Figure 1. Laser processing of NP chains in water yields new plasmon mode.

(a) Schematic process. Charge-stabilized gold NPs exhibit single particle plasmons (SP) at 532 nm resonance. Particles glued into chains by CB molecules give CCPs (745 nm resonance). Illumination with femtosecond laser pulses (schematic b) connects chains by metal thread into strings, producing TCPs (1,100 nm resonance). (c) TEM images of NP chain gaps before and after femtosecond irradiation. (d) After adding CB molecules to the NPs, spectra are different with/without femtosecond laser irradiation. Single NP response is subtracted from spectra. As the threads develop, a spectral dip around the laser wavelength and a peak at the rising TCP emerge. (e) Numerical simulations of resonant six-NP-long chains display TCP mode and indicate the range of nanothread widths contributing to the signal in d.