Facile design of a plasmonic nanolaser

Abstract

A spaser consists of a plasmonic noble-metal nanostructure that acts as nanocavity, when incorporated or surface-coupled two-level emitters constitute the nanoscale gain medium. Suited two-level emitters are, for instance, laser dyes. Optical pumping may provide efficient excitation energy transfer between the two-level emitters in the gain medium and the surface plasmons sustained in the nanocavity. Strong resonant coupling of the surface plasmon modes to the gain medium may establish an inherent feedback amplification mechanism which finally drives the spaser action. In this contribution, we demonstrate that spaser emission can be generated by amplifying longitudinal surface plasmon modes in gold nanorods by optically pumping surface-attached resonantly-coupled laser dyes. Therefore, we synthesized gold nanorods whose longitudinal surface plasmon resonance peak was adjusted between 680 and 700 nm. The gain medium was realized by electrostatically attaching the laser dye phthalocyanine tetrasulfonate via the positively-charged CTAB (cetyltrimethylammonium bromide) bilayer to the gold-nanorod surface. Phthalocyanine tetrasulfonate exhibits fluorescence at 700 nm. Fluorescence quenching experiments unambiguously gave indication of resonant excitation energy transfer. The fluorescence intensity ratio IF0/I F follows the Stern–Volmer relationship, and the Stern–Volmer coefficient was determined as KSV = 1.22 × 106 M−1. The spaser emission was observed in fs transient absorption spectra as an ultrafast decaying narrow emission peak around 716 nm.