Coherent control of phonon quantum beats

The coherent control of excitons coupled to longitudinal optical phonons is analyzed in the quantum kinetic regime. Different models allowing for different levels of sophistication in the theoretical treatment are compared. Within a simple two-level model of the exciton system analytical results relevant for short-time excitation in the low-density regime are derived, which are nonperturbative with respect to the exciton–phonon interaction. For pulses of finite duration and high intensity, numerical calculations are presented realizing a nonperturbative treatment with respect to both the exciton–phonon as well as the exciton–photon coupling. It is shown that for two-level systems, the use of high intensity pulses opens new possible control scenarios that combine the effects of phase control with Rabi oscillations. These findings are compared with quantum kinetic calculations for a two-band semiconductor model. Here we present results for one, two and three dimensions. Within this model, the coupling to the continuum of states above the band edge provides a dephasing mechanism missing in few-level models. It is shown that also the dephasing properties can be coherently controlled. Furthermore, it is demonstrated that the strength and controllability of phonon beats characteristically depend on the dimension of the system.     

Determination of hyperfine parameters using quantum beats

We have observed the delayed time spectra of the coherently scattered radiation following nuclear Bragg diffraction of incident synchrotron radiation pulses. The measurements were carried out using single crystals of⁵⁷Fe-YIG and⁵⁷FeBO₃ by conventional coincidence techniques. Although the intention of these measurements was only to show effects like speed-up and quantum beats, the spectra also provide the possibility of a sensitive direct measure of the hyperfine interaction parameters. It is demonstrated that even with a counting rate of only 1 Hz it was possible to obtain within 1–2 hours (in the case of⁵⁷FeBO₃) spectra of sufficient statistics to derive precise hyperfine parameters.     

The origin of quantum beats in photoionization

The origin of quantum beats in photoionization of a coherent superposition of excited states is examined. Two types of beat are distinguished; one in the production of polarized final states, and the other a higher-order Raman-induced multiphoton beat in the total photoionization signal.     

Photon beats from a single semiconductor quantum dot

Single-photon interference is observed on the ultranarrow long-term stable exciton resonance of an individual semiconductor quantum dot. This interference is related to the fine-structure splitting and allows direct conclusions about the coherence properties of the exciton. When selectively addressing a particular dot by quasiresonant phonon-assisted excitation, despite a rapid orientation relaxation on a 1-ps time scale, coherence is partly maintained. No significant further decoherence occurs when the ground state is reached until the exciton recombines radiatively (approximately 300 ps).     

Interaction-induced beats of Friedel oscillations in quantum wires

We analyze the spectrum of electron density oscillations in an interacting one-dimensional electron system with an impurity. The system's inhomogeneity is characterized by different values of Fermi wave vectors kF=k L/R on left or right side of the scatterer, leading to a Landauer dipole formation. We demonstrate, that while in the noninteracting system the Friedel oscillations possess only one periodicity related to the local kF, say kL on the left side, the interplay of the interactions and the Landauer dipole generates an additional peak in the spectrum of density oscillations at the counterpart kR. Being only present in correlated systems, the position and shape of this spectral feature, which in coordinate space is observable as a beating pattern in the Friedel oscillations, reveals many important details about the nature of interactions. Thus it has a potential to become an investigation tool in condensed matter physics.     

Mixing collisions and quantum beats in photon echoes

The spectral line shifts Δ^(κ) due to the mixing collisions by means of the quantum beats of the photon echoes are analysed.     

Stark quantum beats under pulsed dye laser excitation

The method of quantum beat spectroscopy is extended to the study of pure electric field splittings. As an example Stark splittings of the 6s6p ¹P₁ term of Ba I are investigated by observing quantum beats after excitation by a short resonant dye-laser pulse. The derived value for the tensorial polarizability is in good agreement with previous work. Occurence and shapes of Stark quantum beat signals under different experimental conditions are discussed.     

Polariton spin beats in semiconductor quantum well microcavities

Time-resolved Kerr (Faraday) rotation experiments allow for the observation of polariton spin beats in both InGaAs and CdMnTe quantum well (QW) microcavities. The existence of these beats is an unambiguous manifestation of the coherent energy exchange between exciton and photon components of polariton states created by a circularly polarized and spectrally wide femtosecond laser pulse. The polariton states are also shown to be split into a linearly polarized doublet. This splitting is responsible for the polarization transfer between linearly and circularly polarized states. In a highest-quality sample, the resulting spin dynamics could be detected.     

Fine-structure measurements of4He by zero-field quantum beats

The complete finestructure beat frequency patterns ofn ³ P states of⁴He I (n=3–7) are measured by observation of zero field quantum beats using the beam-foil technique. The results provide a new experimental value for the 7³ P ₂–7³ P _o separation, confirm the beat amplitude ratios to be in accord with theory and allow comparison with the results of other techniques.     

Quantum beats of fine-structure states in InP quantum dots

Different types of quantum beats were experimentally observed in the photoluminescence kinetics of semiconductor nanostructures with InP quantum dots characterized by strong inhomogeneous broadening of the optical transitions. Specific types of beats were selected by varying the magnitude and orientation of the magnetic field, the applied bias voltage, and the frequencies of the exciting and detected light. Parameters of the fine structure of electrons and holes in the system under study and characteristic relaxation times of the spin coherence are determined from the experimental data.     

Electron spin beats in InGaAs/GaAs quantum dots

Time-resolved picosecond spectroscopy is used for the first time to study optical orientation and spin dynamics of carriers in self-organized In(Ga)As/GaAs quantum-dot (QD) arrays. Optical orientation of carriers created by 1.2 ps light pulses, both in the GaAs matrix and wetting layer, and captured by QDs is found to last a few hundreds of picosecond. The saturation of electron ground state at high-excitation-light intensity leads to electron polarization in excited states close to 100% and to its vanishing in ground state. Electron-spin quantum beats in a transverse magnetic field are observed for the first time in semiconductor QDs. We thus determine the quasi-zero-dimensional electron g factor in In_0.5Ga_0.5As/GaAs QDs to be: |g _⊥|=0.27±0.03. Fiz. Tverd. Tela (St. Petersburg) 41, 871–874 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Optical exciton Stark effect and quantum beats at exciton quasienergy levels in quantum wells

The optical Stark effect and quantum beats in a GaAs/AlGaAs quantum well are investigated theoretically for the case when the first two electron size-quantization levels are mixed dynamically by a high-intensity CO₂ laser pulse polarized perpendicular to the plane of the quantum well. The quasienergy spectrum of heavy-hole excitons and the ratio between the probabilities of exciton transition with and without a strong electromagnetic field are obtained. The time-dependent intensity of absorption of the sensing light is determined. It exhibits quantum beats at twice the electron Rabi frequency. Fiz. Tverd. Tela (St. Petersburg) 39, 1291–1294 (July 1997)