Elsevier

Optical Materials

Volume 86, December 2018, Pages 433-440
Optical Materials

A new fabrication process of pedestal waveguides based on metal dielectric composites of Yb3+/Er3+ codoped PbO-GeO2 thin films with gold nanoparticles

https://doi.org/10.1016/j.optmat.2018.10.044Get rights and content

Highlights

  • A new fabrication process for waveguides amplifiers based on the pedestal architecture produced without chromium mask.

  • Propagation losses reduction on pedestal optical waveguides due to a significant reduction of the micromasking effect.

  • Yb3+/Er3+ codoped PbO-GeO2 pedestal waveguides with and without gold nanoparticles for optical amplifiers applications.

  • Enhanced local field due to gold nanoparticles contributes for 180% relative gain growth at 1530nm (6 μm waveguide width).

  • The transmission electronic microscopy is used to investigate gold nanoparticles.

Abstract

This work reports the signal enhancement of Yb3+/Er3+ codoped PbO-GeO2 pedestal waveguides due to gold nanoparticles deposited over the core layer. The pedestal structure was obtained by conventional photolithography and plasma etching with a new procedure that does not use metallic hard-masks that normally introduce roughness, leading to light scattering. This new procedure brings advantages that benefit light guiding, reducing the propagation losses. Yb3+/Er3+ codoped PbO-GeO2 thin film was obtained by RF Magnetron Sputtering deposition and was used as core layer (410 nm height). In order to cover the core with gold nanoparticles the sputtering technique was used, followed by annealing at 400 °C during 1 h. The minimum propagation losses obtained were of 1.0 dB/cm at 1068 nm. Scanning Electron Microscopy (SEM) was employed for the waveguides structure inspection and transmission electronic microscopy (TEM) was used to verify the presence of gold nanoparticles on the waveguides. It was observed an enhancement of 180% for the relative gain that reached 7.8 dB/cm at 1530 nm, for an optical waveguide with 6 μm core width, under 980 nm excitation (pump power of 60 mW), attributed to the local field enhancement in the vicinity of the gold nanoparticles. The new fabrication process presented in this work opens possibilities for optical amplifiers with low propagation losses based on different metal dielectric composites, as well as other waveguide-based devices.

Introduction

The search for new materials with metallic nanoparticles (NPs) has grown in the last 20 years due to the several applications for different technological proposals. In this context plasmonics appeared as a new field to study the metallic nanoparticles, the effects of their optical excitation that produces the collective oscillation of the free electrons, giving rise to the localized surface plasmons (LSP), with specific wavelengths that depends on size, shape and chemical environment of the NPs. Moreover, because of the mismatch between the dielectric function of the metallic NPs and the glass host, there is a confinement of the electromagnetic field near the NPs’ surface originating enhanced near field intensities with interesting photonics applications. These high field intensities nearby the metallic NPs may cause the rare-earth ions photoluminescence (PL) enhancement. Germanium oxide glasses have demonstrated to be adequate hosts for the nucleation of metallic NPs [1]; their enhanced linear and nonlinear optical properties due to the nucleation of gold or silver NPs have shown interesting applications for white light generation [2], color displays [3], optical limiters, all-optical switches [[4], [5], [6], [7]] and waveguide amplifiers [[8], [9], [10]]. Their high linear refractive index (∼1.9) is interesting for ultrafast response devices based on the nonlinear response and their low phonon energy (700 cm−1) is important for the up-conversion processes as the multi-phonon relaxation becomes less probable. Finally, their large transmission window (visible to mid infrared), high mechanical durability, large mechanical resistance and high vitreous stability are important features for several photonic applications.

The 60PbO-40GeO2 (in wt. %) glass composition doped with rare earth ions and containing silver or gold NPs has been extensively investigated. Under different excitation conditions the enhanced optical properties were studied using that composition singly-doped (Pr3+, Er3+, Tm3+, Nd3+) [[11], [12], [13], [14], [15]], doubly-doped (Yb3+/Er3+) [16], and triply-doped (Yb3+/Tm3+/Er3+, Yb3+/Tm3+/Ho3+) [2,3] containing silver NPs. Also, applications as saturable absorbers or optical limiters in the visible range were demonstrated for bulk samples and thin films with and without metallic NPs; thin films with gold NPs showed applications for all-optical switching in the infrared region [[4], [5], [6], [7]]. In all cases the optical properties enhancement was attributed to the increased local-field nearby the NPs and/or to the energy transfer from the metallic NPs to the rare earth ions.

The 60PbO-40GeO2 (in wt.%) glass composition was also used for the fabrication of optical waveguide amplifiers with different architectures making use of the conventional photolithography and plasma etching. There has been overwhelmingly strong evidence that the definition of the waveguide core is very important, since the resulting geometry of the device has great influence on its performance. Different Yb3+/Er3+ codoped PbO-GeO2 pedestal waveguides fabrication processes were reported [8,9] with and without gold NPs. For example, pedestal waveguides present advantages with respect to rib waveguides [17] as it is not necessary to etch the core material: the etching is performed on the lower cladding layers and the core material is deposited over it. So, the pedestal architecture represents an important alternative to obtain the lateral confinement in optical waveguides based on silicon technology.

