Investigation of nonlinear optical properties of large area monolayer molybdenum disulfide (MoS2) and its application as planar waveguide saturable absorber / Chew Jing Wen

Chew , Jing Wen (2023) Investigation of nonlinear optical properties of large area monolayer molybdenum disulfide (MoS2) and its application as planar waveguide saturable absorber / Chew Jing Wen. PhD thesis, Universiti Malaya.

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Abstract

Transition metal dichalcogenides are a type of two-dimensional (2D) material that has been widely studied in the last decade. Among the transition metal dichalcogenides, 2D molybdenum disulfide (MoS2) has attracted much attention among the scientific community due to its unique electrical and optical properties. To obtain MoS2 in its 2D form, top-down exfoliation technique from its bulk material or bottom-up synthesis method can be used. One of the common methods to obtain 2D MoS2 that have been used in photonics studies is chemical vapor deposition (CVD), which is a bottom-up synthesis approach. It relies on the chemical reaction between molybdenum oxide and sulfur precursors at high temperature to produce 2D MoS2. This method provides advantages such as better control in MoS2 thin film thickness uniformity, which allows one to control the properties of 2D MoS2 with higher confidence level. In this work, the nonlinear optical (NLO) properties, particularly the nonlinear refractive index, of a CVD-grown large area monolayer MoS2 is investigated at the telecommunication wavelength band (or C-band) using time-resolved Z-scan technique. The result shows that it exhibits a nonlinear refractive index of 1.40  10-13 m2 W-1, which is a few orders of magnitude larger than that of common bulk dielectric materials. It shows that monolayer MoS2 is a good 2D material candidate for NLO application in C-band wavelength range. To utilize the NLO properties of the monolayer MoS2, different coating lengths of the MoS2 film is transferred onto a planarized optical waveguide using a polymer-assisted transfer method. These MoS2-coated waveguides show saturable absorption properties measured using I-scan technique. Q-switched pulse lasers are produced when they are integrated into an erbium-doped fiber laser cavity. It is found that performance of the Q-switched fiber laser (in terms of the repetition rate, pulse duration, average output power, pulse energy and wavelength tunability range) does not only scale monotonically with the coating length but also depends on the coating quality of the coated MoS2 thin film as well. Among the saturable absorbers (SAs), the waveguide with MoS2 coating length of 7.2 mm shows the widest wavelength tunability. This observation can be attributed to the concentration of vacancy-defect in the thin film. This is verified upon inspections using X-ray photoelectron spectroscopy and optical transmission spectroscopy. Therefore, it can be concluded that a long coating length of MoS2 thin film with minimum exposed edge sites enhances the performance of Q-switching and its wavelength tunability. This work is valuable such that it paves a way for the development of a compact laser source using MoS2 as the SA, which in turn will bring benefits for the development in all-planar waveguide laser technology.

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