methods and sensors for detecting the presence and / or concentration of an analyte are disclosed . in one aspect , a sensing element for use in a sensor is disclosed . the sensing element comprises a resonant cavity device configured to emit optical radiation at an initial power level , a sensing layer exhibiting an initial refractive index , and a detector . the sensing layer is configured to absorb or adsorb an analyte and , in response to absorbing or adsorbing the analyte , exhibit a modified refractive index that differs from the initial refractive index . the resonant cavity device is further configured to , in response to the sensing layer absorbing or adsorbing the analyte , emit optical radiation at a modified power level based on the modified refractive index . the detector is configured to detect the modified power level .