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Abstract
Integrated optical filters show outstanding capability in integrated reconfigurable photonic applications, including wavelength division multiplexing (WDM), programmable photonic processors, and on-chip quantum photonic networks. Present schemes for reconfigurable filters either have a large footprint or suffer from high static power consumption, hindering the development of reconfigurable photonic integrated systems. Here, a reconfigurable hybrid Bragg grating filter is elaborately designed through a precise, modified coupling mode theory. It is also experimentally presented by integrating non-volatile phase change material (PCM) Sb2Se3 on silicon to realize compact, low-loss, and broadband engineering operations. The fabricated filter holds a compact footprint of 0.5 µm × 43.5 µm and maintains a low insertion loss of < 0.5 dB after multiple levels of engineering to achieve crystallization. The filter is able to switch from a low-loss transmission state to the Bragg reflection state, making it a favorable solution for large-scale reconfigurable photonic circuits. With a switching extinction ratio over 30 dB at 1504.85 nm, this hybrid filter breaks the tradeoff between insertion loss and tuning range. These results reveal its potential as a new candidate for a basic element in large-scale non-volatile reconfigurable systems.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Reconfigurable photonic integrated circuits (PICs) have recently gained momentum to satisfy multiple applications such as optical communication [1,2], quantum computing [3], and neuromorphic systems [4]. Large-scale programmable photonics integrated systems (LSPPISs), including neuromorphic computing [5] and optical field programmable gate arrays (OFPGA) [6] increase urgent demands of compact, low-loss, non-volatile features for fundamental reconfigurable components. The integrated photonic filter represents a fundamental component of the reconfigurable PIC. Over the past decade, optical filtering has been achieved by arrayed-waveguide gratings (AWGs) [7–9], diffraction gratings [10–12], Mach-Zehnder interferometer (MZI) [13,14], micro-rings resonators [15–18], and Bragg gratings [19]. The AWG, diffraction grating, and MZI are widely used in multiple-channels wavelength division multiplexers (WDMs), but their large footprint limits the applications in high-density programmable integrated photonic systems [20]. Although the micro-ring resonator has been used in WDM-based optical computing, including spiking neural networks [21], convolution core [22], and neuromorphic computing [23], the limited free spectral range (FSR) of the micro-ring resonator degrades its capability of channel selection, and the process-dependent resonance peaks of the micro-ring resonator hinder their scalability. In contrast, owing to its intrinsic FSR-free characteristics, the Bragg grating has the potential to significantly enhance the bandwidth for optical communication [19] and sensing applications [24–26]. By incorporating either a Fabry–Pérot (F-P) cavity scheme [27] or a subwavelength grating segment [28], the miniaturized Bragg grating can achieve narrowband filtering, leading to enhanced spectral efficiency. This makes it particularly useful for advanced LSPPISs [29,30].
On the other hand, silicon photonics possesses several distinct properties, such as high refractive index contrast and excellent compatibility with the complementary metal oxide semiconductor (CMOS) processes, rendering it appropriate for reconfigurable integrated filters (RIFs) and further LSPPICs. Present RIF mainly relies on the electro-optic (EO) or thermo-optic (TO) effects of silicon [31–33]. Nevertheless, both of them, require a continuous power supply to maintain the state of RIF, leading to high-power consumption. Additionally, due to their weak refractive index modulation, a considerable footprint is required to realize a broad tuning range, thereby hindering the scalability of LSPPICs. As such, there is a need for a compact, energy-efficient RIF.
Due to their large refractive index contract and stable, non-volatile crystallization state, many phase change materials (PCMs) have recently been used in reconfigurable devices. The advantages of low power consumption and compact footprint have been fully compared with other tuning mechanisms in previous works [32]. Some conventional PCMs, especially GST, based on CMOS-compatible fabrication processes have been made efforts in the last few years [34]. However, the significantly increased extinction coefficient of GST during crystallization hinders its application in pure phase modulation. Even though the low-loss PCMs like GSST family have been studied both in theoretical and experimental works [35–37], absorption in the crystalline phase is still non-negligible. Recently, a new family of low-loss PCMs (Sb2S3, Sb2Se3) was discovered [38], and their reconfigurable features have been explored thermally, electrically, and optically [39–43]. We have also experimentally demonstrated a $\pi $ phase shift with 9.3-µm length PCM and propagation loss of 0.036 dB/µm with crystal Sb2Se3 on a silicon waveguide [44]. By combining silicon-on-insulator (SOI) and low-loss, zero stable power consumption non-volatile PCM, the Silicon-PCM hybrid platform performs properties of both strong spectra tuning ability and CMOS-compatible fabrication process. However, the investigation of compact, low-loss PCM-assisted on-chip filters remains insufficient.
