Efficient and Controllable Emission of Circularly Polarized Light From Resonant Metasurfaces



For the applications of classical and quantum optics information processing, an ultra-compact circularly polarized light source is an essential element. The advancement of two technologies, chiral optical cavities and quantum materials, is essential to the growth of this subject.

Large radiating angles, restricted DOP, and incoherent broadband emission are drawbacks of traditional methods for circularly polarized photoluminescence. Low efficiency, energy waste, undesirable handedness, and emission directions all limit their usefulness. Only in certain power ranges can chiral microlasers produce directional output and substantial DOPs. In particular, their subthreshold performances suffer greatly. The method for managing chiral spontaneous emission and chiral lasing simultaneously is currently lacking.

Researchers from the Australian National University and Harbin Institute of Technology use the physics of chiral quasi bound states in the continuum (BICs) in a new study that was just published in the journal Science today, September 8, to show how effectively and controllably resonant metasurfaces can emit circularly polarized light.

Incident light with a single circular polarization state can be linked into the nanostructures at the C sites, producing noticeably stronger local electromagnetic fields. The alternative polarization state transmits almost flawlessly and is disconnected. Although these qualities are well recognized, light emissions hardly ever use them. "This is primarily due to the C points' typical deviation from the band's bottom. Because of their low Q factors, they cannot be stimulated for laser activities, according to Zhang.

Combining the local state density with the inherent chirality at C sites is a crucial first step in realizing chiral light emission. The Q factor of the associated chiral quasi-BIC can be maximized by shifting one C point to the bottom of the band. Fermi's golden rule states that one circularly polarized spontaneous emission radiates at a higher rate while the other polarization is suppressed. As the emission angle increases, the Q factor and radiation rate both drop precipitously. High-purity and very directed light emission is therefore predicted to occur close to the G point.

"Of course, the other C point can enable opposite handedness and high chirality identical to this. However, that point likewise deviates from the highest Q factor and can only be slightly improved. Therefore, with high directionality in the normal direction, our metasurface only generates one near-unity circular polarization, according to Zhang.

The maximizing of chirality in the normal direction and the control of C points in momentum space are closely related. In theory, breaking both in-plane and out-of-plane mirror reflection symmetries simultaneously contributes to the manifestation of chirality. The scientists in this study have included an out-of-plane asymmetry, or the tilt of nanostructures. One out-of-plane asymmetry can relocate one C point to a G point for an in-plane asymmetry. "We observe linear dependence between two types of asymmetries. This makes improving chirality in the normal direction quite simple, according to Zhang.

In the experiment, the scientists created the metasurfaces using a single phase of slanted reactive ion etching and then they measured the emissions. With a DOP of 0.98 and a far-field divergence angle of 1.06 degrees, they have effectively demonstrated the chiral emissions under the excitation of a nanosecond laser. The control of the C point in momentum space and the local density of state enable the realization of our circular light source. According to Zhang, "This is why we can obtain the high Q, high directionality, and high purity circular polarization emission from spontaneous emission to lasing. It is independent of the excitation power."

The chiral quasi-BIC, in contrast to conventional methods, offers a method for simultaneously altering and controlling the radiation patterns, spectra, and spin angular momentum of photoluminescence and lasing without any spin injection. This strategy might enhance the chiral light sources that are currently designed and increase their usefulness in photonic and quantum systems.

By HARBIN INSTITUTE OF TECHNOLOGY 

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