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Abstract
On-chip nonvolatile photonic switches enabled by phase change materials (PCMs) are promising building blocks for power-efficient programmable photonic integrated circuits. However, large absorption loss in conventional PCMs (such as Ge2Sb2Te5) interacting with weak evanescent waves in silicon waveguides usually leads to high insertion loss and a large device footprint. In this paper, we propose a 2×2 photonic switch based on two-mode interference in a multimode slot waveguide (MSW) with ultralow loss Sb2S3 integrated inside the slot region. The MSW supports two lowest order TE modes, i.e., symmetric TE00 and antisymmetric TE01 modes, and the phase of Sb2S3 could actively tune two-mode interference behavior. Owing to the enhanced electric field in the slot, the interaction strength between modal field and Sb2S3 could be boosted, and a photonic switch containing a ∼9.4 µm-long Sb2S3-MSW hybrid section could effectively alter the light transmission between bar and cross ports upon the phase change of Sb2S3 with a cross talk (CT) less than −13.6 dB and an insertion loss (IL) less than 0.26 dB in the telecommunication C-band. Especially at 1550 nm, the CT in the amorphous (crystalline) Sb2S3 is −36.1 dB (−31.1 dB) with a corresponding IL of 0.073 dB (0.055 dB). The proposed 2×2 photonic switch is compact in size and compatible with on-chip microheaters, which may find promising applications in reconfigurable photonic devices.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Programmable photonic integrated circuits (PICs), which could be configured to perform specific tasks after fabrication, are highly desired in optical communication technology as well as emerging optical information applications including optical neural networks [1], quantum information processing [2], and microwave photonics [3]. Silicon photonics is considered as a promising platform for reaching this goal due to its compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication and high integration capacity [4]. Various optical switches, the essential building blocks for controllable routing in programmable PICs, have been proposed on silicon platforms based on the configurations of Mach-Zehnder interferometers [5] and micro-ring resonators [6], etc. Controlling the modal phase in the waveguiding structure through tuning the effective mode index is key to achieve the switching capability, which is usually relying on the thermo-optic [7] or electro-optic effects [8]. However, the refractive index change in these effects is only on the order of 1×10−2 or even less which is basically limited by the internal material properties [7,8], and thus a large footprint with a device length on the order of hundreds of micrometers is usually required to achieve enough phase shifting [9]. In addition, the switched state can only be maintained with continuous power supply, which inevitably increase the power consumption. These drawbacks would severely impede the development of high-density, energy efficient programmable PICs for advanced photonics devices.
Hybrid integration with functional materials provide an important approach to enrich the functionalities of silicon photonics circuits. In order to reduce the footprint of the optical switch, material with large reversible index change is demanded. Chalcogenide phase-change material (PCM) with the advantages of inherent non-volatility, high refractive index contrast between its amorphous and crystalline states, reversible switching with a response time ranging from nanoseconds to microseconds scale [10] is one of the best candidates meeting this requirement. Phase change could be triggered by rapid thermal effect induced by either electrical [11–13] or laser heating [14,15], while the former is usually preferred in large scale on-chip arrays due to it providing easier way to address individual phase change unit, which could be readily accomplished with microheaters such as ITO [16,17], graphene [18,19] and PIN [20] structure. Conventional PCMs, such as Ge2Sb2Te5 (GST) and Ge2Sb2Se4Te1 (GSST), enable the reconfigurability in various silicon photonics devices [21–28]. However, they have relatively large absorption loss in the communication C-band, especially in their crystalline (Cr) phase, leading to a large insertion loss.
Recently, a new type of two elements chalcogenide phase change material Sb2S3 attracts much research interest due to their extremely low loss (extinction coefficient is less than 1×10−5 in Cr state), and a moderate refractive index contrast of about 0.6 upon phase change [29]. In addition, Sb2S3 also demonstrate the multilevel phase change possibility upon judiciously excitation control [30]. Numerous ultralow loss PCM enabled reconfigurable devices have been realized including on-chip Bragg gratings [31,32], Mach-Zehnder interferometer [33], ring resonator [34] and mode converters [35]. Nevertheless, most of the proposed on-chip device rely on weak interaction between the PCMs and evanescent wave [33,34,36,37], leading to a large device footprint. Slot waveguides have the advantage of enhanced modal field in the slot region [38], which has been widely explored in on-chip sensing [39] and nonlinear processes [40]. Integrating Sb2S3 into slot waveguides opens new possibilities for compact reconfigurable on-chip photonic devices with enhanced interaction strength between modal field and PCMs.
