Electrochemical nanoimpacts monitoring, developed within the last decade, is based on the time-resolved detection of stochastic collisions of individual NanoParticles (NPs) in micro/nano-confined electrochemical cells[1]. Although electrochemical techniques are now able to measure electron transfer processes associated to single NP impacts, they are not spatially resolved and fail to characterize complex multiple chemical events.
Superlocalization optical microscopies have recently allowed a complementary visualization of the transformation of NPs during electrochemical reactions. We proposed the coupling of holographic microscopy to electrochemistry[2] to record scattering by individual NPs. Real-time holograms are reconstructed in order to superlocalize and track several individual NPs in 3D with 3x3x10 nm3 accuracy. During Ag electrodissolution experiments, it allowed the 3D monitoring of Brownian or phoretic motion and individual dissolution. In situations where chemical transformations occur, spectroscopy associated to finite element or FDTD modelling can provide additional information on individual NPs during electrochemical processes.
We will illustrate the possibilities of this coupled characterization in several systems, where 3D particle tracking can provide a NP size estimate during the Brownian approach of the particle. During chemical reactions on the metal interface, we will show that holography, spectroscopy, associated to an optical model of the scattering, gives access to relevant information on the size, position, and composition of the NPs.
[1]Y.Zhou et al, Angew. Chem. 2011,
X.Xiao et al. J. Am. Chem. Soc. 2007
[2] V.Brasiliense et al., Acc. Chem. Res., 2016
V.Brasiliense et al. J. Am. Chem. Soc., 2016
A.N Patel et al. Nano Lett., 2015
We study the nonlinear dynamics of semiconductor micropillar lasers with intracavity saturable absorber in the excitable regime. The excitable regime is characterized by an all-or-none type of response to an input perturbation: when the perturbation amplitude is below the excitable threshold, the system remains in its quiet, stable state; when the perturbation exceeds the excitable threshold, a calibrated response pulse is emitted. It is believed to have great potential for fast neuromorphic optical processing, in addition to being also interesting for the study of nonlinear wave propagation. Fast excitable, neuron-like, dynamics is experimentally evidenced with response times in the 200ps range. We also show the presence of an absolute and a relative refractory periods in this system, analog to what is found in biological neurons but with several orders of magnitude faster response times. The absolute refractory period is the amount of time after a first excitable pulse has been emitted during which it is not possible to excite the system anymore. The relative refractory period is the time after a first excitable pulse during which an inhibited response is emitted and has been often overlooked in optical systems. Both these times are of fundamental importance regarding the propagation of stable excitable waves, and in view of designing spike-time based optical signal processing systems. The experimental results are well described qualitatively by a simple model of a laser with saturable absorber.
In high finesse semiconductor microcavities containing quantum wells, photons emitted by the quantum well excitons
can oscillate long enough inside the cavity to be reabsorbed reemitted again and so forth. The system enters the so-called
strong coupling regime, with the formation of entangled exciton-photon eigenstates, named cavity polaritons, which
governs all the physics of the system. After an introduction to cavity polaritons, we will review in this paper some of
their original physical properties and discuss their potential in terms of new photonic devices. In a first part, we will
show how polaritons can massively occupy a single quantum state, thus acquiring spatial and temporal coherence
reflected in the emitted light. Such polariton laser could provide a low threshold source of coherent light. Then the
properties of polariton diodes will be addressed and in particular we will describe a new optical bistability based on the
control of the light matter coupling via the intra cavity electric field.
We develop a model that describes the optical response of a semiconductor quantum dot microcavity pumped above
transparency but kept slightly below threshold. The model takes into account the inhomogeneous broadening of the dot
emission, the coupling mechanisms between quantum dots and the wetting layer and incorporates gain asymmetry
factors in the thermo-emission and capture coefficients. The role of asymmetries with respect to alpha factor and pattern
formation is investigated. We then study the conditions for the onset of bistability and modulational instability and
characterize the patterns formed.
The availability of high power semiconductor lasers makes it possible to optically pump large area cavities with a good spatial homogeneity and with an arbitrary profile, which is otherwise difficult to obtain with electrical injection. In addition, a high pumping efficiency may be obtained with reduced heat generation thanks to the absence of Joule heating. However, in order to fully benefit from these advantages it is necessary to pay special attention to the spectral characteristics of the cavity and to design it accordingly. We present and extend a Bragg mirrors optimization technique to control both the absorption and the transmission of the cavity around the pump wavelength. The absorption coefficient reaches close to 80% over a 30nm width pumping window around 800nm while keeping the cavity transmission below 10% at the pump wavelength. Laser action is obtained at 890 nm with an almost flat pumping (and hence gain) profile over a diameter of 80μm and a laser threshold of 11.5kW/cm2. We point out that the method may be employed in the design of vertical external cavity surface emitting lasers.
Theoretical studies on cavity transverse nonlinear dynamics have shown the possibility of exploiting the self-organizing properties of light response in the form of periodic patterns and cavity solitons. While very few experimental confirmations exist - only in macroscopic systems - we report on the first experimental results obtained in bulk and multi-quantum-well AlGaAs microresonators. These systems combine the advantage of : a monolithic character deriving from their well-controlled epitaxial growth conditions, a variety of nonlinear optical properties near the band gap edge, and a high Fresnel number. We review the general properties of semiconductor microresonators that lead to optical self-organization, emerging from the interplay between the dispersive or saturable absorptive nonlinearities and transverse mechanisms such as light diffraction and carrier diffusion. We show the first observation of periodic rolls, rhombs and hexagons patterns, stress the strong interaction of these patterns with the transverse fluctuations of the cavity thickness. Finally, we present the observation of precursor forms of cavity solitons, and evidence the strong thermal contribution they involve.
Epitaxial methods allow the fabrication of nonlinear interferomethc structures which include the mirrors and the GaAs Multiple Quantum Wells (MQW) nonlinear medium in a single crystal. This results in very compact nonlinear microcavities with typically 5im overall thickness. Optical bistability is observed in these structures at mW optical power. We review our latest experimental results on these structures including the observation of high contrast switching in the reflection mode the direct measurement of dispersive optical nonlinearities in MQWs evidencing their saturation behavior at high intensities. Finally we report external-beam switching experiments which allow to assess the potential of these structures as all-optical gates.
GaAs/A1GaAs epitaxial structures allow the fabrication of monolithic bistable devices with very
appealing characteristics for all-optical switching applications, such aslow threshold power and good thermal
stability. They rely upon the excitonic and band-gap resonant nonlinearities in bulk GaAs or MQW. Very
compact devices of a few micron size with A1As/A1GaAs integrated Bragg reflectors can be grown during
a single epitaxial process. We report linear and nonlinear reflectivity measurements on such a monolithic
étalon with a MQW active layer grown by MOVPE. A bistability power threshold lower than 3 mW at
836 nm was observed with a reflective contrast ratio as high as 30: 1 .We show evidence that the refractive
index dependence on optical intensity is strongly sublinear, indicating that a substantial degree of saturation
occurs at intensity levels of iO W/cm2. We discuss the origin of this saturation and its implications on the
design of future devices.
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