Trans-Spectral Orbital Angular Momentum Transfer via Four-Wave Mixing in Rb Vapor

We report the transfer of phase structure and, in particular, of orbital angular momentum from near-infrared pump light to blue light generated in a four-wave-mixing process in ${}^{85}\mathrm{Rb}$ vapor. The intensity and phase profile of the two pump lasers at 780 and 776 nm, shaped by a spatial light modulator, influences the phase and intensity profile of light at 420 nm, which is generated in a subsequent coherent cascade. In particular, we observe that the phase profile associated with orbital angular momentum is transferred entirely from the pump light to the blue. Pumping with more complicated light profiles results in the excitation of spatial modes in the blue that depend strongly on phase matching, thus demonstrating the parametric nature of the mode transfer. These results have implications on the inscription and storage of phase information in atomic gases.

Figure 1

Experimental setup, with abbreviations: NPBS (nonpolarizing beam splitter), PBS (polarizing beam splitter), DM (dichroic mirror), TA (tapered amplifier), SLM (spatial light modulator), and CCD (charge-coupled device). Inset: ${}^{85}\mathrm{Rb}$ FWM level scheme.

Figure 2

Evidence of OAM transfer: Example CCD data at maximum blue intensity and minimum Kerr effects for ${\Phi }_0^1$ input modes at 780 (a) and 776 nm (b). Resulting intensity profile (c) and interferogram (d) at 420 nm. Both (c) and (d) are consistent with a blue ${\Phi }_0^2$ mode.

Figure 3

OAM transfer from IR pump light with $l_{\mathrm{NIR}}=0–5$ to blue light at 420 nm beams, with intensity profile (top) and interferograms (bottom) consistent with $l_B=l_{780}+l_{776}$.

Figure 4

Phase transfer of superposition modes: Experimental realization of the pump superposition modes detailed on the left (a) and corresponding observed blue output modes (c). Theoretical simulation of pump modes (b) and anticipated output modes detailed on the right (d).