A New Refractive Index Sensor Based on Enhanced Surface Field of Zero-Admittance Layer in Dielectric Multi-Layers

Optical field enhancement plays a key role in applications such as chemical and biological detection, imaging or alternatively exchange of information and communication. Historically, thanks to plasmonic resonances, metals have dominated applications related to field enhancement and confinement for sensing and imaging [1]-[4]. Dielectrics, on the other hand, found a prominent role mainly in communication and waveguiding applications because of the associated low optical losses [5], [6]. With the growing demands for chemical and biological sensing at higher sensitivity and selectivity, recent designs such as hybrid systems with the Tamm plasmons [7], [8] or resonant all-dielectric multilayers [9]-[11] are gaining prominence. Such systems can be synthesized and optimized with the aid of the admittance formalism, commonly practiced in optical thin film design, leading to huge optical fields when working under total internal reflection [12]-[16]. Following our recent synthesis method, wherein a zero-admittance layer is deposited on a Bragg mirror [12], [17], we will explore the resulting field enhancement and confinement parameters, and therefore fully characterize the associated large ultra-sharp optical resonances. The strength and sharpness of these resonances are assets for low detection applications but require a well-controlled incident illumination and a highly accurate control over the geometric parameters of the multilayers. After evaluating such boundaries [18], [19], we will fabricate and operate with optimized resonant dielectric multilayers in sensing configurations to evaluate both the achievable sensitivity and limit of detection. We will present our first experimental results involving different solutions with concentrations as low as 0.03%, corresponding to a refractive index variation of 6×10
^-5
[17]. The strong agreement with the theoretical predictions demonstrates the validity and utility of the presented sensing approach.