In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometre-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the infrared and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technology.

Subwavelength integrated photonics.

Propagation of light beams through scattering or multimode systems may lead to the randomization of the spatial coherence of the light. Although information is not lost, its recovery requires a coherent interferometric reconstruction of the original signals, which have been scrambled into the modes of the scattering system. Here we show that we can automatically unscramble optical beams that have been arbitrarily mixed in a multimode waveguide, undoing the scattering and mixing between the spatial modes through a mesh of silicon photonics tuneable beam splitters. Transparent light detectors integrated in a photonic chip are used to directly monitor the evolution of each mode along the mesh, allowing sequential tuning and adaptive individual feedback control of each beam splitter. The entire mesh self-configures automatically through a progressive tuning algorithm and resets itself after significantly perturbing the mixing, without turning off the beams. We demonstrate information recovery by the simultaneous unscrambling, sorting and tracking of four mixed modes, with residual cross-talk of −20 dB between the beams. Circuit partitioning assisted by transparent detectors enables scalability to meshes with a higher port count and to a higher number of modes without a proportionate increase in the control complexity. The principle of self-configuring and self-resetting in optical systems should be applicable in a wide range of optical applications. A silicon photonics chip featuring a mesh of tunable beam splitters can unscramble mode mixing that occurs in multimode waveguides. Scattering or multimode systems can randomize the spatial coherence of light beams. Francesco Morichetti and co-workers from Politecnico di Milano, Italy, and Stanford University, USA, have fabricated a chip-based descrambler that can automatically unscramble optical beams. A progressive tuning algorithm that monitors the output of the chip enables the mesh to self-configure so that it can unscramble and sort different spatial modes. In a demonstration of the device, four optical beams containing mixed modes were unmixed and separated into outputs with a residual crosstalk of less than −20 dB between the modes. The approach is scalable to a higher number of modes and is promising for optical communication systems employing mode division multiplexing.

/pdf/unscrambling-light-automatically-undoing-strong-mixing-4z5wwk1a86.pdf

Unscrambling light—automatically undoing strong mixing between modes

Segmenting silicon waveguides at the subwavelength scale produce an equivalent homogenous material. The geometry of the waveguide segments provides precise control over modal confinement, effective index, dispersion and birefringence, thereby opening up new approaches to design devices with unprecedented performance. Indeed, with ever-improving lithographic technologies offering sub-100-nm patterning resolution in the silicon photonics platform, many practical devices based on subwavelength structures have been demonstrated in recent years. Subwavelength engineering has thus become an integral design tool in silicon photonics, and both fundamental understanding and novel applications are advancing rapidly. Here, we provide a comprehensive review of the state of the art in this field. We first cover the basics of subwavelength structures, and discuss substrate leakage, fabrication jitter, reduced backscatter, and engineering of material anisotropy. We then review recent applications including broadband waveguide couplers, high-sensitivity evanescent field sensors, low-loss devices for mid-infrared photonics, polarization management structures, spectral filters, and highly efficient fiber-to-chip couplers. We finally discuss the future prospects for subwavelength silicon structures and their impact on advanced device design.

/pdf/subwavelength-grating-metamaterial-structures-for-silicon-253awniogk.pdf

Subwavelength-Grating Metamaterial Structures for Silicon Photonic Devices

Ultra-compact, densely integrated optical components manufactured on a CMOS-foundry platform are highly desirable for optical information processing and electronic-photonic co-integration. However, the large spatial extent of evanescent waves arising from nanoscale confinement, ubiquitous in silicon photonic devices, causes significant cross-talk and scattering loss. Here, we demonstrate that anisotropic all-dielectric metamaterials open a new degree of freedom in total internal reflection to shorten the decay length of evanescent waves. We experimentally show the reduction of cross-talk by greater than 30 times and the bending loss by greater than 3 times in densely integrated, ultra-compact photonic circuit blocks. Our prototype all-dielectric metamaterial-waveguide achieves a low propagation loss of approximately 3.7±1.0 dB/cm, comparable to those of silicon strip waveguides. Our approach marks a departure from interference-based confinement as in photonic crystals or slot waveguides, which utilize nanoscale field enhancement. Its ability to suppress evanescent waves without substantially increasing the propagation loss shall pave the way for all-dielectric metamaterial-based dense integration.

/pdf/controlling-evanescent-waves-using-silicon-photonic-all-4pku13aywr.pdf

Controlling evanescent waves using silicon photonic all-dielectric metamaterials for dense integration.

