Time-reversed nonlinear optical wave mixing

Abstract

Time-reversal symmetry is ubiquitous in physics; it allows a physical process to reverse in a backward direction of time. The study of time symmetry in optics has been attracting increasing attention, since they provide alternative and substantial ways to manipulate light. Recently, it has been theoretically and experimentally demonstrated that a time-reversed process of laser emission, or coherent perfect absorber (CPA), can enable total absorption of coherent light fields inside an optical cavity of loss by time-reversing the original gain medium. Recent efforts have studied CPA properties with different geometries, though most in the linear optics regime. Nonlinearity, however, can often destroy such symmetry in nonlinear optics, making it difficult to study time-reversal symmetry with nonlinear optical wave mixings. Here we experimentally demonstrate time-reversed wave mixings for optical second harmonic generation (SHG) and optical parametric amplification (OPA) by exploring this well-known but underappreciated symmetry, characterize their nonlinear properties as opposite to their time-reversal counterparts, and reveal the nontrivial dynamics of phase varying in time-reversed nonlinear wave-mixing schemes. These backward-nonlinear wave mixings lead to the unique property of annihilation of coherent beams in a nonlinear quadratic medium by time reversal. Unlike the case of CPA in the linear regime where incident fields are totally absorbed and converted into heat, annihilation of incident fields can lead to the generation of new fields. This allows us to observe the annihilation of coherent beams. More interestingly, a flexible phase control can be utilized to probe nonlinear dynamics during wave mixing, and redirect wave mixing forward or backward in time. Our study offers new avenues for flexible control in nonlinear optics and has potential applications in efficient wavelength conversion, all-optical computing, etc‥