Optical Gain in Eu-Doped Hybrid Nanocrystals Embedded SiO₂-HfO₂-ZnO Ternary Glass-Ceramic Waveguides

Abstract

Rare-earth doped transparent glass-ceramic waveguides are playing a very crucial role in integrated optics. We fabricated ZnO-HfO₂ hybrid nanocrystals embedded with 70 SiO₂–(30-x) HfO₂–x ZnO (x = 0, 2, 5 and 7 mol %) ternary transparent glass-ceramic waveguides doped with 1 mol % Eu-ions. The formation and size of the nanocrystals evolved with an increase in ZnO concentration in the glass-ceramic waveguides. In this context, key factors of such nanocrystals embedded active glass-ceramic waveguides were optical losses and transparency. A lab-built m-line experimental set-up was used for the characterization of the waveguides. On the other hand, optical gain measurements of the Eu-doped hybrid nanocrystals embedded glass-ceramic waveguides were performed using the variable stripe length method. The optical amplification of the waveguides was investigated on the red emission line (⁵D₀ → ⁷F₂) of Eu-ions pumped by a 532 nm laser in a stripe-like geometry generated by a cylindrical lens. Here, we report, the optical gain in rare-earth activated glass-ceramic waveguides with nanocrystals of varying sizes formed in the waveguides with increasing ZnO concentration.

1. Introduction

“Glass-ceramics” are composite materials where nanoparticles are embedded within an amorphous glass matrix, providing the benefits of a crystalline structure within the glass environment. These materials retain the shape of the original glass but exhibit significantly improved thermal and mechanical properties [1,2]. The unique combination of properties found in glass ceramics, which share characteristics of both single crystals and glasses, makes them highly suitable as host materials for rare-earth (RE) ions, unlike traditional glasses. The crystalline environment within glass ceramics allows for the incorporation of higher concentrations of RE-ions, minimizing the risk of chemical clustering, which can lead to luminescence quenching. In this regard, glass-ceramic waveguides are particularly valuable, as they combine the mechanical and optical advantages of glasses with crystal-like environments for RE-ions.

RE-doped glass-ceramic waveguides have emerged as advanced materials for developing various optoelectronic devices, including integrated optical amplifiers, lasers, etc. [3,4] because of their narrow and intense emissions from the 4f-intrashell transitions [5]. The formation of nanoparticles in glasses and their potential photonic applications have been studied in great detail in the literature [6,7,8,9,10]. However, scattering losses in these waveguides are a key concern, and numerous composite systems have been explored over recent decades to fabricate low-loss glass-ceramic waveguides [11,12].

Among various functional materials, ZnO is very promising as an active optical material for the fabrication of photonic devices [13,14], due to its direct band-gap with a band-gap energy of ~3.3 eV, large exciton binding energy of ~60 meV at room temperature, and also having high optical transparency in both the visible and near-infrared regions of the electromagnetic spectrum.

RE-doped glass-ceramic materials are commonly regarded as stable gain media due to their excellent photoluminescence efficiency and thermal stability. In the past, Er-doped glasses have been demonstrated as potential candidates for the development of optical amplifiers in the near-infrared [15,16]. While there have been few studies exploring optical amplification in RE-doped glasses and glass-ceramics for the visible spectrum [17,18]. In this context, Eu-doped glass-ceramic waveguides with high transparency present a promising system to investigate optical gain in the visible spectral range. In this work, we report the formation and growth of SiO₂–HfO₂ hybrid nanocrystals and their influence on amplified spontaneous emission in Eu-doped SiO₂–HfO₂–ZnO ternary waveguides.

2. Materials and Methods

Planar ternary waveguides, doped with 1 mol% Eu and composed of 70 mol% SiO₂, (30-x) mol% HfO₂ and x mol% ZnO (where x = 0, 2, 5, and 7 mol%), were fabricated by the sol-gel method combined with a dip-coating technique on silicon and vitreous SiO₂ (v-SiO₂) substrates. The preparation began with the formation of a SiO₂ sol, synthesized by mixing ethanol (EtOH), tetraethylorthosilicate (TEOS), de-ionized H₂O, and HCl as a catalyst, in the molar ratio of TEOS:EtOH:HCl:H₂O = 10:1:0.03:4. This mixture was pre-hydrolyzed at 65 °C for 1 h.

