Birefringent surface gratings for ultrafast spin-VCSELs

The data rates required for optical interconnection in datacenters continue to increase. On the other hand, intensity-modulated vertical-cavity surface-emitting lasers (VCSELs), which are the preferred light source for this task, have a limited bandwidth of at present about 35 GHz [1]. This limit can be overcome with spin-VCSELs that show much faster polarisation and modulation dynamics [2]. In these devices, spin-polarised electrons are injected, by which the degree of circular polarisation of the emitted light can be changed and thus information be encoded. The characteristic eigenfrequency of the laser, corresponding to the resonance frequency in conventional laser diodes, is determined by the birefringence splitting B, namely the frequency difference between the two fundamental polarisation modes of a single-mode VCSEL. Recently a polarisation oscillation frequency of 212 GHz was demonstrated [2]. In that laser the birefringence was induced by bending the sample and employing the elastooptic effect. Alternatively we have shown that the cavity birefringence can be tuned by mounting VCSELs on piezoelectric substrates or by asymmetric heating, which, however, leads to much lower B. Up to now no truly integrated B-inducing technique existed for a spin-VCSEL that would allow to maintain great similarity to the successful high-efficiency device platform of present oxide-confined VCSELs. In this work, for the first time, we present a VCSEL in which high B of almost 100 GHz stems from a tailored integrated surface grating.