In this work, we propose a compact Silicon-PCM hybrid filter based on Bragg grating. Thanks to the ultra-low-loss non-volatile phase changing and large contrast between amorphous and crystalline states of the PCM, such Silicon-Sb2Se3 hybrid Bragg filter breaks the tradeoffs between insertion loss and modulation depth, which is difficult for traditional filters based on high-loss PCM. To meet the requirements of large-scale ASICs, the existing theoretical model was improved to provide more reliable and universal projections of filtering characteristics rapidly. This method was employed to accurately predict the multiple distinct non-volatile states. And the experimental results of the photonic crystal bandgap trimming exhibit a high-level consistency with the predictions. By introducing optical or electrical triggered phase transition of Sb2Se3, it is promising to realize on-chip multi-level reconfigurable miniaturized optical filters. Considering the rigorous demands of loss, tuneability, scalability, and filtering performance, this hybrid filter presents a favorable candidate in reconfigurable LSPPISs.
2. Device design and operation principle
To promote the large-scale application of the low-loss RIF in LSPPISs, a scheme of the design procedure and predictions of their key operation performances are preferred. Figure 1(a) shows the diagram image of the RIF, consisting of a Bragg grating formed by the periodic arrangement of Sb2Se3 elements and a silicon waveguide. The cross-section and the top view of the RIF are shown in Fig. 1(b). The silicon waveguide was etched in the depth of ${H_{etch}}$ (130 nm) and the width of ${W_{wg}}$ (500 nm). The width and thickness of the Sb2Se3 are ${W_{\textrm{pcm}}}$, and ${H_{pcm}}$, respectively. The small extinction coefficient (k) of our developed Sb2Se3 could be neglected, and the refractive index contrast before and after the phase transition at 1550 nm is as large as 0.823, which is consistent with previous works [38,44] as shown in Fig. 1(c).
Fig. 1. (a) Schematic image of RIF with low-loss phase change material Sb2Se3 on top of the waveguide. (b) Cross-section image of the hybrid SOI-Sb2Se3 waveguide and top view of the hybrid waveguide. (c) Complex refractive index of amorphous Sb2Se3 and crystallized Sb2Se3.
To develop this RIF, the Bragg law is commonly applied. When the wavelength is much greater than the central wavelength of Bragg grating λB, low-loss transmission can be achieved, which is regarded as an equivalent medium, while the wavelength approaching λB will be reflected. The Bragg wavelength λB of the Bragg filter is obtained through the modal analysis and follows the formula:
where $\mathrm{\Lambda }$ is the period of the grating and ${n_{eff}}$ is the effective refractive index of the Bragg grating. ${n_{eff}}$ can be obtained by: where $FF$ is the duty cycle of the grating, ${n_{SOI - SbSe}}$, and ${n_{SOI}}$ are the effective refractive index of the waveguide with and without the phase change material, respectively.Eigenmode Expansion Method (EME) is commonly implemented for designing periodic devices, providing accurate results with much lower computational cost than the brute-force Finite Difference Time Domain (FDTD) algorithm for solving Maxwell’s equations (Fig. 2(b) and (d)). Another popular method, the Coupling Mode Theory (CMT), can also provide qualitative predictions rapidly. Although both EME and CMT have a time complexity of O(n) when executing eigenmode computations of periodic elements, CMT is more computationally efficient, especially in the case of corrugation optimization. Using an Inter Core i5-9600 K, a single corrugation calculation took less than 15 seconds with CMT, and 75 seconds with the EME method. According to the CMT [45], the mode propagation in a Bragg grating could be considered as interference between forwarding and backward waves. The maximum reflectivity can be determined with the following formula [46]:
where R, $\kappa $ and L are the reflectivity at the Bragg wavelength, the coupling magnitude and the length of the Bragg grating. The bandwidth between the zero reflection points of the central lobe is given by:Fig. 2. (a) Simulated transmission spectrum by modified CMT for filters with different widths of amorphous Sb2Se3 (Hpcm = 60 nm). (b) Comparisons of the transmissions at the central wavelength and the bandwidth through EME, original CMT, and modified CMT for filters with different widths of amorphous Sb2Se3. (c) Simulated transmission spectrum by modified CMT for filters with different heights of amorphous Sb2Se3 (Wpcm = 30 nm). (d) Comparisons of the transmissions at the central wavelength and the bandwidth through EME, original CMT, and modified CMT for filters with different heights of amorphous Sb2Se3. (e) Effective index of the hybrid Silicon-PCM waveguide in various crystalline states (Inset: electric fields of the hybrid platform in the amorphous state (top) and crystal state (down)). (f) Electrical fields of the hybrid filter along the propagation direction at z = 110 nm with PCM in amorphous (top) and crystalline (bottom) states.