The ZnO and HfO₂ sols were prepared by dissolving zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) and hafnium dichloride oxide octahydrate (HfOCl₂·8H₂O) in ethanol. Europium nitrate pentahydrate (Eu(NO₃)₃·5H₂O) was used as the dopant. Both the ZnO and HfO₂ sols were then mixed with the TEOS-containing solution, and the final solution was stirred continuously at 40 °C for 16 h to ensure homogeneity [19].

A 0.2 μm Whatman Puradisc syringe filter was used to filter the sol and then it was deposited onto v-SiO₂ and Si substrates using a dip-coating technique. After the deposition of each layer, the samples were annealed in a quartz-tube furnace at 900 °C for 1 min in air before proceeding with the next layer. After the deposition of 25 layers, the desired thickness of the waveguide was achieved, which is influenced by the dipping speed, number of deposited layers, and heat treatment process. Figure 1a,b presents the schematic diagrams describing the synthesis of the sol and the fabrication of thin film waveguides using the dip-coating method, respectively.

3. Characterizations

Transmission electron microscopy (TEM) was used to analyze the size, distribution, and fringe pattern of the nanocrystal lattices. For TEM sample preparation, the thin film deposited on a silicon substrate was polished and dimpled, followed by milling using Ar⁺ ions. Bright-field TEM imaging was then performed on these samples by mounting them on a copper grid. The imaging was conducted using a Philips CM200FEG/ST microscope operating at an acceleration voltage of 200 kV.

A custom-built prism-coupling setup was used to measure the thickness (d) and refractive index (n) of glass-ceramic planar waveguides deposited on a silica-on-silicon substrate [20]. This setup featured a Gadolinium Gallium Garnet (GGG) prism of refractive index 1.965 at 632.8 nm, positioned on a motorized rotational stage with 0.007° resolution. The setup utilized a diode-pumped solid-state laser operating at 532.0 nm and a silicon photodetector. The photometric detection technique was used to determine the propagation losses of the fundamental mode (TE₀) at 632.8 nm in the glass-ceramic waveguides by recording the light scattered out of the plane of the waveguide. The image of the propagating streak through the waveguide in the prism coupling configuration was captured using a CCD camera (Stingray F-146B).

Amplified spontaneous emission measurements were carried out by focusing a diode-pumped solid-state (DPSS) continuous wave 532 nm laser onto the waveguide in a stripe-like geometry using a cylindrical lens having a focal length of 50 mm. The emission from the edge of the waveguides was dispersed using a monochromator (Horiba Jobin-Yvon iHR 320) and detected by a photomultiplier tube attached to the monochromator for detection. The UV-visible transmission spectra of the glass-ceramic waveguides were recorded using a fiber-coupled spectrometer (Avantes AvaSpec-ULS3648-USB2) with a spectral resolution of 0.4 nm.

4. Results and Discussions

The nanocrystal structure and size details are important for tailoring them to meet the objectives of any required application [21]. The details of X-ray diffraction studies on the 70 SiO₂–(30-x) HfO₂–x ZnO glass-ceramic waveguides have been presented elsewhere [19]. TEM imaging was conducted to analyze the size and distribution of nanocrystals within the ternary matrix. Figure 2a–c presents analytical TEM images of 1 mol % Eu-doped 70 SiO₂–(30-x) HfO₂–x ZnO (mol %) ternary films for x values of 2, 5, and 7 mol %, respectively. The TEM images confirmed the formation and growth of hybrid nanocrystals in the SiO₂-HfO₂ matrix, particularly for samples with 5 and 7 mol % ZnO, as illustrated in Figure 2b,c. The average sizes of the nanocrystals were measured to be 3, 17, and 22 nm for x = 2, 5, and 7 mol %, respectively. Interestingly, increasing the ZnO concentration from x = 5 to 7 mol % resulted in the development of ZnO-HfO₂ hybrid nanocrystals, composed of ZnO nanocrystals encapsulated by HfO₂ nanocrystals. The formation of HfO₂ nanocrystals was further explored through a lattice fringe analysis of the TEM images, shown in Figure 2d, which revealed lattice fringe spacings of 0.295 nm and 0.179 nm for the ZnO (100) and HfO₂ (022) lattice planes, respectively.

