Nanosphere lithography: Tunable localized surface plasmon resonance spectra of silver nanoparticles

The wavelength corresponding to the extinction maximum, lambda (max), of th e localized surface plasmon resonance (LSPR) of silver nanoparticle arrays fabricated by nanosphere lithography (NSL) can be systematically tuned from -300 nm to 6000 nm. Such spectral manipulation was achieved by using (1) p recise lithographic control of nanoparticle size, height, and shape, and (2 ) dielectric encapsulation of the nanoparticles in SiOx. These results demo nstrate an unprecedented level of wavelength agility in nanoparticle optica l response throughout the visible, near-infrared, and mid-infrared regions of the electromagnetic spectrum. It will also be shown that this level of w avelength tunability is accompanied with the preservation of narrow LSPR ba ndwidths (fwhm), Gamma. Additionally, two other surprising LSPR optical pro perties were discovered: (1) the extinction maximum shifts by 2-6 nm per 1 nm variation in nanoparticle width or height, and (2) the LSPR oscillator s trength is equivalent to that of atomic silver in gas or liquid phases. Fur thermore, it will be shown that encapsulation of the nanoparticles in thin films of SiOx causes the LSPR lambda (max) to red shift by 4 nm per nm of S iOx film thickness. The size, shape, and dielectric-dependent nanoparticle optical properties reported here are likely to have significant impact in s everal applications including but not limited to the following: surface-enh anced spectroscopy, single-molecule spectroscopy, near-field optical micros copy, nanoscopic object manipulation, chemical/biological sensing, informat ion processing, data storage, and energy transport in integrated optical de vices.