Frequency tuning of polarization oscillations in spin-lasers

Abstract

Polarization oscillations can be observed as resonant oscillations of the coupled spin-photon system in spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs). They are a reasonable measure of the polarization dynamics and provide insights to the polarization modulation bandwidth of these devices. These oscillations can be generated using pulsed spin injection and have proven to be much faster than the relaxation oscillations for the intensity dynamics under the same conditions. The oscillation frequency mainly depends on the cavity birefringence, which can be tuned by applying mechanical strain to the VCSEL structure. This provides a direct tool to considerably increase the polarization oscillation frequency and thus the modulation bandwidth. Following this approach we were able to experimentally tune the frequency over a range of 34 GHz. We demonstrated polarization oscillations in spin-VCSELs with frequencies up to 44 GHz recently, only limited by the used mechanical strain setup.1 By measuring the polarization oscillation frequency and the birefringence-governed mode splitting as a function of the applied strain simultaneously, we investigated the correlation between birefringence and polarization oscillations. Here we use an optimized and simplified mount, which potentially allows for larger strain values. The experimental findings are compared to numerical calculations based on the spin-flip model. Taking our previously reported record value of more than 250 GHz for the birefringence splitting in VCSEL cavities into account,2 this technique may pave the road toward high-speed polarization modulation in VCSELs for bit rates above 100 Gb/s.