Acoustic phonon strain induced mixing of the fine structure levels in colloidal CdSe quantum dots observed by a polarization grating technique

Coherent Two-Dimensional and Broadband Electronic Spectroscopies

Over the past few decades, coherent broadband spectroscopy has been widely used to improve our understanding of ultrafast processes (e.g., photoinduced electron transfer, proton transfer, and proton-coupled electron transfer reactions) at femtosecond resolution. The advances in femtosecond laser technology along with the development of nonlinear multidimensional spectroscopy enabled further insights into ultrafast energy transfer and carrier relaxation processes in complex biological and material systems. New discoveries and interpretations have led to improved design principles for optimizing the photophysical properties of various artificial systems. In this review, we first provide a detailed theoretical framework of both coherent broadband and two-dimensional electronic spectroscopy (2DES). We then discuss a selection of experimental approaches and considerations of 2DES along with best practices for data processing and analysis. Finally, we review several examples where coherent broadband and 2DES were employed to reveal mechanisms of photoinitiated ultrafast processes in molecular, biological, and material systems. We end the review with a brief perspective on the future of the experimental techniques themselves and their potential to answer an even greater range of scientific questions.

Enhanced Emission from Bright Excitons in Asymmetrically Strained Colloidal CdSe/CdxZn1-xSe Quantum Dots

Colloidal CdSe quantum dots (QDs) designed with a high degree of asymmetric internal strain have recently been shown to host a number of desirable optical properties including subthermal room-temperature line widths, suppressed spectral diffusion, and high photoluminescence (PL) quantum yields. It remains an open question, however, whether they are well-suited for applications requiring emission of identical single photons. Here we measure the low-temperature PL dynamics and the polarization-resolved fluorescence line narrowing spectra from ensembles of these strained QDs. Our spectroscopy reveals the radiative recombination rates of bright and dark excitons, the relaxation rate between the two, and the energy spectra of the quantized acoustic phonons in the QDs that can contribute to relaxation processes. In comparison to conventional colloidal CdSe/ZnS core/shell QDs, we find that in asymmetrically strained CdSe QDs over six times more light is emitted directly by the bright exciton. These results are therefore encouraging for the prospects of chemically synthesized colloidal QDs as emitters of single indistinguishable photons.

Coherent Exciton Dynamics in Ensembles of Size-Dispersed CdSe Quantum Dot Dimers Probed via Ultrafast Spectroscopy: A Quantum Computational Study

Interdot coherent excitonic dynamics in nanometric colloidal CdSe quantum dots (QD) dimers lead to interdot charge migration and energy transfer. We show by electronic quantum dynamical simulations that the interdot coherent response to ultrashort fs laser pulses can be characterized by pump-probe transient absorption spectroscopy in spite of the inevitable inherent size dispersion of colloidal QDs. The latter, leading to a broadening of the excitonic bands, induce accidental resonances that actually increase the efficiency of the interdot coupling. The optical electronic response is computed by solving the time-dependent Schrodinger equation including the interaction with the oscillating electric field of the pulses for an ensemble of dimers that differ by their size. The excitonic Hamiltonian of each dimer is parameterized by the QD size and interdot distance, using an effective mass approximation. Local and charge transfer excitons are included in the dimer basis set. By tailoring the QD size, the excitonic bands can be tuned to overlap and thus favor interdot coupling. Computed pump-probe transient absorption maps averaged over the ensemble show that the coherence of excitons in QD dimers that lead to interdot charge migration can survive size disorder and could be observed in fs pump-probe, four-wave mixing, or covariance spectroscopy.

Simultaneous Existence of Confined and Delocalized Vibrational Modes in Colloidal Quantum Dots

Coupling to phonon modes is a primary mechanism of excitonic dephasing and energy loss in semiconductors. However, low-energy phonons in colloidal quantum dots and their coupling to excitons are poorly understood because their experimental signatures are weak and usually obscured by the unavoidable inhomogeneous broadening of colloidal dot ensembles. We use multidimensional coherent spectroscopy at cryogenic temperatures to extract the homogeneous nonlinear optical response of excitons in a CdSe/CdZnS core/shell colloidal quantum dot ensemble. A comparison to the simulation provides evidence that the observed lineshapes arise from the coexistence of confined and delocalized vibrational modes, both of which couple strongly to excitons in CdSe/CdZnS colloidal quantum dots.

