Lead halide perovskites open great prospects for optoelectronics and a wealth of potential applications in quantum optical and spin-based technologies. Precise knowledge of the fundamental optical and spin properties of charge-carrier complexes at the origin of their luminescence is crucial in view of the development of these applications. On nearly bulk Cesium-Lead-Bromide single perovskite nanocrystals, which are the test bench materials for next-generation devices as well as theoretical modeling, we perform low-temperature magneto-optical spectroscopy to reveal their entire band-edge exciton fine structure and charge-complex binding energies. We demonstrate that the ground exciton state is dark and lays several millielectronvolts below the lowest bright exciton sublevels, which settles the debate on the bright-dark exciton level ordering in these materials. More importantly, combining these results with spectroscopic measurements on various perovskite nanocrystal compounds, we show evidence for universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanostructures, regardless of their chemical composition. These scaling laws solely based on quantum confinement effects and dimensionless energies offer a general predictive picture for the interaction energies within charge-carrier complexes photo-generated in these emerging semiconductor nanostructures.
When a UV photon is absorbed by a single semiconductor nanocrystal (NC), two or more excitons can be simultaneously generated through the carrier multiplication (CM) process. It is still highly debated whether the CM efficiency is truly enhanced in semiconductor NCs because all the routine CM measurements performed exclusively at the ensemble level are incapable of completely excluding the false CM signals contributed by the charged excitons. Here we place single CdSe NCs above an aluminum film and successfully resolve their UV-excited photoluminescence time trajectories where the true and false CM signals are contained in the blinking “on” and “off” levels, respectively. When the UV photon energy is ~2.46 times of the NC energy gap, an average CM efficiency of ~20.2% can be reliably estimated. The ability to detect UV-excited photoluminescence from a single NC will surely provide a great guidance for the CM applications in various light-to-electric conversion devices.
In order to provide a green alternative to QDs for bioimaging purposes and aiming at designing bright nanoparticles combining both large one- and two-photon brightness, a bottom-up route based on the molecular engineering of dedicated red to NIR emitting dyes that spontaneously form fluorescent organic nanoparticles (FONs) has been implemented. These fully organic nanoparticles built from original quadrupolar dyes are prepared using a simple, expeditious and green protocol that yield very small molecular-based nanoparticles (radius ~ 7 nm) suspension in water showing a nice NIR emission (λem=710 nm). These FONs typically have absorption coefficient more than two orders larger than popular NIR-emitting dyes (such as Alexa Fluor 700, Cy5.5 ….) and much larger Stokes shift values (i.e. up to over 5500 cm-1). They also show very large two-photon absorption response in the 800-1050 nm region (up to about 106 GM) of major promise for two-photon excited fluorescence microscopy. Thanks to their brightness and enhanced photostability, these FONs could be imaged as isolated nanoparticles and tracked using wide-field imaging. As such, thanks to their size and composition (absence of heavy metals), they represent highly promising alternatives to NIR-emitting QDs for use in bioimaging and single particle tracking applications. Moreover, efficient FONs coating was achieved by using a polymeric additive built from a long hydrophobic (PPO) and a short hydrophilic (PEO) segment and having a cationic head group able to interact with the highly negative surface of FONs. This electrostatically-driven interaction promotes both photoluminescence and two-photon absorption enhancement leading to an increase of two-photon brightness of about one order of magnitude. This opens the way to wide-field single particle tracking under two-photon excitation
Being a scanning microscopy, Stimulated Emission Depletion (STED) needs to be parallelized for fast wide-field
imaging. Here, we achieve large parallelization of STED microscopy using well-designed Optical Lattice (OL) for
depletion, together with a fast camera for detection. Depletion optical lattices with 100 intensity “zeros” are generated by
four-beam interference. Scanning only a unit cell, as small as 290 nm by 290 nm, of the depletion OL is sufficient for
STED imaging. The OL-STED microscopy acquires super-resolution images with 70 nm resolution and at the speed of
80 ms per image.
Spectroscopically resolved emission from single nanocrystals at cryogenic temperatures provides unique insight into photophysical 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 photophysical stability have been reported. Here we review our recent work that shows 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 discussed. In addition, we show how high resolution excitation spectroscopy can provide information on external processes that ultimately limit the coherence of the nanocrystal band-edge states. Overall we demonstrate how cryogenic single nanocrystal spectroscopy can be used as a vital tool for understanding fundamental photophysics and guiding the synthesis of new nanocrystal materials.
Using single-molecule microscopy, we present a method to quantify the number of single autofluorescent proteins when they cannot be optically resolved. This method relies on the measurement of the total intensity emitted by each aggregate until it photobleaches. This strategy overcomes the inherent problem of blinking of green fluorescent proteins. In the case of small protein aggregates, our method permits us to describe the mean composition with a precision of one protein. For aggregates containing a large number of proteins, it gives access to the average number of proteins gathered and a signature of the inhomogeneity of the aggregates' population. We applied this methodology to the quantification of small purified citrine multimers.
We applied single-molecule fluorescence microscopy (using organic dyes or semiconductor quantum dots) to study the lateral diffusion of glutamate receptors (AMPA and NMDA) in live synapses. We directly imaged glutamate receptors movements inside and outside synapses of live cultured hippocampal neurons. We could record exchanges of receptors through lateral diffusion between these different membrane compartments. In addition, our data suggest that this lateral diffusion might be regulated by neuronal activity. To overcome the photobleaching problem inherent to fluorescence techniques we recently developed new optical methods for the detection of individual metallic nanoparticles. We can now detect signatures of diffusing AMPA receptors labeled with 10 nm gold nanoparticles on live neurons.
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.
Photothermal Heterodyne Imaging (PHI) is a highly sensitive optical detection method of individual absorptive nano-objects. It can be applied to absorption spectroscopy measurements Surface Plasmon Resonance spectra of individual gold nanoparticules with diameters down to 5nm were recorded. Intrinsic size effects which result in a broadening of the Resonance are unambiguously observed and analyzed within the frame of Mie theory. Preliminary results obtained with silver nanoparticles are also presented.
We have developed a photothermal method for far-field optical detection of nanometer-sized metal particles, combining high-frequency modulation and polarization interference contrast. We can image gold colloids down to 5 nm in diameter, with a signal-to-noise ratio higher than 10. This is a considerable improvement over commonly used optical methods based on resonance plasmon scattering which, for background reasons, are limited to particles of more than about 40 nm in diameter. We also show that in addition to its intrinsic sensitivity, our photothermal method is totally insensitive to non-absorbing scatterers as 10 nm nanoparticles can be imaged in cells.
The use of diode lasers to cool and trap Cesium atoms in a low Cs pressure cell allows the construction of a relatively simple and reliable atomic fountain frequency standard. Here we discuss the design and the potentialities of the Cs clock frequency standards being built at L.P.T.F..
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