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Abstract
In this paper, we present the design of a compact reconfigurable polarization beam splitter (PBS) enabled by ultralow-loss phase-changing Sb2Se3. By harnessing the phase-change-mediated mode coupling in a directional coupler (DC), guided light with different polarizations could be routed into different paths and this routing could be dynamically switched upon the phase change of Sb2Se3. With an optimized DC region, the proposed PBS demonstrates efficient polarization splitting with crosstalk less than −21.3 dB and insertion loss less than 0.16 dB at 1550 nm for both phase states of Sb2Se3, and features energy efficient property benefitting from the nonvolatile phase change of Sb2Se3, which holds great potentials for on-chip applications involving polarization control, including polarization-division multiplexing system, quantum photonics, microwave photonics, etc.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Silicon photonics has gained significant research interest owing to its intriguing features of high-density integration, CMOS compatibility, and scalability, opening up promising prospects for applications spanning optical computing to quantum information processing. Highly birefringent planar silicon waveguides showcase excellent polarization-maintaining ability, effectively mitigating the impact of polarization state randomness encountered in optical fibers. Polarization-division multiplexing (PDM), alongside wavelength- and mode-division multiplexing technologies, presents an appealing and cost-effective approach to enhance the on-chip processing capacity. Polarization beam splitter (PBS), separating light with orthogonal polarizations into different output routes, is a key component in on-chip polarization control for PDM systems, finding extensive employment in diverse photonic technologies including microwave photonics [1], coherent optical transceivers [2] and quantum photonics [3], etc. PBS has been realized based on various configurations such as directional couplers [4–12], Mach-Zehnder interferometer [13–16], subwavelength/Bragg gratings [17–20], and multimode interferometers [21]. However, a significant drawback of these PBS implementations is their inherent lack of reconfigurability after fabrication, while a PBS with reconfigurability is essential for simultaneous routing and switching different polarized modes into desired paths in a single component, which provide a flexible way for polarization handling in photonic applications [22], such as on-chip optical processing of polarization-encoded signals in the classical [23] or quantum regime [24], and polarization-multiplexed optical phased array [25], etc.
Reconfigurable photonics usually relies on dynamic control of the refractive index of waveguide medium through carrier injection, thermal tuning, or electro-optic effects. A recent study by Yin et al. demonstrated a reconfigurable PBS using thermal-optics effect of silicon and the output path of TE and TM modes could be exchanged with a switching ratio exceeding 12 dB [26]. However, due to the small thermo-optical coefficients of silicon (1.8 × 10−4 K−1 [27]), the proposed device requires a lengthy waveguide of 516.8 µm to achieve the efficient polarization splitting. Moreover, when densely packed on the same chip, thermal crosstalk could pose a challenge, limiting the overall on-chip system size.
To overcome these limitations, integration of functional materials with large refractive index change has been explored, with chalcogenide phase-change materials (PCMs) emerging as promising candidates. For instance, Ge2Sb2Te5 (GST) exhibits a refractive index contrast of up to 2.6 at 1550 nm [28]. The nonvolatile phase change between crystalline and amorphous states in PCM materials requires energy only during the phase change, offering energy-efficient photonic device construction in contrast to thermal-optic or electro-optic effects. Moreover, chalcogenide PCMs have a low processing temperature, and can be heterogenous integrated on existing photonic platforms without the requirement for lattice matching, which are backend-compatible with CMOS fabrication processes [29–31]. In addition, PCMs such as GST have found applications in electronic memory systems like the 3D XPoint memory architecture, demonstrating their potential for high-volume manufacturing in reconfigurable silicon photonics. Recently, Zhang et al. reported that the GST-based reconfigurable polarization beam splitter [32]. However, GST suffers from significant absorption losses in the C-band of communication, particularly in its crystalline phase. A newly developed two-element PCM, Sb2Se3/Sb2S3, has garnered attention due to its ultra-low loss in the telecommunication band, characterized by an extinction coefficient (k) of less than 1 × 10−5 [33]. The phase transition in Sb2Se3/Sb2S3 can be triggered optically or electrically [34]. These materials have found applications in optical switches [35,36], Bragg filters [37,38], and reconfigurable directional couplers [39].
