Reconfigurable metasurface-based 1 &#215; 2 waveguide switch

Amged Alquliah; Mohamed Elkabbash; Jinluo Cheng; Gopal Verma; Chaudry Sajed Saraj; Wei Li; Chunlei Guo

doi:10.1364/PRJ.428577

Journals >Photonics Research >Volume 9 >Issue 10 >Page 2104 > Article

Photonics Research
Vol. 9, Issue 10, 2104 (2021)

Reconfigurable metasurface-based 1 × 2 waveguide switch

Amged Alquliah^1、2, Mohamed Elkabbash^{3、4、5、*}, Jinluo Cheng^{1、2、6、*}, Gopal Verma^1、2, Chaudry Sajed Saraj^1、2, Wei Li^{1、2、7、*}, and Chunlei Guo^3、8、*

Author Affiliations

¹GPL, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China

²University of Chinese Academy of Sciences, Beijing 100049, China

³The Institute of Optics, University of Rochester, Rochester, New York 14627, USA

⁴Current address: The Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

⁵e-mail: melkabba@mit.edu

⁶e-mail: jlcheng@ciomp.ac.cn

⁷e-mail: weili1@ciomp.ac.cn

⁸e-mail: guo@optics.rochester.edu

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DOI: 10.1364/PRJ.428577 Cite this Article Set citation alerts

Amged Alquliah, Mohamed Elkabbash, Jinluo Cheng, Gopal Verma, Chaudry Sajed Saraj, Wei Li, Chunlei Guo. Reconfigurable metasurface-based 1 × 2 waveguide switch[J]. Photonics Research, 2021, 9(10): 2104 Copy Citation Text

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Abstract

Reconfigurable nanophotonic components are essential elements in realizing complex and highly integrated photonic circuits. Here we report a novel concept for devices with functionality to dynamically control guided light in the near-visible spectral range, which is illustrated by a reconfigurable and non-volatile (

1 \times 2

) switch using an ultracompact active metasurface. The switch is made of two sets of nanorod arrays of

{TiO}_{2}

and antimony trisulfide (

{Sb}_{2} S_{3}

), a low-loss phase-change material (PCM), patterned on a silicon nitride waveguide. The metasurface creates an effective multimode interferometer that forms an image of the input mode at the end of the stem waveguide and routes this image toward one of the output ports depending on the phase of PCM nanorods. Remarkably, our metasurface-based

1 \times 2

switch enjoys an ultracompact coupling length of 5.5 μm and a record high bandwidth (22.6 THz) compared to other PCM-based switches. Furthermore, our device exhibits low losses in the near-visible region (

\sim 1 dB

) and low cross talk (

- 11.24 dB

) over a wide bandwidth (22.6 THz). Our proposed device paves the way toward realizing compact and efficient waveguide routers and switches for applications in quantum computing, neuromorphic photonic networking, and biomedical sensing and optogenetics.

1. INTRODUCTION

The advancement of nanofabrication, coupled with the attainment of a high level of complexity in photonic integrated circuits (PICs), triggered tremendous interest toward realizing miniaturized all-optical interconnects that are superior to electronic circuits in terms of bandwidth density, speed, and energy efficiency as well as mitigating the von Neumann data transmission bottleneck [1 –3]. Particularly, the reconfigurable control of light propagating in PICs is crucially important for many emerging applications such as programmable PIC [4], neuro-inspired computing [5,6], quantum information processing [7,8], optical communication [9,10], microwave photonics [11], and sensor applications [12,13].

Reconfigurable photonic computing cores are conventionally implemented using waveguide meshes of Mach–Zehnder interferometers (MZIs) where the interference is controlled via two phase shifters that are independently tuned through volatile and weak modulation of the waveguide refractive index commonly using electro-optic or thermo-optic effects [4], leading to devices with a limited tunability, high energy consumption [several milliwatts (mW)], and large footprints [hundreds of micrometers (μm)] [14]. On the other hand, micro-ring resonators (MRRs) [15 –20] and micro-electromechanical systems (MEMS) [21,22] offer high modulation depth and a relatively small footprint. Still, they suffer from a narrow operational bandwidth (less than 3 dB) [20], low tolerance to temperature variations and fabrication errors, as well as a large actuation voltage ( $> 40 V$ ) [23]. Notably, systems based on exciting surface plasmon resonances (SPRs) enjoy the highest switching rates and smallest footprints. However, they suffer from high insertion and propagation losses as they require coupling from/to a photonic waveguide [24,25], and plasmonic metals are highly lossy [25], which hinder their widespread usage. The common volatility of these schemes necessitates an “always-on” power supply to retain the switching state, rendering them energy inefficient [14,26].

