Among various laser architectures currently used to make lasers out of organic materials (distributed feedback lasers or organic vertical cavity surface-emitting lasers, ....), vertical EXTERNAL cavities have several distinctive features that enable making lasers with a high brightness, resulting from a combination of high efficiency and good beam quality, and also offer a superior flexibility to monitor the laser spectrum.
In this talk I will highlight a few recent results on external-cavity organic lasers and reveal their potential through the example of a single mode organic laser device with an ultranarrow linewidth (< pm) corresponding to coherence lengths of several meters under diode pumping (typically 2-3 orders of magnitude longer than the state-of-the-art). From the material point of view, I will also show how ink-jet printing can be successfully used in vertical external-cavity organic lasers to make thick and optical-quality films that have the potential to be easily produced with a high throughput.
Organic lasers offer the promise to build compact, inexpensive, broadly tunable solid-state lasers in the visible range,
with potential applications in spectroscopy, bio/chemo sensing or short-haul data telecommunications. Among existing
laser architectures of optically-pumped organic lasers, external-cavity resonators enable the highest conversion
efficiencies, excellent beam quality, power scalability and versatility due to the open cavity. Recently, we reported on an
open-cavity laser architecture using a thin film of dye-doped polymer as the gain medium, named Vertical External
Cavity Surface-emitting Organic Laser (VECSOL). The very high gain of organics make these lasers highly efficient
even for macroscopic cavities, even though the pulse buildup time must be short enough to fit within the gain time
window defined by the pump pulse duration and the fluorescence lifetime.
In this paper we analyze the laser turn-on dynamics of organic VECSELs. A simple theoretical framework is presented,
based on the Statz-DeMars coupled rate equations. Simulations are compared to the experimental pulse shapes of the
pump and laser beams, recorded with the same fast photodiode. We observe that the laser pulse is both shifted and
broadened with respect to the 0.5-ns-long pump pulse when the cavity length is increased, together with a drop of
efficiency. Efficiency curves are presented, showing a higher threshold and lower slope efficiency when the cavity length
increases, which is well accounted by the model. Finally, an optimized VECSOL is presented, with a 25 ns-pulsewidth
pump source, enabling reaching conversion efficiencies up to 61%.
White light can be obtained from Organic Light Emitting Diodes by mixing three primary colors, (i.e.
red, green and blue) or two complementary colors in the emissive layer. In order to improve the
efficiency and stability of the devices, a host-guest system is generally used as an emitting layer.
However, the color balance to obtain white light is difficult to control and optimize because the
spectrum is very sensitive to doping concentration (especially when a small amount of material is
used). We use here an ultra-thin mixed emitting layer (UML) deposited by thermal evaporation to
fabricate white organic light emitting diodes (WOLEDs) without co-evaporation. The UML was
inserted in the hole-transporting layer consisting of 4, 4'-bis[N-(1-naphtyl)-N-phenylamino]biphenyl
(α-NPB) instead of using a conventional doping process. The UML was formed from a single
evaporation boat containing a mixture of two dipolar starbust triarylamine molecules (fvin and fcho)
presenting very similar structures and thermal properties and emitting in complementary spectral
regions (orange and blue respectively) and mixed according to their weight ratio. The composition of
the UML specifically allows for fine tuning of the emission color despite its very thin thickness
down to 1 nm. Competitive energy transfer processes from fcho and the host interface toward fvin
are key parameters to control the relative intensity between red and blue emission. White light with
very good CIE 1931 color coordinate (0.34, 0.34) was obtained by simply adjusting the UML film
In this work we study optically pumped polymer-based lasers doped by various organic dyes in two different
configurations, namely plane cavities of various shapes and vertical external cavities ("VECSOL"). The intrinsic
fluorescence anisotropy of specific dye molecule together with the polarization state of the pump beam defines the basic
emission properties of such dye-doped polymer system. The nonlinear enhancement of amplified spontaneous emission
(ASE) and lasing results in the forcing of the emission properties and particularly of their polarization features. However,
we demonstrate experimentally that it is possible to release this constraint and to obtain laser emission with a
polarization state different from that of the pump.
Although optically pumped semiconductor organic lasers have been reported for a decade, no electrically pumped
organic laser diode has been till now realized. Charge-induced and triplet excited state absorption have been identified as
major bottlenecks: in this context exciton dynamic plays a key role. We report on the measurement of exciton diffusion
lengths in the archetypal ambipolar material CBP, in the presence of an injected current in a working multilayer device.
The technique is based on moving a thin red phosphorescent layer away from the recombination zone. The amount of
emitted light depends on the layer position via the diffusion of triplet excitons. We demonstrate the crucial importance of
designing the structure according to optical field calculations in order to measure diffusion lengths LD. We measured a
value of LD = 16 nm +/- 4 nm in CBP, and also report on the variation of LD with the injected current.
We report on direct, absolute and spatially resolved temperature measurements in various diode-end-pumped laser crystals, using an infrared camera. Our measurement method requires careful calibrations of the camera, to take into account the emissivity of the crystals. We tested the repeatability of the calibration process, and the linearity of calibrations curves was verified to up to 100°C. We obtained good agreement between experimental results and finite elements analysis simulations done with LASCAD. We also studied and compared different types of thermal contacts and to measure the corresponding heat transfer coefficients using an Yb:YAG crystal. Finally we tried to highlight one of the major controversy concerning the comparison of the thermal behaviours of Nd:YVO4 and Nd:GdVO4 crystals.
We demonstrate for the first time the use of a new borate crystals: Yb:GdCOB and Yb:BOYS in femtosecond mode-locked oscillators. Pulses as short as 69 fs for the Yb:BOYS and 89 fs for the Yb:GdCOB have been obtained near 1 µm.