Marc A. Carnahan

Ultrafast terahertz probes of transient conducting and insulating phases in an electron–hole gas

Many-body systems in nature exhibit complexity and self-organization arising from seemingly simple laws. For example, the long-range Coulomb interaction between electrical charges has a simple form, yet is responsible for a plethora of bound states in matter, ranging from the hydrogen atom to complex biochemical structures. Semiconductors form an ideal laboratory for studying many-body interactions of electronic quasiparticles among themselves and with lattice vibrations and light. Oppositely charged electron and hole quasiparticles can coexist in an ionized but correlated plasma, or form bound hydrogen-like pairs called excitons. The pathways between such states, however, remain elusive in near-visible optical experiments that detect a subset of excitons with vanishing centre-of-mass momenta. In contrast, transitions between internal exciton levels, which occur in the far-infrared at terahertz (1012 s-1) frequencies, are independent of this restriction, suggesting their use as a probe of electron–hole pair dynamics. Here we employ an ultrafast terahertz probe to investigate directly the dynamical interplay of optically-generated excitons and unbound electron–hole pairs in GaAs quantum wells. Our observations reveal an unexpected quasi-instantaneous excitonic enhancement, the formation of insulating excitons on a 100-ps timescale, and the conditions under which excitonic populations prevail.

Far-infrared optical conductivity gap in superconducting MgB2 films

We report the first study of the optical conductivity of MgB2 covering the range of its lowest-energy superconducting gap. Terahertz time-domain spectroscopy is utilized to determine the complex, frequency-dependent conductivity sigma(omega) of thin films. The imaginary part reveals an inductive response due to the emergence of the superconducting condensate. The real part exhibits a strong depletion of oscillator strength near 5 meV resulting from the opening of a superconducting energy gap. The gap ratio of 2Delta0/k(B)TC approximately 1.9 is well below the weak-coupling value, pointing to complex behavior in this novel superconductor.

Transient terahertz spectroscopy of excitons and unbound carriers in quasi two-dimensional electron-hole gases

We report a comprehensive experimental study and detailed model analysis of the terahertz (THz) dielectric response and density kinetics of excitons and unbound electron-hole pairs in GaAs quantum wells. A compact expression is given, in absolute units, for the complex-valued THz dielectric function of intra-excitonic transitions between the 1s and higher-energy exciton and continuum levels. It closely describes the THz spectra of resonantly generated excitons. Exciton ionization and formation are further explored, where the THz response exhibits both intra-excitonic and Drude features. Utilizing a two-component dielectric function, we derive the underlying exciton and unbound pair densities. In the ionized state, excellent agreement is found with the Saha thermodynamic equilibrium, which provides experimental verification of the two-component analysis and density scaling. During exciton formation, in turn, the pair kinetics is quantitatively described by a Saha equilibrium that follows the carrier cooling dynamics. The THz-derived kinetics is, moreover, consistent with time-resolved luminescence measured for comparison. Our study establishes a basis for tracking pair densities via transient THz spectroscopy of photoexcited quasi-2D electron-hole gases.

Parity-Forbidden Excitations of Sr2CuO2Cl2 Revealed by Optical Third-Harmonic Spectroscopy

We present the first study of nonlinear optical third-harmonic generation (THG) in the strongly correlated charge-transfer insulator Sr(2)CuO(2)Cl(2). For fundamental excitation in the near infrared, the THG spectrum reveals a strongly resonant response for photon energies near 0.7 eV. Polarization analysis reveals this novel resonance to be only partially accounted for by three-photon excitation to the optical charge-transfer exciton, and indicates that an even-parity state at 2 eV, with a(1g) symmetry, participates in the third-harmonic susceptibility.

Ultrafast THz Spectroscopy of Excitons in Multi-Component Carrier Gases

We discuss time-resolved experiments that study photo-excited e-h pairs in GaAs quantum wells via their THz response. Resonant generation of excitons leads to characteristic THz spectra dominated by 1s-2p intra-excitonic transitions, which provide a direct density gauge. Fundamental differences between the intra-excitonic and free-carrier conductivity enable quantitative analysis of multicomponent e-h gases. We examine exciton ionization dynamics, where the exciton fraction approaches a quasi-equilibrium value in agreement with the Saha equation. A Saha model that takes into account thermal excitation of both heavy- and light-hole pairs is derived. Such experiments identify non-equilibrium phases of e-h gases and their underlying quasi-particle densities and dynamics.

Terahertz probes of transient conducting and insulating phases in quasi-2D electron-hole gases

We employ ultrafast terahertz (THz) pulses to study the dynamical interplay of optically-induced excitons and unbound electron-hole pairs in GaAs/AlGaAs quantum wells. A distinct low-energy oscillator appears upon resonant excitation of heavy-hole excitons, linked to transitions between their internal degrees of freedom. Time-resolving changes in the THz conductivity, we can observe dynamical transitions between conducting and insulating phases as excitons form or ionize on ultrashort timescales.

Terahertz intra-exciton transitions: new views of ultrafast exciton dynamics

We discuss experiments that resolve the time evolution of exciton populations by sensing transitions between the excitons internal levels. The dynamical interplay of optically-induced excitons and unbound electron-hole pairs is investigated under various initial conditions.

Linear and nonlinear optics of Sr2CuO2Cl2

Abstract We present evidence from both linear and nonlinear optical spectroscopy for the existence of distinct exciton types at the charge-transfer gap of Sr 2 CuO 2 Cl 2 , which are linked by strong, phonon-mediated coupling. We show how several basic optical properties of this material depend on these excitations.

Third harmonic generation at the charge transfer gap in Sr/sub 2/CuO/sub 2/Cl/sub 2/

Summary form only given. Progress towards understanding high transition-temperature superconductivity in the copper oxides (cuprates) is hindered by the coexistence of complex spin, charge and lattice degrees of freedom. Even in the undoped parent compounds a detailed microscopic description of excitations within the basic energy bandstructure as observed in electron and optical spectroscopies remains a formidable task. Nonlinear optical spectroscopy, however, allows precise control over excitation energies and symmetries, revealing important new information about such systems. Here, we employ third-harmonic generation (THG) spectroscopy to investigate the electronic structure around the charge-transfer gap in the strongly correlated cuprate Sr/sub 2/CuO/sub 2/Cl/sub 2/.