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Abstract
High-refractive-index-contrast potassium double tungstate waveguides have been experimentally demonstrated. A bulk KY(WO4)2 layer was successfully bonded onto a lower refractive index carrier using a UV curable optical adhesive and polished down to the thickness of 2.4 µm. A set of rib waveguides with ~2 μm width and 0.85 μm slab thickness were fabricated on the thin transferred KY(WO4)2 layer by focused-ion-beam milling. The upper-limit of the propagation losses of the fabricated waveguides is estimated to be 1.5 dB/cm at the wavelength of 1.55 μm using the Fabry-Perot method.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The family of monoclinic potassium double tungstates, KY(WO4)2, KGd(WO4)2 and KLu(WO4)2, are excellent host materials for lanthanide ions (Yb3+, Er3+, Gd3+, Lu3+, etc.) thanks to their optical properties such as high refractive index (n~2) in comparison with other host materials—Al2O3 [1], fluoride [2], phosphate [3] and soda-lime silicate [4] glasses—high thermal conductivity (~3.3 Wm−1K−1) [5], and high emission and absorption cross-sections for the lanthanide ions doped into these materials [6]. The crystalline structure of the potassium double tungstates offers sufficient inter-ionic distance between the active ions in the crystal lattice (~0.5 nm) so that clustering of doped ions can be prevented. As a result, high modal gain (circa 1000 dB/cm) in an Yb3+ doped waveguide amplifier [7] as well as efficient (~80%) high power (~1.6 W) waveguide lasers at ~2 μm [8] have recently being reported in a low-contrast waveguide configuration. These devices have been fabricated on rare-earth ion doped layers grown onto undoped KY(WO4)2 substrates by liquid phase epitaxy [9]. The refractive index contrast between the doped and undoped layers is typically smaller than 0.02.
Low contrast waveguides have several disadvantages, such as large mode field diameter (i.e., ranging from 6 μm [7] to 25 μm [8]), large bend radii and long devices (i.e., typically a few millimeters), which make them not suitable for integration with other optical functions onto different passive photonic technology platforms such as silicon-on-insulator or Si3N4 on SiO2. High-contrast waveguides (i.e., refractive index difference between core and cladding of >0.5), on the other hand, exhibit a tight field confinement in the core, which enhances light-matter interaction, reducing the pump power requirements to achieve inversion of the material and enabling the exploitation of non-linear effects that would require too high power in a low-contrast waveguide configuration. However, the crystalline nature of the double tungstate host materials is disadvantageous for the integration with low refractive index dielectrics, such as SiO2, required for the realization of high-contrast waveguides. Direct growth of crystalline KY(WO4)2 material onto amorphous substrates is prevented by the requirement of lattice matching to achieve optical quality crystalline layers [9]. Heterogeneous integration has been developed in recent years for the integration of dissimilar materials including III-V semiconductors on silicon [10], LiNbO3 on silicon nitride [11], and magnetorresistive materials on silicon [12]. Either direct bonding technique [10] or adhesive bonding, typically with bonding material such as bencocyclobutene (BCB) [13], polyimide [14] or SU8 [15] are the techniques typically employed.
In this work, high-refractive-index-contrast passive KY(WO4)2 waveguides are demonstrated for the first time. The waveguides are fabricated by heterogeneous transfer of a thin KY(WO4)2 layer followed by focused-ion-beam (FIB) milling of the rib waveguides [16]. The propagation losses of the fabricated waveguides are estimated from the measured reflected Fabry-Perot fringes. In the following sections, the steps for the fabrication of high-contrast KY(WO4)2 waveguides will be explained in detail. Propagation losses <1.5 dB/cm were achieved for high contrast uncladded waveguides.
2. Fabrication of high-contrast rib KY(WO4)2 waveguides
A new micro-fabrication process flow has been developed to realize high-contrast passive KY(WO4)2 waveguides. KY(WO4)2 substrates 1 mm thick and with 1x1 cm2 surface area are purchased from Altechna (LT). The monoclinic KY(WO4)2 crystal has three optical axes [9] as well as different coefficients of thermal expansion along the different optical axes [17].
