Ultra-long-working-distance spectroscopy of single nanostructures with aspherical solid immersion microlenses.

In light science and applications, equally important roles are played by efficient light emitters/detectors and by the optical elements responsible for light extraction and delivery. The latter should be simple, cost effective, broadband, versatile and compatible with other components of widely desired micro-optical systems. Ideally, they should also operate without high-numerical-aperture optics. Here, we demonstrate that all these requirements can be met with elliptical microlenses 3D printed on top of light emitters. Importantly, the microlenses we propose readily form the collected light into an ultra-low divergence beam (half-angle divergence below 1°) perfectly suited for ultra-long-working-distance optical measurements (600 mm with a 1-inch collection lens), which are not accessible to date with other spectroscopic techniques. Our microlenses can be fabricated on a wide variety of samples, including semiconductor quantum dots and fragile van der Waals heterostructures made of novel two-dimensional materials, such as monolayer and few-layer transition metal dichalcogenides. Tiny elliptical microlenses efficiently focus light onto and extract it from nano-sized light emitters, enabling an ultra-long-working-distance measurements. Aleksander Bogucki from the Faculty of Physics, University of Warsaw and colleagues tested their microlenses by 3D printing them on top of light-emitting quantum dots and a more fragile layered structure made of novel 2D materials. They demonstrated the microlenses can be deterministically placed onto specific parts of a sample, or can be printed as a large-scale array of hundreds of lenses on samples containing a large number of light emitters. It can also be printed onto fragile materials without affecting their properties. The findings could pave the way for their use in a wide range of applications, including single-nanowire lasers, photoconductive antennas, optical setups in high magnetic fields, and in integrated photonic circuits.