To circumvent these hurdles, phase-change materials (PCMs) emerged as candidates to demonstrate photonic reconfigurability owing to their unique tunable properties [27,28]. PCMs possess high contrasts in the electrical resistivity and refractive index between the resonant-bonded crystalline and covalent-bonded amorphous phase states over a wide spectral range. They are non-volatile, reversible, and they provide fast and low energy actuation ( $< 10 aJ / {nm}^{3}$ [29]) by ultrashort electrical or optical pulses (up to subnanosecond) [30] and stable switching ability of more than $10^{15}$ cycles [31]. In addition, the scalability of PCMs makes their nanofabrication relatively approachable and compatible with other substrates, as their amorphous state is used during deposition [23]. Several PCM-based integrated photonic devices were recently demonstrated, e.g., photonic memories [32,33], optical modulators [34,35], optical switches [14 –20], and optical computing [36,37]. In these applications, a top-cladding layer of ${Ge}_{2} {Sb}_{2} {Te}_{5}$ (GST) and, recently, ${Ge}_{2} {Sb}_{2} {Se}_{4} {Te}_{1}$ (GSST) was deposited onto a waveguide. However, the high absorption losses in one of the phases of such materials fundamentally restrict their use in phase modulation schemes for large-area PICs [4] and deep-neural networks [5,6], where light would propagate through several PCM-based interconnects. Although losses can be alleviated in devices working at the telecom wavelengths, this comes with the cost of sacrificing the device footprint. In addition, such approach is impractical at the visible and near-visible wavelengths due to the large intrinsic losses. Finally, the use of large-area PCMs creates a considerable barrier to the actuation mechanism (see Appendix A for more details).

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On the other hand, optical metasurfaces enable unprecedented flexibility in controlling the propagation of light through a spatially dependent and abrupt phase change at an interface that is imposed by ultrathin artificial arrays of engineered subwavelength nanoantennas [38]. Metasurfaces have realized a plethora of ultracompact, broadband, and efficient on-chip photonic devices, such as mode converters [39], polarization rotators [39], mode-pass polarizers [40], power splitters [41], asymmetric power transmitters [39], second-harmonic generators [42], remote near-field controllers [43], and guided waves to free space wave couplers [44 –46]. However, these devices are passive and application specific because the optical properties of the constitutional meta-atoms are permanent once fabricated. To actively tune the optical properties of metasurfaces, several approaches have been introduced [47,48] with considerable interest in using PCMs [28,49] due to their advantages as discussed above. So far, such investigations on reconfigurable metasurfaces are focusing on controlling light propagating in free space, e.g., for bichromatic and multifocus Fresnel zone plates [50], beam steering [31], tunable color generators [51], dynamic spectrum controllers [52], switchable spectral filters [53], information processing [54], communication [55], imaging [56], hologram and augmented reality [57 –59], vortex beam generators, and illusion and cloaking [60].

In this paper, we extend the concept of reconfigurable metasurfaces to PICs by reporting a novel dynamic near-visible nanophotonic ( $1 \times 2$ ) switch enabled by combining two nanorod arrays of ${TiO}_{2}$ and a novel ultralow-loss PCM (antimony trisulfide ${Sb}_{2} S_{3}$ ) superimposed on silicon nitride waveguide. Our designed device provides unprecedented functionality as it facilitates the non-volatile dynamic routing of light inside a multimode waveguide toward predefined outputs. The device shows a wide bandwidth of 22.6 THz, low loss ( $\sim 1 dB$ ), and low cross talk ( $< - 11.29 dB$ ) with a compact active length (5.5 μm). To the extent of our knowledge, our design enjoys a record footprint compared to PCM-based optical switches, which would overcome the challenges associated with large-area PCM actuation. Table 1 compares the design principles, the type of PCM designs, and performance of the reported ( $1 \times 2$ ) PCM-based switches and of our metasurface PCM-based switch. Appendix B discusses the need for visible/near-visible PICs.Table 1.

