FWM Signal Research Articles

Coherent Interaction of Three-Dimensionally Confined Electron–Hole Pairs with LO-Phonons

The spectrally resolved FWM signal of CdSe quantum dots with sizes below the bulk excitonic Bohr radius has been studied as a function of temperature and excitation intensity and compared with bulk CdSe. A clear oscillation with a period of 163 fs (≈︂ 25 meV) is found in the transient FWM signal and attributed to a coherent coupling of confined electron–hole pairs with LO-phonons. Two side bands are observed in the spectrum 25 meV below and above the excitonic resonance. The strong coupling to LO-phonons is observed even at excitation densities around 1×1016 cm—3 which is close to the bulk value of the Mott density.

Coherent Interaction of Three-Dimensionally Confined Electron–Hole Pairs with LO-Phonons

The spectrally resolved FWM signal of CdSe quantum dots with sizes below the bulk excitonic Bohr radius has been studied as a function of temperature and excitation intensity and compared with bulk CdSe. A clear oscillation with a period of 163 fs (≈ 25 meV) is found in the transient FWM signal and attributed to a coherent coupling of confined electron–hole pairs with LO-phonons. Two side bands are observed in the spectrum 25 meV below and above the excitonic resonance. The strong coupling to LO-phonons is observed even at excitation densities around 1×1016 cm—3 which is close to the bulk value of the Mott density.

Four-Wave-Mixing in Radiatively Coupled Multiple-Quantum-Well Structures: A Model Study

The influence of radiative coupling on the four-wave-mixing signals of multiple-quantum-well structures is investigated theoretically for excitation at the excitonic resonance. It is shown that amplitude and shape of the FWM signals depend strongly on the number of quantum wells N in the sample and on the periodicity of the structure. For a fixed delay time τ12, the signal amplitude of a Bragg structure increases quadratically with N due to constructive interference as long as the radiative decay rate Nγrad is small compared to the inverse delay time τ—112. For large N, if the enhanced radiative decay rate exceeds the inverse delay time, superradiant emission leads to a decrease of the observed signals with increasing N. The signals of anti-Bragg structures with an odd number of quantum wells decrease with increasing N due to destructive interference effects whereas anti-Bragg structures with an even number of quantum wells behave similarly to Bragg structures.

Nonlinear optical studies of Bloch oscillations

We have investigated the coherent dynamics of wave packets in semiconductor superlattices with an applied electric field. In particular, we have performed four-wave mixing experiments with time-resolving of the diffracted beam. The temporal behavior of the signal shows the oscillation dynamics of the Bloch wave packet. Additionally, the delayed rise of the signals clearly demonstrates the influence of many-body effects due to Coulomb coupling in the nonlinear optical response of the system. We show that the influence of the many-body effects can be tuned by changing the static applied field: for low field (small exciton binding energy), the FWM signal rises fast; for high field (large exciton binding energy), the signal rises slowly. The experimental results are well described by theory.

Four‐wave‐mixing theory beyond the semiconductor Bloch equations

AbstractFour‐wave‐mixing (FWM) experiments using a dynamical density matrix model of the semiconductor band edge are discussed. Higher‐order correlation functions are retained which are neglected in the commonly used RPA treatment leading to the semiconductor Bloch equations. In order to terminate the hierarchy of the equations of motion for the higher‐order density matrices systematically a truncation scheme controlled by orders in the driving field is applied. For any prescribed order n in the exciting field a closed set of equations is obtained from which the dielectric response up to order n can be calculated exactly. In addition it turns out that in a coherently driven system part of the remaining density matrices become redundant and can be eliminated. Four‐wave‐mixing experiments are dominated by third‐order contributions. Applying the above‐described results one ends up with only two functions in this case. These are the excitonic and the biexcitonic transition densities. As an application of our method the example of a GaAs single quantum well is studied. Two pulses with finite lengths are assumed such that both heavy‐and light‐hole excitons are excited. The influence of the biexciton contribution on the polarization properties of the FWM signal is analyzed and compared with experimental results.

Detuning characteristics and conversion efficiency of nearly degenerate four-wave mixing in a 1.5- mu m traveling-wave semiconductor laser amplifier

The optical nonlinearity in a semiconductor laser gain medium is investigated through copropagating nearly degenerate four-wave mixing (NDFWM) in a 1.5- mu m InGaAsP traveling-wave laser amplifier (TWA). The FWM signal output powers vary symmetrically with the sign of probe detuning with respect to the pump frequency, while the pump and probe output powers vary asymmetrically. The NDFWM conversion efficiency from the probe input to the FWM signal output is a maximum of 8.3 dB around zero detuning and has positive gain in the range of +or-6 GHz. This demonstrates highly efficient nonlinear interaction due to both large optical gain and large third-order susceptibility. The NDFWM efficiency is also investigated in connection with the TWA gain saturation characteristics and is found to be a maximum for operation around the saturation intensity of the TWA. >