Carrier (exciton) multiplication in colloidal InAs/CdSe/ZnSe core-shell quantum dots (QDs) is investigated using terahertz time-domain spectroscopy, time-resolved transient absorption, and quasi-continuous wave excitation spectroscopy. For excitation by high-energy photons (∼2.7 times the band gap energy), highly efficient carrier multiplication (CM) results in the appearance of multi-excitons, amounting to ∼1.6 excitons per absorbed photon. Multi-exciton recombination occurs within tens of picoseconds via Auger-type processes. Photodoping (i.e., photoinjection of an exciton) of the QDs prior to excitation results in a reduction of the CM efficiency to ∼1.3. This exciton-induced reduction of CM efficiency can be explained by the twofold degeneracy of the lowest conduction band energy level. We discuss the implications of our findings for the potential application of InAs QDs as light absorbers in solar cells.

In our earlier paper, 1 we reported carrier multiplication (CM) in colloidal InAs quantum dots (QDs) and CM in dots with a pre-population of one exciton. The occurrence of CM in relaxed InAs QDs was concluded from the results of time-resolved TeraHertz (THz). Both THz and quasi-continuous wave (quasi-CW) experiments were performed to study the CM in preexcited dots. Recent attempts to reproduce the observations of CM using THz spectroscopy on InAs based QDs of two sizes were unsuccessful. These attempts followed transient absorption measurements in Jerusalem by one of us (S. R.) and co-workers reported elsewhere, 2 which have not yielded evidence for CM in these QDs.Time-resolved THz measurements were repeated for InAs/ CdSe/ZnSe core/shell-1/shell-2, synthesized as reported elsewhere. 3 The investigated QDs had an InAs core of 4.4 nm diameter (E g ) 1.1 eV) onto which one atomic layer of CdSe and four layers of ZnSe were deposited. As in our original paper, the presence or absence of CM was investigated by comparing two excitation wavelengths above and below the theoretical onset for CM (for InAs: 2.05 times E g ). 4 CM is characterized by the presence of relatively short-lived biexcitons (lifetime tens of picoseconds), 1,4,5 which are created by the absorption of one photon. However, biexcitons are also readily created by sequential multiphoton absorption. Hence, the relative yield of biversus single excitons has to be determined for fluences where multiexciton generation by multiphoton absorption is negligible. To disentangle effects of multiphoton excitation versus CM, we use various excitation fluences. Single excitons are longlived (∼100 ns) as compared to the time frame of our experiment.For the 4.4 nm particles, the scaled signals for excitation wavelengths of 400 and 800 nm, corresponding to 2.74 and 1.35 times the gap are shown in Figure 1 (offset for clarity).The data in Figure 1 correspond to 400 and 800 nm excitation fluences, which result in approximately the same average number of absorbed photons per particle (considering the optical density of the sample at the two wavelengths and the ratio of absorption cross sections σ at 400 and 800 nm, σ 400 nm/σ 800 nm ) 10.0). It is apparent from the data that there is no significant bi-exciton decay visible at low fluence. This points directly to the absence of CM. A conservative estimate, considering both the fluence dependence of the 400 nm signal, and a comparison of the signals at 400 and 800 nm at roughly the same excitation densities, provides an upper limit for CM of 10%, well below the factor of 1.6 concluded previously, 1 and also lower than the factor of 1.2 concluded in ref 6 under similar conditions. Summarizing, we could not reproduce our earlier results and the conclusions regarding the presence of CM. One or a combination of the following effects may explain these contradictory observations:•The QDs in previous and recent measurements were synthesized at different times. We cannot exclude the possibility that QDs from different synthesis ba...

Adaptive femtosecond pulse shaping in an evolutionary learning loop is applied to a bioinspired dyad molecule that closely mimics the early-time photophysics of the light-harvesting complex 2 (LH2) photosynthetic antenna complex. Control over the branching ratio between the two competing pathways for energy flow, internal conversion (IC) and energy transfer (ET), is realized. We show that by pulse shaping it is possible to increase independently the relative yield of both channels, ET and IC. The optimization results are analyzed by using Fourier analysis, which gives direct insight to the mechanism featuring quantum interference of a low-frequency mode. The results from the closed-loop experiments are repeatable and robust and demonstrate the power of coherent control experiments as a spectroscopic tool (i.e., quantum-control spectroscopy) capable of revealing functionally relevant molecular properties that are hidden from conventional techniques. coherent control ͉ energy transfer ͉ quantum-control spectroscopy ͉ artificial photosynthesis A rtificial photosynthesis is an important challenge of science and technology today. Numerous applications include solar cells and other artificial power sources, light-emitting materials, sensor systems, and other electronic and photonic nanodevices that use the conversion of light energy into chemical potentials (1). Over the last decade, major technological advances have been made by using biomimicry, an approach that makes use of teachings from studies on nature's wide-ranging selection of highly efficient pigment-protein complexes (2). It has been shown that integrating light-harvesting antennae with electrontransfer relay systems is a potent way to emulate photosynthesis (3). Thus, biomimicry has inspired systems based on complicated natural light-harvesting complexes (LHCs) reduced to their basic elements, and efficient antenna systems based on polymer polyenes covalently attached to tetrapyrroles have been synthesized (4, 5).The antennae are responsible for the first step of photosynthesis, capturing energy of the sun and transferring it to subsequent photosynthetic structures where the energy is transformed in chemical potential. Within various natural and synthetic LHCs, blue-green photons are absorbed by carotenoid molecules, from which the energy is transferred to neighboring porphyrin molecules (6). This energy transfer (ET) step from the carotenoid donor to the accepting molecular species is the primary process in using energy in the 450-to 550-nm window and contributes significantly to the functioning of the complex. The efficiency of ET over competing loss processes, such as internal conversion (IC), is a crucial factor in the overall quantum yield of (artificial) photosynthesis. Hence, a high priority is given to understanding the mechanisms of energy flow and mediating processes to allow development of more efficient artificial systems.In this study, we use adaptive femtosecond pulse shaping in a learning loop (7, 8) to control the pathways of energy flow i...