1. Introduction

Silicon photonics has been a subject of immense research interest over the last few decades due to its promise in realizing low cost, integrated optical chips that leverage well-established, state-of-the-art CMOS manufacturing technology [1–4]. However, silicon still faces several challenges as a photonic material—particularly low light-emission efficiency due to its indirect band gap, short non-radiative recombination lifetime and intrinsic second-order non-linearity [4–6]. Rather than doping silicon with optically active materials to alter its intrinsic properties and overcome these challenges, hybridization of silicon with functional cladding materials provides a comparatively simple approach with less fabrication complexity. Various on-chip devices have utilized this process, including silicon wire waveguides [7–8], slot waveguides [9–10], nanophotonic beam splitters [11] and microring resonators [12–14].

Subwavelength grating (SWG) metamaterials on the silicon-on-insulator (SOI) platform are attractive photonic structures due to their simple fabrication process and adjustable waveguide core effective index. The periodic nature of these waveguides can enable various behaviours of light, including low loss Bloch mode propagation, slow light propagation, Bragg reflections and diffraction, depending on which regime the operating point of the device within the dispersion diagram is located [15]. The cladding material surrounding the SWG waveguide core plays an important role in determining the core effective index due to the index averaging effect, according to effective medium theory [16]. Thus far, SWG devices have been fabricated using air, water, silicon dioxide and various polymers as cladding materials, yielding a plethora of silicon-based integrated optical devices, including fiber-to-chip couplers [17–22], waveguide crossings [23], sensors [24–25], modulators [26], and multiplexers [27], to name a few. However, the refractive indices of these materials are relatively low, limiting the achievable overlap of an SWG waveguide mode with the cladding material.

Tellurium oxide (TeO₂) is an amorphous glass material with significant potential as a high index cladding (n ∼ 2.1) for integrated optical devices. Various on-chip thin-film waveguides [28–30], lasers [31], optical amplifiers [31–34], microring resonators [35], sensors [36] and non-linear devices [37] have incorporated TeO₂ due to its wide optical transparency and excellent rare-earth solubility [38]. TeO₂ employed as a cladding material for SWG waveguides would provide a useful guiding mechanism for the expanded mode caused by the relatively lower SWG waveguide core index, as well as a buffer against substrate leakage that often threatens modal guidance in SWG waveguides due to the mode’s preference of expanding into the higher index TeO₂ top cladding [39]. This significantly increases the modal overlap with the top cladding material, which is beneficial for amplification and sensing, and can reduce light interaction with the silicon waveguide sidewalls that contributes to waveguide propagation loss caused by scattering. Scattering loss is also reduced due to the lower index contrast between the waveguide core and cladding materials. Furthermore, different TeO₂ film thicknesses, from tens of nanometers to microns, can be applied to enable fine control of the Bragg condition for applications such as slow light devices [40]. This new design parameter can also be exploited to scale down the waveguide geometry without sacrificing mode effective index, thus increasing design flexibility for SWG waveguides across a wide array of wavelengths that extend beyond the standard telecommunication bands. This advantage is paralleled with the wide optical transparency window of TeO₂ that allows light propagation up to the mid-infrared wavelength range, making it a versatile material across multiple platforms.

In this work, we present a new waveguide platform consisting of TeO₂ cladding on silicon SWG waveguides. The photonic bandstructure of the periodic SWG waveguides is engineered by tuning the device geometry. The fabrication processes of both the waveguides and the TeO₂ film are discussed and considered during the design phase. Modal guidance is demonstrated in the TeO₂-clad structure with a propagation loss of 1.0 ± 0.1 dB/mm reported above the bandgap. Similar losses can be achieved across other regimes of the device bandstructure, which can be tuned accordingly over a wide range of wavelengths used in telecommunications.

2. Theory and simulations

SWG channel waveguides are 1-D periodic structures composed of two or more media with different refractive indices alternating along the axis of light propagation. Their periodicity creates an opening in the electromagnetic dispersion relation of the structure where light in a certain frequency range is forbidden to propagate [41]. A flattened first-order band exists below the bandgap near the Brillouin zone edge where the wavevector k represents a wavelength that is twice the grating period (k =π/Λ), indicating slow light with near-zero group velocity and highly dispersive properties. As the frequency decreases further, along with the periodicity relative to the wavelength of light, light efficiently propagates through the waveguide in the form of Bloch modes [42].

