[Show abstract][Hide abstract] ABSTRACT: We use nominally spheroidal CdSe nanocrystals with a zincblende crystal structure to study how shape perturbations lift the energy degeneracies of the band-edge exciton. Nanocrystals with a low degree of symmetry exhibit splitting of both upper and lower bright-state degeneracies due to valence band mixing combined with the isotropic exchange interaction, allowing active control of the level splitting with a magnetic field. Asymmetry-induced splitting of the bright states is used to reveal the entire 8-state band-edge fine structure, enabling complete comparison with band-edge exciton models.
[Show abstract][Hide abstract] ABSTRACT: The band-edge exciton in elongated CdSe nanocrystals is composed of an upper and lower manifold associated with heavy and light holes in which the energy separation is sensitive to the nanocrystal shape. Using resonant photoluminescence excitation we probe the upper heavy hole exciton manifold and find rapid relaxation to the lower light hole manifold on a 5 ps timescale. State selective excitation allows preparation of single quantum states in this system. We used this to map the hole spin relaxation pathways between the fine structure sublevels which have energy splittings incommensurate with either optical or acoustic phonon energies. This reveals a hitherto unexpected hole spin-relaxation channel in these materials.
[Show abstract][Hide abstract] ABSTRACT: The direct coupling of excited electronic states to optical phonons in single CdSe colloidal quantum dots is explored using both photoluminescence emission and excitation spectroscopies. We find a broad optical phonon spectrum associated with a single fine structure state. Multiple peaks in the optical phonon sideband are ascribed to different optical phonon types emanating from both the core and shell layers. A mixed emission process that involves the simultaneous generation of two different types of optical phonon is also observed. In general, rather than a single mode, each designated phonon type is associated with a dispersed family of modes. Narrow optical phonon sidebands, consistent with the dominant LO mode, are observed in some nanocrystals. A linewidth analysis indicates that optical phonon lifetimes are in the 10 picosecond range. We demonstrate the ability to selectively excite a specific band-edge state by directly exciting its LO phonon sideband.
[Show abstract][Hide abstract] ABSTRACT: Measurements of the emission linewidth of single nanocrystals are usually limited by spectral diffusion. At cryogenic temperatures, the origin of this instability was revealed to be photo-induced, suggesting that the spectral peak position may be stable in the limit of vanishing optical excitation. Here we test this stability using resonant photoluminescence excitation and find there is persistent spectral broadening, which ultimately limits the emission linewidth in these materials. The spectral broadening is shown to be consistent with spontaneous fluctuations of the local electrostatic field within the disordered environment surrounding the nanocrystal.
[Show abstract][Hide abstract] ABSTRACT: As colloidal semiconductor nanocrystals are developed for a wider range of diverse applications, it becomes more important to gain a deeper understanding of their properties in order to direct synthetic efforts. While most synthetic developments are guided by changes in ensemble properties, certain applications such as those in nano-electronics and nano-photonics rely on properties of nanocrystals at the individual level. For such applications and even for a more detailed understanding of the ensemble behavior, single nanocrystal spectroscopy becomes a vital tool. This review looks at how single nanocrystal spectroscopy has been applied to materials based on modern synthetic techniques and how these studies are elucidating properties that remain hidden at the ensemble level. First, recent theoretical models that are important for understanding many observed phenomena are explored. Then the review highlights new insights into many of the photophysical properties that are of interest in semiconductor nanocrystal materials, such as the ubiquitous spectral instability, magneto-optical identification of the band-edge exciton fine structure, emission from multi-excitons, and the spectroscopic properties of charged nanocrystals that challenge long standing theories on photoluminescence blinking behavior. To date most of the research has been conducted on materials based on cadmium selenide primarily due to its many years of development as a prototypical nanocrystal system. The review ends with a discussion of new materials that would also benefit from a detailed photophysical understanding afforded by single nanocrystal spectroscopy.
Chemical Society Reviews 10/2013; 43(4). DOI:10.1039/c3cs60209e · 30.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Charged quantum dots provide an important platform for a range of emerging quantum technologies from spin qubits to single phonon sources. Colloidal quantum dots in particular could offer unique advantages for such applications (eg. facile synthesis, manipulation and compatibility with a wide range of environments), especially if stable charged states can be harnessed in these materials. This possibility has been encouraged by recent reports of trion emission from colloidal quantum dots at cryogenic temperatures [1,2] indicating that trion states can be bright with quantum yields approaching unity . However, these reports relied on random ionization to produce the trion state and did not provide any insight into the charging mechanism.
