Spin-polarized lasers are highly attractive spintronic devices providing characteristics superior to their conventional purely charge-based counterparts. Spin-polarized vertical-cavity surface emitting lasers (spin-VCSELs) promise to offer lower thresholds, enhanced emission intensity, spin amplification, full polarization control, chirp control and ultrafast dynamics. In particular, the ability to control and modulate the polarization state of the laser emission with extraordinarily high frequencies is very attractive for many applications like broadband optical communication and ultrafast optical switches. After briefly reviewing the state of research in this emerging field of spintronics, we present a novel concept for ultrafast spin-VCSELs which has the potential to overcome the conventional speed limitation for directly modulated lasers and to reach modulation frequencies significantly above 100 GHz. The concept is based on the coupled spin-photon dynamics in birefringent micro-cavity lasers. By injecting spin-polarized carriers in the VCSEL, oscillations of the coupled spin-photon system can by induced which lead to oscillations of the polarization state of the laser emission. These oscillations are decoupled from conventional relaxation oscillations of the carrier-photon system and can be much faster than those. Utilizing these polarization oscillations is thus a very promising approach to develop ultrafast spin-VCSELs for high speed optical data communication in the near future.
Spin-optoelectronic devices have become a field of intensive research in the past few years. Here we present
electrical spin injection into spin light-emitting diodes both at room temperature and in magnetic remanence.
Our devices consist of a Fe/Tb multilayer spin injection structure with remanent out-of-plane magnetization, a
MgO tunnel barrier for efficient spin injection and an InAs quantum dot light-emitting diode. The ground state
emission and first excited state emission both show circularly polarized emission in remanence, i.e. without
external magnetic fields which is due to spin injection from our ferromagnetic contact. Using a series of samples
with varying transport path lengths between the spin injector and the active region, we investigate the spin
relaxation length during vertical carrier transport through our devices. Due to our spin injector with remanent
out-of-plane magnetization this spin relaxation can be investigated without the need for external magnetic fields
which would possibly influence the spin relaxation process. The decrease in circular polarization with increasing
injection path length is found to be exponential, indicating drift-based transport which is in accordance with
theoretic calculations. From the exponential decay the spin relaxation length of 26 nm as well as a lower bound
for the spin injection efficiency of 25% are calculated. Additionally, influences of magnetic field, temperature
and current density in the devices on the spin relaxation process are discussed.
Spin-controlled vertical-cavity surface-emitting lasers (VCSELs) have been intensively studied in recent years because
of the low threshold feasibility and the nonlinearity above threshold, which make spin-VCSELs very promising for
spintronic devices. Here we investigate the circular polarization dynamics of VCSELs on a picosecond time scale after
pulsed optical spin injection at room temperature. A hybrid excitation technique combining continuous-wave (cw)
unpolarized electrical excitation slightly above threshold and pulsed polarized optical excitation is applied. The
experimental results demonstrate ultrafast circular polarization oscillations with a frequency of about 11 GHz. The
oscillations last inside the first undulation of the intensity relaxation oscillations. Via theoretical calculations based on a
rate equation model we analyze these oscillations as well as the underlying physical mechanisms.
Spin-polarized lasers offer new encouraging possibilities for future devices. We investigate the polarization dynamics of
electrically pumped vertical-cavity surface-emitting lasers after additional spin injection at room temperature. We find
that the circular polarization degree exhibits faster dynamics than the emitted light. Moreover the experimental results
demonstrate a strongly damped ultrafast circular polarization oscillation due to spin injection with an oscillation
frequency of approximately 11GHz depending on the birefringence in the VCSEL device. We compare our experimental
results with theoretical calculations based on rate-equations. This allows us to predict undamped long persisting ultrafast
polarization oscillations, which reveal the potential of spin-VCSELs for ultrafast modulation applications.
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