Low-Loss Integrated Photonic Switch Using Subwavelength Patterned

Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX

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Low-Loss Integrated Photonic Switch Using Subwavelength Patterned Phase Change Material Changming Wu,† Heshan Yu,‡ Huan Li,§ Xiaohang Zhang,‡ Ichiro Takeuchi,‡ and Mo Li*,†,§,∥ †

Department Department § Department ∥ Department Downloaded via UNIV OF SOUTH DAKOTA on December 22, 2018 at 10:00:10 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

of of of of

Electrical and Computer Engineering, University of Washington, Seattle, Washington 98195, United States Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States Physics, University of Washington, Seattle, Washington 98195, United States

S Supporting Information *

ABSTRACT: Efficient integrated photonic switches play a critical role in both interchip optical interconnects and data center networks that need to be dynamically reconfigured. Here, we demonstrate a 1 × 2 switch using phase change material Ge−Sb−Te (GST) combined with a silicon nitride microring resonator. The switch operates by utilizing the dramatic difference in the optical refractive index and extinction coefficient between the crystalline and amorphous phases of GST. By patterning and encapsulating the GST into subwavelength structures, the device achieves a low insertion loss of less than 1 dB in both output ports and can be switched reliably both photothermally and electrothermally. KEYWORDS: silicon photonics, phase-change materials, optical switch, microring resonator, germanium antimony telluride

R

a common feature of them is that sustained power consumption is required to maintain the switched state (on or off). Although the energy consumption per bit decreases as the bitrate increases, the total power consumption of a largescale switch system may still be prohibitive. Therefore, a nonvolatile switch technology that consumes no additional power to sustain the switched state and has compelling performances in the loss, speed, and size is highly desirable. Phase change materials (PCMs) has been conceived as a very promising candidate material to realize reconfigurable photonic devices as they afford drastic, nonvolatile changes in the optical properties when the phase transition occurs.19,20 Particularly, germanium antimony telluride alloys (Ge−Sb−Te or GST) have been widely used in optical information storage by using optical heating to induce phase transition.21−23 The family of GST PCMs has many compelling characteristics that make them very promising for switching and many other optical applications. First, the drastic change of their refractive index and extinction coefficient between the two phases can lead to strong phase or amplitude modulation within a small device footprint. Second, the phase change is nonvolatile, hence it requires zero-power to sustain the switched phase. Third, the phase transition speed can be faster than a nanosecond and thus ultrafast switching speed is possible.24 Finally, the phase transition can be induced both electrically and optically, thus both electrical and all-optical operation can

ecent advancement of large-scale silicon photonics promises integrated optical interconnects with dramatic bandwidth density advantage over electrical connections.1,2 The availability of extreme communication bandwidth between processors, memory, and storage units would revolutionize the architecture of both high-performance computing systems and data centers.3−7 For many of these applications, on-chip highdensity photonic network fabric that can be dynamically reconfigured will be essential to efficiently routing data between different functional units of the system. The key building-block components to realize such an adaptive optical network are photonic relay switches with high-performance.7−9 These switches need to have a sufficiently low loss, high energy efficiency, and high speed, as well as a small footprint and compatibility with silicon fabrication processes. On the silicon photonics platform, optical switches have been demonstrated by using carrier injection or depletion in a p−i−n diode structure or photothermal effects by locally heating silicon waveguides. Both Mach−Zehnder interferometer and microring resonator configurations have been employed, while the former provides a wide operation bandwidth,10−12 the latter enhances the efficiency from optical resonances.13−16 Notably, large arrays of waveguide cross-bar switches actuated by microelectromechanical systems (MEMS) have also been developed on the silicon photonic platform, demonstrating remarkably low insertion loss and crosstalk.17,18 However, a large actuation voltage up to 40 V may cause a problem to many applications. Despite the pros and cons of various optical switching technologies listed above, © XXXX American Chemical Society

Received: November 1, 2018 Published: December 19, 2018 A

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Figure 1. Operation principle and measurement scheme. (a) The 1 × 2 optical switch operates by triggering the phase transition of the GST film embedded in a microring resonator. When the GST is in the amorphous phase, the signal resonantly couples into the microring and outputs from the drop port with a very low loss. When the GST is in the crystalline phase, the signal is decoupled from the microring and outputs directly from the through port to avoid any optical loss. (b) The measurement scheme uses pulses from a pump laser to optically control the phase transition of the GST, thereby switches the output of the probe laser between the through and drop ports.

