We show a new approach for achieving precise control over the internal structure of phase-change materials (PCMs) using the glancing angle deposition (GLAD) technique, which offers a foundry-friendly bottom-up growth alternative to commonly used lithography or chemical modification methods that introduce unwanted defects and impurities. We show that by adjusting deposition angle and rotation speed during growth, GLAD can enable a precise and unprecedented engineering of refractive index and extinction coefficient, in both amorphous and crystalline phases of commonly used GeTe and GST PCM films, without the need to alter their chemical composition.
Phase-change materials (PCMs), capable of non-volatile electrically or optically induced transitions, are being actively explored as a promising option for use in silicon photonic neuromorphic integrated circuits and compact modulators in telecom networks to overcome the limitations in footprint and power consumption imposed by the utilization of weak and volatile thermo-optic effects in current architectures. We present the first-ever broadband measurement of the thermo-optic effect in a number of widely explored chalcogenide PCMs across visible to telecom frequencies. Our measurements show that beyond their non-volatile phase change properties, PCMs also possess giant switchable broadband thermo-optic coefficients.
Silicon photonics has emerged as the dominant technology platform for short distance, inter-chip communication for a variety of photonic computing and sensing applications due to its efficiency in modulation and confinement of light across telecom frequencies in addition to its inherent CMOS compatibility. The integration of metallic nanogaps within silicon photonic architectures provides a promising route for scaling this platform through the extreme confinement offered by plasmonics while providing an efficient route to interfacing future photonic integrated circuits with electronics. However, fabricating the gap sizes (< λg/10) required of plasmonic resonating nanogaps for efficient operation across telecommunication frequencies is highly challenging. Efficient coupling from waveguides to plasmonic nanogaps also remains a major source of loss. Here, we show that the key to merging these platforms lies in applying metamaterial/metasurface engineering principles to the design of the nanogap. Over the last decade, metamaterials and metasurfaces have emerged as a versatile toolkit for control and enhancement of light-matter interaction at application-driven wavelengths of interest in nanophotonic device platforms. We show that integrating a metagrating within a waveguide-coupled plasmonic nanogap made from Au, can enhance coupling to and from the silicon waveguides. Furthermore, the incorporation of the metasurface within the gap allows resonant response to be maintained at user-specified wavelength of interest with gaps as large as λg/5, drastically easing fabrication. Finally, we show that by incorporating a reconfigurable phase change chalcogenide alloy into the gap, non-volatile signal switching with modulation contrasts of up to 10:1 can be achieved across telecom frequencies.
Silicon photonics has matured over the last decade as a unique platform for highly miniaturized photonic integrated systems seamlessly integrated with electronics allowing the realization and commercialization of highly compact devices with ultrafast data transfer rates and significantly reduced power consumption. Although such submicron scale photonic and waveguide structures enable dense on-chip device integration, they also result in reduced efficiency in fiber-to-waveguide coupling mainly due to increased mode area mismatch. As a result, one of the key challenges faced in current technology platforms has been to efficiently couple light without additional fabrication, post-processing, and complex optical alignment. In-plane grating couplers (GC) have been a widely preferred coupling platform, mainly because of low fabrication costs, ease of alignment and high-level of flexibility in circuit design. A wide range of coupling platform designs have been investigated in the last decade including both passive and active designs. In passive design the coupling efficiency (CE) is fixed once the device is fabricated, however in active designs various tuning mechanism have been explored to modulate the CE, but at the expense of increased power consumption and reduced CE. Using inverse design techniques, we demonstrate CMOS compatible, reconfigurable phase change chalcogenide-on-insulator based apodised GC having maximized CE of more than 50% at λ=1550nm when the phase of the chalcogenide is in amorphous state. When the phase is switched to crystalline state, a near zero CE is shown allowing the design to be both non-volatile and reversibly reconfigurable with highest transmission modulation contrast of more than 50db.
Alloys of sulphur, selenium and tellurium, often referred to as chalcogenide semiconductors offer a highly versatile, compositionally-controllable material platform for reconfigurable metamaterial applications. They present various high- and low-index dielectric, low-epsilon and plasmonic properties across ultra-violet (UV), visible and infrared frequencies, in addition to an ultra-fast, non-volatile, electrically-/optically-induced switching capability between phase states with markedly different electromagnetic properties. We show that by integrating chalcogenide metasurfaces on the tip and side of optical fibers as well as silicon photonic waveguide platforms a range of wavelength-tunable modulators for telecommunication networks and synaptic weights for emerging neuromorphic computing applications can be realized.
Crossbar architectures are a highly popular platform in the electronics industry for enabling high-component density at the nanoscale, in today’s constantly shrinking electronic devices. These structures are akin to metal-insulator-metal (MIM) architectures widely used in nanophotonics and are key to the realization of a range of reconfigurable and addressable metasurfaces. Therefore, the application of nanophotonic design principles to such electronic platforms provides an unexplored path towards the integration of nanophotonic technologies into telecommunication and computing platforms. We show here that these crossbar-architectures can be engineered to act as addressable metasurfaces exhibiting, multispectral optical resonances forming the basis for next-generation optical computing systems, while still preserving their electronic functionality.
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