UC San Diego

The development of ultrafast spectroscopy gives us access to the time dimension to study the dynamics of condensed matter systems. Different from conventional condensed matter experimental techniques, the pump-probe technique allows us not only to study the relaxation of nonequilibrium to equilibrium states, but also the possibility to selectively control the properties of a material using ultrafast light. In my thesis, I will introduce the fundamental of optics in the first chapter. Second chapter consists of the experimental set-ups I developed and built during my Ph.D.: Terahertz time-domain spectroscopy (THz-TDS), broadband THz source from two-color laser-induced gas plasma, intensive mid-infrared pump Kerr rotation probe and data acquisition (DAQ) system. The third chapter is the background of spin-orbit coupled Mott insulator Sr2IrO4,and the last chapter includes my work of ultrafast magnetic control in Sr2IrO4: we study the magnetic dynamics of the Jeff = 1/2 Mott state using strong mid-Infrared 9 µm (below the charge gap), and near-infrared 1.3 µm (above the charge gap) circularly polarized excitations, and monitor the pump induced Kerr signal. For both pump wavelength, the 2D in-plane B2g coherent magnon oscillation of frequency ~ 0.5 THz was observed in the pump-induced Kerr rotation signal. The circularly polarized 9 µm pumps of opposite helicities excite oscillations of opposite phase, while 1.3 µm pumps excite oscillations of same phase. The quadratically scaling of the fluence dependent magnon amplitude for the 9 µm pump indicates a novel photon-two-magnon coupling mechanism for the magnon generation. The directly excitation (9 µm) of the spin spectrum without photodoping electrons permits extremely efficient magnon generation, almost ten times better than the resonant 1.3 µm pump.