Two-Dimensional Visible Spectroscopy For Studying Colloidal Semiconductor Nanocrystals

Sub-8-fs pulses in the visible to near-infrared by a degenerate optical parametric amplifier

This work presents a single-stage optical parametric amplifier (OPA) operating at degeneracy (DOPA) and pumped by the third harmonic of a Yb:KGW laser system. This DOPA exploits the broad amplification bandwidth that occurs with type-I phase-matching in β-barium borate (BBO) when signal and idler overlap in the spectrum. The output pulses span from 590 to 780 nm (1.59-2.10 eV) with 7.75-fs duration after compression. Ultrashort pulses with similar bandwidths in this spectral window complement the existing array of optical parametric amplifiers that cover either the visible or the near-IR spectral regions with sub-10-fs pulses. This source of ultrashort optical pulses will enable the application of sophisticated spectroscopy techniques to the study of electronic coherences and energy migration pathways in biological, chemical, and condensed matter systems.

Narrow homogeneous linewidths and slow cooling dynamics across infrared intra-band transitions in n-doped HgSe colloidal quantum dots

Intra-band transitions in colloidal quantum dots (QDs) are promising for opto-electronic applications in the mid-IR spectral region. However, such intra-band transitions are typically very broad and spectrally overlapping, making the study of individual excited states and their ultrafast dynamics very challenging. Here, we present the first full spectrum two-dimensional continuum infrared (2D CIR) spectroscopy study of intrinsically n-doped HgSe QDs, which exhibit mid-infrared intra-band transitions in their ground state. The obtained 2D CIR spectra reveal that underneath the broad absorption line shape of ∼500 cm^-1, the transitions exhibit surprisingly narrow intrinsic linewidths with a homogeneous broadening of 175-250 cm^-1. Furthermore, the 2D IR spectra are remarkably invariant, with no sign of spectral diffusion dynamics at waiting times up to 50 ps. Accordingly, we attribute the large static inhomogeneous broadening to the distribution of size and doping level of the QDs. In addition, the two higher-lying P-states of the QDs can be clearly identified in the 2D IR spectra along the diagonal with a cross-peak. However, there is no indication of cross-peak dynamics indicating that, despite the strong spin-orbit coupling in HgSe, transitions between the P-states must be longer than our maximum waiting time of 50 ps. This study illustrates a new frontier of 2D IR spectroscopy enabling the study of intra-band carrier dynamics in nanocrystalline materials across the entire mid-infrared spectrum.

Ultrafast Coherent Delocalization Revealed in Multilayer QDs under a Chiral Potential

In recent years, it was found that current passing through chiral molecules exhibits spin preference, an effect known as Chiral Induced Spin Selectivity (CISS). The effect also enables the reduction of scattering and therefore enhances delocalization. As a result, the delocalization of an exciton generated in the dots is not symmetric and relates to the electronic and hole excited spins. In this work utilizing fast spectroscopy on hybrid multilayered QDs with a chiral polypeptide linker system, we probed the interdot chiral coupling on a short time scale. Surprisingly, we found strong coherent coupling and delocalization despite having long 4-nm chiral linkers. We ascribe the results to asymmetric delocalization that is controlled by the electron spin. The effect is not measured when using shorter nonchiral linkers. As the system mimics light-harvesting antennas, the results may shed light on a mechanism of fast and efficient energy transfer in these systems.

Single particle level dynamics of photoactivation and suppression of Auger recombination in aqueous Cu-doped CdS quantum dots

Cu-doped CdS quantum dots (QDs) have been synthesized in water using 3-mercaptopropionic acid (3-MPA) as the capping agent. They exhibit intense photoluminescence and excellent color tunability, unlike most of the QDs synthesized/dispersed in water so far. Complete characterization of these aqueous doped CdS QDs has been performed for the first time, along with a single particle level elucidation of their exciton dynamics using fluorescence correlation spectroscopy. Photoactivation via dim/dark to bright particle conversion is observed at higher excitation powers. Dispersive blinking kinetics in undoped QDs reflects the involvement of a broad distribution of trap states. A lesser extent of dispersity is observed for doped QDs, in which hole-capture by Cu-defect states predominates. Excitation fluence dependence of the blinking rate highlights the role of Auger recombination in undoped QDs, which is suppressed significantly upon doping, due to disruption of the electron-hole correlation.

