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Universal micro-trench resonators for monolithic integration with silicon waveguides

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We present a systematic study of micro-trench resonators for heterogeneous integration with silicon waveguides. We experimentally and numerically demonstrate that the approach is compatible with a large variety of thin film materials and that it does not require specific etching recipe development, thus making it virtually universal. The microresonators are fabricated through in-foundry silicon-on-insulator processing and in-house backend processing. We also report ultra-compact chalcogenide microresonators with radius as small as 5µ and quality factors up to 1.8 × 105. We finally show a proof-of-concept of a novel multilayer waveguide using the micro-trench technique.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Figures (8)

Fig. 1.
Fig. 1. (a) Schematic of the process flow for the fabrication of micro-trench waveguides co-integrated with silicon waveguides. The steps are as follows: (1) Lithography and etching of the silicon structures, (2) deposition of the top silica cladding, (3) lithography and etching of the micro-trench, (4) thin film deposition, (5) optional process to remove the top layer and leave a rectangular waveguide as shown in (6). In this work, when there is a step (5), it consists of thermal dewetting. (b,c) type I/II waveguide schematic (left) and top view micrograph of a fabricated device (right). The silicon waveguide (Si wg) on the micrographs is faintly visible through the chalcogenide cladding. The materials are identified using different colors: brown for photoresist (PR), light blue for silicon dioxide (SiO$_2$), grey for silicon (Si) and red for the material to co-integrate (Mat).
Fig. 2.
Fig. 2. (a) Effective index of the guided modes in a waveguide with $w_t = 2.0$ $\mathrm {\mu }$m and $t_f = 1.0$ $\mathrm {\mu }$m with varying refractive index $n$. (b) Effective index of the guided modes in a waveguide with a fixed refractive index $n=2.05$ and $t_f = 1.0$ $\mathrm {\mu }$m. Both figures consider type II waveguides.
Fig. 3.
Fig. 3. Simulated quality factor limited by (a,b) substrate leakage $Q_l$ and (c,d) bending leakage $Q_b$. Type I waveguides are shown in (a) and (c) while type II waveguides are shown in (b) and (d). The waveguide dimensions are constant at $w_t = 2.5$ $\mathrm {\mu }$m and $t_f = 1.0$ $\mathrm {\mu }$m.
Fig. 4.
Fig. 4. Effective index of (left panel) the fundamental quasi-TE mode in a 220 nm thick silicon waveguide with varying width and of (right panels) the guided (up to 20) quasi-TE modes in type II waveguides with $t_f = 1$ $\mathrm {\mu }$m, varying width and different refractive index: from left to right, $n=1.65$, $n=2.05$, $n=2.25$, $n=2.45$.
Fig. 5.
Fig. 5. Cross-section SEM images of a (a) type I waveguide and (d) type II waveguide. (b,c,e,f) Perspective SEM images of MRs with $R = 100$ $\mathrm {\mu }$m using different thin film materials with refractive index: (b) Ta2O5, $n = 2.07$ (c) Ge23Sb7S70 $n=2.17$, (e) As20S80 $n=2.19$ and (f) As20Se80 $=2.57$. (b) and (c) are examples of type I waveguides while (e) and (f) are examples of type II waveguides.
Fig. 6.
Fig. 6. Measured transmittance of micro-trench resonators with $R=100$ $\mathrm {\mu }$m and various materials (a) Type I and Ta2O5, (b) Type I and Ge23Sb7S70, (c) Type II and As40S60 and (d) Type II and As20S80.
Fig. 7.
Fig. 7. (a,d) Top view micrograph of As20S80 and As20Se80 MR with $R = 5$ $\mathrm {\mu }$m, respectively. (b,c) Measured transmittances with an inset showing the simulated electric field intensity mode profile at 1550 nm. (c,f) Single resonance with a Lorentzian fit to extract $Q$.
Fig. 8.
Fig. 8. (a) Schematic of the multilayer waveguide combining Ta2O5 and As20S80. (b) Measured transmittance, (c) quality factor (d) group index and (e) temperature dependent wavelength shift $TDWS$ of the multilayer with $R = 100$ $\mathrm {\mu }$m. The insets in (d) show a perturbed resonance (i) and an unperturbed resonance (ii).

Tables (1)

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Table 1. Review of micro-trench resonator demonstrations

Equations (3)

Equations on this page are rendered with MathJax. Learn more.

Q 1 = Q c 1 + Q s 1 + Q a 1 + Q b 1 + Q l 1 .
Q i = 2 Q 1 + T 0 ,
Q i 2 π n g α λ r ,


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