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Even J.,CNRS Foton Laboratory | Pedesseau L.,CNRS Foton Laboratory | Katan C.,CNRS Chemistry Institute of Rennes
Physical Chemistry Chemical Physics | Year: 2014

Yun Wang et al. used density functional theory (DFT) to investigate the orthorhombic phase of CH3NH3PbI3, which has recently shown outstanding properties for photovoltaic applications. Whereas their analysis of ground state properties may represent a valuable contribution to understanding this class of materials, effects of spin-orbit coupling (SOC) cannot be overlooked as was shown in earlier studies. Moreover, their discussion on optical properties may be misleading for non-DFT-experts, and the nice agreement between experimental and calculated band gap is fortuitous, stemming from error cancellations between SOC and many-body effects. Lastly, Bader charges suggest potential problems during crystal structure optimization. © 2014 the Owner Societies.

Ding Y.,Technical University of Denmark | Peucheret C.,CNRS Foton Laboratory | Ou H.,Technical University of Denmark | Yvind K.,Technical University of Denmark
Optics Letters | Year: 2014

We design and fabricate an ultrahigh coupling efficiency (CE) fully etched apodized grating coupler on the siliconon- insulator (SOI) platform using subwavelength photonic crystals and bonded aluminum mirror. Fabrication error sensitivity and coupling angle dependence are experimentally investigated. A record ultrahigh CE of -0.58 dB with a 3 dB bandwidth of 71 nm and low back reflection are demonstrated. © 2014 Optical Society of America.

Najar A.,King Abdullah University of Science and Technology | Charrier J.,CNRS Foton Laboratory | Pirasteh P.,CNRS Foton Laboratory | Sougrat R.,King Abdullah University of Science and Technology
Optics Express | Year: 2012

High density vertically aligned Porous Silicon NanoWires (PSiNWs) were fabricated on silicon substrate using metal assisted chemical etching process. A linear dependency of nanowire length to the etching time was obtained and the change in the growth rate of PSiNWs by increasing etching durations was shown. A typical 2D bright-field TEM image used for volume reconstruction of the sample shows the pores size varying from 10 to 50 nm. Furthermore, reflectivity measurements show that the 35% reflectivity of the starting silicon wafer drops to 0.1% recorded for more than 10 μm long PSiNWs. Models based on cone shape of nanowires located in a circular and rectangular bases were used to calculate the reflectance employing the Transfert Matrix Formalism (TMF) of the PSiNWs layer. Using TMF, the Bruggeman model was used to calculate the refractive index of PSiNWs layer. The calculated reflectance using circular cone shape fits better the measured reflectance for PSiNWs. The remarkable decrease in optical reflectivity indicates that PSiNWs is a good antireflective layer and have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection. ©2012 Optical Society of America.

Even J.,CNRS Foton Laboratory | Pedesseau L.,CNRS Foton Laboratory | Katan C.,CNRS Chemistry Institute of Rennes
Journal of Physical Chemistry C | Year: 2014

Solution-processable metal-halide perovskites recently opened a new route toward low-cost manufacture of photovoltaic cells. Converting sunlight into electrical energy depends on several factors among which a broad absorption across the solar spectrum and attractive charge transport properties are of primary importance. Hybrid perovskites meet such prerequisites, but despite foremost experimental research efforts, their understanding remains scanty. Here we show that in these materials the appropriate absorption and transport properties are afforded by the multibandgap and multivalley nature of their band structure. We also investigate the nature of the photoexcited species. Our analysis suggests exciton screening by collective orientational motion of the organic cations at room temperature, leading to almost free carriers. Molecular collective motion is also expected to couple to carrier diffusion at room temperature. In mixed halides, our interpretation indicates that doping might hinder collective molecular motions, leading to good transport properties despite alloying and local lattice strain. © 2014 American Chemical Society.

Pedesseau L.,CNRS Foton Laboratory | Katan C.,CNRS Foton Laboratory | Even J.,CNRS Foton Laboratory
Applied Physics Letters | Year: 2012

An extended and complete thermodynamical model of third-order electro-elastic coupling is proposed with symmetry analyses and density functional theory (DFT) calculations to evaluate consistently the various linear and non-linear coefficients. It is shown that in non-centrosymmetric materials, electrostrictive and non-linear piezoelectric phenomena are strongly coupled, except for materials crystallizing in a cubic lattice associated to the 432 point group. Thorough numerical results are given for GaN and AlN compounds in the Wrtzite structure. Electrostriction dominates, but non-linear elasticity and non-linear piezoelectricity must be taken into account for strain evaluation whereas non-linear piezoelectricity yields a significant correction for electric field. © 2012 American Institute of Physics.