We employed 3D finite-difference-time-domain (FDTD) simulations using Lumerical FDTD Solutions to model the bandstructure of our TeO₂-coated SWG waveguides and calculate their Bloch mode effective and group indices. The simulation window surrounds a single unit cell as shown in the inset of Fig. 1(a) with silicon (red), TeO₂ (blue) and air (white) media indicated. Periodic boundary conditions are used along the propagation direction to reduce simulation time, while perfectly matched layer (PML) boundaries are used in the transverse directions of the simulation window. The bandstructure of our SWG device with waveguide width (W_SWG) of 450 nm, period (Λ) of 360 nm, duty cycle (δ) of 0.5 and TeO₂ cladding thickness (t_clad) of 100 nm is depicted in Fig. 1(a). The dispersion diagram shows the flattening of the first-order band (in red), as the band edge is approached and the bandgap region (shaded in yellow) begins. The grey shaded area located above the black lightline of silicon dioxide (SiO₂, n ≈ 1.44) corresponds to a continuum of leaky modes. Various mode profiles along the first-order band are shown in Figs. 1(b)–1(c) to demonstrate the different Bloch mode behaviours that can be exploited in TeO₂-coated SWG structures. Figure 1(c) shows the mode profile near the band-edge where the mode intensity is mostly concentrated in and around the high-index silicon grating. The mode is more delocalized in Fig. 1(b), far from the band-edge and the smaller period-to-wavelength ratio results in effective homogenization of index contrast.

 

Fig. 1. (a) 3D FDTD simulated photonic bandstructure of proposed device. Inset depicts top view index profile at half waveguide height of simulated unit cell with length Λ (black dashed box). Top view Bloch mode intensity distribution profile for periodicities and wavenumbers (b) Λ = 300 nm & k = 0.33 and (c) Λ = 360 nm & k = 0.47 at λ = 1480 nm. Yellow and green star on the first order band show the location of (b) and (c), respectively. Grey and yellow shaded areas correspond to a continuum of unguided modes above the SiO₂ lightline (in black) and the photonic bandgap, respectively.

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The Bragg condition states n_eff = λ_Bragg/2Λ, where n_eff is the Bloch mode effective index, λ_Bragg is the Bragg wavelength that marks the boundary between subwavelength regime and photonic bandgap and Λ is the period (pitch) of the waveguide gratings. For a grating with a period of 360 nm, a Bloch mode effective index of ∼ 2.06 is achieved at a Bragg wavelength of 1480 nm, as indicated in Fig. 2(b) (in black). Conformal cladding materials with lower refractive index, such as SiO₂, and micron-scale thickness cannot achieve a Bloch mode effective index this high without having to increase the volume of the silicon waveguide, which would yield a multimode waveguide and also reduce light-matter interaction with the cladding material that is essential for our aimed application (light amplification). In contrast to conformal SiO₂ cladding, high-index TeO₂ cladding only a few tens of nanometers thick is sufficient to reach this Bloch mode effective index value. Figure 2 shows the dispersion relation, Bloch mode effective index and group index, with respect to wavelength, for our SWG device with the same parameters as in Fig. 1, but with varying cladding thicknesses of 90 nm (red), 100 nm (green) and 110 nm (blue). The corresponding Bragg wavelengths for these cladding thicknesses are 1444 nm, 1465 nm and 1481 nm, respectively, yielding a 15–20 nm shift in Bragg wavelength with only 10 nm variation in TeO₂ cladding thickness. This is a significant Bragg wavelength shift, especially compared to equivalent variations in low index cladding thickness, indicating that TeO₂ provides a useful design parameter for bandgap tunability of SWG waveguides.

 

Fig. 2. 3D FDTD simulated (a) dispersion relation, (b) Bloch mode effective index and (c) group index for the TeO₂-coated SWG waveguide with W_SWG = 450 nm, Λ = 360 nm and δ = 0.5. The red, green and blue lines correspond to TeO₂ cladding thicknesses of 90, 100 and 110 nm, respectively. The black dotted line in (b) corresponds to the Bragg condition at Λ = 360 nm. Shaded areas represent slow light regions where n_g > 10 for each case.