In this paper, we propose a compact reconfigurable 2×2 photonic switch utilizing two-mode interference in a MSW with ultralow loss Sb2S3 incorporated into the gap region. We investigate the characteristics of two lowest order TE modes with opposite symmetry in the Sb2S3-integrated MSW and show their different mode indices behaviors upon the phase change, and then we study the coupling between single mode waveguide and MSW as well as the two modes interference (TMI) effect in the multimode region. By controlling geometric parameters of the MSW, a compact 2×2 optical switch could be realized with its output mediated by the phase change of Sb2S3. Owing to the strong interaction between modes and the ultralow loss PCM, the PCM region is only ∼9.4 µm in length and the insertion loss (IL) of proposed switch is less than 0.26 dB in the telecommunication C-band.
2. Structure and design
Figure 1 shows the proposed 2×2 photonic switch fabricated on a standard on a silicon-on-insulator (SOI) platform with a 220-nm-thick silicon top layer. The switch consists of a multimode slot waveguide (MSW) supporting two lowest order TE modes, and four single mode waveguides connecting either side of the multimode region as input and output ports. The length and width of the slot waveguide is L and Wm with the slot located in the center with a width of Wss. An ultralow loss phase change material, namely, Sb2S3, is embedded in the slot region. The single mode waveguide (SMW) has a width of Wm/2 and is designed as a Bézier S curved shape as illustrated in Fig. 1(c), the separation between two input (output) single mode waveguides is d and the total length of S bend is Ls. In this paper, we consider Ls = 8 µm and d = 4 µm, respectively. The whole device is based on partially etched rib waveguides with a thin pedestal layer with a thickness of h = 50 nm, as illustrated in Fig. 1(b), and hss is 170 nm. The pedestal layer could be heavily doped by boron and phosphorus ion implantation on either side of the MSW, forming an on-chip high energy-efficient PIN (p-type, intrinsic, n-type junction) heater for electrically control the phase of Sb2S3 [20,41]. In the telecommunication C-band, the refractive indices of Sb2S3 are taken from [29], and the refractive indices of Sb2S3 in amorphous and crystalline states are 2.712 and 3.308 at 1550 wavelength, respectively. Meanwhile, the extinction coefficient of Sb2S3 in both phases is less than 1×10−5 [29]. The design could be realized by standard electron-beam lithography (EBL) and dry etching process, and the sandwiched Sb2S3 layer could fabricated by using RF sputtering deposition followed by a lift-off process [42–44]. The p++ (n++) doping and metal contact deposition can be conducted by conventional protocols [21]. To improve the long-term stability of the proposed switch, a thin capping layer could be deposited on top of Sb2S3 in the practical fabrication to prevent oxidization and loss of sulfur atoms [36].
Fig. 1. (a) Schematic of the 2×2 photonic switch based on two-mode interference. (b) Cross section of multimode slot waveguide region. (c) Top view of the proposed photonic switch.
The basic working principle of the proposed switch is illustrated in Fig. 1(a): Light is injected from the Port 1. When Sb2S3 is in amorphous state, the light will output at the Bar Port 3, after Sb2S3 is switched to crystalline state, the interference effect in the multimode region will be modified accordingly due to the refractive index change of Sb2S3 so that the light will be output at the Cross Port 4. In order to achieve the desired functionality, we firstly show the modal characteristics of the MSW by a finite-difference eigenmode solver (Lumerical Mode Solutions). The modes in the multimode waveguide propagating along x axis could be expressed as E(y, z)exp(iβ x), where E is the vectorial modal electric field, β is the propagation constant equaling to k0neff with k0 as the free space wavevector and neff as effective mode index. The effective mode indices neff of TE00, TE01, and TE02 modes as a function of Wm at 1550 nm is plotted in Fig. 2(a), here, the slot width Wss is 100 nm. The solid (dashed) curves correspond Sb2S3 in crystalline (amorphous) state. With the width decreasing, the effective mode indices of all three modes drops, and TE02 mode cut off at Wm =1020 nm and Wm =1050 nm when Sb2S3 was in amorphous and crystalline states, respectively, which indicate the number of supported TE modes in the multimode region could be controlled by the Wm. neff of all modes experience an increase upon the phase change of Sb2S3 from amorphous to crystalline state. In order to keep the multimode region only support two lowest order TE modes, we choose Wm = 900 nm, and the corresponding width of the SMW is 450 nm.
