NIR Luminescence from Sol-Gel Er³⁺ Doped SiO₂:GeO₂ Transparent Gels, Nanostructured Powders and Thin Films for Photonic Applications

Abstract

Samples of (1 – x)SiO₂:xGeO₂ compositions, containing x = 10 and 20, and doped with Er³⁺, were prepared by a simple sol-gel route. Homogeneous and transparent gels were synthesized and xerogels were obtained by gel annealing from 700 ºC to 1100 ºC. Films exhibiting high transmittance in the visible and near infrared were deposited by spin-coating technique using the colloidal precursors. The materials were characterized structurally and microstructurally by some techniques. Spherical completely amorphous and tetragonal GeO₂ nanocrystals from 3.7 nm to 25.0 nm in diameter embedded in silica-rich amorphous phase materials were obtained. The optical properties were studied by transmission spectra from ultraviolet to near infrared region, photoluminescence measurements in the infrared region, and the average lifetime of the metastable state ⁴I_13/2 of Er³⁺ ions were determined. The 1 mol% Er³⁺ doped host with x = 20 have an infrared emission 4.9 times higher than the compound containing x = 10, and the Er³⁺⁴I_13/2 level present lifetime between 15.0 ms and 7.5 ms. The optical band gap and refractive index values were determined, as well as the Sellmeier parameters. Finally, rib channel waveguides using femtosecond laser etching technique on silica substrates were obtained with width of 15 mm. The 1 mol% Er³⁺ doped 80SiO₂:20GeO₂ compounds can be applicable in integrated optical systems.

Keywords:
Er-doped SiO₂:GeO₂; simple sol-gel route; nanocomposite; optical and structural properties; optical waveguide

Introduction

Growing demand of optical systems and their future potentialities stimulate research for photonic devices development, which are characterized by a network system with excellent flexibility and larger information capacities at much faster rates. Among several devices in telecommunication systems, the invention of optical amplifiers can be compared to that of the transistors in electronics in terms of its technological impact.¹ The technology to directly amplify the light signal without the conversion of light/electricity/light has been achieved with rare earth doped fibers which realize ideal amplification with high gain and low noise.² Optical properties of rare-earth ions incorporated in glass hosts are of great interest in opto-electronic technology.³ Rare earth trivalent ions in some solid compounds emit light at characteristic wavelengths due to intra-4f or internal 4f-5d transitions. In the case of Er³⁺, the emission at 1540 nm corresponds to a dipole forbidden intra 4f transition from ⁴I_13/2 to ⁴I_15/2, coinciding with the low-loss window of standard optical telecommunication silica fiber, allowing its use in optical amplifiers for C telecommunication band.

Recently, remarkable progress has been achieved in the development of single-mode Er-doped optical fiber amplifiers and lasers.^4-6 Development of Er-doped planar waveguide amplifiers has also been investigated to be applied in integrated optical systems, but integrated optical amplifiers should be as small as possible (a few centimeters),^7,8 so that much higher rare-earth concentrations are required than in Er-doped fiber amplifiers (EDFAs). However, due to the onset of concentration quenching at low doping levels in silica hosts, the relatively low gains/unit length which can be achieved has made such development difficult. Consequently, SiO₂ is an unsuitable host in small, compact amplifiers due to the low erbium solubility.

However, there is a great interest to insert other elements into the SiO₂ host, for example GeO₂, resulting in (1 – x)SiO₂:xGeO₂ binary system, which is promising for integrated optics due to low propagation losses in the visible and infrared regions and their photosensitivity that allow the channel waveguides and Bragg mirrors photo-inscription using ultraviolet light.^9,10

Furthermore, there is a motivating aspect for the use of homogeneous glassy or nanostructured thin films as waveguides because there are no grain boundaries, which can cause a high optical loss as in polycrystalline films. Recent results presented on the literature confirm that ceramic laser materials became an attractive alternative to single crystal due to their easy manufacture and low cost.¹¹ In this way, the nanostructured Er³⁺ doped SiO₂:GeO₂ host are interesting materials for integrated amplifiers and laser medium.

The sol-gel process allows the preparation of bulk materials, thin films as well as powders, which can be used to prepare glasses, glass-ceramics and ceramics. The major advantages of this low temperature chemical route are the large variety of materials exhibiting not only a multicomponent host but also can be doped to modify their properties, the excellent control of the chemical purity, low cost of fabrication and possibility to obtain nanostructured powders with a controlled size particle for transparent ceramics manufacture. Since thin sol-gel films can be easily made through dip-coating or spin-coating techniques, this method is considered ideal for the fabrication of active integrated optical devices like optical waveguide lasers and amplifiers.^12-15

The sol-gel process has been used for the preparation of SiO₂:GeO₂ compounds.^16-20 Jing et al.¹⁶ studied thick films of 30SiO₂:70GeO₂ obtained from tetraethylorthogermanate (Ge(OC₂H₅)₄, TEOG) and diethylorthosilicate (DEOS). Jang et al.¹⁸ have used TEOG and tetraethylorthosilicate (Si(OC₂H₅)₄, TEOS) to obtain thin films by spin-coating technique. Ge-doped SiO₂ glasses was synthesized by sol-gel process using TEOS and 3-trichologermanium propanoic acid (Cl₃GeCH₂CH₂COOH) as starting materials.¹⁹ Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane mixed with germanium isopropoxide are used as matrix precursors to prepare Nd³⁺ doped SiO₂:GeO₂ thin films by spin-coating technique.²⁰ Also, glasses of 90SiO₂:10GeO₂ composition, in mol%, have been prepared by the sol-gel method in which TEOS and TEOG were used as starting materials.¹⁷ In all these procedures, the authors have used germanium alkoxides, which are very reactive with water molecules, promoting very fast hydrolysis and condensation reactions, and consequently, an atmosphere control is required for obtaining a transparent, homogeneous and nanostructured material.

