Biexciton effects in femtosecond nonlinear transmission of semiconductor quantum dots

Klimov, Victor I.; Hunsche, S.; Kurz, H.

doi:10.1103/physrevb.50.8110

Cited by 200 publications

(199 citation statements)

References 23 publications

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“…Furthermore, the observed complete bleaching of the lowest transition under high-intensity pumping could only be interpreted as a coincidence when excitation of a limited number of NCs is assumed. Finally, the DTS of our sample exhibit the same structures after excitation with a pump photon energy of 4 eV as under 2 eV excitation [33].…”

Section: Discussionmentioning

confidence: 56%

“…In our DTS measurements, which have been performed with much larger excitation densities than the fast-scan experiment, we observed a reduction of the bleaching amplitude (at positive delay time) within less than a minute. In an experiment using frequency-doubled pump pulses [33], the visible luminescence decayed within a few seconds. All nonlinear spectra shown in the following are obtained after strong illumination of the probe spot for a few minutes, so that the sample is completely darkened.…”

Section: Photodarkeningmentioning

confidence: 99%

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Ultrafast dynamics of carrier-induced absorption changes in highly-excited CdSe nanocrystals

et al. 1996

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Abstract. We use femtosecond time-resolved transmission spectroscopy to study the density and time dependence of transient absorption changes of CdSe nanocrystals. Our data show pronounced absorption saturation up to complete bleaching of the lowest optical transition. At high carrier density the nonlinear spectra show several peaks that can be related to the two lowest quantized electron states. Thetime dependence of the carrier-induced absorption changes indicates an ultrafast relaxation process within the strongly broadened absorption lines. 78.47.+p; 73.20.Dx; 71.35.+z In recent years, there has been a strong interest in lowdimensional semiconductor structures such as quantum wells, quantum wires, and quantum dots. The electronic states in these artificial materials are strongly influenced by size quantization effects, which occur when the dimensions of the structures become comparable to or smaller than the exciton Bohr radius aB. Then, the electronic and optical properties of the structures become determined by their geometry. Thus, the controlled fabrication of low-dimensional semiconductor structures may allow to obtain materials with predefined properties. PACS:In addition, size quantization has been shown to lead to a significant enhancement of excitonic effects and to strongly enhanced optical nonlinearities in two-dimensional quantum wells, where the carriers are confined in only one dimension [1]. It can be expected that a confinement of carriers in two or three dimensions will lead to a further enhancement of the optical nonlinearities. Therefore, semiconductor quantum dots appear as a promising new material for nonlinear optical applications. It has been found that semiconductor crystallites with radii on the order of a few nanometer, denoted as nanocrystals (NCs), can be rather easily produced in semiconductor-doped glasses (which are commercially available as color-glass filters) or in organic solution [2][3][4]. The linear absorption of such samples generally shows a correlation with the average NC size, as the lowest absorption feature shifts towards higher energies with decreasing size [3,[5][6][7][8][9][10][11]. While this clear sign of size quantization is easily observed, the absorption spectra of semiconductordoped glasses usually fail to show sharp exciton peaks (in contrast with two-dimensional quantum wells) or even wellseparated absorption lines that reflect the expected discrete energy spectrum of a "zero-dimensional" structure. It is commonly accepted that the presence of discrete electronic states is masked in the linear absorption by strong homogeneous and inhomogeneous broadening. The nanocrystals may vary in size, geometry, and chemical composition. These variations cause a strong inhomogeneous broadening of the optical transitions. Variations of the NC size are typically on the order of 25% [12]. This value may be reduced to a few percent by sophisticated preparation techniques, but even then the size distribution has a significant influence on the optical properties [4,10,1...