Figure 2. Nanopatterning enables complete and repeatable switching and encapsulating prevents deforming of the GST film. (a) Optical microscope image of a Si3N4 waveguide device with an integrated GST film. (b, c) Scanning electron microscope (SEM) image of five GST nanodisks with a diameter of 500 nm patterned directly on the waveguide, (b) before and (c) after many cycles of optically induced phase transition. (d) The change of the waveguide transmission when the GST is changed between the amorphous phase and the crystalline phase in multiple steps. The device with encapsulated disks shows much higher contrast than that with unpatterned GST patches without encapsulation. Inset: SEM image of the deformed GST patch after one optical pulse; scale bar: 500 nm.

uses a typical microring add-drop filter configuration with two coupling waveguides. In this study, the photonic devices are made in silicon nitride, but silicon can also be used. The device has two output ports: a through (or cross) port and a drop (or bar) port. A layer of GST film is directly patterned on top of the waveguide forming the microring so its optical mode is evanescently coupled with the GST layer. The microring is designed to be near-critically coupled with the coupling waveguides when the GST is in the amorphous phase with a negligibly low absorption such that the resonance of the microring assumes a high-quality factor and a high extinction ratio. Input optical signal tuned to the resonance wavelength will then be coupled into the ring and output from the drop port with a very low insertion loss. When the GST is switched to its crystalline phase, both of its refractive index and extinction coefficient increase significantly. The former causes a large red-shift of the microring’s resonance, far detuned from the input signal’s wavelength, such that the input signal no longer couples into the ring thus is unaffected by the high optical absorption in the crystalline GST. In this phase, the

be realized for components deployed in different depth of an optical network: the former at the interfacial region between electronics and photonics, and the latter for inside the optical fabric. Recently, GST has been integrated directly on silicon photonic waveguides to implement a photonic memory, mimic neural synapses and emulate an abacus calculator.25−30 A 1 × 2 switch has also been demonstrated by integrating GST in a microring resonator but the demonstrated performance in insertion loss and switching extinction ratio are still limited.31 In this work, we demonstrate a 1 × 2 optical switch with a low insertion loss of less than 1 dB in both output ports and a switching extinction ratio up to 20 dB. The performance is achieved by using a GST film with ultralow loss and utilizing its change in both refractive index and extinction coefficient between two phases. Patterning the GST material into subwavelength-scale nanostructures and placing them at the field maximum of the optical mode also significantly facilitates complete phase transition, improves switching performance and prevents deformation that causes excessive optical loss. Figure 1a illustrates the working principle of our device, which B

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Figure 3. (a) Through port and (b) drop port transmission spectra near a resonance of the microring when the GST is in a transition from the crystalline phase (blue) to the amorphous phase (red). (c, d) Log scale spectra highlighting the low insertion loss of 0.46 dB (0.75 dB) and crosstalk −14.1 dB (−22 dB) at the through (drop) port.

First, the small GST disks will be more uniformly heated than a patch with a relatively large area. Encapsulating the disks conformally on all sides also prevents the GST film from reflowing and deforming to minimize surface energy when it is melted at an elevated temperature. Second, the GST disks of subwavelength dimensions can be placed at the field maximum of the waveguide mode so that optically heating them can be more efficient and faster. With this design, we found that a single optical pulse with a duration of 50 ns and total energy of 220 pJ is enough to melt all the five GST disks shown in Figure 2b and “quench” them to fully amorphous phase (aGST). Because of aGST’s low extinction coefficient, hence low absorption, the waveguide transmission increases significantly after quenching, as shown in Figure 2d. Even though aGST has a very low optical absorption, it can still be recrystallized step-by-step in the socalled “annealing” process when the film is heated with much longer laser pulses. Each step is achieved by sending into the waveguide a train of 50 laser pulses with 50 ns duration each and gradually decaying energy. In Figure 2d, we plot the change of waveguide transmission normalized to that when the GST is fully crystallized (T0): ΔT/T0 = (T − T0)/T0, where T is the waveguide transmission with GST in an arbitrary state, to manifest the switching contrast between the aGST and cGST phases. Figure 2d shows that 87% contrast can be achieved in devices including five nanostructured GST disks with a total area of 0.98 μm2. For comparison, we also fabricated devices with a rectangular GST patch covering the full width of the waveguide with a total area of 3.6 μm2. Despite more than three times in the area, the latter device can only achieve 31% contrast. Further examination of the latter device with SEM

