Dynamics of excitons in CuBr nanocrystals: Spectral-hole burning and transient four-wave-mixing measurements

Abstract

CuBr nanocrystals ͑NC's͒ embedded in a borosilicate glass matrix have been studied by transmission pump-and-probe experiments with both spectral and temporal resolution. Transient as well as persistent spectral hole-burning phenomena are observed in the excitonic absorption of small NC's ͑mean radius less than 4 nm͒ when picosecond and nanosecond dye-laser pulses excite selectively NC's of a specific size at low temperatures. The holes consist of a zero-phonon line ͑at the photon energy of the excitation͒ and marked phonon sidebands. This structure is explained by a strong exciton-optical-phonon interaction. A value of about 1 of the Huang-Rhys factor S is obtained from the Stokes shifts between absorption and photoluminescence spectra. From the width of zero-phonon holes, a lower limit of 2.3 ps of the exciton dephasing time T 2 is deduced. This value is compared to the one obtained by transient four-wave-mixing measurement ͑6.4 ps͒ at low excitation intensities. When the radii of NC's decrease, both types of spectral holes become broader and the persistent spectral-hole burning is more efficient.

Biexciton, single carrier, and trion generation dynamics in single-walled carbon nanotubes

“…The promoted carrier could then gain enough energy to be ejected from the nanotube, in its vicinity or at trapping sites on the surface, following a mechanism that has been largely identified for semiconductor nanocrystals. 44,45 The other carrier remaining in the nanotube is thus not subject to recombination. It may localize at local fluctuations of the electrostatic potential along the nanotube and be involved in the binding with an exciton to form a trion.…”

Méthodes expérimentales de l'optique non linéaire

“…Cette partie décroît avec la constante de temps T 2 et parce que l'intensité du signal est proportionnelle à |P| 2 , l'écho émis à l'instant 2t décroît avec la constante de temps de 4T 2 . D a été intéressant de mesurer les constantes de temps par mélange de quatre ondes dans des nanocristaux de CuBr [45] et de comparer les résultats avec ceux obtenus par " spectral hole burning ". Nous utilisons les mêmes échantillons que précédemment et les excitons avec des impulsions d'un laser Ti : saphir d'une durée de 100 fs dans la région de la résonance excitonique.…”

“…Cette différence est systématique si l'on mesure à des énergies de photons d'excitation différentes. Ceci est dû aux techniques que l'on emploie : dans le cas du mélange d'onde, on détermine une valeur moyenne de T 2 qui dépend faiblement de l'énergie de photon [45]. Dans le cas du " hole burning " les impulsions laser sont si longues que la distribution des charges dans la matrice peut se réarranger en déplaçant la fréquence de résonance de l'oscillateur.…”

Longitudinal optical phonons in the excited state of CuBr quantum dots

“…The energies of the LO and transverse-optical ͑TO͒ phonons in the bulk CuBr material are 21.1 and 17.0 meV, respectively, which were measured by Raman scattering. 18 Recently, by resonant Raman scattering, [19][20][21][22] the energy of LO phonon in the CuBr QD's in glass was measured to be 20.3 meV, which was almost the same as that of the bulk CuBr crystal. The energy of the LO phonon side bands for small 2.4-nm-radius CuBr QD's in glass was found to be lower than that of the bulk CuBr crystal by 2.5 meV ͑12%͒.…”

“…2, the energies of LO phonons measured by PSHB spectroscopy are clearly smaller than those of LO phonons obtained by Raman scattering. [19][20][21][22] The absorption and PSHB spectra of CuBr QD's in NaBr crystals are shown in Figs. 3͑a͒ and ͑b͒ at 2 K. Unlike CuCl QD's in NaCl crystals, 4,13 no clear oscillatory fine structure is observed in the absorption spectra of CuBr QD's in NaBr crystals.…”