Here, we theoretically investigate a reconfigurable on-chip PBS enabled by the ultralow loss PCM Sb2Se3. By leveraging the phase change-induced refractive index modulation of the Sb2Se3 integrated on the waveguide top, we can effectively control the coupling behavior of TE and TM modes in a directional coupler (DC) consisting of a pair of single-mode waveguides. Through careful design of the waveguide cross-section, we achieve efficient splitting of different polarizations at the output, and the output path for TE and TM polarizations can be switched by manipulating the phase of the Sb2Se3 layer. Furthermore, we analyze the impact of the bending radius of the input/output waveguides on bending loss as well as mode coupling to enhance the performance of the proposed PBS while maintaining its compact footprint. To minimize insertion loss, we introduce a tapered PCM structure that reduces scattering losses caused by the PCM layer. The proposed reconfigurable PBS design exhibits low cross-talk less than −21.3 dB and low insertion loss less than 0.16 dB with a 41.6 µm-long DC structure, which hold great potentials for on-chip polarization control.
2. Principle and design
Figure 1(a) illustrates the schematic diagram of our proposed reconfigurable PBS, which is constructed using partially-etched ridge waveguides that are fabricated on a silicon-on-insulator (SOI) wafer with a 250-nm-thick silicon top layer. The refractive indices of Si and SiO2 are 3.477 and 1.444 at 1.55 µm, respectively. The PBS consists of a directional coupler (DC) structure with Sb2Se3 layer which is connected by four 90-degree bent waveguides as the input and output ports. The DC structure serves as the core component of the reconfigurable PBS, responsible for routing TE and TM polarizations into different output ports and switching the output paths of two polarizations upon the phase change of Sb2Se3.
Fig. 1. (a) Schematic diagram of the reconfigurable PBS. (b) Cross-section view of the DC region. (c) Top view of the reconfigurable PBS.
Figure 1(b) provides a cross-sectional view of the DC region. The etch depth to form the ridge waveguides is 200 nm, and the width of the ridge waveguide is w, the separation between the two parallel waveguides is wg. The phase-change material Sb2Se3 is deposited on top of ridge waveguides with a thickness of hs. The length of the DC region is L, and the bending radius of the input/output waveguides is r. Notably, there is a tapered Sb2Se3 transition region with a length of lt, as depicted in Fig. 1(c). A PIN-based microheater can be implemented in the 50 nm-thick pedestal layer [36]. The PIN junction is formed through ion implantation, where heavily doped regions of boron and phosphorus are introduced to create the p-type, intrinsic, and n-type regions [40]. By applying an appropriate electrical current to the PIN junction, the microheater generates localized heating, triggering the phase transition of the Sb2Se3 [34,41]. At the telecommunication wavelength of 1550 nm, the refractive indices of Sb2Se3 in the amorphous and crystalline states are 3.285 and 4.050, respectively [33]. The device fabrication can be achieved through a standard nanofabrication process [34,42] (see Supplement 1 for detail). The entire device is covered with an upper layer of silicon dioxide to improve the long-term stability of the phase change material [43].
We first demonstrate the Sb2Se3-mediated mode coupling in the DC region. Figure 2 shows the mode profiles in the coupled ridge waveguides at 1550 nm. The waveguide width w is 410 nm to ensure each waveguide works in the single mode regime. The coupling of single-mode waveguides leads to the hybridization of the fundamental TE and TM modes, which forms two kinds of modes with opposite symmetries. The symmetric modes have larger effective mode indices, which is defined as neff = β/k0 with β as the propagation constant and k0 as the free space wavevector, and the antisymmetric modes have lower ones. The calculated effective mode indices for each hybridized TE and TM modes are labeled in Fig. 2. Here, waveguide separation wg is 120 nm, and the thickness of Sb2Se3 hs is 60 nm. In the following discussion, these TE and TM modes are labeled with subscripts “s” and “a” representing the symmetric and anti-symmetric field profile. The difference in neff’s between the symmetric and antisymmetric modes lead to different phase velocities. As the guided modes with certain polarization propagate through the DC structure, whenever the accumulated propagation phase difference between two modes undergoes a change of π, the power distribution in the DC cross-section would flip between two waveguides, resulting in a periodic power coupling effect which could be characterized by a coupling length defined as
Fig. 2. The transverse electric field distributions and effective indices (neff) of the TEs and TEa (left), and TMs and TMa (right) modes of DC based on (a) a-Sb2Se3 and (b) c-Sb2Se3 at 1550 nm.
Realization of a compact reconfigurable PBS requires 1) large Lc contrast between two phase states of Sb2Se3 for single polarization, which ensures switching between output ports with a minimal DC length L, and 2) large Lc contrast between TE and TM polarizations, which facilitate an efficient polarization splitting with a minimal DC length L.