Recently, Sigoli et al.²¹ have published results on the Er³⁺ and Yb³⁺ co-doped SiO₂:GeO₂ planar waveguide onto silica on silicon substrates obtained by the sol-gel process combined with a spin-coating technique. The compositions of studied samples were 95.8SiO₂:4.2GeO₂ doped with 0.1-2.0 mol% of Er³⁺ and 0.7-1.4 mol% of Yb³⁺, which is based in a low Ge content host. For the sample preparation, firstly silica nanoparticles were prepared by hydrolysis of tetraethyl orthosilicate (TEOS) in basic media, and the germanium precursor solution was obtained by adding GeO₂ into ethylene glycol with NH₄OH. The SiO₂-GeO₂ precursors mixture were doped with different amounts of Er and Yb chloride salts, followed by addition of 30 wt.% aqueous solution of polyvinyl alcohol (PVA), resulting in a transparent sol. The films were prepared using a multilayer process, followed by a posterior heat treatment at 1150 ºC for 60 s using a rapid thermal annealing (RTA) furnace. As a conclusion, the authors claimed that the rapid thermal annealing under nitrogen flows avoided the crystallization of the films and the rare-earth cluster formation, being effective for material densification, however the effect of the Ge content on the Er³⁺ emission was not evaluated.

In this paper, we present the development of a simple and relatively low cost sol-gel route for obtaining homogeneous Er³⁺ doped (1 – x)SiO₂:xGeO₂ with x = 10 and 20 nanometer-sized powders and transparent thin films, which are cracks and porous free with relatively thick layers. We describe and discuss the thermal, structural, micro-structural, optical transmission properties and their correlation and influence on the near infrared (NIR) emission of Er³⁺ ions from the samples. Besides, the paper reports on the preparation of planar and channel waveguides based on Er³⁺ doped 80SiO₂:20GeO₂. This work contributes to establish a new sol-gel route, without requirement of a specific atmosphere control, which can be used to prepare materials containing germanium.

Experimental

Sample preparation

Crystalline GeO₂ has mainly two crystalline structures: one is hexagonal and the other is tetragonal. Hexagonal GeO₂ is somewhat soluble in water (0.453 g per 100 g of water, 25 ºC), while tetragonal GeO₂ is insoluble.²² On the other hand, GeO₂ is soluble in H₂SO₄/NH₄OH basic solutions at pH around 9-10,²² but insoluble in acid solutions.²³ Based on these information, initially we tested several possibilities to mix TEOS in the germanium oxide (GeO₂) previously dissolved in a solution of pH ca. 10, but a fast precipitation takes place in all tests. Another chemical route was dissolve GeO₂ in an aqueous solution containing ethylenediamine in which could be obtained a transparent and stable sol, but a fast precipitation takes place when TEOS is included. Finally, we could prepare a stable sol with tetramethylammonium hydroxide (TMAH) solution (aqueous solution containing 25% in mass of TMAH). The TMAH solution is very efficient to dissolve GeO₂, and it allowed include TEOS, as will show below.

Sols preparation

The starting solutions was prepared using the tetraethylorthosilicate (TEOS, Si(OC₂H₅)₄, Alfa Aesar, 99% purity), germanium oxide (GeO₂, Sigma-Aldrich, 99.999% purity), erbium chloride (ErCl₃, Reacton, 99.9% purity), tetrametylammonium hydroxide solution (TMAH, (CH₃)NOH, Sigma-Aldrich, solution of 25 wt.% in water), isopropanol (iPrOH, Chemis, P.A.), and D-sorbitol (Sigma-Aldrich, 99% purity).

Firstly, the TEOS was pre-hydrolysed (1Si(OC₂H₅)₄:2H₂O, molar ratio) during 1 h, for x = 10 and x = 20 was used 3.838 mL and 1.706 mL of TEOS, respectively (named Sol1). 200 mg of GeO₂ was diluted in 2 mL of TMAH solution, 2 mL of iPrOH, and 2 mL of deionized H₂O (named Sol2). Considering the addition of rare-earth chlorides (as ErCl₃, YbCl₃ or EuCl₃) in basic medium was necessary include a small amount of D-sorbitol to prevent rare earth hydrolysis and precipitation. The ErCl₃ was dissolved in D-sorbitol and deionized water (1ErCl₃:5 D-sorbitol:500H₂O, molar ratio) named Sol3. A mixture of Sol1, Sol2, and Sol3 were performed under vigorous stirring during 1 h and aged for 24 h. During the aging period occurs the formation of some precipitates. Finally, 1 mL of deionized water was added under vigorous stirring for around 30 min, and we could obtain a transparent and stable sol. All procedure was performed in atmospheric air, and did not require special atmosphere.

Films deposition

The sols were filtered with Millipore 0.2 mm and used for thin films deposition at 25 ºC. Before the film depositions, the substrates were cleaned with a detergent (biodegradable anionic surfactant), rinsed with deionized water and placed inside a HNO₃ + HCl solution (16 HNO₃ + 28 HCl + 56 H₂O, molar ratio) during 5 min. The substrates were then rinsed once again in deionized water, ethyl alcohol and dried with anhydrous air.

Thin films were deposited on sodalime glass substrates using the spin-coating technique. Rotation speed and spin time were fixed at 1500 rpm, and 10 s, respectively. Before further coating, each layer was annealed in air for 2 h at 400 ºC using a heating rate of 2 ºC min^-1. The final films were obtained after 3 coatings and heat-treated at 700 ºC during 2 h using a heating rate of 5 ºC min^-1 in atmospheric air.