The interest in pedestal-type waveguide has emerged recently since they present favorable characteristics for optical field guiding. The main advantageous characteristic is the fact that the core layers sidewalls are made up of deposited surfaces rather than etched ones, as will be discussed over the next session. This reduces propagation losses since less overall surface roughness translates into smaller scattering losses, which play a prominent role in these types of wave guiding structures. The other key advantage of the pedestal process is related to the simple fact that the core material is not etched at all. This allows materials that would not be used otherwise, due to incompatibility with conventional plasma etching techniques, to be explored in order to fabricate low loss waveguides for a breadth of applications, including rare earth doped optical amplifiers [18,19].

In a typical fabrication process, the pedestals are defined by RIE technique on the lower cladding layer (in general silicon oxide). This step has been done using chromium as hard-mask, as reported in previous works [8,9,17] due to its high selectivity in the oxide etching. Unfortunately, the use of chromium leads to the formation of pillar shaped structures in the form of “grass” in the film being etched and to larger values of sidewall roughness [20]. These effects are related to the micromasking effect [21], in which chromium particles are sputtered off from the hard-mask, and may be redeposited over the surrounding region, acting locally as micro-masks. Then the core layer is deposited in a subsequent step. However, the chromium particles that might have been originated in the previous step contributes for the growth of an irregular film (columnar growth) [22], increasing optical losses due to the augmented roughness and radiation leakage [23]. Advances on the pedestal fabrication processes based on the reduction of the chromium mask thickness and 3 steps of CHF3+O2 plasma etching allowed an enhancement of 50% on the relative gain that reached 6 dB/cm, at 1530 nm, for a 70 μm waveguide width, under 980 nm excitation [8]. More recently the same procedure was used to produce Yb3+/Tm3+ codoped PbO-GeO2 waveguides with and without gold NPs [9]. Relative gain reached 22 dB/cm, at 805 nm, and represented an enhancement of about 100% due to the nucleation of gold NPs; these results demonstrated the possibility of developing optical amplifiers to be used in integrated optics mainly for short distance optical networks.

The present study demonstrates a new and improved procedure to fabricate pedestal-type waveguides, that does not use metallic hard-masks. We demonstrate in this work that this new method improves light guiding by reducing the propagation losses originated from light scattering due to the micromasking. Yb3+/Er3+ codoped PbO-GeO2 were employed as the core of the waveguides and passive and active characterization results are presented. We also show results that compare these waveguides performance employed as optical amplifiers, at 1530 nm, with and without the presence of a layer based on gold NPs, deposited over the waveguides core. In a recent review [17] we presented results of pedestal waveguides produced with chromium mask; so, the present study shows advances with respect to that fabrication process and represents another alternative for waveguides amplifiers production. The results obtained in this work opens possibilities that are relevant for the development of new optical amplifiers based on different metal dielectric composites.

Section snippets

Pedestal optical waveguide fabrication process

The optical waveguides were fabricated on a p-type single-crystal silicon wafer with (100) crystallographic orientation (Fig. 1a). The first step is the growth of a 150 nm-thick thermal silicon dioxide (n = 1.46) in a furnace at 1150 °C using O2 gas (Fig. 1b). The total oxidation time was 50 min. We used 150 nm for the silicon dioxide that served as hardmask in order to obtain a layer that was thick enough to withstand the total etch time without compromising the structure of the silicon

Results and discussion

The measured concentrations of Er3+ and Yb3+ obtained using Rutherford Backscattering Spectrometry (RBS) and Particle Induced X-Ray Emission (PIXE) analysis were 1.86 × 1020 ions/cm3 and 2.87 × 1021 ions/cm3, respectively.

A typical SEM image of a Yb3+/Er3+ codoped PbO-GeO2 pedestal waveguide produced without chromium mask is shown in Fig. 2. We note that the core surface with 410 nm height presents low roughness. The SiO2 thermally grown layer has approximately 1.6 μm thickness, and the

Conclusions

The present study shows a novel procedure to fabricate Yb3+/Er3+ codoped PbO-GeO2 pedestal waveguides, with and without gold NPs, obtained by conventional photolithography and plasma etching, which does not use metallic masks that normally introduce roughness. We observed an enhancement of 180% for the relative gain that reached 7.8 dB/cm (6 μm core width and 410 nm core height), at 1530 nm, under 980 nm excitation (pump power of 60 mW), due to the presence of gold NPs. The gain enhancement can

Acknowledgements

We acknowledge the financial support by the Brazilian agency, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, 303548/2015-0). The work was performed in the framework of the National Institute of Photonics (INCT de Fotônica, 465763/2014-6). The Nanotechnology National Laboratory (LNNano), CNPEM-Campinas/Brazil, is also acknowledged for the TEM measurements.

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