Ultra-compact, densely integrated optical components manufactured on a CMOS-foundry platform are highly desirable for optical information processing and electronic-photonic co-integration. However, the large spatial extent of evanescent waves arising from nanoscale confinement, ubiquitous in silicon photonic devices, causes significant cross-talk and scattering loss. Here, we demonstrate that anisotropic all-dielectric metamaterials open a new degree of freedom in total internal reflection to shorten the decay length of evanescent waves. We experimentally show the reduction of cross-talk by greater than 30 times and the bending loss by greater than 3 times in densely integrated, ultra-compact photonic circuit blocks. Our prototype all-dielectric metamaterial-waveguide achieves a low propagation loss of approximately 3.7 dB/cm, comparable to those of silicon strip waveguides. Our approach marks a departure from interference-based confinement as in photonic crystals or slot waveguides, which utilize nanoscale field enhancement. Its ability to suppress evanescent waves without substantially increasing the propagation loss shall pave the way for all-dielectric metamaterial-based dense integration.

/pdf/controlling-evanescent-waves-using-silicon-photonic-all-45tlhd4mrj.pdf

Controlling evanescent waves using silicon photonic all-dielectric metamaterials for dense integration

Nanophotonic beamsplitters are fundamental building blocks in integrated optics, with applications ranging from high speed telecom receivers to biological sensors and quantum splitters. While high-performance multiport beamsplitters have been demonstrated in several material platforms using multimode interference couplers, their operation bandwidth remains fundamentally limited. Here, we leverage the inherent anisotropy and dispersion of a sub-wavelength structured photonic metamaterial to demonstrate ultra-broadband integrated beamsplitting. Our device, which is three times more compact than its conventional counterpart, can achieve high-performance operation over an unprecedented 500 nm design bandwidth exceeding all optical communication bands combined, and making it one of the most broadband silicon photonics components reported to date. Our demonstration paves the way toward nanophotonic waveguide components with ultra-broadband operation for next generation integrated photonic systems.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201600213

Ultra-broadband nanophotonic beamsplitter using an anisotropic sub-wavelength metamaterial

Subwavelength grating (SWG) structures are an essential tool in silicon photonics, enabling the synthesis of metamaterials with a controllable refractive index Here we propose, for the first time to the best of our knowledge, tilting the grating elements to gain control over the anisotropy of the metamaterial Rigorous finite difference time domain simulations demonstrate that a 45° tilt results in an effective index variation on the fundamental TE mode of 023 refractive index units, whereas the change in the TM mode is 20 times smaller Our simulation predictions are corroborated by experimental results We furthermore propose an accurate theoretical model for designing tilted SWG structures based on rotated uniaxial crystals that is functional over a wide wavelength range and for both the fundamental and higher order modes The proposed control over anisotropy opens promising venues in polarization management devices and transformation optics in silicon photonics

/pdf/tilted-subwavelength-gratings-controlling-anisotropy-in-1uuddb5wqm.pdf

Tilted subwavelength gratings: controlling anisotropy in metamaterial nanophotonic waveguides.

Surface grating couplers enable efficient coupling of light between optical fibers and nanophotonic waveguides. However, in conventional grating couplers, the radiation angle is intrinsically wavelength dependent, thereby limiting their operation bandwidth. In this Letter, we present a zero-order surface grating coupler in silicon-on-insulator which overcomes this limitation by operating in the subwavelength regime. By engineering the effective refractive index of the grating region, both high coupling efficiency and broadband operation bandwidth are achieved. The grating is assisted by a silicon prism on top of the waveguide, which favors upward radiation and minimizes power losses to substrate. Using a linear apodization, our design achieves a coupling efficiency of 91% (-0.41  dB) and a 1-dB bandwidth of 126 nm.

Broadband fiber-chip zero-order surface grating coupler with 0.4 dB efficiency.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201900172

An Ultracompact GRIN-Lens-Based Spot Size Converter using Subwavelength Grating Metamaterials

/pdf/a-review-of-silicon-subwavelength-gratings-building-break-54chaj4osh.pdf

José Manuel Luque-González

Papers

Ultra-broadband nanophotonic beamsplitter using an anisotropic sub-wavelength metamaterial

Tilted subwavelength gratings: controlling anisotropy in metamaterial nanophotonic waveguides.

Broadband fiber-chip zero-order surface grating coupler with 0.4 dB efficiency.

An Ultracompact GRIN-Lens-Based Spot Size Converter using Subwavelength Grating Metamaterials

A review of silicon subwavelength gratings: building break-through devices with anisotropic metamaterials