Nonlinear transmission using highly nonlinear Bragg mirrors

Nonlinear transmission upon the formation of an optically induced photonic band gap (PBG) is demonstrated by using periodic layers of optical polymers doped with highly nonlinear transition metal oxides. The refractive indices of the alternating layers are designed to be close and no PBG is formed at low power densities. Under high power illumination, the index difference becomes large because of the high optical nonlinearities of the transition metal oxides. Consequently, nonlinear transmission is accomplished with the formation and the broadening of the PBG. Compared to typical optical limiters based on a PBG approach, our devices provide a large dynamic range and a broad operation wavelength range. The experiments on a nonlinear Bragg mirror consisting of only 4 pairs of PVA:Co₃O₄-PVK, each with a layer thickness of 85 nm, show a linear transmittance of greater than 50% throughout the visible, and nonlinear transmission for a 10 ns laser pulse at 523 nm with a threshold of 30 mJ/cm² and a minimum transmission of about 10%. The minimum transmission reduces to 5% for a 12-pair device. Improving the uniformity of each layer and adding more pairs can result in even lower transmission at high intensities. The threshold can be further reduced through precise design and control of the thickness of each layer. The device and material approach is promising for applications such as protection for broadband detectors and human eyes.