Gain properties of dye-doped polymer thin films

Hybrid pumping appears as a promising compromise in order to reach the much coveted goal of an electrically pumped organic laser. In such configuration the organic material is optically pumped by an electrically pumped inorganic device on chip. This engineering solution requires therefore an optimization of the organic gain medium under optical pumping. Here, we report a detailed study of the gain features of dye-doped polymer thin films. In particular we introduce the gain efficiency $K$, in order to facilitate comparison between different materials and experimental conditions. The gain efficiency was measured with various setups (pump-probe amplification, variable stripe length method, laser thresholds) in order to study several factors which modify the actual gain of a layer, namely the confinement factor, the pump polarization, the molecular anisotropy, and the re-absorption. For instance, for a 600 nm thick 5 wt\% DCM doped PMMA layer, the different experimental approaches give a consistent value $K\simeq$ 80 cm.MW$^{-1}$. On the contrary, the usual model predicting the gain from the characteristics of the material leads to an overestimation by two orders of magnitude, which raises a serious problem in the design of actual devices. In this context, we demonstrate the feasibility to infer the gain efficiency from the laser threshold of well-calibrated devices. Besides, temporal measurements at the picosecond scale were carried out to support the analysis.Comment: 15 pages, 17 figure

Red-emitting fluorescent Organic Light emitting Diodes with low sensitivity to self-quenching

International audienceConcentration quenching is a major impediment to efficient organic light-emitting devices. We herein report on Organic Light-Emitting Diodes (OLEDs) based on a fluorescent amorphous red-emitting starbust triarylamine molecule (4-di(4'-tert-butylbiphenyl-4-yl)amino-4'-dicyanovinylbenzene, named FVIN), exhibiting a very small sensitivity to concentration quenching. OLEDs are fabricated with various doping levels of FVIN into Alq3, and show a remarkably stable external quantum efficiency of 1.5% for doping rates ranging from 5% up to 40%, which strongly relaxes the technological constraints on the doping accuracy. An efficiency of 1% is obtained for a pure undoped active region, along with deep red emission (x=0.6; y=0.35 CIE coordinates). A comparison of FVIN with the archetypal DCM dye is presented in an identical multilayer OLED structure

Trace formula for dielectric cavities II: Regular, pseudo-integrable, and chaotic examples

Dielectric resonators are open systems particularly interesting due to their wide range of applications in optics and photonics. In a recent paper [PRE, vol. 78, 056202 (2008)] the trace formula for both the smooth and the oscillating parts of the resonance density was proposed and checked for the circular cavity. The present paper deals with numerous shapes which would be integrable (square, rectangle, and ellipse), pseudo-integrable (pentagon) and chaotic (stadium), if the cavities were closed (billiard case). A good agreement is found between the theoretical predictions, the numerical simulations, and experiments based on organic micro-lasers.Comment: 18 pages, 32 figure

Laser à semiconducteur à 852 nm bifrequence pompé optiquement pour les horloges atomiques CPT (poster)

National audienceNous présentons un laser à semiconducteur en cavité externe pompé optiquement, émettant sur deux fréquences optiques polarisées perpendiculairement, destiné au piégeage cohérent d'atomes (CPT) de Cs. L'émission est accordable autour de 852 nm. La différence de fréquence est ajustée grâce à une lame électro-optique autour de 9,2 GHz. La longueur d'onde du mode ordinaire est stabilisée sur la raie D2 du Cs et la différence de fréquence est asservie sur un signal de référence RF. En fonctionnement stabilisé, nous caractérisons les sources de bruits du laser afin d'évaluer les performances du laser en vue de son application dans une horloge atomique CPT

An insight into polarization states of solid-state organic lasers

The polarization states of lasers are crucial issues both for practical applications and fundamental research. In general, they depend in a combined manner on the properties of the gain material and on the structure of the electromagnetic modes. In this paper, we address this issue in the case of solid-state organic lasers, a technology which enables to vary independently gain and mode properties. Different kinds of resonators are investigated: in-plane micro-resonators with Fabry-Perot, square, pentagon, stadium, disk, and kite shapes, and external vertical resonators. The degree of polarization P is measured in each case. It is shown that although TE modes prevail generally (P>0), kite-shaped micro-laser generates negative values for P, i.e. a flip of the dominant polarization which becomes mostly TM polarized. We at last investigated two degrees of freedom that are available to tailor the polarization of organic lasers, in addition to the pump polarization and the resonator geometry: upon using resonant energy transfer (RET) or upon pumping the laser dye to an higher excited state. We then demonstrate that significantly lower P factors can be obtained.Comment: 12 pages, 12 figure

Evaluation of the noise properties of a dual-frequency VECSEL for compact Cs atomic clocks (Poster)

International audienceWe evaluate a dual-frequency and dual-polarization optically-pumped semiconductor laser emitting at 852 nm as a new laser source for compact atomic clocks based on the coherent population trapping (CPT) technique. The frequency difference between the laser modes is tunable to 9.2 GHz corresponding to the ground state hyperfine-split of Cs. Impact of the laser noise has been investigated. Laser relative intensity noise is limited by the pump-RIN transfer to a level of-110 dB/Hz. Laser frequency noise shows excess mechanical and technical noise resulting in a laser linewidth of 1 MHz at 1 s in lock operation. The noise performance and spectral properties of the laser are already adequate to realize CPT experiments and should result in Allan standard-deviation of the clock below 1 × 10-12 at 1 second

Grain growth: The key to understand solid-state dewetting of silver thin films

International audienceThe dynamics of solid-state dewetting of polycrystalline silver thin films in oxygen atmosphere was investigated with in situ and real-time environmental Scanning Electron Microscopy at high temperature combined with Atomic Force Microscopy. Three steps were identified during dewetting : induction, hole propagation without specific rim and sintering. Moreover, it was observed that a very selective grain growth, promoted by surface diffusion, plays a key role all along the process

Scalable simple liquid deposition techniques for the enhancement of light absorption in thin films: Distributed Bragg reflectors coupled to 1D nanoimprinted textures

Light trapping within a light absorbing medium is a key to highly efficient thin film solar cells. We propose a large-scale procedure based on materials with low absorption for the fabrication of combined Distributed Bragg Reflector (DBR) and grating light trapping structures. Using Rigorous Coupled Wave Analysis (RCWA) numerical simulations we designed a combined DBR and 1D grating structure allowing to significantly improve the absorption in a aSi:H film as thin as 100 nm. The optimized light trapping structure was fabricated. The enhancement of light absorption in thin aSi:H film was experimentally proven and discussed quantitatively with respect to the theoretical expectations

On the solid-state dewetting of polycrystalline thin films: Capillary versus grain growth approach

International audienceSolid-state dewetting of polycrystalline silver thin films was investigated with in situ and real time Environmental Scanning Electron Microscopy at High Temperature (HT-ESEM) in different annealing atmospheres: secondary vacuum or oxygen-rich (partial pressure ≥100 Pa) environment. A model where oxygen plays a key role is proposed to explain the very different observed morphologies; oxygen favours hole creation and isotropic hole propagation as well as grain selection. But, whatever the atmosphere, dewetting does not proceed through the propagation of a rim but instead involves the growth of specific grains and shrinkage of others. Models based on macroscopic curvature to account for the propagation speed of the dewetting front fail to fit the present observations. This points to a paramount role of the grain size and stability in the dewetting morphology