We discuss how the poor metals Al, Ga, In, Sn, Tl, Pb, and Bi can be used for plasmonics in the near to far ultraviolet (UV) range, similar to the noble metals Ag and Au in the visible (Vis) range. We first discuss the empirical dielectric functions of the poor metals, contrasting them with Ag and Au, and also fitting them to a Drude and multiple Lorentz oscillator form. Using Mie theory, we then compare the optical responses of spherical poor metal nanoparticles to noble metal ones. Finally, nanoparticle dimers are studied using a vectorial finite element method. We show how the poor metals exhibit large electric field enhancements in the UV, comparable to Au in the Vis, which makes them particularly attractive for sensing applications, such as surface enhanced Raman spectroscopy.

Plasmonics in the ultraviolet with the poor metals Al, Ga, In, Sn, Tl, Pb, and Bi

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

Solar-to-chemical energy conversion via heterogeneous photocatalysis is one of the sustainable approaches to tackle the growing environmental and energy challenges. Among various promising photocatalytic materials, plasmonic-driven photocatalysts feature prominent solar-driven surface plasmon resonance (SPR). Non-noble plasmonic metals (NNPMs)-based photocatalysts have been identified as a unique alternative to noble metal-based ones due to their advantages like earth-abundance, cost-effectiveness, and large-scale application capability. This review comprehensively summarizes the most recent advances in the synthesis, characterization, and properties of NNPMs-based photocatalysts. After introducing the fundamental principles of SPR, the attributes and functionalities of NNPMs in governing surface/interfacial photocatalytic processes are presented. Next, the utilization of NNPMs-based photocatalytic materials for the removal of pollutants, water splitting, CO2 reduction, and organic transformations is discussed. The review concludes with current challenges and perspectives in advancing the NNPMs-based photocatalysts, which are timely and important to plasmon-based photocatalysis, a truly interdisciplinary field across materials science, chemistry, and physics.

Non-Noble Plasmonic Metal-Based Photocatalysts.

The physical and electronic structure of the (1 0 0), (0 1 0), (0 0 1) and ( 1 0 1 ¯ ) faces of β-Ga 2 O 3 are addressed using ab initio theory. Restricted Hartree–Fock calculations, with large-core Ga and O pseudopotentials, are done to optimize the structure of first the bulk and then of slabs “cut” in the required orientations. The slab unit cells are fully relaxed during optimization, and the displacements of all atoms from the ideally-terminated positions are obtained as functions of depth into the bulk. For the relaxed slabs, single-point density functional theory calculations using the B3LYP functional and all-electron basis sets are performed to obtain surface energies, ionic charges and bond overlap populations. All surfaces exhibit a decrease in surface energy upon relaxation, and the local bonding at the surface is analyzed by comparing nearest-neighbor bond lengths and overlap populations with those in the bulk. The ( 1 0 1 ¯ ) surface, which exhibits a high energy when ideally terminated, undergoes large displacements and changes in bonding during relaxation leading to a substantial lowering of the surface energy. The band structure is also obtained for the lowest-energy surface, which is one of the possible non-polar terminations of the (1 0 0). The results provide insight into the growth and structure of β-Ga 2 O 3 nanoribbons.

The structure of low-index surfaces of β-Ga2O3

Gallium has recently been demonstrated as a phase-change plasmonic material offering UV tunability, facile synthesis, and a remarkable stability due to its thin, self-terminating native oxide. However, the dense irregular nanoparticle (NP) ensembles fabricated by molecular-beam epitaxy make optical measurements of individual particles challenging. Here we employ hyperspectral cathodoluminescence (CL) microscopy to characterize the response of single Ga NPs of various sizes within an irregular ensemble by spatially and spectrally resolving both in-plane and out-of-plane plasmonic modes. These modes, which include hybridized dipolar and higher-order terms due to phase retardation and substrate interactions, are correlated with finite difference time domain (FDTD) electrodynamics calculations that consider the Ga NP contact angle, substrate, and native Ga/Si surface oxidation. This study experimentally confirms previous theoretical predictions of plasmonic size-tunability in single Ga NPs and demonstrates that the plasmonic modes of interacting Ga nanoparticles can hybridize to produce strong hot spots in the ultraviolet. The controlled, robust UV plasmonic resonances of gallium nanoparticles are applicable to energy- and phase-specific applications such as optical memory, environmental remediation, and simultaneous fluorescence and surface-enhanced Raman spectroscopies.

/pdf/gallium-plasmonics-deep-subwavelength-spectroscopic-imaging-31efptu92w.pdf

Gallium plasmonics: deep subwavelength spectroscopic imaging of single and interacting gallium nanoparticles.

The optical properties of clean, evaporated films of gallium have been studied at several temperatures. The observed spectra were interpreted on the basis of a pseudopotential calculation of the band structure. The position of characteristic peaks in the spectra agreed well with the predictions from the optimized model potential for gallium recently published by Appapillai and Williams (1973). The shift of the peaks with temperature was too large to be accounted for by shifts in the atomic form factor alone; the Debye-Waller factor must be included to explain the observations.

http://iopscience.iop.org/article/10.1088/0305-4608/4/11/032/pdf

Band structure and optical properties of gallium

The optical properties of gallium quench condensed onto substrates at 10K, 30K and 80K have been studied. It is found that films condensed at 10K had a spectrum which could be described extremely well by the Drude equations. At higher temperatures two different spectra were observed, which the authors believe belong to the orthorhombic alpha dn and the monoclinic beta phases of gallium.

O. Hunderi

Papers

Band structure and optical properties of gallium

Amorphous gallium-a free electron metal

A simple automatic ellipsometer for a wide energy range

Ellipsometric study of lithium hydroxide overlayers on lithium

Optical properties of Cu dispersed in solid Xe