The fabrication process flow can be divided into three main processing steps: (1) bonding the KY(WO4)2 onto a SiO2 carrier chip; (2) thinning of the bonded material to the required thickness, which defines the waveguide height; and (3) ion beam milling of rib waveguides.
2.1 Bonding
When the thermal expansion coefficients of the substrate and the bonded sample are significantly different from each other, the stress on the bonding interface causes cracks and/or delamination either on the bonded layer or on the substrate layer. Typical adhesive bonding methods require relatively high temperatures (> 100°C) to achieve a strong adhesion between sample and substrate. In direct bonding, spontaneous adhesion via van der Waals forces between two ultra-smooth (<0.5 nm rms) and ultra-flat materials [18] permits permanent bonding between two dissimilar materials. Although the bonding process in some cases can take place at room temperature [19], usually a post-bond annealing step (200°C – 400°C) is still required to form strong chemical bonds and ensure high bonding strength [20]. In this work, the UV curable adhesive material NOA81 (Norland Products Inc., USA) is used to eliminate the requirement of elevated bonding temperature while providing high bonding strength.
The bonding process starts by cleaning a 2 cm by 2 cm, 0.5 mm thick fused silica carrier substrate and a 1 mm thick KY(WO4)2 sample with lateral dimensions of 1 cm by 1 cm in an ultrasonic bath filled with DI water for 15 minutes in order to remove particles on the surfaces. The sample and the carrier are then chemically cleaned in 99% HNO3 for 10 minutes. A NOA 81 adhesive layer is then spin-coated on the KY(WO4)2 sample using a two-step process: 500 rpm for 5 seconds followed by 4000 rpm for 20 seconds. The spinning process gives a ~6.5 μm thick adhesive layer. The adhesive layer is then pre-cured under a hand-held UV light source (peak wavelength ~365 nm) with an intensity of 50 μJ/cm2 for 5 seconds. This process step increases the viscosity of the adhesive liquid, which reduces the spreading of excess liquid from the corners of the sample during the bonding step. The bonding between fused silica carrier and undoped KY(WO4)2 is realized in a Finetech Lambda flip-chip bonder (Finetech GmbH, DE) with a nominal placement accuracy of 0.5 μm applying 10 N of bonding force. A ~6 μm thick adhesive layer is obtained at the bonding interface. The adhesive material residues on the carrier and on the corners of the KY(WO4)2 are removed in an oxygen plasma (50% 200 sccm O2 flow with 500 W plasma power for 10 minutes under 1.2 mbar pressure). The bonded sample is then exposed to UV light for a total fluence of 2 J/cm2 to fully cure the adhesive layer between the sample and substrate. Finally, the sample surface is cleaned with ethanol for 5 minutes to dissolve any adhesive material residues decomposed in the oxygen plasma step.
2.2 Thinning
A thinning step is required to reduce the thickness of the bonded 1 mm thick KY(WO4)2 material to the required waveguide thickness. The thinning process consists of two steps. A first coarse lapping step reduces the sample thickness rapidly to ~15 μm. It is followed by a polishing step to slowly obtain an optical-quality layer of the target thickness. The thinning was performed using a Logitech PM5 precision lapping and polishing system (Logitech, UK). The bonded sample was fixed on a 5.75 mm thick ultra-parallel glass plate with Alcowax (Nikka Seiko, JP). The corners of the KY(WO4)2 sample were also covered with wax to avoid any undesirable cracks appearing during the thinning process. In the lapping step, a non-drying colloidal silica suspension with 9 µm diameter particles was utilized. During the polishing process, a slurry with 40 nm size particles was utilized (OP-U polishing suspension, Struers, DE). The layer thickness is aimed at supporting a single transverse electric (TE) mode at the operation wavelength (λ = 1.55 µm), for which a KY(WO4)2 layer of <2.5 µm is required according to finite-difference (FD) mode simulations. Figure 1(a) shows an image of a thin KY(WO4)2 layer transferred onto a SiO2 substrate. The thicknesses of the adhesive and KY(WO4)2 layers are ~6 µm and ~2.4 µm respectively. The RMS surface roughness of the polished surface is as low as 0.17 nm according to atomic force microscope measurements [Fig. 1(b)]. The sample was diced from the two sides to create direct access to the KY(WO4)2 layer.
