Sébastien Chénais

Recent advances in solid-state organic lasers

Organic solid-state lasers are reviewed, with a special emphasis on works published during the last decade. Referring originally to dyes in solid-state polymeric matrices, organic lasers also include the rich family of organic semiconductors, paced by the rapid development of organic light-emitting diodes. Organic lasers are broadly tunable coherent sources, potentially compact, convenient and manufactured at low cost. In this review, we describe the basic photophysics of the materials used as gain media in organic lasers with a specific look at the distinctive features of dyes and semiconductors. We also outline the laser architectures used in state-of-the-art organic lasers and the performances of these devices with regard to output power, lifetime and beam quality. A survey of the recent trends in the field is given, highlighting the latest developments in terms of wavelength coverage, wavelength agility, efficiency and compactness, and towards integrated low-cost sources, with a special focus on the great challenges remaining for achieving direct electrical pumping. Finally, we discuss the very recent demonstration of new kinds of organic lasers based on polaritons or surface plasmons, which open new and very promising routes in the field of organic nanophotonics. Copyright

Thermal lensing in diode-pumped ytterbium Lasers-Part I: theoretical analysis and wavefront measurements

A theoretical and experimental study of thermal lensing in Yb-doped crystals is presented. In this first part, we focus on theoretical considerations and we describe an original technique suitable for thermal lensing measurements in end-pumped materials. We first derive an expression of the temperature distribution with account of absorption saturation and pump beam divergence inside the crystal, and we address a more general discussion on the particularities of quasi-three-level lasers, as far as thermal effects and fracture issues are concerned. The thermal lens was then measured using a simple technique based on a Shack-Hartmann wavefront analyzer, under lasing and nonlasing conditions. We demonstrate that the technique allows precise wavefront measurements even on small spots. Thermal lensing measurements are finally presented in Yb-doped YAG, GGG, YCOB, GdCOB, KGW, and YSO crystals.

Implementation of PT symmetric devices using plasmonics: principle and applications

The so-called PT symmetric devices, which feature ε((-x)) = ε((x))* associated with parity-time symmetry, incorporate both gain and loss and can present a singular eigenvalue behaviour around a critical transition point. The scheme, typically based on co-directional coupled waveguides, is here transposed to the case of variable gain on one arm with fixed losses on the other arm. In this configuration, the scheme exploits the full potential of plasmonics by making a beneficial use of their losses to attain a critical regime that makes switching possible with much lowered gain excursions. Practical implementations are discussed based on existing attempts to elaborate coupled waveguide in plasmonics, and based also on the recently proposed hybrid plasmonics waveguide structure with a small low-index gap, the PIROW (Plasmonic Inverse-Rib Optical Waveguide).

White organic light-emitting diodes with fine chromaticity tuning via ultrathin layer position shifting

Nondoped white organic light-emitting diodes using an ultrathin yellow-emitting layer of rubrene (5,6,11,12-tetraphenylnaphtacene) inserted on either side of the interface between a hole-transporting 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (α-NPB) layer and a blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) layer are described. Both the thickness and the position of the rubrene layer allow fine chromaticity tuning from deep blue to pure yellow via bright white with CIE coordinates (x=0.33, y=0.32), an ηext of 1.9%, and a color rendering index of 70. Such a structure also provides an accurate sensing tool to measure the exciton diffusion length in both DPVBi and NPB (8.7 and 4.9nm, respectively).

Thermal lensing in diode-pumped ytterbium Lasers-Part II: evaluation of quantum efficiencies and thermo-optic coefficients

A theoretical and experimental study of thermal lensing in Yb-doped crystals is presented. This papers follows the presentation of theoretical considerations and experimental wavefront measurements, which have been the subject of Part I. In this paper, we study the evolution of thermal lensing versus absorbed pump power, and we derive two parameters valuable for laser design and power scaling. The quantum efficiency and the thermo-optic coefficient, in Yb-doped YAG, GGG, GdCOB, YCOB, KGW and YSO. The clear difference between thermal lensing under lasing and nonlasing conditions is the proof that nonradiative effects occur in all the crystals under investigation. An analytical model which takes into account the laser extraction efficiency enables to explain all the experimental features and allows to infer the fluorescence quantum efficiency of the samples (in the range 0.7-0.96). Under nonlasing conditions, the thermal lens dioptric power experiences a roll-off, for which we propose an explanation based on the theory presented in Part I. These results are then used to yield the thermo-optic coefficient of the crystals. At last, we propose a simple analytical formulation useful for a rough estimation of the focal length.

Highly efficient, diffraction-limited laser emission from a vertical external-cavity surface-emitting organic laser

We report on a solid-state laser structure functioning as the organic counterpart of a vertical external-cavity surface-emitting laser (VECSEL) design. The gain medium is a poly(methyl methacrylate) film doped with Rhodamine 640, spin casted onto the high-reflectivity mirror of a plano-concave resonator. Upon pumping by 7 ns pulses at 532 nm, a diffraction-limited beam (M(2)=1) was obtained, with a conversion efficiency of 43%; higher peak powers (2 kW) could be attained when resorting to shorter (0.5 ns) pump pulses. The spectrum was controlled by the thickness of the active layer playing the role of an intracavity etalon; tunability is demonstrated at over and up to 20 nm.

Laser operation in nondoped thin films made of a small-molecule organic red-emitter

Stimulated emission in small-molecule organic films at a high dye concentration is generally hindered by fluorescence quenching, especially in the red region of the spectrum. Here we demonstrate the achievement of high net gains (up to 50 cm−1) around 640 nm in thermally evaporated nondoped films of 4-di(4′-tert-butylbiphenyl-4-yl)amino-4′-dicyanovinylbenzene, which makes this material suitable for green-light pumped single mode organic lasers with low threshold and superior stability. Lasing effect is demonstrated in a distributed Bragg resonator configuration, as well as under the form of random lasing at high pump intensities.

Organic Solid-State Lasers

Fundamentals of laser physics.- Organic materials for solid-state lasers.- Organic Lasers resonators.- Prospects for electrical pumping.- Organic lasers at the nanoscale.- Applications of organic solid-state lasers.

Tunable ultraviolet vertically-emitting organic laser

A solid-state organic thin-film laser with intracavity frequency doubling is reported. Tunable ultraviolet emission from 309 to 322 nm is achieved from a vertical external cavity surface-emitting organic laser, with 2% efficiency (1 μJ at 315 nm). The laser comprises a poly(methyl methacrylate) layer doped with Rhodamine 640, spun-cast onto a plane mirror, a remote concave mirror, a nonlinear crystal, and a dichroic separator. The output is spectrally narrow (<0.5 nm full width at half maximum) and tunable through phase-matching selection of the fundamental radiation lasing modes. These results highlight a low-cost and portable alternative to tunable UV laser sources, useful for spectroscopic applications.

Highly efficient multilayer organic pure blue light emitting diodes with substituted carbazoles compounds in the emitting layer

Bright-blue organic-light-emitting-diodes based on carbazolic-compounds as emitting layers or dopant in DPVBi matrix are described. Pure-blue light with CIE coordinates xy = (0.158;0.169) and external quantum efficiency as high as etaext = 3.3% were obtained.