input signal outputs from the through port with a very low insertion loss. Therefore, this device design affords a low insertion loss in both output ports, a high switching extinction ratio and low crosstalk. Figure 1b shows an optical microscope image of a device and the measurement schematics to achieve all-optical operation of the switch. A pump laser, connected to an electro-optic modulator (EOM) and an erbium-doped fiber amplifier (EDFA), is used to provide optical pulses to heat the GST film and control its phase transition. The output of the optical signal, generated from the probe laser, at the through port and the drop port is monitored with two photodetectors. The key to implement the proposed optical switch is to be able to control the phase of the GST film integrated on the waveguide in an efficient and reliable way, which has been realized with both electrical and optical heating. To demonstrate all-optical switching of GST, we used a simple waveguide device as shown in Figure 2a. The 1.2 μm wide waveguide is made from 330 nm thick stoichiometric silicon nitride film deposited on an oxidized silicon wafer. A layer of 10 nm thick Ge2Sb2Te5 with a layer of 10 nm thick SiO2 film on top was deposited by sputtering on the wafer. Different from previously demonstrated devices in which simple rectangular patches of GST film were used,30,31 we patterned the GST film into disks with 500 nm diameter and encapsulated them conformally with 50 nm Al2O3 deposited by atomic layer deposition (ALD). Figure 2b shows a scanning electron microscopy (SEM) image of the GST disks on the silicon nitride waveguide, taken when they were just fabricated and in the fully crystalline phase (cGST). Patterning and encapsulating the GST into nanostructures serve two purposes: C

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Figure 4. Optical switching with an integrated electrical heater to facilitate annealing of the GST. (a) Optical image of the switch with an electrical heater made of ITO film. (b) Cross-sectional schematic of the device layer structure. (c) SEM image of the GST and ITO heater area. (d) Realtime switching and resetting operation of the device. Switching is achieved with optical pulses while resetting is done with the electrical heater. The lowquality ITO used for making the heater causes additional optical loss and thus compromises the performance.

1 nm during the full phase transition process, accompanied by an increase in quality factor from 1300 to 5200, and an increase in extinction ratio from 2 to 14 dB. These changes are consistent with the optical properties of the two phases of the GST (Supporting Information). Optimal performance is achieved when the signal is tuned to the resonance wavelength at the aGST phase, which is at 1554.46 nm, as shown in Figure 3c,d plotted in dB scale. The insertion loss is 0.46 dB at the through port (cGST phase) and 0.75 dB at the drop port (aGST phase). The crosstalk is −14 dB at the through port (aGST phase) and −22 dB at the drop port (cGST phase). Therefore, the switching contrast (or on/off ratio) is 13.5 dB at the through port and 21.3 dB at the drop port. These are significant improvements from the previous demonstration of a similar 1 × 2 switch using GST.31 The performance is enabled essentially by the improved properties of the GST film. Particularly, the ultralow absorption in the aGST phase of the material is critical to the low insertion loss at the drop port, which is given by

reveals that, as shown in the inset of Figure 2d, the asfabricated GST film has reflowed and deformed to many worm-shaped stripes after the first optical pulse to quench it. Atomic force microscopy (AFM) measurement also confirms this deformation (see Supporting Information). The deformed aGST with an uneven topography is likely to cause significant scattering loss, reduce transmission, and thus compromise the switching contrast. The uneven film also makes further phase transition cycles less repeatable, as heating of the film will be nonuniform. In stark contrast, Figure 2c shows SEM images of the encapsulated GST disks after many cycles of operation. The disks appear to remain undeformed and even. Intuitively, subwavelength patterning allows GST to be heated more uniformly and efficiently, Al2O3 encapsulation prevents melted GST from reflowing in all directions and quenching into a deformed shape. As a result, the encapsulated GST disks can be completely and repeatably switched between two phases with an optimal transmission contrast. In addition, in the quenching process, multiple intermediate quasi-stable states of waveguide transmission can be reached as the GST is partially crystallized with a mixed phase. Figure 2d shows that eight intermediate states can be reached. The multistate operation can enable novel application of PCM-based photonic devices.28,29 The efficient and precise control of GST phase transition enabled us to achieve optimal performance in the proposed 1 × 2 optical switches. In the device shown in Figure 1b, a 20 nm thick modified Ge2Sb2Te5 film with an ultralow loss in the aGST phase was deposited and patterned on the microring resonator, with a layer of 10 nm thick SiO2 film on top. (See Supporting Information for the deposition method and the optical properties of the GST film.) Figure 3a and b show the transmission spectra near one resonant mode of the microring at the through and the drop ports of the switch, respectively. The spectra are normalized to the maximum transmission in the range so the fiber to grating coupler coupling efficiency, which is about −8 dB, is not considered here. The GST was initially in fully cGST phase and quenched to the aGST phase in steps with single 50 ns, 250 pJ optical pulses tuned to the resonance wavelength. As shown, the resonance blue shifts by

