Subwavelength Engineering Research Articles

Multiple High‐Q Optical Modes in a Polymer‐Lithium Niobate Integrated Metasurface

AbstractWith the development of miniaturization and integration, multifunctional optical devices are necessary. However, its practical realization is a critical challenge due to the lack of simple structures with multiple functionalities. Here, a multimode metasurface is proposed by simply placing polymer nanocubes on the lithium niobate (LN) thin film. The intriguing properties of the integrated structure include high‐quality (Q) factors and different spectral characteristics, associated with quasi‐guide modes (GM) and Mie modes, respectively. The experiment demonstration reveals that Q factors of four modes are larger than 1000 and a second harmonic generation (SHG) enhancement factor of 25.4 is obtained, featuring strong light‐matter interaction. Importantly, the abundant spectral characteristics greatly boost three degrees of freedom, i.e., polarization, incident angle, and slot, to realize multifunctional designs. The results provide a sophisticated platform for developing high‐performance LN metasurfaces with the advantages of simplified fabrication processes and multifunctionality. The strategy can also be extended to other material systems, enabling the flexibility of metasurface‐inspired subwavelength engineering.

Highly Efficient Gradient Solid Immersion Lens with Large Numerical Aperture for Broadband Achromatic Deep Subwavelength Focusing and Magnified Far Field

AbstractAchromatic deep subwavelength lens with large numerical aperture (NA) is in urgent need in several fields, from optical imaging, photolithography, spectroscopy, nanophotonics to microwave engineering and sensing. However, the previous proposed approaches to achieve deep subwavelength lens suffer from severe chromatic aberrations, limited bandwidth, and limited efficiency. In this paper, enlightened by Maxwell's fish‐eye lens, a semicircular gradient solid immersion lens is proposed and manufactured by gradient isotropic dielectrics. Highly efficient (above 85%) achromatic (7–13 GHz) deep subwavelength focusing with full width at half maximum around 0.15λ and large NA of 2.4 as well as magnified far‐field radiation (4–12 GHz) with high recognition are achieved. Theoretical analysis reveals that electric field enhancement at the lens/air interface and magnification of high spatial frequency deep subwavelength information contribute to deep subwavelength focusing and magnified far‐field radiation, respectively. Measured results of near field and far field agree well with those of theoretical predictions, which makes the presented lens having the potential to be applied in deep subwavelength engineering systems.

Subwavelength grating waveguide filter based on cladding modulation with a phase-change material grating.

Subwavelength engineering and utilizing phase-change materials with large contrast in their optical properties have become powerful design tools for integrated silicon photonics. Reversible phase-transition of phase-change materials such as Ge2Sb2Te5 (GST) provide a new degree of freedom and open up the possibility of adding new functionalities to the designed devices. We present an optical filter based on a silicon subwavelength grating (SWG) waveguide evanescently coupled to phase-change material loading segments arranged periodically around the SWG core. The effect of the GST loading segments' geometry and their distance from the SWG core on the filter's central wavelength and bandwidth are studied with three-dimensional finite-difference time-domain simulations. The employment of GST in the structure adds a switching functionality with an extinction ratio of 28.8 dB. We also examine the possibility of using the proposed structure as a reconfigurable filter by controlling the partial crystallization of the GST offering a blueshift of more than 4 nm.

Subwavelength engineering for Brillouin gain optimization in silicon optomechanical waveguides.

Brillouin optomechanics has recently emerged as a promising tool to implement new functionalities in silicon photonics, including high-performance opto-RF processing and nonreciprocal light propagation. One key challenge in this field is to maximize the photon-phonon interaction and the phonon lifetime, simultaneously. Here, we propose a new, to the best of our knowledge, strategy that exploits subwavelength engineering of the photonic and phononic modes in silicon membrane waveguides to maximize the Brillouin gain. By properly designing the dimensions of the subwavelength periodic structuration, we tightly confine near-infrared photons and GHz phonons, minimizing leakage losses and maximizing the Brillouin coupling. Our theoretical analysis predicts a high mechanical quality factor of up to 700 and a remarkable Brillouin gain yielding 3500(W⋅m)-1 for minimum feature size of 50 nm, compatible with electron-beam lithography. We believe that the proposed waveguide with subwavelength nanostructure holds great potential for the engineering of Brillouin optomechanical interactions in silicon.

Perfectly vertical surface grating couplers using subwavelength engineering for increased feature sizes.

