Bing Lei

Ultra-directional forward scattering by individual core-shell nanoparticles

We study the angular scattering properties of individual core-shell nanoparticles that support simultaneously both electric and optically-induced magnetic resonances of different orders. In contrast to the approach to suppress the backward scattering and enhance the forward scattering relying on overlapping electric and magnetic dipoles, we reveal that the directionality of the forward scattering can be further improved through the interferences of higher order electric and magnetic modes. Since the major contributing electric and magnetic responses can be tuned to close magnitudes, ultra-directional forward scattering can be achieved by single nanoparticles without compromising the feature of backward scattering suppression, which may offer new opportunities for nanoantennas, photovoltaic devices, bio-sensing and many other interdisciplinary researches.

Invisible nanowires with interfering electric and toroidal dipoles.

By studying the scattering of normally incident plane waves by a single nanowire, we reveal the indispensable role of toroidal multipole excitation in multipole expansions of radiating sources. It is found that for both p-polarized and s-polarized incident waves, toroidal dipoles can be effectively excited within homogenous dielectric nanowires in the optical spectrum regime. We further demonstrate that the plasmonic core-shell nanowires can be rendered invisible through destructive interference of the electric and toroidal dipoles, which may inspire many nanowire-based light-matter interaction studies, and incubate biological and medical applications that require noninvasive detections and measurements.

Elusive Pure Anapole Excitation in Homogenous Spherical Nanoparticles with Radial Anisotropy

For homogenous isotropic dielectric nanospheres with incident plane waves, Cartesian electric and toroidal dipoles can be tunned to cancel each other in terms of far-field scattering, leading to the effective anopole excitation. At the same time however, other multipoles such as magnetic dipoles with comparable scattered power are simultanesouly excited, mixing with the anopole and leading to a nonnegligible total scattering cross-section. Here, we show that, for homogenous dielectric nanospheres, radial anisotropy can be employed to significantly suppress the other multipole excitation, which at the same time does not compromise the property of complete scattering cancallation between Cartesian electric and toroidal dipoles. This enables an elusive pure anopole excitation within radially anisotropic dielectric nanospheres, which may shed new light on many scattering related fundamental researches and applications.

Phase locking of an array of three fiber lasers by an all-fiber coupling loop.

Constructing a phase-locking array of fiber lasers by an all-fiber coupling loop is presented, and the loop is composed of multiple 2x2 fiber couplers which are connected with the output ends of component fiber lasers of the array. Due to the mutual injection locking among the fiber lasers by the coupling loop, they can achieve the phase-locking states and then are coherently combined in the far field. Phase locking of an array of three fiber lasers with this configuration has been demonstrated experimentally, and obvious interference patterns and stable coherent output have been observed.

Efficient excitation and tuning of toroidal dipoles within individual homogenous nanoparticles

We revisit the fundamental topic of light scattering by single homogenous nanoparticles from the new perspective of excitation and manipulation of toroidal dipoles. It is revealed that besides within all-dielectric particles, toroidal dipoles can also be efficiently excited within homogenous metallic nanoparticles. Moreover, we show that those toroidal dipoles excited can be spectrally tuned through adjusting the radial anisotropy parameters of the materials, which paves the way for further more flexible manipulations of the toroidal responses within photonic systems. The study into toroidal multipole excitation and tuning within nanoparticles deepens our understanding of the seminal problem of light scattering, and may incubate many scattering related fundamental researches and applications.

Q-factor and absorption enhancement for plasmonic anisotropic nanoparticles.

We investigate the scattering and absorption properties of anisotropic metal-dielectric core-shell nanoparticles. It is revealed that the radially anisotropic dielectric layer can accelerate the evanescent decay of the localized resonant surface modes, leading to Q-factor and absorption rate enhancement. Moreover, the absorption cross section can be maximized to reach the single resonance absorption limit. We further show that such artificial anisotropic cladding materials can be realized by isotropic layered structures, which may inspire many applications based on scattering and absorption of plasmonic nanoparticles.

