Studies On Optical Nonlinearity Of Transition Metal Chalcogenides And Bismuth Oxyselenide

The modern photonics and optoelectronics technology is greatly accelerated by the development and application of optical materials that have novel nonlinearity and wide-band saturable absorption in infrared region.The prerequisity and basics for desing semiconductor materials with novel characteristic and wide-band nonlinearity in the infrared region are the band-gap and carrier density.In this thesis,Z-scan tenique and transient absorption spectroscopy are employed to investigate the novel optical nonlinearity as well as ultrafast carrier dynamics in transition metal chalcogenides（TMDCs,including MoSe₂,WSe₂ and TiS₂）and bismuth oxyselenide（Bi₂O₂Se）,which provide fundamental information for desiging photonics device based on these materials.There are two parts according to the categories in the thesis.The first part focuses on the novel optical nonlinearity as well as ultrafast carrier dynamics in TMDCs,including MoSe₂,WSe₂ and TiS₂.1.The preparation of monolayer MoSe₂ by CVD technique and the investigation on wide-band saturable absorption based on band-gap dynamic variation.Because of increasing dielectric shielding,photo-induced carriers tend to form many-particle electronic states,such as excitons（electron-hole）and trions for few-layer or monolayer TMDCs.These states can be altered in the presence of elevated exciton densities,naming bandgap renormalization which may be used to investigate the saturable absorption induced by those lasers far below than the band gap.In order to verify the hypothesis,monolayer MoSe₂ with uniform thickness were prepared by a CVD method and then were investigated using Z-scan technique,indicating saturable absorption from 800 nm（1.55 eV）to 1550 nm（0.80 eV）.To explain the mechanism,the calculation of carrer density in monolayer MoSe₂ reveals a Mott transition from a semiconductor regime to an electron-hole plasma:on the one hand,the band-gap decreases by0.5 eV（1550 nm）due to the polulation inversion;on the other hand,the elevated carrier temperature can also reduce the band-gap by 0.51 eV.Finally,the band-gap decrease from1.53 eV to 0.52 eV by subjecting monolayer MoSe₂ to strong excitation.The part experimentally proves the wide-band saturable absorption in the infrared region and explains the phenomenon using band-gap dynamic variation model based on many-particle interaction model.2.The preparation of multi-layer WSe₂ by CVD technique and the investigation on phonon-assitsted anti-stokes process.Since the exciton states locate near the conduction band edge,the exciton-phonon interaction and optical nonlinearity enhance when the excitation laser is injected resonantly.The thickness of multi-layer WSe₂ is⁵5 nm to increase the light-matter interaction.The band-gap is calculated to be about 1.44 eV,which is supported by PL spectrum.Z-scans indicate reverse saturable absorption even under the excitation wavelength of 800 nm（1.55 eV）.Under the condition of E_laser>E_B near the band edge,the resonance effect enhances the coupling to induce the absorption of photons and the annihilation of phonons to realize the reverse saturable absorption.Furthermore,the optical limiting threshold is obtained to be²1.6 mJ cm^-2,which is lower than those reported for other limiting materials currently for femtosecond laser pulse at 800 nm.The part proves the reverse saturable absorption induced by phonon-assisted anti-Stokes in multilayer WSe₂ under E_laser>E_B near the band edge.3.Ultrafast saturable absorption in Ti S₂ induced by non-equilibrium electrons and generation of femtosecond mode-locked laser.Non-equibibrium electrons induced by ultrafast laser excitation in a correlated electron material can disturb the Fermi energy as well as optical nonlinearity.Here,non-equilibrium electrons translate semiconductor TiS₂ into a plasma to generate the broad band nonlinear optical saturable absorption with sub-picosecond recovery time of⁷68 fs（corresponding to modulation frequencies over 1.3 THz）and modulation depth up to¹45%.Based on this optical nonlinear modulator,a stable femtosecond mode-locked pulse with pulse duration of⁴02 fs and pulses train with a period of¹75.5 ns is observed in the all-optical system.The findings indicate that non-equilibrium electrons can promote TiS₂-based saturable absorber to be ultrafast switch for femtosecond pulse output.The second part of the thesis is about ultra-broadband mid-infrared pulsed optical switch and carrier dynamics.Pulsed lasers operating in the mid-infrared（3-25μm）are increasingly becoming the light source of choice for a wide range of industrial and scientific applications such as spectroscopy,biomedical research,sensing,imaging and communication.Up to now,one of the factors limiting the mid-infrared pulsed lasers is the lack of optical switch with a capability of pulse generation,especially for those with wideband response.Here we exhibit a semiconductor material of bismuth oxyselenide（Bi₂O₂Se）with a facile processibility,constituting an ultra-broadband satruable absorber for the mid-infrared（actually from the near-infrared to mid-infrared:0.8-5.0μm）.Significantly,we find that the optical response is associated with a strong nonlinear character,showing picoseconds response time and modulation depth up to³30.1%at 5.0μm.Combined with facile processibility and low-cost,these solution-processed Bi₂O₂Se materials may offer a scalable and printable mid-infrared optical switch to open up the long sought parameter space which is crucial for the exploitation of compact and high-performance mid-infrared pulsed laser sources.