  Optical responsivity grants biomedical capsules additional capabilities,
promoting them towards multifunctional theragnostic nanodevices. In this
endeavor, screening candidates under conditions that closely resemble in situ
environments is crucial for both the initial optimization and the subsequent
inspection stages of development and operation. Optical tweezers equipped with
dark-field spectroscopy are among the preferable tools for nanoparticle imaging
and refractometry. However, the effectiveness of conventional illumination and
light collection arrangements for inspecting anisotropic complex inner
composition particles is quite limited due to reduced collection angles, which
can result in the omission of features in scattering diagrams. Here we
introduce an endoscopic dark-field illumination scheme, where light is launched
on an optically trapped particle from a single-mode fiber, immersed into a
fluid cell. This arrangement disentangles illumination and collection paths,
thus allowing the collection of scattered light with a very high numerical
aperture. This methodology is applied to vaterite nanocapsules, which are known
to possess strong anisotropic responses. Tweezer configuration allows revealing
optical properties for different crystallographic orientations of vaterite,
which is complex to do otherwise. Furthermore, endoscopic dark-field images
reveal the emergence of polarization-dependent long-range photonic nanojets,
which are capable of interacting with nearby particles, demonstrating a new
pathway for nanojet image formation.