Optical and Physical Probing of Thermal Processes in Semiconductor and Plasmonic Nanocrystals

This article reviews thermal properties of semiconductor and emergent plasmonic nanomaterials, focusing on mechanisms through which hot carriers and phonons are produced and dissipated as well as the related impacts on optoelectronic properties. Elevated equilibrium temperatures, of particular relevance for implementation of nanomaterials in devices, affect absorptive and radiative transitions as well as emission efficiency that can present reversible and irreversible changes with temperature. In noble metal or doped semiconductor/insulator nanomaterials, hot carriers and lattice heating can substantially influence localized surface plasmon resonances and yield large ultrafast changes in transmission or strongly oscillatory coherences. Transient optical and diffraction characterizations enable nonequilibrium investigations of phonon dynamics and cooling such as lattice expansion and crystal phase stability. Timescales of nanoparticle thermalization with surroundings and transport of heat within films of such materials are also discussed.

Thermal Excitation Control over Photon Emission Rate of CdSe Nanocrystals

Temperature-dependent photoluminescence lifetimes of electron-hole pairs (excitons) in CdSe nanocrystals are governed by the energetic ordering and spacing of slowly emitting, spin-forbidden "dark" exciton states and rapidly emitting "bright" states. Here, infrared pulses that are resonant with hydrocarbon surface ligand vibrational transitions are shown to offer a route to manipulate the instantaneous emission rate of CdSe nanocrystals at cryogenic temperature. Transient heating of the inorganic nanocrystal core is achieved via resonant excitation of ligand vibrations, followed by heat flow to the nanocrystal lattice. Heating of the nanocrystal core is demonstrated using transient absorption spectroscopy, which shows a time-dependent red-shift of the quantum dot electronic absorption resonances, consistent with heating. Transient heating of the nanocrystal above the bath temperature increases the instantaneous radiative rate of the nanocrystals via a combination of thermal occupation of bright states as well as phonon-assisted emission. The lifetime of this infrared-pumped, fast-emitting sample condition is dictated by particle thermalization, which is multiple orders of magnitude shorter lived than the dark exciton state. This work demonstrates the feasibility of using heat control pulses to manipulate electronic recombination rates of excitons.

Exciton Fine Structure and Lattice Dynamics in InP/ZnSe Core/Shell Quantum Dots

Nanocrystalline InP quantum dots (QDs) hold promise for heavy-metal-free optoelectronic applications due to their bright and size-tunable emission in the visible range. Photochemical stability and high photoluminescence (PL) quantum yield are obtained by a diversity of epitaxial shells around the InP core. To understand and optimize the emission line shapes, the exciton fine structure of InP core/shell QD systems needs be investigated. Here, we study the exciton fine structure of InP/ZnSe core/shell QDs with core diameters ranging from 2.9 to 3.6 nm (PL peak from 2.3 to 1.95 eV at 4 K). PL decay measurements as a function of temperature in the 10 mK to 300 K range show that the lowest exciton fine structure state is a dark state, from which radiative recombination is assisted by coupling to confined acoustic phonons with energies ranging from 4 to 7 meV, depending on the core diameter. Circularly polarized fluorescence line-narrowing (FLN) spectroscopy at 4 K under high magnetic fields (up to 30 T) demonstrates that radiative recombination from the dark F = ±2 state involves acoustic and optical phonons, from both the InP core and the ZnSe shell. Our data indicate that the highest intensity FLN peak is an acoustic phonon replica rather than a zero-phonon line, implying that the energy separation observed between the F = ±1 state and the highest intensity peak in the FLN spectra (6 to 16 meV, depending on the InP core size) is larger than the splitting between the dark and bright fine structure exciton states.

Polarons Explain Luminescence Behavior of Colloidal Quantum Dots at Low Temperature

Luminescence properties of colloidal quantum dots have found applications in imaging, displays, light-emitting diodes and lasers, and single photon sources. Despite wide interest, several experimental observations in low-temperature photoluminescence of these quantum dots, such as the short lifetime on the scale of microseconds and a zero-longitudinal optical phonon line in spectrum, both attributed to a dark exciton in literature, remain unexplained by existing models. Here we propose a theoretical model including the effect of solid-state environment on luminescence. The model captures both coherent and incoherent interactions of band-edge exciton with phonon modes. Our model predicts formation of dressed states by coupling of the exciton with a confined acoustic phonon mode, and explains the short lifetime and the presence of the zero-longitudinal optical phonon line in the spectrum. Accounting for the interaction of the exciton with bulk phonon modes, the model also explains the experimentally observed temperature-dependence of the photoluminescence decay dynamics and temperature-dependence of the photoluminescence spectrum.

