66 research outputs found
White organic light-emitting diodes with an ultra-thin premixed emitting layer
We described an approach to achieve fine color control of fluorescent White
Organic Light-Emitting Diodes (OLED), based on an Ultra-thin Premixed emitting
Layer (UPL). The UPL consists of a mixture of two dyes (red-emitting
4-di(4'-tert-butylbiphenyl-4-yl)amino-4'-dicyanovinylbenzene or fvin and
green-emitting 4-di(4'-tert-butylbiphenyl-4-yl)aminobenzaldehyde or fcho)
premixed in a single evaporation cell: since these two molecules have
comparable structures and similar melting temperatures, a blend can be
evaporated, giving rise to thin films of identical and reproducible composition
compared to those of the pre-mixture. The principle of fine color tuning is
demonstrated by evaporating a 1-nm-thick layer of this blend within the
hole-transport layer (4,4'-bis[N-(1-naphtyl)-N-phenylamino]biphenyl
(\alpha-NPB)) of a standard fluorescent OLED structure. Upon playing on the
position of the UPL inside the hole-transport layer, as well as on the premix
composition, two independent parameters are available to finely control the
emitted color. Combined with blue emission from the heterojunction, white light
with Commission Internationale de l'Eclairage 1931 color coordinates (0.34,
0.34) was obtained, with excellent color stability with the injected current.
The spectrum reveals that the fcho material does not emit light due to
efficient energy transfer to the red-emitting fvin compound but plays the role
of a host matrix for fvin, allowing for a very precise adjustment of the red
dopant amount in the device
Enhanced production of coherent pulsed radiation at 125 nm: the route towards a tabletop VUV laser
A novel approach is used to enhance by nearly two orders of magnitude the
conversion efficiency of a 125 nm-coherent source, based on four-wave mixing in
room-temperature mercury vapor. Saturation issues are observed and discussed
White Organic Light-Emitting Diodes with fine chromaticity tuning via ultrathin layer position shifting
Non-doped 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 NPB
(4,4'-bis[N-(1-naphtyl)-N-phenylamino]biphenyl) layer and a blue-emitting DPVBi
(4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl) 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), a external quantum efficiency 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 4.9 nm respectively)
On thermal effects in solid state lasers: the case of ytterbium-doped materials
A review of theoretical and experimental studies of thermal effects in
solid-state lasers is presented, with a special focus on diode-pumped
ytterbium-doped materials. A large part of this review provides however general
information applicable to any kind of solid-state laser. Our aim here is not to
make a list of the techniques that have been used to minimize thermal effects,
but instead to give an overview of the theoretical aspects underneath, and give
a state-of-the-art of the tools at the disposal of the laser scientist to
measure thermal effects. After a presentation of some general properties of
Yb-doped materials, we address the issue of evaluating the temperature map in
Yb-doped laser crystals, both theoretically and experimentally. This is the
first step before studying the complex problem of thermal lensing (part III).
We will focus on some newly discussed aspects, like the definition of the
thermo-optic coefficient: we will highlight some misleading interpretations of
thermal lensing experiments due to the use of the dn/dT parameter in a context
where it is not relevant. Part IV will be devoted to a state-of-the-art of
experimental techniques used to measure thermal lensing. Eventually, in part V,
we will give some concrete examples in Yb-doped materials, where their
peculiarities will be pointed out
Time-domain Compressed Sensing
Ultrashort time-domain spectroscopy, particularly field-resolved
spectroscopy, are established methods for identifying the constituents and
internal dynamics of samples. However, these techniques are often encumbered by
the Nyquist criterion, leading to prolonged data acquisition and processing
times as well as sizable data volumes. To mitigate these issues, we have
successfully implemented the first instance of time-domain compressed sensing,
enabling us to pinpoint the primary absorption peaks of atmospheric water vapor
in response to tera-hertz light transients that exceed the Nyquist limit. Our
method demonstrates successful identification of water absorption peaks up to
2.5 THz, even for sampling rates where the Nyquist frequency is as low as 0.75
THz, with a mean squared error of 12*10-4. Time-domain sparse sampling achieves
considerable data compression while also expediting both the measurement and
data processing time, representing a significant stride towards the realm of
real-time spectroscop
Passively Q-switched diode-pumped Cr4+:YAG/Nd3+:GdVO4 monolithic microchip laser
the realization of high repetition rate passively Q-switched monolithic
microlaser is a challenge since a decade. To achieve this goal, we report here
on the first passively Q-switched diode-pumped microchip laser based on the
association of a Nd:GdVO4 crystal and a Cr4+:YAG saturable absorber. The
monolithic design consists of 1 mm long 1% doped Nd:GdVO4 optically contacted
on a 0.4 mm long Cr4+:YAG leading to a plano-plano cavity. A repetition rate as
high as 85 kHz is achieved. The average output power is approximately 400 mW
for 2.2 W of absorbed pump power and the pulse length is 1.1 ns
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