Nonlinear Optics

We demonstrate anti-parity-time (anti-PT) symmetry inherently in the nonlinear optical interaction based upon forward four-wave mixing in a laser-cooled atomic ensemble with negligible linear gain and loss. We observe that the pair of frequency modes undergo a nontrivial anti-PT phase transition between coherent power oscillation and optical parametric amplification in presence of a large phase mismatch. As no appreciable linear gain or loss involved, a scheme of this sort can be readily extended to the quantum optical domain that is challenging to standard gain-loss PT systems. We also found that in this phase-unbroken regime, anti-PT symmetry automatically compensates large phase-mismatching and hence provides a new way for efficient nonlinear conversion by the relaxation of desired phase matching. Besides the span of research in PT and anti-PT, our framework theory also provides an interesting and alternative interpretation to parametric nonlinear processes. All these attributes are believed to deserve further investigations and explorations on counter-intuitive optical phenomena as well as a new generation of (anti-)PT-enabled optical devices for quantum information applications.

Based on the EIT-enhanced backward four-wave mixing process in cold atoms, we demonstrate narrow-band Mirrorless optical parametric oscillation (MLOPO) with a tunable threshold. The pump threshold can be tuned by varying the operating parameters: the pump detuning, the coupling laser power, and the atomic density. We achieved a pump threshold as low as 15 μW, which approaches the lowest record of OPO. Our theoretical analysis reveals that with larger OD and smaller dephasing rate, the pump threshold can be further suppressed. Compared with other schemes, our method greatly reduces system complexity without degrading the performance. We also study the normalized two-photon correlation between Stokes and anti-Stokes photons and confirm its transition from non-classical biphoton regime to the classical coherent state regime.

We construct a nontraditional quantum heat engine using electromagnetically induced transparency effect. By making use of quantum interference, we are able to break the symmetry of photon absorptive and emissive cross-section, thus enabling the system to absorb photons from a “thermal” reservoir, with wide spectrum and random wave vector, and convert them into “work”, which has a certain direction of radiation and with an ultra-narrow spectrum. There are some unique properties of this quantum heat engine. One is that the emissivity at central frequency is 9 times greater than that of a blackbody at the same ambient temperature, indicating the violation of Kirchhoff’s law. In addition, the temperature of cold reservoirs can be even higher than hot reservoirs, which is not allowed in classical heat engines.