Optical far-field characterisation of LESPR sensor. (a-b) The emission spectrum evolution of a LESPR device in varied refractive index environment of ethanol (a) and propyl alcohol (b). At a fixed refractive index of interaction layer, the device show typical lasing behaviour, while there is a clear lasing peak shift with altered refractive index. (c) The light-light curves of the LESPR device shown in (a-b). The spontaneous emission becomes saturated after the onset of lasing due to the gain saturation effect, while the stimulated emission experiences a nonlinear increase to the fully lasing state. Data points in olive: device in ethanol; Data points in red: device in propyl alcohol. (d) The linewidth evolution with the pump power. There is a significant line width narrow width narrowing effect from a FWHM of ~20 nm below threshold to ~0.3 nm above. Olive: device in ethanol; Red: device in propyl alcohol.