1、 The 'dilemma' of chiral lasers: the contradiction between symmetry and efficiency
The core value of chiral lasers lies in their "chirality" - the helical polarization state of left-handed (LHC) or right-handed (RHC), which enables specific interactions with chiral molecules (such as amino acids and drug molecules) and enables "precise recognition" in biosensing; In quantum optics, its orbital angular momentum can be used to encode quantum information and increase computational capacity. However, traditional design concepts have inherent flaws: in order to achieve chirality, it is necessary to break the symmetry of the light field (such as using vortex structures), but this structure is very sensitive to environmental disturbances and can easily lead to mixing of chiral states (such as simultaneous emission of left-handed and right-handed rotations); On the other hand, complex nanostructures can increase the transmission loss of light within the resonant cavity, leading to an increase in laser threshold and a decrease in efficiency. For example, the "annular resonant cavity" structure of traditional vortex lasers requires multiple reflections of light to form vortex modes, resulting in a light loss of about 30% and an efficiency that is difficult to exceed 30%. These issues seriously constrain the industrial application of chiral lasers.
2、 Photonic crystal scheme: using "collective oscillation" to improve efficiency, using "asymmetric pumping" to maintain chirality
The innovation of Zheng Wanhua's team lies in combining the "collective oscillation effect" of photonic crystals with "asymmetric pumping" technology, fundamentally solving the contradiction between symmetry and efficiency.
Circular active photonic crystal: improving transmission efficiency: The team has designed a circular boundary photonic crystal made of semiconductor gain medium (such as InGaAs), whose periodic refractive index distribution forms a "micro resonant cavity". When the pump light is irradiated, the modes in the photonic crystal undergo "collective oscillation" - multiple photon modes vibrate synchronously, forming a highly concentrated beam (with a spot size 50% smaller than traditional structures), greatly reducing light divergence losses, and increasing transmission efficiency to over 60% (twice as high as traditional structures).
Asymmetric pumping: Achieving single chirality: Traditional uniform pumping can lead to simultaneous oscillation of both left-handed and right-handed modes, making it impossible to achieve single chirality. The team uses "asymmetric pumping" (such as strong and weak laser irradiation on one side) to break the symmetry of photonic crystals, allowing only one chiral mode to meet the "phase matching condition", thereby achieving single left-handed or right-handed emission. Experiments have shown that the chiral purity is over 95% (far higher than the 70% of traditional methods).
3、 Experimental verification: "Fork shaped stripes" and "phase singularities" clarify chiral characteristics
To verify the characteristics of the new chiral laser, the team conducted a "self interference experiment" - dividing the laser into two beams through a beam splitter, superimposing them, and observing the interference fringes. The results show that:
Fork shaped stripes: The stripe fork of left-handed laser is clockwise, while right-handed laser is counterclockwise. This is a typical characteristic of the orbital angular momentum of vortex light (when two vortex light beams are superimposed, the difference in orbital angular momentum causes the stripes to bifurcate);
Phase singularity: There is a clear dark spot at the center of the stripe, known as the "phase singularity", indicating that the phase of light is discontinuous at the center (with a sudden change from 0 to 2 π), which is the core sign of vortex rotation.
The experimental results show that the new chiral laser has clear chiral specificity and significantly better stability than traditional methods (when the ambient temperature changes by ± 10 ℃, the chiral purity only decreases by 3%).
4、 From "laboratory" to "application end": more compact and efficient chiral lasers
Compared with traditional vortex lasers, the new chiral laser has three major advantages:
Compact structure: Adopting photonic crystal structure, it can achieve "chip level" integration (with a size of only a few millimeters), suitable for portable devices or large-scale array applications;
Higher efficiency: The transmission efficiency reaches over 60%, which is twice as high as traditional solutions and reduces energy consumption;
Flexible regulation: By adjusting the pump mode (such as changing the pump intensity distribution), rapid switching between left and right rotation can be achieved, providing new degrees of freedom for chiral regulation.
These advantages make it have broad application prospects in multiple fields:
Quantum computing: The orbital angular momentum of chiral lasers can encode more quantum information, increasing computational capacity;
Biosensing: Accurately identify chiral molecules (such as left-handed isomers in drugs) to avoid the side effects of right-handed isomers;
Optical communication: By switching chiral states, information channels can be increased and communication bandwidth can be doubled (theoretically).
Conclusion
The new method of "photonic crystal+asymmetric pumping" chiral laser developed by Academician Zheng Wanhua's team has successfully solved the two major problems of "loss of chiral specificity" and "low efficiency" in traditional chiral lasers, achieving the combination of single chiral emission and efficient transmission. This new type of chiral laser not only has a compact structure and high integration, but also provides new degrees of freedom for chiral regulation, and is expected to become a "core light source" in fields such as quantum computing and biosensing. With further technological optimization (such as lowering thresholds and improving stability), mass production of chip level chiral lasers can be achieved in the next 3-5 years, promoting the practical process of chiral optics and providing important support for China's development in quantum technology, biomedical and other fields.
