Achievement of Sub-Diffractive Detection Depth in Absorption Spectroscopy Using Excitation from an Upconverting Particle

Conventionally, a collimated visible laser beam can only be focused to a transverse region volume with a waist of about 200 nm and an axial waist of about 1200 nm due to the wave nature of light. Several techniques have been used to bypass the diffraction limit, including a recently developed one using the emission from an optically trapped upconverting nanoparticle. This nanoparticle has a diameter smaller than 200 nm, such that it emits like a dipole into a 45° cone. Thus, not only is the emission coming from a particle smaller in size than the waist of the diffraction-limited spot but also the cone is smaller than that of the volume of a very tightly focused beam. Here, the technique is developed even further where the emission coming from a specific region of the cone is selected by an optical fiber-based pinhole in the detection path. Using this technique, a confocal depth of about 200 nm is achieved as the overlap between the emission cone and the detection volume. This would correspond to a volume of about 250 attoliters for a 1.5 μm diameter particle which can be reduced to 40 attoliters by using a 500 nm diameter particle.