Session H10: Focus Session: Optical Properties of Nanostructures I: Quantum Dots

Coherent Optical Phenomena in Quantum Dots

The quantum confinement provided by a semiconductor quantum dot suppresses much of the many body physics associated with the coherent nonlinear optical response observed in higher dimensional systems. This makes them attractive for potential device applications where atomic like properties, such as high Q resonances, strong optical interactions, or long quantum coherence times, could be important. In this talk, we present recent results demonstrating high field effects beyond Rabi oscillations including the Mollow absorption spectrum showing gain without inversion, dark state formation in single electron doped dots, and suppression of nuclear fluctuations by the hyperfine interaction leading to longer electron spin coherence times.   

Creating an artificial periodic table using quantum dots

Confinement of carriers in quantum dots results in hydrogenic like states for the exciton. Thus a single excitation in a quantum dot bears resemblance to a hydrogen atom; these materials are often referred to as ``artificial atoms.'' A pair of excitons will form a four body biexciton, akin to a helium atom. The excitonic `He atom should have an eigenstate spectrum in the vein of atomic orbitals. The eigenstate spectrum of the biexciton has remained elusive due to the ultrafast timescale of relaxation processes in quantum dots which mask observation of the excited states. Here, we show the first, direct observation of spectrum of states of the biexciton, completing the analogy of excitons in quantum dots to atomic and molecular systems. We report on the first observation of a biexciton Stokes shift, which we will discuss in terms of non-Aufbau filling and biexciton fine structure. The observation of biexciton Stokes shift underpins the physics of optical gain in quantum dots.   

Selection rules for optical transitions in lead salts semiconductor nanocrystals: Drastic effect of structure inversion asymmetry

We resolve a paradox according to which some of the optical transitions in PbSe and PbS semiconductor nanocrystals seemingly forbidden by the parity selection rule exhibit a large oscillator strength in optical absorption. We calculate oscillator strengths for various optical transitions in the framework of the effective mass approximation.   

The appearance of localized resonances above the continuum in quantum dots

We investigate the nature of hole and electron states in self-assembled InAs/GaAs and (In,Ga)As quantum dots, using a multi-band atomistic pseudopotential approach. We offer a classification of both hole and electron states based on an analysis of their localization both in the $z$ and $xy$-directions. We show that the coherent dot-matrix strain present in self-assembled quantum dots distorts the electron confining potential, creating ``wings'' in the vicinity of the dot. This results in the appearance of dot-confined electronic states that lie above the continuum of the matrix material. The spectroscopic manifestation of these resonant states is investigated, by calculating the inter-band as well as the intra-band absorption spectra. We find, in both cases, that clear finger-prints of the resonances appear, in the form of sharp, well-defined peaks. In contrast, the previously suggested ``cross-transitions'' between wetting-layer states and dot states are shown to disappear once realistic strain is included.   

Effect of Atomic-Scale Alloy Randomness on the Optical Polarization of Semiconductor Quantum Dots

Alloyed Ga$_{1-x}$In$_x$As system consists of different random assignments $\sigma$ of the Ga and In atoms to the cation sublattice sites; each configuration having, in principle, distinct physical properties. For self-assembled dots made of finite number of cations ($\leq$10$^5$), self-averaging of configurations may not be complete, so single-dot spectroscopy can observe the atomic-scale alloy randomness effects. We examine the effect of such atomic-scale randomness on the fine structure-splitting (FSS) of the exciton observed via the polarization anisotropy of its components. We find: (i) The FSS of the monoexciton X$^0$ changes by more than a factor of 7 with $\sigma$. Thus, finite nanostructure systems provide clear evidence for the effects of atomic-scale randomness on physical properties. (ii) The polarization anisotropy of two X$^0$ transitions is affected both by $\sigma$ variations and from possible QD base elongation. Thus, the polarization anisotropy cannot be used as a measure of geometrical anisotropy alone, (iii) Polarization directions of different multiexciton emission lines are determined by $\sigma$.   

Blinking suppression on millisecond-to-minutes time scales in giant nanocrystal quantum dots

Fluorescence intermittency (blinking) is an intrinsic feature of molecular-like fluorophores, including nanocrystal quantum dots (NQDs). The effect complicates applications of NQDs in areas such as quantum informatics, bio-imaging, and real-time tracking. Previously we developed ``giant'' NQDs in which a small emitting core is overcoated with a thick shell of a wider-gap material and observed strong blinking suppression on a time scale of 100s ms and longer. In this work, we employ time-tagged correlated single photon counting to detect photoluminescence (PL) traces from individual ``giant'' CdSe/CdS NQDs with resolution better than 1 ms. We observe a strong dependence of the fluorescence on/off times on shell thickness and almost complete blinking suppression on all measured time scales for NQDs coated with more than $\sim $10 monolayers of CdS. Further systematic analysis of our PL traces reveal a photon statistics that differs significantly from a power-law distribution of on/off times typically observed for ``regular'' NQDs.   

