NOVELTY - A microsphere whispering-gallery mode evanescent-wave sensor comprises a tunable laser capable of emitting laser light at light wavelengths; a microsphere sensor emitting at least a portion of the emitted laser light; a microsphere compression system acting to compress the microsphere along a polar axis; a fast modulation unit in electronic communication with the laser and/or the compression system; and a fast modulation lock-in and control unit.    USE - Determining whether a sample absorbs light at a particular light frequency (claimed), preferably for detection of carbon monoxide, carbon dioxide, and atmospheric trace gases including methane and ammonia.    ADVANTAGE - The sensor can rival the performance of a multipass cell and can be made part of a much more compact and rugged system.    DETAILED DESCRIPTION - A microsphere whispering-gallery mode evanescent-wave sensor comprises:    (a) a tunable laser capable of emitting laser light at several light wavelengths;    (b) a microsphere sensor, wherein the tunable laser is positioned to emit laser light toward the microsphere sensor and the microsphere sensor emits at least a portion of the laser light;    (c) a microsphere compression system acting to compress the microsphere along a polar axis;    (d) a fast modulation unit in electronic communication with at least one of the tunable laser and the microsphere compression system and imposing a high frequency oscillation; and    (e) a fast modulation lock-in and control unit sensing at least a portion of the light emitted from the microsphere and acting to cause the microsphere compression system to compress the microsphere in concert with a light frequency emitted by the tunable laser.    An INDEPENDENT CLAIM is also included for a method for determining whether a sample absorbs light at a particular frequency by:    (i) exciting a whispering-gallery light mode within a sensor at the particular light frequency;    (ii) exposing the sensor to the sample to produce a responsive intensity at which the sensor reflects or transmits light; and    (iii) determining whether the sample absorbs light at the particular frequency using at least the responsive intensity.    DESCRIPTION OF DRAWING(S) - The figure shows an absorption spectroscopy system.