We report a comprehensive experimental study and detailed model analysis of the terahertz 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 terahertz dielectric function of intraexcitonic transitions between the 1s and higher-energy exciton and continuum levels. It closely describes the terahertz spectra of resonantly generated excitons. Exciton ionization and formation are further explored, where the terahertz response exhibits both intraexcitonic 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 terahertz-derived kinetics is, moreover, consistent with time-resolved luminescence measured for comparison. Our study establishes a basis for tracking pair densities via transient terahertz spectroscopy of photoexcited quasi-two-dimensional electron-hole gases.

Energy- and time-resolved spectroscopy reveals a photoinduced softening of the charge-transfer gap in the insulating copper oxide Sr2CuO2Cl2 that indicates rapid and efficient photoproduction of optical phonons. By relating the pump-probe signal amplitude to the thermal difference spectrum, we estimate that eleven to twenty optical phonons are created for every one 3 eV photon. Assuming relaxation to the optical absorption edge at 1.5 eV, this corresponds to 70-130 meV per boson. While the lower limit is consistent with relaxation exclusively through optical phonons, the upper limit suggests a significant role for magnetic excitations. We observe a photoinduced bleaching of the gap excitation that we associate with phase space filling, and estimate the excluded area of the photoexcited state to be about nine copper oxide plaquettes. The temporal decay of the pump-probe signal is consistent with anharmonic phonon decay.

We report a photoinduced femtosecond change in the magnetization direction in the ferromagnetic semiconductor GaMnAs, which allows for the detection of a four-state magnetic memory on the femtosecond time scale. The temporal profile of the magnetization exhibits a discontinuity that reveals two distinct temporal regimes, marked by the transition from a carrier-mediated nonthermal regime within the first 200 fs to a thermal, lattice-heating picosecond regime.

We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semiconductor GaMnAs, which allows for detection of a four-state magnetic memory at the femtosecond time scale. The temporal profile of this cooperative magnetization rotation exhibits a discontinuity that reveals two distinct temporal regimes, marked by the transition from a highly non-equilibrium, carrier-mediated regime within the first 200 fs, to a thermal, lattice-heating picosecond regime.

We report on the first observation of photo-induced femtosecond cooperative magnetization rotation at the non-equilibrium, charge distribution timescale, in ferromagnetic semiconductor GaMnAs, enabling the ultrafast detection of four-state magnetic memory.

Few‐cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many‐body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density‐dependent renormalization of the binding energy of a high‐density exciton gas in GaAs/Al 0.3 Ga 0.7 As quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu 2 O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10 –14 cm 2 . Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two‐dimensional THz emission spectroscopy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the first observation of femtosecond response in the photoinduced cooperative magnetization rotation in ferromagnetic semiconductor GaMnAs, which enables the detection of four-state magnetic memory at the femtosecond time scale.

Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a nonradiative coherence between the discrete Landau levels of an undoped quantum well and study its dynamics. We observe quantum beats that reflect the time evolution of the coherence between the two lowest Landau level magnetoexcitons. We interpret our observations using a many-body theory and find that the inter-Landau level coherence decays with a time constant, substantially longer than the corresponding interband magnetoexciton dephasing times. Our results indicate an intraband excitation dynamics that cannot be described in terms of uncorrelated interband excitations.

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.

We report on the dramatic ultrafast photo-enhancement of ferromagnetism on a 100 picosecond timescale in ion-implanted semiconductor GaMnAs via photoexcited transient carriers. This non-thermal, transient cooperative magnetic process surprisingly quenches at low temperatures, significantly bellow the Curie temperature.

Excitons are of fundamental interest and of importance for opto-electronic applications of bulk and nano-structured semiconductors. This paper discusses the utilization of ultrafast terahertz (THz) pulses for the study of characteristic low-energy excitations of photoexcited quasi-2D electron-hole (e-h) gases. Optical-pump THz-probe spectroscopy at 250-kHz repetition rate is employed to detect characteristic THz signatures of excitons and unbound e-h pairs in GaAs quantum wells. Exciton and free-carrier densities are extracted from the data using a two-component model. We report the detailed THz response and pair densities for different photoexcitation energies resonant to heavy-hole excitons, light-hole excitons, or the continuum of unbound pairs. Such experiments can provide quantitative insights into wavelength, time, and temperature dependence of the low-energy response and composition of optically excited e-h gases in low-dimensional semiconductors.

We report on ultrafast photoenhanced ferromagnetism and para- to ferromagnetic phase transition, on a 100-ps time scale, due to a transient enhancement of Curie temperature Tc via the population of photoexcited carriers in III-Mn-V semiconductor GaMnAs.

In three-pulse four-wave mixing experiments of undoped GaAs quantum wells in large perpendicular magnetic fields, we see a small signal with unexpectedly long-lived beats. Using a microscopic many-body theory, we identify the beats as due to non-radiative Raman coherences, thereby extracting the dephasing time of the Raman coherence.

Excitons, Coulomb bound pairs of one electron with one hole, are among the most important elementary excitations in condensed matter physics. They are often seen as analogous to the hydrogen atom. However, a complex phase diagram due to intricate many-body interactions sets limits to this picture. Up to now, excitons have almost exclusively been investigated by optical techniques resonant to the band gap. Since these studies involve generation or annihilation of electron-hole pairs, they are only indirectly sensitive to existing populations. In contrast, femtosecond terahertz (THz) technology (1 THz ap 4.1 meV) has been advanced in recent years to directly trace the complex-valued dielectric response of low-energy resonances in extreme non-equilibrium systems (Huber et al., 2005). The internal quantum fine structure of excitons has thus become accessible (Kaindl et al., 2003). We exploit THz spectroscopy to test the quasi-particle concept of excitons in a direct, time-resolved way and explore new quantum optical processes.

We report on the observation of ultrafast photoenhanced ferromagnetism in GaMnAs. It is manifested as a transient magnetization increase on a 100 ps time scale, after an initial subpicosecond demagnetization. The dynamic magnetization enhancement exhibits a maximum below the Curie temperature T-c and dominates the demagnetization component when approaching T-c. We attribute the observed ultrafast collective ordering to the p-d exchange interaction between photoexcited holes and Mn spins, leading to a correlation-induced peak around 20 K and a transient increase in T-c.

Using 3-pulse four-wave mixing spectroscopy, we excite an intra-band coherence of the quantum Hall system. By comparison with an undoped sample (no two-dimensional electron gas) and high intensity measurements, we identify features in the 4-wave mixing signal from this coherence. A many-body theory is used to understand our results.

With 3-pulse wave-mixing ultrafast spectroscopy we probe simultaneously the intra and inter-band coherent dynamics in GaAs/AlGaAs quantum wells in a perpendicular magnetic field. By comparing to theory, we identify the role of inter-Landau-level coherences.

We report the first observation of stimulated emission of terahertz radiation from internal transitions of excitons. The far-infrared electromagnetic response of Cu2O is monitored via broadband terahertz pulses after ultrafast resonant excitation of three-dimensional 3p excitons. Stimulated emission from the 3p to the energetically lower 2s bound level occurs at a photon energy of 6.6 meV, with a cross section of ∼10-14 cm2. Simultaneous excitation of both exciton levels, in turn, drives quantum beats, which lead to efficient terahertz emission sharply peaked at the difference frequency.