in a Flow

“…While a microsphere is known to act as a focusing microlens 6,7,8,9 , when it has a refractive index contrast relative to the fluid medium that is less than 2:1 and a diameter between several to tens of wavelengths (λ), a highly-focused propagating beam from the shadow-side surface of the microsphere is generated due to constructive interference of the light field 10,11 . This beam is termed as "photonic nanojet" and has a sub-λ full-width-at-half-maximum (FWHM) transverse dimension and typically is several λ in length 11,12 .…”

“…This beam is termed as "photonic nanojet" and has a sub-λ full-width-at-half-maximum (FWHM) transverse dimension and typically is several λ in length 11,12 . A very interesting predicted property of a photonic nanojet is that the presence of a particle, much smaller than λ and positioned within the nanojet, significantly enhances back-scattering of the light through the microsphere 13,14 .…”