2D Electronic Spectroscopic Techniques for Quantum Technology Applications

2D electronic spectroscopy (2DES) techniques have gained particular interest given their capability of following ultrafast coherent and noncoherent processes in real-time. Although the fame of 2DES is still majorly linked to the investigation of energy and charge transport in biological light-harvesting complexes, 2DES is now starting to be recognized as a particularly valuable tool for studying transport processes in artificial nanomaterials and nanodevices. Particularly meaningful is the possibility of assessing coherent mechanisms active in the transport of excitation energy in these materials toward possible quantum technology applications. The diverse nature of these new target samples poses significant challenges and calls for a critical rethinking of the technique and its different realizations. With the confluence of promising new applications and rapidly developing technical capabilities, the enormous potential of 2DES techniques to impact the field of nanosystems, quantum technologies, and quantum devices is here delineated.

Observing Multiexciton Correlations in Colloidal Semiconductor Quantum Dots via Multiple-Quantum Two-Dimensional Fluorescence Spectroscopy

Correlations between excitons, that is, electron-hole pairs, have a great impact on the optoelectronic properties of semiconductor quantum dots and thus are relevant for applications such as lasers and photovoltaics. Upon multiphoton excitation, these correlations lead to the formation of multiexciton states. It is challenging to observe these states spectroscopically, especially higher multiexciton states, because of their short lifetimes and nonradiative decay. Moreover, solvent contributions in experiments with coherent signal detection may complicate the analysis. Here we employ multiple-quantum two-dimensional (2D) fluorescence spectroscopy on colloidal CdSe_1-xS_x/ZnS alloyed core/shell quantum dots. We selectively map the electronic structure of multiexcitons and their correlations by using two- and three-quantum 2D spectroscopy, conducted in a simultaneous measurement. Our experiments reveal the characteristics of biexcitons and triexcitons such as transition dipole moments, binding energies, and correlated transition energy fluctuations. We determine the binding energies of the first six biexciton states by simulating the two-quantum 2D spectrum. By analyzing the line shape of the three-quantum 2D spectrum, we find strong correlations between biexciton and triexciton states. Our method contributes to a more comprehensive understanding of multiexcitonic species in quantum dots and other semiconductor nanostructures.

High-Efficiency FRET Processes in BODIPY-Functionalized Quantum Dot Architectures

Efficient FRET systems are developed combining colloidal CdSe quantum dots (QDs) donors and BODIPY acceptors. To promote effective energy transfer in FRET architectures, the distance between the organic fluorophore and the QDs needs to be optimized by a careful system engineering. In this context, BODIPY dyes bearing amino-terminated functionalities are used in virtue of the high affinity of amine groups in coordinating the QD surface. A preliminary QD surface treatment with a short amine ligand is performed to favor the interaction with the organic fluorophores in solution. The successful coordination of the dye to the QD surface, accomplishing a short donor-acceptor distance, provides effective energy transfer already in solution, with efficiency of 76 %. The efficiency further increases in the solid state where the QDs and the dye are deposited as single coordinated units from solution, with a distance between the fluorophores down to 2.2 nm, demonstrating the effectiveness of the coupling strategy.

Quantum Device Emulates the Dynamics of Two Coupled Oscillators

Our quantum device is a solid-state array of semiconducting quantum dots that is addressed and read by 2D electronic spectroscopy. The experimental ultrafast dynamics of the device is well simulated by solving the time-dependent Schrödinger equation for a Hamiltonian that describes the lower electronically excited states of the dots and three laser pulses. The time evolution induced in the electronic states of the quantum device is used to emulate the quite different nonequilibrium vibrational dynamics of a linear triatomic molecule. We simulate the energy transfer between the two local oscillators and, in a more elaborate application, the expectation values of the quantum mechanical creation and annihilation operators of each local oscillator. The simulation uses the electronic coherences engineered in the device upon interaction with a specific sequence of ultrafast pulses. The algorithm uses the algebraic description of the dynamics of the physical problem and of the hardware.

Atomic fluctuations in electronic materials revealed by dephasing

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.

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

Understanding and controlling the electronic structure of nanomaterials is the key to tailoring their use in a wide range of practical applications. Despite this need, many important electronic states are invisible to conventional optical measurements and are typically identified indirectly based on their inferred impact on luminescence properties. This is especially common and important in the study of nanomaterial surfaces and their associated defects. Surface trap states play a crucial role in photophysical processes yet remain remarkably poorly understood. Here we demonstrate for the first time that broadband electronic sum frequency generation (eSFG) microspectroscopy can directly map the optically bright and dark states of nanoparticles, including the elusive below gap states. This new approach is applied to model cadmium selenide (CdSe) quantum dots (QDs), where the energies of surface trap states have eluded direct optical characterization for decades. Our eSFG measurements show clear signatures of electronic transitions both above the band gap, which we assign to previously reported one- and two-photon transitions associated with the CdSe core, as well as broad spectral signatures below the band gap that are attributed to surface states. In addition to the core states, this analysis reveals two distinct distributions of below gap states, providing the first direct optical measurement of both shallow and deep surface states on this system. Finally, chemical modification of the surfaces via oxidation results in the relative increase in the signals originating from the surface states. Overall, our eSFG experiments provide an avenue to directly map the entirety of the QD core and surface electronic structure, which is expected to open up opportunities to study how these materials are grown in situ and how surface states can be controlled to tune functionality.

