Nanophotonics has garnered intensive attention due to its unique capabilities in molding the flow of light in the subwavelength regime. Metasurfaces (MSs) and photonic integrated circuits (PICs) enable the realization of mass-producible, cost-effective, and highly efficient flat optical components for imaging, sensing, and communications. In order to enable nanophotonics with multi-purpose functionalities, chalcogenide phase-change materials (PCMs) have been introduced as a promising platform for tunable and reconfigurable nanophotonic frameworks. Integration of non-volatile chalcogenide PCMs with unique properties such as drastic optical contrasts, fast switching speeds, and long-term stability grants substantial reconfiguration to the more conventional static nanophotonic platforms. In this review, we discuss state-of-the-art developments as well as emerging trends in tunable MSs and PICs using chalcogenide PCMs. We outline the unique material properties, structural transformation, electro-optic, and thermo-optic effects of well-established classes of chalcogenide PCMs. The emerging deep learning-based approaches for the optimization of reconfigurable MSs and the analysis of light-matter interactions are also discussed. The review is concluded by discussing existing challenges in the realization of adjustable nanophotonics and a perspective on the possible developments in this promising area.

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Tunable nanophotonics enabled by chalcogenide phase-change materials

Leveraging the spatial modes of multimode waveguides using mode-division multiplexing on an integrated photonic chip allows unprecedented scaling of bandwidth density for on-chip communication. Switching channels between waveguides is critical for future scalable optical networks, but its implementation in multimode waveguides must address how to simultaneously control modes with vastly different optical properties. Here we present a platform for switching signals between multimode waveguides based on individually processing the spatial mode channels using single-mode elements. Using this wavelength-division multiplexing-compatible platform, we demonstrate a 1×2 multimode switch for a silicon chip that routes four data channels with low (<−16.8  dB) crosstalk. We show bit-error rates below 10−9 and power penalties below 1.4 dB on all channels while routing 10 Gb/s data when each channel is input and routed separately. The switch exhibits an additional power penalty of less than 2.4 dB when all four channels are simultaneously routed. These results enable individual processing of multimode signals and high-bandwidth, flexible optical networks.

On-chip mode-division multiplexing switch

A dual-polarization 10-channel mode (de)multiplexer is proposed and realized with cascaded dual-core adiabatic tapers on a silicon-on-insulator (SOI) platform. The mode demultiplexer has a 2.3 μm-wide multimode bus waveguide, which supports six mode-channels of TE polarization and four mode-channels of TM polarization. These ten mode-channels are (de)multiplexed with five cascaded dual-core adiabatic tapers based on SOI nanowires. The widths for these dual-cores are chosen optimally according to the dispersion curves of the dual-core SOI nanowire, so that the desired highest-order modes of TE- and TM-polarizations are extracted simultaneously. These two extracted mode-channels are coupled very efficiently to the fundamental modes of TE- and TM-polarizations (TE0 and TM0) in the narrow waveguide, respectively, which are then separated by using a polarization beam splitter based on bent directional couplers. A chip consisting of a pair of 10-channel mode (de)multiplexers is fabricated and then tested with data transmission of 30Gbps/channel. The measurement results show that all TM- and TE mode-channels have low crosstalks (–15∼–25 dB) and low excess losses (0.2∼1.8 dB) over a broad wavelength band of ∼90 nm, which makes it WDM (wavelength-division-multiplexing)-compatible and thus suitable for high capacity on-chip optical interconnects.

10-Channel Mode (de)multiplexer with Dual Polarizations

Abstract Multimode silicon photonics is attracting more and more attention because the introduction of higher-order modes makes it possible to increase the channel number for data transmission in mode-division-multiplexed (MDM) systems as well as improve the flexibility of device designs. On the other hand, the design of multimode silicon photonic devices becomes very different compared with the traditional case with the fundamental mode only. Since not only the fundamental mode but also the higher-order modes are involved, one of the most important things for multimode silicon photonics is the realization of effective mode manipulation, which is not difficult, fortunately because the mode dispersion in multimode silicon optical waveguide is very strong. Great progresses have been achieved on multimode silicon photonics in the past years. In this paper, a review of the recent progresses of the representative multimode silicon photonic devices and circuits is given. The first part reviews multimode silicon photonics for MDM systems, including on-chip multichannel mode (de)multiplexers, multimode waveguide bends, multimode waveguide crossings, reconfigurable multimode silicon photonic integrated circuits, multimode chip-fiber couplers, etc. In the second part, we give a discussion about the higher-order mode-assisted silicon photonic devices, including on-chip polarization-handling devices with higher-order modes, add-drop optical filters based on multimode Bragg gratings, and some emerging applications.

https://www.degruyter.com/downloadpdf/j/nanoph.2019.8.issue-2/nanoph-2018-0161/nanoph-2018-0161.xml

Multimode silicon photonics

Recently the developments in the field of II-VI-oxides have been spectacular. Various epitaxial methods has been used to grow epitaxial ZnO layers. Not only epilayers but also sufficiently good-quality multiple quantum wells (MQWs) have also been grown by laser molecular-beam epitaxy (laser-MBE). We discuss mainly the experimental aspect of the optical properties of excitons in ZnO-based MQW heterostructures. Systematic temperature-dependent studies of optical absorption and photoluminescence in these MQWs were used to evaluate the well-width dependence and the composition dependence of the major excitonic properties. Based on these data, the localization of excitons, the influence of exciton-phonon interaction, and quantum-confined Stark effects are discussed. 
The optical spectra of dense excitonic systems are shown to be determined mainly by the interaction process between excitons and biexcitons. The high-density excitonic effects play a role for the observation of room-temperature stimulated emission in the ZnO MQWs. The binding energies of exciton and biexciton are enhanced from the bulk values, as a result of quantum-confinement effects.

