Wide detection range and sensitivity enhanced dual eccentric-core D-shaped photonic crystal fiber surface plasmon resonance sensor

Surface plasmon resonance (SPR) phenomenon occurs when the wave vectors of incident light and surface plasmon polariton (SPP) mode are equal, the energy of incident light is coupled into SPP mode and results in a loss peak of energy at the resonance wavelength[1]. In the past twenty years, SPR sensor has become a research hotspot because of its merits of real-time monitoring, anti-electromagnetic interference and label-free detection[2-3]. However, the traditional fiber based SPR sensor has drawbacks such as low sensitivity, small detection range, inconvenience in manufacturing and difficulty adapting to different applications due to the limitations of traditional fiber[4-5]. The emergence of photonic crystal fiber (PCF) in the 1990s[6] provided more possibilities for fiber based SPR sensor. PCF-SPR sensor is more flexible in structural design and with the advanced manufacturing processes, making it easier to achieve high sensing performance than conventional SPR sensor[7]. Additionally, due to its characters such as small size and variable structure etc, PCF-SPR sensor shows its great potentialities in various fields[8-9].

In recent years, various structures of PCF-SPR sensor have been proposed. Normally, the sensing methods of PCF-SPR sensor can be divided into internal and external sensing[10]. The plasmonic metal layer and the analyte channel are set inside and outside of the fiber for internal and external sensing respectively. But internal sensing PCF-SPR sensors are often difficult to fabricate as they have to coat the metal film on the inner surface of the air holes and during the measurement, it is also necessary to change the liquid analyte in the air holes, which increases the difficulty of use[11]. The external sensing PCF-SPR sensor has more advantages in practical applications, the D-shaped PCF-SPR sensor is the general external sensing PCF-SPR sensor and it has gained much attention from scholars for its outstanding performance in the sensing field[12-13]. In 2021, De et al.[14] proposed a high birefringent D-shaped PCF-SPR sensor. The sensor uses a single silica core as the optical channel, uses the gold and titanium dioxide film as the plasma layer, the sensor achieves the wavelength sensitivity of 9,245 nm/RIU and the RI detection range of 1.29-1.36. The single core PCF-SPR sensor has only one fibre core for transmitting the core mode, therefore partly restricting the performance of sensor[15]. Compared to single core PCF, multi-core PCF including dual-core PCF can enhance the coupling of energy between core mode and SPP mode to obtain better loss conditions[16]. Many PCF-SPR sensors based on dual-core structure have been designed to improve the performance of the PCF-SPR sensors these years. In 2021, Sakib et al.[17] reported a D-shaped PCF-SPR sensor with the dual-core structure and a gold film on the D-shaped surface. The sensor works in even mode and reaches the wavelength sensitivity of 8,000 nm/RIU, the detection range of 1.45-1.48 and the figure-of-merit (FOM) of 138 RIU-1. In 2020, Momtaj et al.[18] proposed a D-shaped dual-core PCF-SPR sensor works in even mode for x and y polarization. The sensor coats a gold film in an open air hole on D-shaped surface to excite the SPR. The sensor has high sensitivity of 16,000 nm/RIU and 17,000 nm/RIU for x and y polarization respectively, with the maximum FOM of 320 RIU-1 and the detection range of 1.33-1.44. Nevertheless, dual-core D-shaped SPR sensors still have considerable potential for improvement in sensitivity, detection range and FOM.

In this paper a D-shaped dual eccentric-core PCF-SPR sensor is proposed. We used the eccentric core structure based on the dual-core PCF to further enhance the sensing performance. We present a detailed data analysis of the optimization process of the structural parameters and the performance indicators of the sensor. The proposed sensor works in odd mode and even mode, which achieves the sensitivity of 44,000 nm/RIU and 39,000 nm/RIU for odd mode and even mode respectively. The sensor reaches the detection range of 1.00-1.37, the optimal FOM of 325 RIU-1 and the maximum refractive index (RI) resolution of 2.72× 10-6 RIU. The proposed sensor has wide detection range, high sensitivity and can meet the application requirements of miniaturization and low cost, it is potential in industrial production, gas and liquid detection and other fields.