Epsilon-near-zero mode enhanced higher-order optical Kerr effects in transparent conducting oxides

We experimentally observe for the first time higher-order Kerr optical nonlinearity in epsilon-near-zero (ENZ) transparent conducting oxide thin films when the plasmon-polariton mode near the ENZ regime is excited. Utilizing the z-scan nonlinearity measurement technique, we show that the refractive index exhibits a sign flip upon exiting the ENZ mode at a large angle of incidence. This sign flip stems from the enhanced field intensity within the ENZ film, which results in the 5th-order Kerr nonlinearity making a comparable contribution to the material’s nonlinear refraction. Our findings introduce an additional tuning mechanism for ultrafast optical switching, ultrafast photonics, Kerr lensing, and mode-locking based on ENZ materials.