Gels and powders preparation

In parallel, the sol obtained after films preparation was maintained in an oven at 45 ºC during 6 days in atmospheric air, to obtain transparent monoliths (transparent dried gels). Next, the samples were heat treated 100 ºC for around 24 h, annealed in an electrical furnace at 400 ºC for 5 h, and 900 ºC for 2 h or 1100 ºC for 2 h using a heating rate of 5 ºC min^-1 in atmospheric air, to obtain nanocomposite powders.

Characterization

The dried gels at 45 ºC during 6 days, were subjected to thermal analysis using an SDTQ600 TA Instruments Model thermoanalyzer which records differential thermal analysis (DTA) and thermogravimetry (TGA) simultaneously. Samples (25 mg) were put into α-alumina crucibles, using calcined α-alumina as a reference material. The heating rate was 10 ºC min^-1 and the thermal analysis was carried out in flowing synthetic air (50 mL min^-1) as the measurements were taken.

The gel and powder samples were mixed with dried KBr and pressed into pellets, and the corresponding Fourier transform infrared (FTIR) spectra were collected in the 4000-400 cm^-1 range using a FTIR Perkin Elmer 2000 spectrometer with 4 cm^-1 resolution.

The crystalline phases were analyzed by X-ray diffraction (XRD) technique. XRD measurements were performed at room temperature in a transmission geometry using a Siemens D5000 equipped with a nickel filter using Cu Kα radiation (λ = 1.5406 Å) and a graphite monochromator. XRD scans were collected from 5 to 60º (2θ) using a scan rate of 0.6 degree min^-1 and an increment of 0.02º.

Images of the powder samples were obtained by high-resolution transmission electron microscopy (HRTEM) using a Philips CM200 microscope.

Optical transmission spectra in the UV-Visible-NIR near regions (200-2000 nm) were recorded at room temperature using a Cary 500 spectrophotometer (600 nm min^-1, resolution of 0.5 nm).

The photoluminescence (PL) spectra at room temperature were registered on a spectrofluorometer (SpectraPro 300i, Roper Scientific), equipped with an InGaAs detector (ID-441C) and a double monochromator with resolution of 1.8 nm (Spex 212L). The excitation source was the 488 nm line of an Ar⁺ laser (500 mW) (Innova 300C, Coherent Inc.).

The lifetime at room temperature was acquired with a InGaAs detector using the 488 nm line of an Ar⁺ laser (500 mW) (Innova 300C, Coherent Inc.), a chopper (SR540, Stanford Research Systems Inc.) operating at 10 Hz, a digital oscilloscope (54600B, Hewlett Packard), and an optical filter RG1000 to eliminate the excitation signal.

Channels were written onto the surface of films deposited on silica substrates using a femtosecond laser etching technique. To this end, an 800 nm Ti-sapphire laser emitting 150 fs, 76 MHz mode-locked pulses was focused on the film surface with an objective lens. The spot size of the focused beam was 5 mm at the focal point, and the average power of the laser was 10 mW. The scanning speed was 0.5 mm s^-1. 3D surface image of the channel waveguide was measured on a FORMTRACER profilometer from Taylor Hobson Precision model SV-C525.

Results and Discussion

Thermal properties

Figure 1 presents the TGA and DTA curves of the 90SiO₂:10GeO₂ (x = 10) and 80SiO₂:10GeO₂ (x = 20) gels doped with 1 mol% of Er³⁺. As can be seen, a mass loss (16.5% for x = 10, and 12.5% for x = 20) was observed between 35 and 160 ºC due to desorption of the adsorbed moisture and the evaporation of solvents and dehydration. The samples showed a mass loss of approximately 33.8% for x = 10 and 45.5% for x = 20 between 160 and 330 ºC due to an elimination of residual solvents (peak at 210 ºC for x = 10 and x = 20) and combustion/oxidation process (exothermic peaks at 264 and 317 ºC for x = 10, and at 244, 264 and 297 ºC for x = 20). From 330 to 500 ºC, exothermic reactions was observed centered at 371 and 423 ºC for x = 10, and at 350, 450 and 480 ºC for x = 20, with mass loss of 2.1% for both samples, due to final oxidation process. Between 500 and 700 ºC, any reactions occurred.

Figure 1
TGA and DTA curves for gels dried at 45 ºC during 6 days of (1 – x)SiO₂:xGeO₂ with x = 10 and 20 compositions. Reference and sample crucibles of alumina, air atmosphere and heating rate of 5 ºC min^-1.

Structural properties

In order to better understand the structural changes from gels (45 ºC and 100 ºC) and amorphous powders (900 ºC) to partially crystallized powders (1100 ºC) for x = 10, FTIR spectroscopy was used. IR spectra are reported in Figure 2 between 4000 and 400 cm^-1. For x = 20 only samples heat-treated at 900 ºC and 1100 ºC were presented and compared with x = 10 composition.

Figure 2
(a) FTIR spectra for gels dried at 45 ºC during 6 days and powders of (1 – x)SiO₂:xGeO₂ with x = 10 and 20 compositions heat treated 100, 900 and 1100 ºC; (b) area of the absorption at ca. 3430 cm^-1 as a function of temperature for x = 10.