Fig. 1 (a) End-facet of a thin (2.4 μm) KY(WO4)2 layer bonded onto a fused silica substrate by NOA81 adhesive. The end-facet has been polished using focused ion beam; (b) Atomic force microscope AFM) scan of the surface of the transferred KY(WO4)2 layer (AFM Fast Scan, Bruker, DE). A 0.9x0.9 μm2 area was scanned with an 8 nm radius tip. The surface root-mean-square roughness is as low as 0.17 nm.
2.3 Waveguide patterning
Numerous reports describe the patterning of KY(WO4)2 by ion beam etching (IBE) technique [7-8, 21]. However, the lateral resolution is limited by the required thickness of the resist used as etch mask. In focused-ion-beam milling (FIB), accelerated gallium ions are used to sputter the material and it can be utilized to pattern optical structures with nanometer resolution [22]. FIB milling is a serial technique and therefore not suited for mass-production applications. In this work, FIB was used to develop the first proof-of-concept high-contrast waveguides in KY(WO4)2.
Two-dimensional (2-D) FD mode calculations were carried out using OptoDesigner (Phoenix Software BV, NL) to find the cross-sectional dimensions of the waveguides that support single mode propagation at the operation wavelength. Rib waveguides with a width of ~2 μm, thickness 2.4 μm and slab height of 0.85 μm [Fig. 2(a)] were fabricated on a transferred KY(WO4)2 layer. The waveguides were aligned along the Ng axis. Sets of waveguides with lengths of 300 μm, 250 μm and 200 μm were produced [Fig. 2(b)]. The milling beam current was set to 2.7 nA to obtain high quality, low roughness waveguide sidewalls within a reasonable process time (2 hours 20 minutes for a set of three waveguides). In order to minimize processing time, the waveguides were placed inside a 10 μm wide trench as shown in Fig. 2(a). According to the FD mode calculations, the trench width is sufficient to prevent any cross-talk between adjacent waveguides. The patterning was followed by FIB polishing of the input and output facets to ensure high and equal reflectivity from input [Fig. 2(a)] and output [Fig. 2(c)] waveguide end-facets. Figure 2(d) shows the simulated mode profile supported at the cross-section of the rib waveguides. Table 1 shows the refractive indices of the different materials utilized for the simulations.
Fig. 2 (a) Scanning electron microscope (SEM) image of the cross-section of a FIB milled rib passive KY(WO4)2 channel waveguide. The corresponding dimensions are given on the image; (b) Tilted SEM image of the waveguides on the layer. The waveguides covered with a yellow solid line with labels “W1”, “W2” and “W3” were characterized in this study. The length of the “W1”, “W2” and “W3” waveguides are 200 μm, 250 μm and 300 μm, respectively. At the end of each waveguide, a reflector is etched to ensure the same reflectivity from both waveguide end-facets. Triangular anti-reflectors are etched away at the end of the waveguides to minimize the possible reflection that goes back to the waveguide end-facets; (c) SEM image of the deep reflectors at the output end-facets; (d) Simulated 2-D transverse-electric (TE) mode profile at the cross-section of the rib waveguide given at λ = 1.55 μm. The optical axes of the KY(WO4)2 crystal are shown. The calculated effective refractive index is 1.9492 at the wavelength of 1.55 μm.
Table 1. Refractive indices of the materials utilized in the simulations, indicating the Sellmeier equation utilized to fit the refractive index and its coefficients. Lambda in micrometers.
