M.Sc Thesis

M.Sc StudentBronstein Tojen Michal
SubjectTwo-Photon Excitation of Nanocrystals by Intense
Femtosecond Pulses
DepartmentDepartment of Chemistry
Supervisor ASSOCIATE PROF. Zohar Amitay
Full Thesis textFull thesis text - English Version


Semiconductor nanocrystal quantum dots (QDs) are spherical crystalline particles of semiconductor material having a size of a few nanometers, such that quantum-confinement size effects significantly affect many of their properties. One such process is the femtosecond nonresonant two-photon absorption (fs-NR-TPA).

The fs-NR-TPA process in QDs generates an electron-hole pair (exciton) via the absorption of two photons in a nonresonant two-photon interband transition that is induced by a field having its photon energy smaller than the QD's bandgap energy. It was previously investigated experimentally; however the focus was on low pulse intensities corresponding to the single-excitonic regime. Clearly, in order to fully understand and utilize the fs-NR-TPA process, it is important to extend its studies to higher pulse intensities corresponding to the multi-excitonic regime, where several excitons might be generated in a QD via the intrapulse absorption of several photon-pairs.

The present research includes two experimental parts of direct multi-excitonic ultrafast spectroscopy, with the benchmark system being an ensemble of colloidal CdTe QDs of 3.7-nm diameter in room-temperature hexane solution. The corresponding experiments follow a pump-probe scheme.

The first part presents the observation of the phenomenon of intrapulse multi-excitonic state-filling in fs-NR-TPA in QDs. The results exhibit a saturation behavior, where the average-number-of-excitons-per-QD generated by the multi-excitonic fs-NR-TPA strongly deviates from a quadratic dependence on the pump pulse intensity. Our modeling quantitatively explains this effect to originate from intrapulse multi-excitonic state filling of the electronic quantum level into which the two-photon excitation occurs, as dictated by the level degeneracy and the Pauli Exclusion Principle. The results also yield the weak-field two-photon absorption cross-section as well as the bi-excitonic and tri-excitonic non-radiative (Auger) decay lifetimes of the investigated CdTe QDs.

The second part of the research investigates multi-excitonic fs-NR-TPA in QDs in the presence of pre-generated excitons. The corresponding experiments follow a preparation-pump-probe three-pulse scheme, where a first femtosecond pulse prepares the QDs with a variable number of initially generated excitons, a subsequent time-delayed intense femtosecond pump pulse induces the fs-NR-TPA process under the prepared conditions, and a last weak femtosecond probe pulse measures the ensemble's excitons at variable time delays via transient intraband one-photon absorption. The results present absorption inhibition that depends on the number of pre-generated excitons per QD. Our analysis and modeling explain this phenomenon to originate, mainly, from the filling of the state providing the initial-to-final off-resonant coupling for the fs-NR-TPA process.