Fig. 2. (a) Effective mode indices neff of TE00, TE01 and TE02 modes at 1550 nm as a function of Wm (here Wss = 100 nm, solid line: crystalline state, dashed line: amorphous state). (b), (c) The cross-sectional electric field distribution and neff of TE00 and TE01 of the MSW with (b) a-Sb2S3 and (c) c-Sb2S3 at 1550 nm. The width of the Sb2S3 and the MSW are Wss =100 nm and Wm =900 nm, respectively. (d) Field distribution of co-propagating TE00 and TE01 modes in a MSW with Wm = 900 nm. The cross-sectional field profiles at position i and ii denoted by the dashed line is plotted on the right.
Figures 2(b), (c) show the cross-sectional electric field profiles of TE00 and TE01 modes with amorphous Sb2S3 (a-Sb2S3) and crystalline Sb2S3 (c-Sb2S3) at Wm = 900 nm and Wss = 100 nm. TE00 mode has a symmetric field distribution with respect to z axis, while the TE01 mode has an anti-symmetric one manifested by a field node in the center. Figure 2(d) shows the field distribution with two modes co-propagating along x in the MSW. Due to the different propagating constants, two mode interference could be observed, manifested by an oscillatory field pattern along the propagating direction with a period of 2Lc, where Lc represents the distance over which the phase difference of the two modes experiences a π changes, which is also dependent on the phase state of Sb2S3 and we will discuss later.
Since the refractive index of the Sb2S3 layer in both states is smaller than that of the silicon layer, enhanced field in the slot could also be observed for TE00 mode with as depicted in Figs. 2(b), (c), the power ratio in the Sb2S3 region in TE00 mode is 11.7% (a-Sb2S3) and 13.5% (c-Sb2S3). However, for TE01 mode, the field node locates near the slot, and no field enhancement exhibits in this region, and the power ratio of TE01 mode inside Sb2S3 is only 0.25% (a-Sb2S3) and 0.40% (c-Sb2S3). The stark contrast between TE00 and TE01 field profile renders their neff change differently upon the phase change of Sb2S3. For example, at Wm = 900 nm, the effective mode index contrast of TE00 mode Δneff between two phase states of Sb2S3 is 0.0981, which is ∼2.8 times larger than that of the TE01 mode (Δneff = 0.0348). This difference provides an efficient way to tune the interference behavior in the MSW.
Next, we demonstrate the Sb2S3-mediated interference behavior between TE00 and TE01 modes in the MSW. When guided wave is injected into the multimode waveguide supporting TE00 and TE01 modes from the input single mode waveguide, the electric field in the multimode waveguide could be expressed as
Fig. 3. (a), (b) Excitation coefficients Ai of TE00 and TE01 modes in the multimode region as a function of slot width Wss in (a) amorphous and (b) crystalline at λ=1550 nm. The part highlighted in orange indicates the slot width range in the optimization process.
When the excitation coefficients of the two modes are approximately equal, i.e. A1 ≈ A0 = A, the modal field at the end of the multimode region at x = L could be expressed as
As depicted in Fig. 2(d), we show that changing the modal superposition from E1 + E0 to E1 − E0 will induce field pattern flipped over the x-axis. Since a refractive index change of Sb2S3 could tune the propagation constants of TE00 and TE01 modes differently, a change of Δβ and thus a π change in ΔβL in a suitably designed multimode region could be anticipated upon the phase change of Sb2S3, which could further facilitate switching the output between bar and cross ports.
To achieve low CT as well as compact size for the proposed 2×2 switch, we employ an approximated yet efficient method to determine Wss and L of the MSW. The CT is defined as the contrast of transmission ratio between cross and bar output ports [22]. Here, we first define a transition length Ltr of the multimode region: after the superposed modal field propagates a distance of Ltr in the MSW, the light is predominantly output from the cross port, i.e. the lowest CT between bar and cross ports, as depicted in Fig. 4(a), and Ltr is dependent on Δβ, which is related to Wss and the phase of Sb2S3. Figure 4(b) shows the CT as a function of the length of multimode region. For c-Sb2S3, we could determine the Ltr = 0.8 µm at Wss = 100 nm and 0.82 µm at Wss = 200 nm, while for a-Sb2S3, Ltr = 0.96 µm at Wss = 100 nm and 1.1 µm at Wss = 200 nm. Since the difference of Ltr between two Wss values is relatively small (ΔLtr = 0.02 µm for c-Sb2S3, and 0.15 µm for a-Sb2S3), we average transition lengths under different Wss so that Ltr is only dependent on the phase state of Sb2S3, i.e. Ltr,am = 0.81 µm and Ltr,cr = 1.03 µm. Next, we calculated the effective mode indices of TE00 and TE01 modes as a function of Wss, as depicted in Fig. 4(c), and thus the intermodal coupling length Lc under amorphous and crystalline Sb2S3 could be obtained. The total length L of the multimode region should satisfy Lam = Ltr,am +nLc,am and Lcr = Ltr,cr + (n + 1)Lc,cr when Sb2S3 is in amorphous or crystalline state, respectively, in order to output the light from cross or bar port. We found the minimum n value equals to 3 could ensure Lam and Lcr have one intersecting point, as depicted in Fig. 4(d), at Wss = 175 nm with a corresponding length L of ∼9.4 µm.