Scaling of Multiexciton Nonradiative and Radiative Decay Rates with Exciton Number in Semiconductor Nanocrystals

Rapid multiexciton decay by nonradiative Auger recombination places strong constraints on potential applications of semiconductor nanocrystals (NCs) in lasing and solar energy conversion exploiting carrier multiplication. Hence, it is important to understand the scaling of the Auger recombination rate with exciton number in NCs. Likewise, understanding the scaling of multiexciton radiative rates with exciton number is important both for potential applications of, e.g., ordered multiphoton emission and for interpreting experimental measurements of time-resolved photoluminescence. We report measurements of the scaling of Auger and recombination rates in CdSe and PbSe NCs and of multiexciton radiative rates in PbSe NCs. The more rapid scaling of Auger rates with exciton number N in PbSe compared to CdSe can be understood in terms of the different symmetries of N-excitons with N$>$2 due to the different degeneracies of the lowest-energy excitonic states. The scaling of the multiexciton radiative rates in PbSe can be interpreted in terms of a ``free-carrier'' model.   

Carrier Multiplication in PbSe Nanocrystals and Extraneous Processes

Spatial confinement of electronic wave functions in semiconductor nanocrystals (NCs) can enhance the efficiency of carrier multiplication (CM), a process whereby multiexcitons are generated from single absorbed photons. In the last year, a controversy has emerged due to large discrepancies between values of CM efficiencies reported by different groups using different techniques -- transient absorption (TA) and photoluminescence upconversion (uPL). We report studies of CM in solutions of PbSe NCs using measurements of exciton dynamics by both TA and uPL and find excellent agreement between the CM efficiencies extracted by both techniques. Moderate variations in efficiencies are observed for nominally similar samples. More dramatically, measurements of static and stirred solutions can display large differences in dynamics. This indicates that extraneous effects such as NC photoionization can distort the results of CM studies and are a likely contribution to the discrepancies between previously reported CM efficiencies.   

Universal size dependence of Auger constants in direct- and indirect-gap semiconductor nanocrystals

We compare Auger recombination rates in several direct- and indirect-gap semiconductor nanocrystals including Ge, PbSe, InAs, and CdSe. Our size-dependent biexciton lifetime measurements indicate that the most important factor determining recombination rates is nanocrystal size, while details of the materials' electronic structure such as the width of the energy gap or its direct/indirect nature play only a minor role. We observe that the effective Auger constants for all semiconductor nanocrystals in this study exhibit a universal cubic dependence on particle radius (R), C$_{A }\sim $ R$^{3}$. Moreover, absolute values of nanocrystal Auger constants are comparable across different materials despite a dramatic difference (up to 4-5 orders of magnitude) in C$_{A}$ values in the respective bulk solids. Our results can be explained by confinement-induced relaxation of momentum conservation, diminishing the difference between direct- and indirect-gap semiconductors at the nanoscale.   

Steady-state fluorescence spectroscopy of multiexcitons in single `giant' nanocrystal quantum dots

Due to ultrafast nonradiative Auger recombination, emission of multiexciton states is not pronounced in steady-state spectra of nanocrystal quantum dots (NQDs). Here, we report the first observation of multiexcitonic signatures in steady-state photoluminescence (PL) from single `giant' core/shell NQDs$^{\ast }$, in which a CdSe core is overcoated with a thick ($>$10 monolayers) CdS shell. At low temperature, we observe the emergence of multiple high-energy PL peaks with increasing pump power. Analysis of intensity scaling of these PL peaks with pump power allows us to assign them to bi-, tri- and higher order multiexcitons. Lifetimes of these multiexciton states obtained by single-dot time- and wavelength-resolved PL further corroborate this assignment. These results suggest that in `giant' NQDs Auger recombination is greatly suppressed compared to regular NQDs, which likely stems from their large effective volume and decreased spatial overlap between electrons (occupy entire NQD volume) and holes (localized in CdSe core). $^{\ast }$Y.F. Chen et al. \textit{J. Am. Chem. Soc}. \textbf{130}, 5026 (2008)