Researchers at MIT Lincoln Laboratory and the MIT Department of Material Science and Engineering (DMSE) are exploring new strategies for protecting eyesight and sensitive equipment from short intense laser pulses utilizing unusual nanoshapes in transparent matrices. Complex spined gold nanoparticles, dubbed "nanourchins", have been synthesized in Prof. Stellacci's laboratory (DMSE). Because of their spiked nanoantenna shape, these nanoparticles exhibit strong plasmonic extinction resonances. Irradiating an aqueous solution of these nanourchins with nanosecond laser pulses revealed a greatly enhanced optical limiting effect, that is, a substantial decrease in transmission with increased laser intensity, over similar-sized spherically shaped gold particles.

While the optical limiting phenomenon is not new, the unique feature of this work is connecting the nonlinear transmission of the plasmonic suspension with the electric field enhancement inside the plasmonic nanostructure. Both plasmonic material and its shape play a role in this connection. The surface E-field plasmonic enhancement is a) stronger in a silver nanosphere than in a gold nanosphere, and b) stronger in a gold nanourchin shape than in a gold nanosphere shape. Experimentally, these are just the trends that have been observed in nonlinear transmission coefficient measurements for suspensions of these nanostructures (see top image). Researchers postulate that the increased electric fields just below the surface of the metal nanostructures boost nonlinear absorption inside the material and, hence, the optical limiting effect. Based on rigorous electromagnetic calculations performed using a Finite Difference Time Domain method, spined nanostructures show great electric field enhancement at the surface as a function of spine length. Spatially, the fields are localized along the tips. While the performance of currently available optical limiters is still not adequate, this work shows a path to the rational design of plasmonic nanoshapes towards improved performance.

Full details can be found in Journal of Optics.