Since 2000, there has been an explosion of activity in the field of plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). In this Review, we explore the mechanism of PERS and discuss PERS hotspots — nanoscale regions with a strongly enhanced local electromagnetic field — that allow trace-molecule detection, biomolecule analysis and surface characterization of various materials. In particular, we discuss a new generation of hotspots that are generated from hybrid structures combining PERS-active nanostructures and probe materials, which feature a strong local electromagnetic field on the surface of the probe material. Enhancement of surface Raman signals up to five orders of magnitude can be obtained from materials that are weakly SERS active or SERS inactive. We provide a detailed overview of future research directions in the field of PERS, focusing on new PERS-active nanomaterials and nanostructures and the broad application prospect for materials science and technology. Assisted by rationally designed novel plasmonic nanostructures, surface-enhanced Raman spectroscopy has presented a new generation of analytical tools (that is, tip-enhanced Raman spectroscopy and shell-isolated nanoparticle-enhanced Raman spectroscopy) with an extremely high surface sensitivity, spatial resolution and broad application for materials science and technology.

Nanostructure-based plasmon-enhanced Raman spectroscopy for surface analysis of materials

Infrared spectroscopy is the technique of choice for chemical identification of biomolecules through their vibrational fingerprints. However, infrared light interacts poorly with nanometric-size molecules. We exploit the unique electro-optical properties of graphene to demonstrate a high-sensitivity tunable plasmonic biosensor for chemically specific label-free detection of protein monolayers. The plasmon resonance of nanostructured graphene is dynamically tuned to selectively probe the protein at different frequencies and extract its complex refractive index. Additionally, the extreme spatial light confinement in graphene—up to two orders of magnitude higher than in metals—produces an unprecedentedly high overlap with nanometric biomolecules, enabling superior sensitivity in the detection of their refractive index and vibrational fingerprints. The combination of tunable spectral selectivity and enhanced sensitivity of graphene opens exciting prospects for biosensing.

Mid-infrared plasmonic biosensing with graphene

Core–shell nanoparticles are at the leading edge of the hot research topics and offer a wide range of applications in optics, biomedicine, environmental science, materials, catalysis, energy, and so forth, due to their excellent properties such as versatility, tunability, and stability. They have attracted enormous interest attributed to their dramatically tunable physicochemical features. Plasmonic core–shell nanomaterials are extensively used in surface-enhanced vibrational spectroscopies, in particular, surface-enhanced Raman spectroscopy (SERS), due to the unique localized surface plasmon resonance (LSPR) property. This review provides a comprehensive overview of core–shell nanoparticles in the context of fundamental and application aspects of SERS and discusses numerous classes of core–shell nanoparticles with their unique strategies and functions. Further, herein we also introduce the concept of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail because it overcomes the long...

Core–Shell Nanoparticle-Enhanced Raman Spectroscopy

Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals (NCs) that results in resonant absorption, scattering, and near field enhancement around the NC can be tuned across a wide optical spectral range from visible to far-infrared by synthetically varying doping level, and post synthetically via chemical oxidation and reduction, photochemical control, and electrochemical control In this review, we will discuss the fundamental electromagnetic dynamics governing light matter interaction in plasmonic semiconductor NCs and the realization of various distinctive physical properties made possible by the advancement of colloidal synthesis routes to such NCs Here, we will illustrate how free carrier dielectric properties are induced in various semiconductor materials including metal oxides, metal chalcogenides, metal nitrides, silicon, and other materials We will highlight the applicability and limitations of the Drude model as applied to semiconductors considering the complex band structures 

Localized Surface Plasmon Resonance in Semiconductor Nanocrystals

The discovery, design and development of materials are critically linked to advances in many areas of research, and optics is no exception. Recently, the spectral region in which the index of refraction of a material approaches zero has become a topic of interest owing to fascinating phenomena, such as static light, enhanced nonlinearities, light tunnelling and emission tailoring. As a result, such near-zero-index (NZI) materials bridge materials development and optical research. Here, we review recent advances in various classes of NZI platforms, with particular focus on homogeneous materials, including metals, semi-metals, doped semiconductors, phononic and interband materials, discussing the novel optical phenomena that they can produce. We also overview the developments in a key area for NZI materials, nonlinear optics, and survey some of the future goals in the field, such as the development of tailorable NZI materials in the visible range and the improvement of the theoretical description of the nonlinear enhancements in these materials. Materials with a near-zero optical refractive index have stimulated much interest, as they can be used to investigate fundamental light–matter interactions. This Review surveys the wide range of near-zero-index homogeneous materials that have been explored and highlights the key experimental advancements they have enabled.

Near-zero-index materials for photonics

The field of plasmonics has the potential to enable unique applications in the mid-infrared (IR) wavelength range. However, as is the case regardless of wavelength, the choice of plasmonic material has significant implications for the ultimate utility of any plasmonic device or structure. In this manuscript, we review the wide range of available plasmonic and phononic materials for mid-IR wavelengths, looking in particular at transition metal nitrides, transparent conducting oxides, silicides, doped semiconductors, and even newer plasmonic materials such as graphene. We also include in our survey materials with strong mid-IR phonon resonances, such as GaN, GaP, SiC, and the perovskite SrTiO 3 , all of which can support plasmon-like modes over limited wavelength ranges. We will discuss the suitability of each of these plasmonic and phononic materials, as well as the more traditional noble metals for a range of structures and applications and will discuss the potential and limitations of alternative plasmonic materials at these IR wavelengths.

https://www.spiedigitallibrary.org/journals/journal-of-nanophotonics/volume-9/issue-1/093791/Review-of-mid-infrared-plasmonic-materials/10.1117/1.JNP.9.093791.pdf

Review of mid-infrared plasmonic materials

: The sensitivity of infrared (IR) focal plane arrays (FPAs) is often limited by a low pixel ﬁ ll factor. Solid immersion microlens arrays address this problem by focusing the light reaching each pixel into the most sensitive part of that pixel. This strategy is used in CMOS image sensors but has not been industrially adopted for IR FPAs due to signi ﬁ cant di ﬃ culties in integration with compound semiconductors. Here, we present an all-in-one solution for producing solid immersion microlens arrays compatible with various IR FPAs regardless of their substrate material. Our strategy is to use refractive lenses made of SiO 2 and Si 3 N 4 with very broad-band and e ﬃ cient focusing abilities. Notably, our strategy works across a broad range of wavelengths with little performance degradation, meaning it is scalable to various applications. We implemented our method in short-wavelength IR FPAs and demonstrated 7.4 times improvement in quantum e ﬃ ciency. This is the ﬁ rst demonstration of an immersion microlens array in a non-silicon infrared FPA.

Travis Hamilton

Papers

Review of mid-infrared plasmonic materials

Substrate-Independent Broad-Band Immersion Microlens Arrays with a High Coupling Efficiency for Infrared Focal Plane Arrays