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3. Fabrication and layout

Our waveguides were fabricated on a 200 mm multi-project wafer (MPW) on an SOI platform through Advanced Micro Foundry (AMF) in Singapore. A 193 nm deep-UV (DUV) lithographic process was used with a 220 nm-thick silicon waveguide layer on a 2 µm-thick SiO₂ buried oxide (BOX) layer. Deep silicon trenches were created along the chip edges to facilitate fiber-chip edge coupling. Wafers with and without top oxide cladding deposited by AMF were fabricated, allowing SiO₂-cladded reference structures and exposed waveguide structures for a separate post-process TeO₂ thin film deposition, respectively. It is noted that the DUV lithography process used for our manufacturing has a minimum feature size (MFS) of 180 nm, which is comparatively large [43]. This MFS allows a minimum SWG period of 360 nm with a corresponding 50% duty cycle (Λ = 360 nm, δ = 0.5), which we used in all of our SWG structures. Figure 3(a) shows the overall schematic of our SWG waveguide test structure. A series of these straight SWG structures were fabricated with different SWG waveguide length (L_SWG) in order to measure loss using the cutback method. The SWG waveguide section is connected with two wire waveguide sections that vary in length between devices so that a constant device length of 3 mm (L_device = 3 mm) is maintained for all test structures. A standard wire waveguide width of 450 nm (W_wire = 450 nm) is used for single mode operation [44]. 50 µm-long mode converters (L_MC = 50 µm) leveraging the SWG waveguide engineering concept first proposed in [19] are used to connect SWG and wire waveguide sections to gradually transform the mode along the device. The silicon wire taper in the mode converter ends with a width of 180 nm, due to MFS restrictions of the fabrication, before joining the SWG region. A series of 450 nm-wide wire waveguides were also fabricated in order to determine loss of the wire sections. Paperclip structures with 50 µm bend radius are used to obtain different Si-wire waveguide lengths required for loss measurements. All waveguide structures have 75 µm-long tapers (L_T = 75 µm) from the wire waveguide sections up to the chip edge where the waveguide width is reduced to 180 nm (MFS limit), in order to enhance the fiber-coupling efficiency.

 

Fig. 3. (a) Top view schematic of fabricated SWG device with silicon wire (solid) and SWG (striped) waveguides sections. Dark dotted lines indicate the locations of the SEM images of the fabricated b) uncoated and c) 105 nm-thick TeO₂-coated mode converter-SWG transitions, d) TeO₂-coated SWG waveguide (side view), and e) uncoated and f) TeO₂-coated SWG waveguides (angled view). Image in d) was taken 52° to the surface normal while all others were taken 45° to the surface normal.

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The wafer was diced into chips following wafer-scale fabrication. Figures 3(b) and 3(e) show SEM images of the uncladded mode converter and SWG waveguide structures on the chip, respectively. We deposited a 101 ± 5 nm-thick amorphous tellurium oxide (TeO₂) thin film using a radio frequency (RF) reactive magnetron sputtering process [35,45]. Figures 3(c) and 3(f) depict the structures mentioned above following the TeO₂ thin film deposition. This backend deposition procedure enables room temperature deposition at a chip-scale that yields low loss TeO₂ thin films compatible with active silicon photonic devices fabricated in CMOS foundries [46–47]. With the given pitch and duty cycle available for our SWG device, a thin layer of approximately 100 nm is required in order to assure an appropriate Bloch mode effective index. This also helps minimize air gap formation between the silicon segments caused by nucleation of the film on top of the gratings. This is particularly evident in gratings with increased duty cycles or reduced pitches.

4. Experimental results

We measured the optical propagation loss of the fabricated structures by fiber edge coupling to a set of test structures with different propagation lengths. A tunable laser source was coupled into the chip via lensed fiber with a 2.5 ± 0.5 µm mode field diameter. The input polarization was controlled using polarization paddles at the input of the chip and the transmitted signal at the output was measured using an InGaAs photodetector. Figure 4(a) shows the measured insertion loss of the waveguide with 3 µm-thick SiO₂ cladding and 100 nm-thick TeO₂ cladding over a 1460–1640 nm wavelength range. Measurements were referenced to a 450 nm-wide wire waveguide structure equivalent to the wire waveguide sections on the SWG device to account only for the loss of the SWG waveguides of varying lengths (L_SWG) and the two 50 µm- long mode converters connecting them to the wire waveguide sections. A Savitsky-Golay smoothing filter was applied to the overall spectral data to reduce Fabry-Perot ripple caused by facet reflectivity. Each spectrum displays a bandgap with high reflectivity surrounded by regions of high transmission, as expected. The misalignment of the spectra between the different claddings is caused by differences in Bloch mode effective index, where the SiO₂-clad spectra have a lower wavelength bandgap than the TeO₂-clad spectra, i.e. a lower Bragg wavelength, as expected. The position of the TeO₂-clad waveguide spectra is significantly redshifted in comparison to the simulation results with a Bragg wavelength of ∼ 1530 nm instead of 1465 nm. From the SEM images in Fig. 3, we observe rounded waveguide grating structures that result from limited resolution of the DUV lithographic patterning process. The round shapes are emphasized following the deposition of the TeO₂ film, which displays excellent uniformity.

