Ways to break through the diffraction limit and into the regime of subwavelength imaging have been growing ever-more creative. One tool that has proven useful in this endeavour is the photonic nanojet (PNJ) – an extremely narrow and intense beam of radiation at the interface between a dielectric particle and its surrounding medium. Now, a pair of researchers have developed a way to simultaneously minimize the particles and enhance the PNJs, which could benefit many imaging, microscopy and sensing applications, offering a resolution five times better than that of traditional PNJ imaging systems.
Victor Pacheco-Peña from Newcastle University, UK, and Miguel Beruete from the Public University of Navarra in Spain conducted numerical simulations of titanium dioxide (TiO2) particles surrounded by air. The material was chosen specifically for its very high refractive index (around 9.95) and low absorption when excited with 50 GHz radiation, as used in these simulations. This is the key to the success of the study – traditional PNJs are produced in particles that have a lower index (around 2) and must be around five times larger than the wavelength. Thanks to the higher index of TiO2 the particles studied by Pacheco-Peña and Beruete are just over half a wavelength across, bringing the technology to the mesoscale.
Freeing the nanojet
The first dielectric particle tested was an infinitely long cylinder. Initial results showed that a PNJ was indeed produced when the cylinder was illuminated from one side with a plane wave. However it was trapped inside the cylinder, as opposed to resting on the surface as in lower-index particles.
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In order to release the PNJ from its dielectric prison, the researchers applied the Weierstrass formula, normally used in the design of solid immersion lenses, to find the distance from the centre at which the cylinder should be truncated. Simulations show that the PNJ at the cylinder’s surface enhances the power of the backscattered signal by a factor of 2.5, with a full-width at half-maximum (FWHM) of 0.14 times the wavelength.
For the second shape, Pacheco-Peña and Beruete studied something a little easier to realise experimentally; a truncated dielectric sphere with equivalent dimensions to that of the cylinder. When illuminated by a plane wave incident on the curved side, a PNJ appears at the flat face of the truncated sphere that is even narrower (FWHM of 0.06 times the wavelength) but slightly less intense (power enhancement of 1.8) than that produced by the truncated cylinder.
Soaring past the diffraction limit
Next the researchers investigated the ability of the PNJs to capture an image with subwavelength resolution. In another numerical model, two small gold spheres are positioned under a truncated dielectric sphere, and the backscattered signal measured from the combined system. The gold spheres are scanned around, which produces an image similar to that from a scanning probe microscope. After testing gold spheres with various separations, the researchers found that they could clearly distinguish spheres with separations as little as 0.06 times the wavelength, a resolution 5 times better than that of traditional PNJ imaging systems.
This study shows a promising future for PNJs in subwavelength imaging and sensing systems. The researchers are currently delving more deeply into other possible geometries for high-index particles, such as cubes and ellipsoids. Perhaps they can beat their own resolution record!
Read more about this work in the article published in the Journal of Applied Physics.