Coherent mid-infrared vortex generation at room temperature for manipulation of microparticles

We investigate the generation of mid-infrared (mid-IR) vortex beams carrying orbital angular momentum (OAM) through nonlinear processes in an inhomogeneously broadened 85 Rb atomic ensemble. By employing a four-level atomic system featuring two strong control fields and a weak probe field, we generate a non-degenerate four-wave mixing signal at a wavelength of 5.23 µm. Applying the density-matrix formalism, we derive an analytical expression for the nonlinear atomic coherence which facilitates the transfer of vortex characteristics such as topological charge and intensity and phase profiles from the probe field to the mid-IR signal. Numerical solutions of Maxwell’s wave equation confirm the generation of mid-IR vortex beams with adjustable topological charges and beam widths at different spatial positions. This technique offers significant potential for applications in mid-IR communication, providing additional bandwidth and improved data transmission rates, as well as in fields such as microfluidics, biophysics, and nanotechnology, where OAM-carrying beams can manipulate microparticles with precision.