Comparison of Previously Reported PCM-Based ( $1 \times 2$ ) Switches with Our Metasurface-Based Switch

Design Principle	PCM Design	Bandwidth (THz)	Cross Talk (dB)	Operating Wavelength Range	Reference
Directional coupler	GST layer	3.74	$- 10$	IR	[14]
Directional coupler	GSST layer	4.4	$- 32$	IR	[61]
Contra-directional coupler	GST-Si grating	0.275	$- 30$	IR	[62]
Micro-ring resonator	GST layer	0.125	$- 6$	IR	[16]
Micro-ring resonator	GST layer	0.125	$- 33$	IR	[17]
Micro-ring resonator	GST layer	0.125	$- 42$	IR	[18]
Micro-ring resonator	GST layer	0.125	$- 5$	IR	[15]
Micro-ring resonator	GST nanodisk	0.125	$- 5$	IR	[20]
Micro-ring resonator	GST layer	0.125	$- 14.1$	IR	[19]
Micro-ring resonator	${Sb}_{2} S_{3}$ layer	0.125	NA	Visible	[63]
Y branch	Sb₂S₃/TiO₂metasurface	22.6	$- 11.24$	Near visible	This work

2. MATERIALS AND METHODS

Figure 1(a) schematically illustrates the structure and functionality of our device, which is a Y-branch waveguide with a metasurface located on the surface of its stem. From bottom to top, the waveguide is formed by a silicon substrate, a 3 μm thick ${SiO}_{2}$ layer, and a ridge SiN waveguide. The metasurface consists of two sets of nanorods. Each set includes 28 equally spaced nanorods with a length $L$ and a center-to-center distance between neighbor nanorods $Λ$ . These two sets are aligned to form two adjacent rectangles separated by a gap $g$ . The total footprint of the metasurface is $L_{x}$ .

Figure 1.Design of the proposed metasurface-based reconfigurable ( $1 \times 2$ ) integrated switch working around $λ = 800 nm$ . (a) 3D illustrations of the device structure and its functionality when the ${Sb}_{2} S_{3}$ metasurface structure is in a crystalline or amorphous state. The top-left inset shows the top view of the device with dimensions of the metasurface consisting of a set of passive ${TiO}_{2}$ nanorods and another set of PCM ( ${Sb}_{2} S_{3}$ ) nanorods. The top middle inset shows the conditions required for the ${Sb}_{2} S_{3}$ to undergo a reversible structural transition from amorphous to crystalline states. (b) Cross section of the device. (c) and (d) Wavelength dependence of the complex optical constants (n, k) of amorphous ${Sb}_{2} S_{3}$ , crystalline ${Sb}_{2} S_{3}$ , amorphous titanium dioxide, and silicon nitride. Highlighted parts in red indicate the spectral region of interest where ${Sb}_{2} S_{3}$ exhibits low loss and high switching contrast.

The materials are chosen to realize a device appropriate for near-visible integrated photonics. The SiN waveguide has the advantages of thermodynamic stability, wide spectral range of transparency ( $λ = 0.25 - 8 μm$ ), higher fabrication flexibility, and lower propagation losses compared to silicon waveguides [64]. The materials of the two sets of nanorods are chosen as PCM antimony trisulfide ( ${Sb}_{2} S_{3}$ ) for one set and amorphous titanium dioxide ( $a - {TiO}_{2}$ ) for the other. Compared to the prototypical PCM (GST), the new class of PCM ( ${Sb}_{2} S_{3}$ ) has a bandgap tunable from 2.0 eV (crystalline) to 1.7 eV (amorphous), and thus it works in the near-visible spectrum with a low absorption coefficient and a relatively higher contrast in the change of the real part of its refractive index $Δ n$ [65 –67]. $A - {TiO}_{2}$ is chosen because it is lossless in the near-visible range and enjoys a high refractive index to provide strong guided light–nanorod interaction [68]. The top middle inset in Fig. 1(a) shows the reversible optical switching of the ${Sb}_{2} S_{3}$ from the crystalline to amorphous phases. The optical switching [65 –67] can be done using a 630 nm laser source with a power of up to 90 mW. This laser source provides stable performance and cycling durability of ${Sb}_{2} S_{3}$ during the experiment [67]. The reversible switching between both structural states of ${Sb}_{2} S_{3}$ is realized by controlling the width and focus of the laser pulse [67]. The crystallization process requires pulses with long time duration (100 ms) [67] to reach the ${Sb}_{2} S_{3}$ glass transition temperature at 270°C [67]. In contrast, the re-amorphization process requires pulses with shorter time duration (400 ns) [67] to reach the ${Sb}_{2} S_{3}$ melting temperature at 527.85°C [67] and then quickly cool it at a rate $> 20 ° C / ns$ [53,67,69].