The bands at 595, 1404 and 1490 cm^-1 in the spectra of x = 10 at 45 ºC and 100 ºC are due to absorptions of C−C, C−CH₂, N−O and N−CH₃ groups stretching, and CH and CH₂ groups deformation. These organic compounds were completely eliminated after annealing at 700, 900 and 1100 ºC. In the 1400-400 cm^-1 range, the most intense bands refer to the SiO₂ network vibrational modes. Besides that, vibrational modes related to Si−OH, Si−O−Ge, and Ge−O−Ge vibrational modes also appear in this region. The SiO₂ network can be characterized by three main Si−O−Si vibrational modes, which can be observed in detail in Figure 2, at 460, 800 and 1090 cm^-1. The peak at 460 cm^-1 is attributed to transverse-optical (TO) rocking motions perpendicular to the Si−O−Si plane of the oxygen bridging two adjacent Si atoms. The second one located at 800 cm^-1 is due to the TO symmetric stretching of the O atom along a line bisecting the Si−O−Si angle. The broad band at 1090 cm^-1 corresponds to the back and forth motion of the oxygen atom along a line parallel to the Si−Si axis, which results in a TO antisymmetric stretching. The latter band is accompanied by a shoulder at 1250 cm^-1, characteristic of an admixture of TO and LO (longitudinal optical) components, which is more intense in porous materials.^24,25 In the case of the samples annealed at low temperature, the bands around 1030 and 1200 cm^-1 show a shift and broadening in comparison with the samples annealed at higher temperature, corroborating with the presence of organic groups and an incomplete hydrolysis and condensation process, originating a porous material.

Concerning the presence of germanium oxide, in the literature is well reported that the infrared absorption band of GeO₂ show a broad band centered at 900 cm^-1, attributed to Ge−O vibrations in GeO₄ tetrahedral in which every oxygen atom is bonded to another germanium atom.²⁶ In Figure 2, a shoulder at 1000 cm^-1 and 900 cm^-1 appears increasing in intensity with the germanium content, and can be attributed to the Ge−O−Ge and Si−O−Ge vibrational modes. The presence of GeO₂ can be also attested analyzing the spectra region between 500-650 cm^-1. Vibrational modes attributed to the Ge−O links in this region are also reported in the literature, and can be seen in the samples annealed at higher temperatures.

Bands at 1670 cm^-1 and 3440 cm^-1 are also observed due to vibrational modes of the water molecules or hydroxyl groups that aware reduced gradually by increasing the annealing temperature, as previously observed in the TGA results and discussions. For the samples heat treated at 45 and 100 ºC, a vibrational mode attributed to the Si-OH groups (silanols) appears at around 950 cm^-1, which disappear at higher temperatures.^24,25

Similar FTIR spectra of compounds with x = 10 and x = 20 annealed at 900 ºC were observed, suggesting the presence of structure similarity. However, increasing the temperature to 1100 ºC, peaks at 592 and 665 cm^-1 for x = 10 and 20 appears, which can be attributed to GeO₂ particle formation in silica host. On the other hand, the band at 808 cm^-1 for 900 ºC shifts to 792 cm^-1 for 1100 ºC (x = 10) and from 802 cm^-1 for 900 ºC to 792 cm^-1 for 1100 ºC (x = 20), which indicates some contribution of Ge−O to the band around 800 cm^-1.

In Figure 3 are shown the XRD patterns for x = 10 and 20 doped with 1 mol% of Er³⁺ heat treated at 700, 900 and 1100 ºC, where the peaks are indentified and compared with those reported in the literature. A structural change clearly occurs as a function of the heat treatment temperature, which denotes a direct dependence of the crystalline phase contents. A large background between 15º and 35º in 2θ was detected in all diffractograms, which corresponds to the amorphous silica-based host, as also observed by Ferrariet al.²⁷ for SiO₂-Ta₂O₅ compounds. In accordance with JCPDS card number 73-1306, the peaks at 22.5; 31.5; 36.1; 48.7; and 57.1 degree are due to (101); (102); (112) and (200); (212); (213) and (301) planes of GeO₂ nanocrystals with tetragonal structure (space group P4₁2₁2 (92)) and the cell parameters are a = 4.99 Å, and c = 7.06 Å (C = 1.4148 and Z = 4). From the XRD results, the GeO₂ crystals into the silicate host appear only after heat treatment at 1100 ºC.

Figure 3
X-ray diffraction patterns for powders of (1 – x)SiO₂:xGeO₂ with x = 10 and 20 heat treated at 700, 900 and 1100 ºC.

Morphological properties

Figures 4a and 4d show TEM images of the compounds annealed at 900 ºC for 2 h containing x = 10, in which spherical GeO₂ nanoparticles of 3.7 ± 0.1 nm in diameter is dispersed into the silicate host (which were not detected by XRD results). For the compounds with x = 20 annealed at 900 ºC, Figures 4b and 4e, spherical GeO₂ nanoparticles of around 4.6 ± 0.2 nm in diameter can be observed, and for x = 20 heat-treated at 1100 ºC the diameter size increase to 25.0 ± 0.8 nm (Figures 4c and 4f). In fact, the diameter size of GeO₂ nanocrystals is almost the same as the content of the germanium oxide increases, at least for concentration of 10 and 20 mol%. Due to low crystallinity and the small size of the GeO₂ nanoparticles, they were not detected by XRD patterns (Figure 3) after annealing at 900 ºC. Increasing the annealing to 1100 ºC for x = 20, GeO₂ nanoparticles with higher diameter were crystallized and consequently could be detected by XRD patterns (Figure 3).

Figure 4
TEM images and GeO₂ nanoparticles size distribution of powders from (1 – x)SiO₂:xGeO₂ compositions, with: (a) x = 10 and heat-treated at 900 ºC; (b) x = 20 and heat-treated at 900 ºC; (c) x = 20 and heat-treated at 1100 ºC; (d) x = 10 and heat-treated at 900 ºC; (e) x = 20 and heat-treated at 900 ºC; (f) x = 20 and heat-treated at 1100 ºC.

