In anisotropic spin-lasers, the ultrafast dynamics of a coupled system between carrier and photon spins will be exploited to realize spin and polarization modulation at frequencies above 200 GHz, far beyond the current limits for conventional current-modulated laser devices. This makes spin-VCSELs excellent candidates not only for the next generation of ultrafast optical communication systems, but also for many other emerging applications such as polarization-based optical communication, neuromorphic computing, chaos-based random bit generation, or microwave and THz generation. Here we present our recent developments on ultrafast spin and polarization control in anisotropic spin-lasers and discuss the prospects and challenges of this new technology on its way to application.
We report high frequency (20-100 GHz range) optical field intensity oscillations in laterally-coupled-cavity verticalcavity surface-emitting lasers with several different techniques. The oscillation frequency is defined by the photon energy splitting of the coupled states. The resonance effect is stable in an extended current range and can enable modulation frequency resonances at higher frequencies as compared to the conventional relaxation oscillation frequency of the laser. This paves a way towards high-speed data transmission solutions at data rates beyond ~200 Gb/s with the advantage of better laser stability, as the resonance observed can reach high frequencies even at low current densities. A ~75 GHz intensity modulation between optical modes of a coupled-cavity VCSEL array was first reported by the authors in a two-aperture configuration in 2023 applying optical excitation [1]. Studies of 4- and 10-element coupled VCSEL arrays give further insight into the effects observed. New 3D numerical simulations and electrical modulation techniques have been applied to address the specific nature of the photon-photon resonance studies.
Spin-controlled lasers are highly interesting photonic devices and have been shown to provide ultra-fast polarization dynamics in excess of 200 GHz. Another class of modern semiconductor lasers are high-beta emitters which benefit from enhanced light-matter interaction due to strong mode confinement in low-mode-volume microcavities. We combine the advantages of both laser types to demonstrate spin-lasing in high-beta microlasers for the first time. For this purpose, we realize bimodal high-beta quantum dot micropillar lasers for which the mode splitting and the polarization-oscillation frequency can be engineered via the pillar cross-section. The microlasers show very pronounced spin-lasing effects with polarization oscillation frequencies up to 15 GHz.
Spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) have proven to be a highly promising device technology for high-speed optical communication systems. In spin-lasers, the polarization state of the laser emission can be controlled by the carrier spin state exploiting the transfer of angular momentum between carriers and photons. The resonance frequency of the polarization dynamics can be increased by inducing birefringence into the resonator. Here we discuss the role of the photon lifetime and show results on the influence of this parameter on the static polarization behavior.
Spin-controlled lasers are highly interesting photonic devices and have been shown to provide ultra-fast polarization dynamics in excess of 200 GHz. Another class of modern semiconductor lasers are high-beta emitters which benefit from enhanced light-matter interaction due to strong mode confinement in low-mode-volume microcavities. We combine the advantages of both laser types to demonstrate spin-lasing in high-beta microlasers for the first time. For this purpose, we realize bimodal high-beta quantum dot micropillar lasers for which the mode splitting and the polarization-oszillation frequency can be engineered via the pillar cross-section. The microlasers show very pronounced spin-lasing effects with polarization oscillation frequencies up to 16 GHz.
Vertical-cavity surface-emitting lasers (VCSELs) are widely used in optical data communication mainly in data centers for short-haul transmissions. However, their intensity modulation resonance frequency does not exceed 40 GHz which also limits the achievable modulation bandwidth and data rate. In contrast, spin-VCSELs can overcome these bandwidth limitations by modulating spin and polarization instead of current and intensity. In spin-VCSELs, the birefringence determines the resonance frequency of the polarization dynamics as well as the modulation bandwidth. We control the birefringence and thus the polarization dynamics via the elasto-optic effect by mechanically or thermally induced strain providing polarization oscillation frequencies up to more than 200 GHz. Detailed analysis shows that spin-VCSELs offer polarization dynamics with good signal strength even when operating close to threshold and at high temperatures. Here, we analyze devices with integrated surface gratings. VCSELs with different grating periods as well as mesa diameters and resulting different oxide apertures were investigated.
