Oh Seung-Hoon, Kim Jinuk, Ha Junseo, Son Gibeom, An Kyungwon
Department of Physics and Astronomy & Institute of Applied Physics, Seoul National University, Seoul, Korea.
Korea Research Institute of Standards and Science, Daejeon, Korea.
Light Sci Appl. 2024 Sep 5;13(1):239. doi: 10.1038/s41377-024-01591-2.
Lasing threshold in the conventional lasers is the minimum input power required to initiate laser oscillation. It has been widely accepted that the conventional laser threshold occurring around a unity intracavity photon number can be eliminated in the input-output curve by making the so-called β parameter approach unity. The recent experiments, however, have revealed that even in this case the photon statistics still undergo a transition from coherent to thermal statistics when the intracavity mean photon number is decreased below unity. Since the coherent output is only available above the diminished threshold, the long-sought promise of thresholdless lasers to produce always coherent light has become questionable. Here, we present an always-coherent thresholdless laser based on superradiance by two-level atoms in a quantum superposition state with the same phase traversing a high-Q cavity. Superradiant lasing was observed without the conventional lasing threshold around the unity photon number and the photon statistics remained near coherent even below it. The coherence was improved by reducing the coupling constant as well as the excited-state amplitude in the superposition state. Our results pave a way toward always-coherent thresholdless lasers with more practical media such as quantum dots, nitrogen-vacancy centers and doped ions in crystals.
传统激光器中的激光阈值是启动激光振荡所需的最小输入功率。人们普遍认为，通过使所谓的β参数接近1，可以在输入-输出曲线中消除在腔内光子数约为1时出现的传统激光阈值。然而，最近的实验表明，即使在这种情况下，当腔内平均光子数降至1以下时，光子统计仍会从相干统计转变为热统计。由于相干输出仅在降低后的阈值以上可用，长期以来无阈值激光器始终产生相干光的承诺变得值得怀疑。在此，我们展示了一种基于处于量子叠加态且具有相同相位的两能级原子超辐射的始终相干无阈值激光器，该原子穿过一个高Q腔。观察到超辐射激光在光子数约为1时没有传统的激光阈值，并且即使在其以下光子统计仍保持接近相干。通过降低耦合常数以及叠加态中的激发态振幅，相干性得到了改善。我们的结果为使用更实用的介质（如量子点、氮空位中心和晶体中的掺杂离子）制造始终相干无阈值激光器铺平了道路。
