  The study of nanomaterials is an active area of research for technological
applications as well as fundamental science. A common method for studying
properties of isolated nanoparticles is by an in-vacuum particle beam produced
via an aerodynamic lens. Despite being common practice, characterization of
such beams has proven difficult as light scattering detection techniques fail
for particles with sizes beyond the diffraction limit. Here we present a new
technique for characterizing such nanoparticle beams using strong field
ionization. By focusing an ultrafast, mJ-level laser into the particle beam, a
nanoparticle within the laser focus is ionized and easily detected by its
ejected electrons. This method gives direct access to the nanoparticle density
at the location of the focus and, by scanning the focus through the transverse
and longitudinal profiles of the particle beam, the 3-dimensional particle
density distribution can be attained. Further, we show that strong field
ionization is effective in detecting spherical nanoparticles as small as 10 nm
in diameter. Additionally, this technique is an effective tool in optimizing
the particle beam for specific applications. As an example we show that the
particle beam density and width can be manipulated by restricting the gas flow
into the aerodynamic lens.
