NIR window and near-field detection of gold nanoshells

The optical properties of metal nanoparticles play a fundamental role for their use in a wide range of applications. In hyperthermia treatment, for example, the nanoshells (core+metal shell) pre-embedded in a tumor cell absorb energy when exposed to the appropriate wavelengths of a laser beam and heat up thus destroying the tumor cell; nevertheless, the healthy tissues along the laser path are not affected. This is because most biological soft tissues have a relatively low light absorption coefficient in the NIR regions, characteristic known as the tissue optical window. Over such window, NIR light transmits through the tissues with the scattering-limited attenuation and minimal heating preventing the healthy tissues. In this paper, we place emphasis on the NIR response of ultrasmall metal nanoparticles and nanoshell clusters. The necessity to focus the attention on such ultra-small aggregates is based on the availability of experimental SNOM data recorded in the recent past. The optical responses of such aggregates have been analytically deduced by the reformulation of the Mie theory in evanescent wave regime. In addition, FDTD simulations were addressed to validate the numerical results. In turn, the experimental results obtained with an aperture SNOM were also enlightened.