Using reflection electron energy-loss spectroscopy (EELS), we unveil a direct correlation between the thickness-dependent oscillatory charge spilling of quantum-well states in epitaxial Mg(0001) films, and the linear dispersion coefficient of the monopole surface plasmon mode. In addition, the spectral intensities of the photoemission threshold excitation and multipole surface plasmon mode follow a similar but mutually anticorrelated quantum oscillatory pattern. These oscillatory features of the collective response are attributed to quantum oscillations in the ground-state charge-density profile, specifically its decay into the vacuum. Because the decay lengths can be controlled through the quantum-size effects and appropriate choice of the quantum-size boundary conditions, these insights are expected to be significant in the design of quantum-sized plasmonic devices.
The optical properties of metals are mainly determined by their plasmonic excitations, with various intriguing phenomena associated with systems in reduced dimensions. In this paper, we present a systematic study of the plasmonic excitations in ultrathin metal films on dielectric substrates using two different theoretical approaches, and with Mg thin films on Si as prototype systems. The bulk of the results are obtained using the first approach within firstprinciples time-dependent local density approximation. We show that the presence of the substrate substantially modifies the plasmon hybridization of the metal films; in turn, the plasmon excitation in the films strongly enhances the absorption of the substrate. The detailed absorption spectra contain several intriguing features. Above the Mg surface plasmon mode, we observe a broad resonance due to the hybridization between the antisymmetric surface plasmon and multipole surface plasmon. Furthermore, below the Mg surface plasmon Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. mode, there also exists a broad absorption feature, caused by individual electron-hole pair excitations. In the second approach, we use a semi-classical local optics model to reveal an intrinsic connection between the broad absorption feature and the multipole surface plasmon modes, which result from the singleparticle and collective excitations of the same surface electrons, respectively. Our theoretical predictions on the plasmon dispersions and absorption spectra are also shown to be qualitatively consistent with the latest experimental observations using electron energy loss spectroscopy for Mg thin films grown on Si substrates.Keywords: ultrathin metal film, surface plasmon, multipole surface plasmon, plasmon hybridization, local optics model New J. Phys. 16 (2014) 065014 X Li et al
Axial optical chain (optical bottle beams) beams are widely used in optical micromanipulation, atom trapping, guiding and binding of microparticles and biological cells, etc. However, the generation of axial optical chain beams are not very flexible at present, and its important characteristics such as periodicity and phase shift cannot be easily regulated. Here, we propose a holographic method to achieve the axial optical chain beams with controllable periodicity and phase. A double annular phase diagram is generated based on the gratings and lenses algorithms. The beam incident to the double annular slits was tilted from the optical axis to produce concentric double annular beams. The annular beam with different radius will produce the zero-order Bessel beam with different axial wave vector. Axial optical chain beams is produced by interference of two zero-order Bessel beams with different axial wave vectors. The phase and periodicity of the axial optical chain beams can be changed by changing the initial phase difference and radius of the double annular slits of the double annular phase diagram, respectively. The feasibility and effectiveness of the proposed method are demonstrated by theoretical numerical analysis and experiments. This method will further expand the application of axial optical chain beams in optical tweezers, optical modulation and other fields.
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