Optical nonlinear properties of novel two dimensional transition metal dichalcogenides

Abstract

Apart from the development of scientific theories and mechanisms, the sustained investigation on the material properties of newly synthesised substance had also underpinned the advancement of modern technology. In the optical aspect, according to the evolution of high power pulsed laser, it had opened up a new research branch on the material optical properties, which is known as nonlinear optics. Under the illumination of high-intensity light, the optical response of nonlinear materials show unique phenomenons as compared to that observed in conventional optics, for instance, incident light intensity-dependent absorbance and wavelength modulation of the output light. In the same time, these unique nonlinear properties of the novel materials can be applied to synthesise advanced optical devices, including saturable absorber for inducing laser pulses with picoto femtosecond pulse duration, and photonic crystal fibre for supercontinuum generation. Therefore, the exploration and investigation of novel nonlinear materials is a vital test for promoting modern optics development. Contributed by the works on graphene, it had motivated the study on low-dimensional materials in the past two decades, for example, two-dimensional layered material, nanotube, and quantum dot. The previous literature had shown that these nanomaterials possess superior material properties, such as high charge carrier mobility and mechanical strength, especially strong nonlinear optical response. In this work, the research objective is mainly focused on the exploration of the nonlinear optical properties of novel two-dimensional layered transition metal dichalcogenides(TMDs), where the TMDs materials were applied as the saturable absorber(SA) and installed into different laser cavity to induce ultrashort laser pulses generation. The applied laser systems include of diode-end-pumped solid-state (DPSS) Nd: YVO4 laser and Erbium-doped fiber laser (EDFL). The studies on nonlinear optical absorption(NOA) properties of TMDs materials began with the promising nonlinear group 6 tungsten disulfide (WS2). The WS2 saturable absorber was inserted within a DPSS Nd: YVO4 laser cavity to test the corresponding NOA properties. In the experiment, Q-switched operation was demonstrated, where maximum single pulse energy of 145 nJ was achieved and is comparable to the results of other WS2-SA-based Q-switched lasers. This work has enriched the understanding of the applicability of WS2-SA in diverse laser systems. Meanwhile, in the synthesis process, experiences of TMDs material properties modulation were obtained. The synthesis parameters, such as the thickness of the precursor thin film, post-annealing time and temperature profile will significantly affect to the TMDs nanosheets growth direction, crystalline structure and stoichiometry ratio of the synthesised thin film. These experiences had built the foundation of the successor works of applying vertically aligned group 10 TMDs to fabricate high-efficient photodetector and image sensor, which was demonstrated and published in high-quality Journal. Remarkably, the NOA study had also extended to group 10 air-stable TMDs candidates, which both platinum disulfide (PtS2) and platinum ditelluride (PtTe2) saturable absorber-based passively mode-locked EDFLs were experimentally demonstrated. The achieved pulse duration of the mode-locked EDFLs are 2.064 ps and 1.66 ps corresponding to the PtS2 and PtTe2 saturable absorber, respectively, which are comparable to other low dimensional materials-based results. It indicates that both of the platinum-based TMDs can apply as a promising nonlinear saturable absorber. To the best of our knowledge, the demonstrations of mode-locked EDFLs based on PtS2 and PtTe2 saturable absorber are a pioneer study within the nonlinear photonic field and has not been reported previously. Furthermore, the NOA dependence on the lateral size and layer thickness of PtS2 nanosheets was also demonstrated. By using the individual centrifugation method with distinct centrifugal forces, four PtS2-NMP suspension samples with different distributions of lateral dimension and thickness were prepared. By the Z-scan measurement, it shows that as the PtS2 nanosheets lateral dimension and layer thickness reduced, it will enhance the excited state photon absorption (ESA) effect and two-photon absorption (TPA) effect, which promoted the reverse saturable absorption (RSA) response of the PtS2 samples. If the RSA response of the nonlinear sample is already close to the saturation limit, further enhancement of photon absorption will result in a weakening of the RSA response, and eventually induced a change of the NOA behavior from RSA to saturable absorption response. This work provides the method and strategy for further optimising the group 10 Pt/Pd-based saturable absorber performance by modifying the morphological features of the TMDs nanosheets.