In our designed metasurface, the nanorod antennas cause a spatial linear phase shift of the optical mode in the waveguide in addition to the propagation phase along the propagation direction ( $x$ axis). The accumulative local phase shifts from the two nanorod sets result in a constructive interference that focuses the input fundamental mode in the stem multimode waveguide. Following that, the focused input mode partially converts the input fundamental mode to higher-order modes, creating an effective asymmetric multimode interferometer (MMI). Consequently, an image of the input mode is formed at the end of the stem multimode waveguide due to the self-imaging principle of the MMI [70]. The produced image is then routed in one of the predefined output ports depending on the phase of the PCM nanorods (specifically depending on the effective refractive index of the nanorod arrays). Therefore, when ${Sb}_{2} S_{3}$ is in the crystalline state, it has a significantly higher effective index, causing a larger phase delay at the side where the PCM nanorods are located. In contrast, when the ${Sb}_{2} S_{3}$ is in the amorphous state, the ${TiO}_{2}$ causes a larger phase delay at the other side of the waveguide, leading to the field constructively interfering at the opposite side.

The optical properties of the device were simulated using the finite-difference time-domain method (FDTD, Mode Solutions, Lumerical Ansys Inc.). The simulation domain was enclosed by eight standard perfectly matched layers as boundary conditions. To minimize the numerical dispersion and to enhance the interfaces’ resolution, we chose the nonuniform auto mesh setting with a minimum size of conformal mesh cell of 2.5 nm. The fundamental TE mode was launched into the stem SiN waveguide using a broadband mode source and enabling multifrequency calculations. The frequency-domain time power monitors were used to record the profile of the normalized electric field intensity, transmission at output ports, reflection, and scattering. For calculating the effective indices as well as the dispersion in the SiN waveguide, the finite-difference eigenmodes solver (FDE, Mode Solutions, Lumerical Ansys Inc.) was used. To estimate the effective indices of metasurface nanorod arrays, the effective medium theory was considered by using Rytov’s approximations.

To optimize the device performance, the ridge SiN multimode waveguide is chosen to have a rectangular shape with a height $h = 220 nm$ , a thickness of under-etched layer $s = 200 nm$ , and a sufficiently large width $d = 3.5 μm$ to support the multimode interference and to reduce the cross talk between the output ports [70] [see Fig. 1(b)]. Note that the two output waveguides are narrower to match the spot size of the produced single self-images and optimize the device cross talk. The mode of the output waveguides can be coupled with other devices with arbitrary widths using techniques such as waveguide tapers [71 –74] or integrated mode size converters [75 –77]. Appendix C shows the dependence of effective refractive indices and dispersion of modes on the width of the SiN waveguide. The two output ports have a width $d_{out} = 1.65 μm$ and are attached to the end of the stem waveguide at a position where the self-image is formed. During the simulation, the complex optical constants of SiN, ${SiO}_{2}$ , Si, and ${TiO}_{2}$ are obtained from the database of Palik [78], while the experimental values of ${Sb}_{2} S_{3}$ are taken from Refs. [67,79]. Figures 1(c) and 1(d) list the optical constants of $a - {TiO}_{2}$ , SiN, and two phases of ${Sb}_{2} S_{3}$ . To compromise between the absorption coefficient (k) and the refractive index contrast ( $Δ n$ ), the operating bandwidth is set around 800 nm as shown in the red bars in Figs. 1(c) and 1(d).

