Metallic nanoparticles exhibit fascinating optical properties due to the surface plasmons – the collective oscillation of electron cloud within the particle. Surface plasmon can be at a resonant condition with the driving electromagnetic wave, and its resonant peak is shown to be tunable at various wavelengths of light with the shape and size of nanoparticles. For example, gold nanorods exhibit a longitudinal surface plasmon resonance (SPR) peak that can be tuned from 540 nm to around 1.5 micron or more depending on the length of the rod.

Femtosecond pulsed laser irradiation on those nanorods with a wavelength at the SPR peak position can induce the shape change of the rods, which results in a significant reduction in SPR peak. Such a shape change could only occur on sub-population of rods with a narrow aspect ratio range that satisfies the SPR condition, hence the spectral encoding on a rod mixture with varying aspect ratios could be possible. Such technology would have many possible applications such as security labeling and optical data storage. In this paper, we demonstrate the spectral encoding technique of gold nanorods and its potential application to the future high-density optical data storage.

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