E. Daran

High angular tolerance and reflectivity with narrow bandwidth cavity-resonator-integrated guided-mode resonance filter

Guided mode resonance filters (GMRFs) are a promising new generation of reflective narrow band filters, that combine structural simplicity with high efficiency. However their intrinsic poor angular tolerance and huge area limit their use in real life applications. Cavity-resonator-integrated guided-mode resonance filters (CRIGFs) are a new class of reflective narrow band filters. They offer in theory narrow-band high-reflectivity with a much smaller footprint than GMRF. Here we demonstrate that for tightly focused incident beams adapted to the CRIGF size, we can obtain simultaneously high spectral selecitivity, high reflectivity, high angular acceptance with large alignment tolerances. We demonstrate experimentally reflectivity above 74%, angular acceptance greater than ±4.2° for a narrow-band (1.4 nm wide at 847 nm) CRIGF.

CaF2:Er3+ molecular beam epitaxial layers as optical waveguides

CaF2:Er thin films grown by molecular beam epitaxy on CaF2 substrates have been characterized as optical waveguides. The characterization has been carried out by measuring the synchronous angles in attenuated total reflection experiments. It has been found that the incorporation of Er3+ ions to the CaF2 crystal produces an increase of the refractive index of the material giving rise to the formation of a steplike planar optical waveguide. The refractive index increase shows a linear dependence with Er3+ concentration up to 35 mol %, at which point saturation is observed. The obtained results compared well with previously reported data on the lattice parameter of CaF2:Er3+ bulk crystals and fluorescence quenching from Er3+ ions in these layers. The results of this work show that the preparation of CaF2:Er3+ layers on CaF2 substrates by molecular beam epitaxy is an ideal method to produce monomode active optical waveguides useful for optoelectronic devices.

Fabrication and characterization of microlens arrays using a cantilever-based spotter

We present a quantitative study on the fabrication of microlenses using a low-cost polymer dispending technique. Our method is based on the use of a silicon micro-cantilever robotized spotter system. We first give a detailed description of the technique. In a second part, the fabricated microlenses are fully characterized by means of SEM (Scanning Electron Microscope), AFM (Atomic Force Microscopy) non contact optical profilometry and Mach-Zehnder interferometry. Diameters in the range [25-130mum] are obtained with an average surface roughness of 2.02nm. Curvature radii, focal lengths as well as aberrations are also measured for the first time: the fabricated microlenses present focal lengths in the range [55-181mum] and exhibit high optical quality only limited by diffraction behaviour with RMS aberration lower than lambda/14.

Er3+ doping of CaF2 layers grown by molecular beam epitaxy

Molecular beam epitaxy of CaF2 monocrystalline layers Er3+ doped up to a concentration of 5 wt % is demonstrated on CaF2 substrates. Separated effusion cells containing CaF2 and ErF3 were used. The photoluminescence spectra of the samples show emissions from centers of different symmetry identified by reference to published results obtained on CaF2:Er3+ bulk crystals. No influence of the substrate orientation—(100) or (111)—on the luminescence characteristics was observed.

1.54 μm wavelength emission of highly Er‐doped CaF2 layers grown by molecular‐beam epitaxy

CaF2:Er layers have been grown by molecular‐beam epitaxy on (100)‐oriented CaF2 substrates; the Er concentration ranges from 1% to 50% (mole fraction). The 1.54 μm emission observed under excitation around 800 nm was studied by photoluminescence. Up to 35% Er concentration the integrated emission increases monotonously, quenching appearing for higher doping levels. Photoluminescence results are discussed within the framework of previous studies of Er3+ emission in the near‐infrared range (830–860 nm) in order to gain insight into the Er centers involved in the 1.54 μm emission.

Molecular beam epitaxy of Nd‐doped CaF2 and CaSrF2 layers on Si and GaAs substrates

The incorporation of Nd3+ in CaF2 layers grown on Si and GaAs substrates by molecular beam epitaxy is studied by photoluminescence spectroscopy. The results are in qualitative agreement with those obtained on CaF2:Nd homoepitaxial layers. A lower emission intensity (∼70%) at λ=1.0475 μm is attributed to residual stress and crystalline defects. Concentration quenching of photoluminescence appears at concentrations higher than 3.6 wt % Nd. The use of (Ca,Sr)F2 for lattice matching to GaAs leads to a significant inhomogeneous broadening of Nd3+ emissions due to disorder in the cationic sublattice.

Strain relaxation in [001]‐ and [111]‐GaAs/CaF2 analyzed by Raman spectroscopy

Study is devoted to a complete characterization of GaAs/CaSrF2/CaF2 heterostructures. Due to the transparency of CaF2 substrate to visible light, Raman spectra have been obtained at both interface and surface sides of the 2 μm GaAs layer. Moreover, penetration depth of light varying with wavelength allows one to perform a tomography of this layer. The crystalline quality at the vicinity of the surface is analyzed through Raman selection rules for both [001] and [111] growth directions. In the latter case, a stress profile has been realized in order to determine its relaxation into the GaAs layer: It occurs in the first 40 nm from the interface. Finally, this methodology is applied to optimize growth conditions in order to obtain stable highly strained systems. By comparison with photoluminescence data, the Raman probe is shown to be very efficient for this purpose.

EFFECT OF GROWTH TEMPERATURE AND DOPING CONCENTRATION ON THE DISTRIBUTION OF THE EMITTING CENTERS IN CAF2:ER MOLECULAR BEAM EPITAXIAL LAYERS

Molecular beam epitaxy of Er‐doped CaF2 layers on (100)oriented CaF2 substrates was performed using CaF2 and ErF3 evaporation cells. The effect of growth temperature and Er concentration on the distribution of the different emission centers observed in these epitaxial Er‐doped layers was investigated. Photoluminescence analyses were performed in the 830–860 nm wavelength range in which the 4S3/2→4I13/2 transitions of the Er3+ ions take place. The evolution of the relative emission intensity between single and aggregate Er3+ centers as a function of growth temperature shows that the emission from isolated Er3+ ions is favored in a growth temperature range of 500–520 °C. Emission lines from complex Er3+ centers are found to rise relative to those of isolated sites as Er concentration increases. Finally, no quenching of the integrated luminescence intensity occurs in the concentration range investigated, 0.05–6 mol % (0.1–16 wt%).

Collective Micro-Optics Technologies for VCSEL Photonic Integration

We describe the main recent technological approaches that associate micro-optical elements to VCSELs in order to control their output beam and to improve their photonic integration. These approaches imply either a hybrid assembly or a direct integration technique. They are compared with regards to their tolerance to alignment errors and to their ease of implementation onto arrays of devices at a wafer level. In particular, we detail the integration techniques we have developed for self-aligned polymer microlens fabrication for beam collimation and short distance beam focusing. Finally, designs to achieve active micro-optics or to exploit novel nanophotonic effects are discussed.

RAMAN SCATTERING STUDY OF HHK-GAAS/(SI OR CAF2) STRAINED HETEROSTRUCTURES

Raman spectroscopy is used to measure the frequency shift, symmetry, and activity of long‐wavelength optical phonons in several GaAs strained epilayers. The results are compared with theoretical evaluations using the elastic compliances, phonon deformation potentials, and Raman tensors. The effect of growth direction ([001], [111], and [112]) and the substrate nature (Si or CaF2) is analyzed. The importance of nonstandard growth directions, [111] or [112], on residual stress and piezoelectric effect is discussed.