Sensitivity and figure of merit of a LESPR sensor. (a) The emission peak wavelength evolution of a LESPR device in varied refractive index environment. While there is a slight ~0.08 nm blue shift of the lasing wavelength over the entire pump region owing to the free carrier plasma effect, the absolute value of the wavelength shift about ~0.43 nm is independent of the pumping power, indicating the stable performance of the devices. Olive: device in ethanol; Red: device in propyl alcohol. (b) The spectrum shifts with the refractive index at fully lasing state. Olive: device in ethanol; Red: device in propyl alcohol. (c) The intensity detection figure of merit of LESPR device is ~84000, 400 times higher than state-of-the-art surface plasmon sensor at same wavelength. Such a record high FOMI originates from the three exclusive features of LESPR sensor comparing to their SPR and LSPR sensors counterparts: extremely narrow linewidth, nearly zero background emission by decoupling of the lasing emission and spontaneous emission and the Gaussian shape of the lasing emission.