Circularly polarized light (CPL) emission and detection have emerged as critical functionalities in modern optoelectronic systems, particularly in the context of chiral two-dimensional materials and twisted heterostructures. These materials exhibit unique optical and electronic properties due to their structural asymmetry or engineered interlayer interactions, enabling selective interaction with left- or right-handed circularly polarized light. The ability to generate, modulate, and detect CPL with high fidelity has significant implications for applications such as quantum encryption and stereoscopic displays, where polarization control is paramount.

Chiral 2D materials, such as certain transition metal dichalcogenides (TMDCs) or engineered graphene derivatives, possess inherent asymmetry that breaks mirror symmetry, leading to preferential absorption or emission of one helicity of CPL. This property arises from spin-momentum locking or valley-selective optical transitions. For instance, monolayer WS2 or MoS2 with induced chirality through chemical functionalization or strain engineering can exhibit circular dichroism with dissymmetry factors reaching 0.1 to 0.3 in the visible spectrum. Such materials enable compact CPL sources without the need for external optical elements like quarter-wave plates, which are bulky and limit device miniaturization.

Twisted heterostructures, formed by stacking atomically thin layers with a controlled rotational misalignment, introduce moiré patterns that modify the electronic and optical properties of the system. At specific twist angles, such as the magic angle in graphene-based systems, correlated electronic states emerge, but even at non-magic angles, the broken symmetry can lead to pronounced CPL sensitivity. For example, a twisted bilayer of MoSe2/WSe2 at 15 degrees exhibits valley-polarized emission with a degree of circular polarization exceeding 80% at low temperatures due to interlayer exciton formation. The twist angle serves as a tunable parameter to optimize CPL interaction, offering a versatile platform for polarization-sensitive optoelectronics.

In the context of CPL emission, chiral 2D materials can directly generate polarized light through electroluminescence. When an electric current injects spin-polarized carriers into a chiral material, the recombination process emits light with a net circular polarization. Recent studies have demonstrated electroluminescence with a circular polarization degree of up to 40% at room temperature in chirally modified TMDCs. This performance is competitive with conventional CPL sources while offering superior integration potential. Twisted heterostructures, on the other hand, leverage interlayer excitons with long-lived valley polarization, enabling sustained CPL emission even under ambient conditions.

For CPL detection, chiral materials function as polarization-discriminating photodetectors. The absorption asymmetry between left- and right-handed CPL translates into a photocurrent imbalance, allowing direct polarization readout without external optics. Devices based on chiral graphene hybrids have achieved polarization discrimination ratios of 10:1 in the near-infrared range. Twisted heterostructures enhance this capability through moiré-induced asymmetry, with some systems demonstrating wavelength-specific CPL detection tunable via the twist angle. The spectral selectivity and compact form factor make these detectors ideal for integrated photonic circuits.

Quantum encryption represents a major application for CPL-active 2D materials. The polarization state of photons can encode quantum information, and chiral materials enable the generation and decoding of such states at the chip level. The inherent scalability of 2D materials allows for the fabrication of arrays of polarization-entangled photon emitters, a requirement for quantum key distribution systems. Experimental implementations have shown that chiral TMDCs can produce entangled photon pairs with a polarization concurrence above 0.7, meeting the threshold for practical quantum communication protocols. The compatibility of these materials with silicon photonics further facilitates their integration into existing infrastructure.

Stereoscopic displays also benefit from advances in CPL emission from 2D materials. Current 3D displays rely on passive polarization filters, which reduce brightness and limit viewing angles. Active CPL emitters based on chiral materials can directly project left- and right-handed images with high polarization purity, enabling brighter and more energy-efficient displays. Prototype devices using chirally patterned perovskites have achieved polarization purities of 95% at display-relevant brightness levels of 1000 cd/m². The ultrafast response times of 2D materials additionally eliminate motion artifacts, a common issue in conventional 3D displays.

The performance metrics of CPL-active 2D materials and heterostructures are continually improving through advances in material synthesis and device engineering. For emission, key figures of merit include the degree of circular polarization, quantum efficiency, and spectral range. State-of-the-art chiral emitters cover wavelengths from 400 nm to 1600 nm, addressing both visible and telecom applications. For detection, the polarization discrimination ratio, responsivity, and response speed are critical, with best-in-class devices offering nanosecond-scale response times and responsivities exceeding 0.1 A/W.

Challenges remain in achieving uniform chirality over large areas and maintaining performance at elevated temperatures. Scalable fabrication techniques such as chemical vapor deposition with chiral precursors or template-assisted growth are under development to address these issues. For twisted heterostructures, precise angle control during stacking is essential, with advanced transfer techniques achieving angular accuracy below 0.1 degrees. Environmental stability is another consideration, particularly for materials susceptible to oxidation, necessitating encapsulation strategies compatible with device integration.

The future trajectory of CPL-active 2D materials points toward hybrid systems combining chiral and twisted components for enhanced performance. For example, a chirally functionalized twisted heterostructure could leverage both molecular asymmetry and moiré effects for unprecedented polarization control. Such systems could push the degree of circular polarization toward unity while maintaining high quantum yields, unlocking new applications in quantum computing and ultra-secure communication.

In summary, chiral 2D materials and twisted heterostructures provide a versatile platform for CPL emission and detection, with performance metrics that meet or exceed conventional technologies. Their atomic thickness and compatibility with on-chip integration make them ideal candidates for next-generation quantum encryption and stereoscopic display systems. As synthesis and device fabrication techniques mature, these materials are poised to enable compact, efficient, and high-performance optoelectronic systems that leverage the unique properties of circularly polarized light.