Comment on ‘Sensitivity analysis of a photonic crystal waveguide for refraction index sensing’

Bougriou et al demonstrated a photonic crystal waveguide (PCW) design to enhance the sensitivity of refractive index sensing [1]. In their paper they declared that a very high sensitivity of more than 818 nm per refractive index unit can be achieved by utilizing the improved PCW structure (as shown in figure 6 in [1]). This improved PCW has a sensing area where each hole has a 1.5 times larger radius than before. The purpose of this comment is to show that Bougriou uses incorrect arguments to achieve their incorrect conclusions. First, the authors states that the analysis they performed concentrated on the transverse-electric (TE) polarization (the electric field component parallel to the air hole axis). However, in our calculations (the software we used is the same Rsoft software as the authors used) we find that they employ the TE mode (the magnetic field component parallel to the air hole axis [2]) in their research, or they cannot get figure 2 in [1]. Second, the authors obtain the conclusion that a 270 nm wavelength position of the upper band edge shift was observed, corresponding to a high sensitivity of 818 nm RIU from figure 7 in [1]. However, in our calculations, the data in figure 7 of [1] are incorrect. The solid curve in figure 7 in [1], which cannot be repeated by us, is the same as the solid curve in figure 2 in [1]. We conjecture that the authors of [1] used incorrect data in figure 7. In fact, we also calculate the transmission spectra of the improved PCW for different refractive indexes (n = 1.0, n = 1.2, n = 1.33) of the sensing area. The results are shown in figure 1. From figure 1, one can clearly see that even for n = 1.0 the transmission spectra (the blue curve) still has two band gaps, which is quite different from the solid curve in figure 7 in [1]. In the solid curve in figure 7 in [1], there is only one band gap at 1.7 μm, which is same as the solid curve in figure 2 in [1]. However, in our results there is an additional sharp valley at 1.38 μm in the blue curve. This valley comes from localized modes which are sensitive to the sensing area. The transmission spectra for n = 1.33 (the red curve) is very similar to the dashed curve in figure 7 in [1]. The reason for the difference between these two curves is because we use different parameters of the light source and power monitor, which in principle does not change the positions of the band gaps in the transmission spectra. With the refractive index of the sensing area increasing, the whole curve shifts to larger wavelength. Moreover, when the refractive index increases from 1.0 to 1.33, the position of this valley shifts from 1.38 μm to 1.42 μm. So the shift is S1 = 40 nm as shown in figure 1. This shift is far below the value of 270 nm in [1]. If we focus on the shift of the upper band edge for the second band gap (the position is at 1.74 μm for the blue curve), we only obtain a shift of S2 (as shown in figure 1) that is no more than 80 nm. So the conclusion in [1] is surely wrong, and the sensitivity of the improved PCW sensor cannot reach such a high value of 818 nm RIU. | Royal Swedish Academy of Sciences Physica Scripta