Fig. 4. (a) A cross output field pattern in a 2×2 switch with a shortest-lengthed MSW defined as Ltr. (b) In both states of Sb2S3, the crosstalk as a function of the length of MSW is at Wss= 100 nm (solid) and Wss= 200 nm (dashed), respectively. (c) In both states of Sb2S3, the effective mode indices of TE00 and TE01 modes as a function of Wss (Wm = 900 nm) at 1550 nm. (d) The MSW length L as a function of the Sb2S3 width Wss in amorphous and crystalline states, respectively.
3. Results and discussions
We carried out a 3D finite-difference time-domain simulation to validate the switching performance of our design. The parameters for the designed MSW are Wss = 175 nm, L∼ 9.4 µm and Wm = 900 nm, and the footprint of the entire device is only ∼4.9 µm × 25.4 µm, including the input/output SMWs. Figures 5(a), (b) show the transmission spectra of the proposed 2 × 2 TMI-based photonic switch with Sb2S3 in its amorphous and crystalline states, respectively. When Sb2S3 is in the amorphous state, the light is mainly output from the bar port. Figure 5(c) depicts the corresponding propagating electric field in the proposed switch at the designed wavelength of 1550 nm. The injected modal field from the single mode waveguide propagates a distance of 3Lc,am + Ltr,am, and then output from the bar ports. The overall insertion loss (IL) is less than 0.26 dB, and CT ranging from −13.6 dB to −36.1 dB within the telecommunication C-band from 1530 nm to 1565 nm. When Sb2S3 is switched to the crystalline state, the output changes from bar port to cross port. Figure 5(d) depicts the propagating field in the switch at 1550 nm, owing to the larger refractive index of c-Sb2S3, the field in the multimode region alternates four times (i.e. a propagation distance of 4Lc,cr + Ltr,cr) and finally output from the cross port. The overall IL is less than 0.18 dB and CT ranges from −15.3 dB to −31.2 dB within the telecommunication C-band. It should be noted that due to the mode mismatch between the input SMW and MSW, slight reflection back into Port 1 and 2 may occur, manifested by the weak interference pattern in the input SMW (see Supplement 1). Different from the structure of the previous asymmetric directional couplers (DC) [21,22], switching performance is wavelength dependent for both phase states. At the designed wavelength of 1550 nm, an extremely low CT is −36.1.1 dB (−31.1 dB) in the amorphous (crystalline) state and the corresponding IL in the amorphous (crystalline) state is 0.073 dB (0.055 dB).
Fig. 5. (a), (b) Transmission spectra at bar and cross ports with (a) a-Sb2S3 and (b) c-Sb2S3. (c), (d) Normalized electric field intensity distribution of the 2×2 switch with (c) a-Sb2S3 and (d) c-Sb2S3 at 1550 nm.
Next, we discuss the influence of possible manufacturing errors on the performance of the proposed switch. Figure 6(a) show the CT as a function of the slot width perturbation ΔWss, i.e. the slot width deviation from the designed Wss. A larger ΔWss error would increases the CT value, and CT is more sensitive on ΔWss when Sb2S3 is in its amorphous state. To maintain a CT lower than −20 dB for both Sb2S3 states, | ΔWss | should be less than 19 nm. Figure 6(c) shows the dependence of CT on slot position shift Δy from the center of the multimode region. The overall CT is relative robust to slot position shift for both Sb2S3 states when | Δy | is less than 10 nm. The device could tolerate ±20 nm position shift while still maintaining a low CT which less than −20 dB. It should be noted that for TMI structures, the sensitivity to fabrication imperfections can be reduced by a factor of 2, compared with directional coupler (DC) structures [45]. In addition, TMI structure has a much shorter coupling length than the traditional DC structure, which can further shorten the structure of the device, and has been used in the design of optical switches [46,47], polarization controllers [48] and wavelength multiplexer [49]. The refractive indices of Sb2S3 films may slightly varies due to different preparation processes [50]. Figure 6(c) shows the CT as a function of the refractive index variation dn, which is defined as the deviation of refractive index from the reported value in Ref. [29]. CT is sensitive to the refractive index variations, while a relatively low CT less than −10.3 dB could be maintained within a ±0.1 refractive index variation. For the actual device fabrication, the switch should be designed using the measured refractive index of Sb2S3 in order to obtain the best device performance.
