15 January 2018

Many nonlinear optical applications use resonators to boost local intensities, enhancing the nonlinear effect of interest. Higher resonator finesse generally gives greater enhancement, but makes more challenging the task of maintaining optical resonance which, to date, requires electronic solutions. 

In the study published in Optics Letters, and highlighted as one of the top downloads of the month of December 2017, the ICFO researchers Joanna Zielińska and ICREA Prof. at ICFO Morgan W. Mitchell have demonstrated a nonlinear optical resonator that tunes itself onto resonance with an input beam. 

In their experiment, the team of researchers employed a rubidium-doped periodically poled potassium titanyl phosphate crystal to create a monolithic Fabry–Perot cavity. They then pumped the crystal with an input laser, and at low powers observed the usual Fabry-Perot resonances. At higher laser powers, however, the intensity-dependent refractive index pulled the cavity resonance toward the laser frequency, a self-tuning effect related to optical bistability. In contrast to previous experiments, in this RKTP device, the achievable cavity shift was multiple free-spectral ranges, meaning that if one nonlinear resonance condition became unstable, the system would simply jump to the next stable nonlinear resonance condition. In this way, the team demonstrated stable second harmonic generation over hours with no feedback at all to the cavity or to the laser. 

The results from the study demonstrate stable second-harmonic generation with no active feedback to the laser or cavity and support the possibility of using dispersive optical nonlinearities for all-optical functionalities, including optical memories, optical switches and optical signal processors. Such implementations could help reduce the complexity of chip-level optical interconnects, among others. 

• Link to the paper
• Link to the research group led by ICREA Prof. at ICFO Morgan Mitchell