Optical Gain Spectrum and Confinement Factor of a Monolayer Semiconductor in an Ultrahigh-Quality Cavity

Tianhua Ren; Junyong Wang; Kaizhen Han; Yuye Kang; Annie Kumar; Gong Zhang; Zhe Wang; Rupert F Oulton; Goki Eda; Xiao Gong

doi:10.1021/acs.nanolett.3c03357

Optical Gain Spectrum and Confinement Factor of a Monolayer Semiconductor in an Ultrahigh-Quality Cavity

Nano Lett. 2023 Dec 27;23(24):11601-11607. doi: 10.1021/acs.nanolett.3c03357. Epub 2023 Dec 8.

Authors

Tianhua Ren^{1

2}, Junyong Wang^{3

4}, Kaizhen Han^{1

2}, Yuye Kang^{1

2}, Annie Kumar¹, Gong Zhang¹, Zhe Wang^{1

2

5}, Rupert F Oulton⁶, Goki Eda^{2

3

5}, Xiao Gong^{1

2}

Affiliations

¹ Department of Electrical and Computer Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Singapore.
² Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore.
³ Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore.
⁴ CAS Key Laboratory of Nanophotonic Materials and Devices & Key Laboratory of Nanodevices and Applications, i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, People's Republic of China.
⁵ Department of Chemistry, National University of Singapore, 2 Science Drive 3, Singapore 117543, Singapore.
⁶ The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, U.K.

PMID: 38063776
DOI: 10.1021/acs.nanolett.3c03357

Abstract

Two-dimensional (2D) semiconductors have attracted great attention as a novel class of gain materials for low-threshold, on-chip coherent light sources. Despite several experimental reports on lasing, the underlying gain mechanism of 2D materials remains elusive due to a lack of key information, including modal gain and the confinement factor. Here, we demonstrate a novel approach to directly determine the absorption coefficient of monolayer WS₂ by characterizing the whispering gallery modes in a van der Waals microdisk cavity. By exploiting the cavity's high intrinsic quality factor of 2.5 × 10⁴, the absorption coefficient spectrum and confinement factor are experimentally resolved with unprecedented accuracy. The excitonic gain reduces the WS₂ absorption coefficient by 2 × 10⁴ cm^-1 at room temperature, and the experimental confinement factor is found to agree with the theoretical prediction. These results are essential for unveiling the gain mechanism in emergent, low-threshold 2D-semiconductor-based laser devices.

Keywords: TMD; absorption; confinement factor; gain; optical cavity; quality factor.