All the samples show GeO₂ nanoparticles with diameter lower than λ/20, considering as λ the wavelength of the Er³⁺ pumping (λ = 980 nm, λ/20 = 49 nm) and emission (λ = 1540 nm; λ/20 = 77 nm) used in telecommunication systems as optical amplifiers. Therefore, the presence of GeO₂ nanoparticles will not reduce significantly the NIR propagation signal, i.e., the contribution to the losses due to volume scattering is not significant considering these compositions in waveguides for infrared light.^28,29 Besides, films heat-treated around 700 ºC could be even better for waveguide application, since at these temperatures any GeO₂ nanoparticles are distributed in the silicate host.

To evaluate the dispersion behavior of Er³⁺ ions in x = 20 compounds heat-treated at 900 ºC and 1100 ºC, were used the energy dispersive X-ray (EDX) to collect spectra on the silica-rich matrix and on the GeO₂ nanoparticles, as can see in Figure 5. For 900 ºC, note that a higher erbium peak intensity in GeO₂ nanoparticles in comparison to that in silicate matrix region. However, for 1100 ºC in which nanoparticles have diameter of around 25.0 ± 0.8 nm, a much higher concentration of Er³⁺ ions around/into the GeO₂ nanoparticles than in silica-rich matrix is observed, indicating that Er³⁺ solubility in GeO₂ nanoparticles are higher than in silicate host, especially at higher temperatures.

Figure 5
EDX spectra of x = 20 doped with 1 mol% of Er³⁺ heat-treated at 900 ºC and 1100 ºC. The thin lines are EDX spectra in a light gray region (matrix) and the thick lines are EDX spectra in a dark gray region (nanoparticles = black spheres) measured by TEM (Figures 4b and 4c, respectively).

The chemical procedure developed here allows obtaining stable and uniform −Si−O−Ge−O−Si− network in sols, gels and films as well as powders synthesized at relative low temperatures. With heat-treatment phase separation was observed. It is well known that different ions segregate in silica network, as shown for SiO₂-ZrO₂,^28,30 SiO₂-TiO₂,³¹ and SiO₂-Ta₂O₅^27,29 leading to nanocrystals. The same effect occurs for SiO₂-GeO₂ system studied here. Increasing the energy (heat-treatment temperature) and the Ge content into the matrix, more ions quantity are expelled from SiO₂ network progressively and due to this the GeO₂ particles into silica matrix increase in size significantly only at 1100 ºC. On the other hand, the literature have mentioned that rare-earth solubility in silica is < 10¹⁸ ions cm^-3,³² and we have shown in this work that larger amounts of Er³⁺ are located around/into GeO₂ particles by EDX results, confirming the Er³⁺ migration in samples heat treated at high temperatures.

Optical properties

Gels

An illustrative image of the obtained gel is included in the Figure 6 took by a digital camera. Also, in Figure 6 we present a UV-Vis-NIR spectrum of the 90SiO₂:10GeO₂ gel doped with 1 mol% of Er³⁺. Similar results were obtained for different concentrations of Er³⁺ and for x = 20 (not shown here). The gels were well transparent with high mechanical resistance allowing be handled. The f-f transitions of the Er³⁺ ions were detected at 269, 322, 366, 378, 406, 452, 489, 521, 547, 653, 886, 981 and 1490 nm from ⁴I_15/2 level to ⁴G_9/2,²P_3/2, ²G_9/2, ⁴G_11/2, ²H_9/2, ⁴F_3/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 level, respectively. The bands centered at 1161, 1226, 1339, 1350, 1363, 1600, 1664, 1703, 1720, 1745, 1782, 1812, 1857, and 1937 nm are due to organic compounds (CH₃, CH₂, C−H, C−C, C=C, C−O, C=O, C−N, C=N, O−H, N−H, N-O, and/or N=O, etc.) into the gel from precursor compounds.

Figure 6
Transmission spectrum in the UV-Vis-NIR regions for gels with x = 10 doped with 1 mol% of Er³⁺ and dried at 45 ºC during 6 days.

Powders

Photoluminescence emission spectra in the near infrared region under excitation at 488 nm from the different powders changing composition and doping levels were obtained. In Figure 7a is illustrated the PL emission due to ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions in the 90SiO₂:10GeO₂ powders heat-treated at 900 ºC for different Er³⁺ concentrations, in the inset were included the integrated band intensity of these spectra as a function of Er³⁺ concentration. Note that the PL emission increase from 0.05 to 0.5 mol% Er³⁺, and higher concentrations the PL emission decrease gradually, showing that for x = 10 and annealing at 900 ºC the best rare-earth doping is around 0.5 mol%, being for this matrix the solubility limit. Increasing the heat treatment to 1100 ºC (Figure 7b), the PL emission maximum is observed for 0.8 mol% (almost the same intensity of 0.5 mol% sample) but the emission intensities are lower than that observed for 900 ºC.

Figure 7
PL emission spectra under 488 nm excitation for powders doped with different erbium concentrations of: (a) x = 10 composition, and heattreated at 900 ºC; (b) x = 10 composition, and heat-treated at 1100 ºC; (c) x = 10 and 20, doped with 1 mol% of Er³⁺, and heat-treated at 1100 ºC.

