Spectroscopy
Andrew H. Proppe, Kelvin Lee Kin Long et al. Phys. Rev. B 2022, 106, 04525.
Second-order photon correlation measurements [g(2)(τ) functions] are widely used to classify single-photon emission purity in quantum emitters or to measure the multiexciton quantum yield of emitters that can simultaneously host multiple excitations – such as quantum dots – by evaluating the value of g(2)(τ=0). Accumulating enough photons to accurately calculate this value is time-consuming and could be accelerated by fitting of few-shot photon correlations. Here, we develop an uncertainty-aware, deep adversarial autoencoder ensemble (AAE) that reconstructs noise-free g(2)(τ) functions from noise-dominated, few-shot inputs. This work demonstrates the advantage of machine learning models to perform uncertainty-aware, fast, and accurate reconstructions of simple Poisson-distributed photon correlation functions, allowing for on-the-fly reconstructions and accelerated materials characterization of solid-state quantum emitters.
Lifetime-resolved photon-correlation Fourier spectroscopy.
Hendrik Utzat et al. Optics Express 2021, 29 (10), 14293-14303.
The excited state population of single solid-state emitters is subjected to energy fluctuations around the equilibrium driven by the bath and relaxation through the emission of phonons or photons. Simultaneous measurement of the associated spectral dynamics requires a technique with a high spectral and temporal resolution with an additionally high temporal dynamic range. We propose a pulsed excitation-laser analog of photon-correlation Fourier spectroscopy (PCFS), which extracts the linewidth and spectral diffusion dynamics along the emission lifetime trajectory of the emitter, effectively discriminating spectral dynamics from relaxation and bath fluctuations. This lifetime-resolved PCFS correlates photon-pairs at the output arm of a Michelson interferometer in both their time-delay between laser-excitation and photon-detection T and the time-delay between two photons τ. We propose the utility of the technique for systems with changing relative contributions to the emission from multiple states, for example, quantum emitters exhibiting phonon-mediated exchange between different fine-structure states.
Coherent single-photon emission from colloidal lead halide perovskite quantum dots.
Hendrik Utzat et al. Science 2019, 363, 1068-1072
The development of many optical quantum technologies is dependent on the availability of solid-state single quantum emitters with near-perfect optical coherence. Light-emitting defects in diamond and quantum dots grown by molecular beam epitaxy have demonstrated transform-limited emission linewidths. However, they are limited in terms of production scalability and reproducibility between individual emitters. We now demonstrate that individual colloidal lead halide perovskite quantum dots (PQDs) display highly efficient single-photon emission with optical coherence times as long as 80 picoseconds, an appreciable fraction of their 210-picosecond radiative lifetimes. These results suggest that perovskite quantum dots can overcome these limitations and provide unprecedented versatility for the generation of indistinguishable single photons or entangled photon pairs for quantum information processing.
Multiexciton Lifetimes Reveal Triexciton Emission Pathway in CdSe Nanocrystals.
Katherine E. Shulenberger et al. Nano Letters 2018, 18 (8), 5153-5158.
Multiexcitons in emerging semiconducting nanomaterials play a critical role in potential optoelectronic and quantum computational devices. We describe photon resolved single molecule methods to directly probe the dynamics of biexcitons and triexcitons in colloidal CdSe quantum dots. We confirm that biexcitons emit from a spin-correlated state, consistent with statistical scaling. Contrary to current understanding, we find that triexciton emission is dominated by band-edge 1Se1S3/2 recombination rather than the higher energy 1Pe1P3/2 recombination.
Supramolecular Lattice Deformation and Exciton Trapping in Nanotubular J-Aggregates,
Megan D Klein et al. J. Phys. Chem. C 2022,126 (8), 4095-4105.
Interactions between excitons and molecular vibrational modes lead to exciton self-trapping and subsequently change their emission behavior. Certain amphiphilic cyanine dyes form nanotubular aggregates that demonstrate high exciton transport rates and show no such coupling between excitons and molecular vibrational modes. However, under sustained illumination these aggregates undergo photobrightening (PB) and can show a doubling in quantum yield. We investigate this reversible PB process through spectral- and time-resolved photoluminescence (PL) measurements under low illumination intensities. We propose a model of PB through large polaron formation, leading to trapping or shielding of these long coherence length excitons through interactions with supramolecular vibrations rather than the intramolecular vibrations typically observed in other aggregates. Finally, we demonstrate control over PB behavior through rigidification of the aggregate with a silica shell, potentially enabling the development of long-term-photobrightened devices utilizing molecular aggregates with significantly higher photoluminescence quantum yields.
Katherine E. Shulenberger, Sophie C. Coppieters ‘t Wallant, Megan D. Klein et al. Nano Letters 2021, 21 (18), 7457-7464.
