Part VII Advanced Trends of Nanophotonics

Wei Ting Chen and Din Ping Tsai

Introduction

Nanophotonics is the study of the behavior of light-matter interaction at the nanometer scale. By adding the dimensions of optical devices and components to sub-wavelength scale, nanophotonics provides new opportunities for fundamen- tal science and practical applications. One of the goals of nanophotonics devel- opment is to manipulate light at the nanoscale, which may not be limited by the chemical composition of natural materials and the diffraction limit of electromag- netic wave. Nanophotonics has several advantages with such diffraction-unlimited properties for functional applications: (i) nanoscale footprints-smaller compo- nents and devices; (ii) photon-electron process in nanoscale—faster processing speed, and (iii) nanoscale confinement of optical radiation and electromagnetic fields—enhancing the light-matter interactions and dramatically reducing the optical energy consumption. The characterization of drastic optical localization within such components strongly enhances the typically weak interaction between light and matter, which increases the energy efficiency to obtain desired effects and phenomena. This chapter covers two major parts of the latest trends of nano- photonics, plasmonics, and metamaterials. Several cutting-edge approaches har- vested from the extraordinary properties of nanophotonics, which are conducted to advanced trends relate to: Micro/nano-lasers with the smallest plasmonic nano- laser, theoretical models of the micro/nano-cavity, and semiconductor micro-lasers with tuning functions on a flexible substrate (Chap. 16, 17 and 19), nanostructures light-emitting diode (LED) with better light extraction and reduced piezoelectric field induced by strain (Chap.18 , 24), one-dimensional photonic crystal nanowires with small footprints and ultrahigh Q-factors (Chap. 20), nano-structured wave- guides with slow light effect (Chap. 21), negative refraction index generated by glancing angle deposition (Chap. 22), improving the light harvest of solar cell with anti-reflective nanostructures (Chap. 23), high-sensitive plasmonic biosensors with Fano-like resonance (Chap. 25) are addressed. 356 Part VII: Advanced Trends of Nanophotonics

Plasmonics, the coherent electrons oscillation of noble materials driven by photons, has abilities to confine electromagnetic field to be smaller than the wave- length of incident electromagnetic wave. There are two branches of plasmonics: “surface plasmon polaritions” (SPP) and “localized surface plasmons” (LSP). Surface plasmon polaritions are supported by metallic thin film, and can be usu- ally excited by either grating coupling or total internal reflection to provide addi- tional wave vector to match the phase difference between free space propagating wave and surface plasmon polaritions. On the contrary, the localized surface plasmons of a given metallic nanoparticles can be directly excited by free space propagating wave, and its resonance frequency can be tailored by its geometrical dimensions and the refraction index of environment as well as polarization state of incident electromagnetic wave. At both surface plasmon and localized surface plasmon resonance, the electromagnetic field is strongly enhanced in the imme- diate vicinity, and therefore the light-matter interaction is enhanced. As a result, plasmonics show a wide range of potential applications—nanocavity, high-sensi- tive bio-sensing, LED, nano-laser, etc. Artificial materials with sub-wavelength structure, which are so-called metama- terials, have attracted a lot of attention. The optical properties of metamaterials are determined by their artificial structures rather than their material composition. The Greek word “meta” is translated as “beyond,” which means that the central con- cept is to construct new materials with those optical properties that are not found or hardly observed in nature. For instance, the negative refraction is the typical example of metamaterials that reword the formula of Snell’s law. Moreover, the function of metamaterials as artificial atoms or molecules provides an entirely new route to further enhance the capability to design and create novel material prop- erties. While through the near-field interaction between metamaterials, they can offer and generate more fascinating physical and optical properties unavailable in nature or chemically synthesized materials. Metamaterials therefore enable us to tailor the propagation of electromagnetic wave, even more transformation optics and optical cloaking. Till date, the research agenda on metamaterials is shifting from fundamental researches to functionalities for practical applications, such as tunable, switchable, and biosensing devices. It will also pave a new way to inte- grate photonics with electronics in not only telecommunication systems but also opto-electron circuit devices and applications. Finally, I hope this chapter proves to be a useful guideline for both current and future researchers, and inspires people toward cutting-edge breakthroughs in the field of nanophotonics. Any comment and suggestion related to this chapter is highly appreciated.