Periodic structures with a sub-wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high-precision lithography techniques that sub-wavelength structures have found widespread application in the field of photonics. This review first provides an introduction to the physics of sub-wavelength structures. An overview of the applications of sub-wavelength structures is then given including: anti-reflective coatings, polarization rotators, high-efficiency fiber-chip couplers, spectrometers, high-reflectivity mirrors, athermal waveguides, multimode interference couplers, and dispersion engineered, ultra-broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub-wavelength structures and their impact in photonics.

We propose a silicon waveguide-fiber grating coupler that uses a subwavelength microstructure to achieve a continuously variable grating strength yet can be fabricated using only a single etch step. By adjusting the subwavelength microstructure at every point along the grating, the grating coupler can be optimized to give high field overlap with the optical fiber mode and also minimize backreflections along the incident waveguide path. Our design example is optimized for quasi-TM mode in a silicon photonic-wire waveguide, as required for waveguide evanescent-field-sensing applications. A field overlap of up to 94% with a standard single-mode optical fiber (SMF-28) is achieved by coupler apodization. Backreflection from the grating is reduced to ~0.1%, and the total predicted photonic wire to fiber coupling efficiency is 50%.

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 highperformance 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 ultrabroadband operation for next generation integrated photonic systems.

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 everimproving 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 Manuscript

Directional couplers are extensively used devices in integrated optics, but suffer from limited operational wavelength range. Here we use, for the first time, the dispersive properties of sub-wavelength gratings to achieve a fivefold enhancement in the operation bandwidth of a silicon-on-insulator directional coupler. This approach does not compromise the size or the phase response of the device. The sub-wavelength grating based directional coupler we propose covers a 100 nm bandwidth with an imbalance of ≤ 0.6 dB between its outputs, as supported by full 3D FDTD simulations.

We present several fundamental photonic building blocks based on suspended silicon waveguides supported by a lateral cladding comprising subwavelength grating metamaterial. We discuss the design, fabrication, and characterization of waveguide bends, multimode interference devices and Mach-Zehnder interferometers for the 3715 - 3800 nm wavelength range, demonstrated for the first time in this platform. The waveguide propagation loss of 0.82 dB/cm is reported, some of the lowest loss yet achieved in silicon waveguides for this wavelength range. These results establish a direct path to ultimately extending the operational wavelength range of silicon wire waveguides to the entire transparency window of silicon.

We propose a novel multimode interference coupler (MMI) design for high index contrast technologies, based on a shallowly etched multimode region, which is, for the first time, directly coupled to deeply etched input and output waveguides. This reduces the phase errors associated with the high index contrast, while still allowing for a very compact layout. Using this structure, we demonstrate a 2 × 4 MMI operating as a 90 • hybrid, with a footprint of only 0.65 mm × 0.53 mm including all the structures necessary to couple light to a fibre array. The hybrid exhibits a common mode rejection ratio (CMRR) better −20 dBe and phase errors better than ±5 • in a ∼ 50 nm bandwidth. • hybrids for coherent optical receivers [3][4][5][6]. The latter enable optical fibre based long haul transmissions with drastic increases in data rates without sacrificing additional bandwidth, by using complex quadrature and phase modulations like QPSK (quaternary phase shift keying). High index contrast technologies, such as Silicon-on-Insulator (SOI) and deeply etched Indium-Phosphide (InP) are attractive platforms for the implementation of such components, since they allow for very compact designs and small waveguide curvature radii, that allow for complex interconnections.However, the high index contrast of deeply etched InP and SOI platforms hinders the design of high performance MMIs, as explained in the following. The basic operation of MMIs consists in launching light from one of the access waveguides (numbered 1 and 2 in Fig. 1), into the wide multimode section where it expands into multiple modes. These travel with different propagation constants, and, at certain imaging distances, interfere to form one or several replicas of the input field, which are coupled to the output waveguides (numbered 3 to 6 in Fig. 1). The formation of these images is governed by the self-imaging theory [7], which essentially requires that all the modes excited in the multimode section exhibit quadratically related propagation constants, i.e. β m = β 1 − (m 2 − 1)π/(3L π ), where m = 1, 2, 3, ... is the mode number and L π is the beat length of the two lowest order modes. In waveguides with high lateral (x direction) index contrast, such as deeply etched InP ridges and silicon wires, this relation between the propagation constants only holds for the lower order modes, resulting in strong phase errors for the higher order modes [8]. These phase errors result in low quality imaging, and become especially detrimental as the number of MMI inputs or outputs grows. A number of techniques have been proposed to overcome this problem. First, by increasing the access waveguide width, only the lowest order modes, which exhibit almost ideal propagation constants, are excited [9][10][11][12]. This requires careful design of the access waveguide width, since increasing it too much results in unnecessarily large devices [9]. Second, using shallowly etched waveguides [8] reduces the index contrast, thus alleviating the phase error. However, this yields larger devices, ...

We propose an ultra-broadband multimode interference (MMI) coupler with a wavelength range exceeding the O, E, S, C, L and U optical communication bands. For the first time, the dispersion property of the MMI section is engineered using a subwavelength grating structure to mitigate wavelength dependence of the device. We present a 2 × 2 MMI design with a bandwidth of 450nm, an almost fivefold enhancement compared to conventional designs, maintaining insertion loss, power imbalance and MMI phase deviation below 1dB, 0.6dB and 3°, respectively. The design is performed using an in-house tool based on the 2D Fourier Eigenmode Expansion Method (F-EEM) and verified with a 3D Finite Difference Time Domain (FDTD) simulator.