We present perfectly vertical grating couplers for the 220 nm silicon-on-insulator platform incorporating subwavelength metamaterials to increase the minimum feature sizes and achieve broadband low back-reflection. Our study reveals that devices with high coupling efficiencies are distributed over a wide region of the design space with varied back-reflections, while still maintaining minimum feature sizes larger than 100 nm and even 130 nm. Using 3D-finite-difference time-domain simulations, we demonstrate devices with broadband low back-reflection of less than -20dB over more than 100 nm bandwidth centered around the C-band. Coupling efficiencies of 72% and 67% are achieved for minimum feature sizes of 106 nm and 130 nm, respectively. These gratings are also more fabrication tolerant compared to similar designs not using metamaterials.

Stretching the spectra of Kerr frequency combs with self-adaptive boundary silicon waveguides

Dispersion engineering of optical waveguides is among the most important steps in enabling the realization of Kerr optical frequency combs. A recurring problem is the limited bandwidth in which the nonlinear phase matching condition is satisfied, due to the dispersion of the waveguide. This limitation is particularly stringent in high-index-contrast technologies such as silicon-on-insulator. We propose a general approach to stretch the bandwidth of Kerr frequency combs based on subwavelength engineering of single-mode waveguides with self-adaptive boundaries. The wideband flattened dispersion operation comes from the particular property of the waveguide optical mode that automatically self-adapts its spatial profile at different wavelengths to slightly different effective spatial spans determined by its effective index values. This flattened dispersion relies on the squeezing of small normal-dispersion regions between two anomalous spectral zones, which enables it to achieve two Cherenkov radiation points and substantially broaden the comb, achieving a bandwidth between 2.2 and 3.4 μm wavelength. This strategy opens up a design space for trimming the spectra of Kerr frequency combs using high-index-contrast platforms and can provide benefits to various nonlinear applications in which the manipulation of energy spacing and phase matching are pivotal.

Self-Adaptive Waveguide Boundary for Inter-Mode Four-Wave Mixing

We propose a universal approach to provide wideband multimode four-wave mixing independently of the intrinsic dispersion of the involved waveguide modes. Concepts from quantum mechanics and subwavelength engineering are adopted to design an effective lateral confinement photonic well, i.e., with a graded potential along the waveguide cross section providing flexible control over the modes’ confinement. The self-adaptive nature of the waveguide boundary allows different spatial modes with equi-spaced frequencies and shared propagation wavevector, thus automatically fulfilling both energy conservation and wavevector phase matching conditions. Capitalizing on this concept, we show phase-matching among modes separated by 400 nm (bridging from telecom wavelengths to almost ${2}\;{\rm{\mu m}}$ ), with less than 5% deviation in a remarkably large bandwidth exceeding 300 nm. Furthermore, we also show the flexibility of the proposed approach that can be seamlessly adapted to different technology platforms. This strategy opens a new design space for versatile on-chip nonlinear applications in which the manipulation of energy spacing and phase matching is pivotal, e.g., all-optical signal processing with four-wave mixing, mid-infrared supercontinuum or frequency comb light generation, or Brillouin scattering with selectable phonon energy.

Subwavelength Engineering in Silicon Photonic Devices

In the past decade, there has been tremendous progress in using subwavelength-scale nanostructures with elaborately designed periodic and disordered photonic materials for applications in integrated photonics. In this paper, we review the advances in subwavelength engineering used in silicon photonic devices, with an emphasis on our own contributions on the use of subwavelength gratings and hyperuniform disordered photonic structures to attain state-of-the-art performances for near- and mid-infrared applications in fiber–chip coupling, slot waveguides for refractive-index sensing, mode conversion, wavelength filtering, integrated resonators, and ultracompact high-extinction and broadband integrated polarizers.

Subwavelength-Grating Metamaterial Structures for Silicon Photonic Devices

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.

Subwavelength engineering and asymmetry: two efficient tools for sub-nanometer-bandwidth silicon Bragg filters.

Bragg filters stand as key building blocks of the silicon-on-insulator (SOI) photonics platform, allowing the implementation of advanced on-chip signal manipulation. However, achieving narrowband Bragg filters with large rejection levels is often hindered by fabrication constraints and imperfections. Here, we show that the combination of single-side corrugation asymmetry and subwavelength engineering provides a narrowband response with large corrugations, overcoming minimum feature size constraints of conventional Si Bragg filters. We comprehensively study the impact of the corrugation asymmetry in conventional and subwavelength single-etched SOI Bragg filters, showing their potential for bandwidth reduction. Finally, we experimentally demonstrate novel subwavelength geometry based on shifted corrugation teeth, achieving null-to-null bandwidths and rejections of 0.8nm and 40dB for the symmetric configuration and 0.6nm and 15dB for the asymmetric case.

Subwavelength engineering and asymmetry: two efficient tools for sub-nanometer-bandwidth silicon Bragg filters

Subwavelength engineering and asymmetry: two efficient tools for sub-nanometer-bandwidth silicon Bragg filters