Infrared transient absorption of excitons and biexcitons confined in CuCl quantum dots

Abstract

We have measured the mid-infrared transient absorption of CuCl quantum dots embedded in a NaCl matrix with picosecond temporal resolution. The transient absorption exhibited two decay components that are attributable to confined excitons and biexcitons. This result indicates the existence of stable excited states of biexcitons confined in the quantum dots, although the energy separation between the ground and excited states of a biexciton is larger than its binding energy in bulk crystals.1 Introduction The optical properties of semiconductor quantum dots (QDs) have attracted much interest in recent years. The three-dimensional confinement of electrons and holes dramatically changes the electronic structure with a continuous energy spectrum in bulk crystals to that with discrete energy levels in QDs. To date, the optical properties of the lowest-energy state of an electron−hole (e−h) pair (1S Rydberg exciton) confined in QDs have been investigated extensively. However, it is also very important and interesting to study excited-state properties of confined excitons because of larger or anisotropic extension of their wavefunctions. In QDs, the transitions between sublevels of confined excitons are expected to show large optical nonlinearity because of the discrete energy levels and large correlation between excited carriers that are spatially confined in a QD. Utilizing this nonlinearity, the optical response concerned with the confined excited states is applicable to novel infrared (IR) optical devices. A number of studies on the excited exciton states were carried out in materials in the 'strong-confinement' regime, in which the properties of confined excitons are much different from those of free excitons in bulk crystals [1,2]. On the other hand, there are fewer reports on the excited exciton states in the 'weakconfinement' regime, in which the quantum confinement affects essentially the center-of-mass motion of the excitons with less influence on the relative motion of the e−h pair.CuCl QDs show a typical character of the weak confinement because of the very small exciton Bohr radius a B = 0.7 nm. For CuCl QDs, the infrared transient absorption (IRTA) ascribed to the transition from the lowest Rydberg 1S state to the 2P state of the confined exciton was observed and the results showed that the Rydberg energy of the confined exciton depends on the size of the QD [3,4]. However, there are a number of unresolved questions to be clarified. First, since the temporal resolution of the previous measurements was not sufficient to resolve the decay time of the IRTA, the attribution of the IRTA to the confined exciton was still ambiguous. Thus, it is necessary to confirm that the IRTA is actually induced by the exciton, and to examine other possibilities of the origin of the IRTA. Especially, it is interesting to examine the excited states of the confined excitons in the case where two or more excitons are created in a QD. Second, the observed IRTA spectra had large spectral widths and the absorption

Infrared transient absorption and excited-states of excitons and biexcitons confined in CuCl quantum dots

“…However, the authors do not know of any report for the confined biexcitons. Experimentally, the IRTA spectral shape of the biexcitons was quite similar to that of the excitons predicted by the theoretical calculation [5]. Moreover, the obtained peak energy of the biexcitons eventually almost coincides with the transition energy of the Rydberg 1s-2p state of the confined excitons [5].…”

“…In addition, the concept of a biexciton as a quasiparticle is effective in large quantum dots (R>3 nm) [4]. For this material, we found the appearance of infrared transient absorption (IRTA) that was attributed to the biexciton state [5]. In this paper, we have measured the photoluminescence (PL) spectra and the IRTA under two-photon resonant excitation of the confined biexcitons in order to confirm the origin of IRTA as due to biexcitons.…”