Extremely long nanojet formation from a ballpoint photonic pen

Abstract

Certain microelements can facilitate light manipulation in the form of an intense electromagnetic beam known as a “photonic nanojet” (PNJ), which is an emerging candidate in the field of spectroscopy, nanolithography, and bio/chemical sensing. However, inherent limitations of the PNJs such as short jet length, focal point, and working length minimize their application areas. Herein, by considering readily available materials and technologies, we report a simple and physically realizable “photonic pen” (PP)-like microstructure to yield extremely long PNJs under plane-wave illumination. Essentially, by semi-immersing a sphere of certain diameter (acting as pen tip) in a barrel material with the width equal to and height 5 times the sphere diameter, a “ballpoint pen”-like microstructure is formed where illuminated light acts as the ink. Finite-difference time-domain analysis shows the use of low refractive index (RI) spherical tips ( n = 1.48 ) with 10 µm diameter, and a high RI SU-8 barrel ( n = 1.62 ) creating a net index difference of Δ n = 0.14 to effectively form PNJs with extremely long jet length, working distance, and full width at half-maximum of over 330 λ (121.6 µm), 319.45 λ (116.6 µm), and 4.22 λ (1.54 µm) respectively, and a figure of merit of ∼ 155 in water environment. S i O 2 / SU- 8 PPs show over ∼ 3 times longer jet length than previous studies. We show that such SU- 8 / SiO 2 PPs with extremely long PNJs can act as epidermal photonic imagers to scan biological tissues such as trilayer human skin to detect melanoma (skin cancer), both in contact and in noncontact mode with efficiency enhancement reaching ∼ 45 % compared to plain microspheres without barrels.