Refractive index sensing with Al2O3-nanocoated long-period gratings working at dispersion turning point: temperature cross-sensitivity

Abstract

In this work is discussed an effect of nanocoating long-period grating (LPG) with an aluminum oxide (Al 2 O 3 ) on its refractive index (RI) and temperature (T) sensitivity. For comparison two LPGs, namely one before and after the nanocoating process were optimized towards working at dispersion turning point (DTP) of higher order cladding modes. The DTP was reached with two methods, i.e., by wet etching of LPG's cladding and by optimized deposition of Al 2 O 3 thin overlay using atomic layer deposition (ALD) method. For both the cases we show significant increase in RI sensitivity at DTP. When Al 2 O 3 is deposited the RI sensitivity reaches 9270 nm/RIU for traced only one resonance out of the pair and RI in range 1.33–1.34 RIU, what is over 4-fold higher than for the bare LPG. Moreover, T sensitivity for both the structures was compared and the one for Al 2 O 3 -coated sample was found to be only up to 12% higher than for the bare sample. Higher RI sensitivity and low temperature cross-sensitivity effects allow for more accurate RI measurements than in case of bare LPGs. The Al 2 O 3 -nanocoated LPGs can further find applications in label-free biosensing, due to their optimized sensitivity in RI in range close that of water.