  Efficiently collecting light emitted or scattered from nanoscale systems,
which can be embedded in a high-index medium, is a challenge for fundamental
spectroscopic studies and commercial applications of quantum dots, color
centers, single molecules, plasmonic antennas, etc. Solid immersion lenses
offer a powerful solution to increase the numerical aperture of the optical
system, while being robust and compatible with cryogenic temperatures. However,
they suffer from strong chromatic aberrations, limiting their applications to
narrow-band excitation and emission spectral windows. Here, we design and
optimize an achromatic lens assembly based on a solid immersion lens and a
cemented doublet for broadband confocal spectroscopy. We provide an example
with an achromatic range between 600 and 750 nm and show that it achieves
diffraction limited spot size in excitation and in collection (reimaging of a
point source) over more than 100 nm bandwidth. We expect our results to be
useful to the broad community involved in spectroscopy and technological
applications of nano-emitters for single photon sources, quantum information
processing and quantum sensing.
