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.
Photonic modulators have seen widespread use in optical circuitry, optical processing, and next-generational computing regimes such as neuromorphic computing. Prior research has focused on the incorporation of high-index functional materials on or adjacent to photonic circuit components such as modulators to enhance signal detection, modulation, and generation. The reversible, non-volatile transitions between optically and electrically unique amorphous and crystalline material phases inherent to chalcogenide phase-change materials (PCMs) present a promising material platform for this integration. However, current methods of incorporation combined with lossy material properties lead to integrations having large insertion losses and device footprints. Here we demonstrate that applying metamaterial effective medium theory enables dispersive engineering to drastically reduce insertion losses and footprints in PCM-loaded optical circuitry. Two configurations are explored, a metagrating and a multilayer, in which full-π modulator phase shifts are achieved in compact footprints down to 4.36 and 4.7 𝜇m with low insertion losses at a 1550nm wavelength.
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.
Obtaining high-resolution images using an optical microscope is critical when dealing with micro/nanoscale objects. Current techniques use high magnification objective lenses with high numerical apertures to resolve closely spaced objects at the micron/nanoscale. However, these lenses often require additional optics and have a narrow depth of field, preventing ease of use. To date, scanning electron microscopy (SEM) is used for imaging beyond the diffraction limit and has led to various breakthroughs in semiconductor physics and nanotechnology. An alternative to an SEM is using artificial intelligence (AI) to enable super-resolution techniques with correlated image sets. We utilize a convolutional neural network (CNN) and generative adversarial network (GAN) to train correlated images gathered from higher magnification SEM and lower magnification SEM, resulting in a model that enables resolving nanoscale features. We demonstrated that by training a neural network with SEM images, we are able to aid the optical microscope to image beyond the diffraction limit with a resolution closer to the SEM.
The prohibitively large inclusions of micro-ring resonators, interconnected waveguide crossbar arrays, and multi-port multi-mode interferometers components demonstrated in the latest integrated photonic neural network hardware accelerators, pose a significant scaling issue for implementing large number of neural connections required to accurately represent the cognitive functions of a biological brain. In this investigation we utilize phase-change chalcogenide material GST to replicate a non-volatile synaptic weight with built-in memory functionality by employing metamaterial design principles for wavelength-division multiplexing photonic architectures. The transmission response of the optimized GST metamaterial gives rise to contrast ratios of 6dB in both positive and negative weighting values.
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.
Previous approaches for achieving asymmetric transmission (AT) of light rely on nonreciprocal systems, which are limited to having external bias or high input powers. We show AT from a metasurface comprising dielectric nanogratings on two sides of a silicon nitride membrane. The proposed metagrating provides wideband AT over multiple operational windows, and we demonstrate that such metasurfaces can be realized using both noble plasmonic metals and chalcogenide phase change materials to achieve reconfigurable AT. The proposed structure has various applications, from enhancing efficiencies in photovoltaic systems to tunable isolators and electromagnetic shielding.
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.
Recently reconfigurable phase change chalcogenide based metamaterials/metasurfaces have shown great promise in the realization of high speed large contrast all-optical switching and beam-steering devices with built-in memory functionality at a fraction of a wavelength in size across the ultraviolet to infrared frequency range. To incorporate these devices into current telecommunication platforms, integration with photonic waveguide architectures is a must as they present the most mature, widely used commercial photonic device platform today. Here, we present a new class of waveguide-integrated reconfigurable all-dielectric metasurfaces utilizing high refractive index phase change chalcogenides and discuss the unique considerations, design, and physical principles that are essential for integrating such nanostructures into waveguides where illumination is provided through controlled evanescent coupling of guided modes into the metamaterial structure.
Due to their small physical footprint, fibre integrated metamaterials and metadevices made from phase change chalcogenide semiconductors that can be dynamically reconfigured using optical or electrical stimuli present the most promising platform for integration into future telecommunication networks to alleviate the data latency and high power consumption associated with current network configurations. Here, through numerical simulations, we present reconfigurable metadevices that can be integrated onto the tip and side of commercial optical fibres showing tunable behavior across the entire telecommunication band. Such devices can be used for dynamic dispersion control and signal switching.
Photonic metamaterials have driven the development of innovative devices with novel functionalities. Research in this field has relied upon noble plasmonic metals that suffer from optical loss, low-melting-points, cost and incompatibility with CMOS technology. Alternative material platforms such as chalcogenide semiconductors, oxides, and nitrides have been explored to overcome these challenges. Refractory metal oxides are highly versatile and are often overlooked in the realization of metamaterials and metadevices. Metal-oxide bilayers grown by vapor deposition, followed by annealing, enable a class of metamaterial coatings (meta-coatings) that offer tuneable resonant behaviour across visible frequencies. These meta-coatings are fabricated without nanopatterning allowing for large-scale fabrication.
Chalcogenide glasses offer a unique platform for the realization of reconfigurable and tunable metasurfaces, as evidenced through the emergence of reconfigurable phase change metamaterials. Reconfiguration in these devices involves a thermally intensive melt/quench phase transition process which can reduce device lifetimes and degrade performance. Notably, metal doped chalcogenide semiconductors also exhibit photo-induced long-range movement of their constituent metal ions in their amorphous phase, resulting in non-volatile changes to refractive index and conductivity, removing the need for a phase transition. We utilize this photo-ionic movement in amorphous nanostructured silver-doped germanium selenide metasurfaces to demonstrate reversible non-volatile switching over optical frequencies.
Photoluminescence (PL) and excitation spectra of Bi melt-doped oxide and chalcogenide glasses are very similar, indicating the same Bi center is present. When implanted with Bi, chalcogenide, phosphate and silica glasses, and BaF2 crystals, all display characteristically different PL spectra to when Bi is incorporated by melt-doping. This indicates that ion implantation is able to generate Bi centers which are not present in samples whose dopants are introduced during melting. Bi-related PL bands have been observed in glasses with very similar compositions to those in which carrier-type reversal has been observed, indicating that these phenomena are related to the same Bi centers, which we suggest are interstitial Bi2+ and Bi clusters.
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