  The physical origin of subwavelength photonic nanojet in
retrograde-reflection mode (retro-PNJ) is theoretically considered. This
specific type of photonic nanojet emerges upon sequential double focusing of a
plane optical wave by a transparent dielectric microparticle located near a
flat reflecting mirror. For the first time to the best of our knowledge, we
report a unique property of retro-PNJ for increasing its focal length and
intensity using microparticles with the optical contrast exceeding limiting
value (n > 2) that fundamentally distinguishes retro-PNJ from regular PNJ
behavior. This may drastically increase the trapping potential of PNJ-based
optical tweezers.