Fang H.-H.,Zernike Institute for Advanced Materials | Raissa R.,Zernike Institute for Advanced Materials | Abdu-Aguye M.,Zernike Institute for Advanced Materials | Adjokatse S.,Zernike Institute for Advanced Materials | And 3 more authors.
Advanced Functional Materials | Year: 2015

Hybrid organometal halide perovskites have been demonstrated to have outstanding performance as semiconductors for solar energy conversion. Further improvement of the efficiency and stability of these devices requires a deeper understanding of their intrinsic photophysical properties. Here, the structural and optical properties of high-quality single crystals of CH3NH3PbI3 from room temperature to 5 K are investigated. X-ray diffraction reveals an extremely sharp transition at 163 K from a twinned tetragonal I4/mcm phase to a low-temperature phase characterized by complex twinning and possible frozen disorder. Above the transition temperature, the photoluminescence is in agreement with a band-edge transition, explaining the outstanding performances of the solar cells. Whereas below the transition temperature, three different excitonic features arise, one of which is attributed to a free-exciton and the other two to bound excitons (BEs). The BEs are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon at high power excitation. The long lifetime and the saturation effect make us attribute these low temperature features to bound triplet excitons. This results in a description of the room temperature recombination as being due to spontaneous band-to-band radiative transitions, whereas a diffusion-limited behavior is expected for the low-temperature range. Low-temperature photophysical investigations of CH3NH3PbI3 single crystals indicate that the recombination in these perovskites is due to spontaneous band-to-band radiative transition at room temperature and to singlet-free-exciton and bound-triplet excitons below the phase transition temperature. The bound-triplet excitons are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon due to many-body interactions. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Dumeige Y.,CNRS Foton Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We theoretically analyze the second-harmonic generation process in a sequence of unidirectionnaly coupled doubly resonant whispering gallery mode semiconductor resonators. By using a convenient design, it is possible to coherently sum the second-harmonic fields generated inside each resonator. We show that resonator coupling allows the bandwidth of the phase-matching curve to be increased with respect to single-resonator configurations simultaneously taking advantage of the resonant feature of the resonators. This quasi-phase-matching technique could be applied to obtain small-footprint nonlinear devices with large bandwidth and limited nonlinear losses. The results are discussed in the framework of the slow-light-effect enhancement of second-order optical nonlinearities. © 2011 American Physical Society.

Dumeige Y.,CNRS Foton Laboratory | Feron P.,CNRS Foton Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

Coupled nonlinear resonators have potential applications for the integration of multistable photonic devices. The dynamic properties of two coupled-mode nonlinear microcavities made of Kerr material are studied by linear stability analysis. Using a suitable combination of the modal coupling rate and the frequency detuning, it is possible to obtain configurations where a hysteresis loop is included inside other bistable cycles. We show that a single resonator with two modes both linearly and nonlinearly coupled via the cross-Kerr effect can have a multistable behavior. This could be implemented in semiconductor nonlinear whispering-gallery-mode microresonators under modal coupling for all optical signal processing or ternary optical logic applications. © 2011 American Physical Society.

Even J.,CNRS Foton Laboratory | Pedesseau L.,CNRS Foton Laboratory | Jancu J.-M.,CNRS Foton Laboratory | Katan C.,CNRS Chemistry Institute of Rennes
Physica Status Solidi - Rapid Research Letters | Year: 2014

3D hybrid organic perovskites, CH3NH3PbX3 (X = halogen), have recently been used to strongly improve the efficiency of dye sensitized solar cells (DSSC) leading to a new class of low-cost mesoscopic solar cells. CsSnI3 perovskite can also be used for hole conduction in DSSC. Density functional theory and GW corrections are used to compare lead and tin hybrid and all-inorganic perovskites. The room temperature optical absorption is associated to electronic transitions between the spin-orbit split-off band in the conduction band and the valence band. Spin-orbit coupling is about three times smaller for tin. Moreover, the effective mass of relevant band edge hole states is small (0.17). The high temperature phase sequence of CsSnI3 leading to the room temperature orthorhombic phase and the recently reported phases of CH3NH3MI3 (where M = Pb, Sn) close to the room temperature, are also studied. Tetragonal distortions from the ideal cubic phase are analysed by a k · p perturbation, including spin-orbit effect. In addition, the non-centrosymmetric phases of CH3NH3MI3 exhibit a splitting of the electronic bands away from the critical point. The present work shows that their physical properties are more similar to conventional semiconductors than to the absorbers used in DSSC. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Trebaol S.,CNRS Foton Laboratory | Dumeige Y.,CNRS Foton Laboratory | Feron P.,CNRS Foton Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

We present a simple model to describe the transient response of two coupled resonators probed by a monochromatic wave whose frequency is rapidly swept across the resonances with respect to their characteristic photon lifetimes. The model is applied to analyze the dynamic behavior of the modal coupling between two degenerate resonances of the same cavity. In particular, this can be used to describe the coupling of counterpropagating whispering gallery modes (WGMs) by Rayleigh scattering. The theory is successfully compared to experiments carried out in silica microspheres. These results show that this ringdown technique can be extended to accurately measure linear properties and frequency splittings of high-quality factor WGM microresonators. © 2010 The American Physical Society.

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