Session L1: Recent Advances in Ultrafast Studies of Condensed Matter

Ultrafast and Nonlinear Optical Spectroscopy of Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs) provide a variety of unique opportunities for studying the dynamics and interactions of one-dimensional (1-D) electrons and phonons. We have carried out a series of ultrafast and nonlinear optical experiments on SWNTs, revealing novel properties of high- density 1-D excitons as well as coherent lattice vibrations.\footnote{Y.~Murakami and J.~Kono, Phys.~Rev.~Lett.~ {\bf 102}, 037401 (2009); Phys.~Rev.~B {\bf 80}, 035432 (2009); A.~Srivastava and J.~Kono, Phys.~Rev.~B {\bf 79}, 205407 (2009); J.-H.~Kim {\it et al}., Phys.~Rev.~Lett.~{\bf 102}, 037402 (2009); G.~D.~Sanders {\it et al}., Phys.~Rev.~B~{\bf 79}, 205434 (2009); Y.-S.~Lim {\it et al}., ACS Nano {\bf 4}, 3222 (2010); L.~G.~Booshehri {\it et al}., arXiv:1007.3144v1.} We have shown that there exists an upper limit on the density of 1-D excitons in SWNTs, which results in photoluminescence saturation. Using a model based on diffusion-limited exciton- exciton annihilation, we provided realistic estimates for the exciton densities in the saturation regime. We also predicted and demonstrated that there is an optimum temperature at which the exciton density can be maximized, due to the existence of a dark exciton state. Using ultrashort pulses, we have also investigated the dynamics of coherent phonons (CPs) in SWNTs, including both the low frequency radial breathing mode and high frequency G-mode phonons. Pulse shaping techniques allowed us to generate and detect CPs in SWNTs in a chirality-selective manner, which provided insight into the chirality dependence of light absorption, phonon generation, and phonon-induced band- structure modulations. Finally, we observed novel large- amplitude CPs through near-band-edge excitations as well as strongly polarization-dependent CP signals in highly-aligned SWNTs.   

Femtosecond magnetism and spin manipulation on a time-scale of the exchange interaction

The dynamics of phase transformations on the time-scale pertinent to atomic, orbital and spin motion is a rather unexplored field in physics. This is also a particularly interesting problem of modern magnetism, a study of which may have tremendous consequences for future development of magnetic recording technology. However, generation of magnetic field pulses much shorter than 100 ps and strong enough to reverse magnetization (more than 1T) is an extremely challenging technical problem.As a result the dynamics of the magnetic phase transitions at the sub-100 ps time-scale remains to be one of the most intriguing areas of modern magnetism [1,2]. Recently it has been observed that a 40 fs laser pulse influences spins in a magnet as an equally short pulse of effective magnetic field with a strength up to 20 T [3,4]. In my talk I will discuss how these opto-magnetic pulses can be used to excite a magnet on a time-scale of the exchange interaction between the spins [5-7]. Novel insights into the physics of non-equilibrium magnetism will be provided, showing that two exchange-coupled magnetic sublattices of a ferrimagnet may have totally different spin dynamics [8]. As a result, ultrafast spin reversal of two antiferromagnetically coupled magnetic sub-lattices appears to proceed via a novel ferromagnet-like transient state. \\[4pt] [1] J. St\"ohr and H. C. Siegmann, \textit{Magnetism: from fundamentals to nanoscale dynamics} (Springer, Berlin, 2006). \\[0pt] [2] A. Kirilyuk, A. V. Kimel, Th. Rasing, \textit{Rev. Mod. Phys.} \textbf{82} 2731 (2010). \\[0pt] [3] A. V. Kimel et al,\textit{ Nature }\textbf{435 }655 (2005) \\[0pt] [4] C. D. Stanciu et al,\textit{ Phys. Rev. Lett. }\textbf{99}, 047601 (2007). \\[0pt] [5] K. Vahaplar et al \textit{Phys. Rev. Lett.} \textbf{103}, 117201 (2009). \\[0pt] [6] A.V. Kimel et al, \textit{Nature-Physics} \textbf{5} 727 (2009). \\[0pt] [7] A. H. M. Reid et al \textit{Phys. Rev. Lett.}\textbf{105} 107402 (2010). \\[0pt] [8] I. Radu et al (submitted).   

