We report novel effect of solvents on optical coherence properties of CdSe/ZnS semiconductor quantum dots (QDs) in solution detected by femtosecond four-wave mixing (FWM) spectroscopy. The FWM signals of QDs show different time profiles and femtosecond coherence decays in three solvents. Our findings provide definite evidence that the interaction with solvent molecules predominantly determines optical coherence and line broadening properties of QDs in solution.
We report novel effect of solvents on optical coherence properties of CdSe/ZnS semiconductor quantum dots (QDs) in solution detected by femtosecond four-wave mixing (FWM) spectroscopy. The FWM signals of QDs show different time profiles and femtosecond coherence decays in three solvents. Our findings provide definite evidence that the interaction with solvent molecules predominantly determines optical coherence and line broadening properties of QDs in solution.
We elaborate methods of femtosecond time resolved spectroscopy of nanocomposites. Features of photon echo (PE) and time resolved four-wave mixing (TRFWM) signals in nanocomposites consisted of semiconductor quantum dots (QD) incorporated into nonlinear dielectric matrix are studied. In particular, impacts of value and angular distribution of permanent dipole moments (PDM) as well as their spatial dispersion on two pulse responses are under investigation. As is known, large values of permanent dipole moments are inherent to many semiconductor QDs, including ZnO QDs. Moreover, the presence of giant dipole moment in cellulose nanocrystals what reaches 4400+400 Debye has been demonstrated [1]. To the best of our knowledge, the PDM impact on the signals of time resolved four-wave mixing as a whole, and, in particular, photon echo has been yet unrevealed.
Our recent work on optical two-dimensional coherent spectroscopy (2DCS) of semiconductor materials is reviewed. We present and compare two approaches that are appropriate for the study of semiconductor nanostructures. The first one is based on a non-collinear geometry, where the Four-Wave-Mixing (FWM) signal is detected in the form of a radiated optical field. This approach works for samples with translational symmetry, such as Quantum Wells (QWs), or large and dense ensembles of Quantum Dots (QDs). The second method is based on a collinear geometry, where the FWM is detected in the form of a photocurrent. This second approach enables 2DCS of samples where translational symmetry is broken, such as single QDs, nanowires, or nanotubes, and small ensembles thereof. For each method, we provide an example of experimental results obtained on semiconductor QWs. In particular, it is shown how 2DCS can reveal coherent excitonic coupling between adjacent QWs.
Coherent optical phenomena such as the optical Stark effect, Rabi flopping, photon echo and quantum beating which are well-known in atomic spectroscopy can also be observed in semiconductors by using femtosecond laser pulses. On these short time scales, the quantum coherence of the optical excitations in the solid do not only influence the optical properties but change at the same time the relaxation and dephasing kinetics. The quasi-classical Boltzmann kinetics has to be replaced by quantum kinetics. Coherence leads to the appearance of memory in the scattering integrals. For femtosecond four-wave mixing and pump-and-probe spectroscopy the use of quantum kinetics for LO-phonon and for carrier-carrier scattering will be reviewed.
We review our recent work on multi-dimensional coherent optical spectroscopy (MDCS) of semiconductor nanostructures. Two approaches, appropriate for the study of semiconductor materials, are presented and compared. A first method is based on a non-collinear geometry, where the Four-Wave-Mixing (FWM) signal is detected in the form of a radiated optical field. This approach works for samples with translational symmetry, such as Quantum Wells (QWs) or large and dense ensembles of Quantum Dots (QDs). A second method detects the FWM in the form of a photocurrent in a collinear geometry. This second approach extends the horizon of MDCS to sub-diffraction nanostructures, such as single QDs, nanowires, or nanotubes, and small ensembles thereof. Examples of experimental results obtained on semiconductor QW structures are given for each method. In particular, it is shown how MDCS can assess coupling between excitons confined in separated QWs.
Abstract Multidimensional coherent optical spectroscopy (MDCS) is an elegant and versatile tool to measure the ultrafast nonlinear optical response of materials. Of particular interest for semiconductor nanostructures, MDCS enables the separation of homogeneous and inhomogeneous linewidths, reveals the nature of coupling between resonances, and is able to identify the signatures of many-body interactions. As an extension of transient four-wave mixing (FWM) experiments, MDCS can be implemented in various geometries, in which different strategies can be used to isolate the FWM signal and measure its phase. I review and compare different practical implementations of MDCS experiments adapted to the study of semiconductor materials. The power of MDCS is illustrated by discussing experimental results obtained on semiconductor nanostructures such as quantum dots, quantum wells, microcavities, and layered semiconductors.