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Amplified spontaneous emission is a physical phenomenon that entails the amplification of the light spontaneously emitted by excited particles, due to photons of the same frequency triggering further emissions. This phenomenon is central to the functioning of various optoelectronic technologies, including lasers and optical amplifiers (i.e., devices designed to boost the intensity of light).

The excitation of a material with high-energy photons can produce what is known as an electron-hole plasma. This state is characterized by the dense presence of negatively charged particles (i.e., electrons) and positively charged vacancies (i.e., holes).

Researchers at Wuhan University recently observed amplified spontaneous emission originating from degenerate electron-hole plasma in a 2D semiconductor, namely suspended bilayer tungsten disulfide (WS₂). Their paper, in Â鶹ÒùÔºical Review Letters, could pave the way for the development of new optoelectronic technologies based on 2D semiconductors.

"This paper builds on our earlier studies of highly excited states in 2D transition metal dichalcogenide materials, where we observed an anomalous sharp increase in photoluminescence (PL) intensity at a threshold excitation power," Yiling Yu, senior author of the paper, told Â鶹ÒùÔº. "This phenomenon indicated a significant phase change in the excited electron-hole system, which we hypothesized would lead to a dramatic change in the optical dielectric function."

The primary objective of the recent study by Yu and her colleagues was to better understand the evolution of the dielectric function during the sharp increase in photoluminescence that they observed as part of their earlier research. In addition, the team hoped to uncover physical mechanisms that drive this phase transition-related PL increase.

"To achieve this, we performed two key experiments," explained Yu. "First, we used transient differential transmission spectroscopy on bilayer WS₂ under continuous-wave laser excitation. This allowed us to detect optical gain occurring simultaneously with a sharp increase in PL intensity."

Following this first experiment, Yu and her colleagues set out to better understand the origins of the amplified spontaneous emission they observed in WS₂. To do this, they measured the photoluminescence spectrum of a WS₂ sample integrated with a Fabry-Pérot cavity, which ultimately unveiled signatures of an electron-hole plasma phase.

"Together, these experiments confirmed that the optical gain and amplified spontaneous emission originate from the electron-hole plasma state in the highly excited WS₂ system," said Yu.

The most notable achievement of this recent work is that it demonstrated the existence of an amplified spontaneous emission emerging from degenerate electron-hole plasma in 2D semiconductors for the first time. In addition, the researchers gathered insight into the evolution of the optical dielectric response across the phase transition of the excited electron-hole system they studied.

"The capability of the strong many-body interaction to sustain the degenerate electron-hole plasma and resulting optical gain, highlights the potential of this excited electron-hole phase to achieve novel macroscopic quantum states," said Yu. "This could advance both fundamental understanding and optoelectronic applications."

The findings gathered by Yu and her colleagues could soon inspire more research groups to explore the emergence of ASE in 2D semiconductors, which could lead to more interesting discoveries. In addition, they could contribute to the future design and fabrication of advanced optoelectronics based on 2D materials.

"We plan to leverage this excited electron-hole phase as a platform to establish pathways toward superfluorescence and Bardeen-Cooper-Schrieffer-like macroscopic quantum phenomena, and also achieve efficient lasing by combining these plasma with tailored photonic structures," added Yu.