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 nonlinear 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 × increase of the nonlinear signal. This is caused by laser-induced electromigration of gold nanoparticles, a process that is feedgap-sizedependent, 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 × 10⁶ as compared with an unpatterned gold film. By experimentally quantifying the electric field enhancement inside the nanoantenna feedgap to be >2000 ×, 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.