 

Fig. 4. (a) Measured insertion loss of SWG waveguides with 3 µm-thick SiO₂ cladding and 100 nm-thick TeO₂ cladding, with W_SWG = 450 nm, Λ = 360 nm and δ = 0.5 and varying SWG waveguide lengths (L_SWG) indicated in legend. (b) Measured propagation loss at λ = 1630 nm and λ = 1480 nm for the SiO₂-clad and TeO₂-clad waveguides, respectively.

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The areas of high transmission correspond to the second- and first-order bands below 1480 nm and above 1600 nm, respectively. Fabry-Perot resonance patterns starting at a wavelength of 1540 nm are noted with a gradual ascent in transmission towards higher wavelengths, making it difficult to determine the exact location of the band edge. These oscillations likely arise from the mode converters approaching Bragg resonance near the junction with the silicon wire waveguide. The Bloch mode effective index is significantly raised near the silicon wire taper and any rounding of the described structures caused by the lithographic patterning might leave behind excess silicon, thus contributing to the large redshift in the Bragg wavelength and overall dispersion relation. Bragg wavelengths of 1560 nm and 1670 nm are estimated in the mode converter at the SWG end [Fig. 3(c)] and the wire waveguide end, respectively. This shift affects the transmission spectrum over the wavelength range up to 1670 nm, decreasing transmission and introducing Fabry-Perot fringes. This effect can be readily mitigated by chirping the period along the mode converter to reduce the Bloch mode effective index near the SWG-wire junctions. However, we could not use this technique in our current design due to the comparatively large minimum feature size of the DUV patterning process.

We analyzed losses in the S-band for the TeO₂-clad waveguide and the far end of the L-band for the SiO₂-clad waveguide since Fabry-Perot fringes are minimal in these regions. The propagation losses are fitted using the cut-back method and linear regression, as seen in Fig. 4(b), and are reported to be 1.0 ± 0.1 dB/mm at λ = 1480 nm for the TeO₂-clad SWG waveguide and 1.5 ± 0.1 dB/mm at λ = 1630 nm for the SiO₂-clad SWG waveguide. We attribute the loss primarily to the scattering loss of the silicon waveguide structure due to limited resolution of the lithographic process used. The TeO₂ coating has been shown to have low planar waveguide propagation loss on the order of 0.1 dB/cm [45]. According to waveguide scattering theory [48], loss scales with a factor of (n₁²-n₂²)², where n₁ and n₂ are the material indices of the waveguide core and cladding, respectively. Based on this factor, the reduced index contrast of Si-TeO₂ compared to Si-SiO₂ is expected to reduce the scattering losses by approximately 40% for equal mode overlap with the interface, which is in good agreement with the observed loss reduction. The fiber-chip coupling loss was also extracted from these measurements, with 4.8 ± 0.2 dB at λ = 1480 nm for the TeO₂-clad waveguide and 2.5 ± 0.1 dB at λ = 1630 nm for the SiO₂-clad waveguide. Higher coupling losses are expected for the TeO₂-clad devices due to the higher modal index of the waveguide at the chip edge lowering the overlap with the fiber mode.

5. Summary

In summary, we have demonstrated a new type of subwavelength grating waveguide on an SOI platform coated with tellurium oxide. Measured propagation loss and fiber-chip coupling loss of 1.0 ± 0.1 dB/mm and 4.8 ± 0.2 dB are reported. Our future design of TeO₂-clad SOI SWG waveguides will aim to reduce this propagation loss by utilizing a writing process with finer resolution, such as electron-beam or immersion deep-ultraviolet lithography. Together, the benefits of both SWG waveguides and a high index cladding such as TeO₂ that can serve as an efficient host for active elements such as erbium ions will likely open new avenues for compact on-chip active devices in SOI photonic integrated circuits.