End-to-End Self-Assembly of Gold Nanorods in Water Solution for Absorption Enhancement at a 1-to-2 μm Band for a Broadband Saturable Absorber

The redshift of the long wavelength plasmonic absorption band is essential to the use of gold nanorods (GNRs) for the implementation of the infrared broadband saturable absorbers (SAs). A common method to obtain the GNR-based SAs with a substantially broad absorption bandwidth from 1 to 2 μm is to use GNRs with a large aspect ratio. Here, a simple self-assembly technique is demonstrated, which enables us to obtain a significant redshift of the long wavelength plasmonic absorption peak of GNRs with a small aspect ratio, together with substantial spectral broadening of the absorption band. The proposed technique is based on an end-to-end self-assembly of the GNRs through hot air induced rapid drying of the GNRs in a deionized water solution, which makes the GNRs reassemble by attaching to one another by their ends. Using the proposed end-to-end self-assembly method for GNRs with an aspect ratio of 5.1, a single broadband SA that can operate from 1.5 to 1.9 μm was fabricated on a fiber ferrule based sandwich platform. The saturable absorption performance was tested by incorporating the fabricated GNR-based SA into two different optical fiber based ring cavities: One was based on an erbium-doped fiber and the other was a thulium-holmium codoped fiber. Our fabricated single SA readily produced stable Q-switched pulses from both the cavities.