Since todays Internet traffic is more and more concentrated in hyperscale datacenters,1 new concepts for shortrange optical communication systems with high modulation bandwidth, high temperature stability, and low energy consumption are urgently needed. Birefringent spin-lasers, in particular spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs), are a novel type of ultrafast laser devices which promise to serve as ultrafast transmitters for the next generation of optical communication systems. While current-driven intensitymodulated VCSELs are state-of-the-art laser devices for short-range communication, their modulation bandwidth is limited to values below 40 GHz.2, 3 Recently, we were able to demonstrate that modulating carrier spin and light polarization in spin-VCSELs instead of carrier density and light intensity in conventional devices enables ultrafast polarization dynamics and a modulation bandwidth of more than 200 GHz.4 This high modulation bandwidth was achieved by increasing the resonance frequency of the coupled carrier spin-photon system by implementing high values of birefringence to the cavity of 850 nm GaAs/GaAlAs VCSELs. Here, we show experimental results for the intensity and polarization dynamics in highly birefringent spin-VCSELs as a function of bias current, birefringence, and temperature and demonstrate the capability of spin-VCSELs for ultralow energy consumption and high temperature stability. Furthermore, we present first results on polarization dynamics in 1.3 μm VCSELs for potential long-range communication systems and discuss novel concepts for future integrated and electrically pumped devices.
Data centers play an important role in the ongoing demand for higher data rates. Here intensity-modulated vertical-cavity surface-emitting lasers (VCSELs) are the emitters of choice. The intensity dynamics resonance is currently limited to around 30 GHz. Using the much faster polarization dynamics in VCSELs can be a promising alternative. The polarization dynamics resonance is mainly determined by the frequency difference of the two orthogonal linearly polarized modes, which is defined by the birefringence. We have experimentally investigated mechanisms for birefringence manipulation from large-scale down to on-chip solutions. Polarization oscillations with frequencies in excess of 200 GHz have been observed.
Vertical-cavity surface-emitting lasers (VCSELs) are commonly used in optical data communication mainly for short-haul transmissions in data centers. Spin-VCSELs can be a promising solution in order to overcome the bandwidth limitations of conventional VCSELs by utilizing the spin and polarization instead of current and intensity. Recently, their polarization dynamics have been enhanced to resonance frequencies of more than 200 GHz by implementing a large amount of birefringence into the laser cavity. For future applications onchip solutions to control the birefringence are preferred. For this purpose, a keyhole-shaped mesa-structure on standard wafer material for an 850nm oxide-confined AlGaAs-VCSEL is used. A variable heating current is driven into the semiconductor ridge connected to the mesa at a constant pump current. This creates an asymmetrical heat gradient. Here we investigate the polarization behaviour in a spin-VCSEL with thermally induced birefringence. We analyze the hysteresis in the heating and pump current of the sample to identify optimized working points near the polarization switching points.
In single-mode vertical-cavity surface-emitting lasers (VCSELs) the frequency difference between the two orthogonal modes, which is defined by the birefringence present in the cavity, is the key factor to enable ultrafast polarization dynamics in spin-lasers. This could be a promising alternative to overcome the bandwidth limitations in short-haul data transmission. Therefore, controlling the birefringence is indispensable to utilize the full potential of the polarization dynamics. Splittings of around 100GHz were realized with an on-chip approach by integration of a surface grating in an oxide-confined AlGaAs-based VCSEL. In this paper we present further details of the parameter search process using a three-dimensional vectorial optical VCSEL electro-magnetics (VELM) model. We also show the geometrical properties of the processed grating structure.
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.
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.
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.
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.
While the high-frequency performance of conventional lasers is limited by the coupled carrier-photon dynamics, spin-polarized lasers have a high potential to overcome this limitation and to push the direct modulation bandwidth beyond 100 GHz. The key is to utilize the ultrafast polarization dynamics in spin-polarized vertical cavity surface-emitting lasers (spin-VCSELs) which is decoupled from the intensity dynamics and its fundamental limitations. The polarization dynamics in such devices, characterized by the polarization oscillation resonance frequency, is mainly determined by the amount of birefringence in the cavity. Using an approach for manipulating the birefringence via mechanical strain we were able to increase the polarization dynamics to resonance frequencies of more than 40 GHz. Up to now these values are only limited by the setup to induce birefringence and do not reflect any fundamental limitations. Taking our record results for the birefringence-induced mode splitting of more than 250 GHz into account, the concept has the potential to provide polarization modulation in spin-VCSELs with modulation frequencies far beyond 100 GHz. This makes them ideal devices for next-generation fast optical interconnects. In this paper we present experimental results for ultrafast polarization dynamics up to 50 GHz and compare them to numerical simulations.
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.
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.
Spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) have a high potential to overcome limitations of conventional purely charge-based lasers. Probably the most important feature of such spin-lasers lies in their ultrafast spin and polarization dynamics which are decoupled from the intensity dynamics and their limitations. This yields the potential to modulate the polarization state of spin-VCSELs with frequencies far above the barriers known for the intensity modulation dynamics of conventional VCSELs. Such a quality makes them ideal devices for fast optical interconnects. While in conventional devices relaxation oscillations provide insights in the intensity dynamics and modulation bandwidth, in spin-VCSELs oscillations in the circular polarization degree are an ideal measure for investigating the dynamics of the coupled spin-photon system. These polarization oscillations (POs) can be generated using pulsed spin injection and have been proven to be much faster than intensity dynamics in the devices. Their frequency is mainly dependent on the birefringence in the cavities and can be increased by adding mechanical strain. Using an approach for manipulating the birefringence via mechanical strain we demonstrated tunable POs with frequencies up to 44 GHz, recently. Taking our results for strain-induced birefringence splitting of more than 250 GHz into account, the concept has the potential to overcome conventional limitations and to provide polarization modulation in VCSELs with bit rates beyond 100 Gbit=s. In this paper we investigate numerically the in uence of the spin decay rate on the PO amplitude and frequency in order to investigate potential limitations for future ultrafast polarization modulation schemes.
Controlling the coupled spin-photon dynamics in vertical-cavity surface-emitting lasers (VCSELs) is an attractive opportunity to overcome the limitations of conventional, purely charge based semiconductor lasers. Such spin-controlled VCSELs (spin-VCSELs) offer several advantages, like reduced threshold, spin amplification and polarization control. Furthermore the coupling between carrier spin and light polarization bears the potential for ultrafast polarization dynamics. By injecting spin-polarized carriers, the complex polarization dynamics can be controlled and utilized for high-speed applications. Polarization oscillations as resonance oscillations of the coupled spin- photon system can be generated using pulsed spin injection, which can be much faster than the intensity dynamics in conventional devices. We already demonstrated that the oscillations can be switched in a controlled manner. These controllable polarization dynamics can be used for ultrafast polarization-based optical data communication. The polarization oscillation frequency and therefore the possible data transmission rate is assumed to be mainly determined by the birefringence-induced mode-splitting. This provides a direct tool to increase the polarization dynamics toward higher frequencies by adding a high amount of birefringence to the VCSEL structure. Using this technique, we could recently demonstrate experimentally a birefringence splitting of more than 250 GHz using mechanical strain. Here, we employ the well-known spin-flip model to investigate the tuning of the polarization oscillation frequency. The changing mechanical strain is represented by a linear birefringence sweep to values up to 80πGHz. The wide tuning range presented enables us to generate polarization oscillation frequencies exceeding the conventional intensity modulation frequency in the simulated device by far, mainly dependent on the birefringence in the cavity only.
Spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) provide novel opportunities to overcome several limitations of conventional, purely charge-based semiconductor lasers. Presumably the highest potential lies in the spin-VCSEL's capability for ultrafast spin and polarization dynamics which can be significantly faster than the intensity dynamics in conventional devices. By injecting spin-polarized carriers, these coupled spin-photon dynamics can be controlled and utilized for high-speed applications. While relaxation oscillations provide insights in the speed and direct modulation bandwidth of conventional devices, resonance oscillations in the circular polarization degree step in for the spin and polarization dynamics in spin-VCSELs. These polarization oscillations can be generated using pulsed spin injection and achieve much higher frequencies than the conventional intensity relaxation oscillations in these devices. Furthermore polarization oscillations can be switched on and off and it is possible to generate short polarization pulses, which may represent an information unit in polarization-based optical communication. The frequency of polarization oscillations is mainly determined by the birefringence-induced mode splitting between both orthogonal linearly polarized laser modes. Thus the polarization modulation bandwidth of spin-VCSELs can be increased by adding a high amount of birefringence to the cavity, for example by incorporating mechanical strain. Using this technique, we could demonstrate tunable polarization oscillations from 10 to 40 GHz in AlGaAs-based 850nm VCSELs recently. Furthermore a birefringence-induced mode splitting of more than 250 GHz could be demonstrated experimentally. Provided that this potential for ultrafast dynamics can be fully exploited, birefringent spin-VCSELs are ideal devices for fast short-haul optical interconnects. In this paper we review our recent progress on polarization dynamics of birefringent spin-VCSELs and investigate numerically how ultrafast polarization oscillations can be utilized for data communication using simulations based on the spin-flip model.
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.
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.
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.
Compared to purely charge based devices, spintronic lasers offer promising perspectives for new superior device concepts. Especially vertical-cavity surface-emitting lasers with spin-polarization (spin-VCSELs) feature ultrafast spin and polarization dynamics. Oscillations in the circular polarization degree can be generated using pulsed spin-injection. The oscillations evolve due to the carrier-spin-photon system that is coupled for the linear modes in the VCSEL's cavity via the birefringence. The polarization oscillations are independent of the conventional relaxation oscillations and have the potential to exceed frequencies of 100 GHz. The oscillations are switchable and can be the basis for ultrafast directly modulated spin-VCSELs for, e.g., communication purposes. The polarization oscillation frequency is mainly determined by the birefringence. We show a method to tune the birefringence and thus the polarization oscillation frequency by adding mechanical strain to the substrate in the vicinity of the laser. We achieved first experimental results for high-frequency operation using 850 nm oxide-confined single-mode VCSELs. The results are compared with simulations using the spin-flip-model for high birefringence values.
