Loss compensation in a LESPR sensor by gain and analytes introduced resonant wavelength shift. (a) Full wave simulation result of the plasmon laser for the refractive index sensing. The pumping produced gain increasing process is mimicked by increasing the imaginary part of refractive index of gain materials. At a gain coefficient of zero, the cavity quality factor is about 20, which is mainly limited by the non-radiative Drude damping. With the increasing of the gain coefficiency, the cavity quality factor increases by orders of magnitude, indicating that the Drude damping has been compensated by gain. (b) Scaling law of the lasing emission quality factor. Over 60 devices with their physical volume spanning over two orders of magnitude were measured experimentally to map the lasing quality factor of LESPR device. (c) Full wave simulated wavelength shift of LESPR at a change of analyte refractive index of 0.0213. In our sensing scheme, the change of the environment refractive index produces a cavity resonance wavelength shift which can be monitored in the optical far-field in terms of lasing emission spectra. Such a shift becomes dramatic when the physical volume is smaller than λ3.