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Two-quantum variants of two-dimensional electronic spectroscopy (2DES) have previously been used to characterize multi-exciton interactions in molecules and semiconductor nanostructures though many implementations are limited by phasing procedures or non-resonant signals. We implement 2DES using phase-cycling to simultaneously measure one-quantum and two-quantum spectra in colloidal CdSe quantum dots. In the pump–probe geometry, fully absorptive spectra are automatically acquired by measuring the sum of the rephasing and nonrephasing signals. Fifth-order two-quantum spectroscopy allows for direct access to multi-exciton states that may be obscured in excited state absorption signals due to population relaxation or third-order two-quantum spectra due to the non-resonant response.

The microscopic origin and timescale of the fluctuations of the energies of electronic states has a significant impact on the properties of interest of electronic materials, with implication in fields ranging from photovoltaic devices to quantum information processing. Spectroscopic investigations of coherent dynamics provide a direct measurement of electronic fluctuations. Modern multidimensional spectroscopy techniques allow the mapping of coherent processes along multiple time or frequency axes and thus allow unprecedented discrimination between different sources of electronic dephasing. Exploiting modern abilities in coherence mapping in both amplitude and phase, we unravel dissipative processes of electronic coherences in the model system of CdSe quantum dots (QDs). The method allows the assignment of the nature of the observed coherence as vibrational or electronic. The expected coherence maps are obtained for the coherent longitudinal optical (LO) phonon, which serves as an internal standard and confirms the sensitivity of the technique. Fast dephasing is observed between the first two exciton states, despite their shared electron state and common environment. This result is contrary to predictions of the standard effective mass model for these materials, in which the exciton levels are strongly correlated through a common size dependence. In contrast, the experiment is in agreement with ab initio molecular dynamics of a single QD. Electronic dephasing in these materials is thus dominated by the realistic electronic structure arising from fluctuations at the atomic level rather than static size distribution. The analysis of electronic dephasing thereby uniquely enables the study of electronic fluctuations in complex materials.

Strong confinement in semiconductor quantum dots enables them to host multiple electron–hole pairs or excitons. The excitons in these materials are forced to interact, resulting in quantum-confined multiexcitons (MXs). The MXs are integral to the physics of the electronic properties of these materials and impact their key properties for applications such as gain and light emission. Despite their importance, the electronic structure of MX has yet to be fully characterized. MXs have a complex electronic structure arising from quantum many-body effects, which is challenging for both experiments and theory. Here, we report on the investigation of the electronic structure of MX in colloidal CdSe QDs using time-resolved photoluminescence, state-resolved pump–probe, and two-dimensional spectroscopies. The use of varying excitation energy and intensities enables the observation of many signals from biexcitons and triexcitons. The experiments enable the study of MX structures and dynamics on time scales spanning 6 orders of magnitude and directly reveal dynamics in the biexciton manifold. These results outline the limits of the simple concept of binding energy. The methods of investigations should be applicable to reveal complex many-body physics in other nanomaterials and low-dimensional materials of interest.

The electronic structure of multiexcitons significantly impacts the performance of nanostructures in lasing and light-emitting applications. However, these multiexcitons remain poorly understood due to their complexity arising from many-body physics. Standard transient-absorption and photoluminescence spectroscopies are unable to unambiguously distinguish effects of sample inhomogeneity from exciton-biexciton interactions. Here, we exploit the energy and time resolution of two-dimensional electronic spectroscopy to access the electronic structure of the band-edge biexciton in colloidal CdSe quantum dots. By removing effects of inhomogeneities, we show that the band-edge biexciton structure must consist of a discrete manifold of electronic states. Furthermore, the biexciton states within the manifold feature distinctive binding energies. Our findings have direct implications for optical gain thresholds and efficiency droop in light-emitting devices and provide experimental measures of many-body physics in nanostructures.

Coherent Multi-dimensional Spectroscopy is ideally suited to investigate many-body effects in semiconductor nanostructures. Here we employ 2D optical spectroscopy on the model system of CdSe quantum dots to reveal the structure of the bandedge biexciton.

We present a detailed analysis of a setup capable of arbitrary amplitude, phase, and polarization shaping of broadband visible femtosecond pulses at 1 kHz via a pair of actively phase stabilized acousto-optic programmable dispersive filters arranged in a Mach-Zehnder interferometer geometry. The setup features phase stability values around λ/225 at 580 nm as well as degrees of polarization of at least 0.9 for any polarization state. Both numbers are important metrics to evaluate a setup's potential for applications based on polarization-shaped femtosecond pulses, such as fully coherent multi-dimensional electronic spectroscopy.

The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the vario...

