Purcell Effect in the Stimulated and Spontaneous Emission Rates of Nanoscale Semiconductor Lasers

Nanoscale semiconductor lasers have been developed recently using either metal, metallo-dielectric or photonic crystal nanocavities. While the technology of nanolasers is steadily being deployed, their expected performance for on-chip optical interconnects is still largely unknown due to a limited understanding of some of their key features. Specifically, as the cavity size is reduced with respect to the emission wavelength, the stimulated and the spontaneous emission rates are modified, which is known as the Purcell effect in the context of cavity quantum electrodynamics. This effect is expected to have a major impact in the 'threshold-less' behavior of nanolasers and in their modulation speed, but its role is poorly understood in practical laser structures, characterized by significant homogeneous and inhomogeneous broadening and by a complex spatial distribution of the active material and cavity field. In this work, we investigate the role of Purcell effect in the stimulated and spontaneous emission rates of semiconductor lasers taking into account the carriers' spatial distribution in the volume of the active region over a wide range of cavity dimensions and emitter/cavity linewidths, enabling the detailed modeling of the static and dynamic characteristics of either micro- or nano-scale lasers using single-mode rate-equations analysis. The ultimate limits of scaling down these nanoscale light sources in terms of Purcell enhancement and modulation speed are also discussed showing that the ultrafast modulation properties predicted in nanolasers are a direct consequence of the enhancement of the stimulated emission rate via reduction of the mode volume.Comment: 12 pages, 5 figure

Is playing alone in the darkness sufficient to prevent informational cascades?

Models of herd behaviour and informational cascades were theoretically developed in 1992 respectively by Banerjee (A simple model of herd behavior) and Bikhchandani, Hirshleifer and Welch (A Theory of Fads, Fashion, Custom and Cultural Change as Informational Cascades). Both articles pointed out the existence of an information externality that causes a welfare loss, and both proposed the idea that destroying an amount of information may turn out in a social improvement. Although this is an old idea and in the last years many features of herd behaviour and informational cascades were studied, this particular aspect was never developed or extensively analysed. In this article we will try to investigate this hypothesis both theoretically and experimentally.Informational Cascades; Individual Decision Making; Experiments; Information Externality

On the absorbability of herd behaviour and informational cascades: an experimental analysis

A theory is said to be fully absorbable whenever its own acceptance by all of the individuals belonging to a certain population does not question its predictive validity. This accounts for strategic equilibria and can be related to the logic underlying convergence of behaviour and intentional herding in sequential games. This paper discusses the absorbability of informational cascades' theory by bounded rational decision-makers and analyses whether providing individuals with theoretic information on informational cascades affects overall probability of herding phenomena to occur as well as whether an incorrect cascade can be reversed because of bounded rational adapting of the theory's prescriptive. --Theory absorption,Herd behaviour,Informational cascades

ON THE ABSORBABILITY OF HERD BEHAVIOUR AND INFORMATIONAL CASCADES: AN EXPERIMENTAL ANALYSIS

A theory is said to be fully absorbable whenever its own acceptance by all of the individuals belonging to a certain population does not question its predictive validity. This accounts for strategic equilibria and can be related to the logic underlying convergence of behaviour and intentional herding in sequential games. This paper discusses the absorbability of informational cascades’ theory by bounded rational decision-makers and analyses whether providing individuals with theoretic information on informational cascades affects overall probability of herding phenomena to occur as well as whether an incorrect cascade can be reversed because of bounded rational adapting of the theory’s prescriptive.Theory absorption, Herd behaviour, Informational cascades

