Browsing by Author "Ali, Farhan"

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Open Access
Investigation of dual-narrowband plasmonic perfect absorbers at visible frequencies for biosensing
(2019-12) Ali, Farhan
Since the introduction of first plasmonic perfect absorber (PA) in early 2008 by Landy et al., numerous studies have demonstrated their superior optical performance in frequencies ranging from terahertz to visible region of electromagnetic spectrum. In the literature broadband PAs are studied in more detail compared to narrowband PAs as their large absorption bandwidths make them a prime candidate for energy harvesting applications or security and defense. Recently scientists have shown a great interest in designing narrowband PAs by controlling the optical losses of the plasmonic materials as the narrowband resonances with a high quality-factor is particularly important for label-free biosensing. However, given the lossy optical properties of metals, this task has been challenging and requires delicate investigation and parameter control in contrast to broadband perfect absorbers. In this research, we numerically studied and experimentally fabricated a narrow-band plasmonic perfect absorber based on a metal-insulator-metal con- figuration. We analyzed the origin of perfect absorption for our proposed system and investigated the parameters that effect the optical properties. The purposed plasmonic structure comes up with a dual narrow-band absorption peaks at visible and near-infrared region of electromagnetic spectrum with near unity absorption e ciency. The physical origin of these absorption peaks is shown to be the excitation of propagating and localized surface plasmon resonances at certain individual frequencies, that leads to impedance matching and critical coupling when certain conditions are satisfied. Finally, we analyzed the sensing capabilities of PA by embedding nanostructure into different background refractive index, resulting in sensitivity of 500 nm/RIU, making such a platform suitable for biosensing and spectroscopic applications. This work analyzes the perfect absorption phenomena in visible frequencies in detail and will be a go to guide for researchers in the perfect absorber community.
Open Access
A narrow-band multi-resonant metamaterial in near-ir
(MDPI AG, 2020) Ali, Farhan; Aksu, S.
We theoretically investigate a multi-resonant plasmonic metamaterial perfect absorber operating between 600 and 950 nm wavelengths. The presented device generates 100% absorption at two resonance wavelengths and delivers an ultra-narrow band (sub-20 nm) and high quality factor resonance. The studied perfect absorber is a metal–insulator–metal configuration where a thin MgF spacer is sandwiched between an optically thick gold layer and uniformly patterned gold circular nanodisc antennas. The localized and propagating nature of the plasmonic resonances are characterized and confirmed theoretically. The origin of the perfect absorption is investigated using the impedance matching and critical coupling phenomenon. We calculate the effective impedance of the perfect absorber and confirm the matching with the free space impedance. We also investigate the scattering properties of the top antenna layer and confirm the minimized reflection at resonance wavelengths by calculating the absorption and scattering cross sections. The excitation of plasmonic resonances boost the near-field intensity by three orders of magnitude which enhances the interaction between the metamaterial surface and the incident energy. The refractive index sensitivity of the perfect absorber could go as high as nm/RIU. The presented optical characteristics make the proposed narrow-band multi-resonant perfect absorber a favorable platform for biosensing and contrast agent based bioimaging.