Since the launching of the photonic bandgap concept in 1987, the development of the corresponding structures has expanded very rapidly, in particular for two-dimensional semiconductor-based structures. In the case of sol-gel derived materials, the main emphasis has been on one-dimensional mutilayer stacks and, in particular, on three-dimensional structures of the opal and inverse opal type. In this work, one-dimensional multilayer stacks of periodically alternating low refractive index (SiO2) and high index (TiO2) materials have been deposited by spin-coating onto silica or single crystal Si substrates, in the form of dielectric mirrors (distributed Bragg reflectors) and Fabry-Perot microcavities, both single and coupled (double). Some of the microcavities were doped with rare-earth elements (Er and Yb). These structures have been characterized by X-ray diffraction, infrared spectroscopy and field emission-scanning electron microscopy; their optical properties have also been measured, namely the stop band of high reflectivity and, in the case of the microcavities, the cavity modes and the photoluminescence behavior of the rare-earth ions inserted in the cavity layers. The presence of Er in both cavity layers of coupled microcavities was found to lead to a substantial increase in photoluminescence signal intensity and width, compared to the cases where Er was present only in one of the cavities. The possibility of an antenna effect between Yb3+ and Er3+ ions is also examined.
SiO2-TiO2:Er3+-Yb3+ waveguides were prepared by rf-sputtering technique. The active films were deposited on silica-on-silicon and v-SiO2 substrates. The parameters of preparation were chosen in order to optimize the waveguides for operation in the NIR region with particular attention to the minimization of losses. The thickness of the waveguides and the refractive index at 632.8 and 543.5 nm were measured by an m-line apparatus. The losses, for the TE0 mode, were evaluated at 632.8 and 1300 nm. Roughness measurements were carried out by means of a stylus profilometer. The structural properties were investigated with several techniques such as Energy Dispersive Spectroscopy and Raman Spectroscopy. All waveguides were single-mode at 1550 nm. An attenuation coefficient equal or lower than 0.2 dB/cm was measured both at 632.8 nm and 1300 nm. The emission 4I13/2 →4I15/2 of Er3+ ion transition with a 40 nm bandwidth was observed upon excitation in the TE0 mode at 981 and 514.5 nm. Back energy transfer from Er3+ to Yb3+ was demonstrated by measurement of Yb3+ emission upon Er3+ excitation at 514.5 nm. Photoluminescence excitation spectroscopy was used to obtain information about the effective excitation efficiency of Er3+ ions by co-doping with Yb3+ ions. Channel waveguides in rib configuration were obtained by etching the active film by a wet etching process. Scanning Electron Microscopy was used to analyze the morphology of the waveguides.
One-dimensional (1-D) photonic bandgap (PBG) structures remain one of the most practical ways of applying the PBG concepts to the solution of many urgent problems in laser physics and optical technologies. The sol-gel method is an inexpensive and flexible liquid phase processing technique that is suitable for the deposition of multilayer stacks. The multilayer stacks can be designed as 1-D PBG structures, such as distributed Bragg reflectors (DBR), or single and coupled microcavities, as reported in this work. Spin-coated TiO2 and SiO2 layers acted as the high and low refractive index materials, respectively. Each layer was heat-treated at a high temperature (~1000 degree(s)C) for a short period of time (~90 s) in order to increase the index contrast, while preserving relatively smooth interfaces between the consecutive layers. Ellipsometry, X-ray diffraction, micro-Raman spectroscopy, transmittance/reflectance spectroscopy, and atomic force microscopy were used to characterize both the individual layers and the whole structures. Strong PBG properties are demonstrated, with an omni-directional stop band for a 5.5-pair DBR ((lambda) equals 550 - 600 nm, gap to mid-gap ratio equals 7.6%) and sharp pass bands within the stop bands for the microcavities.
Sol-gel processing is a very good method for the preparation of silica and other oxide glass planar optical waveguides, including those doped with rare-earth ions like Nd3+ or Er3+, for applications such as integrated optics lasers and amplifiers. In active integrated optics devices for 1.54 micrometers , involving Er3+ doping, a low vibrational energy glass matrix is desirable, in order to increase the quantum efficiency of the Er3+ emission; however, this is even more important in the case of Pr3+. Here, therefore, the use of a low vibrational energy sulfide glass becomes particularly adequate, although the sol-gel processing of non-oxide glasses is still in its infancy.
