Design of an ultra-compact, energy-efficient non-volatile photonic switch based on phase change materials

The on-chip photonic switch is a critical building block for photonic integrated circuits (PICs) and the integration of phase change materials (PCMs) enables non-volatile switch designs that are compact, low-loss, and energy-efficient. Existing switch designs based on these materials typically rely on weak evanescent field interactions, resulting in devices with a large footprint and high energy consumption. Here we present a compact non-volatile 2 by 2 switch design leveraging optical concentration in slot waveguide modes to significantly enhance interactions of light with PCMs, thereby realizing a compact, efficient photonic switch. To further improve the device's energy efficiency, we introduce an integrated single-layer graphene heater for ultrafast electrothermal switching of the PCM. Computational simulations demonstrate a 2 by 2 switch with crosstalk (CT) down to -24 dB at 1550 nm wavelength and more than 55 nm 0.3 dB insertion loss (IL) bandwidth. The proposed photonic switch architecture can constitute the cornerstone for next-generation high-performance reconfigurable photonic circuits.


Introduction
In the past few decades, the rapid development of photonic integrated circuits (PICs) has demonstrated their potential in telecommunication and data communications.Moreover, the von Neumann bottleneck in electronics 1 suggests that scalable programmable PICs could be an alternative solution for energy-efficient classical and/or quantum information storage and processing. 2,3Programmable PIC reported to date predominantly rely on thermo-optic effects, 4 free-carrier effects, 5 or electro-optic effects 6 of materials.The small change in refractive index afforded by these effects, however, limits the tunability and scalability of these methods, leading to a large device footprint and excessive energy consumption.Although plasmonic light confinement can mitigate the issue that results in lossy devices, 7 making the approach unsuitable for large-scale PICs.Moreover, these effects are volatile and demand a constant power supply (∼10 mW).This disqualifies them for applications where only sporadic re-programming or reconfiguration is needed, such as optical switching and routing in data centers, 8 optical neural networks, 9 and photonic memories. 10,11halcogenide-based phase change materials (PCMs) emerge as promising candidates to enable ultra-compact and energy-efficient reconfigurable photonics.2][13][14] Phase transition in PCMs can be triggered by ultrashort optical or electrical pulses, 15 and a multitude of intermediate states (between fully amorphous and crystalline) can be accessed by changing the pulse parameters. 16In addition, PCMs offer compatibility with large-scale integration, as they can be conveniently prepared using various large-area deposition methods 11,12,17,18 onto different photonic integrated circuit (PIC) platforms in a CMOS backend-compatible manner. 12,19Despite these advantages, conventional PCMs, such as Ge 2 Se 2 Te 5 (GST) and GeTe, display significant absorption in both phases at optical communication wavelengths, limiting their effectiveness in photonic phase shifters-a crucial component of programmable PICs.Recently, interest has been growing in wide-bandgap PCMs, such as GeSbSeTe (GSST), 20,21 antimony selenide (Sb 2 Se 3 ), 22 and antimony sulfide (Sb 2 S 3 ). 22,23For example, Sb 2 Se 3 exhibits minimal losses at 1550 nm and a substantial index contrast (Δn ≈ 0.77). 22These characteristics position Sb 2 Se 3 as a promising phase-change material for applications in programmable photonics within the telecommunication bands.
One essential design element in PCM-based configurable devices is the heating mechanism.Electro-thermal heating using resistive micro-heaters facilitating scalable on-chip integration have been investigated in numerous recent studies.Various heater materials have been employed, including metals, 24,25 transparent conducting oxides (TCOs), 26 and doped Si. [27][28][29] While metals prove effective for free-space reflective devices, they introduce notable optical losses in transmissive or waveguide components.Doped silicon stands out as an excellent choice for integrating PCMs into the silicon-on-insulator (SOI) platform.However, applying it to Si 3 N 4 -based devices, another widely used photonic platform, or to other non-silicon waveguide platforms poses challenges.TCO heaters, while suitable for devices operating in the visible spectrum, encounter exacerbated optical losses in the infrared due to free carrier absorption.To address these challenges, graphene has emerged as a promising heating material due to its exceptional thermal and electrical conductivity, versatile integration compatibilities, and remarkable stability. 30,31In addition, the infrared optical losses associated with graphene can be minimized by leveraging the doping-induced Pauli blocking effect.Recent theoretical analysis and experimental reports [32][33][34] indicate that graphene heaters exhibit two orders-of-magnitude higher figures of merit for overall performance (heating efficiency and induced loss) than that of doped Si or TCO heaters when applied to PCM switching.
Here, we present the design of a compact non-volatile photonic 2 × 2 switch on the SOI platform utilizing Sb 2 Se 3 and a single-layer graphene heater.The design exploits a configuration involving a slotted waveguide filled with PCM.Compared to traditional layouts where the PCM typically interacts only with evanescent fields, the design leverages strong field concentration in the slot region to boost light-PCM interactions, 35 thus simultaneously achieving low insertion loss, a compact form factor, high extinction ratio, and zero-static power consumption.

