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A quantum spin liquid state is a Mott insulator without any long-range magnetic order down to the lowest temperature.For the typical candidate material κ-(ET)2Cu2(CN)3,which is believed to be described by an effective U(1) gauge theory,many experimental evidences suggest the existence of a large spinon Fermi surface.Therefore,we study the optical properties and find that the optical conductivity at energies above the charge gap has a unique feature that distinguishes them from ordinary insulators.In particular,we propose the existence of a long-life surface plasmon mode propagating along the interface between a linear medium and the spin liquid at frequencies above the charge gap,which can be detected by the widely used Kretschmann-Raether three-layer configuration.Then the theory is generalized to include nonlinear effects,and the obtained third order nonlinear susceptibility may also induce through process of third harmonic generation propagating EM waves with triple frequency inside the spin liquids.These results will provide a straightforward test that distinguishes between ordinary insulator and spin liquids with large spinon Fermi surfaces.