Sol-gel SiO2 - TiO2 multilayers (containing 20 mol% TiO2) have been deposited by spin-coating onto single crystal Si substrates previously covered with a SiO2 buffer layer (approximately 4 micrometers), also prepared by sol-gel. The silica-titania films were first densified at 900 degrees Celsius and were then subjected to selected crystallization heat treatments at 1000 degrees Celsius, in order to precipitate different volume fractions of anatase (TiO2) crystallites, between 2.5 and 15%. The optical loss of these nanocomposites was measured at different wavelengths, using argon ion and He-Ne laser light. The experimental loss values, after removing the intrinsic Rayleigh term and surface scattering, were compared to scattering losses calculated by means of the Rayleigh-Mie theory, for light scattering by spherical particles, which was used to examine the influence of different parameters: radiation wavelength, nanocrystallite size and volume fraction of nanocrystals. The theoretical calculations show that, for the wavelengths of interest ((lambda) on the order of or greater than 1 magnitude), nanocrystallite scattering losses remain below 0.5 dB/cm, even for volume fractions as high as 15%, as long as their diameter is below 11 nm. The experimental results agree reasonably well with the theoretical predictions, considering the approximations made. The extension of the model to the study of residual film porosity led to the conclusion that typical porosity present has a negligible influence on the total waveguide loss.
There is a strong need for the development of cheap component technologies for optical functions such as switching, demultiplexing and amplification. Silica-on-silicon integrated optics using sol-gel processing is probably the best technology for such low cost applications. This review focuses on the sol-gel based thin film fabrication technologies for integrated optics (IO) lasers and amplifiers, using Nd3+ and Er3+ as the active species. Special emphasis is given to the work performed under the European Union sponsored projects NODES (ESPRIT) and CAPITAL (ACTS), in particular to the processing and characterization of Nd3+ and Er3+-doped silica-titania planar waveguides for IO lasers and amplifiers.
Infrared absorption spectra were recorded for silica and silica-titania sol-gel films spin-coated on silicon substrates. Information of two types is derived from those spectra concerning the porosity of the films, which was independently calculated from ellipsometry results: (1) the IR spectra allow the determination of the `infrared thickness' of the porous films, i.e., the thickness that they will exhibit after full densification (independent of the film porosity), which is always less than the thickness obtained by mechanical profilometry (or ellipsometry); also, the volume percent porosity can be estimated from those two thickness values; (2) as the films are densified at increasing temperatures, the IR thickness remains constant, but the profilometer (and ellipsometer) thickness decreases, whereas the frequency of the dominant Si- O stretching mode near 1070 cm-1 passes through a minimum, at approximately equals 500 degree(s)C for silica films and at approximately equals 450 degree(s)C for silica-titania (20 mol% TiO2), suggesting a maximum porosity for those heat treatment temperatures; this was confirmed by a minimum in the refractive index, in the case of pure silica films only. The significance of these findings is discussed.
For applications such as integrated optical devices, it is desirable to fabricate thick optical films, e.g. by the sol-gel process. while avoiding some of the problems associated with multilayer deposits. In the present work, alkoxide-derived 5i02 films were deposited by spin-coating onto single crystal silicon wafers, while varying independently several experimental parameters, namely the water/tetraethoxysilane (TEOS) molar ratio R, the volume percentage of TEOS, the aging period of the solutions and the rotational speed of the substrate. It was concluded that the thickness of the as-deposited porous silica gel films increased with decreasing R, as well as with increasing TEOS volume percentage or aging period of the solution (up to a certain point), whereas it showed a maximum when plotted as a funtion of the substrate rotational speed. Maximum thicknesses in excess of 370 urn were obtained for porous films, for which densification did not cause cra.cking,while porous undensified films up to 1.2 pm thick were also prepared. The conditions under which maxiimiin film thickness is achieved are discussed.
A series of silica gel films were spin-coated on single crystal silicon
(c-Si) substrates and their structure was characterized by vibrational spectroscopy.
The films were either dried at room temperature or partially densified
at 450 0C. Fourier transform infrared absorption spectra have been obtained
for each film and they are compared to the spectrum of thermal SiO2
films. The gel films (ca. 150 nm thick) show the presence of residual OH
groups, but very little molecular water or organic species and the fundamental
Si-O-Si vibrations exhibit shifts toward lower frequencies, compared to the
thermal oxide. The Si-O-Si antisymmetric stretch near 1070 cm was narrower
for the gels and the shoulder on the high frequency side was stronger. The nature
of this feature is discussed based also on oblique incidence transmission
and reflection-absorption spectra taken with polarized infrared light.
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