Structure and Design
Figure 1 shows the proposed 2 × 2 photonics directional-coupler switch design in a semi-standard SOI platform.The switch consists of a multimode slotted waveguide (the two-waveguide coupling zone) attached to four single-mode waveguides serving as input and output ports, on either side of the multimode section.The height and width of the single-mode waveguides are h ¼ 240 nm and W wg ¼ 450 nm, respectively.The slot waveguide has a length L slot ¼ 10 μm and a centrally located slot with a width W slot ¼ 100 nm, which is completely filled with Sb 2 Se 3 .In the telecommunication C-band, the refractive indices of Sb 2 Se 3 are taken from Ref. 22 as 3.825 and 4.050 at 1550 nm wavelength for the amorphous Sb 2 Se 3 and crystalline Sb 2 Se 3 , respectively.The loss of crystalline Sb 2 Se 3 was also reported in the same paper to be as low as 0.01 dB μm −1 .The whole device is cladded all around by SiO 2 with a thickness of 2 μm.Directly on top of the multimode slot waveguide, there is a single-layer graphene heater.The dimensions of the graphene layer are L gr ¼ 9 μm and W gr ¼ 3 μm, and it is symmetrically positioned on the slot waveguide.Ti/Au contact pads are placed on both sides of the graphene heating layer to minimize contact resistance.Figure 1(b) demonstrates the working principle of the switch as the PCM is switched reversibly by a sequence of voltage pulses applied to the graphene heater.Short pulses (several hundreds of nanoseconds) with high voltage will reset the PCM back to the amorphous state while long pulses (several milliseconds) with moderate voltage are used to crystallize the material.Sb 2 Se 3 was reported to be successfully amorphized at T a ¼ 620°C for 400 ns and crystallized at T c ¼ 200°C for minimally 0.1 ms. 12 In this study, the fundamental transverse electric (TE) mode at the telecommunication wavelength of 1550 nm was targeted.Yet, the design principle is not wavelength-sensitive and can be applied to a broadband device, as we will show later.The whole design could be realized by standard lithography and dry etching processes.Sb 2 Se 3 be deposited in the slot by conformal coating methods, such as atomic layer deposition 36 and solution processing. 37Alternatively, thermal reflow has been demonstrated as an effective means for filling thin slots with chalcogenide materials. 38The graphene heater can be fabricated by transferring chemical-vapor-deposition (CVD) grown single-layer graphene via the standard wet transfer technique, 39  from a bar state to a cross state.Due to the difference in propagating constants, the interference between odd mode and even mode results in the oscillation between bar state and cross state when we increase the slot length.The ultralow loss exhibited by Sb 2 Se 3 allows the PCM to have a strong overlap with the supermodes in the slot region without incurring excessive losses, thereby enhancing the phase modulation effect.The effective index differences between the even mode and the odd mode are 0.2951 and 0.3875 for amorphous and crystalline Sb 2 Se 3 states, respectively, corresponding to different beating lengths in the two states.
Figure 2(c) plots transmission through the ports 1 and 2 in the amorphous Sb 2 Se 3 state with different PCM-filled slot length L slot , simulated using 3D finite-difference time-domain (FDTD) and fitted to sine curves.Following the result, we take the slot length to be 10 μm, which yields the maximum transmission in the amorphous (bar) state.To realize switching with maximum contrast, we selectively crystallize the center section of the PCM as shown in Fig. 2(d).In practice, this can be implemented by controlling the temperature profile along the y-axis, as the center portion of the PCM slot experiences the highest temperature and hence preferentially crystallizes first (refer to Fig. 4 and discussions for more details).Figure 2(d) shows the simulated transmission through the output ports as a function of crystallized Sb 2 Se 3 slot length.The plot implies that an optical crystallization section length of 8.5 μm (i.e., leaving 0.75 μm of amorphous region on either side) would result in maximal switching contrast.This leads to an overall device (including the four single-mode waveguide ports) footprint of 5.5 μm × 24 μm.1][42] The main difference between our assumption here and the actual experiment implementation is that there is no abrupt interface between the amorphous and crystalline Sb 2 Se 3 sections.Instead, a gradual transition region with varying crystalline fraction is likely present.This contributes to lowering reflection and scattering from the abrupt interface and can lead to even lower insertion losses than the simulations presented here.

