Two-frequency emission and polarization dynamics at lasing threshold in vertical-cavity surface-emitting lasers

We investigate experimentally and theoretically the polarization dynamics in vertical-cavity surface-emitting lasers in the vicinity of the lasing threshold. If the net gain anisotropy between the two orthogonally polarized modes is close to zero, the laser operates in a partially polarized state. This state occurs at and slightly above the lasing threshold. It is characterized by anticorrelated dynamics of the two modes which result from completely anticorrelated dynamics at low frequencies and correlated dynamics on the time scale of the relaxation oscillations. If the net anisotropy is changed by a change of the detuning between the cavity resonance and the gain maximum, one of the two linear polarization states is selected depending on the sign of the net gain. For one sign the well-known phenomenon of polarization switching is recovered at higher currents. The experimental results show very good agreement with simulations based on a model that takes into account nonlinear spin dynamics and linear temperature-dependent detuning effects.