This is an indication that the Er³⁺ ions have higher solubility around/into GeO₂ in comparison with silicate host. This is supported by EDX results (Figure 5) that show high concentrations of Er³⁺ ions around/into GeO₂ nanoparticles at 1100 ºC. However, increasing the temperature, it can favor Er−O−Er clusters formation, which increases energy migration. Besides, the crystallization process, in which Ge−O−Ge, Si−O−Ge and Si−O−Si links breakdown, leads to the formation of defects and non-binding oxygens that favor Er³⁺ ions non-radiative de-excitation and reduce its infrared emission. Probably the energy migration (due to Er−O−Er cluster formation) combined with presence of defects are the main contributions to the observed decrease in intensity for the samples containing higher than 0.8 or 0.5 mol% Er³⁺.

Note that, the annealing temperature is fundamental to activate Er³⁺ ions in the 90SiO₂:10GeO₂ host, in fact the 900 ºC is the most appropriate temperature, when compared to 1100 ºC, being limited to 0.5 mol% of Er³⁺ and presenting 30 nm of FWHM (full width at half medium). This concentration limit value is higher than of Er³⁺ doped silica, aluminates and phosphates materials.^33-35 The SiO₂:GeO₂ compositions have potential use as optical amplifiers in integrated systems. In addition, their refractive index are slightly higher than that of silica allowing low losses in silica optical fibers coupling, and the presence of germanium enable diffraction gratings recording using UV lasers by photo-contraction phenomenon.^9,10

At 1100 ºC the FWHM of the emission centered at 1535 nm is around 13 nm, which are smaller than those observed for 900 ºC, due to the presence of Er³⁺ ions around/into GeO₂ nanoparticles with higher size and crystallinity (Figures 3 and 4).

Besides, when the GeO₂ concentration is increased to x = 20 (80SiO₂:20GeO₂) and doped with 1 mol% is observed an emission 4.9 times superior than for x = 10, as presented in Figure 7c. Also, the NIR emission spectra were collected for both samples x = 10 and 20 containing 1 mol% of Er³⁺ and heat-treated at 1100 ºC under excitation at 980 nm light from a diode laser (spectra were not shown here), in which was observed the same emission intensity relation among both samples and same spectral shape in comparison to those spectra collected under 488 nm excitation. In previous EDX results, it was observed a higher erbium concentration around/into GeO₂ nanoparticles, leading us to think that GeO₂ crystalline nanoparticles possess higher erbium solubility in comparison to silicate host, preventing clustering and formation of Er−O−Er links (at least for the samples containing Er³⁺ concentration lower than 0.5 mol%).

Since the luminescence intensity for the both samples (x = 10 and x = 20) annealed at 900 ºC doped with 1 mol% of Er³⁺ are similar in NIR region, it was presented in Figure 7 only emission spectra concerning the x = 10 samples. However, when the samples are annealed at 1100 ºC the x = 20 sample have higher emission intensity in comparison with that containing x = 10. This can be explained by the hydroxyl groups reduction and relevant portion of Er³⁺ ions migration to nanocrystals (as shown by EDX results). Moreover, GeO₂ compounds have lower phonon energy (740 cm^-1)³⁶ than SiO₂ compounds (1075 cm^-1)²⁷ favoring an improvement of the Er³⁺ NIR emission. The EDX technique has no high precision to determine the exact Er³⁺ position, i.e., it is not known exactly if they are in higher quantity into or around GeO₂ particles. Also, in samples with x = 20 heat-treated at high temperatures (1100 ºC) occurs the formation of higher crystalline GeO₂ particles size than for x = 10, which gives a different light scattering (in excitation and emission), but a higher Er³⁺ solubility associated to low phonon energy can compensate some intensity decrease due to scattering.

Likewise, to study the heat treatment effect and GeO₂ content, the PL decay curves were measured at 1535 nm from ⁴I_3/2 level of Er³⁺, the medium lifetime values were listed in Table 1. In x = 10 with 1 mol% of Er³⁺ occurs the lifetime changes from 15.0 ± 0.5 ms for 900 ºC to 7.5 ± 0.5 ms 1100 ºC, but when GeO₂ is increased (x = 20) the lifetime is about 12.8 ± 0.5 ms for 1100 ºC, corroborating with our previous results and discussions, i.e., low GeO₂ concentration associate to high temperature promote some Er−O−Er clustering or defects generation that reduce active Er³⁺ ions, and high temperatures with higher GeO₂ concentration increase active Er³⁺ ions. In fact, the Er³⁺ infrared PL emissions, lifetime values and structural properties of the host are strongly correlated.

Table 1
Lifetime value for ⁴I_13/2 level of Er³⁺ ions in the compositions with x = 10 and 20, and heat-treated at 900 or 1100 ºC

Sample	Er3+ doping / mol%	Heat treatment / oC	Er3+4I13/2 lifetime / ms
x = 10	1.0	900	15.0 ± 0.5
x = 10	1.0	1100	7.5 ± 0.5
x = 20	1.0	1100	12.8 ± 0.5

The lifetime value for ⁴I_13/2 level of Er³⁺ ions in the x = 20 composition heat-treated at 1100 ºC is comparable to those values measured by Sigoli et al.²¹ for 95.8SiO₂:4.2GeO₂ host, which changed between 13.2 and 11.3 ms depending on the Er³⁺ and Yb³⁺ contents.

Thin films

Optical transmission spectra of the 80SiO₂:20GeO₂ films doped with 1 mol% of Er³⁺ were measured continuously from 200 nm to 2000 nm with an UV-Vis-NIR spectrophotometer. Figure 8 a displays the transmission spectra of film with 3 layers heat-treated at 700 ºC. A high transparency in the visible and infrared regions was detected, higher than 92% at 632.8 nm (the He-Ne laser wavelength used as reference). Other optical property, such as the band gap, was obtained indirectly. Considering the high absorption region, the transmittance (T) followed a simple correlation with absorption coefficient, ³⁷ equation 1, where C is approximately equal to the unity at the absorption edge and d is the thickness of the thin films. The relation between the absorption coefficient α and incident photon energy hν for allowed direct transition can be written as equation 2,³⁷ where A is constant and E_g is the direct band gap.

