Twisted Photonics Enables Novel Chiral Lasing for Optical Applications.

The manipulation of light’s inherent angular momentum presents significant opportunities across diverse fields, from advanced imaging to optical sensing. Researchers are increasingly focused on harnessing not only the spin angular momentum, a property linked to polarisation, but also the orbital angular momentum, which relates to the spatial distribution of the light’s wavefront. A team led by Mingjin Wang from the Institute of Semiconductors, CAS, and Nianyuan Lv from Peking University, alongside Yuri Kivshar of the Australian National University and colleagues, report the first observation of orbital chiral lasing within a twisted bilayer photonic structure. Their work, detailed in a forthcoming publication titled “Orbital chiral lasing in twisted bilayer metasurfaces”, demonstrates single-mode lasing across a broad spectral range, achieved through the precise fabrication and optical pumping of a Moire-patterned metasurface created by bonding and rotating two semiconductor membranes.

The observed lasing emission exhibits distinct orbital chiral characteristics, stemming from the helical and non-Hermitian coupling of clockwise and counter-clockwise rotating guided resonances, a phenomenon confirmed through detailed polarisation analysis and self-interference measurements.
Researchers have demonstrated the generation of orbital angular momentum (OAM) through lasing within a twisted bilayer metasurface, establishing a novel method for light manipulation and expanding the scope of photonic applications. The fabricated device achieves single-mode operation across a 250 nanometre spectral range, emitting light characterised by a doughnut-shaped beam, a clear indicator of OAM. This innovative design diverges from conventional techniques that induce chirality by utilising the spin angular momentum of photons, offering a new paradigm for controlling light’s angular momentum with increased precision.

The observed lasing originates from helical and non-Hermitian couplings between collective guided resonances within the twisted bilayer structure, creating a unique interaction between light and the engineered material. Detailed polarisation-resolved imaging and the observation of off-center self-interference patterns corroborate these couplings, establishing a direct link between the structural chirality of the device and the chiral characteristics of the emitted light. Structural chirality, in this context, refers to the asymmetry of the device’s structure, which is crucial for inducing the desired optical properties.

Researchers confirm that the twisted bilayer structure effectively breaks mirror symmetry, inducing chirality in the generated OAM and enabling the creation of light beams with a helical phase front. This structural asymmetry introduces a phase gradient across the beam profile, causing the light to twist as it propagates. The achieved single-mode operation ensures a clean and well-defined beam profile, critical for applications demanding precise control of light’s properties, such as optical communications and high-resolution imaging.

Fabrication involves precise nanofabrication techniques to create the twisted bilayer structure with nanoscale precision, ensuring the desired optical properties are realised. Researchers carefully control layer thickness, twist angle, and material composition to optimise device performance, maximising OAM generation and minimising unwanted optical effects.

The theoretical understanding of the observed phenomena relies on the principles of non-Hermitian physics, which describes systems where energy is not necessarily conserved, and allows for the existence of exceptional points – singularities in the system’s parameter space. Operating near an exceptional point enhances sensitivity and contributes to the observed behaviour, allowing for enhanced control over the generated OAM and enabling the creation of novel optical effects.

Researchers are now focused on exploring the tunability of the OAM generated by varying the twist angle and metasurface design, aiming to create devices with increased power and efficiency. Investigating different materials with enhanced optical properties could further improve performance, enabling the generation of OAM with even greater control and precision. Exploring integration with other photonic components, such as waveguides and resonators, could lead to complex photonic circuits with unprecedented functionality.

The ability to generate and control OAM in a compact and efficient manner presents new possibilities for manipulating light across a variety of technological contexts, and offers a significant advancement over traditional methods of generating and controlling light’s angular momentum. Researchers envision integrated photonic circuits incorporating these structurally chiral metasurfaces, enabling the creation of compact and versatile optical devices for a wide range of applications.

The development of structurally chiral metasurfaces represents a significant step forward in photonics, offering a new paradigm for controlling light’s angular momentum and opening up new possibilities for advanced optical technologies. Researchers are confident that this innovative approach will lead to applications in communications, imaging, sensing, and fundamental science.

Researchers are also exploring the potential for using these metasurfaces to create novel optical elements, such as lenses and polarisers, with enhanced performance and functionality. The ability to tailor the optical properties of the metasurface with nanoscale precision allows for the creation of optical elements with unprecedented control over light’s behaviour.

The development of these structurally chiral metasurfaces is a testament to the power of interdisciplinary research, bringing together expertise in nanofabrication, optics, and materials science. This collaborative approach has been instrumental in overcoming the challenges associated with designing and fabricating these complex structures.

Sensitive sensing platforms could utilise the chiral characteristics of the emitted light to detect subtle environmental changes, such as the presence of specific molecules or the polarisation of light, opening up new possibilities for environmental monitoring and biomedical diagnostics. High-resolution imaging could benefit from the ability to image structures beyond the diffraction limit.