The quantum dot, one of the central materials in nanoscience, is a semiconductor crystal with a physical size on the nanometer length scale. It is often called an "artificial atom" because researchers can create nanostructures which yield properties similar to those of real atoms. By virtue of having a size in between molecules and solids, the quantum dot offers a rich palette for exploring new science and developing novel technologies. Although the physical structure of quantum dots is well known, a clear understanding of the resultant electronic structure and dynamics has remained elusive. However, because the electronic structure and dynamics of the dot, the "excitonics", confer its function in devices such as solar cells, lasers, LEDs, and nonclassical photon sources, a more complete understanding of these properties is critical for device development. In this Account, we use colloidal CdSe dots as a test bed upon which to explore four select issues in excitonic processes in quantum dots. We have developed a state-resolved spectroscopic approach which has yielded precise measurements of the electronic structural dynamics of quantum dots and has made inroads toward creating a unified picture of many of the key dynamic processes in these materials. We focus on four main topics of longstanding interest and controversy: (i) hot exciton relaxation dynamics, (ii) multiexcitons, (iii) optical gain, and (iv) exciton-phonon coupling. Using this state-resolved approach, we reconcile long standing controversies related to phenomena such as exciton cooling and exciton-phonon coupling and make surprising new observations related to optical gain and multiexcitons.

Single dot spectroscopy is performed on two-color CdSe/ZnS/CdSe core/barrier/shell nanostructures. Unlike quantum dots cores, these systems have two phases with which to emit and ultimately examine for blinking analysis. These particles are brighter than conventional quantum dots and also show the photoluminescence (PL) intensity and energy fluctuations characteristic of quantum dots. Single dot spectral diffusion analysis yields no measureable energy shift correlation between the core and the shell on the 200 ms time scale. In contrast, the single dot PL from the CdSe shell has narrower linewidths than the CdSe core, indicating differences in its spectral diffusion on shorter timescales.

The first observation of state-resolved exciton−phonon couplings are reported for the lowest four eigenstates in strongly confined colloidal quantum dots for both optical and acoustic phonons. State-resolved pump/probe experiments show that the excitonic eigenstate has a strong influence upon the Frohlich coupling to optical phonons and a moderate influence on the deformation potential coupling to acoustic phonons. These experiments provide a general approach to measuring exciton−phonon couplings in quantum dots with eigenstate-specificity.

In this chapter, the influence of quantum confinement upon light amplification is reviewed. Emphasis is placed upon the two broad classes of three-dimensionally confined nanostructures: self-assembled quantum dots and colloidal quantum dots. Fabrication, carrier dynamics, and state-of-the-art-device results for these classes of quantum dots are reviewed.

The elementary excitation in semiconductor quantum dots is the exciton, an excited electron-hole pair. The size and geometry of the dot confines the exciton thereby yielding quantum confinement effects. The simplest examples of size quantization effects include the spectrum of single exciton states which dominate the linear absorption spectrum and the Stokes shift for the spontaneous photoluminescence spectrum. Here, we report on our recent results exploring the structure and dynamics of both single excitons and multi-excitons in colloidal CdSe quantum dots using femtosecond state-resolved pump/probe spectroscopy. These experiments have revealed tremendous insight into the relaxation pathways of hot excitons, new aspects of exciton-phonon interactions, and in the first observation of the electronic structure of multi-excitons.

We report on the electronic structure of biexcitons in CdSe quantum dots using state-selective femtosecond pump/probe spectroscopy. The pump/probe experiments are compared to direct probing of biexcitons via two-dimensional electronic spectroscopy.

Employing real time observation of coherent acoustic phonons, we demonstrate a novel extrinsic piezoelectric response of quantum dots, that is quantized, tunable and an order of magnitude larger than their intrinsic piezo response.

Optical gain in strongly confined colloidal semiconductor quantum dots is measured using state resolved pump/probe spectroscopy. Though size tunable optical amplification has been previously reported for these materials, the influence of confinement enhanced multiexcitonic interactions has limited prior demonstrations to specific particle sizes or host media. Here we show that the influence of the interfering multiexcitonic interactions, and hence the development of optical gain, is dependent on the identity of the initially prescribed excitonic state. By maintaining a constant excitonic state in the size tunable electronic structure of these materials, we recover the predicted universal development of optical gain, reflected by size-independent occupation thresholds, and differential gains. In addition, we explicitly compare the influence of surface passivation on the development and lifetime of the optical gain. Furthermore, we introduce a general, state-resolved pumping scheme which enables control over the optical gain spectrum. The capacity to manipulate the optical gain spectra of these spherically confined systems is evident in both the measured stimulated emission and amplified spontaneous emission. We anticipate that state-resolved optical excitation will be a useful method of enabling the development and manipulation of optical gain in any quantized nanostructure.

We report on the electronic structure of the band-edge biexciton in colloidal CdSe quantum dots using femtosecond spectroscopy and atomistic many-body pseudopotential calculations. Time-resolved spectroscopy shows that optical transitions between excitonic and biexcitonic states are distinct for absorptive and emissive transitions, leading to a larger Stokes shift for the biexciton than for the single exciton. The calculations explain the experimental results by showing that there is a previously unobserved electronic substructure to the band-edge biexciton which yields two distinct families of transitions.

Excitonic state-resolved optical pumping experiments were performed on strongly confined semiconductor quantum dots. We demonstrate for the first time that optical gain is dependent upon the initial excitonic state. By prescribing the specific multiexcitonic states which can create, block, and ultimately control optical gain spectra, we recover the theoretically predicted size independence, even in systems which previously showed zero gain. In addition, we show for the first time that stimulated emission in quantum dots can be controlled via specific multiexcitonic interactions.