Theory of ultrafast pump-probe phenomena in high-temperature superconductors

The physics underlying the pairing mechanism for high-temperature superconductors remains a topic of current interest. The complexity lies with the existence of competing interactions in these strongly correlated electronic materials. The ultra-fast pump-probe technique can make a stride to untangle the competing degrees of freedom (DOF). In this talk, the theoretical underpinning for this technique will be reviewed. In particular, we have developed a three-temperature model [1] to simulate the real time dependence of the electron and phonon temperatures in high-temperature superconductors. The model considers anisotropic electron-phonon coupling [2]. Based on this model, we have calculated the time-resolved spectral function, which exhibits interesting features with time delay. It has been found that the excitation of phononic DOF can provide a defining signature for the evidence of electron-vibration mode coupling [1]. In addition, the time-resolved optical conductivity and Raman spectra will also be discussed within the same model [3]\\[4pt] [1] Jianmin Tao and Jian-Xin Zhu, Phys. Rev. B \textbf{81}, 224506 (2010);\\[0pt] [2] T. P. Devereaux \textit{et al}., Phys. Rev. Lett. \textbf{93}, 117004 (2004); Jian-Xin Zhu \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 177001 (2006);\\[0pt] [3] Jianmin Tao and Jian-Xin Zhu \textit{et al}., unpublished (2010).   

Ultrafast Optical Excitation in YBa$_2$Cu$_3$O$_{7-\delta}$: Tracing the Optical Phonons

The time-resolved spectroscopy of nonequilibrium states proved to be a powerful tool for observation of the electron-phonon scattering dynamics and the recombination of photoexcited quasiparticles (QP), particularly in high-temperature cuprate superconductors. However, most of the reported experiments monitor only the electronic subsystem [1-5]. Thus, a detailed dynamics of the various phonon modes during an initial non-thermal regime has been beyond reach. Here we utilize the field-resolved ultrabroadband THz spectroscopy to resonantly trace ultrafast phonon and QP dynamics of optimally doped single crystals of YBa$_2$Cu$_3$O$_{7-\delta}$ [6]. The superconducting state is perturbed by 12-fs optical pump pulses, and the induced changes in the mid-infrared optical conductivity are probed by THz transients. Thus, we simultaneously observe the dynamics of nonequilibrium QPs and two specific phonon modes with a time resolution of 40 fs. A quantitative line shape analysis of the apex oxygen vibration allows us to separately follow its transient occupation and coupling to the Josephson plasma resonance. A strong phonon population and the maximum QP density are reached within the same time scale of 150 fs demonstrating that the lattice absorbs a major portion of the pump energy before the QPs are thermalized. Our results indicate substantial electron-phonon scattering in YBa$_2$Cu$_3$O$_{7-\delta}$ and introduce a powerful approach for characterizing transient phonon dynamics in a broad variety of solids.\\ [4pt] [1] S. G. Han et al., Phys. Rev. Lett. \textbf{65}, 2708 (1990);\\ [0pt] [2] R. A. Kaindl et al., Science \textbf{287}, 470 (2000);\\ [0pt] [3] R. D. Averitt et al., Phys. Rev. B \textbf{63}, 140502 (2001);\\ [0pt] [4] L. Perfetti et al., Phys. Rev. Lett. \textbf{99}, 197001 (2007);\\ [0pt] [5] R. P. Saichu et al., Phys. Rev. Lett. \textbf{102}, 177004 (2009);\\ [0pt] [6] A. Pashkin et al., Phys. Rev. Lett. \textbf{105}, 067001 (2010).