Metamorphic plasmonic nanoantennas for self-enhanced nonlinear light generation

Nanoplasmonic antennas are well established for focusing light from the far field into subwavelength-sized dimensions, while simultaneously increasing strongly the local electromagnetic field-an important ingredient for boosting non-linear optical effects. Here, we study both the optical and structural properties of gold bowtie nanoantennas under illumination with femtosecond laser pulses and observe a pronounced metamorphism of the antennas, while the upconverted incoherent nonlinear emission increases simultaneously. Scanning electron microscopy images recorded before and after illumination show a clear modification of the antenna feedgap, accompanied by an 100 x increase of the nonlinear signal. This is caused by laser-induced electromigration of gold nanoparticles, a process that is feedgap-size-dependent, self-limiting, and irreversible. Moreover, it is identified as the root cause for the strong enhancement of the nonlinear conversion efficiency by a factor of >= 4 x 10(6) as compared with an unpatterned gold film. By experimentally quantifying the electric field enhancement inside the nanoantenna feedgap to be > 2000 x, we demonstrate consistency with the observed enhancement of the nonlinear signal. Complete switching off of the nonlinear response of such metamorphic nanoantennas with a degree of polarization > 99% indicates their potential for novel, nonlinear all-optical devices. Furthermore, we envision the controlled, laser-induced modification of plasmonic nanoantennasmay provide a promising route to realize antennas with even higher field enhancements and, moreover, might enable deterministic quantum plasmonic experiments that require sub-nanometer-sized feedgaps. (C) 2016 Optical Society of America