Correlated spatially resolved two-dimensional electronic and linear absorption spectroscopy

A multimodal method for correlating linear and nonlinear optical spectra with a spatial resolution is presented. Using a partially collinear pump-probe geometry and two-frame phase-cycling, ultrafast two-dimensional electronic spectroscopy (2DES) was performed with transverse-spatial and temporal resolutions of 17 μm and 80 fs, respectively. Time-resolved 2DES maps were spatially correlated with linear extinction spectra obtained in the same imaging platform, enabling the examination of state-resolved dynamics of spatially heterogeneous materials. Thin films of aggregated CdSe nanocrystals were studied to demonstrate the combined spectral, temporal, and imaging capabilities of this method.

Optical Gain from Biexcitons in CsPbBr3 Nanocrystals Revealed by Two-dimensional Electronic Spectroscopy

Perovskite semiconductor nanocrystals (NCs) exhibit highly efficient optical gain, which is promising for laser applications. However, the intrinsic mechanism of optical gain in perovskite NCs, particularly whether more than one exciton per NCs is required, remains poorly understood. Here, we use two-dimensional electronic spectroscopy to resonantly probe the interplay between near-band-edge transitions during the buildup of optical gain in CsPbBr₃ NCs. We find compelling evidence to conclude that optical gain in CsPbBr₃ NCs is generated through stimulated emission from strongly interacting biexcitons. The threshold is largely determined by the competition between stimulated emission from biexcitons and excited-state absorption from single exciton to biexciton states. The findings in this work may guide future explorations of NC materials with low-threshold optical gain.

Investigating exciton structure and dynamics in colloidal CdSe quantum dots with two-dimensional electronic spectroscopy

Two-Dimensional Electronic Spectroscopy (2DES) is performed on CdSe colloidal quantum dots. These experiments reveal new observations on exciton structure and dynamics in quantum dots, expanding upon prior transient absorption measurements of excitonics in these systems. The 2DES method enables the separation of line broadening mechanisms, thereby better revealing the excitonic lineshapes and biexcitonic interactions. 2DES enables more information rich spectral probing of coherent phonons and their coupling to excitons. The data show spectral modulations and drifts, with differences based upon whether one monitors the excitation energy (E₁) or emission energy (E₃). These measurements reveal both homogeneous and inhomogeneous broadenings, as well as static and dynamic line broadening. The longitudinal optical phonon modulates the dynamic absorption spectrum both in energy and linewidth. These experiments enable measurement of hot exciton cooling with improved resolution in energy and time. These 2DES results are consistent with prior excitonic state-resolved transient absorption measurements, albeit with the addition of contributions due to coherent phonons. Finally these 2DES experiments enable disentangling of coupling versus relaxation contributions to the signals, further offering a test of electronic structure theory.

Deciphering hot- and multi-exciton dynamics in core-shell QDs by 2D electronic spectroscopies

2D electronic spectroscopy maps acquired in different configurations unveil intraband hot carrier cooling and interband multi-exciton recombination dynamics.

Although the harnessing of multiple and hot excitons is a prerequisite for many of the groundbreaking applications of semiconductor quantum dots (QDs), the characterization of their dynamics through conventional spectroscopic techniques is cumbersome. Here, we show how a careful analysis of 2DES maps acquired in different configurations (BOXCARS and pump–probe geometry) allows the tracking and visualization of intraband Auger relaxation mechanisms, driving the hot carrier cooling, and interband bi- and tri-exciton recombination dynamics. The results obtained on archetypal core–shell CdSe/ZnS QDs suggest that, given the global analysis of the resulting datasets, 2D electronic spectroscopy techniques can successfully and efficiently dispel the intertwined dynamics of fast and ultrafast recombination processes in nanomaterials. Hence, we propose this analysis scheme to be used in future research on novel quantum confined systems.

Deconvolution of optical multidimensional coherent spectra

Optical coherent multidimensional spectroscopy is a powerful technique for unraveling complex and congested spectra by spreading them across multiple dimensions, removing the effects of inhomogeneity, and revealing underlying correlations. As the technique matures, the focus is shifting from understanding the technique itself to using it to probe the underlying dynamics in the system being studied. However, these dynamics can be difficult to discern because they are convolved with the nonlinear optical response of the system. Inspired by methods used to deblur images, we present a method for deconvolving the underlying dynamics from the optical response. To demonstrate the method, we extract the many-particle diffusion Green's functions for excitons in a semiconductor quantum well from two-dimensional coherent spectra.

