Seeing the ultrasmall, capturing the ultrafast!
We are developing and using advanced optical techniques for investigating ultrafast processes in complex material systems. Experimental techniques that we use include time-resolved ARPES, ultrafast electron diffraction, pump-probe spectroscopy, transient grating spectroscopy, second harmonic generation and time-resolved terahertz spectroscopy. We are especially interested in using these techniques to understand fascinating properties of strongly correlated electron systems and topological insulators.
You can find more information about our group by following the links on the left.
Selected Recent Papers
Large, valley-exclusive Bloch-Siegert shift in monolayer WS2 . Coherent interaction with off-resonance light can be used to shift the energy levels of atoms, molecules, and solids. The dominant effect is the optical Stark shift, but there is an additional contribution from the so-called Bloch-Siegert shift that has eluded direct and exclusive observation in solids. We observed an exceptionally large Bloch-Siegert shift in monolayer tungsten disulfide (WS2) under infrared optical driving. By controlling the light helicity, we could confine the Bloch-Siegert shift to occur only at one valley, and the optical Stark shift at the other valley, because the two effects obey opposite selection rules at different valleys. Such a large and valley-exclusive Bloch-Siegert shift allows for enhanced control over the valleytronic properties of two-dimensional materials.
Selective scattering between Floquet-Bloch and Volkov states in a topological insulator. Solid-state systems driven periodically by light are promising platforms to realize novel quantum states of matter. In this case, photons coherently interact with Bloch electrons in the solid to generate hybrid states known as Floquet-Bloch states. Similarly, light also couples to free electrons near the surface of the solid to create Volkov states. In this work we use time and angle resolved photoemission spectroscopy (Tr-ARPES) on the topological insulator Bi2Se3 to quantify the scattering between Floquet-Bloch and Volkov states. We find that the interaction between the two is strongly dependent on the initial electron momentum. More importantly, by controlling the polarization of the driving electric field, we can negate Volkov states so as to generate pure Floquet-Bloch states. This work establishes a systematic path for the coherent manipulation of solids via light-matter interaction.
Valley-selective Optical Stark Effect in Monolayer WS2. Monolayer semiconductors such as WS2 have two different valleys in their electronic band structures. Their corresponding energy gaps are normally locked in equal magnitude. Separating these valleys in energy is of great interest because it would allow for control in valleytronic applications to carry information. In this work, we show that circularly polarized light can be used to shift the energy of one valley but not the other by means of the optical Stark effect.
Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene. We have demonstrated the ability to control the way light influences the conductive properties of graphene by electrically tuning the number of electrons in the material. Graphene is a single atomic layer of carbon with remarkable optical and electronic properties with potential for future device applications. We showed that an intense incident light pulse can either increase or decrease the electrical conductivity of graphene, depending on how many electrons are initially in the material. By using ultrashort laser pulses, modulation of conductivity can be controlled in one trillionth of a second. This work also revealed that the conducting capacity of graphene depends on temperature in a manner distinct from that of conventional metals or semiconductors, due to the unique crystal symmetry of this two-dimensional carbon material. These results may potentially be useful for developing next-generation optoelectronic devices using graphene.
Observation of Floquet-Bloch states on the surface of a topological insulator. The unique electronic properties of the surface electrons in a topological insulator are protected by time-reversal symmetry. Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort mid-infrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. These photon dressed surface bands exhibit polarization-dependent band gaps at avoided crossings. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal symmetry on the surface. These observations establish the Floquet-Bloch bands in solids and pave the way for optical manipulation of topological quantum states of matter.
Spin-Induced Optical Conductivity in the Spin-Liquid Candidate Herbertsmithite. Kagome lattice Herbertsmithite ZnCu3(OH)6Cl2 is one of the best candidates for the experimental realization of a quantum spin liquid state, but the details of the ground state remain elusive, in particular, the existence of a spin gap. In this work, we probe the optical conductivity of Herbertsmithite at THz frequencies using THz time-domain spectroscopy. We find a power-law contribution to the conductivity whose origin, we conclude, is the highly frustrated spin system. This result agrees with recent theoretical predictions for the low frequency conductivity in a U(1) Dirac spin liquid state on the kagome lattice.
