Optical seminar | 25 September 2020

Standard experimental setup for spectroscopic studies of single semiconductor nanostructures (e.g. QDs) requires a microscope objective in front of the studied sample. In order to avoid photon losses, the numerical aperture (NA) of the objective should be as high as possible. On the other hand, longer working distance from the sample to the light collecting optics is often desirable. It is particularly important in systems with restricted access to the sample, e.g., in the case of split-coil superconducting magnets. In this work, we present aspherical micro-lenses which redirect emitted photons from semiconductor nanostructure into an ultra-narrow light cone of NA=0.016 - the outcoming light can be collected by a 1 inch-diameter lens at the distance of 590 mm from the sample [1]. The resulting working distance is more than 70 times longer than the one offered by conventional microscope objectives. Micro-lenses were fabricated by Two-Photon Polymerisation Direct Laser Writing (TPP-DLW), the technique which allows for three-dimensional printing of micro-objects made of transparent non-conductive resin that withstands cryogenic temperatures [2]. We demonstrate the usability of the proposed solid immersion lenses with single self-assembled CdTe/ZnTe quantum dots (λ≈600 nm) containing a single manganese or cobalt ion. The lenses were also tested at shorter wavelengths with single CdSe/ZnSe quantum dots (λ≈500 nm). Finally, we show that our solution works for monolayers of transition metal dichalcogenides (TMDs), in particular MoSe2 and WSe2 (λ≈750 nm), thus confirming the feasibility of broadband operation of fabricated lenses. Presented micro-lenses could be especially useful for single nanostructure spectroscopy in very high magnetic fields - due to the absence of metal elements - as well as in microwave cavities or optical dilution refrigerator systems. Bibliography: [1] A. Bogucki, Ł. Zinkiewicz, et al., Light Science & Applications, 9, 48 (2020). [2] A. Bogucki, Ł. Zinkiewicz, et al., Optics Express 26, 11513-11518 (2018).