Transient Electronic Depletion and Lattice Expansion Induced Ultrafast Bandedge Plasmons

Strong optical excitation of plasmonic nanostructures may induce simultaneous interband and intraband electronic transitions. However, interaction mechanisms between interband, intraband, and plasmon-band processes have not been thoroughly understood. In particular, optical-heating-induced lattice expansion, which definitely leads to shift of the Fermi level, has not been taken into account in plasmonic studies. Here, it is shown that plasmonic bandedge shift is responsible for the optical modulation on the boundary between plasmonic electron oscillation and interband transitions via investigations on gold nanofilms and nanoparticles. Strong optical excitation induces transient depletion of the conduction band just below the Fermi level through intraband transitions, while the subsequent lattice heating induces transient thermal expansion and hence lowers the Fermi level. Both effects reduce the threshold for interband transitions and therefore push the plasmonic bandedge to the red. These discoveries introduce a first correlation between plasmonic response and optical excitation induced thermal expansion of lattices. The revealed Fermi-level adjustment mechanism allows alignment of electronic levels at the metal-semiconductor interfaces, which applies to all conductive materials and renders reliable physics for the design of plasmonic or optoelectronic devices.