Figure 3(a) shows the Lc contrast between Crystalline (Cr) and Amorphous (Am) states ΔLc as a function of the ratio k, which is defined as the ratio of the width of the Sb2Se3 to that of the silicon waveguide. Here, we set silicon waveguide width W as 410 nm. For TE polarization, the ΔLc monotonously increase with k, while for TM polarization, ΔLc reaches the maximum at k = 0.62, and slightly falls with k further increasing to 1. As for the contrast of Lc between TE and TM polarizations, ΔLc has the maximum value for both phase states of Sb2Se3, as depicted in Fig. 3(b). Therefore, we choose the width of Sb2Se3 is the same as the width of the waveguide to ensure Lc contrast between TE and TM polarizations and between amorphous and crystalline state of Sb2Se3 both have a large value.
Fig. 3. (a) Lc contrast between two phase states of Sb2Se3 for single polarization and (b) Lc contrast between TE and TM polarizations as a function of the width ratio k between the Sb2Se3 layer and the silicon waveguide. (c) Lc contrast between TE and TM polarizations as a function of the separation wg of the coupled waveguides. (d) The coupling length for TE and TM polarization as a function of the waveguides separation wg. Solid and dashed curves correspond to Sb2Se3 in its amorphous and crystalline states.
The Lc contrast between TE and TM polarizations is also dependent on the waveguide separation wg, as depicted in Fig. 3(c), ΔLc increases with wg for both phase state of Sb2Se3, which is preferred for efficient switching with a minimal DC length L. The compactness of the device is also directly related to the coupling length Lc, and Fig. 3(d) shows that the coupling length Lc increases with wg. Therefore, we choose a moderate value of wg within the range of 100 nm to 150 nm in our following discussion to guarantee the compactness of the PBS design.
To achieve reconfigurable polarization beam splitting, the length of DC region L should be carefully chosen to simultaneously satisfy two conditions: 1) For Sb2Se3 in its amorphous state, L and Lc’s should have the relation as Eq. (2),
2) When Sb2Se3 experiences a phase transition into the crystalline state, the relation between L and Lc’s would be described as Eq. (3), where p and q are integers with opposite parity and m is an odd number. The underlying physics for these integer numbers is that, when the light is injected into the one waveguide in the DC region from one input port, light energy would totally couple into another waveguide after the guided mode propagates at a distance of Lc, and couple back into the original waveguide after another distance of Lc. Therefore, if the length of DC region L equals an even number times of Lc, the light energy will output from the same waveguide as the input port. If L equals to an odd number times of Lc, the light energy will output from the neighboring waveguide. The splitting of guided modes with different polarizations requires L should equals nLc with the integer n being opposite parity for TE and TM modes. While the reconfigurability of PBS requires n change its parity upon the phase change of Sb2Se3 for TE and TM modes, so as to exchange the output of different polarizations.Next, we aim to find the optimal DC waveguide cross-section to obtain the shortest L satisfying Eq. (2,3). We calculated effective mode indices of TE and TM modes with amorphous and crystalline phase of Sb2Se3, and obtained the corresponding Lc’s as a function of the waveguide width W and the thickness of Sb2Se3 layer hs, as depicted in Fig. 4. Here, we choose the waveguide separation wg = 120 nm, which yields the most favorable result in our following discussions with wg from 100 to 150 nm. The smaller difference between p and q enables a shorter L. Therefore, we set q = p - 1, indicating the crystallization of the Sb2Se3 layer induces one more coupling in the DC region. We first select the DC cross-section parameters W and hs rendering the coupling lengths Lc’s of TE and TM modes satisfying Eq. (2) with the integer p ranging from 1 to 9 and m equaling to 3, 5, and 7, which are circled out in the Fig. 4(a) and (b). A DC region with the circled values of W and hs as well as the corresponding L would act as a PBS with Sb2Se3 in its amorphous state. Next, we select the W and hs values to obtain Lc’s satisfying Eq. (3) with Sb2Se3 in its crystalline state and the same q and m values as Fig. 4(a), which are circled out in Fig. 4(c) and (d), indicating a DC with c-Sb2Se3 layer would split TE and TM mode from the opposite output ports upon phase change of Sb2Se3. A reconfigurable PBS requires LAm = LCr, and we find that when w = 410 nm and hs = 60 nm, the coupling lengths Lc’s of TE and TM modes with Sb2Se3 in its both phase states would best satisfy the condition required by Eq. (2) and (3) where the total length of DC region L = 43.45 µm, as depicted in Fig. 4(e). In such case, differently polarized modes are split at the DC end and switched between the output port upon phase change.
