Studies On Ultrafast Carrier Dynamics Of Three-dimensional Dirac Semimetal Cd₃As₂ And Single-wall Carbon Nanotubes

The modern photonics technology is greatly accelerated by the development and the application of novel quantum materials.One prerequisite for designing and engineering quantum material-based optoelectronic devices is the understanding of their ultrafast carrier dynamics.In this thesis,ultrafast photocarrier dynamics of two quantum materials,three-dimensional topological Dirac semimetal(3D TDS)Cd3As2 and single-wall carbon nanotubes(SWNTs),have been investigated by transient absorption spectroscopy.The results provide fundamental information that is very important for designing and engineering their electronic and photonic devices.There are two main aspects in this thesis.The first part focuses on the ultrafast saturable absorption of 3D TDS Cd3As2 thin films and their relaxation time control.It is well known that an important technique bottleneck of current mid-infrared(mid-IR)photonics technology is the lack of robust,wideband and tunable saturable absorber above 3 ?m wavelength.To deal with this problem,we demonstrate that the high quality Cd3As2 thin films exhibit saturable absorption behavior over broad wavelengths from 1.6 to 6 ?m.In addition,the photocarrier relaxation time of this material is about several picoseconds.These results indicate that Cd3AS2 is an excellent optical material that can be potential used as saturable absorber application in mid-IR wavelength.Furthermore,the hot-carrier relaxation dynamics of Cd3As2 are well described by a semiclassical two-temperature model.The carrier thermalization time about-300-500 fs and carrier-phonon coupling factor g about 5.3 × 1015 W m-3 K-1 can be obtained.The g value of Cd3As2 is much smaller than that of graphite.A smaller g is the reason why Cd3As2 has a slower recovery rate than graphene or graphite.Relaxation time is a key parameter for saturable absorbers.Hence,to confirm this parameter can be flexible controlled,we introduce chromium(Cr)as a dopant to the Cd3As2 film and perform the pump-probe measurements with the wavelength range from 3 ?m to 6 ?m.When the Cr concentration about 0-2 at.%is used,tuning of the photocarrier recovery time over an order of magnitude can be achieved.The observed faster relaxation time(at higher Cr concentrations)are assigned to additional doping-induced scattering channels,arising from the alteration of the band structure near the Dirac point.These experimental results demonstrate that 3D TDS Cd3As2 is a wideband and tunable mid-IR saturable absorption material that can be used to solve a long sought problem,the lack of high-performance saturable absorber above 3 ?m wavelength,for the development of compact and high-performance mid-IR ultrafast sources.The second part of this thesis is about the photocarrier dynamics of SWNTs.SWNTs exhibit fascinating physical properties that are highly relevant to optoelectronic and photonic applications.However,up till now several carrier dynamic features of SWNTs is still under debate.Here,we performed broadband(1-2.4 pm)degenerate and non-degenerate pump-probe measurements on SWNTs of different chiralities and morphologies.Through spectral and temporal analysis,we conclude that the ultrafast photoresponse features of SWNTs originate from the combined effects of Pauli blocking and bandgap renormalization.More importantly,our results indicate that the bandgap renormalization is likely to be caused by electron-hot phonon interactions,in contrast to the hot electron-hole plasma origin that has been identified in other low-dimensional systems.The sometimes'inconsistent' dynamics obtained from degenerate and non-degenerate measurements are as a result of different sensitivity to the Pauli blocking and bandgap renormalization effects,respectively.In addition,we also provide direct evidence for the existence of competition between Pauli blocking and bandgap renormalization effects.Our findings provide a simple and self-contained physical model for interpreting the broadband optical response of carbon nanotubes.In summary,the ultrafast photocarrier dynamics of three-dimensional topological Dirac semimetal Cd3As2 and single-wall carbon nanotubes,have been experimental investigated.The results obtained in my dissertation provide a strong foundation for understanding their electronic and optical properties,as well as their devices operation mechanism.