Figure 8
(a) Transmission spectra in the UV-Vis-NIR region for film with x = 20 doped with 1 mol% of Er³⁺, deposited by spin-coating onto silica substrate (3 layers), and heat-treated at 700 ºC; (b) refractive index values dispersion and Sellmeier curve for x = 20 and values for silica substrate.

The (ahν)²versus hν plot for the thin film is shown in Figure 8a. A linear behavior can be observed in a certain range of the curves, supporting the interpretation of direct E_g band gap for thin film.³⁷ Therefore, the E_g band gap can be obtained by extrapolating relation (2) around 5.79 eV (214 nm). This value is comparable with those reported by Jing et al.¹⁶ of 5.5 eV (225 nm) for 30SiO₂:70GeO₂ thick films. On the other hand, works on glasses with 90SiO₂:10GeO₂ composition developed by Nishii et al.³⁸ and Shigemura et al.¹⁷ obtained an optical band gap of ca. 7.1 eV (175 nm), and ca. 6.0 eV (207 nm), respectively. All these values is influenced by the GeO₂ nanosized into the matrix and an absorption band at ca. 5 eV (248 nm) assigned to different kinds of intrinsic germanium-related oxygen-deficient centers in glasses, which consist of neutral oxygen monovacancy [(NOMV)≡Ge-Ge≡ or ≡Ge-Si≡] and neutral oxygen divacancy [(NODV)-Ge²⁺].^16,17,39 In our optical transmission spectra (Figure 8a) was not observed the band associated to these defects, indicating that we have films with high optical quality without structural defects.

The refractive index (n) and film thickness could be determined from the interference fringes by the envelope method, using the expression reported by Manifacier et al.⁴⁰ and modified by Peng and Desu,⁴¹ equation 3, where N is defined with equation 4. T_max and T_min are the transmission maximum and the corresponding minimum at a wavelength λ of the interference fringes (Figure 8a), while n_s is the substrate refractive index silica at the same wavelength.⁴² The refractive index values, n, calculated from equations 3 and 4, are shown in Figure 8b for 80SiO₂:20GeO₂ films annealed at 700 ºC. The refractive index results have been fitted with the following Sellmeier curve,⁴³ equation 5.

The fit of the experimental results with equation 5 provides the following set of parameters: A = 2.2315; B = 0.0040; C = 0.0030; and D = 0.0005 (λ in nm). This Sellmeier curve is shown in Figure 8b. The calculated refractive index presents a good dispersion and it can be well fitted by the Sellmeier relation. The extrapolated values of the Sellmeier curves can be used to calculate the admittance angle and coupling angle when 80SiO₂:20GeO₂ amorphous is used in waveguiding experiments.

Another application of transmission spectra is the film thickness estimation. The thickness d of the layer can be calculated from two adjacent maxima or minima of interference fringes at λ₁ and λ₂ by equation 6,⁴¹ where λ₁, n(λ₁) and λ₂, n(λ₂) are the corresponding wavelengths and refractive indexes calculated above by the envelope method. The average value of d determined by the equation 6 is 1214 ± 35 nm.

The refractive index of the bulk glass material is 1.489 at 589.3 nm wavelength.⁴⁴ In this work we obtained a refractive index of 1.498, which is similar than that of 80SiO₂:20GeO₂ glass, indicating a high density of our films annealed at 700 ºC. This film heat-treated at 700 ºC is very important for waveguide design; in the present case, one has a high Δn = 0.040 at 589.3 nm, Δn = 0.040 at 632.9 nm, Δn = 0.050 at 1535 nm wavelengths between the waveguiding layer and the SiO₂ buffer layer (or cladding), such that a guiding layer thickness of 787 nm is already sufficient for single mode propagation (TE₀ or TM₀ mode) at 1535 nm. To excite the second mode (TE1 or TM1), it is necessary to have a film with 2790 nm thickness (these thickness limit values were calculated by relation in the reference 25).

Sigoli et al.²¹ have obtained a refractive index of ca. 1.453 at 1550 nm for the film thermally treated at 1150 ºC with an average thickness of ca. 3.9 mm. The refractive index was lower than that obtained in our work (ca. 1.49) because we used larger amounts of GeO₂ that allow prepare thinner films to have only one guided mode, i.e., thickness of ca. 1.2 mm is enough to guide the first mode (TE₀ or TM₀ mode) in the films of 80SiO₂:20GeO₂ matrix. Probably, films of x = 20 composition having thickness around ca. 2500 nm (6 layers) should be adequate to be applied as waveguides in the NIR region.

An 800 nm Ti-sapphire laser emitting 150 fs, 76 MHz mode-locked pulses was focused on the film surface by an objective lens. The spot size of the focused beam was 5 mm at the focal point, and the average power of the laser was 10 mW. The scanning speed was 0.5 mm s^-1. Optical surface image using a microscopy by the reflected visible light and 3D surface profilometry of the channel waveguide measured using a FORMTRACER profilometer from Taylor Hobson Precision model SV-C525 are shown in Figures 9a and 9b. The images illustrated in Figures 9a and 9b show a surface profile and air grooves created by the femtosecond laser etching technique and between two air grooves there is the waveguide consisting of the Er-doped 80SiO₂:20GeO₂ ribs on silica substrate. The air grooves with a width around 10 mm and a depth of 5 mm, were created by the laser irradiation and the shape and structure of channels are highly reproducible and sharply defined (Figures 9a and 9b). The width and depth of the channels can be tailored changing the position of the spot of the focused beam in order to produce mono or multimodal channel waveguides. In our preliminary experiment we obtained ribs with width of 15 mm.

