Relaxation in the Exciton Fine Structure of Semiconductor Nanocrystals

Abstract

A spectroscopic study is reported of relaxation in the exciton fine structure of CdSe nanorods measured using ultrafast cross-polarized heterodyned third-order transient grating (CPH-3TG) spectroscopy. The CPH-3TG spectroscopy probes the dynamics of population transfer between states accompanying exciton spin flip (radiationless transitions in the exciton fine structure associated with changes in the sign of the total angular momentum). Analysis of the data enabled elucidation of pathways for exciton fine structure relaxation (EFSR) involving all of the exciton fine structure states. The mechanism and origin of the EFSR dynamics in CdSe nanorods are discussed in the exciton state picture with an analogy to molecular radiationless transition processes such as internal conversion and intersystem crossing. Key conclusions are that the fast transitions are between states with total angular momentum of (1 and -2 and that the rate of this transition follows an inverse diameter to the fourth power size dependence, which originates from the Dresselhaus spin-orbit coupling effect.

Spin relaxation in zinc blende and wurtzite CdSe quantum dots

“…One exponential, associated with exciton recombination had a long nanosecond timescale that could not be fully resolved in our experiments, which had a maximum pump-probe time delay, t p , of 850 ps. The other two exponentials likely correspond to surface trapping dynamics and were on the order of picoseconds to tens of picoseconds, consistent with previous results [10,31,32,11,12]. The timescales and relative amplitudes obtained from the VVVV data were then used to analyze the VHVH and VHHV traces, allowing us to account for the exciton population relaxation dynamics in the CPH-3TG signals and to obtain the exciton spin flip rate.…”

“…The laser pulse was tuned to be resonant with the lowest energy exciton transition of each of the QD samples. Further details of the experiment have been described elsewhere [10]. All measurements were performed at 293 K using low incident laser intensity ($6 nJ/pulse) to avoid higher order, biexciton or thermal grating induced effects.…”

“…Inhomogeneous broadening due to the inherent size-distribution associated with colloidal QD samples can easily obscure this small energy separation. Despite the small energy differences, relaxation dynamics in the fine structure of colloidal QD samples can be measured using a time-resolved spectroscopic technique, cross-polarized heterodyned third-order transient grating (CPH-3TG), recently developed by our group [10][11][12][13][14][15][16].…”

“…This signal becomes zero when the system reaches a dynamic equilibrium where there are an equal number of excitons with flipped and conserved spins. Thus, the CPH-3TG technique follows the history of an exciton spin by measuring whether or not the total angular momentum of an exciton state has changed since its initial excitation [10].…”

Two‐Dimensional Visible Spectroscopy For Studying Colloidal Semiconductor Nanocrystals

“…[ 48 ] On the other hand, acoustic phonons allow spin fl ip between bright and dark states within the exciton fi ne structure via spin-orbit couplings. [ 87 ] In inorganic nanostructures, the vibrational modes (phonon modes) have relatively low frequency compared to the intramolecular vibrational modes of organic systems. Therefore, it is usually diffi cult to resolve the phonon replica from the zero-phonon line at room temperature in ensemble measurements.…”