The performance metrics of a waveguide optical switch are the cross talk and the transmission, where the former is defined as the contrast transmission ratio between the two output ports [14]. Although it is not easy to define a qualitative quantity for the performance metric, an efficient optical switch should exhibit low cross talk and high transmission. The metasurface was engineered adopting the direct design approach [80]. To obtain the target performance, first, the width ( $W_{{Sb}_{2} S_{3}}$ ) and height ( $h_{s}$ ) of ${Sb}_{2} S_{3}$ nanorods were fixed at ( $W_{{Sb}_{2} S_{3}} = 55 nm$ , $h_{s} = 50 nm$ ) to support Mie resonant modes in the nanoantennas [39,42,81,82] and to meet the current state of manufacturing capacity [65,66]. The period ( $Λ$ ) was set at 200 nm to avoid diffraction effects [83,84]. Following that, the dimensions of ${TiO}_{2}$ -nanorods (width and height) were optimized to realize the desired optical routing response (see Appendix D for further details). The gap width ( $g$ ) between the two nanorod sets was kept at 180 nm to bring about degrees of freedom to the metasurface fabrication. Finally, $L_{x}$ , $L$ , $g$ , and $Λ$ were parametrically swept in sequential order as shown in Figs. 2(a)–2(d). The final dimensions of the metasurface are as follows: the TiO₂ nanorods have a width of 72 nm and a thickness of 250 nm; $Λ = 200 nm$ , $L = 1060 nm$ , $g = 180 nm$ , and $L_{x} = 5.5 μm$ , indicating an ultracompact footprint (Fig. 2, inset). The suggested fabrication method and working setup of the proposed device are provided in Appendix E.

Figure 2.Metasurface parametric sweep. (a)–(d) Device cross talk as a function of metasurface footprint ( $L_{x}$ ), nanorod length ( $L$ ), separated gap width ( $g$ ), and the nanorods’ center-to-center distance ( $Λ$ ), respectively, for $a - {Sb}_{2} S_{3}$ and $c - {Sb}_{2} S_{3}$ . The dotted gray lines show the dimensions used in our metasurface.

3. RESULTS AND DISCUSSION

Before presenting the functionality of the metasurface, it is instructive to characterize how the single metasurface nanoantenna confines the light inside the waveguide. Figures 3(a) and 3(b) show the calculated field distribution ( $E_{z}$ components) and the electric field intensity ${| E |}^{2}$ distribution in the nanorods and for both phases of ${Sb}_{2} S_{3}$ . In our design, the dimensions of the ${TiO}_{2}$ nanorods are larger than those of the ${Sb}_{2} S_{3}$ nanorods in order to increase the magnitude of phase delay (i.e., effective refractive index) and compensate for its lower refractive index compared to $a - {Sb}_{2} S_{3}$ . Therefore, for $a - {Sb}_{2} S_{3}$ , the field is mostly confined in the ${TiO}_{2}$ nanorod. When the PCM changes its state to $c - {Sb}_{2} S_{3}$ , the field becomes strongly confined in the ${Sb}_{2} S_{3}$ rods. Figure 3(c) shows the length dependence of the calculated effective refractive index for different nanorod antennas positioned on the SiN substrate at $λ = 800 nm$ . The FDE method was implemented to obtain the effective refractive indices caused by metasurface nanoantennas. A significant $n_{eff}$ difference is obtained between $a - {Sb}_{2} S_{3}$ and $c - {Sb}_{2} S_{3}$ antennas due to their high refractive index contrast. The $n_{eff}$ of ${TiO}_{2}$ lies between the $n_{eff}$ of $a - {Sb}_{2} S_{3}$ and $c - {Sb}_{2} S_{3}$ , which in turn partly explains the switching functionality of our device. Appendix F discusses the method used for calculating the equivalent effective indices of the ${Sb}_{2} S_{3}$ and ${TiO}_{2}$ nanorod arrays. These results show that employing high-index dielectric/PCM nanoantennas on a waveguide offers a unique ability to independently and dynamically tune the localization of guided light by engineering the relative effective indices induced by the metasurface nanoantennas.

