We report an experimental study of slow light diffraction from a resonant nonlinear grating in cold Rb atoms. Alternatively, the diffracted light can be viewed as generated from an EIT enhanced six-wave mixing process. Electromagnetically induced transparency (EIT) may be used to obtain vanishing linear absorption and large nonlinear susceptibilities as well as slow light speed in an absorbing medium, which renders it possible to study nonlinear optics near the resonant frequencies of the medium at low light levels. We report an experimental study of six-wave mixing resonantly enhanced by EIT at low light intensities. The experiment is done on a four-level EIT system that has been shown to exhibit vanishing linear absorption and greatly enhanced optical nonlinearities along with reduced group velocities. The four-level HIT system is formed in the 87Rb D1 transition lines. A coupling laser driving the transition D1 F=2→F′=2 and a probe laser driving the transition D1 F=l→F′=2 create the standard A-type EIT. A standing-wave pump laser drives the transition D 1 F=2→F′=1 and facilitates a six-wave mixing process that results in the emission of photons at the probe frequency ωp and propagating in the opposite direction of the probe photons. The light generation from the six-wave mixing process can be viewed as from diffraction of slow probe photons from a nonlinear grating induced by the standing-wave pump field. When both the coupling laser detuning and the probe laser detuning are zero, the linear absorption and the linear dispersion are suppressed by EIT. The sanding-wave pump field induces a spatial modulation of the nonlinear susceptibilities and creates a pure nonlinear grating resonantly enhanced by EIT. Our experiment is directly related to a recently proposed Bragg scattering scheme in the four-level system, in which a photonic band gap is formed and the near perfect Bragg scattering is predicted for a suitably dense EIT medium [1]. The dynamic controlled Bragg scattering is achieved with an off-resonant standing-wave field that creates an EIT enhanced nonlinear index grating. While in our experiment, a resonant standing-wave field is used to create a mixed nonlinear index and population grating and facilitates the six-wave mixing process. Our experiment is done in a sequential mode at a repetition rate of 5 Hz with cold 87Rb atoms confined in a magneto-optical trap (MOT). All lasers are turned on and off by Acousto-Optic Modulators (AOM). For each period of 200 ms, ∼198 ms is used for cooling and trapping of the Rb atoms, during which the trap laser and the repump laser are turned on by two separate AOMs while the coupling laser, the probe laser, and the standing-wave pump laser are off. The time for the measurement of the six-wave mixing lasts ∼ 2 ms, during which the trap laser and the repump laser are turned off, and the coupling laser and the pump laser are turned on. 100 μs after the coupling laser and the pump laser are turned on, a probe laser pulse of 0.6 μs duration is turned by a separate AOM. The generated signal pulse and the transmitted probe pulse are then recorded versus the time. We measured that the probe pulse propagating through the EIT medium (without the standing-wave pump laser) is delayed by 25±8 ns relative to the reference probe pulse. The probe pulse propagating through the EIT medium modified by the standing-wave pump laser is delayed by 20±10 ns. The generated signal pulse matches the temporal shape of the input probe pulse. Under the low light intensity conditions (the Rabi frequencies of the three laser fields are ∼10 MHz (coupling), ∼0.4 MHz (probe), and ∼3 MHz (pump), respectively), the measured efficiency of the six-wave-mixing emission is about 3%.
