# Microchannel-Embedded D-Shaped Photonic Crystal Fiber-Based Highly Sensitive Plasmonic Biosensor
## 1 Introduction
For example, the human kidney has an RI of 1.41, and the bovine muscle has an RI of 1.41. In recent years, intensive efforts have been devoted towards designing D-shaped PCF sensors because they enhance the sensing performance as the energy transfer increases from core to plasmonic mode due to a closer distance between the plasmonic film and core of the PCF [19]. To fabricate the D-shaped PCF sensors, a portion of PCF is polished by the side-polishing process, and the plasmonic film is placed over the flat surface [19]. For example, a flat surface-based D-type PCF sensor with a hollow core was proposed in [20] and using spatially distributed bimetallic layers was proposed in [21]. However, a deep polishing depth increases the fragility of the PCF, and the flat surface of D-type PCF needs to be smooth, otherwise a nonuniform sensing surface arises that affects sensing performance. The sensing performance increases significantly by introducing a microchannel in the PCF sensor because of the reduction in separation of the sensing layer and core region. For example, a microchannel-based D-shaped sensor showed maximum sensitivity of 27,800 nm/RIU for RI range of 1.37 to 1.41 [22]. A hexagonal lattice D-shaped PCF sensor containing an open ring channel was proposed in [12] that can detect RI from 1.20 to 1.29 with a low wavelength sensitivity of 11,055 nm/RIU. This sensor requires extra care to make a D-shape, otherwise at the stage of the side-polishing process of the PCF side, air holes of the open ring may be damaged. A PCF sensor with two microchannels was designed to detect RI from 1.23 to 1.29 with a maximum sensitivity of 13,000 nm/RIU [23]. The problem with the sensor is that double sides of the PCF need side polishing to obtain a D-shape with microchannels on both sides of the PCF. Moreover, the sensor has low sensitivity and a low detection range. Microchannels not only improve the performance of the D-shaped sensors but also boost the sensing performance of the circular-shaped PCF sensors. For example, a U-shaped microchannel in a circular-shaped PCF next to the core showed maximum sensitivity of 66,000 nm/RIU and a wider sensing range [13]. However, the problem of the microchannel-embedded PCFs is the difficulty of the fabrication of the channel by using a focused ion beam or laser micromachining process. The sensing layer can also be placed close to the core in the case of exposed core graprefruit fiber-based sensors to increase performance [24; 25; 26]. Additionally, in suspended core microstructured-fiber, the sensing layer is close to the core that can detect multi analyte [27]. Besides SPR-based RI sensors, other new RI sensors such as long-range SPP and Bloch surface wave based sensors are proposed to detect RI effectively [28; 29]. The aim of this work is to detect a wide range of analyte RI with very high sensitivity by incorporating the benefits of a D-shaped PCF sensor and avoiding its surface roughness effect. Therefore, an improved design of a D-shaped PCF-based SPR sensor is proposed in this work with a large semicircular open channel. The construction of the novel D-shaped PCF sensor is easier because of its hexagonal lattice structure. The channel open-up process can be completed along with the fabrication of D-shaped PCF by the side-polishing method. To avoid the fragility of the PCF, we use a shallow polishing depth. Plasmonic material gold is deposited on the interior surface of the large semicircular channel. Since the plasmonic layer is deposited inside the curved channel, we can ignore the surface roughness effect of the D-shaped PCF on the sensing performance. Due to the poor adhesion of gold with the background silica, a thin adhesive coating of TiO\({}_{2}\) is placed over the curved surface of the channel. Numerical analysis shows that the PCF-based sensing device has a wider detection range of analytes with wavelength sensitivity of 63,000 nm/RIU and maximum amplitude sensitivity of 1439 RIU\({}^{-1}\).