Researchers at Swinburne University of Technology used an advanced spectroscopy technique to quantify the energy required to bind two excitons into a biexciton state for the first time, they reported. The Swinburne team collaborated with researchers at Australian National University to directly measure the biexciton binding energy in tungsten disulfide (WS2), a 2D material that belongs to the transition metal dichalcogenide (TMDC) family of semiconductors. The team said its findings could be used to develop future applications based on the flow of biexcitons in TMDCs. The researchers used two-quantum, multidimensional coherent spectroscopy (2Q-MDCS), a technique used for probing doubly excited states, to identify and separate the optically excited biexciton in monolayer WS2. To unambiguously measure biexcitonic signatures in the atomically thin TMDC material, the researchers ran a sequence of ultrashort optical pulses with a precisely controlled phase relation and well-defined wave vectors. “By using multiple pulses with a high degree of precision, we can selectively and directly probe the doubly excited biexciton state while eliminating any contributions from singly excited exciton states,” Swinburne professor Jeff Davis said. The 2Q-MDCS method enabled the team to perform a direct experimental measurement of the biexciton binding energy. “This ability to directly excite the biexciton is inaccessible to more common techniques such as photoluminescence spectroscopy,” Davis said. Professor Jeff Davis, a corresponding author of the study on quantifying biexciton binding energy, leads Swinburne’s ultrafast spectroscopy lab. Researchers at the university used an advanced spectroscopic method to quantify the energy required to bind two excitons into a biexciton state. The work holds implications for the development of new quantum materials and quantum simulators. Courtesy of FLEET. When the researchers used 2Q-MDCS to observe the biexciton, a signal was generated from correlated excitons, which is an exciton pair that is interacting but unbound. The researchers consider the energy difference between the unbound two-exciton state and the biexciton to be the fundamental definition of biexciton binding energy, which is measured to be 26 ± 2 meV. “The energy difference between the biexciton peak and the correlated two-exciton peak is the best means to measure biexciton binding energy,” researcher Mitchell Conway said. “This was an exciting observation, since other spectroscopic techniques don’t observe these correlated excitons.” In addition, the researchers identified the nature of the biexciton in monolayer WS2. When using 2Q-MDCS to resolve the biexciton peaks, they observed a biexciton comprising two bright excitons in opposite spin, called a bright-bright intervalley biexciton. In contrast, photoluminescence measurements reporting biexcitons in monolayer WS2 are unable to identify the specific excitons involved. Previous techniques used to identify the biexciton have been limited to measuring photons from the biexciton-to-exciton transition. This transition may not reflect the precise energy of either the biexciton or exciton, relative to the ground state. Beyond increasing scientific understanding of biexciton dynamics and characteristic energy scales, the findings could support the development of biexciton-based devices such as more compact lasers and chemical sensors. The binding energy of excitons and exciton complexes such as biexcitons is enhanced in 2D materials due to the materials' reduced dimensionality. This increased binding energy makes the biexcitons more accessible, even at room temperature, and introduces the possibility of using biexcitons in new materials for a range of low-energy technologies. “Before we can apply these two-dimensional materials to the next generation of low-energy electronic devices, we need to quantify the fundamental properties that drive their functionality,” Conway said. The ability to accurately identify biexciton signatures in monolayer semiconductors could also help to advance development of new quantum materials and quantum simulators. The research was published in 2D Materials (www.doi.org/10.1088/2053-1583/ac4779).