  The nanoscale mode volumes of surface plasmon polaritons have enabled
plasmonic lasers and condensates with ultrafast operation. Most plasmonic
lasers are based on noble metals, rendering the optical mode structure inert to
external fields. Here, we demonstrate active magnetic-field control over lasing
in a periodic array of Co/Pt multilayer nanodots immersed in an IR-140 dye
solution. We exploit magnetic circular dichroism (MCD) at the excitation
wavelength to modify the optical absorption of the nanodots as a function of
their magnetization. Under circularly polarized excitation, angle-resolved
photoluminescence measurements reveal a transition between lasing action and
non-lasing emission as the nanodot magnetization is reversed. Our results
introduce magnetization as a means of externally controlling plasmonic
nanolasers, complementary to the modulation by excitation, gain medium, or
substrate. Further, the results show how the effects of magnetization on light
that are inherently weak become prominent at the lasing regime, inspiring
studies of topological photonics.
