NP injection can either be continuous or pulsed, and ~10 mTorr of argon buffer gas is added to the trap to collisionally damp NP motion. During NP injection, the trap center is irradiated with either a CO 2 laser or one of several solid-state lasers, depending on the absorption spectrum of the NP material. The lasers are loosely focused (beam waist diameters of 0.3 to 1 mm) to ensure that the NPs remain in the focal volume as they oscillate within the trap. Light emission from a ~50 μm volume at the trap center is collected over a solid angle of ~0.3 steradians and focused onto a silicon avalanche photodiode (APD) photon counting module sensitive to wavelengths (λ) below ~1 μm. The background count rate of the APD is typically ~20 photons/second, and when an NP becomes trapped and heated by the laser, the count rate jumps above ~2000 photons/second. Both NP injection and laser irradiation are automatically terminated within ~50 milliseconds of detection of above-background emission.