Compared with EME, original CMT allows rapid analysis quantitatively without requiring eigenmode expansion calculations but still maintaining reasonable accuracy, especially in cases where the perturbation is not sufficiently weak. Our study used different methods to compare the normalized transmission of the filters at the central wavelength and their bandwidths. The larger geometry of Sb2Se3 results in increased effective index contraction, which leads to greater inaccuracies, as illustrated in Fig. 2(b) and(d) and previously published work [47]. The perturbation affecting the field distributions of the hybrid waveguides can be corrected by introducing a confinement factor. With this universal modification, more accurate $\kappa $ could be calculated for the predictions of Bragg gratings:
where $\varDelta {n_{eff}} = {n_{SOI - SbSe}} - {n_{SOI}}$, and $\mathrm{\Gamma } = \textrm{(}{\mathrm{\Gamma }_{SOI}} - {\mathrm{\Gamma }_{PCM}})/({{\mathrm{\Gamma }_{SOI}} + {\mathrm{\Gamma }_{PCM}}} )$ . ${\mathrm{\Gamma }_{SOI}}$ and ${\mathrm{\Gamma }_{PCM}}$ are the confinement factor of silicon waveguide and PCM element.As shown in Fig. 2(a-d), the influence of ${W_{\textrm{pcm}}}$, and ${H_{\textrm{pcm}}}$ on the transmission at the central wavelength and the bandwidth of Bragg grating (period = 290 nm, L = 43.5 µm) has been provided by this CMT(M) method, respectively. Comparing with EME method, the deviation of CMT(M) prediction is smaller than that of the original CMT. The bandwidth error can be less than 1 nm, and the extinction ratio error is less than 0.08 dB. Considering the fabrication and filter performance, ${W_{\textrm{pcm}}}$ of 300 nm, ${H_{\textrm{pcm}}}$ of 60 nm, and a duty cycle of 0.5 has been fixed. The effective index of the RIF with these structure parameters has been shown in Fig. 2(e). Obviously, the electric field confinement in PCM of the hybrid waveguide increases with crystallization, as shown in the insets of the figure. Also, the electrical fields of the hybrid filter along the propagation direction with PCM in both amorphous and crystalline states are simulated by 3D-FDTD (Fig. 2(f)). As shown in the figure at the bottom, when the RIF has crystalline PCMs on top, most of the power is reflected backward, whereas the hybrid RIF with amorphous PCM works as a low-loss subwavelength grating waveguide. Thus, a reconfigurable transition between complete reflection and low-loss propagation could be achieved.
3. Fabrication and results
The devices were fabricated using a user-defined multi-project wafer (MPW) run in order to determine the low-loss filter performance and potential feasibility of this RIF in large-scale reconfigurable systems. It was provided by the Institute of Microelectronics of the Chinese Academy of Sciences (IMECAS) with a homemade low-loss phase-change material thin-film process. Firstly, the rib waveguide was patterned with deep ultra-violet (DUV) lithography followed by inductive coupling plasma (ICP) etching process. Secondly, the nanostructure of Bragg grating was patterned by electron beam lithography (EBL, Raith Voyager) with a positive tone photoresist (PMMA A4). Thirdly, using a lift-off process, a 60-nm Sb2Se3 was deposited on top of the waveguide by thermal evaporation (PVD-II). The process and the fabricated FIR’s SEM image are shown in Fig. 3(a) and (b), respectively. The fabricated devices were characterized by a broadband tunable laser system (Santec full-band TSL-550) with an output power of 0 dBm. The resolution of the measured spectra is 1 pm. The measured spectra were normalized by subtracting the spectrum of a straight waveguide with surface grating couplers on the same chip.
Fig. 3. (a) Schematic overview of the Sb2Se3 RIF fabrication process. (b) SEM image of the hybrid Bragg grating waveguide. (c) Experimental (solid) and simulation (dash) transmission spectra of RIFs with different periods (Number of periods: 200). (d) Experimental (solid) and simulation (dash) transmission spectra of RIFs with different numbers of PCM periods (Period: 290 nm). (e) Experimental (solid) and simulation (dash) transmission spectra of RIFs after annealing at different temperatures. (f) Predicted filtering performances (Line) and fitted experimental result (Scatter: Central wavelength (Circles); Zeros of bandwidth (Crosses)) through CMT (M) (Period: 290 nm, Length: 43.5 µm).