Fig. 6. The influence of fabrication imperfection on CT of the 2×2 switch at 1550 nm. (a) CT as a function of slot width perturbation ΔWss. (b) CT as a function of slot position shift Δy. (c) CT as a function of refractive index variation dn.
We also compare the performance of previously reported PCM-based 2×2 photonic switches in Table 1. Owing to the low loss nature of Sb2S3 in the telecommunication C band, our design demonstrates an extremely low insertion loss less than 0.26 dB. Despite the relatively small refractive index contrast of Sb2S3, the enhanced interaction between Sb2S3 and the modal field by the slot waveguide could still lead to a strong phase modulation upon the phase change of Sb2S3, and switching between two output ports could be achieved with only a ∼9.4 µm-long PCM-integrated MSW. The proposed device has a compact footprint of ∼4.9 × 25.4 µm2, which is the smallest size among PCM-based broadband 2×2 photonic switches. In addition, larger fabrication tolerance of TMI structures would also be beneficial for practical implementation, whereas the 2×2 switches based on DC structure requires high parallelism and width control of the coupled waveguides to ensure effective mode matching. It is worth mentioning that proposed switch is also compatible with the PIN heater which has the advantage of low power consumption [25]. While the large heat volume in the switch design would increase the switching time due to slow thermal relaxation. Sb2S3 generally requires a switching time of hundreds of nanoseconds for amorphization and tens of microseconds for complete crystallization in reconfigurable photonic devices [34,51]. Our electro-thermal shows that the PIN heater driven by a 100 ns-duration electrical pulse with a voltage of 6 V could facilitate the amorphization of Sb2S3 in the slot, and the energy consumption of the amorphization switch operation is 9.59 nJ (see Supplement 1). In addition, a similar two-element phase change material Sb2Se3 exhibits ultralow loss in the wavelength range considered in this paper, which also demonstrates a better switching endurance [29]. However, Sb2Se3 has a larger refractive index in both amorphous and crystalline states compared with Sb2S3, which would lead to a degraded switching performance especially in its crystalline state (see Supplement 1).
Table 1. Comparison of PCM-based 2×2 switches
4. Conclusion
In conclusion, we proposed a compact nonvolatile 2×2 photonic switch enabled by phase change material Sb2S3. To fully exploit the ultralow loss nature of Sb2S3, we hybrid integrate Sb2S3 into the slot region of a MSW, which boosts the interaction strength between PCM and modal field. The propagation constants of TE00 and TE01 modes change differently upon the phase change of Sb2S3, due to different modal field symmetry. Thus, the interference pattern in the multimode region could be effectively mediated by the phase change of Sb2S3. To achieve the optimal switching performance, we employ an approximated yet efficient method to fast determine the geometric parameters of the functional slot multimode waveguide, which is further validated by full wave 3D-FDTD simulation. Despite a relatively small refractive index contrast of Sb2S3, a ∼9.4 µm-long Sb2S3-MSW hybrid section is utilized in our design to realize a reconfigurable 2×2 switch with extremely low CTs of −31.1 dB (−36.1 dB) and ILs of 0.073 dB (0.055 dB) for a-Sb2S3 (c-Sb2S3) at 1550 nm. Our switch design also demonstrates a broadband operation capability with CT less than −13.6 dB and IL less than 0.26 dB in the telecommunication C band. The proposed 2×2 photonic switch can be integrated with on-chip PIN microheaters, could find promising applications in electro-optical tunable devices for on-chip optical communications, photonic neuromorphic computing, etc.
Funding
National Natural Science Foundation of China (62105172, 61875099); Natural Science Foundation of Zhejiang Province of China (LQ21F050004); Ningbo Natural Science Foundation (202003N4102); 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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