Tobias Pusch

Frequency tuning of polarization oscillations: Toward high-speed spin-lasers

Spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) offer a high potential to overcome several limitations of conventional purely charged-based laser devices. Presumably, the highest potential of spin-VCSELs lies in their ultrafast spin and polarization dynamics, which can be significantly faster than the intensity dynamics in conventional devices. Here, we experimentally demonstrate polarization oscillations in spin-VCSELs with frequencies up to 44 GHz. The results show that the oscillation frequency mainly depends on the cavity birefringence, which can be tuned by applying mechanical strain to the VCSEL structure. A tuning range of about 34 GHz is demonstrated. By measuring the polarization oscillation frequency and the birefringence governed mode splitting as a function of the applied strain simultaneously, we are able to investigate the correlation between birefringence and polarization oscillations in detail. The experimental findings are compared to numerical calculations based on ...

Monolithic vertical-cavity surface-emitting laser with thermally tunable birefringence

The birefringence splitting in vertical-cavity surface-emitting lasers offers an opportunity for spintronic-based high-frequency operation. By means of coupling of the carrier spin in the active region with the photons of the laser mode, the device can be excited to oscillations in the degree of circular polarization with a frequency corresponding to the birefringence splitting. On-chip frequency tunability of those oscillations is desirable for future applications. By asymmetric current-induced heating using the elasto-optic effect, we demonstrate a reversible tuning of the birefringence splitting of 45 GHz with less than 3 dB output power penalty.

Birefringence tuning of VCSELs

Using the elasto-optic effect we increase the frequency difference between the two orthogonally polarized modes, the so-called birefringence splitting, in standard single-mode oxide-confined GaAs-based vertical-cavity surface-emitting lasers (VCSELs). The birefringence may play an important role in the realization of ultrafast polarization modulation for high-speed data transmission. For practical implementation it is necessary to miniaturize the strain-inducing mechanism for birefringence tuning in VCSELs. The goal is the realization of integrated structures on the VCSEL chip. In this paper we discuss our work on miniaturized bending devices as the next step in achieving extremely high birefringence splitting. Furthermore measurements with integrated hotspot structures on VCSEL chips were made to reach much smaller scales for birefringence fine-tuning.

Investigations on polarization oscillation amplitudes in spin-VCSELs

Compared to conventional vertical-cavity surface-emitting lasers (VCSELs), spin-pumped VCSELs offer the possibility of polarization control and fast polarization dynamics. It has been demonstrated that oscillations in the circular polarization degree can be excited. In short, the frequency of these polarization oscillations is determined by the frequency splitting between the two orthogonal linearly polarized cavity modes and therefore by the cavity birefringence. The polarization oscillation frequency is the resonance frequency of the VCSEL’s polarization dynamics and can be compared to the conventional resonance frequency for intensity modulation. We have demonstrated polarization oscillations up to 44 GHz, exceeding the direct intensity resonance frequency of the investigated devices by far. As the polarization oscillation frequency can be increased by increasing the cavity birefringence and a VCSEL cavity birefringence of more than 250 GHz has been demonstrated, using polarization dynamics is a possible way of substantially increasing the modulation speeds of VCSELs. This is for instance interesting for high-bandwith short-haul optical interconnects. The experimental results associated with the polarization oscillation effects can be simulated by the widely used spin-flip model. In this work we focus on the amplitude of the polarization oscillations. Previous publications have shown a decrease with increasing oscillation frequency. Here, we show amplitude dependencies on several system parameters like the photon and carrier lifetimes as well as pumping conditions. Based on this, we investigate how to increase the polarization oscillation amplitude, since a significant amplitude is necessary for, e.g., data transmission applications.

Influence of birefringence splitting on ultrafast polarization oscillations in VCSELs

Spin-VCSELs offer numerous advantages over conventional lasers like reduced threshold, spin amplification and ultrafast polarization dynamics. The latter have the potential to generate polarization modulation frequencies far above the conventional intensity relaxation oscillation frequency of one and the same device and thus can be an interesting basis for ultrafast optical data transmission. We have shown that fast polarization oscillations can be generated by pulsed spin injection. Furthermore the oscillation frequency can be tuned via modification of the VCSEL’s cavity strain. Using this technique, oscillation frequencies with a tuning range from nearly zero up to 40 GHz can be demonstrated. In the device under study, this is more than six times the intensity relaxation oscillation frequency, which is nearly independent of the strain. Now we demonstrate the influence of the strain-induced birefringence splitting on the oscillation frequency. We find that the polarization oscillation frequency is directly corresponding to the birefringence splitting. The reason is that the polarization oscillates according to the beating frequency of the two orthogonal linearly polarized cavity modes in the VCSEL. In the case of spin-pumping, those two modes form the circular polarization output of the laser by superposition. Their frequencies are shifted by birefringence manipulation and form the basis of birefringence splitting. The measurement results are compared with simulations employing the spin-flip model. Our results show that high-frequency polarization oscillations can not only be generated with the help of external strain but with high birefringence splitting in general.