Various periods will cause various central wavelengths of the filter, so multiple devices with different periods have been fabricated. Low-loss and reconfigurability experimental performances have been evaluated (solid line) and compared with corresponding simulation results (dash line). The normalized test and simulation spectra of RIFs with periods of 270 nm, 280 nm, and 290 nm are shown in Fig. 3(c). The ripples in the figures mainly comes from the reflections between the grating couplers. Overall, the filtering performance of RIFs is consistent well with the simulation. Also, the extinction ratio is another important parameter and influenced by the numbers of PCM units. The RIFs with 100, 150, and 200 PCM units are experimentally characterized, and the extinction ratio of the filters increased with the PCM units, shown in Fig. 3(d), which agreed well with the numerical prediction. As the prediction of CMT(M), over 30 dB “on-off” switching ratio could be easily achived with the 43 µm RIF in Fig. 3(e). Therefore, this device was selected for the following research.
The bandgap engineering property of the filter was studied by thermal annealing on a hotplate in the glove box. With dry nitrogen filled inside, Sb2Se3 was isolated from water and oxygen during annealing. Since no phase transition is observed under 453 K, the amorphous PCM on the waveguide was annealed separately at 453 K, 463 K, and 473 K for 10 mins. The fully crystalline PCM after 473 K annealing has been confirmed by further annealing. The spectra of the RIF have been traced after each annealing as shown in Fig. 3(e). Before annealing, a low insertion loss was obtained at the wavelength of 1505.26 nm, while the switching extinction ratio and the bandwidth were up to 31.44 dB and 23 nm, respectively. And the wavelength outside the bandgap remains low-loss propagation (−0.32 dB) due to the low extinction coefficient of crystal Sb2Se3. With the help of the transmission matrix method (TMM), the filtering performances of the RIFs after annealing at 453 K and 463 K are consistent with the predicted RIFs with crystallization fractions of 5% and 45% in CMT(M) results, respectively (Fig. 3(f)). The spectra of RIFs with intermedia crystalline PCMs (Fig. 3(e) (dashed line)) has been compared with the experimental results and CMT(M) has provided an accurate prediction for Bragg grating design. The properties of intermedia phases of simulation results are evaluated by the approach mentioned in the paper [48]. Due to the different material densities of the phases, the shrinkages during crystallization are considered. 30% reduction of the film thickness in the crystalline state has been verified by ellipsometry and linearly corrected in simulation. Since a limited extinction ratio will increase the difficulty of electrical or optical pulse distinguishing [49], this work’s high “On-Off” switching contrast enables more switching levels. More refined temperature gradients are expected to be manipulated with more levels of polymorphic regulation in the future. With elaborate theoretical research on the reconfigurable filter, its characteristics of low-loss, reconfigurable tunable bandwidth, and extinction ratio have been verified experimentally, which increases the potential of Bragg gating in large-scale reconfigurable PICs.
4. Conclusion
In this paper, we present a new Bragg filter based on low-loss PCM, which is one of the desirable elements for compact, low-loss and low-power consumption large-scale reconfigurable photonic circuits. Dynamic multi-level regulation of center wavelength, bandwidth, and extinction ratio can be realized stably with less sacrifice in optical and electrical performance by tuning the polymorphism of the material. To promote the application of RIF, a correction of the theory has been provided for rapid design, corrugation optimization, and polymorphism prediction. Currently, with this precise design protocol, the low-loss, high extinction ratio filtering was realized, enabling broad tuning. To the author’s knowledge, this is the first time a low-loss non-volatile Bragg filter has been demonstrated experimentally. Its advantages provide more possibilities for the rapidly developing on-chip large-scale and power-friendly reconfigurable PIC systems and open a pathway for more complex functions of LSPPISs.
Funding
National Key Research and Development Program of China (2021YFB2801300); National Natural Science Foundation of China (61975179, 62105287, 91950204); Natural Science Foundation of Zhejiang Province (LD22F040002); Fundamental Research Funds for the Central Universities (2021QNA5007).
Acknowledgments
We thank Westlake Center Micro/Nano Fabrication, Instrumentation and Service Center for Physical Sciences at Westlake University and ZJU Micro-Nano Fabrication Center at Zhejiang University for the facility support. We would also like to thank Liming Shan for his help in the deposition of Sb2Se3 films.
Disclosures
The authors declare no conflicts of interest.
Data availability
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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