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Section: Discussionmentioning

confidence: 56%

Section: Photodarkeningmentioning

confidence: 99%

Ultrafast dynamics of carrier-induced absorption changes in highly-excited CdSe nanocrystals

et al. 1996

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“…13 A model for MEG in QDs based on the coherent evolution of a single exciton state into multiple exciton states predicts that oscillations (quantum beats) between states of varying numbers of (energy-allowed) exciton states may be observable by ultrafast spectroscopy. 13 While we have not yet obtained unambiguous experimental evidence for such coherent MEG signature oscillations, we demonstrate and describe how the biexciton effect (also known as the transition Stark shift) [14][15][16][17] produces a probe wavelength-dependent transient absorption that complicates the search for coherent oscillations in the ultrafast lowest-exciton bleach signal.…”

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confidence: 87%

Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

et al. 2005

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We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.

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“…9 Here, we determine ∆ XX for an InAs-CdSe core-shell NQD sample using a spectrally resolved TA method. 47,48 Spectrally resolved TA dynamics exhibit a photoinduced absorption feature on the low-energy side of the 1S exciton transient bleach feature, which has a lifetime that is dictated by intraband relaxation of an excited exciton into the 1S state. 30 This photoinduced absorption feature originates from Coulomb-interaction-induced shift of the 1S transition in the presence of a photoexcited exciton (the "biexciton shift" effect).…”

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confidence: 99%

Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit

2007

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Carrier multiplication (CM) is a process in which absorption of a single photon produces not just one but multiple electron−hole pairs (excitons). This effect is a potential enabler of next-generation, high-efficiency photovoltaic and photocatalytic systems. On the basis of energy conservation, the minimal photon energy required to activate CM is two energy gaps (2E g ). Here, we analyze CM onsets for nanocrystal quantum dots (NQDs) based upon combined requirements imposed by optical selection rules and energy conservation and conclude that materials with a significant difference between electron and hole effective masses such as III−V semiconductors should exhibit a CM threshold near the apparent 2E g limit. Further, we discuss the possibility of achieving sub-2E g CM thresholds through strong exciton−exciton attraction, which is feasible in NQDs. We report experimental studies of exciton dynamics (Auger recombination, intraband relaxation, radiative recombination, multiexciton generation, and biexciton shift) in InAs NQDs and show that they exhibit a CM threshold near 2E g .Carrier Multiplication Thresholds in Bulk and Nanocrystalline Semiconductors. It has recently been established that carrier multiplication (CM), the production of multiple excitons following absorption of a single photon of sufficient energy, is efficient within the range of solar photon energies in semiconductor nanocrystal quantum dots (NQDs). [1][2][3][4][5][6][7][8][9] This finding is important because CM can potentially provide increased power conversion efficiency in low-cost, singlejunction photovoltaics via an enhanced photocurrent. 10,11 To obtain optimum photovoltaic power conversion efficiency using a single semiconductor material with energy gap E g , it is desirable that the CM onset energy is minimal and that the CM efficiency above the onset is as large as is allowed by energy conservation.According to energy conservation, the apparent minimal photon energy that is required to activate CM (pω CM ) is simply 2E g . However, the CM thresholds observed in bulk materials are significantly larger than this value. 12,13 In bulk semiconductors, the generally accepted mechanism for multiexciton generation is impact ionization in which a highenergy electron (or a hole) transfers its energy to a valenceband electron that is excited across the energy gap. 14 The CM threshold in this case is determined not only by conservation of energy but also by conservation of translational momentum, which increases the threshold above 2E g . For example, using a free-particle approximation 15,16 and applying both energy and momentum conservation laws, one can obtain that the CM threshold is approximately 4E g (Figure 1a).In NQDs, translational momentum conservation is relaxed. Therefore, one might expect that the CM threshold could reach the 2E g limit as defined by energy conservation. However, available experimental data indicate that pω CM is still greater than twice the energy gap in such materials. For example, in PbSe and PbS NQDs, it ...

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Biexciton effects in femtosecond nonlinear transmission of semiconductor quantum dots

Cited by 200 publications

References 23 publications

Ultrafast dynamics of carrier-induced absorption changes in highly-excited CdSe nanocrystals

Ultrafast dynamics of carrier-induced absorption changes in highly-excited CdSe nanocrystals

Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit

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