Results
We conducted 3D FDTD simulations to validate the switching efficiency of our proposed design.Figures 3(a corresponding in-plane electric field distributions corresponding to amorphous and crystalline Sb 2 Se 3 states.The overall insertion loss (IL) is −0.27 dB (cross state) and −0.11 dB (bar state) at 1550 nm and is consistently <0.5 dB across 1525 to 1575 nm wavelengths.The 0.3 dB IL bandwidth is no <55 nm.The crosstalk (CT), defined as the contrast ratio between the on/off states at the output ports, reaches −23.9 dB (cross state) and −27.4 dB (bar state) at 1550 nm and stays better than −15 dB throughout the 1525 to 1575 nm band.These performance metrics compare favorably to state-of-the-art PCM switches as summarized in Table 1.Next, we discuss the thermal performance of the single-layer graphene heater.The graphene heater has been reported to have outstanding performance as a heating element for integrated photonic devices incorporating PCMs, offering exceptional energy efficiency and high operational speed, 31,32 owing to its ultralow heat capacity and high in-plane thermal and electrical conductivity.Compared to doped Si, which is another popular choice of heater material, graphene heaters claim significantly lower induced loss and higher heating efficiency. 32,33Joule heating employing the graphene heater follows similar phase change dynamics demonstrated in Refs.12 and 29.The pulse width and voltage are contingent on the microheater's properties.With our specific proposed graphene heater, pulses of 2 V, which induces a current of 5.67 mA, with a duration of 100 μs are applied to partially crystallize the Sb 2 Se 3 slot, heating its 8.5 μm long center section to above the crystallization temperature T c (here set as 200°C).It is reported that pulses (as short as 5 μs) can crystallize Sb 2 Se 3 but result in spatially non-uniform crystallization.Longer pulses lasting 100 μs or more are necessary to crystallize the PCM uniformly, 12 which justifies our pulse parameter choice here.To induce amorphization, we investigated two types of pulses, a single 8.6 V (22.4 mA), 100 ns pulse 12,22 or a 6.6 V (18.7 mA), 400 ns pulse.Both can elevate the temperature of the entire PCM-filled slot above the melting point, T m ¼ 620°C (893 K).The total energy consumption for crystallization is 1.13 μJ and 21/49.4nJ (100/400 ns pulse) for amorphization.Clearly, a trade-off between pulse voltage and switch energy exists for the amorphization process.Figure 4(a) demonstrates the temperature evolution over amorphization and crystallization cycles from finite-element method (FEM) simulations using COMSOL Multiphysics.For crystallization, the temperature remains stably higher than T c across the target section.Figure 4(b) plots the temperature across the Sb 2 Se 3 slot at the end of the crystallization and amorphization (100 ns) pulses.The coordinate z ¼ 0 nm refers to the boundary between the silicon waveguide layer and the buried oxide  The out-of-plane temperature variation is particularly relevant for graphene heaters, given that graphene exhibits varying out-of-plane thermal resistance due to the surface polar phonons (SPoPh) scattering effect. 49Consequently, a temperature gradient is established along the out-ofplane direction, as heat transfer occurs more efficiently toward the substrate than toward the top cladding.In order to ensure complete amorphization of Sb 2 Se 3 following the melt-quenching pulse, a series of dynamic simulations was conducted with varying amorphization pulse power.Thermal FEM simulations [Fig.4(b)] suggest that the crystallization length within the slot barely changes at z ¼ 0 and z ¼ 240 nm, implying that the crystallization is uniform along the z-direction.The kinks near the two ends of the orange curve in Fig. 4(b) are attributed to the ends of the graphene sheet.
The quenching rate after the amorphization pulse, which is critical in gauging whether crystallization can be bypassed, is predominantly governed by thermal conductance through cladding material and BOX.As shown in Fig. 4(a), the quenching rate is ∼1 K∕ns, which is sufficient to prohibit re-crystallization of Sb 2 Se 3 .
Finally, we assess the scalability of our design to large switch matrices.2 m × 2 m switches built from 2 × 2 building blocks using the Benes network can be used to estimate the performance of our proposed design. 20Using the values presented in Table 1, we can estimate the total insertion loss and the lower and maximum CT of an m-order switch matrix (assuming that the IL of a waveguide crossing in the C-band as 0.1 dB 50 ): E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 2 ; 1 1 4 ; 1 9 9 CT m ¼ −ð15 dB − 10log 10 m dBÞ: Scaling from our 2 × 2 switch's performance, a 16 × 16 switch is anticipated to exhibit maximal 3.2 dB insertion loss and −24 dB CT at 1550 nm, representing highly promising performance metrics, compared to the state-of-the-art (volatile) on-chip 16 × 16 switch reported by Lu et al. 51 The devices can also be put to good use in creating large-scale, programmable, rectangular and triangular and hexagonal meshes 52 (Fig. 5).