$Characterization of nanoantenna structure. (a) Profile (zy plane) of the normalized near field of the Ez component showing scattering Mie modes in the TiO2 and Sb2S3 nanoantennas for a-Sb2S3 (left panel) and c-Sb2S3 (right panel) at λ=800 nm. (b) Normalized electric field intensity |E|2 distribution (zy plane) in the same nanorods for a-Sb2S3 and c-Sb2S3. The boundaries of the SiN waveguide and nanoantennas are indicated in solid black lines. (c) Effective refractive index of the TE00 mode as a function of TiO2 nanoantenna length and Sb2S3 nanoantenna length for the a-Sb2S3 and c-Sb2S3 phases. The dashed gray line indicates the length of the nanorods (L) used in our simulation. The inset figure shows the FDE simulation setup.$

Figure 3.Characterization of nanoantenna structure. (a) Profile (zy plane) of the normalized near field of the $E_{z}$ component showing scattering Mie modes in the ${TiO}_{2}$ and ${Sb}_{2} S_{3}$ nanoantennas for $a - {Sb}_{2} S_{3}$ (left panel) and $c - {Sb}_{2} S_{3}$ (right panel) at $λ = 800 nm$ . (b) Normalized electric field intensity ${| E |}^{2}$ distribution (zy plane) in the same nanorods for $a - {Sb}_{2} S_{3}$ and $c - {Sb}_{2} S_{3}$ . The boundaries of the SiN waveguide and nanoantennas are indicated in solid black lines. (c) Effective refractive index of the ${TE}_{00}$ mode as a function of ${TiO}_{2}$ nanoantenna length and ${Sb}_{2} S_{3}$ nanoantenna length for the $a - {Sb}_{2} S_{3}$ and $c - {Sb}_{2} S_{3}$ phases. The dashed gray line indicates the length of the nanorods (L) used in our simulation. The inset figure shows the FDE simulation setup.

Figure 4(a) shows the normalized electric field intensity profile ${| E |}^{2}$ in the switch for $a - {Sb}_{2} S_{3}$ at a wavelength 800 nm in the xy plane, when the fundamental TE mode is launched from the stem multimode waveguide to the Y branches along the propagation direction ( $x$ axis). In this case, the field is localized in the ${TiO}_{2}$ nanoantennas as shown in the enlarged view of the inset of Fig. 4(a); the result is consistent with Fig. 3(a), where TiO₂ nanorods show stronger confinement than a-Sb₂S₃ nanorods. Figure 4(b) shows the calculated transmission spectra from ${Port}_{1}$ to ${Port}_{2}$ and from ${Port}_{1}$ to ${Port}_{3}$ . The average simulated insertion loss in the target port is $< 0.706 dB$ . The insertion losses are caused mainly by the reflected optical power resulting from the impedance mismatch with the metasurface nanoantennas and the scattering losses are due to the strong light–antenna interactions at subwavelength intervals. The average value of the cross talk is found to be $- 11.24 dB$ throughout the operating bandwidth.

Figure 4.Simulated device performance for $a - {Sb}_{2} S_{3}$ (upper panel) and $c - {Sb}_{2} S_{3}$ (lower panel). (a) and (d) Full-wave simulation showing the optical field intensity ${| E |}^{2}$ in the switch for the fundamental TE mode in the xy plane at $λ = 800 nm$ . The boundaries of the SiN waveguide and metasurface structure are indicated by dashed lines and rectangles, respectively. Inset: enlarged view of the field profile in the metasurface. (b) and (e) Transmission spectra at two output ports ${Port}_{2}$ and ${Port}_{3}$ . (c) and (f) Total transmission at output ports ( ${Port}_{2} + {Port}_{3}$ ), reflection, and scattering of the device.

To further characterize the device performance, Fig. 4(c) shows the spectra of total transmission (T, ${Port}_{2} + {Port}_{3}$ ), reflection (R), and scattering (S) over the simulated wavelength region.

Figures 4(d)–4(f) show the functionality of the switch for $c - {Sb}_{2} S_{3}$ . In this case, the field is localized in the $c - {Sb}_{2} S_{3}$ nanorods because of its stronger light confinement than TiO₂ nanorods [see Fig. 3(b)]. From the calculated transmission spectra in Fig. 4(e), the average value of cross talk is $- 10.01 dB$ throughout the operating bandwidth. The average calculated insertion loss is $\sim 1 dB$ , which is lower than the efficiency when the switch is in the $a - {Sb}_{2} S_{3}$ . The higher insertion losses are due to increased reflection and scattering losses are because of the higher refractive index of $c - {Sb}_{2} S_{3}$ compared to $a - {Sb}_{2} S_{3}$ . Figure 4(f) shows the spectra of total transmission (T, ${Port}_{2} + {Port}_{3}$ ), reflection (R), and scattering (S) over the simulated wavelength region. We note that the produced modes at ${Port}_{2}$ and ${Port}_{3}$ maintain the polarization of the input ${TE}_{00}$ mode and do not experience any polarization rotation (see Fig. 12 in Appendix G).

