M. Lindberg

Femtosecond optical nonlinearities of CdSe quantum dots

Attract-Femtosecond differential absorption measurements of the quantum confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The bole width increases with pump intensity and the magnitude of the induced transparency saturates at our highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition.

Coherent effects in pump–probe spectroscopy of excitons

Femtosecond measurements of coherent effects arising from exciton bleaching in bulk GaAs are reported. This phenomenon, which is characterized by spectral oscillatory structures, is general and appears whenever the temporal resolution is shorter than the material coherence time. This is shown to be a manifestation of the uncertainty relation in time-resolved spectroscopy.

Nonlinear optical properties of quantum-confined CdSe microcrystallites

Quantum-confinement effects in CdSe microcrystallites in a glass matrix are investigated in the nanosecond time domain. The results of pump–probe and single-beam absorption saturation measurements show strong evidence for electron–hole quantization in the semiconductor microstructures, a different bleaching behavior from those of bulk semiconductors and multiple quantum wells, and a trend of increased saturation intensity with decreased microcrystallite size. A theory for the one- and two-pair states in quantum dots is developed that fully includes the relevant Coulomb interactions. The theory is evaluated numerically, and results are presented for intrinsic quantum dots and for quantum dots with impurities. The experimentally observed nonlinearities are attributed to saturation of one-electron–hole-pair resonances and induced absorption caused by two-pair resonances in the presence or absence of impurities (traps).

Transient and steady-state optical nonlinearities in semiconductors

Theory and experiments on steady-state and femtosecond time-resolved optical nonlinearities in semiconductors are reviewed. A simple description of the physical processes underlying the nonlinearities is given. The discussion is focused on the spectral region around the fundamental absorption edge, and it covers coherent oscillations, the optical Stark effect as well as the bleaching of the exciton resonance with increasing excitation intensity, plasma screening and band-filling phenomena.

Two-photon absorption and third-order nonlinearities in GaAs quantum dots

Third-order optical nonlinearities of GaAs quantum dots are investigated theoretically. Two quantum confinement regimes are analyzed, and large optical nonlinearities are predicted for sufficiently narrow linewidths. The appearance of an induced (two-photon) absorption resonance is predicted energetically above the exciton resonance for quantum-dot radii, which are between the electron and hole Bohr radii.

Oscillatory instability of an induced absorber in a ring cavity

The instabilities of a nonlinear optical medium with induced absorption in a ring cavity are theoretically analyzed. Stable, bistable, and unstable stationary solutions are found for various values of incident intensity and detuning. The dynamic treatment yields, in the unstable regime, oscillations of the transmitted intensity in various modes for constant incident intensity. If the medium relaxation time is short compared with the round-trip time, oscillatory solutions are obtained, which show strong locking of the oscillation frequency to multiples of the inverse round-trip time. If the round-trip time is comparable with the medium relaxation time, numerical integrations yield periodic solutions with substructures because of the mismatch of the two characteristic times.

Dephasing of interacting heavy-hole and light-hole excitons in GaAs quantum wells

We describe a novel three-pulse degenerate four-wave mixing configuration that permits the direct observation of the coherence between the σ+ and the σ− exciton states in a GaAs quantum well excited in a two-photon process. It is found that the phase coherence between the two single heavy-hole exciton states decays with a time constant that is considerably longer than the dephasing time of the coherence between these states and the ground state (interband coherence). All the experimental data are well described by numerical solutions of the optical Bloch equations for a phenomenological multilevel model.

Optical Nonlinearities and Femtosecond Dynamics of Quantum Confined CdSe Microcrystallites

Pump-probe spectroscopic techniques with nanosecond pulses are used to investigate the size quantization effects in CdSe microcrystallites in glass matrices (quantum dots). Nonlinear properties of the transitions between quantum confined electron and hole states are reported for low temperatures and at room temperature. Femtosecond four-wave mixing and differential transmission spectroscopic techniques were also employed to study the excited state dynamics and relaxation times of the quantum dots. The homogeneous and inhomogeneous contributions to the lowest electronic transitions are measured by femtosecond spectral hole burning at various temperatures. The inhomogeneous linewidth is due to size and shape distribution of the crystallites. Our experiments indicate that the hole-width increases with increasing light intensity. The optical nonlinearities as a function of microcrystallite size were investigated using single beam saturation experiment for the three quantum confined samples. A simple absorption saturation model was used to analyze the data. The results indicate that the saturation intensity is larger for smaller semiconductor sizes. Therefore, the index change per unit of intensity, Δn/I which is proportional to (α-αB)/Is is larger for larger sizes. Here, Δn is the index change, α is the absorption at the peak of the transition, αB is the background absorption, and Is is the saturation intensity.

Many-Body Theory of Coherent Optical Effects in Semiconductors

An overview of the physics and the microscopic theory of coherent light-matter interaction in semiconductors and semiconductor heterostructures is presented. After an introduction of the basic concepts, a series of ultrafast coherent phenomena is reviewed, including the optical Stark effect, photon echoes, Rabi oscillations, quantum beats, multi-wave mixing, and pulse propagation effects.

Ultrafast optical nonlinearities in II–VI compounds

We review our experimental and theoretical results on the femtosecond laser excited CdS and CdSe bulk and quantum dots. Optical Stark effect in bulk CdS is reported where the electron-hole continuum states in addition to the bound-exciton states are blue shifted. The nonlinear optical properties of CdSe microcrystallites embedded in glass matrices are investigated using femtosecond and nanosecond laser pulses. Spectral hole-buring is observed which is accompanied by an induced absorption feature on the high-energy side. A 25±10 fs dephasing time is obtained from these measurements. It is concluded that Coulomb effects are important in quantum dots. They give rise to a ground state two-pair resonance on the low energy side of the one-pair state and higher energy two-pair resonances which are spectrally located between the one-pair states. Single-beam saturation experiments show that the intensities required for bleaching are larger for smaller crystallites.