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Register a new userModeling Electrical Switching of Nonvolatile Phase-Change Integrated Nanophotonic Structures with Graphene Heaters
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Progress in integrated nanophotonics has enabled large-scale programmable photonic integrated circuits (PICs) for general-purpose electronic-photonic systems on a chip. Relying on the weak, volatile thermo-optic or electro-optic effects, such systems usually exhibit limited reconfigurability along with high energy consumption and large footprints. These challenges can be addressed by resorting to chalcogenide phase-change materials (PCMs) such as Ge2Sb2Te5 (GST) that provide substantial optical contrast in a self-holding fashion upon phase transitions. However, current PCM-based integrated photonic applications are limited to single devices or simple PICs due to the poor scalability of the optical or electrical self-heating actuation approaches. Thermal-conduction heating via external electrical heaters, instead, allows large-scale integration and large-area switching, but fast and energy-efficient electrical control is yet to show. Here, we model electrical switching of GST-clad integrated nanophotonic structures with graphene heaters based on the programmable GST-on-silicon platform. Thanks to the ultra-low heat capacity and high in-plane thermal conductivity of graphene, the proposed structures exhibit a high switching speed of ~80 MHz and high energy efficiency of 19.2 aJ/nm^3 (6.6 aJ/nm^3) for crystallization (amorphization) while achieving complete phase transitions to ensure strong attenuation (~6.46 dB/micron) and optical phase (~0.28 dB/micron at 1550 nm) modulation. Compared with indium tin oxide and silicon p-i-n heaters, the structures with graphene heaters display two orders of magnitude higher figure of merits for heating and overall performance. Our work facilitates the analysis and understanding of the thermal-conduction heating-enabled phase transitions on PICs and supports the development of the future large-scale PCM-based electronic-photonic systems.
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Reconfigurable photonic systems featuring minimal power consumption are crucial for integrated optical devices in real-world technology. Current active devices available in foundries, however, use volatile methods to modulate light, requiring a constant supply of power and significant form factors. Essential aspects to overcoming these issues are the development of nonvolatile optical reconfiguration techniques which are compatible with on-chip integration with different photonic platforms and do not disrupt their optical performances. In this paper, a solution is demonstrated using an optoelectronic framework for nonvolatile tunable photonics that employs undoped-graphene microheaters to thermally and reversibly switch the optical phase-change material Ge$_2$Sb$_2$Se$_4$Te$_1$ (GSST). An in-situ Raman spectroscopy method is utilized to demonstrate, in real-time, reversible switching between four different levels of crystallinity. Moreover, a 3D computational model is developed to precisely interpret the switching characteristics, and to quantify the impact of current saturation on power dissipation, thermal diffusion, and switching speed. This model is used to inform the design of nonvolatile active photonic devices; namely, broadband Si$_3$N$_4$ integrated photonic circuits with small form-factor modulators and reconfigurable metasurfaces displaying 2$pi$ phase coverage through neural-network-designed GSST meta-atoms. This framework will enable scalable, low-loss nonvolatile applications across a diverse range of photonics platforms.
Active metasurfaces promise reconfigurable optics with drastically improved compactness, ruggedness, manufacturability, and functionality compared to their traditional bulk counterparts. Optical phase change materials (O-PCMs) offer an appealing material solution for active metasurface devices with their large index contrast and nonvolatile switching characteristics. Here we report what we believe to be the first electrically reconfigurable nonvolatile metasurfaces based on O-PCMs. The O-PCM alloy used in the devices, Ge2Sb2Se4Te1 (GSST), uniquely combines giant non-volatile index modulation capability, broadband low optical loss, and a large reversible switching volume, enabling significantly enhanced light-matter interactions within the active O-PCM medium. Capitalizing on these favorable attributes, we demonstrated continuously tunable active metasurfaces with record half-octave spectral tuning range and large optical contrast of over 400%. We further prototyped a polarization-insensitive phase-gradient metasurface to realize dynamic optical beam steering.
Reconfigurability of photonic integrated circuits (PICs) has become increasingly important due to the growing demands for electronic-photonic systems on a chip driven by emerging applications, including neuromorphic computing, quantum information, and microwave photonics. Success in these fields usually requires highly scalable photonic switching units as essential building blocks. Current photonic switches, however, mainly rely on materials with weak, volatile thermo-optic or electro-optic modulation effects, resulting in a large footprint and high energy consumption. As a promising alternative, chalcogenide phase-change materials (PCMs) exhibit strong modulation in a static, self-holding fashion. Here, we demonstrate nonvolatile electrically reconfigurable photonic switches using PCM-clad silicon waveguides and microring resonators that are intrinsically compact and energy-efficient. With phase transitions actuated by in-situ silicon PIN heaters, near-zero additional loss and reversible switching with high endurance are obtained in a complementary metal-oxide-semiconductor (CMOS)-compatible process. Our work can potentially enable very large-scale general-purpose programmable integrated photonic processors.
Inducing a large refractive-index change is the holy grail of reconfigurable photonic structures, a goal that has long been the driving force behind the discovery of new optical material platforms. Recently, the unprecedentedly large refractive-index contrast between the amorphous and crystalline states of Ge-Sb-Te (GST)-based phase-change materials (PCMs) has attracted tremendous attention for reconfigurable integrated nanophotonics. Here, we introduce a microheater platform that employs optically transparent and electrically conductive indium-tin-oxide (ITO) bridges for the fast and reversible electrical switching of the GST phase between crystalline and amorphous states. By the proper assignment of electrical pulses applied to the ITO microheater, we show that our platform allows for the registration of virtually any intermediate crystalline state into the GST film integrated on the top of the designed microheaters. More importantly, we demonstrate the full reversibility of the GST phase between amorphous and crystalline states. To show the feasibility of using this hybrid GST/ITO platform for miniaturized integrated nanophotonic structures, we integrate our designed microheaters into the arms of a Mach-Zehnder interferometer to realize electrically reconfigurable optical phase shifters with orders of magnitude smaller footprints compared to existing integrated photonic architectures. We show that the phase of optical signals can be gradually shifted in multiple intermediate states using a structure that can potentially be smaller than a single wavelength. We believe that our study showcases the possibility of forming a whole new class of miniaturized reconfigurable integrated nanophotonics using beyond-binary reconfiguration of optical functionalities in hybrid PCM-photonic devices.
Phase-change materials (PCMs) have emerged as promising active elements in silicon (Si) photonic systems. In this work, we design, fabricate, and characterize a hybrid Si-PCM optical modulator. By integrating vanadium dioxide (a PCM) within a Si photonic waveguide, in a non-resonant geometry, we demonstrate ~ 10 dB broadband modulation with a PCM length of 500 nm.
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