Optical Properties of Excitons in ZnO-based Quantum Well Heterostructures

A low-loss and broadband silicon thermo-optic switch is proposed and demonstrated experimentally by using a Mach-Zehnder Interferometer with 2×2 3 dB power splitters based on bent directional couplers (DCs). The bent DCs are introduced here to replace the traditional 2×2 3 dB power splitters based on multimode interferometers or straight DCs, so that one achieves a coupling ratio of ∼50%∶ 50%, as well as low excess loss over a broadband. The demonstrated Mach-Zehnder switch (MZS) has a ∼140  nm bandwidth for an excess loss of 20  dB. The present MZS also shows excellent reproducibility and good fabrication tolerance, which makes it promising for realizing N×N optical switches.

Low-loss and broadband 2 × 2 silicon thermo-optic Mach-Zehnder switch with bent directional couplers.

An ultracompact and low-loss TM-pass polarizer on silicon is proposed and demonstrated experimentally with a subwavelength-grating (SWG) waveguide The SWG waveguide is designed to support Bloch mode for TM polarization so that the incident TM-polarized light goes through the SWG waveguide with very low excess loss On the other hand, for TE polarization, the SWG waveguide works as a Bragg reflector, and consequently the incident TE-polarized light is reflected For a fabricated ∼9  μm long polarizer (with the period number N=20), the measured extinction ratio is ∼27  dB and the excess loss is ∼05  dB at the central wavelength 1550 nm The bandwidth to achieve an extinction ratio of 20 dB is about 60 nm (from 1520 to 1580 nm) When increasing the period number to N=40, the measured extinction ratio is up to 40 dB (which is not as high as the expected theoretical value 65 dB due to the limit of the measurement system)

Low-loss ultracompact transverse-magnetic-pass polarizer with a silicon subwavelength grating waveguide.

An improved 8-channel silicon mode demultiplexer is realized with TE-type and TM-type grating polarizers at the output ends, and these gratings serve as fiber-chip couplers simultaneously The present 8-channel silicon mode demultiplexer includes a three-waveguide PBS (for separating the TE0 and TM0 modes) and six cascaded ADCs (for demultiplexing the high-order modes of both polarizations) The grating polarizers with high extinction ratios are used to filter out the polarization crosstalk in the 8-channel hybrid multiplexer efficiently and the measured crosstalk for all the mode-channels of the improved 8-channel mode multiplexer is reduced greatly to ~-20dB in a ~100nm bandwidth

Improved 8-channel silicon mode demultiplexer with grating polarizers.

O R IG IN A L P A P ER Abstract A compact 64-channel hybrid demultiplexer based on silicon-on-insulator nanowires is proposed and demonstrated experimentally to enable wavelength-division-multiplexing and mode-division-multiplexing simultaneously in order to realize an ultra-large capacity on-chip optical-interconnect link. The present hybrid demultiplexer consists of a 4-channel mode multiplexer constructed with cascaded asymmetrical directionalcouplers and two bi-directional 17 × 17 arrayed-waveguide gratings (AWGs) with 16 channels. Here each bi-directional AWG is equivalent as two identical 1 × 16 AWGs. The measured excess loss and the crosstalk for the monolithically integrated 64-channel hybrid demultiplexer are about -5 dB and -14 dB, respectively. Better performance can be achieved by minimizing the imperfections (particularly in AWGs) during the fabrication processes.

Monolithically integrated 64-channel silicon hybrid demultiplexer enabling simultaneous wavelength- and mode-division-multiplexing

High-order microring resonator (MRR) filters with bent directional couplers are proposed and demonstrated to achieve a box-like filter response. When using bent couplers, the coupling ratio can be adjusted easily by choosing the length of the coupling region, and the excess loss is almost zero while the perimeter of the microring length is unchanged. For the present fabricated five-microring filters with bent directional couplers, the excess loss is less than 1.0 dB, the out-of-band extinction ratio is ∼36  dB, and the response has rising and falling edges as sharp as 48  dB/nm. The thermal tunability of the high-order MRR filter with a Ti-microheater is also demonstrated and the thermally tuning efficiency is about 0.10  nm/mW.

Sitao Chen

Papers

Low-loss and broadband 2 × 2 silicon thermo-optic Mach-Zehnder switch with bent directional couplers.

Low-loss ultracompact transverse-magnetic-pass polarizer with a silicon subwavelength grating waveguide.

Improved 8-channel silicon mode demultiplexer with grating polarizers.

Monolithically integrated 64-channel silicon hybrid demultiplexer enabling simultaneous wavelength- and mode-division-multiplexing

High-order microring resonators with bent couplers for a box-like filter response