Alexis Fischer

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).

Experimental and theoretical study of filtered optical feedback in a semiconductor laser

We report on the systematical investigation of the steady-state regime and the dynamical behavior of a semiconductor laser subject to delayed filtered optical feedback. We study a Fabry-Perot (FP) interferometer type of filter placed in the external feedback loop of a diode laser. The effects of the filter on the locking of the diode laser frequency to the external cavity modes are described. We report and observe hysteresis, bistability, and multistability and show that all these are well described by a set of rate equations for the coupled laser and FP cavity system. We also present an experimental stability diagram that summarizes the dynamical behavior of the system.

Experimental stability diagram of a diode laser subject to weak phase-conjugate feedback from a rubidium vapor cell

We report on a systematic experimental investigation of the dynamical behavior of a diode laser subject to delayed feedback from a phase-conjugating mirror based on nearly-degenerate four-wave mixing in rubidium vapor. We present the first experimental stability diagram for this system. In the weak feedback regime, with feedback levels up to -30 dB, me demonstrate locking to the pump frequency with a reduced linewidth smaller than 8 MHz. In contrast to a laser with external injection, and contrary to predictions made for phase-conjugate feedback, the extent of this locking region is found to be limited. For increased feedback rates, the laser leaves the locking region and enters an unstable region through a series of multiple sidebands spectra. This is associated with self-excitation of the relaxation oscillation, and the observed scenario suggests a series of period-doublings after an initial Hopf bifurcation.

Optimal design of a microcavity organic laser device under electrical pumping

The quality factor of microcavity organic lasers, designed for operation under electric pumping, has been numerically investigated. The microcavity structure consists of an organic light emitting diode set in between multilayer dielectric mirrors centered for an emission at 620 nm. In order to optimize the quality factor, different parameters have been studied: the impact of high and low index materials used for the multilayer mirrors, the role of a spacer inserted in between the mirrors to obtain an extended cavity, and the effect of an absorbing electrode made of metallic or transparent conductive oxide layer. The results of our different optimizations have shown a quality factor (Q) as high as 15,000.

Optically pumped lasing from organic two-dimensional planar photonic crystal microcavity

In this letter, we report the realization and characterization of a planar two-dimensional organic photonic crystal microcavity laser. The gain medium consists of a tris(8-hydroxyquinolinato) aluminum doped with 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran and is deposited on a lithographically patterned silicon nitride two dimensional photonic crystal H2 microcavity. The experimental results show a laser emission at 662 nm with a threshold of 9.7 μJ/cm2.

Experimental study of multiwavelength parametric generation in a two-dimensional periodically poled lithium tantalate crystal

In this Letter, we experimentally investigate multiwavelength parametric generation in two-dimensional second-order nonlinear photonic crystals. For this purpose, a 2D periodically poled lithium tantalate crystal with rectangular lattice was fabricated and characterized. We demonstrate multiple and simultaneous wavelength generation due to the contribution of different lattice vectors. Numerical simulations emphasize the agreement of our phase matching scheme with the experimental results and made it possible to assign the observed wavelengths to the reciprocal lattice vectors involved in the parametric generation process. Moreover, our results indicate that some signals are the result of the joint contribution of more than one lattice vector.

Effects of Low Ag Doping on Physical and Optical Waveguide Properties of Highly Oriented Sol-Gel ZnO Thin Films

A sol-gel dip-coating process was used to deposit almost stress-free highly c-axis oriented zinc oxide (ZnO) thin films onto glass substrates. The effects of low silver doping concentration (Ag/Zn < 1%) on the structural, morphological, optical, and waveguide properties of such films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy, UV-Visible spectrophotometry, and M-lines spectroscopy (MLS). XRD analysis revealed that all the films were in single phase and had a hexagonal wurtzite structure. The grain size values were calculated and found to be about 24–29 nm. SEM micrographs and AFM images have shown that film morphology and surface roughness were influenced by Ag doping concentration. According to UV-Vis. measurements all the films were highly transparent with average visible transmission values ranging from 80% to 86%. It was found that the Ag contents lead to widening of the band gap. MLS measurements at 632.8 nm wavelength put into evidence that all thin film planar waveguides demonstrate a well-guided fundamental mode for both transverse electric and transverse magnetic polarized light. Moreover, the refractive index of ZnO thin films was found to increase by Ag doping levels.

