Publications

Silicon photonics is one of the most dynamic fields within photonics, and it has seen huge progress in the last 20 years, addressing applications in data centers, autonomous cars, and sensing. It is mostly focused on the telecommunications wavelength range (1.3 and 1.55 µm), where silicon becomes transparent. In this range, there are excellent light sources and photodetectors, as well as optical fibers operating with extremely low losses and dispersion. It is a technology that hugely benefits from the availability of complementary metal–oxide–semiconductor (CMOS) fabrication infrastructure and techniques used for microelectronics. Silicon and germanium, as another CMOS compatible group IV material, are transparent beyond the wavelength of 2 µm. The mid-IR wavelength range (2–20 µm) is of particular importance as it contains strong absorption signatures of many molecules. Therefore, Si- and Ge-based platforms open up the possibility of small and cost-effective sensing in the fingerprint region for medical and environmental monitoring. In this paper, we discuss the current mid-IR silicon photonics landscape, future directions, and potential applications of the field.

Mid-infrared spectroscopy enabled by silicon photonics has received great interest in recent years as a pathway for a scalable sensing technology. The development of such devices would realise inexpensive and accessible instrumentation for a wide variety of uses over numerous fields. However, not every sensing application is the same; to produce sensors for real-world scenarios, engineers need flexibility in device design but also need to maintain compatibility with scalable fabrication processes. Sub-wavelength gratings can offer a solution to this problem, as they enable the engineering of optical properties using standard fabrication techniques and without requiring new materials. By using sub-wavelength gratings, specific design approaches can be tailored to different applications, such as increasing the interaction of a sensor with an analyte or broadening the bandwidth of an integrated photonic device. Here, we review the development of sub-wavelength grating-based devices for mid-infrared silicon photonics and discuss how they can be exploited for spectroscopic and sensing devices.

Photonic nanostructures such as gratings and ring resonators have become ubiquitous building blocks in Photonics. For example, they are used in filters, they resonantly enhance signals and act as grating couplers. Much research effort is invested in using such structures to create novel functionalities, which often employs electron-beam lithography. An intrinsic issue in this field is the ability to accurately achieve a specific operating wavelength, especially for resonant systems, because nanometer-scale variations in feature size may easily detune the device. Here, we examine some of the key fabrication steps and show how to improve the reproducibility of fabricating wavelength scale photonic nanostructures. We use guided mode resonance grating sensors as our exemplar and find that the exposure condition and the development process significantly affect the consistency of the resonance wavelength, amplitude, and sensitivity of the sensor. By having careful control over these factors, we can achieve consistent performance for all the sensors studied, with less than 10% variation in their resonance behaviors. These investigations provide useful guidelines for fabricating nanostructures more reliably and to achieve a higher success rate in exploratory experiments.

Here we present a roadmap on Photonic metasurfaces. This document consists of a number of perspective articles on different applications, challenge areas or technologies underlying photonic metasurfaces. Each perspective will introduce the topic, present a state of the art as well as give an insight into the future direction of the subfield.

Numerous optical phenomena and applications have been enabled by nanophotonic structures. Their current fabrication from high refractive index dielectrics, such as silicon (Si) or gallium phosphide (GaP), pose restricting fabrication challenges while metals, relying on plasmons and thus exhibiting high ohmic losses, limit the achievable applications. An emerging class of layered, so-called van der Waals (vdW), crystals is presented as a viable nanophotonics platform in this work. The dielectric response of 11 mechanically exfoliated thin-film (20–200 nm) vdW crystals is extracted, revealing high refractive indices up to n = 5, pronounced birefringence up to Δn = 3, sharp absorption resonances, and a range of transparency windows from ultraviolet to near-infrared. Nanoantennas are subsequently fabricated on silicon dioxide (SiO2) and gold, utilizing the compatibility of vdW thin films with a variety of substrates. Pronounced Mie resonances are observed due to the high refractive index contrast on SiO2, leading to a strong exciton-photon coupling regime as well as largely unexplored high-quality-factor, hybrid Mie-plasmon modes on gold. Additional vdW-material-specific degrees of freedom in fabrication are further demonstrated by realizing nanoantennas from stacked twisted crystalline thin-films, enabling control of nonlinear optical properties, and post-fabrication nanostructure transfer, important for nano-optics with sensitive materials.

The interest in high quality factor (high-Q) resonances in metasurfaces has been rekindled with the rise of the bound states in the continuum (BIC) paradigm, which describes resonances with apparently limitlessly high quality-factors (Q-factors). The application of BICs in realistic systems requires the consideration of the angular tolerance of resonances, however, which is an issue that has not yet been addressed. Here, we develop an ab-initio model, based on temporal coupled mode theory, to describe the angular tolerance of distributed resonances in metasurfaces that support both BICs and guided mode resonances (GMRs). We then discuss the idea of a metasurface with a perturbed unit cell, similar to a supercell, as an alternative approach for achieving high-Q resonances and we use the model to compare the two. We find that, while sharing the high-Q advantage of BIC resonances, perturbed structures feature higher angular tolerance due to band planarization. This observation suggests that such structures offer a route toward high-Q resonances that are more suitable for applications.