Conclusion
In conclusion, the non-volatile 2 × 2 photonic switch design takes advantage of a PCM-slot configuration to achieve an ultra-compact footprint (5.5 × 24 μm 2 ) with minimal CT (−23.9 dB), and the low-loss PCM Sb 2 Se 3 enables a low insertion loss of 0.27 dB, and a single-layer graphene heater achieves low switching energies of 1.13 μJ for crystallization and 21 nJ for amorphization.The design further demonstrates its scalability toward large-scale non-blocking switch matrices.Our proposed design, therefore, holds the potential for the development of future large-scale PCM-based programmable PICs.

Fig. 3
Fig. 3 Transmission spectra at two output port and the normalized electric field intensity distribution of the 2 × 2 switch at 1550 nm with (a) amorphous Sb 2 Se 3 (bar state) and (b) crystalline Sb 2 Se 3 (cross state).

a
Additional loss due to PCMs to the total device IL. b Through/drop port IL. c Optical pulse energy.d Experimental results.Dao, Hu, and Soref: Design of an ultra-compact, energy-efficient non-volatile. ..(BOX).The yellow line shows that the entire Sb 2 Se 3 middle section (from y ¼ −4.25 μm to y ¼ 4.25 μm) was elevated to be above its melting point, which guarantees complete amorphization.The 3D temperature profile predicted by thermal FEM simulation at the end of the 100 ns amorphization pulse was shown in Fig.4(c), suggesting that the heat is effectively confined within the target section of Sb 2 Se 3 .

Fig 4
Fig 4 (a) FEM simulated transient temperature during and after the amorphization and crystallization pulses.The shaded areas represent pulse-on times.(b) Simulated temperature across the slot at the end of the amorphization and crystallization pulses.(c) Simulated three-dimension temperature profiles at the end of the amorphization pulse.

Fig. 5 (
Fig. 5 (a) Insertion loss and (b) maximum CT as functions of the switch matrix of order m.

Table 1
Comparison of PCM-based optical switches.