Origin of optical nonlinearity in plasmonic semiconductor nanostructures

TL;DR

This paper demonstrates that nonlinear optical responses in heavily doped semiconductors are primarily due to free-electron effects, which can surpass dielectric nonlinearities, enabling advanced plasmonic functionalities in integrated photonics.

Contribution

It provides experimental and computational evidence that free-electron nonlinearities in doped semiconductors dominate and can be tuned for enhanced nonlinear optical applications.

Findings

Free-electron nonlinearities can exceed dielectric nonlinearities by orders of magnitude.

Doping level controls the spectral location and magnitude of nonlinear efficiency.

Hydrodynamic modeling captures nonlocal effects in free-electron dynamics.

Abstract

The development of nanoscale nonlinear elements in photonic integrated circuits is hindered by the physical limits to the nonlinear optical response of dielectrics, which requires that the interacting waves propagate in transparent volumes for distances much longer than their wavelength. Here we present experimental evidence that optical nonlinearities in doped semiconductors are due to free-electron and their efficiency could exceed by several orders of magnitude that of conventional dielectric nonlinearities. Our experimental findings are supported by comprehensive computational results based on the hydrodynamic modeling, which naturally includes nonlocal effects, of the free-electron dynamics in heavily doped semiconductors. By studying third-harmonic generation from plasmonic nanoantenna arrays made out of heavily n-doped InGaAs with increasing levels of free-carrier density, we discriminate between hydrodynamic and dielectric nonlinearities. As a result, the value of maximum nonlinear efficiency as well as its spectral location can now be controlled by tuning the doping level. Having employed the common material platform InGaAs/InP that supports integrated waveguides, our findings pave the way for future exploitation of plasmonic nonlinearities in all-semiconductor photonic integrated circuits.