Extreme field confinement in zigzag plasmonic crystals.

We propose extreme field confinement in a zigzag plasmonic crystal that can produce a wide plasmonic bandgap near the visible frequency range. By applying a periodic zigzag structure to a metal-insulator-metal plasmonic waveguide, the lowest three plasmonic crystal bands are flattened, creating a high-quality broadband plasmonic mirror over a wavelength range of 526-909 nm. Utilizing zigzag plasmonic crystals in a three-dimensional tapered metal-insulator-metal plasmonic cavity, extreme field confinement with a modal volume of less than 0.00005 λ 3 can be achieved even at resonances over a wide frequency range. In addition, by selecting the number of zigzag periods in the plasmonic crystal, critical coupling between the cavity and the waveguide can be achieved, thereby maximizing the field intensity with an enhancement factor of 105 or more. We believe that zigzag plasmonic crystals will provide a powerful platform for implementing broadband on-chip plasmonic devices.

Nanolayer-embedded pseudo-photonic crystals.

In recent years, novel high-performance nanophotonic devices have been realized by applying ultrathin two-dimensional nanolayer materials to nanophotonics. In this paper, we propose nanolayer-embedded compact pseudo-photonic crystals (PPCs) that enable strong interaction between ultrathin nanolayers and photonic crystal modes. In typical two-dimensional slab photonic crystals, the transverse-magnetic (TM) photonic crystal bandgap is not well formed, making it difficult to operate the TM photonic crystal waveguide modes. However, by utilizing the low-frequency TM PPC bands, a long propagation TM waveguide mode, a slow TM waveguide mode, and a TM photonic bandgap are all readily available. In particular, the insertion of a nanometer-thick low-refractive-index layer in the vertical center of TM PPC waveguide can localize the electric fields tightly in nanometer space, causing strong field interaction with the inserted nanolayer material. Using the TM slow light near PPC band edges, field interaction with the nanolayer is significantly enhanced. We can also realize nanolayer-embedded high-quality-factor (Q-factor > 104) PPC cavities using the TM PPC bandgap. We believe that the proposed TM PPCs will play an important role in the strong interaction of ultrathin nanolayer materials with photonic crystal modes.

Ultrahigh Nonlinear Responses from MXene Plasmons in the Short-Wave Infrared Range.

Surface plasmons in 2D materials such as graphene exhibit exceptional field confinement. However, the low electron density of majority of 2D materials, which are semiconductors or semimetals, has limited their plasmons to mid-wave or long-wave infrared regime. This study demonstrates that a 2D Ti3C2Tx MXene with high electron density can not only support strong plasmon confinement with an acoustic plasmon mode in the short-wave infrared region, but also provide ultrahigh nonlinear responses. The acoustic MXene plasmons (AMPs) in the MXene (Ti3C2Tx)-insulator (SiO2)-metal (Au) nanostructure generate in the 1.5-6.0 µm wavelength range, exhibiting a two orders of magnitude reduction in wavelength compared to wavelength in free space. Furthermore, AMP resonators with patterned Au rods exhibit a record-high nonlinear absorption coefficient of 1.37 × 10-2 m W-1 at wavelength of 1.56 µm, ≈3 orders of magnitude greater than the highest value recorded for other 2D materials. These results indicate that MXenes can overcome fundamental plasmon wavelength limitations of previously studied 2D materials, providing groundbreaking opportunities in nonlinear optical applications, including all-optical processing and ultrafast optical switching.

Light Engineering in Nanometer Space.

Significant advances have been made in photonic integrated circuit technology, similar to the development of electronic integrated circuits. However, the miniaturization of cavity resonators, which are the essential components of photonic circuits, still requires considerable improvement. Over the past decades, various optical cavities have been utilized to implement next-generation light sources in photonic circuits with low energy, high data traffic, and integrable physical sizes. Nevertheless, it has been difficult to reduce the size of most commercialized cavities beyond the diffraction limit while maintaining high performance. Herein, recent advancements in subwavelength metallic cavities that can improve performance, even with the use of lossy plasmonic modes, are reviewed. The discussion is divided in three parts according to light engineering methods: subwavelength metal-clad cavities engineered using intermediate dielectric cladding; implementation of plasmonic cavities and waveguides using plasmonic crystals; and development of deep-subwavelength plasmonic waveguides and cavities using geometric engineering. A direction for further developments in photonic integrated circuit technology is also discussed, along with its practical application.