Ultrasensitive Optical Label-Free Biosensing and Fluorescent Measurements using UV Surface Plasmons in Gold Nanolayers Launched with the Support of a Specially Designed Photonic Crystal

Abstract

Recently, we used Photonic Crystal (PC) - supported surface plasmons (SP) propagating along thin Pd, Co (in red spectral range) and Au (in blue, 405 nm) nanolayers for ultrasensitive gas sensing (Pd, Au) and magnetoplasmonic researches (Co). In all these cases, without the exploitation of a specially designed PC, the propagation length of the plasmons is just of the order of the wavelength hence there would be no reason to speak about SP at all. Here we experimentally show that the same concept is applicable even in UV spectral range at 375 nm despite the extremely non-favorable conditions of SP excitation at bulk gold - dielectric interface: the real part of the dielectric permittivity of gold at this wavelength is minus 1.5, and its imaginary part 5.6. Nevertheless, the use of a specially designed PC, containing seven pairs of transparent Ta2O5/SiO2 layers plus an additional Ta2O5 and 8 nm-thick gold layer, enabled us to attain the angular width of the surface plasmon excitation around 0.250 and thus the UV SP propagation length of tens of microns. This same structure can be used in quite usual fashion as an ultrasensitive optical label-free biosensor. Besides, the short UV wavelength exploited enables also to use it for excitation of fluorescence for very broad class of studied compounds. The perspectives of the application of this approach to deeper UV, at 280 nm, will be discussed. Such structures will enable direct label-free excitation and measurement of fluorescence from many proteins and DNA/RNA molecules/complexes.