Researchers at the Georgia Institute of Technology (Georgia Tech; Atlanta, GA) have created a nonlinear optical metamaterial that has opposite refractive indices at the fundamental and harmonic frequencies of light. Such a material had been predicted for nearly a decade.1 Observation of backward phase matching, creating what is known as a nonlinear mirror, provided proof that this new type of metamaterial had been created.
Experimentally, the researchers struggled to create a negative-index material with a sample size large enough for testing. They had to tailor the refractive indices at both the fundamental and harmonic frequencies simultaneously.
Silver and dielectric
The team exploited two distinct modes in a nonlinear plasmonic waveguide, which was built with a thin dielectric spacer of relatively high refractive index sandwiched between two flat silver films. A large dielectric constant in the gap enabled a pronounced separation of the surface-plasmon and the bulk-plasmon frequencies, while a narrow gap pushed the operating point away from the surface-plasmon frequency and helped balance the magnitudes of the refractive indices of the two modes.
The waveguide operated with the real part of the modes' refractive indices at 3.4 and -3.4 for the fundamental and harmonic waves, respectively. The observed peak conversion efficiency at the excitation wavelength of about 780 nm indicated fulfillment of the phase matching condition in which the coherent harmonic wave emerged along a direction opposite to that of the incoming fundamental light.
Though by itself the discovery may have few immediate practical applications, realization of a material that had been predicted by theorists could lead to new areas of study and prompt a re-evaluation of the fundamental rules governing nonlinear optics.
"Our effort substantially expands the scope of nonlinear light-matter interactions in artificially structured media with engineered, unconventional linear and high-order material parameters," says Wenshan Cai, the team's leader. "The linear responses of metamaterials have substantially augmented the linear properties available from naturally-occurring materials. In the same way, the studies of nonlinear metamaterials may have a revolutionary impact on the field of nonlinear optics. The unconventional electromagnetic parameters made possible by metamaterials will provoke us to rethink and re-evaluate many of the established rules of nonlinear optics."
1. Shoufeng Lan et al., Nature Materials (2015); http://dx.doi.org/10.1038/nmat4324