3. Characterization of waveguides using the Fabry-Perot method
Various methods are typically used to estimate the propagation losses of optical waveguides, including the measurement of the waveguide scattered light streak [23,24], the prism-coupling method [25], insertion loss measurement method [26, 27], and the cut-back method [28]. The aforementioned methods are, however, not suitable for the study of the losses of the short waveguides fabricated in this work. The method based on the measurement of the waveguide scattered light streak depends strongly on the quality of the device surface and the confinement of the waveguide architecture. In the particular case of this work, the short length of the waveguides makes the scattering from the incoupling spot overrun the limited scattering along the waveguide streak due to the high confinement of the waveguide mode. The other methods rely on transmission measurements, which are not possible in the waveguides of this work [Fig. 2 (b)]. Propagation losses can also be estimated based on the Fabry-Perot fringes induced in the waveguide cavity formed between the input and output end-facets [29-30].
The power reflected from the Fabry-Perot cavity by the end-facets of the waveguides is given by Eq. (1) [30]
where δ defines the phase change accumulated in a round trip along the length of the waveguide, L, given by the relation k0neff2L, k0 is the free-space wavenumber (2π/λ), neff effective refractive index, which is calculated to be 1.9492 at 1.55 μm wavelength based on the refractive indices as a function of wavelength of the different materials (Table 1) and the measured dimensions of the waveguides [Fig. 2(a)] using the 2D FD mode solver, α is the propagation loss in cm−1, P0 is the incident beam power on the waveguide end-facet, ƞ is the coupling coefficient, calculated as the mode overlap between the modes of the fiber and waveguide,and r is the wavelength dependent reflection coefficient of the end-facet, which has been approximated as the Fresnel reflection between a material with refractive index neff and air. The term offset defines the non-coupled power reflected from the input end-facet, which contributes as a DC offset to the measured signal (i.e., it therefore does not affect the quality of the fit of the waveguide losses). As mentioned above, FIB milling with Ga+ ions was utilized to pattern the high-contrast KY(WO4)2 waveguides as well as to polish the input and output facets. A square deep trench larger than the size of the mode propagating in the waveguide was etched at the output end-facet to ensure the same reflectivity, r, in both end-facets [Fig. 2 (c)]. Identical waveguides were fabricated with different lengths—200 μm, 250 μm and 300 μm—in order to increase the confidence of the fit to the measured data.The characterization setup consists of a coherent tunable light source with an InGaAs photodetector module (Agilent 8164B Lightwave measurement system with 81600B-201 laser module), and an optical circulator (OC). The OC is utilized to separate the input signal from the reflected signal coming from the KY(WO4)2 waveguides. The schematic of the characterization setup is depicted in Fig. 3. A tunable laser in the wavelength range of interest is connected to the first port of the OC by a single-mode (SM) polarization-maintaining (PM) fiber. The light coming from port 1 is guided to port 2 and then coupled to the waveguide using a bare PM fiber. The distance between the bare fiber and the end-facet of the waveguide was minimized (typically below 1 μm) to minimize the effect of the cavity formed between the fiber and end-facet, which will introduce a slow varying envelop to the measured data. The reflected light coming from the waveguide is circulated to port 3, where the power detector is connected to. Prior to measuring the response of the waveguide, a reference measurement of the characterization setup without the sample is taken to detect and store the reflections originating from the various cavities formed within the system, such as the fiber connections in the light source and the fiber-to-fiber interfaces across the OC. This reference measurement is subtracted from the measured Fabry-Perot reflection of the waveguide.
Fig. 3 Schematic of the optical setup built for the characterization of the propagation loss of KY(WO4)2 waveguides based on Fabry-Perot method around λ = 1.55 μm.