Effect of Electron-Hole Overlap and Exchange Interaction on Exciton Radiative Lifetimes of CdTe/CdSe Heteronanocrystals

Wave function engineering has become a powerful tool to tailor the optical properties of semiconductor colloidal nanocrystals. Core-shell systems allow to design the spatial extent of the electron (e) and hole (h) wave functions in the conduction- and valence bands, respectively. However, tuning the overlap between the e- and h-wave functions not only affects the oscillator strength of the coupled e-h pairs (excitons) that are responsible for the light emission, but also modifies the e-h exchange interaction, leading to an altered excitonic energy spectrum. Here, we present exciton lifetime measurements in a strong magnetic field to determine the strength of the e-h exchange interaction, independently of the e-h overlap that is deduced from lifetime measurements at room temperature. We use a set of CdTe/CdSe core/shell heteronanocrystals in which the electron-hole separation is systematically varied. We are able to unravel the separate effects of e-h overlap and e-h exchange on the exciton lifetimes, and we present a simple model that fully describes the recombination lifetimes of heteronanostructures (HNCs) as a function of core volume, shell volume, temperature, and magnetic fields.

The Strongest Particle: Size-Dependent Elastic Strength and Debye Temperature of PbS Nanocrystals

We investigated the elastic compressibility of PbS nanocrystals (NCs) pressurized in a diamond anvil cell and simultaneously probed the structure using synchrotron-based X-ray diffraction. The compressibility of PbS NCs exhibits bimodal size dependence. The elastic modulus of small NCs increases with increasing diameter and peaks near a particle diameter of approximately 7 nm. For large NCs the elastic modulus decreases toward the bulk value with increasing NC diameter. We explain the bimodal size-dependence of the elastic modulus in terms of a core-shell model based on distinct elasticity of the crystal near the surface and in the core of the particle. We combined insights into the size-dependent elasticity and lattice spacing to determine the Debye temperature of PbS NCs as a function of particle diameter. Understanding the size-dependent elasticity of defect-free colloidal NCs provides new insights into their crystal structure and mechanical properties.

Observation of the full exciton and phonon fine structure in CdSe/CdS dot-in-rod heteronanocrystals

Light emission of semiconductor nanocrystals is a complex process, depending on many factors, among which are the quantum mechanical size confinement of excitons (coupled electron-hole pairs) and the influence of confined phonon modes and the nanocrystal surface. Despite years of research, the nature of nanocrystal emission at low temperatures is still under debate. Here we unravel the different optical recombination pathways of CdSe/CdS dot-in-rod systems that show an unprecedented number of narrow emission lines upon resonant laser excitation. By using self-assembled, vertically aligned rods and application of crystallographically oriented high magnetic fields, the origin of all these peaks is established. We observe a clear signature of an acoustic-phonon assisted transition, separated from the zero-phonon emission and optical-phonon replica, proving that nanocrystal light emission results from an intricate interplay between bright (optically allowed) and dark (optically forbidden) exciton states, coupled to both acoustic and optical phonon modes.

Particle-level engineering of thermal conductivity in matrix-embedded semiconductor nanocrystals

Known manipulations of semiconductor thermal transport properties rely upon higher-order material organization. Here, using time-resolved optical signatures of phonon transport, we demonstrate a "bottom-up" means of controlling thermal outflow in matrix-embedded semiconductor nanocrystals. Growth of an electronically noninteracting ZnS shell on a CdSe core modifies thermalization times by an amount proportional to the overall particle radius. Using this approach, we obtain changes in effective thermal conductivity of up to 5× for a nearly constant energy gap.

Observation of size-dependent thermalization in CdSe nanocrystals using time-resolved photoluminescence spectroscopy

We report heat dissipation times in semiconductor nanocrystals of CdSe. Specifically, a previously unresolved, subnanosecond decay component in the low-temperature photoluminescence decay dynamics exhibits longer decay lifetimes (tens to hundreds of picoseconds) for larger nanocrystals as well as a size-independent, ~25-meV spectral shift. We attribute the fast relaxation to transient phonon-mediated relaxation arising from nonequilibrium acoustic phonons. Following acoustic phonon dissipation, the dark exciton state recombines more slowly via LO-phonon assistance resulting in the observed spectral shift. The measured relaxation time scales agree with classical calculations of thermal diffusion, indicating that interfacial thermal conductivity does not limit thermal transport in these semiconductor nanocrystal dispersions.

A molecular theory for two-photon and three-photon fluorescence polarization

In the analysis of molecular structure and local order in heterogeneous samples, multiphoton excitation of fluorescence affords chemically specific information and high-resolution imaging. This report presents the results of an investigation that secures a detailed theoretical representation of the fluorescence polarization produced by one-, two-, and three-photon excitations, with orientational averaging procedures being deployed to deliver the fully disordered limits. The equations determining multiphoton fluorescence response prove to be expressible in a relatively simple, generic form, and graphs exhibit the functional form of the multiphoton fluorescence polarization. Amongst other features, the results lead to the identification of a condition under which the fluorescence produced through the concerted absorption of any number of photons becomes completely unpolarized. It is also shown that the angular variation of fluorescence intensities is reliable indicator of orientational disorder.