We present a physical setup for realizing all-real-spectrum optical potentials with arbitrary gain-and-loss distributions in a coherent medium consisting of a cold three-level atomic gas driven by control and probe laser fields. We show that by the interference of Raman resonances and the Stark shift induced by a far-detuned laser field, tunable, non-parity-time (non-PT )-symmetric optical potentials with all-real spectra proposed recently by Nixon and Yang [Phys. Rev. A 93, 031802(R) (2016)] can be actualized physically. We also show that when the real parts of the non-PT -symmetric optical potentials are tuned cross certain thresholds, phase transitions—where the eigenspectrum of the system changes from all real to complex—may occur and hence the stability of the probe-field propagation is altered. Our scheme can also be extended to high dimensions and to a nonlinear propagation regime, where stable optical solitons with power of the order of nano-Watts may be generated in the system.
FIG. 1. Energy-level diagram and Raman excitation scheme of the mixed two species of 87Rb and 85Rb atoms with -type configuration of three levels (|g,s, |a,s, |e,s; s = 1,2) interacting with the probe field Ep, the control field Ec, and the Stark field ES .s (s = 1,2) and δ1 are one- and two-photon detunings, respectively.The black points indicate the energy levels initially populated. SLM1and SLM2 are spatial light modulators used to produce the spatial distributions of Ec and ES for acquiring non-PT -symmetric optical refractive indexes of the probe field.