Demonstrating ultrafast polarization dynamics in spin-VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) are used for short-haul optical data transmission with increasing bit rates. The optimization involves both enhanced device designs and the use of higher-order modulation formats. In order to improve the modulation bandwidth substantially, the presented work employs spin-pumped VCSELs (spin-VCSELs) and their polarization dynamics instead of relying on intensity-modulated devices. In spin-VCSELs, the polarization state of the emitted light is controllable via spin injection. By optical spin pumping a single-mode VCSEL is forced to emit light composed of both orthogonal linearly polarized fundamental modes. The frequencies of these two modes differ slightly by a value determined by the cavity birefringence. As a result, the circular polarization degree oscillates with their beat frequency, i.e., with the birefringence-induced mode splitting. We used this phenomenon to show so-called polarization oscillations, which are generated by pulsed spin injection. Their frequency represents the polarization dynamics resonance frequency and can be tuned over a wide range via the birefringence, nearly independent from any other laser parameter. In previous work we demonstrated a maximum birefringence-induced mode splitting of more than 250 GHz. In this work, compared to previous publications, we show an almost doubled polarization oscillation frequency of more than 80 GHz. Furthermore, we discuss concepts to achieve even higher values far above 100 GHz.

Electrical birefringence tuning of VCSELs

The birefringence splitting B, which is the frequency difference between the two fundamental linear polarization modes in vertical-cavity surface-emitting lasers (VCSELs), is the key parameter determining the polarization dynamics of spin-VCSELs that can be much faster than the intensity dynamics. For easy handling and control, electrical tuning of B is favored. This was realized in an integrated chip by thermally induced strain via asymmetric heating with a birefringence tuning range of 45 GHz. In this paper we present our work on VCSEL structures mounted on piezoelectric transducers for strain generation. Furthermore we show a combination of both techniques, namely VCSELs with piezo-thermal birefringence tunability.

Thermally-induced birefringence in VCSELs: approaching the limits

Polarization dynamics in vertical-cavity surface-emitting lasers (VCSELs) are much faster than their intensity-driven counterparts and can be a potential approach to overcome the bandwidth limitation in short-distance data transmission. The birefringence splitting B as the frequency difference between the two fundamental polarization modes is an important factor determining the polarization dynamics in spin-VCSELs. Although B can be strongly influenced by mechanical bending, for later applications an on-chip solution for birefringence tuning is favored. With an electrically driven asymmetric heating device we have demonstrated a thermally induced tuning range of ΔB = 45GHz. The maximum achievable birefringence tuning was not limited by the laser but by material parameters and the fabrication process. In this paper we present an optimized design for thermally induced birefringence tuning and additional possibilities to increase the efficiency of the mechanism.

Spin lasers for optical data communication

For short-haul optical interconnects, state-of-the-art technology are vertical-cavity surface-emitting lasers (VCSELs). To transmit data, direct current modulation is used. The corresponding intensity modulation resonance frequency is determined by design and material parameters of the laser and therefore practically limited to a few tens of GHz. To overcome this limitation, an alternative approach is the utilization of spin-VCSELs. In this case, the information carrier is no longer represented by the intensity, but instead by the polarization. The polarization can be controlled by the carrier spin. The birefringence in the cavity has the strongest impact on the polarization modulation resonance frequency. This can be explained by the generation of resonant polarization oscillations in the circular polarization degree in a spin-VCSEL. The circular polarization is composed of the two orthogonal linearly polarized cavity modes. The electromagnetic fields emitted from the two modes are coupled in phase by birefringence and in amplitude by dichroism. However, dependent on the birefringence in the cavity, their frequencies may differ. Spin pumping, i.e., circularly polarized optical pumping pulses, causes the fact that both modes become active. This results in an oscillation of the circular polarization degree of the emitted light, representing the polarization dynamics resonance frequency of the spin-VCSEL device. We demonstrate that the birefringence can be manipulated in actual VCSEL devices over a broad tuning range. Employing this parameter tuning, we demonstrate a polarization dynamics resonance frequency of 89 GHz, which is much faster than currently obtained intensity dynamics resonance frequencies. Not only the maximum frequency, but also the amplitude of the polarization effects should be optimized. An important factor for the amplitude damping is the dichroism, which represents the difference in the gain of the two orthogonal modes. We investigate the influence of birefringence on dichroism and the polarization oscillation amplitude.

Thermally induced birefringence tuning of 37 GHz in VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) are the transmitters of choice in high-speed optical interconnects. Further progress beyond present 25 Gb/s devices is hampered by the limited modulation bandwidth of not more than 30 GHz. For higher data rates one thus has to resort to higher-order modulation formats [1] with substantially increased system complexity. Polarization modulation of VCSELs is a potential alternative to intensity modulation. In this technique the birefringence splitting B, namely the frequency difference between the two fundamental polarization modes, is a key parameter. By optical spin injection a birefringent VCSEL can be excited to oscillations in the degree of circular polarization, where the oscillation frequency is close to B [2]. In this work we show a large tuning range of B in a VCSEL achieved via anisotropic strain caused by an integrated heater.