A first object of the invention is a radiation detector comprising an energy absorber (203), for absorbing incident radiation (RAD) and thus undergoing a temperature increase; and optical readout means, for detecting said temperature increase; wherein said optical readout means comprises input coupling means (202) for coupling a light beam (2011) to said energy absorber (203) by exciting surface plasmons resonance, a surface plasmons resonance condition being dependent on the energy absorber (203) temperature, and wherein said energy absorber (203) is separated from said input coupling means (202) by a dielectric layer (2032). A second object of the invention is a micromechanical sensor comprising: a micromechanical oscillator and optical readout means (202) for detecting a displacement of said micromechanical oscillator; wherein said optical readout means comprise input coupling means (202) for coupling a light beam (2011) to a conductive surface (2031) by exciting surface plasmons resonance, a surface plasmons resonance condition being dependent on the displacement of said micromechanical oscillator.