Ultrafast optical spectroscopy of high-temperature superconductors

The mechanism of high-temperature superconductivity is still an unsolved mystery in physics, and it is the ″pearl in the crown″ of condensed matter physics. Among the numerous optical methods investigating superconductors, ultrafast spectroscopy is one of the most exquisite methods and the most powerful control means. It can interact with superconductors in all the charge, lattice, spin and orbital degrees of freedom. It can probe the excited state of superconductors. It can uniquely realize investigations of the ultrafast processes of quasiparticles, the coherence control of lattices, electron-phonon coupling strength, and the interface superconductivity. Here we briefly review the ultrafast optical spectroscopy (especially the ultrafast dynamics) investigations of high temperature superconductors, with concrete examples. Particularly, we demonstrate the unique virtues of this experimental method in the observation and realization of quasiparticle excited states, bosonic coherent states, laser-induced superconductivity, and interface superconductivity. We give the prospect of this area at the end. The complexity, profundity and serendipity of superconductivity quite much root in its bridging between both fermions and bosons in the condensed matters—a solid universe. Ultrafast spectroscopy can probe both the electrons and bosonic collective elementary excitations, thus making it feasible for revealing the superconducting mechanism. The time-resolved measurements provide direct evidences of the superconducting Bose-Einstein condensate and clues to distinguish it from the pseudogaps, charge density waves, spin density waves, etc. Delicate ultrafast spectroscopy investigations can also yield testifying information on the gap symmetry, including whether there is a nodal line in the system. The electron-phonon coupling constant can be obtained by directly observing the quasiparticle relaxation, which usually occurs at picosecond scales and marks the rate of energy transferring among carriers and phonons—a direct reflection of electron-coupling strength. The unique way of generating and detection coherent phonons in a solid adds another way of looking into the lattice behavior in a superconductor—if it is phonon glue, which mode plays the major role? The aforementioned methods have been used to study all the cuprates, iron-based superconductors, and interface single-layer superconductors. Furthermore, ultrafast laser pulse can act as a natural controlling tool. It is known photo-doping can be more efficient than chemical doping in some situations, but more dramatically, ultrafast light pulses can induce superconducting phenomenon in a non-superconducting system with even room temperature T c. Since the superconducting feature occurs and evolves in picoseconds, this transient superconductivity can only be observed using ultrafast spectroscopy. This excited state superconductivity is a concrete example of the importance of excited state in superconductivity investigation. The accessibility for ultrafast spectroscopy to excited states (non-equilibrium quantum states) making it an exceptionally feasible experimental means among the all in such investigations. Currently, ultrafast spectroscopy is extending to the THz and mid-IR range to resonantly probe the narrow gaps or phonon excitation, to the X-ray range (RIXS) to achieve momentum-resolved information of bosonic excitations, to adding angle-resolved photoemission spectroscopy to access the momentum-resolved electronic features, to adding Transmission Electron Microscopy (TEM), Scanning Tunnel Microscope (STM) or Scanning Nearfield Optical Microscope (SNOM) for spatially-resolved properties, etc. We foresee that ultrafast spectroscopy of superconductors is going to be mature area in 30 years. During this period, it is going to proceed in a foreseeable way marked by the aforementioned science problems and physics techniques, and in an un-foreseeable way marked by novel exciting results.