Design and fabrication of nano-tapered whispering gallery spiral waveguide for gold nanoparticle detection

The advancement of fabrication techniques, especially in the field of polymer-based photonic devices, has led to the implementation of novel designs with increased flexibility. In this paper, we propose a polymer-based whispering gallery spiral waveguide, which has been tapered down to nano-dimension for the detection of gold nanoparticles. Gold nanoparticles are being explored extensively in the areas of catalysis, imaging, therapeutic agents, drug delivery in various chemical and medical applications. Though, detection of gold nanoparticles in low concentration with high sensitivity is still a challenge. The proposed structure can be used to detect a single gold nanoparticle. It consists of two interleaved spirals; the inner ends of the spirals are connected by semicircular-arcs of a circle forming an S-shaped structure whereas the outer ends of the spirals serve as input and output ports of the waveguide. By nano-tapering, the center of the spiral, we observe enhanced evanescent field in the tapered region. The interaction of the evanescent field with the gold nanoparticle causes a shift in the characteristic wavelength of the device. The capability of the device to detect gold nanoparticle of different radius (10 nm-100 nm) has been theoretically analyzed. As the shift in the characteristic wavelength depends on the size of the gold nanoparticle, it provides a size selective detection of nanoparticles and this is regarded as an extrinsic size effect. The proposed device has been fabricated by two-photon polymerization based direct laser writing technique.