Observation of suppressed terahertz absorption in photoexcited graphene. Graphene, a two-dimensional honeycomb lattice of carbon atoms, is an attractive material for use in fast optoelectronic devices due to its broadband optical absorption and high electron mobility. It is crucial to understand the mechanisms of electron energy relaxation on ultrashort timescales in order to realize these applications. In this work, we apply optical pump - terahertz probe spectroscopy to study the ultrafast response of electrons to an intense light pulse. We find a transient decrease in the electron conductivity, in stark contrast to the response observed in conventional semiconductors.
Fluctuating charge-density waves in a cuprate superconductor.
Measurement of intrinsic Dirac fermion cooling on the surface of the topological insulator Bi2Se3 using time-resolved and angle-resolved photoemission spectroscopy. How the surface electrons of a topological insulator interact with bulk electrons and phonons is important for future electronics and computers that may employ these electrons as information carriers. In this work, we use time- and angle-resolved photoemission spectroscopy developed in our lab to capture 3D movies of their motion in the energy-momentum space with sub-picosecond time resolution. This technique allows us to observe the interaction between surface and bulk electrons at high temperatures.
Observation of a metal-to-insulator transition with both Mott-Hubbard and Slater characteristics in Sr2IrO4 from time-resolved photo-carrier dynamics. Iridates are novel electronic systems in which spin-orbit coupling, electronic bandwidth and on-site Coulomb interactions occur on comparable energy scales. It is important to understand the electronic structure of these Iridates to realize exotic quantum phenomena such as correlated topological insulators. In this work, we use time-resolved optical spectroscopy to study the temperature evolution of the electronic structure of Sr2IrO4. We find a clear change in the ultrafast dynamics across TN indicating a gap opening concomitant with antiferromagnetic order.
Control over topological insulator photocurrents with light polarization. Electrons on the surface of a topological insulator have the unique property that their spin orientation depends on their direction of propagation. We show in this work that the the indirect coupling of light and spin in the topological insualtor Bi2Se3 results in current flow confined to the surface. By changing the polarization of the light, the current direction can be controlled.
Observation of a Warped Helical Spin Texture in Bi2Se3 from Circular Dichroism Angle-Resolved Photoemission Spectroscopy. Topological insulators are characterized by surface states where the electron spin is locked perpendicular to its momentum. We show in this work that performing ARPES with circularly polarized light is a sensitive measure of the spin polarization of the surface states. We discover in n-doped Bi2Se3 that the spin distribution warps from the expected perpendicular spin-momentum locking in both the in-plane and out-of-plane directions.
Selective Probing of Photoinduced Charge and Spin Dynamics in the Bulk and Surface of a Topological Insulator. The spin-polarized electrons on the surface of a topological insulator (TI) provide a promising platform for optical and spintronic devices. To achieve this, it is important to understand the dynamics of surface electrons in a TI in response to light while distinguishing this response from that of bulk electrons. In this work, we use time-resolved fundamental and second-harmonic optical pump-probe measurements to selectively distinguish between bulk and surface response. We optically induce a net transient spin density and resolve different relaxation processes for both bulk and surface electrons following photo-excitation.
Nonlinear optical probe of tunable surface electrons on a topological insulator. The topological insulator is a new phase of quantum matter that hosts conducting electrons on all of its surfaces that are robust against backscattering. However, isolating the surface electrical response has proven difficult owing to the proponderance of conducting electrons in the bulk of these materials. In this work, we demonstrate that the second harmonic generation of light off the the topological insulator Bi2Se3 is sensitive to the surface crystal structure, surface Fermi level, and time-reversal-symmetry breaking.
Band-dependent quasiparticle dynamics in the hole-doped Ba-122 iron pnictides. Iron-pnictide superconductivity exists on five bands, complicating efforts by experimentalists and theorists to gain definitive insight into the nature of pairing. Of utmost importance are the symmetry of the superconducting gap and the interrelationship of the various bands with one another. In this article, we describe how pump-probe spectroscopy can be used to probe relaxation of a photoexcited sample back to its superconducting state with a view towards the separate dynamics occuring in each of the bands. Our results point towards fully gapped hole bands on the inner portions of the Brillouin zone, and either nodal or highly anisotropic electron bands near the zone boundary.
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