Controlled switching and frequency tuning of polarization oscillations in vertical-cavity surface-emitting lasers

Abstract

Spintronic lasers offer promising perspectives for novel concepts and characteristics superior to conventional purely charge-based devices. This applies in particular to spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs), which exhibit ultrafast spin and polarization dynamics. Using pulsed spin-injection, oscillations in the circular polarization degree can be generated, which have the potential to be much faster than conventional relaxation oscillations and may exceed frequencies of 100 GHz. The oscillations originate from the coupled carrier-spin-photon system in birefringent VCSEL cavities. The polarization oscillations are independent from conventional relaxation oscillations and thus can be the cornerstone for ultrafast directly modulated spin-VCSELs in the near future. It is possible to switch the oscillations on and off, depending on phase and amplitude conditions of two consecutive excitation pulses. Even half-cycles can be generated, which is the basis for short polarization pulses, only limited by the polarization oscillation resonance frequency. Experimental results of oscillation switching are given using an 850 nm oxide-confined single-mode VCSEL. In order to increase the polarization oscillation frequency, the birefringence has to be tuned to higher values. We demonstrate a method to manipulate the birefringence by adding mechanical strain to the substrate in vicinity of the VCSEL. With this method the polarization oscillation frequency can be tuned over a wide range. The results are compared to the theory with simulations using the spin-flip-model.