The waveguiding properties of 70 SiO₂–(30-x) HfO₂–x ZnO (with x = 0, 2, 5, and 7 mol %) glass-ceramic waveguides doped with 1 mol% Eu were evaluated using a lab-built prism-coupling setup. The losses of the waveguides were assessed by photometrically detecting the light scattered out of the plane of the waveguide, using a CCD camera.

Table 1. The optical properties of 70 SiO₂–(30 − x) HfO₂–x ZnO (with x = 0, 2, 5 and 7 mol %) glass-ceramic waveguides doped with 1 mol% Eu. Refractive indices (n) at 532.0 nm and propagation losses of waveguides measured at 632.8 nm, the optical gain (g) of waveguides measured at the principal peak of the Eu³⁺ emission spectrum (λ = 612 nm) using variable stripe length method under the excitation of 532 nm, are tabulated.

Waveguide Composition (mol%)	Refractive Index (n) (±0.005)	Thickness (d) (±0.1 µm)	Propagation Loss @ 632.8 nm (±0.2 dB/cm)	Nanocrystal Size (nm)	Optical Gain (g) cm−1
70 SiO2–30 HfO2	1.503	0.9	0.4	-	5.3
70 SiO2–28 HfO2–2 ZnO	1.502	0.9	0.6	3	4.8
70 SiO2–25 HfO2–5 ZnO	1.506	0.9	0.3	17	3.7
70 SiO2–23 HfO2–7 ZnO	1.499	0.9	0.5	22	0.1

summarizes the refractive indices (n) measured at 532.0 nm, thicknesses (d), and propagation losses measured at 632.8 nm of the heat-treated waveguides. A decrease in refractive indices from 1.503 to 1.499 (± 0.005) was observed with increasing ZnO concentrations, attributed to the incorporation of ZnO (n = 2.02) replacing the relatively higher index HfO₂ (n = 2.12). The thickness of the waveguides was found to be 0.9±0.1 μm. Propagation losses from the CCD images were determined to be as low as (0.3 to 0.6) ± 0.2 dB/cm.

The light amplification properties of the 70 SiO₂–(30-x) HfO₂–x ZnO (where x = 0, 2, 5, and 7 mol%) ternary glass-ceramic waveguides doped with Eu ions were investigated using the variable stripe length (VSL) method [22,23,24]. For optical pumping, a DPSS continuous wave laser operating at 532 nm was focused on the waveguide in a stripe-like configuration using a cylindrical lens with a focal length of 50 mm and a laser power of 80 mW. An adjustable slit on a micro-positioner stage was used to vary the stripe length of the optical pump beam. Figure 3 illustrates the schematic diagram of the VSL method experiment conducted on the hybrid nanocrystals embedded in the waveguides. Under this setup, the 612 nm emission from Eu³⁺ (⁵D₀ → ⁷F₂ transition) was collected from the edge of the waveguides, dispersed by a monochromator, and recorded by a photomultiplier tube.

The amplified spontaneous emission (ASE) intensity (I_ASE) measured at the principal peak of the Eu³⁺ emission spectrum (λ = 612 nm) as a function of stripe length (l) is presented in Figure 4a. The optical gain coefficient (g) can be determined by fitting the plot of I_ASE against l using a 1-D amplifier model, described by the following expression [22,25,26]:

$$I_{ASE}={\displaystyle \frac{I_{SP}\times l}{g}}(e^{gl}-1)$$

where I_SP is the spontaneous emission intensity per unit length, l is the stripe length of the pump beam, and g is the net optical gain coefficient. The net optical gain for the 1 mol% Eu-doped 70 SiO₂–30 HfO₂ glass waveguide was found to be 5.3 cm⁻¹. For the glass-ceramic waveguides, the optical gain values were 4.8 cm⁻¹, 3.7 cm⁻¹, and 0.1 cm⁻¹ for ZnO concentrations of 2, 5, and 7 mol%, respectively. These optical gain coefficients were obtained by fitting the experimental data using a 1-D amplifier model up to l = 4 mm. Fitting beyond l > 4 mm was limited due to gain saturation effects in the Eu-doped SiO₂–HfO₂–ZnO glass-ceramic waveguides. It was observed that the optical gain is higher in the SiO₂–HfO₂ waveguide without ZnO. The optical gain reduces drastically at the 7 mol% ZnO concentration, wherein the structure and size of the ZnO and HfO₂ nanocrystals have been observed to exhibit a large change, as discussed in our earlier work [19]. The PL spectra of waveguides containing 0 and 7 mol % ZnO concentrations are shown in Figure 4b,c, respectively, which were obtained under optical excitation for different stripe lengths (0.4 mm, 4.0 mm, and 6.8 mm). For the x = 0 mol % sample in Figure 4b, the PL spectra mainly consist of the well-known Eu³⁺ emission from ⁵D₀ → ⁷F_j (j = 0, 1, 2) transitions; however, for the x = 7 mol % sample in Figure 4c, a broad emission background is also observed, attributed to the presence of Eu²⁺, apart from the discrete features attributed to Eu³⁺. The shape of the PL spectra is found to remain invariant under different lengths of the excitation stripe, confirming the homogeneous distribution of Eu²⁺ and Eu³⁺ ions in the fabricated samples.