4. CONCLUSION

In conclusion, we have proposed a conceptually novel approach for devices that dynamically control guided light using a reconfigurable metasurface. We have demonstrated a broadband compact and low-loss ( $1 \times 2$ ) switch for near-visible wavelengths by integrating a metasurface consisting of two nanorod arrays of ${TiO}_{2}$ and ${Sb}_{2} S_{3}$ on a silicon nitride waveguide. Our demonstrated device enjoys a record high bandwidth (22.6 THz) compared to other phase-change-material-based switches while having low loss ( $\sim 1 dB$ ), low cross talk ( $- 11.24 dB$ ), and ultracompact active length (5.5 μm). The proposed device could be a reliable component in the meshes of future energy-efficient large-scale PICs. Moreover, we believe that integrating active metasurfaces with photonic waveguides, as demonstrated in our example, may provide a step change toward realizing several tunable, efficient, and non-volatile chip-scale devices for applications in neuromorphic computing [5,6], quantum information processing [7,8], optical communication [9,10], microwave photonics [11], and biomedical sensing [12,13].

Acknowledgment

Acknowledgment. A. A. acknowledges the CAS-TWAS Presidents Fellowship Program. M. E. initiated the project and conceived the approach. C. G., W. L., M. E., and J. C. supervised the project. A. A. designed the metasurface and performed simulations. A. A., M. E., W. L., J. C., and G. V. analyzed the data. C. S. contributed to the discussions. A. A. wrote the paper with input from all the authors. All authors discussed the research.

APPENDIX A: THE DRAWBACKS OF USING LARGE-AREA PCMs FOR DEVICES’ ACTUATION MECHANISMS

The use of large-area PCMs creates a considerable barrier to the actuation mechanism because of the following reasons: first, the lack of optimization for additional scaling and integration because of the inaccurate, slow, and diffraction-limited alignment process [85]; second, the difficulty in attaining similar levels of crystallization and amorphization using a thermodynamic mechanism over a large-area PCM [67,86]; third, large-area PCMs prevent achieving the sufficient cooling rate [31,53] that is necessary for the re-amorphization process of PCMs (because PCMs suffer from low thermal conductivity [31,87,88]). These reasons lead to weak and inconsistent switching behavior of large-area PCMs [23,67,86]. In addition, another significant limitation is the filamentation phenomenon associated with electrical switching. Such filamentation causes a nonuniform crystallization throughout large-area PCMs. These limitations eventually lead to devices with weak and inconsistent switching behavior [86,88,89].

1 \times 10^{6}

APPENDIX B: THE NEED FOR VISIBLE/NEAR-VISIBLE PHOTONIC INTEGRATED DEVICES

Δ n

APPENDIX C: THE DESIGN OF A STEM MULTIMODE WAVEGUIDE

W

Figure 5.Multimode waveguide characterization. The dependence of waveguide width on the (a) $n_{eff}$ and (b) dispersion D of different SiN waveguide modes at $λ = 800 nm$ . The shaded region indicates the waveguide widths that support asymmetric multimode. The dashed black lines show the maximal modal index ( $n_{eff}$ ) in the waveguide and the waveguide width that support a single mode. The gray dotted line indicates the width considered in the stem SiN waveguide. (c) $n_{eff}$ and D of the fundamental ${TE}_{00}$ mode as a function of simulated wavelengths. The inset figure shows the $E_{y}$ distribution of the ${TE}_{00}$ mode at $λ = 800 nm$ .

APPENDIX D: THE ROLE OF OPTIMIZING THE HEIGHT AND WIDTH OF TiO2 IN THE DEVICE PERFORMANCE

{TiO}_{2}

Figure 6.Parametric sweep of ${TiO}_{2}$ nanorods for the ${TE}_{00}$ mode at $λ = 800 nm$ . (a) and (b) Simulated device cross talk considering variations of the height and width of ${TiO}_{2}$ nanorods when ${Sb}_{2} S_{3}$ is in the crystalline and amorphous state, respectively. (c) and (d) Calculated transmission at the desired output as a function of height and width of ${TiO}_{2}$ nanorods for the crystalline and amorphous state, respectively. The black dashed lines indicate the dimensions used in our metasurface.

