The development of surface-plasmon-based sensors using arrays of sub-wavelength holes

Abstract

The transmission of normally incident light through arrays of subwavelength holes (nanoholes) in gold thin films is enhanced at the wavelengths that satisfy the surface plasmon (SP) resonance condition. The enhanced transmission is accompanied by strong field localization and has potential for applications in several fields, ranging from quantum information processing to nanolithography. In this work, arrays of nanoholes were used as chemical sensors to monitor the binding of organic and biological molecules to metallic surfaces. In a first approach, the interaction between the adsorbate with the metallic nanostructure modified the SP resonance conditions, leading to a shift in the wavelength of maximum transmission. The sensitivity of this substrate was found to be 400 nm RIU -1 (refractive index units), which is comparable to other grating-based surface plasmon resonance devices. The array of nanoholes was also integrated into a microfluidic system and the characteristics of the solution flow and detection systems were evaluated. The second approach to sensor development using this class of substrate involved the observation of enhanced spectroscopic signal from species located within the SP field. Surface-enhanced Raman scattering and surface- enhanced fluorescence spectroscopy were observed from adsorbed dyes. The enhanced spectroscopic signal was dependent on the fabrication parameters of the array. The largest enhancement was observed when the periodicity of the nanoholes matched the energy of the laser excitation. Among the main advantages of this substrate for chemical sensing is the collinear optical geometry. This simplifies the alignment with respect to the traditional reflection arrangement used in SPR sensing.