All-dielectric materials and related nanophotonic applications

Abstract

Generally, nanophotonics is associated with plasmonic materials and structures made of noble metals such as gold or silver. However, conventional plasmonic materials have several disadvantages restricting their applications. First, plasmonic materials like gold and silver suffer from high optical loss at optical frequencies. Second, noble metals are rare and not proper for large scale fabrication. Third, as a nanoantenna, plasmonic nanoparticles only hold electric dipole-like resonance which cannot tailor and direct the optical field as we want. Therefore, those have driven the intense search for all-dielectric materials (ADMs) which offer unique opportunities for reduced dissipative losses and large resonant enhancement of both electric and magnetic near-fields beyond plasmonic materials.There are usually three types of ADMs, i.e., firstly high-index ADMs such as silicon, germanium and gallium arsenide, secondly mid-index ADMs such as titanium dioxide, silicon carbide and boron and thirdly low-index ADMs such as silicon dioxide and polymer. ADMs with different refractive indexes bring a lot of freedom to design nanostructures with different optical properties. For example, ADMs with high refractive index and low loss can generate strong Mie resonances for far-field related applications, while ADMs with high absorption and plasmonic-like properties are favorable for near-field applications.Although many efforts have been devoted to prepare ADMs and study the related nanophotonic applications, researchers still face fundamental challenges: how to control phase, size and shape of building blocks in the synthesis of ADMs, how to fabricate functional nanostructures by using these building blocks, and further how to achieve the transformation from simple nanoparticles synthesis to functional nanostructures fabrication. To address these issues, we have developed a series of unique techniques based on laser ablation in liquids (LAL) for all-dielectric nanomaterials preparation and all-dielectric nanostructures fabrication in recent years. Using LAL, we have prepared a series of high quality all-dielectric nanomaterials. Meanwhile, optical nanoantennas and devices based on ADMs by LAL have demonstrated appreciable figures-of-merit, manifesting great potential for nanophotonics and the next generation optoelectronics.In this review, we will introduce the latest progresses of ADMs prepared via top-down and bottom-up methods and related applications in nanophotonics. Firstly, we will present the basic optical properties of ADMs. For example, a new branch of nanophotonics has emerged that seeks to manipulate the strong, optically induced electric and magnetic Mie resonances in nanoparticles based on high refractive index ADMs. Then, we will introduce various approaches for the fabrication of ADM-based nanostructures and their merits and demerits, in which we will demonstrate that LAL technique is more suitable to produce different kinds of ADMs compared with traditional top-down and bottom-up fabrication methods. Secondly, we will summarize the nanophotonic applications of ADMs, including the utilization of unique resonant modes in silicon nanoparticles, enhancement of both linear and nonlinear optical signals, biosensing, light trapping and harvesting, and the enhancement of light matter interaction. Finally, we will discuss several strategies for improvement of nanophotonic performances of ADMs. For example, through designing specific ADM nanostructures, we can obtain higher Q factor and better near-field feature. Additionally, recent progresses on active tuning of ADM-based nanodevices are presented which make contributions to the practical use of ADMs.Overall, ADMs have actually opened a window toward building highly efficient nanophotonic devices from their unique attributes. (i) ADMs contain a group of materials which have varied optical properties. This diversity provides us freedom and possibilities of functional structural design. (ii) Most of ADMs are compatible with mature semiconductor processing technology and have much lower cost compared with plasmonic materials of noble metals. (iii) ADM-based nanostructures can generate intriguing resonant modes, such as magnetic dipole, toroidal, anapole modes and others.