2015 Volume 70 Issue 4 Pages 269-274
Although Planck's law of blackbody radiation well describes spectral profiles of thermal radiation from macroscopic objects, it is not applicable to particles of size comparable to optical wavelengths. We experimentally demonstrate that thermal radiation from a micron-sized dielectric particle depends sensitively on its size and shape through the cavity quantum-electrodynamic effect. Our flow-assisted optical trapping levitates a single molten droplet of dielectric materials such as Al_2O_3 and TiO_2 to enable its emission spectroscopy. As the particle becomes smaller, a blackbody-like spectrum turns into a spectrum dominated by multiple peaks resonant with whispering gallery modes of the microsphere, showing that the spontaneous emission rate is modified by the discrete mode density.