TECHNIQUES

Terahertz time-domain spectroscopy (THz-TDS)

Terahertz light conveys small energy (1 THz ~ 4 meV); thus charged quasi-particles oscillate with incoming terahertz lights resonantly. And, many collective motions such as antiferromagnetic resonance and polaron appear in this energy window. More attractively, electrodynamical information for the topological insulators and/or relativistic quasi-particle systems can be effectively investigated in terahertz windows since their scattering rate is in the terahertz ranges. 

Now, we are equipping the terahertz time-domain spectrometer within transmission and reflection types. We can obtain data in the time domain, for which the sample's optical properties can be extracted from data measured by THz-TDS alone. Note that extracted optical properties contain information regarding the aforementioned phenomena. Thus, we are targeting to explore the new physics emerging in topological insulators and/or relativistic quasi-particle systems by using THz-TDS technique. 

Optical pump-THz probe (OPTP)

In our setup, temporal electrodynamics in solid-state materials can be investigated by illuminating NIR pump pulses onto the target samples prior to the THz probe pulse. The optical pump makes photo-excited quasi-particles, altering their optical properties. And, by varying a time dealy between the pump and the probe beams, we are able to trace the electrodynamics as a function of an ultra-short time scale being pico-second scales. 

Band manipulation, the relaxation process of quasiparticles can be explored in this fashion. 

Narrow-band terahertz pump-terahertz probe

In contrast to the NIR pump case, the terahertz pump interacts mostly with the free carriers, so-called intraband transition. Thus, it would expect to induce strong oscillations via the terahertz pump, resulting in hot-electronic temperature which can not be accessed by the common heating process. Within this setup, we are capable of accessing the nonlinear electrodynamics of quasi-particles caused by the ultra-high electronic temperature.