Inherent symmetries of a system lead to multiple degeneracies of its energy spectra. Introducing individual atomic impurities can locally break these symmetries,which is expected to lift the degenerate degrees of freedom around the impurities. Although central to our understanding of the fundamental properties of solids, the broken-symmetry states induced by individual atomic impurities have so far eluded observation. Here, we report nanoscale probing of the broken-symmetry states in graphene induced by two types of individual atomic impurities, i.e., isolated nitrogen dopants and isolated hydrogen atoms chemisorbed on graphene. Our experiments demonstrate that both types of atomic impurities can locally break sublattice symmetry of graphene and generate valley-polarized states, which extends several nanometers around the impurities. For the isolated hydrogen atom chemisorbed on graphene, the enhanced spin-orbit coupling, which arises from the sp 3 distortion of graphene due to the hydrogen chemisorption, further lifts the spin degeneracy, resulting in a fully spin and valley polarized states within about 1 nm around the hydrogen atom. Our result paves the way to control various broken-symmetry states at the nanoscale by various atomic impurities.
We study the excitation modes in parabolic quantum wells via an approach based on the invariant-imbedding method in the random-phase approximation. For large values of wave vectors, several higher-multipole edge plasmon modes have been observed for the first time below the intrasubband plasmon in parabolic quantum wells in addition to the normal surface plasmon modes. It is shown that the edge electron density profile has an important effect on the higher-multipole edge plasmon modes.
Optical nonlinear effect plays an important role in optical communication, optical detection, quantum information and other areas. However, it is constrained by the weakness of the nonlinear optical response of the common materials. The enhancement of the optical nonlinear response on a nanoscale becomes a critical challenge. Over the years, several ways to enhance the optical nonlinear effects have been suggested. In fact, these technologies can slightly enhance the optical nonlinear response. Recently, some research groups focused on the materials with vanished permittivity, which is called epsilon-near-zero (ENZ) material, showing that it can exhibit large optical nonlinearity due to the field enhancement in the material of this type. However, the ENZ material only holds a large optical nonlinear response in a limited spectral range. In order to overcome this limitation, here in this paper we report the ENZ mode which is excited by the ITO film and strongly coupled to the gap surface plasmons excited by the metal-dielectric-metal structure. To acquire the nonlinear refractive index <i>n</i><sub>2</sub>, we first calculate the ITO permittivity through the Drude-Lorentz model and find the wavelength of the ENZ material. Then we calculate the time-dependent electron temperature and lattice temperature of ITO by the two-temperature model. According to the elevated electron temperature, we can calculate the plasma frequency <inline-formula><tex-math id="M1">\begin{document}${\omega _{\rm p}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M1.png"/></alternatives></inline-formula>, and by taking it into the Drude-Lorentz model, we can obtain a new permittivity of ITO compared with the initial one. Finally, we can calculate the variation of the refractive index <inline-formula><tex-math id="M2">\begin{document}$ \Delta n $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M2.png"/></alternatives></inline-formula>, and the nonlinear refractive index <inline-formula><tex-math id="M3">\begin{document}$ {n_2} = \Delta n/{I_0} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M3.png"/></alternatives></inline-formula>. In this paper, our coupled structure exhibits a broadband (~1000 nm bandwidth) enhancement of the nonlinear optical effect in the near-infrared spectrum, a maximum nonlinear refractive index <i>n</i><sub>2</sub> as large as 3.02 cm<sup>2</sup>·GW<sup>–1</sup>, which is nearly 3 orders larger than the previously reported nonlinear refractive index of bare ITO film. As a result, it is possible to realize a dramatically large variation of nonlinear refractive index under a low-power optical field. It is expected to be used in the nano photonic devices such as optical storage, all-optical switches, etc.
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