Interband Dephasing Research Articles

Femtosecond Transfer and Manipulation of Persistent Hot-Trion Coherence in a Single CdSe/ZnSe Quantum Dot.

Ultrafast transmission changes around the fundamental trion resonance are studied after exciting a p-shell exciton in a negatively charged II-VI quantum dot. The biexcitonic induced absorption reveals quantum beats between hot-trion states at 133GHz. While interband dephasing is dominated by relaxation of the P-shell hole within 390fs, trionic coherence remains stored in the spin system for 85ps due to Pauli blocking of the triplet electron. The complex spectrotemporal evolution of transmission is explained analytically by solving the Maxwell-Liouville equations. Pump and probe polarizations provide full control over amplitude and phase of the quantum beats.

Ultrafast acoustic phonon scattering in CH3NH3PbI3 revealed by femtosecond four-wave mixing.

Carrier scattering processes are studied in CH3NH3PbI3 using temperature-dependent four-wave mixing experiments. Our results indicate that scattering by ionized impurities limits the interband dephasing time (T2) below 30 K, with strong electron-phonon scattering dominating at higher temperatures (with a time scale of 125 fs at 100 K). Our theoretical simulations provide quantitative agreement with the measured carrier scattering rate and show that the rate of acoustic phonon scattering is enhanced by strong spin-orbit coupling, which modifies the band-edge density of states. The Rashba coefficient extracted from fitting the experimental results (γc = 2 eV Å) is in agreement with calculations of the surface Rashba effect and recent experiments using the photogalvanic effect on thin films.

Control of the Urbach band tail and interband dephasing time with post-growth annealing in low-temperature-grown GaAs

Femtosecond four-wave mixing experiments on low-temperature-grown (LT-) GaAs for a range of post-growth annealing temperatures indicate that the Urbach band tail abruptly diminishes above 550°C due to the conversion of As-related point defects to As clusters and that the interband dephasing time is limited by scattering with As point defects for annealing temperatures below 550°C. In addition, we observe a complex interplay of polarization source terms associated with the exciton and Urbach band tail for annealing temperatures below 550°C. These experiments shed light on the carrier dynamics and ultrafast nonlinear optical properties of LT-GaAs.

Subcycle-resolved probe retardation in strong-field pumped dielectrics.

The response of a bulk dielectric to an intense few-cycle laser pulse is not solely determined by the pulse envelope, but also by ultrafast processes occuring during each optical cycle. Here, a method is presented for measuring the retardation of a probe pulse in a strong-field pumped, bulk dielectric with subcycle resolution in the pump–probe delay. Comparisons to model calculations show that the measurement is sensitive to the timing of the electronic Kerr response. When conduction band states are transiently populated at the crests of the laser field, the measurement is also sensitive to the interband dephasing time.

Correlation and dephasing effects on the non-radiative coherence between bright excitons in an InAs QD ensemble measured with 2D spectroscopy

Exchange-mediated fine-structure splitting of bright excitons in an ensemble of InAs quantum dots is studied using optical two-dimensional Fourier-transform spectroscopy. By monitoring the non-radiative coherence between the bright states, we find that the fine-structure splitting decreases with increasing exciton emission energy at a rate of 0.1μeV/meV. Dephasing rates are compared to population decay rates to reveal that pure dephasing causes the exciton optical coherences to decay faster than the radiative limit at low temperature, independent of excitation density. Fluctuations of the bright state transition energies are nearly perfectly correlated, protecting the non-radiative coherence from interband dephasing mechanisms.

Interband dephasing and photon echo response in GaMnAs

Coherent carrier dynamics are studied in GaMnAs using time-integrated and time-resolved four-wave mixing techniques. Dephasing is observed to be dominated by spin-flip scattering between the optically injected holes and Mn ions, revealing the rapid time scale of this scattering process in the III-Mn-V diluted magnetic semiconductors. The optical response is shown to exhibit the characteristic signatures of a simple photon echo, despite the complexity of band tail contributions and strong exchange coupling in this system.

Excitonic couplings and interband energy transfer in a double-wall molecular aggregate imaged by coherent two-dimensional electronic spectroscopy

The early stage of molecular excitonics and its quantum-kinetic dynamics in the multiband, bitubular cyanine dye aggregate C(8)O(3) at room temperature are revealed by employing two-dimensional (2D) coherent electronic spectroscopy in the visible spectral region. The sub-20 fs measurements provide a direct look into the details of elementary electronic couplings by spreading spectroscopic transitions into two frequency axes. Correlation spectra of rephasing (k(I) = -k(1) + k(2) + k(3)) and nonrephasing (k(II) = +k(1) - k(2) + k(3)) data in emission (omega(3))-absorption (omega(1)) 2D-frequency space image interband excitons into cross-peak signals and unveil the quantum-dissipative regime of exciton relaxation. Spectral streaking of cross peaks directly reveals interband dephasing and exciton population relaxation on the road to tube-to-tube energy transfer without making recourse to an a priori model. Theory and simulations, based on an effective multilevel scheme and a quantum-dissipative model with experimental pulse envelopes, explain the origin of the cross peaks, reveal the underlying sequences of electronic transitions, recover the streaking patterns of relaxing cross peaks along omega(1), and reconstruct the space-energy pathways of electronic excitation flow.

Excitonic approach to the ultrafast optical response of semiconductors

We present a theoretical approach to treating the coherent dynamics of optically generated charge carriers in semiconductors using an excitonic basis. In contrast to the semiconductor Bloch equations, our approach treats intraband correlations without factorization. It also includes phase space filling effects that have generally been omitted in previous excitonic treatments of coherent dynamics. We show that, in the coherent limit, where the intraband dephasing time and population decay time are both equal to half of the interband dephasing time, our excitonic approach agrees with the semiconductor Bloch equations to at least third order in the optical field, but that it differs significantly in more general situations. Our excitonic equations are shown to be particularly applicable in systems, such as biased semiconductor superlattices, where bound excitons dominate the optical response and where intraband correlations play a central role. Using a simple model of a nanoring, we show how the spectral shifts in the interband response can be explained in terms of phase-space-filling-induced excitonic population dynamics.

Quantum Correlations and Intraband Coherences in Semiconductor Cavity QED

A fully quantized theory of optical interactions with semiconductors reveals quantum correlations between the field and carrier density. These quantum correlations lead to intraband coherences which live much longer than the interband dephasing time; the correlations are observed as oscillations in the time-resolved reflectivity in a pump-probe experiment on a normal-mode microcavity. Coherent control of the microcavity normal modes using phase-locked pump pulses enables considerable enhancement of the intraband oscillations due to quantum interference; the detailed phase dependence is well reproduced by the theory.