Biexcitons in strongly confined, colloidal CdSe quantum dots were investigated with excitonic state selectivity combined with 10 fs temporal precision. Within the first 50 fs, the first excited state of the biexciton was observed. By 100 ps, mixed character biexcitons were observed, comprised of a core exciton and a surface trapped exciton. The size dependence of the biexciton binding energies is reported for these specific biexcitons. Analysis of the spectral signatures of each biexcitonic state yields a quantitative measure of enhanced excited state trapping rates at the surface of the quantum dots. By comparing the biexcitonic signals to the state-filling signals, we show that it is primarily the holes which are trapped at the interface on the 100 ps time scale.

We show that biexcitons in quantum dots posses a coarse and fine structure. The intrinsic fine structure of biexcitons has not been previously observed in any system and is the controlling parameter for optical gain.

Using state-selective excitation to tailor multi-exciton interactions, we have demonstrated that optical gain in strongly confined semiconductor quantum dots is a completely universal, size independent, and intrinsic property of these materials.

Coherent optical and acoustic phonons are simultaneously observed in CdSe quantum dots with excitonic state-specificity for the first time. These experiments yield the size and eigenstate dependent coupling strengths for both modes.

Size dependent electron and hole relaxation dynamics were measured in colloidal CdSe quantum dots with state-to-state specificity. These experiments reveal the electron and hole state-to-state relaxation dynamics with a precision of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, allowing quantitative evaluation of the manifold of pathways by which an exciton relaxes in strongly confined quantum dots. These experiments corroborate previously observed confinement induced femtosecond Auger relaxation channels for electrons, but with sufficient precision to quantitatively and unambiguously determine the size dependence of the Auger mechanism. These experiments also show that the hole energy loss rate increases for smaller quantum dots, contradicting known relaxation mechanisms for holes. We propose a confinement enhanced mechanism for hole relaxation in colloidal quantum dots, overcoming the predicted phonon bottleneck for holes. The relative contributions of the relaxation pathways are identified for electrons and for holes. These state selective experiments produce a unified picture of the manifold of relaxation pathways available to both electrons and holes in strongly confined colloidal quantum dots.

Size dependent hole dynamics are measured in colloidal CdSe quantum dots for a specific state-to-state excitonic transition. These experiments show that the hole energy loss rate increases for smaller quantum dots, contradicting known relaxation mechanisms for holes. These experiments reveal a new mechanism for hole relaxation in colloidal quantum dots which circumvents the expected phonon bottleneck for holes. The data are consistent with a nonadiabatic surface channel as the dominant pathway for hole relaxation in colloidal semiconductor quantum dots.

Femtosecond carrier relaxation dynamics in colloidal CdSe quantum dots are measured which focus on the early time dynamics for different initial excitonic states. Selecting the initial excitonic states produces a clear influence upon the early time dynamics. The difference between the dynamics when initially populating different excitonic states provides a direct measurement of excitonic relaxation times in strongly confined colloidal semiconductor quantum dots with specificity to electrons and holes. These results furthermore show state specific biexciton interactions.

Femtosecond relaxation dynamics of colloidal CdSe quantum dots are measured for different initial excitonic states. These experiments show dramatic, previously unobserved dynamics at all probe wavelengths based upon preparation of the initial state.

We demonstrate a pulse-shaper capable of independent polarization, phase and amplitude control over each pulse. The set-up is compact and easily switchable between pump-probe and collinear geometries.

Multiple exciton recombination (MER) and multiple exciton generation (MEG) are two of the main processes for assessing the usefulness of quantum dots (QDs) in photovoltaic devices. Recent experiments, however, have shown that a firm understanding of both processes is far from well established. By performing surface-dependent measurements on colloidal CdSe QDs, we find that surface-induced charge trapping processes lead to false MER and MEG signals resulting in an inaccurate measurement of these processes. Our results show that surface-induced processes create a significant contribution to the observed discrepancies in both MER and MEG experiments. Spectral signatures in the transient absorption signals reveal the physical origin of these false signals.

The overall symmetry of the envelope functions for the four lowest energy excitonic states in colloidal CdSe quantum dots are assigned using excitonic state-resolved pump/probe spectroscopy. These experiments yield a direct test of the two prevailing approaches to quantum dot electronic structure: the effective mass and the atomistic approaches. The experimental results are in complete agreement with the atomistic approach, whereas the effective mass approach fails to assign the correct electron symmetries for higher excitonic states.

We analyze sources of noise in kilohertz frequency pump/probe experiments and present a method for reducing experimental noise by identifying and filtering noisy shots. The power spectrum of instrumental noise shows high frequency, small amplitude modulations which cannot be averaged out. A histogram analysis shows that low frequency, large amplitude signals pose a serious obstacle to signal averaging for improved signal to noise. In kilohertz frequency pump/probe experiments, this low frequency noise typically arises from laser scatter due to bubbles, dust, and defects. We quantify the effectiveness in analyzing and rejecting these large amplitude signals which can produce a hindrance to the effectiveness of signal averaging.