A bimorph spiral which exhibits a shape-altering response to thermal radiation and is dimensioned to have a focussing effect on light, such as a visible light, by acting as a quasi-Fresnel element. The focussing effect varies as the shape of the bimorph spiral changes due to absorption of thermal radiation. An array of such bimorph spirals can be used for efficient, high-resolution and rapid uncooled photothermal spectroscopy.