Transforming solid-state single photon sources using multifunctional metalenses

Quantum photonics is one of the important research directions in the quantum field, which utilizes the unique properties of light at the quantum level. The core of this field is the deterministic single photon source, which sequentially emits individual photons through spontaneous emission and is the cornerstone of quantum communication, computing, and secure encryption. However, under environmental conditions, the interaction between light and solid-state single photon emitters (SPE, such as quantum dots, diamond nitrogen vacancy color centers, defects in two-dimensional materials) is very weak and difficult to control. 

Therefore, the resulting single photon source has many problems, such as low collection efficiency, lack of directionality, and poor polarization/phase characteristics. To create complex quantum optical states and fully utilize the multiple degrees of freedom of a single photon (such as polarization and orbital angular momentum), it is necessary to construct a complex optical system composed of a series of discrete components (such as polarizers, wave plates, lenses, spatial light modulators, etc.). This method is inherently unfriendly due to its large configuration, difficult alignment, instability, high loss, and limited functionality.

Schematic diagram of multi-dimensional manipulation of hBN quantum emission using multifunctional metalenses

Design and characterization of polarization beam splitting metalenses
Optical metasurfaces are extremely thin nanoantennas arranged in carefully designed patterns, with unprecedented potential in manipulating all properties of classical and non classical light, providing a unique and promising platform for quantum nanophotonics. Especially, optical metasurfaces provide a new platform for generating and manipulating quantum states of photons, and offer new methods for controlling quantum light in integrated quantum photon devices.

It is reported that a joint research team led by Dr. Chi Li and Dr. Haoran Ren from Monash University, Professor Junsuk Rho from Pohang University of Science and Technology, and Professor Igor Aharonovich from Sydney University of Science and Technology has developed a new type of multifunctional metalenses, redefining the control of SPE quantum emission in hexagonal boron nitride (hBN) at room temperature. This research achievement showcases the rapid development of quantum photonics and has been published in the eLight journal under the title "Arrarly structured quantum emission with multifunctional metals".

This designed superlens can simultaneously map quantum emissions from superbright defects in hBN and imprint any wavefront onto the orthogonal polarization state of the light source, while shaping directionality, polarization, and orbital angular momentum (OAM). Therefore, this hybrid quantum superlattice lens system can simultaneously manipulate multiple degrees of freedom of the quantum light source. In its design, researchers used low loss hydrogenated amorphous silicon as the material for constructing the metalens unit. The extinction coefficient of this material in the hBN SPE emission spectrum can be ignored, thus achieving a collection efficiency of up to 0.3. Using this design, researchers created three different polarization separation superlenses and measured them using SPE to verify their ability to simultaneously control the directionality and polarization of single photon emission. In addition, researchers have also implemented more complex superlenses that can encode different helical phase wavefronts (OAM modes) in addition to directionality and polarization.

This study demonstrates the ability of superlenses to manipulate the quantum emission of hBN defects, allowing arbitrary wavefronts to be imprinted onto orthogonal polarization states. The multifunctionality of metalenses provides an important foundation for achieving advanced quantum computing, secure communication, and enhanced quantum sensing. Researchers believe that this quantum metasurface has the excellent ability to independently and synchronously control multiple degrees of freedom of photons, and will rapidly develop as a unique enabling platform for generating, routing, and manipulating quantum optical states.

Despite the pioneering nature of this study, the multifunctional metalens used to manipulate single photon emission from hBN SPE remains an external component, i.e. separate from the photon source. By adding transparent spacers, hBN SPE can be directly integrated into the superlens, but adjusting the device architecture and arrangement method is not an easy task and further research is needed. In addition, there is still room for development of integrated quantum superlattice surface chips that can simultaneously generate photon states and engage in high-dimensional quantum entanglement. In addition, the static properties of quantum metasurfaces that have been demonstrated so far severely limit their functional range, thus requiring the development of spatiotemporal quantum metasurfaces to provide new research avenues and breakthroughs for planar quantum photonics.

Source: China Optical Journal Network