Seeing Multiexcitons through Sample Inhomogeneity: Band-Edge Biexciton Structure in CdSe Nanocrystals Revealed by Two-Dimensional Electronic Spectroscopy

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.

Exciton-Phonon Spectroscopy of Quantum Dots Below the Single-Particle Homogeneous Line Width

We demonstrate that high-dimensionality coherent spectroscopy yields "super-resolved" spectra whereby peaks may be localized far below their homogeneous line width by resolving them across multiple, coherently coupled dimensions. We implement this technique using a fifth-order photon-echo spectroscopy called Gradient-Assisted Multidimensional Electronic-Raman Spectroscopy (GAMERS) that combines resonant and nonresonant excitation to disperse the optical response across three spectral dimensions: two involving excitonic transitions and one that encodes phonon energies. In analogy to super-resolution localization microscopies, which separate spatially overlapping signals in time, GAMERS isolates signals spectrally using combined electronic and nuclear resolution. Optical phonon lines in a colloidal solution of CdSe quantum dots at room temperature separated by less than 150 μeV are resolved despite the homogeneous line width of these transitions being nearly an order of magnitude broader. The frequency difference between these phonon modes is attributed to softening of the longitudinal phonon mode upon excitation to the lowest exciton state. Further, such phonon mode selectivity yields spectra with electronic line widths that approach the single particle limit. Through this enhanced spectral resolution, the GAMERS method yields insights into the nature of coupling between longitudinal optical and acoustic phonons and specific excitonic transitions that were previously hidden.

Broadband two-dimensional electronic spectroscopy in an actively phase stabilized pump-probe configuration

We introduce a novel configuration for two-dimensional electronic spectroscopy (2DES) that combines the partially collinear pump-probe geometry with active phase locking. We demonstrate the method on a solution sample of CdSe/ZnS nanocrystals by employing two non-collinear optical parametric amplifiers as the pump and probe sources. The two collinear pump pulse replicas are created using a Mach-Zehnder interferometer phase stabilized by active feedback electronics. Taking the advantage of separated paths of the two pump pulses in the interferometer, we improve the signal-to-noise ratio with double modulation of the individual pump beams. In addition, a quartz wedge pair manipulates the phase difference between the two pump pulses, enabling the recovery of the rephasing and non-rephasing signals. Our setup integrates many advantages of available 2DES techniques with robust phase stabilization, ultrafast time resolution, two-color operation, long delay scan, individual polarization manipulation and the ease of implementation.

Charge Dynamics and Optolectronic Properties in HgTe Colloidal Quantum Wells

We investigate the electronic and transport properties of HgTe 2D colloidal quantum wells. We demonstrate that the material can be made p- or n-type depending on the capping ligands. In addition to the control of majority carrier type, the surface chemistry also strongly affects the photoconductivity of the material. These transport measurements are correlated with the electronic structure determined by high resolution X-ray photoemission. We attribute the change of majority carriers to the strong hybridization of an n-doped HgS layer resulting from capping the HgTe nanoplatelets by S^2- ions. We further investigate the gate and temperature dependence of the photoresponse and its dynamics. We show that the photocurrent rise and fall times can be tuned from 100 μs to 1 ms using the gate bias. Finally, we use time-resolved photoemission spectroscopy as a probe of the transport relaxation to determine if the observed dynamics are limited by a fundamental process such as trapping. These pump probe surface photovoltage measurements show an even faster relaxation in the 100-500 ns range, which suggests that the current performances are rather limited by geometrical factors.

Resolving Vibrational from Electronic Coherences in Two-Dimensional Electronic Spectroscopy: The Role of the Laser Spectrum

The observation of coherent quantum effects in photosynthetic light-harvesting complexes prompted the question whether quantum coherence could be exploited to improve the efficiency in new energy materials. The detailed characterization of coherent effects relies on sensitive methods such as two-dimensional electronic spectroscopy (2D-ES). However, the interpretation of the results produced by 2D-ES is challenging due to the many possible couplings present in complex molecular structures. In this work, we demonstrate how the laser spectral profile can induce electronic coherencelike signals in monomeric chromophores, potentially leading to data misinterpretation. We argue that the laser spectrum acts as a filter for certain coherence pathways and thus propose a general method to differentiate vibrational from electronic coherences.

Ultrafast exciton dynamics in 2D in-plane hetero-nanostructures: delocalization and charge transfer

The ultrafast dynamics in Cd-based 2D in-plane heterostructures is revealed by femtosecond optical spectroscopy.