Figure 9
(a) Optical surface image; (b) 3D image of the 15 mm width rib channel waveguide for a film with 1214 nm thickness obtained by femtosecond laser; (c) calculated square electric and magnetic field profiles of the TE₀ and TM₀ modes at 632.8 and 1535 nm along z axis (depth) for the waveguides based on the Er-doped 80SiO₂:20SiO₂.

On the basis of the film thickness and refractive index values, the confinement coefficient of the film was estimated. Figure 9c shows the squared electric and magnetic field profiles of the TE₀ and TM₀ modes of the waveguides containing with x = 20 at 632.8 and 1535 nm wavelengths. The values represent the ratio of the integrated intensity, i.e., the square of the electric or magnetic field in the waveguide to the total intensity for each wavelength. The total intensity also includes the squared evanescent fields.³⁰ A light confinement above 82% can be seen for the waveguides excited at 632.8 nm. For the 1535 nm wavelength, the TE₀ and TM₀ confinement are ca. 50% and ca. 46%. Higher confinement coefficients for TE₀ and TM₀ modes are expected by increasing the film thickness and it is possible to obtain a well confined single mode at 1535 nm, which is the wavelength of interest in integrated optical systems as amplifiers.

The differences between the refractive index of the rib channel waveguide and silica optical fibers are crucial to its application as waveguide amplifiers in telecommunication systems, as well as the rib channel waveguide thickness. The losses by coupling with silica optical fibers increases in compounds with high refractive index and low thickness. In fact, it is necessary to tailor the refractive index and thickness of the rib channel in relation to the commercial optical fibers. But, the refractive index of the rib channel cannot be low, because they are deposited onto silica (same material of the optical fibers), if not the pump and emitted light did not maintain confined. On the other hand, the losses in this sol-gel based planar waveguides are not only due to these parameters. During sol synthesis and film deposition undesirable impurities can be incorporated and/or not be eliminated by heat-treatments, resulting in waveguides with losses of about 1.0 dB cm^-1.²⁸ Although full densification is always achieved together with initial crystallization process, crystalline size need be controlled.²⁵ Surface roughness should be considered as another source of losses, to reduce these losses the homogeneous and crack-free films should be prepared. Finally, the best composition reported in our work (80SiO₂:20GeO₂) is close in composition to other systems in the literature, for example, those containing ZrO₂,^28,30 TiO₂,³¹ and T₂O₅,^27,29 however present lower refractive index.

Conclusions

In summary, we have used a simple sol-gel route to prepare stable and transparent sols, gels and films. The gels decompositions to form powders were evaluated by TGA and DTA techniques, and endothermal and exothermal events were analyzed. The materials show mass loss from room temperature to around 500 ºC. Between 500 and 700 ºC, no reactions were observed. The organic and hydroxyl compounds into the matrix were evaluated by FTIR that reduce gradually by increasing the annealing temperature. At 700 ºC, the materials are completely amorphous, but at higher temperatures a silica-rich amorphous phase and GeO₂ nanocrystals with tetragonal structure were obtained. The HRTEM images reveal spherical GeO₂ nanoparticles of around 3.7, 4.6 and 25.0 nm in diameter for x = 10 at 900 ºC, x = 10 and x = 20 at 1100 ºC, respectively. EDX results show high concentrations of Er³⁺ ions around GeO₂ nanoparticles at 1100 ºC. In fact, the GeO₂ nanoparticles formation can allow incorporate higher concentration of active Er³⁺ ions. For x = 10, higher infrared emissions occur in the matrix doped with 0.5 mol% of Er³⁺, at 900 ºC and 1100 ºC. But the 1 mol% of Er³⁺ doped 80SiO₂:20GeO₂ compound have an infrared emission around 4.9 times higher than that for 1 mol% Er³⁺ doped 90SiO₂:10GeO₂. The Er³⁺⁴I_13/2 level has a lifetime between 15.0 ms and 7.5 ms depending on the GeO₂ content and heat-treatment. The optical band gap of 5.79 eV and refractive index values were determined, being ca. 1.498 at wavelength of 589.3 nm, and the Sellmeier parameters were estimated. Films deposited by spin-coating technique present ca. 1214 nm thickness, as calculated by the envelope method. Rib channel waveguides were obtained with width of 15 mm using femtosecond laser radiation, and a light confinement above 82% can be achieved for these waveguides when excited at 632.8 nm.

The chemical procedure using as a precursor TEOS, GeO₂, TMAH is easy, doesn't require special atmosphere, and has relatively low cost. As a final conclusion, the 1mol% of Er³⁺ doped 80SiO₂:20GeO₂ compounds are potentially applicable in integrated optical systems, specially as miniaturized optical amplifiers.

• FAPESP has sponsored the publication of this article.
• Supplementary Information
  Supplementary data (PL spectra) are available free of charge at http://jbcs.sbq.org.br as PDF file.

Acknowledgements

The authors are grateful for the financial support provided by CAPES, CNPq, FAPEG, FAPESP and FUNAPE/UFG Brazilian agencies.

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Data availability

https://minio.scielo.br/documentstore/1678-4790/WHCNmVwjhRvsDRVfd6XSrpj/ba2c7423fd4142560f99875d9075735a61437bc7.pdf

Publication Dates

• Publication in this collection
  Nov 2015

History

• Received
  15 June 2015
• Accepted
  01 Sept 2015