An optical mercury detector with (a) a light source, in particular a laser, for emitting light, (b) a waveguide structure, which is connected to the source, and (c) a detector element, which is arranged in a light path behind the waveguide structure and is designed to detect a property of the light, (d) wherein the waveguide structure has a coating made of a coating material which reacts with mercury to form a reaction product, (e) wherein the waveguide structure is designed in such a way that light guided in the waveguide structure changes in the property when the coating material reacts to form a reaction product, such that the property of the light detected by the detector element depends on the amount of the reaction product, and (f) the waveguide structure features a linear waveguide, (g) wherein the mercury detector comprises a heating device by means of which the coating can be heated to a regeneration temperature above which the reaction product reacts to form the coating material, characterized in that (h) the waveguide structure comprises a resonator, wherein the resonator is coupled with the linear waveguide via the evanescent field of the linear waveguide and wherein the resonator is at least partially coated with the coating material, (i) the waveguide of the waveguide structure forms a Mach-Zehnder interferometer with a first arm and a second arm, wherein the resonator is situated in the vicinity of the arm of the Mach-Zehnder interferometer in whose vicinity the coating material is arranged, and that (j) the coating material comprises gold in the form of gold nanoantennas arranged in such a way that their average spacing can be determined by spectroscopy in the infrared range, wherein an extension of the gold nanoantennas in two spatial directions is smaller than in the third, remaining spatial direction. (a) a light source, in particular a laser, for emitting light, (b) a waveguide structure, which is connected to the source, and (c) a detector element, which is arranged in a light path behind the waveguide structure and is designed to detect a property of the light, (d) wherein the waveguide structure has a coating made of a coating material which reacts with mercury to form a reaction product, (e) wherein the waveguide structure is designed in such a way that light guided in the waveguide structure changes in the property when the coating material reacts to form a reaction product, such that the property of the light detected by the detector element depends on the amount of the reaction product, and (f) the waveguide structure features a linear waveguide, (g) wherein the mercury detector comprises a heating device by means of which the coating can be heated to a regeneration temperature above which the reaction product reacts to form the coating material, characterized in that (h) the waveguide structure comprises a resonator, wherein the resonator is coupled with the linear waveguide via the evanescent field of the linear waveguide and wherein the resonator is at least partially coated with the coating material, (i) the waveguide of the waveguide structure forms a Mach-Zehnder interferometer with a first arm and a second arm, wherein the resonator is situated in the vicinity of the arm of the Mach-Zehnder interferometer in whose vicinity the coating material is arranged, and that (j) the coating material comprises gold in the form of gold nanoantennas arranged in such a way that their average spacing can be determined by spectroscopy in the infrared range, wherein an extension of the gold nanoantennas in two spatial directions is smaller than in the third, remaining spatial direction.