Shanghai Institute Of Optical Mechanics, Chinese Academy Of Sciences, Makes Progress in Symmetry Engineering-based Ultra-low-threshold Rydberg State Single-mode Polarized Exciton Lasing

Recently, a team of researchers from the Research Center for Infrared Optical Materials, Shanghai Institute of Optics and Precision Machinery (SIPM) of the Chinese Academy of Sciences (CAS), Hongxing Dong and Long Zhang, in collaboration with the Huazhong University of Science and Technology (HUST), reported a new mechanism for generating dynamically tunable single-mode lasers from exciton polariton excitations with ultralow thresholds, and the related research results are presented in the paper "Rydberg State Single-Mode Polariton Lasing". Rydberg State Single-Mode Polariton Lasing with Ultralow Threshold via Symmetry Engineering" was published in Nano Letters.
The realization of single-mode nano-lasers with high energy efficiency and tunable bandwidth is essential for many technological applications such as all-optical information processing, optical encryption, super-resolution biomedical imaging and miniaturized smart displays. Several strategies currently adopted to obtain single-mode lasers still have limitations. In addition, the current means of regulating the emission wavelength are inherently static, and new mechanisms capable of generating dynamically tunable single-mode lasers need to be explored. Exciton polarized excitons, which have the properties of both photons and excitons, have received much attention in recent years. Compared with conventional lasers, polarized exciton lasers do not require set-house number inversion and can achieve mode tuning through the modulation of quantized exciton polarized exciton states, which is an ideal platform for the study of dynamically tunable ultra-low-threshold lasers.

Fig. 1 (a) Schematic diagram of a quantum-polarized exciton state in a symmetric trap; (b) simulated PL spectra corresponding to the experimental configuration shown in a; (c) quantum-polarized exciton state in an asymmetric trap; (d) simulated PL spectra corresponding to the experimental configuration shown in c; (e) SEM image of a typical ZnO microrod; (f) angularly-resolved PL image of a typical ZnO microrod with a radius of 1.27 μm. (g) Spatially resolved PL image of a polarized exciton confined in a trap.
The researchers report the realization of tunable single-mode polarized exciton lasing from highly excited Rydberg states through symmetry engineering. Breaking the symmetry of the polarized exciton wave function through a potential trap and controlling the spatial overlap between the gain region and the eigenmode enable the generation of reversible and dynamic single-mode polarized exciton lasers from quantized polarized exciton states. Increasing the asymmetry of the potential well enables single-mode lasing even in highly excited states with a principal quantum number of N = 14. In addition, the lasing threshold can be lowered by six orders of magnitude compared to conventional lasing due to excellent reservoir eigenmode overlap and effective spatial confinement. The mechanism elaborated in the research results does not depend on any specific material and is applicable to various polarized exciton systems, which opens up a new avenue for the development of threshold-free polarized exciton lasers with dynamic tunability.

Fig. 2 (a-d) Pumping position dependence of spatially resolved PL images in a symmetric trap. Δx given in each figure represents the displacement of the excitation laser spot from the trap center; (e) spectra corresponding to the images shown in a-d, respectively; and (f-i) pumping position dependence of spatially resolved PL images in an asymmetric trap. Δx represents the displacement of the pumping laser spot from the antipodal plane of the ground-state polariton wave function. Pumping power: 1.25 Pth; (j) Spectra corresponding to the images shown in f-1, respectively.

Fig. 3 (a-d) Spatially resolved PL images of a highly asymmetric trap selectively pumped at N = 2, 3, 7, and 14 excited states, respectively, with pump power: 1.35 Pth; (e) Spatially resolved PL images of a highly asymmetric well selectively pumped at N = 2, 3, 7, and 14 excited states, respectively, with pump power: 1.35 Pth.