Author: Ahmed, Safayet
Title: Novel two-dimensional metal chalcogenides for nonlinear optics and ultrafast photonics
Advisors: Tsang, Yuen Hong (AP)
Degree: Ph.D.
Year: 2022
Subject: Two-dimensional materials
Transition metal compounds
Nonlinear optics
Hong Kong Polytechnic University -- Dissertations
Department: Department of Applied Physics
Pages: xxxii, 247 pages : color illustrations
Language: English
Abstract: Nonlinear optics is a major field of optics that studies light-matter interactions in nonlinear medium. For fundamental physics and optoelectronic device development, the nonlinear optical (NLO) properties of two-dimensional (2D) materials are intriguing. The excellent NLO properties of 2D materials lead to different photonics applications like transmission, photon generation, imaging and designing of several photonic devices including optical switch, optical memories, optical modulators, ultrafast pulsed laser, and frequency converter. So far, beyond graphene most of the 2D materials-based nonlinear optics research are performed by exploiting the 2D-transition metal dichalcogenide (TMD) materials. However, different other 2D-transition metal chalcogenide (TMC) materials such as transition metal monochalcogenides (TMMCs) and transition metal trichalcogenides (TMTs) are comparatively less explored in the field of nonlinear optics. Hence, in this thesis the NLO properties and ultrafast photonics applications of group III, IVA TMMCs, and group IVB TMTs are investigated.
The first section of the thesis is focused to a detailed analysis of the NLO activities, including second harmonic generation (SHG), two-photon excited fluorescence (TPEF), and NLO absorption, as well as the broadband ultrafast photonics applications of 2D-gallium sulfide (GaS; group III TMMC). The first demonstration of layer-dependent crystallographic symmetrical characteristics of GaS nanomaterials identifies the non-centrosymmetric nature of the odd layers of GaS. Additionally, the superior efficiency of generated SHG signal is realized from the estimated the second-order susceptibility (χ(²)) value of 47.98 pm/V (3-layers of GaS). A potential efficient symmetry breakdown in liquid-phase exfoliated materials results in an enhancement of the SHG signal, providing the researchers with a promising research area of exploration. The two-photon absorption (TPA) characteristic of 2D-GaS material is realized from the generated TPEF signal. The strong saturable absorption behaviour of the GaS-materials is determined from the giant modulation depths (24.4%, 35.3%, and 29.1%) and large nonlinear absorption coefficient (β) values (-9.3×10³, -91.0×10³, -6.05×10³ cm/GW) for three different wavelengths of 800 nm, 1066 nm, and 1560 nm, respectively. Followed by depositing 2D GaS nanosheets on side polished fibers (SPFs), successful saturable absorbers (SAs) are fabricated for the first time. By integrating these GaS-SAs into three different wavelength-based fiber laser cavities, broadband stable mode-locked pulses are achieved, with a pulse duration of 46.22 ps (1 µm), 614 fs (1.5 µm) and 1.02 ps (2 µm), respectively. Additionally, different orders of harmonic mode-locked pulses with the highest repetition rate of 0.55 GHz (45th order) and Q-switched pulses with the shortest pulse duration of 2.2 µs are obtained in the telecommunication waveband. These findings suggest that 2D GaS has a lot of potential for broadband nonlinear photonic devices.
The second section of the thesis discusses the fabrication of tin telluride-quantum dots (SnTe-QDs; group IVA TMMC) as well as the techniques of the ultrafast laser generation in infrared region. The average size of the synthesized SnTe nanoparticles is determined as 74 nm, which is smaller than the Bohr radius of SnTe (94 nm), therefore, the prepared nanoparticles are considered as QDs. By utilizing the prepared QDs in SPFs, SAs are prepared, and their saturable absorption property is determined from the realized modulation depth (saturable intensity) of 9.1% (42.5 MWcm-2) and 2.2% (1.67 GW cm-2) at 1 µm and 1.5 µm wavelength, respectively. These SAs are then employed in ytterbium (Yb)-doped and erbium (Er)-doped fiber laser cavities for the generation of ultrafast lasers. In Yb-doped fiber laser cavity the mode locked pulse with pulse width of 265 ps is generated, whereas Q-switched (pulse width = 1.81 µs), mode locked (pulse width = 691 fs) and harmonic mode locked (fifth order with repetition rate of 62.1 MHz) pulses are generated in Er-doped fiber laser cavity, which indicate the superiority of SnTe-QDs SAs in infrared laser generation.
Finally, the last section discusses the NLO absorption and broadband ultrafast photonics applications of zirconium tritelluride (ZrTe₃; group-IV TMT). The admirable saturable absorption characteristics of ZrTe₃ are realized from the determined highest nonlinear absorption coefficient (β) value of -82.1 × 10³ (1 µm) and -13.05 × 10³ cm/GW (1.5 µm), respectively. Moreover, the superior quality of the ZrTe₃-based SPF SAs is demonstrated from the obtained broadband stable mode-locked pulses with pulse durations of 323 ps (1 µm), 751 fs (1.5 µm), and 1.2 ps (2 µm), respectively. This work, in this thesis, indicates that ZrTe₃ nanoparticles are a promising material for broadband ultrafast photonic devices and NLO applications.
Rights: All rights reserved
Access: open access

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