Mohammad Ramezani

Plasmon-exciton-polariton lasing

Strong light-matter interaction leads to the appearance of new states, i.e. exciton-polaritons, with photophysical properties rather distinct from their constituents. Recent developments in fabrication techniques allow us to make metallic structures with strong electric field confinement in nanoscale mode volumes, allowing for a facile assembly of strongly coupled systems at room temperature based on a hybrid organic-plasmonic architecture. In this research, a planar array of metallic nano-antennas is covered by a polymer layer doped with organic molecules, achieving strong coupling of organic excitons with collective plasmonic resonances in the array. We use photoluminescence spectroscopy to measure an onset in nonlinear emission and polariton lasing in this system. At increasing molecular doping levels we observe an increase of the Rabi splitting caused by strong coupling and a concomitant decrease in the lasing threshold. This behavior is observed in spite of a strong reduction in the photoluminescence lifetime and the quantum yield of the dye. Using angular resolved photoluminescence spectroscopy, we measure the thermalization and condensation of plasmon-exciton-polaritons (PEPs) into a mode which is dark in the linear regime. These measurements can be interpreted in terms of stimulated scattering of PEPs at room temperature in the open cavity defined by the nano-antenna array [1, 2]. The lowest threshold that we measure is lower than previous values reported at room temperature in organic materials using microcavities. These results illustrate the potential of metamaterials and plasmonic systems for polariton lasing in spite of the inherent losses of metals.

Coherent control of the optical absorption in a plasmonic lattice coupled to a luminescent layer

We experimentally demonstrate the coherent control, i.e. the phase controlled enhancement and suppression, of the absorption and optical loss in an array of plasmonic particles covered by luminescent layer.

Electrical transport in C-doped GaAs nanowires: surface effects

The particular morphology and di-mensions of nanowires (NWs) in the last years have opened many perspectives in applications ranging from electronics, optoelectronics to energy harvesting and stor-age [1–3]. The successful incorporation of dopants in the nanowires is key for the implementation of such devices. Doping of nanowires has been a challenging task, due to the particular growth mechanisms [4]. In this Letter, we report on the electrical properties of C-doped GaAs nanowires obtained by the Ga-assisted method [5]. As C is not soluble in Ga, its incorporation in the nanowire core through the droplet remains difficult. Our strategy is then to grow an intrinsic NW core and a doped shell around it. The growth of the shell is similar to the thin films growth [6]. We study the doping and surface effects on the NWs resistivity, field effect mobility as well as the minority car-rier diffusion length.

Hybrid semiconductor nanowire-metallic Yagi-Uda antennas

We demonstrate the directional emission of individual GaAs nanowires by coupling this emission to Yagi-Uda optical antennas. In particular, we have replaced the resonant metallic feed element of the nanoantenna by an individual nanowire and measured with the microscope the photoluminescence of the hybrid structure as a function of the emission angle by imaging the back focal plane of the objective. The precise tuning of the dimensions of the metallic elements of the nanoantenna leads to a strong variation of the directionality of the emission, being able to change this emission from backward to forward. We explain the mechanism leading to this directional emission by finite difference time domain simulations of the scattering efficiency of the antenna elements. These results cast the first step toward the realization of electrically driven optical Yagi-Uda antenna emitters based on semiconductors nanowires.

Interaction and Coherence of a Plasmon–Exciton Polariton Condensate

Polaritons are quasiparticles arising from the strong coupling of electromagnetic waves and elementary excitations in semiconductors. In this frame, localized surface plasmons in metallic nanoparticles have attracted increasing interests in the last decade, since their sub-diffraction-limited mode volume offers the access to extremely high photonic densities and corresponding huge nonlinearities. However, high absorption losses in metals have always hindered the observation of collective coherent phenomena, such as condensation. In this work we demonstrate the formation of a non-equilibrium room temperature plasmon-exciton-polariton condensate with a long range spatial coherence extending well over the excitation area, by coupling Frenkel excitons in organic molecules to a multipolar mode in a lattice of plasmonic nanoparticles. Time-resolved experiments evidence the sub-ps formation dynamics of the condensate and a sizeable blueshift, thus measuring, for the first time, the effect of polariton interactions in plasmonic cavities. Our results pave the way to the observation of room temperature condensation and novel nonlinear phenomena in plasmonic systems, challenging the common belief that absorption losses in metals prevent the realization of macroscopic quantum states.

Plasmon exciton-polariton lasing

Strong light-matter interaction leads to the appearance of new states, i.e. exciton-polaritons, with photophysical properties rather distinct from their constituents. Recent developments in fabrication techniques allow us to make metallic structures with strong electric field confinement in nanoscale mode volumes, allowing for a facile assembly of strongly coupled systems at room temperature based on a hybrid organic-plasmonic architecture. In this research, a planar array of metallic nano-antennas is covered by a polymer layer doped with organic molecules, achieving strong coupling of organic excitons with collective plasmonic resonances in the array. We use photoluminescence spectroscopy to measure an onset in nonlinear emission and polariton lasing in this system. At increasing molecular doping levels we observe an increase of the Rabi splitting caused by strong coupling and a concomitant decrease in the lasing threshold. This behavior is observed in spite of a strong reduction in the photoluminescence lifetime and the quantum yield of the dye. Using angular resolved photoluminescence spectroscopy, we measure the thermalization and condensation of plasmon-exciton-polaritons (PEPs) into a mode which is dark in the linear regime. These measurements can be interpreted in terms of stimulated scattering of PEPs at room temperature in the open cavity defined by the nano-antenna array [1, 2]. The lowest threshold that we measure is lower than previous values reported at room temperature in organic materials using microcavities. These results illustrate the potential of metamaterials and plasmonic systems for polariton lasing in spite of the inherent losses of metals.