Figure 4 shows the measured reflections from the fabricated waveguides with 200 μm length (W1). The separation between the adjacent reflection peaks fits well to the different cavity lengths as expected from the free spectral range formula given as: ΔλFSR = λ2/ (ngℓ), where ng is the group refractive index of the cavity and ℓ is the roundtrip cavity length; ℓ = 2L. The ng obtained from these measurements, 2.06, 2.04 and 2.06 for waveguides W1, W2 and W3 respectively at a wavelength of 1.55 μm, agree well with the calculated value, 2.053 at 1.55 μm, for the measured waveguide cross-section. The high-frequency oscillations on the measured signals are mainly attributable to the cavity formed inside the bare fiber attached to port 2 of the OC. These oscillations were not filtered to avoid any distortions that would affect the result of the fitting. The data was fitted to the theoretical expression for the reflected power [Eq. (1)]. The incident power at the tip of the 9.9 μm-diameter SM fiber (P0) is measured as 0.635 mW using an IR photodetector (Newport 818-IR photodetector). The coupling coefficient between the waveguide fundamental mode and the fiber is calculated as 0.127 using 2D-FD mode solver. The variables used in the fitting were the length, offset and the propagation losses. The results of the fit are shown in Table 2. An R2 value of 0.94 was obtained for the fit. The offset values obtained agree well with expectations. A propagation loss <1.5 dB/cm for the measured wavelength range was obtained.
Fig. 4 Example of measured and fitted reflected signals from the FIB-patterned high-contrast waveguide. W2 (200 μm length).
Table 2. Parameters used for the fits to the measured data. R2 of the fit, R2 = 0.94.
The propagation losses achieved in this first demonstration of high-contrast waveguides in KY(WO4)2 are comparable to the reported propagation losses for low-contrast KY(WO4)2 channel waveguides in the literature. Geskus et al. fabricated channel waveguides with 0.34 dB/cm at 1 μm on Gd3+ and Lu3+ co-doped KY(WO4)2:Yb3+ layer using Ar+ ion beam milling [31]. Borca et al. reported on fabricating KY(WO4)2:Yb3+ waveguides using an ultrashort pulse laser writing, which leads to propagation losses in the range of 2-2.5 dB/cm at 1 µm [32]. Choudhary et al. demonstrated Ar+ ion beam milled channel waveguide 2.7 dB/cm propagation loss at the same wavelength [33]. Several strategies can be implemented to further reduce the propagation losses. Firstly, it is important to note that the high-contrast waveguides in this work are air cladded, which makes the light propagation in the core more sensitive to particles and roughness of the sidewalls. A low refractive index cladding will be applied when the developed waveguides are used in a real device application, which is expected to reduce scattering losses. Furthermore, FIB milling relies on bombarding the surface to be sputtered with a finely focused beam of Ga+ ions. The Ga+ implantation at the waveguide sidewalls is inevitable in the patterning process, and it can also contribute to the measured propagation losses. Further work is underway to understand the effect of the Ga+ implantation on propagation losses and ways to reduce its effect. Although FIB is not the most suitable method for the fabrication of long waveguides, the low losses achieved in this first demonstration show that this technique could be utilized for the realization of waveguide regions where dimensions below what can be achieved with standard UV lithography are required, such as in tapers or waveguide gratings. Work is currently under way to improve the thinning process to achieve thinner layers (<1 μm) in a reproducible manner. The ultimate objective is the realization of high-contrast waveguides with dimensions below 1 μm x1 μm.
4. Conclusion
In this work, the first high-refractive-index-contrast (Δn ≈0.45) KY(WO4)2 waveguides have been experimentally demonstrated. The waveguides were realized by a combination of an heterogeneous thin KY(WO4)2 layer transfer (via adhesive bonding followed by lapping and polishing) and focused-ion-beam milling. The propagation losses of the fabricated waveguides were estimated to be <1.5 dB/cm at a wavelength of 1.55 µm by analyzing the Fabry-Perot reflections of the cavity formed between the waveguide FIB polished end-facets.
Funding
“Stichting voor de Technisch Wetenschappen” (STW) (STW-12832: “High-bit-rate on-chip nanoamplifiers in Er3+ doped double tungstates”) and ERC Consolidator (grant project RENOS “Rare-earth doped novel on-chip sources”- 648978).
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