Fig. 4. The coupling lengths of the TE mode with (a) amorphous and (c) crystalline states, TM mode with (b) amorphous and (d) crystalline states of Sb2Se3 as a function of the w and hs. The colored circles represent structure parameters rendering a best approximate Lc’s satisfying the relationship in Eq. (2) and (3) with m = 3, 5, and 7, p and q being different integers values ranging from 1 to 9. (e) The dependence of the DC length L on the coupling times X for TE (blue) and TM (red) polarizations with Sb2Se3 in the amorphous (dashed) and crystalline (solid) states, and the dotted line indicate the optimal L equaling to 43.45 µm.
The 90° bent waveguide section serves as the interface between the external injected guided modes and the functional DC coupler, and the choice of bend radius has a significant impact on device performance. The radius r of the 90° bent waveguide should satisfy two requirements: 1) r should be as small as possible to reduce coupling between adjacent bent waveguides before guided modes entering and exiting the DC coupler and to improve compactness of the device, and 2) r should be large enough to reduce bending losses. As shown in Fig. 5(a) and (b), we carry out a 3D finite-difference time-domain simulation to calculate the coupling ratio between a pair of symmetric silicon 90° bent waveguides and the associated bending loss as a function of bending radius r. We could observe that a significant bending loss would occur when the bend radius r is less than 5 µm. The increasing r would lower the bending loss, while the coupling between bent waveguides increases. Therefore, in order to balance bending loss and coupling ratio, we choose r = 5 µm. Noted that the Sb2Se3 layer is amorphous in our calculation, and a crystalline Sb2Se3 layer would yield similar results.
Fig. 5. The bending loss of (a) TE and (b) TM polarization of the 90° bent waveguides and the mode coupling ratio between the bent waveguides pair as a function of the bending radius r.
To reduce scattering losses at the junction between Sb2Se3-covered and bare waveguide sections and ensure the coupling length free from evanescent wave scattering perturbation, we extend the Sb2Se3 layer into the bending region and introduce a tapered Sb2Se3 structure on the bending waveguides to reduce back reflection through a smooth transition of the guided modes near the junction. We calculate the transmittance through the junction as a function of the taper length lt, as depicted in Fig. 6. The transmittance increases with increasing lt length and stabilizes when lt is greater than 2 µm, and c-Sb2Se3 would introduce more scattering loss manifested by an overall lower transmittance, due to a larger refractive index compared with its amorphous counterpart. Also, TM mode is more sensitive to the change of lt. In our following discussion, we choose lt = 2 µm to reduce the influence of the junction while maintain the compact device size.
Fig. 6. The transmittance of (a) TE and (b) TM modes through the junction between Sb2Se3-covered and bare waveguide sections as a function of the length of the transition region lt.
3. Results and discussions
We employ a 3D FDTD simulation to validate our reconfigurable PBS design. Figures 7 shows the normalized electric field distribution at 1550 nm. The light is injected from the port on the up-left side. When the Sb2Se3 is in the amorphous state, the injected TE mode is coupled 5 times in the DC region and output from the cross port, as depicted in Fig. 7(a). While the inject mode is TM polarized, the propagating mode is coupled 8 times and output from the bar port. When the Sb2Se3 is phase changed into the crystalline state, the output states of the injected TE and TM mode are switched, with TE mode output from bar port after 4 times of coupling and TM mode output from cross port after 7 times coupling, as depicted in Fig. 7(b). Noting that the PIN microheater would effectively control the phase change of Sb2Se3 in the DC region, which determines the crosstalk performance. While temperature within the waveguide bending region may not high enough to induce complete phase changing, resulting in part of Sb2Se3 maintaining its initial (amorphous) state, which has a negligible impact on the insertion loss, see Section 1 of Supplement 1 for detail.
Fig. 7. Normalized electric field distribution of TE (upper) and TM (lower) polarization of the PBS with (a) a-Sb2Se3 and (b) c-Sb2Se3 at 1550 nm.
Figure 8 shows the transmission spectra of the output ports with Sb2Se3 in amorphous and crystalline states. Here we set the length of DC region L = 41.6 µm to ensure lower crosstalk for both phase state of Sb2Se3. The L is slightly deviated from the value obtained through eigenmode calculation, which is mainly due to the mode coupling near transition region from the bend waveguide to DC region. For a-Sb2Se3, the crosstalk and insertion loss of TE/TM modes are −34.8 dB/−30.3 dB and 0.03 dB/0.12 dB at 1550 nm, respectively. When Sb2Se3 is phase changed into crystalline state, the crosstalk and insertion loss of TE/TM modes are −25.3 dB/−21.3 dB and 0.04 dB/0.16 dB, respectively. Within the bandwidth from 1540 nm to 1560 nm, the overall insertion loss for TE mode is less than 0.27 dB, and the crosstalk is less than −14 dB, while the overall insertion loss for TM mode is less than 0.6 dB, and the crosstalk is less than −10 dB, suggesting the proposed reconfigurable PBS demonstrates a broadband operation capability. Our electro-thermal simulation shows that the PIN microheater driven by a 100 ns-duration electrical pulse with a voltage of 9.8 V could trigger the amorphization of Sb2Se3, and the energy consumption for amorphization is 191 nJ. While for the crystallization, an electric pulse with a duration of several microseconds is usually required to achieve complete crystallization [34,42], which would lead to a longer switching time and a higher power consumption (see Supplement 1 for detail).
