  Dielectric microspheres or microbeads can squeeze light into the
subwavelength scale via photonic nanojet (PNJ) focusing. This enables strong
light-matter interactions within its focus and induces extraordinary effects
such as enhancements in light emission, signal collection and various other
applications. However, critical challenges exist on how to efficiently and
precisely loading the studied objects into the desired location of the PNJ
focusing, and subsequently measure the field and signal of interests precisely
and reliably. Such technique is currently missing. We present for the first
time a unique microwell-decorated microsphere lens (u-well lens), with a
semi-open microwell sample compartment directly fabricated on top of the
microsphere lens. The u-well lens confines PNJ in a semi-open microwell and
allows passive trapping of individual micro-object into the PNJ focusing area
with high efficiency and spatial accuracy. We demonstrate that individual
fluorescent microsphere of different sizes can be readily introduced to the
designated location with loading efficiency >70% and generates reproducible
enhanced fluorescence signals with standard deviation better than that can be
quantitatively measured. A comprehensive analysis on the optical properties of
the u-well lens reveals the synergistic effect of field enhancement and
collection efficiency on the optical enhancement. We finally employ this
special microsphere lens for fluorescent-bead-based biotin concentration
analysis. The results suggest a greatly enhanced sensitivity and highly
improved detection limit, opening the door for its application in highly
sensitive and quantitative detection.