Figure 7.Full-wave simulation showing the optical field intensity ${| E |}^{2}$ in the switch for the fundamental TE mode in the xy plane at $λ = 800 nm$ and for different TiO₂ dimensions for (a)–(c) $c - {Sb}_{2} S_{3}$ and (d)–(f) $a - {Sb}_{2} S_{3}$ , The boundaries of the SiN waveguide and metasurface structure are indicated by dashed lines and rectangles, respectively. Inset: enlarged view of the field profile in the metasurface.

APPENDIX E: THE SUGGESTED FABRICATION METHOD AND EXPERIMENTAL SETUP OF RECONFIGURABLE METASURFACE-BASED (1×2) WAVEGUIDE SWITCH

1. Suggested Fabrication Method

{SiO}_{2} / Si

Figure 8.Suggested fabrication method employing three steps of electron beam lithography: (a) two positive resists followed by LPCVD to transfer the desired patterns of the nanorod arrays onto the developed gaps and (b) a negative resist followed by reactive ion etching (RIE) to define the SiN waveguide.

Figure 9.Device fabrication tolerance. (a) and (b) Simulated device cross talk for the fundamental mode operating at $λ = 800 nm$ , considering possible parallel misalignment (offset) and axial misalignment (gap width), respectively. The gray dotted lines in the figures indicate the dimensions used in our device.

Figure 10.Schematic illustration of the experimental setup suggested for characterizing the performance of the proposed reconfigurable switch. Here, PPG is a programmable pulse generator, PC is a power controller, and M is a mirror.

APPENDIX F: THE EFFECTIVE INDICES OF A NANOROD-ARRAY-LOADED SiN WAVEGUIDE

The effective refractive indices of each nanorod array in our metasurface were calculated using Rytov’s approximation [99] as follows.

{Sb}_{2} S_{3}

{TiO}_{2}

x

$Sketches showing x-periodic Sb2S3 and TiO2 nanoantennas patterned on a SiN waveguide in (a) xy plane and (b), (c) zy plane, where na/c are the effective indices of a-Sb2S3 and c-Sb2S3 nanorods, n2 is the effective index of the TiO2 nanorods, and nbare is the effective index of bare SiN waveguide. (d) Effective refractive indices for the waveguide cross section calculated using FDE. The inset shows the FDE simulation setup. (e) Effective indices for nanorod arrays calculated by Rytov’s approximation.$

Figure 11.Sketches showing $x$ -periodic ${Sb}_{2} S_{3}$ and ${TiO}_{2}$ nanoantennas patterned on a SiN waveguide in (a) xy plane and (b), (c) zy plane, where $n_{a / c}$ are the effective indices of $a - {Sb}_{2} S_{3}$ and $c - {Sb}_{2} S_{3}$ nanorods, $n_{2}$ is the effective index of the ${TiO}_{2}$ nanorods, and $n_{bare}$ is the effective index of bare SiN waveguide. (d) Effective refractive indices for the waveguide cross section calculated using FDE. The inset shows the FDE simulation setup. (e) Effective indices for nanorod arrays calculated by Rytov’s approximation.

Figure 12.Modes in the input and output ports of SiN waveguides. Simulated E_y component at $λ = 800 nm$ for the ${TE}_{00}$ mode in the (a) input stem SiN waveguide before interacting with the metasurface, (b) output ${Port}_{3}$ for $a - {Sb}_{2} S_{3}$ and (c) output ${Port}_{2}$ for $c - {Sb}_{2} S_{3}$ . The arrows indicate the vector diagrams of the electric field component of the modes. The dashed black lines show the boundaries of the SiN waveguides.

APPENDIX G: MODAL PROFILES AT THE DEVICE INPUT AND OUTPUT PORTS

It is important to note that the produced modes at the output ports of the device maintain the polarization of the input TE 00 mode in both Sb 2 S 3 phases. Figure 12 (a-c) shows the electric field component E y for TE 00 mode at the input and output ports of the device. The arrows illustrate the corresponding vector diagrams of the electric fields of the modes at the input and output ports.

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