A Large‐Area, Mushroom‐Capped Plasmonic Perfect Absorber: Refractive Index Sensing and Fabry–Perot Cavity Mechanism

Abstract

causes the spectral shift of the resonance wavelength. In particular, most organic molecules have a higher refractive index than buffer solution, thus as the concentration of these molecules rises, the local refractive index increases, thereby redshifting the resonance wavelength. The spectral shift of resonance results in the change of the transmission, refl ection, or absorption spectrum, which can be monitored by inexpensive spectrometry. [1][2][3][4][9][10][11][12] However, strong induced electric current in these plasmonic resonators also leads to signifi cant ohmic loss, [6][7][8] which gives rise to the broadening of the resonance spectrum with low-quality factor ( Q -factor) and thus degrade the performance of the sensor characterized by a fi gure of merit (FOM*). Over the years, numerous efforts have been made to improve the performance of plasmon-based sensor, for example, using high Q-factor Fano resonance induced by coupling of surface plasmon polariton and LSPR [ 11 ] and reducing the substrate effect by lifting the LSPR resonators. [ 13 ] Among the efforts, it has been demonstrated that the metamaterial perfect absorbers (MPAs) can be used to signifi cantly increase the Q -factor and thus improve the FOM*. [ 12,14 ] The MPA is a recently developed branch of metamaterial which exhibits nearly unity absorption within certain frequency range. [15][16][17][18][19][20] The optically thin MPA possesses characteristic features of angular-independence, high Q -factor and strong fi eld localization that have inspired a wide range of applications including electromagnetic (EM) wave absorption, [ 17,21,22 ] spatial [ 20 ] and spectral [ 19 ] modulation of light, [ 23 ] selective thermal emission, [ 23 ] thermal detecting, [ 24 ] and refractive index sensing for gas [ 25 ] and liquid [ 12,14 ] targets. The MPA is typically comprised of three layers: a metallic resonator's layer, e.g., cross-type resonators, [ 23,26 ] split-ring resonators, [ 17 ] or metallic nanoparticles, [ 27 ] and a highly refl ective layer, e.g., metallic fi lm [ 17,23,26,27 ] or metallic mesh grid, [ 17 ] separated by a subwavelength-thick dielectric fi lm (spacer). The impedance matching between MPA and free space, and high attenuation of light inside the MPA result in the perfect absorption. [ 17 ] In the sensing application, the spectral shift of perfect absorption peak is attributed to the refractive index change of gas [ 25 ] or liquid. [ 12,14 ] However, the gas or liquid used as the sensing target has been so far only on the surface of the MPA-basedIn most plasmon resonance based sensor to date, only the surface of the sensor is accessible to the gas or liquid as the sensing target. In this work, an interferometric, lithographically fabricated, large-area, mushroom-capped plasmonic perfect absorber whose dielectric spacer is partially removed by a reactive-ion-etch process, thereby enabling the liquid to permeate into the sensitive region to a refractive index change, is demonstrated. Findings of this paper demonstrate experimentall...

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