The Building Up of the Black Hole Mass - Stellar Mass Relation

We derive the growth of SMBHs relative to the stellar content of their host galaxy predicted under the assumption of BH accretion triggered by galaxy encounters occurring during their merging histories. We show that, within this framework, the ratio Gamma=(M_BH/M_*)(z)/(M_BH/M_*)(z=0) between the Black Hole mass and the galactic stellar mass (normalized to the local value) depends on both BH mass and redshift. While the average value and the spread of Gamma(z) increase with z, such an effect is larger for massive BHs, reaching values Gamma=5 for massive Black Holes (M>10^9 M_{\odot}) at z>4, in agreement with recent observations of high-redshift QSOs; this is due to the effectiveness of interactions in triggering BH accretion in high-density environments at high redshifts. To test such a model against observations, we worked out specific predictions for sub-samples of the simulated galaxies corresponding to the different observational samples for which measurements of Gamma have been obtained. We found that for Broad Line AGNs at 1<z<2 values of Gamma=2 are expected, with a mild trend toward larger value for increasing BH mass. Instead, when we select from our Monte Carlo simulations only extremely gas rich, rapidly star forming galaxies at 2<z<3, we find low values 0.3<Gamma<1.5, consistent with recent observational findings on samples of sub-mm galaxies; in the framework of our model, these objects end up at z=0 in low-to-intermediate mass BHs (M<10^9 M_{\odot}), and they do not represent typical paths leading to local massive galaxies. The latter have formed preferentially through paths passing above the local M_*-M_BH relation. We discuss how the global picture emerging from the model is consistent with a downsizing scenario, where massive BHs accrete a larger fraction of their final mass at high redshifts z>4.Comment: 13 pages, 9 figures. Accepted to MNRA

Channel-forming activity of syringopeptin 25 A in mercury-supported phospholipid monolayers and negatively charged bilayers

Interactions of the cationic lipodepsipeptide syringopeptin 25 A (SP25A) with mercury-supported dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylserine (DOPS) and dioeleoylphosphatidic acid (DOPA) self-assembled monolayers (SAMs) were investigated by AC voltammetry in 0.1 M KCl at pH 3, 5.4 and 6.8. SP25A targets and penetrates the DOPS SAM much more effectively than the other SAMs not only at pH 6.8, where the DOPS SAM is negatively charged, but also at pH 3, where it is positively charged just as SP25A. Similar investigations at tethered bilayer lipid membranes (tBLMs) consisting of a thiolipid called DPTL anchored to mercury, with a DOPS, DOPA or DOPC distal monolayer on top of it, showed that, at physiological transmembrane potentials, SP25A forms ion channels spanning the tBLM only if DOPS is the distal monolayer. The distinguishing chemical feature of the DOPS SAM is the ionic interaction between the protonated amino group of a DOPS molecule and the carboxylate group of an adjacent phospholipid molecule. Under the reasonable assumption that SP25A preferentially interacts with this ion pair, the selective lipodepsipeptide antimicrobial activity against Gram-positive bacteria may be tentatively explained by its affinity for similar protonated amino-carboxylate pairs, which are expected to be present in the peptide moieties of peptidoglycan strands

Control of the electromagnetic environment of a quantum emitter by shaping the vacuum field in a coupled-cavity system

We propose a scheme for the ultrafast control of the emitter-field coupling rate in cavity quantum electrodynamics. This is achieved by the control of the vacuum field seen by the emitter through a modulation of the optical modes in a coupled-cavity structure. The scheme allows the on/off switching of the coupling rate without perturbing the emitter and without introducing frequency chirps on the emitted photons. It can be used to control the shape of single-photon pulses for high-fidelity quantum state transfer, to control Rabi oscillations and as a gain-modulation method in lasers. We discuss two possible experimental implementations based on photonic crystal cavities and on microwave circuits.Comment: 5 pages, 3 figure

Dynamically controlling the emission of single excitons in photonic crystal cavities

Single excitons in semiconductor microcavities represent a solid-state and scalable platform for cavity quantum electrodynamics (c-QED), potentially enabling an interface between flying (photon) and static (exciton) quantum bits in future quantum networks. While both single-photon emission and the strong coupling regime have been demonstrated, further progress has been hampered by the inability to control the coherent evolution of the c-QED system in real time, as needed to produce and harness charge-photon entanglement. Here, using the ultrafast electrical tuning of the exciton energy in a photonic crystal (PhC) diode, we demonstrate the dynamic control of the coupling of a single exciton to a PhC cavity mode on a sub-ns timescale, faster than the natural lifetime of the exciton, for the first time. This opens the way to the control of single-photon waveforms, as needed for quantum interfaces, and to the real-time control of solid-state c-QED systems.Comment: 8 pages, 4 figure