Fig. 2: Physical mechanism of the engineered Raman-resonant four-wave-mixing (Rr-FWM) process.
a Experimental layout for the apparatus used for studying the physical mechanism. The angles of plate A1 (at length Z1 = 45 mm) and plate B1 (at length Z2 = 95 mm) were each precisely adjusted with an angular resolution of \({0.015}^{\circ }\) in a range of \({35\pm 7.5}^{\circ }\). The high-order Stokes and anti-Stokes generations initiated from the laser radiation, \({\varOmega }_{0}^{{{{{{\rm{T}}}}}}}\), were monitored by beam reflection with plate A2 at Z3 (145 mm). Here, Z1, Z2, and Z3 are interaction lengths in gaseous para-hydrogen. b, c Contour plots of the observed photon number densities at b \({\varOmega }_{-1}^{{{{{{\rm{T}}}}}}}\) and c \({\varOmega }_{+1}^{{{{{{\rm{T}}}}}}}\) as a function of the insertion angles of plates A1 and B1. 1 implies unity quantum conversion efficiency. d–g Numerically calculated photon number densities at d \({\varOmega }_{-1}^{{{{{{\rm{T}}}}}}}\) and e \({\varOmega }_{+1}^{{{{{{\rm{T}}}}}}}\) and corresponding relative phases, f \({\Delta \phi }_{-1}^{{{{{{\rm{T}}}}}}}\) and g \({\Delta \phi }_{+1}^{{{{{{\rm{T}}}}}}}\), at each angular condition of plates A1 and B1. The white cross in d indicates the point that maximizes the photon number density at \({\varOmega }_{-1}^{{{{{{\rm{T}}}}}}}\) in this explored range. h–j Relative phases, \({\Delta \phi }_{q}^{{{{{{\rm{T}}}}}}}\) (q = −1, 0, 1), at the condition marked by the white cross in d, manipulated as a function of the interaction length, Z. k–m Photon number density distributions among the Raman modes, \({\varOmega }_{q}^{{{{{{\rm{T}}}}}}}\) (q = −2 to 3), calculated at the conditions marked by the white cross in d. The arrows and their thicknesses respectively show how the signs and magnitudes of the relative phases, \({\Delta \phi }_{q}^{{{{{{\rm{T}}}}}}}\), are manipulated to enlarge the photon number density, \({\varOmega }_{-1}^{{{{{{\rm{T}}}}}}}\), as a function of the interaction length, Z.