International Quantum Electronics Conference; 05/2013
[Show abstract][Hide abstract] ABSTRACT: Spectroscopically resolved emission from single nanocrystals at cryogenic temperatures provides unique insight into physical processes that occur within these materials. At low temperatures, the emission spectra collapse to narrow lines, revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level. Since these techniques were first used, the technology of nanocrystal synthesis has matured significantly, and new materials with outstanding photostability have been reported. In this perspective, we show how cryogenic spectroscopy of single nanocrystals probes the fundamental excitonic structure of the band edge, revealing spectral fingerprints that are highly sensitive to a range of photophysical properties as well as nanocrystal morphology. In particular, spectral and temporal signatures of biexciton and trion emission are revealed, and their relevance to emerging technologies is discussed. Overall we show how cryogenic single nanocrystal spectroscopy can be used as a tool for understanding fundamental photophysics and guiding the synthesis of new nanocrystal materials.
[Show abstract][Hide abstract] ABSTRACT: Charged quantum dots provide an important platform for a range of emerging quantum technologies. Colloidal quantum dots in particular offer unique advantages for such applications (facile synthesis, manipulation and compatibility with a wide range of environments), especially if stable charged states can be harnessed in these materials. Here we engineer the CdSe nanocrystal core and shell structure to efficiently ionize at cryogenic temperatures, resulting in trion emission with a single sharp zero-phonon line and a mono exponential decay. Magneto-optical spectroscopy enables direct determination of electron and hole g-factors. Spin relaxation is observed in high fields, enabling unambiguous identification of the trion charge. Importantly, we show that spin flips are completely inhibited for Zeeman splittings below the low-energy bound for confined acoustic phonons. This reveals a characteristic unique to colloidal quantum dots that will promote the use of these versatile materials in challenging quantum technological applications.
[Show abstract][Hide abstract] ABSTRACT: Spectral diffusion of the emission line of single colloidal nanocrystals is generally regarded as a random process. Here, we show that each new spectral position has a finite memory of previous spectral positions, as evidenced by persistent anticorrelations in time series of spectral jumps. The anticorrelation indicates that there is an enhanced probability of the charge distribution around the nanocrystal returning to a previous configuration. We show both statistically and directly that this memory manifests as an observable spontaneous “relaxation” in the absence of a pump laser, so that spectral diffusion progresses in a manner of “two steps forward and one step back”.
[Show abstract][Hide abstract] ABSTRACT: By using the zero-phonon line emission of an individual organic molecule, we
realized a source of indistinguishable single photons in the near infrared. A
Hong-Ou-Mandel interference experiment is performed and a two-photon
coalescence probability of higher than 50% at 2 K is obtained. The contribution
of the temperature-dependent dephasing processes to the two-photon interference
contrast is studied. We show that the molecule delivers nearly ideal
indistinguishable single photons at the lowest temperatures when the dephasing
is nearly lifetime limited. This source is used to generate post-selected
polarization-entangled photon pairs, as a test-bench for applications in
Physical Review A 11/2010; 82(6). DOI:10.1103/PHYSREVA.82.063803 · 2.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a new technique to perform high resolution resonant photoluminescence excitation of CdSe/ZnS nanocrystals. The method takes advantage of the long photoluminescence decay times (~ 1 µs) observed in this system at liquid helium temperatures. Resonant photoluminescence excitation can be performed using a tunable pulsed excitation and a time-gated detection. Spectral hole burning investigations on an ensemble of CdSe/ZnS nanocrystals lead to homogeneous linewidths of ~100 µeV for the band edge exciton state.
[Show abstract][Hide abstract] ABSTRACT: We developed a photothermal method based on scattering around a nano-absorber that allows for the unprecedented detection of individual nano-objects such as gold nanoparticles with diameter down to 1.4 nm as well as CdSe nanocrystals. This method relies on the absorptive properties of the nano-object and does not suffer from the drawbacks of luminescence-based methods. We present here two different applications of this versatile detection method. First, we performed absorption spectroscopy of individual gold nanoparticles as small as 5nm and CdSe nanocrystals in the multiexcitonic regime. Second, we show the applicability of our method for new types of gold nanoparticles based DNA microarrays. In addition to the intrinsic signal stability due to the use of gold labelling, our technique does not require silver staining enhancement and permits to push the signal dynamics of such microarrays from the single nanoparticle detection to almost the full surface coverage.
Proceedings of SPIE - The International Society for Optical Engineering 01/2006; DOI:10.1117/12.647808 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The photothermal heterodyne imaging method is used to study for the first time the absorption spectra of individual gold nanoparticles with diameters down to 5 nm. Intrinsic size effects that result in a broadening of the surface plasmon resonance are unambiguously observed. Dispersions in the peak energies and homogeneous widths of the single-particle resonances are revealed. The experimental results are analyzed within the frame of Mie theory.