As luminescence applications of colloidal semiconductor nanocrystals push toward higher excitation flux conditions, there is an increased need to both understand and potentially control emission from multiexciton states. We develop a spectrally resolved correlation method to study the triply excited state that enables direct measurements of the recombination pathway for the triexciton, rather than relying on indirect extraction of rates. We demonstrate that, for core–shell CdSe–CdS nanocrystals, triexciton emission arises exclusively from the band-edge S-like state. These results provide a potential avenue for the control of nanocrystal luminescence, where core–shell heterostructures can be leveraged to control carrier separation and therefore maintain emission color purity over a broader range of excitation fluxes.
Single Nanocrystal Spectroscopy of Shortwave Infrared Emitters.
Sophie Bertram,Boris Spokoyny et al. ACS Nano 2019, 13 (2), 1042–1049.
Short-wave infrared (SWIR) emitters are at the center of ground-breaking applications in biomedical imaging, next-generation optoelectronic devices, and optical communications. However, the lack of single-particle spectroscopic methods accessible in the SWIR has prevented advances in both nanocrystal synthesis and fundamental characterization of emitters. Here, we demonstrate an implementation of a solution photon correlation Fourier spectroscopy (s-PCFS) experiment utilizing the SWIR sensitivity and time resolution of superconducting nanowire single-photon detectors to extract single-particle emission linewidths from colloidal InAs/CdSe core/shell nanocrystals.
Hendrik Utzat et al. Nano Letters 2017, 17, 6838-6846.
Cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently become a promising material for optoelectronic applications due to their high emission quantum yields and facile band gap tunability via both halide composition and size. Here, we report the first comprehensive spectroscopic investigation of single PNC properties using solution-phase photon-correlation methods, including both highly confined and blue-emitting PNCs, previously inaccessible to single NC techniques. Our results suggest a wide range of underlying Auger rates, likely due to transitory charge carrier separation in PNCs with relaxed confinement.
All-optical fluorescence blinking control in quantum dots with ultrafast mid-infrared pulses.
Jiaojian Shi, Weiwei Sun, Hendrik Utzat et al. Nature Nanotechnology 2021, 16, 1355-1361.
Photoluminescence intermittency is a ubiquitous phenomenon, reducing the temporal emission intensity stability of single colloidal quantum dots (QDs) and the emission quantum yield of their ensembles. Here, we demonstrate a deterministic all-optical suppression of QD blinking using a compound technique of visible and mid-infrared excitation. We show that moderate-field ultrafast mid-infrared pulses (5.5 μm, 150 fs) can switch the emission from a charged, low quantum yield grey trion state to the bright exciton state in CdSe/CdS core–shell QDs, resulting in a significant reduction of the QD intensity flicker. Our approach can be integrated with existing single-particle tracking or super-resolution microscopy techniques without any modification to the sample and translates to other emitters presenting charging-induced photoluminescence intermittencies, such as single-photon emissive defects in diamond and two-dimensional materials.
Boris Spokoyny, Hendrik Utzat et al. J. Phys. Chem. Lett. 2020, 11 (4), 1330-1335
Quantum emitters capable of producing single photons on-demand with high color purity are the building blocks of emerging schemes in secure quantum communications, quantum computing, and quantum metrology. Such solid-state systems, however, are usually prone to effects of spectral diffusion, i.e., fast modulation of the emission wavelength due to the presence of localized, fluctuating electric fields. In this study we report measurements of spectral diffusion in a single hexagonal boron nitride (hBN) quantum emitter on the nanosecond to second time scales using photon correlation Fourier spectroscopy. We model the spectral diffusion dynamics and provide a lower limit of ∼0.13 for the ratio of the emitter’s coherence time (T2) to twice its radiative lifetime (2T1) . These results suggest that attaining transform-limited line widths could be achieved with moderate enhancement of the radiative rate.
Settting an Upper Bound to the Biexciton Binding Energy in CsPbBr3 Perovskite Nanocrystals.
Katherine E. Shulenberger et al. J. Phys. Chem. Lett. 2019, 10, 5680-5686
Cesium lead halide perovskite nanocrystals are promising emissive materials for a variety of optoelectronic applications. To fully realize the potential of these materials, we must understand the energetics and dynamics of multiexciton states which are populated under device relevant excitation conditions. We utilized time-resolved and spectrally-resolved photoluminescence studies to investigate the biexciton binding energy as well as a red-shifted emission feature previously reported under high-flux excitation conditions. We determine that this red-shifted emission feature can be ascribed to sample sintering induced by air-exposure and high-flux irradiation. Furthermore, we determine that the biexciton binding energy at room temperature is at most ±20 meV, providing a key insight toward understanding many-body interactions in the lead halide perovskite lattice.