PHOTOTHERMAL SPECTROSCOPY WITH FEMTOJOULE SENSITIVITY USING A MICROMECHANICAL DEVICE

WHEN a material absorbs a photon, a fraction of the energy may be tran sformed into heat. A measurement of photothermal heating as a function of wavelength can provide an absorption spectrum of the material. We have recently(1,2) developed a micromechanical sensor capable of detec ting heat changes of the order of picojoules (10(-12) J). The instrume nt incorporates a bilayer cantilever of micrometre dimensions which be nds in response to heating. Here we show that this device can be used for photothermal spectroscopy with a power sensitivity of 100 pW-two o rders of magnitude better than the sensitivity of conventional phototh ermal deflection spectroscopy(3). The small size of the sensor allows picogram quantities of material to be studied, opening up the possibil ity of spectroscopic studies on individual cells and bacteria. Being b ased on silicon technology, the sensor should be compatible with micro electronic circuitry.