Extremely long nanojet formation from a ballpoint photonic pen

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 microstruc-ture 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 mu m diameter, anda high RI SU-8 barrel (n = 1.62) creating a net index difference of An = 0.14 to effectively form PNJs with extremely long jet length, working distance, and full width at half-maximum of over 3301. (121.6 mu m), 319.451. (116.6 mu m), and 4.221. (1.54 mu m) respectively, and a figure of merit of similar to 155 in water environment. SiO2/SU-8 PPs show over similar to 3 times longer jet length than previous studies. We show that such SU-8/SiO2 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 similar to 45% compared to plain microspheres without barrels. (c) 2023 Optica Publishing Group