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The layered lanthanum silver antimonide LaAgSb 2 was known to experience two charge density (CDW) phase transitions, which were proposed recently to be closely related to the newly identified Dirac cone. We present optical spectroscopy and ultrafast pump probe measurement on the compound. The development of energy gaps were clearly observed below the phase transition temperatures in optical conductivity, which removes most part of the free carrier spectral weight. Time resolved measurement demonstrated the emergence of strong oscillations upon entering the CDW states, which were illuminated to come from the amplitude mode of CDW collective excitations. The frequencies of them are surprisingly low: only 0.12 THz for the CDW order with higher transition temperature and 0.34 THz for the lower one, which shall be caused by their small modulation wave vectors. Furthermore, the amplitude and relaxation time of photoinduced reflectivity stayed unchanged across the two phase transitions, which might be connected to the extremely low energy scales of amplitude modes. PACS numbers: 71.45.Lr, 78.47.+p Charge-density-waves (CDW) is one of the most fundamental collective quantum phenomena in solids. Charge density waves display periodic modulations of the charge with a period which is commensurate or incommensurate to the underlying lattice. Most CDW states are driven by the nesting topology of Fermi surfaces (FSs), i.e., the matching of sections of FS to others by a wave vector q = 2k F , where the electronic susceptibility has a divergence. A single-particle energy gap opens in the nested regions of the FSs at the transition, which leads to the lowering of the electronic energies of the system. Simultaneously, the phonon mode of acoustic branch becomes softened to zero frequency at q = 2k F as a result of electron-phonon interaction, which further leads to the periodic modulation of lattice structure.CDW also has collective excitations referred to as an amplitude mode (AM) and a phase mode. Phase excitation corresponds to the translational motion of the undistorted condensate. In the q=0 limit, the phase mode should locate at zero energy in ideal case since the translational motion does not change the condensation energy [1,2]. In reality, due to the presence of impurity or defects, the phase mode is pinned at finite frequency, usually in the microwave frequency range. The pinning/depinning of phase mode has dramatic effect on charge transport properties. By applying dc electric field, the phase mode can be driven into a current-carrying state, leading to nonlinear current-voltage characteristics [3][4][5]. On the other hand, the amplitude mode involves the ionic displacement and has a finite energy even at q=0 limit. For most CDW materials, the amplitude mode has an energy scale of about 10 meV (or ∼ 2 THz) [6][7][8][9][10][11][12]. Due to presence of such a gap for the amplitude mode (i.e. the mode energy at q=0), its effect on low temperature physical properties of CDW condensate has been much less studied. Generally...

We use time-domain terahertz spectroscopy to measure the low energy conductivity and magnons in RuCl 3 under external magnetic field. At zero field, an oscillation with a frequency of 0.62 THz is clearly observed in time-domain spectrum below T N , which is identified as a magnon excitation in the magnetic order state. The magnon excitation is not affected by the external magnetic field H DC when it is applied along the c-axis, but is clearly suppressed when H DC is applied within ab plane. More interestingly, when the magnetic component of THz wave h(t) is perpendicular to the applied in-plane magnetic field, we observe another coherent oscillation at slightly higher energy scale at the field above 2 T, which is eventually suppressed for H DC >5 T. The measurement seems to indicate that the in-plane magnetic field can lift the degeneracy of two branches of low energy magnons at Γ point. The low energy optical conductivity calculated from the measured transmission spectrum is dominated by a broad continuum contribution, which is not affected by changing either temperature or external magnetic field. The continuum is likely to be related to the fractional spin excitation due to dominated Kitaev interaction in the material.