Carrier mobility in solids is generally limited by electron-impurity or electron-phonon scattering depending on the most frequently occurring event. Three body collisions between carriers and both phonons and impurities are rare; they are denoted supercollisions (SCs). Elusive in electronic transport they should emerge in relaxation processes as they allow for large energy transfers. As pointed out in Ref. \onlinecite{Song2012PRL}, this is the case in undoped graphene where the small Fermi surface drastically restricts the allowed phonon energy in ordinary collisions. Using electrical heating and sensitive noise thermometry we report on SC-cooling in diffusive monolayer graphene. At low carrier density and high phonon temperature the Joule power $P$ obeys a $P\propto T_e^3$ law as a function of electronic temperature $T_e$. It overrules the linear law expected for ordinary collisions which has recently been observed in resistivity measurements. The cubic law is characteristic of SCs and departs from the $T_e^4$ dependence recently reported for metallic graphene below the Bloch-Gr\"{u}neisen temperature. These supercollisions are important for applications of graphene in bolometry and photo-detection

Using far-infrared magnetospectroscopy in self-assembled InAs quantum dots, we have investigated the electronic transitions from the ground s levels to the excited p levels. The experiments consist of monitoring, by means of Zeeman tuning of the excited level, a resonant interaction between the discrete ( p, 0 LO phonon) state and the continuum of either (s, 1 LO phonon) or (s, 2 LO phonons). We show that the electrons and the LO phonons are always in a strong coupling regime and form an everlasting mixed electron-phonon mode. PACS numbers: 73.40.Kp, 73.20.Dx, 78.20.Ls Electrons in excited atomic states can relax towards lower lying levels by spontaneous emission of photons. Artificial atoms like semiconductor quantum dots display discrete levels. For electrons (or holes) placed in excited levels the spontaneous emission of photons is inefficient for the relaxation due to the characteristic energy splitting of the dot states (ϳ50 meV in a ϳ20 nm dot). The carriers bound to these artificial atoms are however in interaction with phonons which display a continuum of finite width, unlike photons. It has been shown that the intradot relaxation through acoustical phonons is totally inefficient, the energy mismatch between electron states being much too large [1,2]. In semiconductors, the most powerful energy relaxation channel is (by far) the irreversible emission of longitudinal optical (LO) phonons through the Fröhlich coupling. Despite its effectiveness, this electron-phonon coupling is weak, to the extent that the initial discrete level (e, 0 phonon) irreversibly decays into the continuum (g, 1 phonon) where e and g, respectively, denote an excited state and the ground electronic state. Such a weak coupling is very well described by the Fermi golden rule in bulk, quantum well (2D) or quantum wire (1D) structures. Because the optical phonons show very little dispersion, it has been argued that the LO phonon assisted relaxation in semiconductor quantum dots could be efficient only if the energy separation between the electronic states differs by one (or several) LO phonons. Here we present experimental evidence supported by theoretical modeling that the very idea of an electron emitting LO phonons and relaxing irreversibly to a less excited state (as in bulk, 2D, and 1D heterolayers) is wrong in a quantum dot. What happens in reality is that the electrons and the LO phonons are in a strong coupling regime and form everlasting mixed electron-phonon modes, as recently suggested by Inoshita and Sakaki in the case of one phonon [3]. Using far-infrared (FIR) magnetospectroscopy, we have investigated the g ! e transition in self-assembled doped InAs quantum dots. The experiments consist of monitoring, by means of Zeeman tuning of the dot excited level e, a resonant interaction between the discrete (e, 0 LO phonon) state and the continuum of either (g, 1 LO phonon) or (g, 2 LO phonons). We show that the (e, 0 LO phonon) state does not dissolve when entering into the continuum but forms a hybrid mode with (g, 1, or 2 LO p...

Motional narrowing refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower when increasing the reservoir fluctuation. A textbook example is found in nuclear magnetic resonance, where the fluctuating local magnetic fields created by randomly oriented nuclear spins are averaged when the motion of the nuclei is thermally activated. The existence of a motional narrowing effect in the optical response of semiconductor quantum dots remains so far unexplored. This effect may be important in this instance since the decoherence dynamics is a central issue for the implementation of quantum information processing based on quantum dots. Here we report on the experimental evidence of motional narrowing in the optical spectrum of a semiconductor quantum dot broadened by the spectral diffusion phenomenon. Surprisingly, motional narrowing is achieved when decreasing incident power or temperature, in contrast with the standard phenomenology observed for nuclear magnetic resonance.PACS numbers: 78.67. Hc, 78.55.Cr, In the seminal work on motional narrowing by Bloembergen et al., relaxation effects in nuclear magnetic resonance were beautifully explained by taking into account the influence of the thermal motion of the magnetic nuclei upon the spin-spin interaction [1]. The general treatment of relaxation processes for a system interacting with a reservoir was later formulated by Kubo in a stochastic theory that assumes random perturbations of the system by a fluctuating environment [2]. Depending on the relative magnitude of the spectral modulation amplitude and the inverse of the modulation correlation time, the relaxation dynamics is either in the slow modulation limit, where the optical line-shape reflects directly the statistical distribution of the different system energies, or in the fast modulation limit where the fluctuation is smoothed out and the line-shape is motionally narrowed into a Lorentzian profile. The relevance of motional narrowing for the description of relaxation phenomena has spread throughout many different fields, such as spin relaxation in semiconductors [3], vibrational dephasing in molecular physics [4], or phase noise in optical pumping [5].The optical spectrum of a material system with localized, zero-dimensional electronic states provides a generic example of the influence of a fluctuating environment on the coherence relaxation dynamics. In that case, the perturbing interactions induce a stochastic shift over time of the optical spectrum, resulting in the so-called spectral diffusion effect, which was observed for rare-earth ions [6], molecules [7], or semiconductor quantum dots [8,9]. In this latter system, impurities, defects or localized charges in the vicinity of a quantum dot induce micro-electric fields that shift the quantum dot emission line through the quantum confined Stark effect. The fluctuation of the quantum dot environment thus randomize the emission energy over a spectral range Σ on a characteristic time scale τ c . Spectral dif...

We show that the spin state of the resident electron in an n-doped self-assembled InAs-GaAs quantum dot can be written and read using nonresonant, circularly polarized optical pumping. A simple theoretical model is presented and accounts for the remarkable dynamics producing counterpolarized photoluminescence.

We investigate theoretically the coupling of a cavity mode to a continuous distribution of emitters. We discuss the influence of the emitters inhomogeneous broadening on the existence and on the coherence properties of the polaritonic peaks. We find that their coherence depends crucially on the shape of the distribution and not only on its width. Under certain conditions the coupling to the cavity protects the polaritonic states from inhomogeneous broadening, resulting in a longer storage time for a quantum memory based on emitters ensembles. When two different ensembles of emitters are coupled to the resonator, they support a peculiar collective dark state, also very attractive for the storage of quantum information.