Experimental optimization of the optical and electrical properties of a half-wavelength-thick organic hetero-structure in a Micro-cavity

In the context of progress towards the organic laser diode, we experimentally investigate the optical and electrical optimization of an OLED in a vertical λ/2 microcavity. The microcavity consists of a quarter-wavelength TiO₂/SiO₂ multilayer mirror, a half-wavelength-thick OLED and a semitransparent aluminum cathode. The Alq3/DCM2 guest-host system is used as the emitting layer. This study focuses on the design and the fabrication of a half-wavelength thick organic hetero-structure exhibiting a high current density despite both the thickness increase and the cathode thickness reduction. The emission wavelength, the line-width narrowing and the current-density are studied as a function of two key parameters: the hetero-structure optical thickness and the aluminum cathode thickness. The experimental results show that a 125 nm thick cavity OLED ended by a 20 nm thick aluminum cathode exhibits a resonance at 606 nm with a full width at half maximum of 11 nm, and with current-densities exceeding 0.5 A/cm². We show that even without a top high-quality-mirror the incomplete microcavity λ/2 OLED hetero-structure exhibits a clear modification of the spontaneous emission at normal incidence.

Post-annealing effects on the physical and optical waveguiding properties of RF sputtered ZnO thin films

ZnO thin films were deposited at room temperature onto glass substrate by RF sputtering technique. Effects of the post-annealing at 300–500°C on the structural, morphological, optical and waveguiding properties were investigated using different characterization techniques. X-ray diffraction (XRD) analyses have shown that all thin films have a hexagonal wurtzite structure with higher c-axis preferred orientation (002), better crystallinity and larger crystallite size as post-annealing temperature increases. Fourier transform infrared (FTIR) spectra of annealed samples confirmed that the ZnO stretching vibration bond was found to be stable at 419 cm−1. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images have revealed that film morphology and surface roughness were influenced by heat treatment temperatures. The UV-Vis-NIR spectrophotometry characterisations have indicated that all the films were highly transparent with average transmittance exceeding 81% within the visible region, and the bandgap energy of the as-deposited film was increased with increasing of the annealing temperature.The obtained results from m-lines spectroscopy (MLS) measurements at 632.8 nm wavelength have demonstrated that all ZnO thin film optical waveguides were single mode and the ordinary and extraordinary refractive index values of the film annealed at 500°C were very close to the corresponding ZnO single-crystal values.

Self-organized nanoparticle photolithography for two-dimensional patterning of organic light emitting diodes

We report a new simple and inexpensive sub-micrometer two dimensional patterning technique. This technique combines a use of a photomask featured with self-organized particles in the micro- to nano-meter size range and a photoresist-covered substrate. The photomask was prepared by depositing monodispersed silicon dioxide (SiO(2))- or polystyrene- spheres on a quartz substrate to form a close-packed pattern. The patterning technique can be realized in two configurations: a hard-contact mode or a soft-contact mode. In the first configuration, each sphere acts as a micro ball-lens that focuses light and exposes the photoresist underneath the sphere. The developed pattern therefore reproduces exactly the same spatial arrangement as the close-packed spheres but with a feature size of developed hole smaller than the diameter of the sphere. In the soft-contact mode, an air gap of few micrometers thick is introduced between the 2D array of self-organized spheres and the photoresist-covered substrate. In this case, a phase mask behavior is obtained which results in an exposure area with a lattice period being half of the sphere diameter. A 2D lattice structure with period and feature size of a developed hole as small as 750 nm and 420 nm, respectively, was realized in this configuration. We further applied this technique to host the deposition of organic films into the 2D nanostructure and demonstrated the realization of green and red nano-structured OLEDs.