Plasmonic Coupling Effect on Spectral Response of Silver Nanoparticles Immobilized on an Optical Fiber Sensor

The localized surface plasmon resonance (LSPR) of silver nanoparticles during immobilization on the U-bent sensor fiber probe has been studied. The gradual wavelength shift of the peak resonance due to the increment of the number of nanoparticles on the fiber surface has been attributed to the plasmonic coupling effect. Since the immobilization is random, the shift of the resonance as well as the broadening of the absorption spectrum depends on the distribution of the nanoparticles. To get the insight of the cause of this spectral behavior, we have incorporated the two-particle coupling approximation and assumed that the particles form pairs with its nearest neighbors. The distribution of pairs was obtained from the microdimensional images of fiber surfaces. The collective response from the contributions of these pairs was calculated using the theoretical framework of discrete dipole approximation (DDA). The shifts of the resonance as well as the broadening of the calculated spectra were found to be in excellent agreement with those of the experimentally observed spectra. Thus, this methodology can be utilized for tuning and enhancing the sensitivity by exploiting the plasmonic coupling between the noble metal nanoparticles for designing and fabricating the LSPR based optical sensor.