The asymmetry ratio (I_rel) given by the intensity ratio of the ⁵D₀→⁷F₂ to the ⁵D₀→⁷F₁ transition indicates a distortion from the inversion symmetry of the sites around the Eu³⁺ ions [27]. This ratio, I_rel is found to be 2.82 and 1.54 for x = 0 and 7 mol % samples, respectively, indicating the embedding of Eu³⁺ into less distorted environments in the glass-ceramic sample with 7 mol % ZnO.

Interestingly, the propagation losses of the Eu-doped glass-ceramic samples (70 SiO₂–(30-x) HfO₂ –x ZnO (x = 2, 5 and 7 mol%)) are not very different from that observed in the 70 SiO₂–30 HfO₂ waveguide, as shown in Table 1. However, the optical gain for the glass-ceramic waveguides is lower in comparison to the 70 SiO₂–30 HfO₂ waveguide. Hence, the reduced optical gain can neither be associated with the losses in the waveguide nor with the other optical scattering phenomenon. These observations can be explained by considering the reduction of Eu³⁺ to Eu²⁺ in the vicinity of ZnO nanocrystals, in waveguides wherein ZnO has been incorporated. This phenomenon has been discussed in our earlier work through photoluminescence (PL) and time-resolved PL analysis [19,28]. Consequently, the concentration of Eu³⁺ ions decreases with increasing ZnO concentration, resulting in lower optical gain in the ternary glass-ceramic waveguides. Also, with higher ZnO concentrations, the reduction of Eu³⁺ ions to Eu²⁺ gives rise to efficient blue light emission [27]. Despite the reduction in net optical gain due to the presence of ZnO, the added functionalities achieved in the 2 and 5 mol% ZnO incorporated 1 mol% Eu-doped ternary glass-ceramic waveguides make them promising candidates for the development of active integrated optical components in the blue and red regions of the visible spectrum.

Figure 5 presents the UV-visible transmission spectra of glass-ceramic waveguides having different ZnO concentrations. The observed modulations are a result of the thin film interference phenomena. The transmittance values exceeding 95% for all the waveguides in the region of 350 nm–900 nm of the electromagnetic spectrum represent the highly transparent nature of the fabricated samples.

5. Conclusions

We have successfully fabricated 70 SiO₂-(30 − x) HfO₂-x ZnO (with x = 0, 2, 5, and 7 mol %) glass-ceramic waveguides doped with 1 mol% Eu, achieving losses as low as 0.3 to 0.6 ( ± 0.2 dB/cm). The incorporation of 2 mol% ZnO into the SiO₂–HfO₂ glass waveguide resulted in the formation of smaller nanocrystals, as confirmed by TEM images. With increasing ZnO concentrations (x = 5 and 7 mol%), we observed the formation and growth of hybrid nanocrystals, where ZnO nanocrystals are encapsulated by HfO₂ nanocrystals. The net optical gain in the SiO₂–HfO₂–ZnO ternary glass-ceramic waveguides was found to be lower compared to the SiO₂–HfO₂ glass waveguides (g = 5.3 cm⁻¹), primarily due to the reduction of Eu³⁺ to Eu²⁺ in the vicinity of the ZnO nanocrystals within the ternary matrix. Despite this reduction in optical gain for Eu³⁺ in the ternary waveguides, the enhanced functionalities provided by the ZnO incorporation, such as the possibility of blue-light emitting waveguides [29], make the 1 mol% Eu-doped ternary glass-ceramic waveguide a viable material system for the fabrication of active on-chip integrated optical components.