Spintronic lasers offer promising perspectives for new concepts superior to options of purely charge-based devices. Especially spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) exhibit ultrafast spin and polarization dynamics. Using pulsed spin-injection, oscillations in the circular polarization degree can be generated, which have the potential to exceed frequencies of 100 GHz. The oscillations evolve due to coupling of the carrier-spin-photon system for linear modes via birefringence in the VCSEL's cavity. They are independent of the conventional relaxation oscillations and thus their usage can be the cornerstone for ultrafast directly modulated spin-VCSELs in the near future. After giving a short overview of the state of scientific and technical knowledge we will outline a method to control the polarization oscillations by multiple spin-injection pulses. It is possible to switch these 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. We investigate influences of the birefringence, which directly determines the oscillation frequency, by means of calculations with the spin-flip-model and experimental verification using 850 nm VCSELs. Furthermore we discuss experimental possibilities of increasing the birefringence and therefore the oscillation frequency, such that ultrashort pulses come into reach.
Spin-polarized lasers and especially spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) are at- tractive novel spintronic devices providing functionalities and characteristics superior to their conventional purely charge-based counterparts. This applies in particular to ultrafast dynamics, modulation capability and chirp control of directly modulated lasers. Here we demonstrate that ultrafast oscillations of the circular polarization degree can be generated in VCSELs by pulsed spin injection which have the potential to reach frequencies beyond 100 GHz. These oscillations are due to the coupling of the carrier-spin-photon system via the optical birefringence for the linearly polarized laser modes in the micro-cavity and are principally decoupled from conventional relaxation oscillations of the carrier-photon system. Utilizing these polarization oscillations is a very promising path to ultrafast directly modulated spin-VCSELs in the near future as long as an effective concept can be developed to modulate or switch these polarization oscillations. After briefly reviewing the state of research in the emerging field of spin-VCSELs, we present a novel concept for controlled switching of polarization oscillations by use of multiple optical spin injection pulses. Depending on the amplitude and phase conditions of the excitation pulses, constructive or destructive interference of polarization oscillations leads to an excitation, stabilization or switch-off of these oscillations. Furthermore even short single polarization bursts can be generated with pulse widths only limited by the resonance frequency of the polarization oscillation. Consequently, this concept is an important building block for using spin controlled polarization oscillations for future communication applications.
Spin-controlled semiconductor lasers are highly attractive spintronic devices providing characteristics superior to their conventional purely charge-based counterparts. In particular, spin-controlled 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. Most important, 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. We present a novel concept for ultrafast spin-VCSELs which has the potential to overcome the conventional speed limitation for directly modulated lasers by the relaxation oscillation frequency 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 these. 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. Different aspects of the spin and polarization dynamics, its connection to birefringence and bistability in the cavity, controlled switching of the oscillations, and the limitations of this novel approach will be analysed theoretically and experimentally for spin-polarized VCSELs at room temperature.
Spin polarized lasers, especially spin polarized vertical-cavity surface-emitting lasers (VCSEL) provide improved performance when compared to conventional, purely charge-based lasers. Advantages of these spin-enhanced devices lie in their reduced laser threshold, increased emission intensity, amplification of spin information, chirp control and possibilities for ultrafast modulation due to their faster dynamics. Utilizing a commercially available conventional VCSEL and additional spin polarized optical pumping we are able to enhance the modulation dynamics of a conventional VCSEL with new spin effects. Our experiments show polarization oscillations in the spin-photon system that result in oscillations of the circular polarization of the VCSEL emission. The resulting polarization oscillations are of significantly higher frequency than the direct modulation bandwidth of the VCSEL and persist for durations longer than the spin lifetime in the active region. Simulations based on a rate-equation model show that with an improved VCSEL layout it should be possible to reach oscillation frequencies well above 100 GHz. Here, we show that with multiple optical spin polarized pulses these oscillations can be coherently excited, amplified and also stopped. Using this excitation scheme, polarization oscillations faster than the purely charge-based dynamics can be achieved with half-cycle to multi-cycle duration. Various influences of unpolarized electrical bias, optical excitation power and delay between pulses will be discussed both theoretically and experimentally. Additionally, we analyze the qualification of this new concept for ultrafast optical communication.
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.
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