Time-integrated Four-wave Mixing Research Articles

Ultrafast coherent spectroscopy of one-dimensional magnetoexcitons in semiconductors

Abstract We investigate the ultrashort coherent dynamics of one-dimensional Fano andvLorentzian magnetoexcitons in GaAs using temporally and spectrally resolved four-wave mixing (FWM). The dephasing time of a Fano resonance is intrinsically limited by the coupling between discrete states and continua which is responsible for Fano interference. In contrast, the coherent emission from Lorentzian magnetoexcitons dephases solely due to quasi-particle scattering, dominated by exciton-exciton scattering at low temperatures. We find a very fast decay of the time-integrated FWM signal from Fano magnetoexciton resonances. This decay is not due to dephasing. The quenching of the FWM signal for non-overlapping excitation pulses is the result of quantum interference. This destructive quantum interference is not observed for purely Lorentzian dynamics. For the simultaneous excitation of Lorentzian and Fano magnetoexciton resonances, destructive quantum interference also quenches the coherent emission from Lorentzian magnetoexcitons due to quantum mechanical coupling between the different resonances.

Strong instantaneous contribution in femtosecond degenerate four-wave mixing from 350 μm InP due to virtual excitation

We have observed strong four-wave mixing (FWM) signals from the third up to the seventh order for a 350 μm undoped InP at 10 K with an excitation at far below the band gap. The third order spectrally resolved FWM signal shifts continuously to blue as time delay moves away from positive to negative. The peak intensities of the third and fifth order time-integrated FWM signals decrease rapidly as the detuning increases, as (detuning)−2 and (detuning)−6, respectively. The FWM signal at far below the band gap is attributable to the strong instantaneous contribution of the excitation pulse.

Dephasing of excitons in ZnSe and ZnSe/ZnS xSe 1− x quantum wells

We investigate the optical dephasing of photoexcited excitons in two different ZnSe/ZnS x Se 1 − x layer structures by means of time-integrated four-wave mixing using subpicosecond pulses. We find dephasing times of the order of 0.5 ps for the exciton ground state at 9 K. The dephasing time shows a dependence on the excitation intensity due to exciton-exciton scattering. Simultaneous excitation of the exciton ground state and higher exciton states leads to the appearance of the quantum beats.

Coherent dynamics of exciton wavepackets in semiconductor heterostructures

The coherent dynamics of exciton wavepackets in (GaIn)As/GaAs as well as (GaIn)As/Ga(PAs) multiple quantum well structures is studied by means of transient four-wave mixing (FWM) experiments. The wavepackets are generated by simultaneous excitation of several exciton transitions with laser pulses of about 100 fs duration. The time-integrated FWM signals exhibit a pronounced modulation superimposed on the overall decay which can be attributed to the quantum interference of the different eigenstates. In the time-resolved FWM signals this interference is not present, reflecting the interplay between many-body Coulomb effects and inhomogeneous broadening. This experimental technique is then employed to extract the exciton binding energies in pseudomorphic symmetrically strained (GaIn)As/GaIPAs) with various In contents.

Polariton effects in four-wave mixing.

We show that polariton effects lead to time-delayed components in the time-resolved four-wave-mixing (FWM) signal. Hence we find that recent observations in FWM experiments that have been ascribed to many-body Coulomb interactions can also be explained from the formation of exciton polaritons. In addition, we find that due to the formation of polaritons, the rate of decay of the time-integrated FWM signal strongly depends on the oscillator strength and the propagation length.