Mutual injection coupling and phase locking of two multiwavelength fiber lasers

Abstract. Passive phase locking of two multiwavelength fiber lasers has been demonstrated by mutual injection coupling and spatial filtering. Since the fiber loop mirrors are employed as two component lasers’ rear reflectors, multiple wavelengths operate simultaneously owing to their broadband reflection and the compound cavity’s mode selection effect. A single-mode filtering fiber is utilized to collect the expected spatial mode’s energy and feed them back into the coupled laser cavity, and stable and high visibility interference fringes have been obtained by the self-phasing process. A contrastive experiment has been made to investigate its coherent output properties when the rear mirrors are replaced by fiber Bragg gratings. Compared with the ordinary coherent combining of fiber lasers lasing at a single-wavelength, higher output power and efficiency have been obtained, and the cost is that the output beams’ coherence and stability decrease slightly.

Efficient phase locking of two dual-wavelength fiber amplifiers by an all-optical self-feedback loop

Efficient phase locking of two dual-wavelength fiber amplifiers has been demonstrated by using a self-feedback coupling and intracavity filtering configuration, and the effect of bandwidth and wavelength spacing on their phase locking performances have been investigated in experiment. Two independent fiber lasers with different operating wavelength were combined incoherently by a 3 dB fiber coupler to form a dual-wavelength seed source laser, which was injected into the fiber amplifiers’ coupling array through the self-feedback loop. The effect of bandwidth and wavelength spacing was researched by altering the seed laser’s pump power and operating wavelengths respectively. As long as the feedback loop and the single-mode fiber filtering configuration were well constructed in the unidirectional ring laser cavity, stable phase locking states and high fringe visibility interference patterns could always be obtained in our experiment. When the spacing of two operating wavelength was varied from 1.6 nm to 19.6 nm, the fringe visibility decreased slightly with the increase of wavelength spacing, and the corresponding fringe visibility was always larger than 0.6. In conclusion, we believe that efficient phase locking of several multi-wavelength laser sources is also feasible by passive self-adjusting methods, and keeping the component laser beams’ phase relationship stable and fixed is more important than controlling their operating wavelengths.

Demonstration of theoretical and experimental simulations in fiber optics course

“Fiber optics” course plays a supporting effect in the curriculum frame of optics and photonics at both undergraduate and postgraduate levels. Moreover, the course can be treated as compulsory for students specialized in the fiber-related field, such as fiber communication, fiber sensing and fiber light source. The corresponding content in fiber optics requires the knowledge of geometrical and physical optics as background, including basic optical theory and fiber components in practice. Thus, to help the students comprehend the relatively abundant and complex content, it is necessary to investigate novel teaching method assistant the classic lectures. In this paper, we introduce the multidimensional pattern in fiber-optics teaching involving theoretical and laboratory simulations. First, the theoretical simulations is demonstrated based on the self-developed software named “FB tool” which can be installed in both smart phone with Android operating system and personal computer. FB tool covers the fundamental calculations relating to transverse modes, fiber lasers and nonlinearities and so on. By comparing the calculation results with other commercial software like COMSOL, SFTool shows high accuracy with high speed. Then the laboratory simulations are designed including fiber coupling, Erbium doped fiber amplifiers, fiber components and so on. The simulations not only supports students understand basic knowledge in the course, but also provides opportunities to develop creative projects in fiber optics.

Exploration and practice of the cultivation of optoelectronic innovative talents based on the Students Innovation Training Program

The Students Innovation Training Program (SITP) has become an effective method to impel the teaching reform and improve undergraduate’s innovative practical ability in Chinese colleges and universities, which is quite helpful for students to understand the social requirement, to grasp the basic means of scientific research and to improve their innovative practical ability and team work spirit. In this paper, three problems have been analyzed and discussed based on our organizing and instructing experience of SITP in recent years. Firstly, the SITP is a synthetically training project, and it is quite suitable to cultivate the students’ innovative practical ability. Because SITP is similar to the real scientific research activity, and both of them include the steps of project application, solution design, research implementation and project summary etc. By making great efforts to these basic training steps, the undergraduates’ innovative practical ability has been improved systemically. Secondly, a new talents cultivation system has been constructed based on SITP by integrating the subject competitions, graduation design and other conventional training activities, which is quite good to improve the training quality and decrease the total training class hours. Thirdly, a series of long-term effective operation and management guidelines have been established to ensure the SITP work normally, including doing a good job of project evaluation, setting up a reward and punishment system and creating a good atmosphere for innovation. In conclusion, great efforts have been made to enhance undergraduates’ innovative ability, and the research results will provide useful reference for improving the training effects and reforming talents cultivating mode further.