Spin-polarized light-emitting diodes (LEDs) and lasers represent a unique class of optoelectronic devices where the emitted light carries a net circular polarization due to spin-polarized carrier injection. Unlike conventional LEDs and lasers, which rely solely on charge carrier recombination, these devices exploit the spin degree of freedom of electrons and holes to generate circularly polarized light. This functionality is critical for applications in quantum communication, spintronic-photonic integration, and chiral light-matter interactions.

The fundamental principle behind spin-polarized LEDs and lasers is the recombination of spin-polarized carriers, which results in the emission of circularly polarized photons. The degree of circular polarization (DCP) is directly linked to the spin polarization of the carriers at the moment of recombination. For instance, when electrons with a preferential spin orientation recombine with holes in a semiconductor, the emitted photons exhibit a net helicity, either left- or right-handed, depending on the spin orientation. The DCP is quantified as the difference between the intensities of left- and right-circularly polarized light, normalized to their sum.

Materials play a pivotal role in achieving efficient spin-polarized emission. Dilute magnetic semiconductors (DMS), such as GaMnAs or ZnMnSe, are widely studied due to their intrinsic spin-splitting properties under an external magnetic field or through exchange interactions with magnetic dopants. These materials enable spin alignment of carriers at relatively high temperatures, though challenges remain in achieving room-temperature operation. Quantum dots (QDs), particularly those made of II-VI or III-V compounds, are another promising platform due to their strong quantum confinement, which suppresses spin relaxation and enhances spin lifetime. For example, CdSe QDs embedded in a magnetic matrix have demonstrated high DCP at cryogenic temperatures.

Spin relaxation mechanisms are a critical factor limiting the performance of spin-polarized LEDs and lasers. The primary mechanisms include the Dyakonov-Perel (DP) effect, Elliott-Yafet (EY) scattering, and Bir-Aronov-Pikus (BAP) processes. In bulk semiconductors and quantum wells, the DP effect dominates at room temperature, where momentum-dependent spin splitting leads to rapid spin dephasing. In contrast, quantum dots exhibit suppressed DP relaxation due to discrete energy levels, but other mechanisms, such as hyperfine interaction with nuclear spins, become significant. Reducing spin relaxation requires careful engineering of material composition, strain, and dimensionality to minimize these effects.

Modulation speed is another key consideration for practical applications. The ability to switch the polarization state of emitted light at high frequencies is essential for data encoding in spintronic-photonic circuits. The modulation bandwidth is fundamentally limited by the spin lifetime of the carriers and the recombination rate. In DMS-based devices, modulation speeds up to several gigahertz have been achieved at low temperatures, but room-temperature operation remains challenging due to accelerated spin relaxation. Quantum dot systems, with their longer spin lifetimes, offer potential for higher-speed modulation, though electrical injection efficiency remains a bottleneck.

Electrical injection of spin-polarized carriers is a major challenge in these devices. Traditional methods involve ferromagnetic contacts or spin filters to inject polarized electrons into the semiconductor. However, conductivity mismatch between ferromagnetic metals and semiconductors often leads to inefficient spin injection. Tunnel barriers, such as MgO, have been employed to mitigate this issue, but their integration with optoelectronic structures adds complexity. Alternatively, using DMS layers as spin aligners can improve injection efficiency, but their Curie temperatures are typically below room temperature, limiting practicality.

Room-temperature operation is a significant hurdle for spin-polarized LEDs and lasers. Most demonstrations to date have been limited to cryogenic temperatures due to rapid spin relaxation and weak spin-splitting effects at higher temperatures. Advances in material design, such as using wide-bandgap semiconductors with strong spin-orbit coupling or engineered heterostructures, are being explored to enhance spin polarization at room temperature. For instance, GaN-based DMS materials show promise due to their high Curie temperatures and robust spin properties.

The efficiency of spin-polarized light emission is another critical metric. Even if spin injection is achieved, non-radiative recombination and spin-flip processes can degrade the DCP. Optimizing the active region’s design, such as using resonant tunneling structures or polarized cavity modes, can enhance the radiative recombination of spin-polarized carriers. Additionally, reducing defects and interfacial disorder in heterostructures is essential to minimize spin scattering.

Future directions for spin-polarized LEDs and lasers include the integration of topological insulators or two-dimensional materials with strong spin-momentum locking. These materials could enable novel spin injection schemes without the need for ferromagnetic contacts. Another avenue is the development of hybrid systems combining organic semiconductors with inorganic spin aligners, leveraging the long spin coherence times of organic materials.

In summary, spin-polarized LEDs and lasers offer unique capabilities for generating circularly polarized light through spin-injected carriers. While significant progress has been made in understanding the underlying physics and demonstrating proof-of-concept devices, challenges in room-temperature operation, electrical injection efficiency, and modulation speed remain. Advances in material science, heterostructure engineering, and spin control mechanisms will be crucial for realizing practical applications of these devices. The continued exploration of new material systems and device architectures holds promise for overcoming these limitations and unlocking the full potential of spin-polarized optoelectronics.