Nonlinear Optics in Space and Time with Novel Materials

The emerging near-zero-index (NZI) materials have recently opened up new chapters of nonlinear optics by enabling strong light-matter interactions in both nanophotonic and hybrid photonic/plasmonic systems as well as greatly enhanced optical phenomena such as negative refraction, time refraction, and photon acceleration. Building up on our work on the extraordinary modulation of the refractive index in transparent conducting oxides (TCOs), we have demonstrated negative refraction in time-varying, strongly coupled optical media. We show large changes in the epsilon-near-zero (ENZ) points in undoped zinc oxide, with 20-ps response time, and utilize the permittivity modulation to enhance the third-harmonic generation, and develop optically controlled polarization switches. We also used the large nonlinearities in NZI TCO films for ultrafast pulse characterizations. Furthermore, we showed that the femtosecond modulation of TCO permittivity can be achieved by a tightly controlled pulse, opening new directions in nonlinear optics where a temporal modulation of permittivity can result in phenomena such as time refraction, reflection, and open bandgaps in the momentum of a photon pulse. This forms the basis of our investigations in photonic time crystals which can pave the way in gain-free lasing, non-reciprocal optics, and many other exciting fields.