  Frequency up-conversion is an effective method of mid-infrared (MIR)
detection by converting the long-wavelength photons to the visible domain,
where efficient detectors are readily available. Here, we generate the MIR
light carrying orbital angular momentum (OAM) from a difference frequency
generation process and perform the up-conversion of it via sum frequency
conversion in a bulk quasi-phase-matching crystal. The maximum quantum
conversion efficiencies from MIR to visible are 34.0%, 10.4%, and 3.5% for
light with topological charges of 0, 1, and 2, respectively, which is achieved
by utilizing an optimized strong pump light. We also verify the OAM
conservation with a specially designed interferometer, and the results agree
well with the numerical simulations. Our study opens up the possibilities for
generating, manipulating, and detecting MIR light that carries OAM, and will
have great potential for optical communications and remote sensing in the MIR
regime.