Fig. 8. Transmission spectra of TE and TM polarization at bar and cross output ports. (a) TE injection, a-Sb2Se3. (b) TM injection, a-Sb2Se3. (c) TE injection, c-Sb2Se3. (d) TM injection, c-Sb2Se3.
It should be noted that PCM layers may experience thickness change during the phase change processes due to the atomic rearrangement. The reported thickness variation ratio of Sb-Se thin films is 5.4% [44], and such small thickness change would subtle influence the CT of the proposed PBS. Therefore, we did not consider phase change induced thickness variation in our design. We notice that thermally-evaporated Sb2Se3 is reported to have a 30% thickness reduction upon crystallization [38]. Large thickness change alters the effective mode indices of the guided modes as well, and using the design route in this paper could compensate the effect of thickness change of Sb2Se3 to maintain an optimal polarization splitting performance. In addition, the fabrication imperfections may also have a profound impact on the performances of the proposed PBS. We numerically analyzed the device’s tolerance to various types of fabrication inaccuracies in Supplement 1, aiming to provide a reference for future practical realizations.
Table 1 presents a performance comparison between our design and previously reported reconfigurable PBS. Our design leverages the ultra-low-loss properties of the phase-change material Sb2Se3, resulting in a low insertion loss of less than 0.16 dB at the target wavelength of 1550 nm. Although the optical contrast between the two states of Sb2Se3 is relatively small compared to GST [33], our device has a device length of ∼52 µm through a careful mode control to manipulate the coupling behavior of TE and TM modes, which has the shortest device length among the previous works [26,32].
Table 1. Comparison of SOI-based reconfigurable polarization beam splitters
4. Conclusion
In conclusion, we have proposed the design of a compact reconfigurable PBS enabled by phase change material Sb2Se3. By incorporating a Sb2Se3 layer on top of a DC structure interacting with evanescent wave, the coupling behavior of propagating TE and TM modes could be effectively tuned, manifested by the changing of coupling length upon phase change of Sb2Se3. Through a careful design of the waveguide cross section in the DC region to fine control the effective mode indices of hybrid TE and TM modes, the efficiently polarization mode splitting could be achieved with a 41.6 µm-long DC structure, and output ports of TE and TM modes could be switched by phase changing the hybrid integrated Sb2Se3 layer, and thus enabling the reconfigurability of the PBS. At the wavelength of 1550 nm, the crosstalk between bar and cross output ports and insertion loss for TE/TM injection are −34.8 dB/−30.3 dB and 0.03 dB/0.12 dB for a-Sb2Se3, and when the Sb2Se3 switches to the crystalline state, the crosstalk and insertion loss for TE/TM polarization are −25.3 dB/−21.3 dB and 0.04 dB/0.16 dB, respectively. Our PBS also features a broadband operation capability, with an overall insertion loss for TE mode of less than 0.27 dB and crosstalk of less than −14 dB, while overall insertion loss for TM mode is less than 0.6 dB, and the crosstalk is less than −10 dB within the wavelength range from 1540 nm to 1560 nm. The proposed PBS is compact in device footprint and the nonvolatile nature of phase change material would reduce energy consumption for the reconfigurability, which holds great potentials for polarization state manipulation in on-chip optical communication links and interconnect networks.
Funding
National Natural Science Foundation of China (62105172); Natural Science Foundation of Zhejiang Province (LQ21F050004); Natural Science Foundation of Ningbo Municipality (202003N4102); National Key Research and Development Program of China (2021YFB2801300); Fundamental Research Funds for the Provincial Universities of Zhejiang; Open Research Fund of the Key Laboratory of All Optical Network and Advanced Telecommunication Network, Ministry of Education, China (Beijing Jiaotong University) (AON2022005); K. C. Wong Magna Fund in Ningbo University.
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.
Supplemental document
See Supplement 1 for supporting content.
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