Here, we review the progress and most recent advances in phonon‐polaritonics, an emerging and growing field that has brought about a range of powerful possibilities for mid‐ to far‐infrared (IR) light. These extraordinary capabilities are enabled by the resonant coupling between the impinging light and the vibrations of the material lattice, known as phonon‐polaritons (PhPs). These PhPs yield a characteristic optical response in certain materials, occurring within an IR spectral window known as the reststrahlen band. In particular, these materials transition in the reststrahlen band from a high‐refractive‐index behavior, to a near‐perfect metal behavior, to a plasmonic behavior – typical of metals at optical frequencies. When anisotropic they may also possess unconventional photonic constitutive properties thought of as possible only with metamaterials. The recent surge in two‐dimensional (2D) material research has also enabled PhP responses with atomically‐thin materials. Such vast and extraordinary photonic responses can be utilized for a plethora of unusual effects for IR light. Examples include sub‐diffraction surface wave guiding, artificial magnetism, exotic photonic dispersions, thermal emission enhancement, perfect absorption and enhanced near‐field heat transfer. Finally, we discuss the tremendous potential impact of these IR functionalities for the advancement of IR sources and sensors, as well as for thermal management and THz‐diagnostic imaging.

Inspired by moth eyes, nature’s most powerful antireflex, we present a subwavelength SiC micropyramid design, which operates in the reststrahlen band of SiC, namely, the spectral band of strong phonon-photon coupling in the SiC material. While within this band, SiC repels electromagnetic waves, we observe here a broad low-reflectivity window with unique attributes, with distinct characteristics different from typical dielectric moth-eye-like structures. To be specific, while the latter systems are entirely symmetric, the reflection response of our SiC micropyramid system can be highly asymmetric. In particular, the SiC micropyramid system can be near reflectionless for light impinging from the tip side of the micropyramids and can exhibit more than 90% reflection for light impinging from the base side of the micropyramids, over a broad wavelength range in the SiC reststrahlen band. This strongly asymmetric reflection response emanates from the cascaded coupling of vortexlike cavity modes at each of the SiC blocks comprising the micropyramids and translates into a strongly unidirectional absorber response. We discuss how, by virtue of Kirchhoff’s law, this strongly unidirectional superabsorber behavior implies a strongly unidirectional emission profile that is important for one-way infrared sources and passive-radiative-cooling systems.

We theoretically investigate mid-IR absorption enhancement with a SiC one-dimensional photonic crystal (PC) microstructure at the frequency regime of the phonon-polariton band gap, where efficient absorption is unattainable in the bulk material. Our study reveals an intricate relationship between absorption efficiency and the energy velocity of light propagation, that is far more complex than hitherto believed. In particular, our findings suggest that absorption peaks away from the photonic-crystal band edge where energy velocity is minimum. While efficient absorption is still associated with a slow-light mode, the latter is faster by at least an order of magnitude in comparison to the bulk material. Moreover, our calculations suggest that absorption becomes optimal when light gradually slow downs as it enters the PC. Relying on this insight, we achieved near-perfect absorption around the phonon-polariton mid-gap frequency with a PC with a suitably terminated end face. We further demonstrate that the near-perfect absorptive property can be tuned with the incident light angle, to be polarization insensitive or polarization selective. We believe our proposed non-metallic paradigm opens up a new route for harnessing infrared absorption with semiconductor and ionic-crystal materials.

In this paper, we show the experimental results of a thermally stable ${\mathrm{Si}}_3{\mathrm{N}}_4$ external cavity (SiN EC) laser with high power output and the lowest SiN EC laser threshold to our knowledge. The device consists of a 250 μm sized reflective semiconductor optical amplifier butt-coupled to a passive chip based on a series of ${\mathrm{Si}}_3{\mathrm{N}}_4$ Bragg gratings acting as narrow reflectors. A threshold of 12 mA has been achieved, with a typical side-mode suppression ratio of 45 dB and measured power output higher than 3 mW. Furthermore, we achieved a mode-hop free-lasing regime in the range of 15–62 mA and wavelength thermal stability up to 80°C. This solves the challenges related to cavity resonances’ thermal shift and shows the possibility for this device to be integrated in dense wavelength-division multiplexing (WDM) and heat-intensive optical interconnects technologies.

In recent years, the monolithic integration of new materials such as SiN, Ge and LiNbO ₃ on silicon (Si) has become important to the Si photonics community due to the possibility of combining the advantages of both material systems. However, efficient coupling between the two different layers is challenging. In this work, we present a spot size converter based on a two-tier taper structure to couple the optical mode adiabatically between Si and SiN. The fabricated devices show a coupling loss as low as 0.058 dB ± 0.01 dB per transition at 1525 nm. The low coupling loss between the Si to SiN, and vice versa, reveals that this interlayer transition occurs adiabatically for short taper lengths (<200 µ m). The high refractive index contrast between the Si and SiN is overcome by matching the optical impedance. The proposed two-tier taper structure provides a new platform for optoelectronic integration and a route towards 3D photonic integrated circuits.

The authors present results on the performance of a hybrid external cavity photonic crystal laser-comprising semiconductor optical amplifier, and a 2D photonic crystal cavity fabricated in low-temperature amorphous silicon. The authors demonstrate that lithographic control over amorphous silicon photonic crystal cavity-resonant wavelengths is possible, and that single-mode lasing at optical telecommunications wavelengths is possible on an amorphous silicon platform.