Tdrop =

(1 − r 2)2 a (1 − r 2a)2

where r is the self-coupling coefficient between ring and bus waveguides, a = e−αl is the single-pass amplitude transmission, and α is the loss coefficient. At the same time, the large switching contrast stems from the large resonance shift due to the complete phase change, which is enabled by the nanopatterning and encapsulation technique as described above. Besides the low insertion loss, low crosstalk, and high contrast, the PCM-based switch is nonvolatile so that no energy supply is needed to sustain the switched state. This is in drastic contrast to other switching devices and thus presents a significant advantage in power consumption. The ultralow absorption of the new GST film in its fully amorphous phase, however, also causes an issue for all-optical switching as it cannot be efficiently heated with optical pulses, which makes it difficult to reset the switch optically, requiring more powerful optical pulses. To address this issue, we integrated an additional electrical heater on the device to heat the GST film locally, as shown in Figure 4a. To reduce the loss D

DOI: 10.1021/acsphotonics.8b01516 ACS Photonics XXXX, XXX, XXX−XXX

ACS Photonics caused by the heater, we choose the indium tin oxide (ITO) as the heater material, which has a much lower optical absorption in the near-infrared spectral range than metals.32 The ITO film was deposited by sputtering and annealed at 300 °C in the atmosphere to make it stable under high temperature. The heater wire is 6 μm wide, 200 nm thick, and placed 1.1 μm away from the microring. Both the GST and the ITO heater are encapsulated with a layer of 60 nm thick Al2O3 layer, which serves both as a protection layer and the heat conduction layer, as shown in Figure 4b,c. The device can be operated alloptically as before by quenching the cGST gradually to aGST phase using optical pulses (50 ns duration, 500 pJ energy). By sending electrical pulses (5s duration, 1 V amplitude) to the ITO heater to provide prolonged heating, the fully amorphous GST can be successfully annealed to the cGST phase, and thus resets the switch. Figure 4d shows the real-time transmission during two cycles of switching and resetting. Up to 20 intermediate transmission states can be resolved during the process. The device’s performance, however, has been compromised by the addition of the ITO heater because of the low quality of the ITO film deposited with a nonideal tool. The through port insertion loss is 2.4 dB and crosstalk is −7.2 dB, while the drop port insertion loss is 1.7 dB and crosstalk is −7.7 dB. These can certainly be improved by using a highquality ITO film, which will also significantly improve the heater’s efficiency and the switching speed. In summary, we have demonstrated a low-loss 1 × 2 photonic switch by using phase change material GST integrated with a microring resonator. The phase transition can be controlled both optically and electrically to reach multiple intermediate mix-phase states, enabling multilevel operation of the switch. Benefiting from the ultralow optical absorption of the amorphous phase GST and nanopatterning and encapsulating of the GST film, the switch features a very low insertion loss and crosstalk in both the through and the drop ports. The demonstrated performance shows that the PCM-based photonic switch is promising for scaling up to multiple cascaded stages toward realizing an N × N switching network for demanding applications.7,33

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ACKNOWLEDGMENTS

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REFERENCES

We thank Prof. C. David Wright from the Department of Engineering, University of Exeter, U.K., for supplying some of the GST films used in this study and discussions on their properties. We acknowledge the funding support provided by the ONR MURI (Award No. N00014-17-1-2661). Part of this work was carried out in the University of Minnesota Nanofabrication Center, which receives partial support from the NSF through the NNIN program, and the Characterization Facility, which is a member of the NSF-funded Materials Research Facilities Network via the MRSEC program.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b01516.

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Details of the GST film deposition method and measured optical properties; atomic force microscope (AFM) measurement of the GST film on the photonic devices; and simulation results of the photonic devices with GST film (DOCX).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Huan Li: 0000-0002-8749-0385 Ichiro Takeuchi: 0000-0003-2625-0553 Mo Li: 0000-0002-5500-0900 Notes

The authors declare no competing financial interest. E

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