Haojun Hu

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.

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.

Study on the generation of a vortex laser beam by using phase-only liquid crystal spatial light modulator

Abstract. The generation of vortex laser beam by using phase-only liquid crystal spatial light modulator (LC-SLM) combined with the spiral phase screen is experimentally and theoretically studied. Results show that Gaussian and dark hollow vortex laser beams can be generated by using this method successfully. Differing with the Gaussian and dark hollow beams, far field intensities of the generated vortex laser beams still exhibit dark hollow distributions. The comparisons between the ideal generation and experimental generation of vortex laser beams with different optical topological charges by using phase only LC-SLM is investigated in detail. Compared with the ideal generated vortex laser beam, phase distribution of the experimental generated vortex laser beam contains many phase singularities, the number of which is the same as that of the optical topological charges. The corresponding near field and far field dark hollow intensity distributions of the generated vortex laser beams exhibit discontinuous in rotational direction. Detailed theoretical analysis show that the main reason for the physical phenomenon mentioned above is the response error of phase only LC-SLM. These studies can provide effective guide for the generation of vortex laser beam by using phase only LC-SLM for optical tweezers and free space optical communication.

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.

Reform of experimental teaching based on quality cultivation

Experimental teaching plays an import part in quality education which devotes to cultivating students with innovative spirit, strong technological talents and practical ability. However, in the traditional experimental teaching mode, the experiments are treated as a vassal or supplementary mean of theoretical teaching, and students prefer focus on theory to practice. Therefore, the traditional experimental teaching mode is difficult to meet the requirements of quality education. To address this issue, the reform of experimental teaching is introduced in this paper taking the photoelectric detector experiment as the example. The new experimental teaching mode is designed from such aspects as experimental content, teaching method and experimental evaluation. With the purpose of cultivating students’ practical ability, two different-level experimental content is designed. Not only the basic experiments used to verify the theory are set to consolidate the students’ learned theoretical knowledge, but also comprehensive experiments are designed to encourage the students to apply their learned knowledge to solve practical problems. In the teaching process, heuristic teaching thought is adopt and the traditional ‘teacher-centered’ teaching form is replaced by ‘student-centered’ form, which aims to encourage students to design the experimental systems by their own with the teacher’s guidance. In addition to depending on stimulating the students’ interest of science research, experimental evaluation is necessary to urge students to complete the experiments efficiently. Multifaceted evaluation method is proposed to test the students’ mastery of theoretical knowledge, practice ability, troubleshooting and problem solving skills, and innovation capability comprehensively. Practices demonstrated the satisfying effect of our experimental teaching mode.

Unidirectional superscattering by multilayered cavities of effective radial anisotropy.

We achieve unidirectional forward superscattering by multilayered spherical cavities which are effectively radially anisotropic. It is demonstrated that, relying on the large effective anisotropy, the electric and magnetic dipoles can be tuned to spectrally overlap in such cavities, which satisfies the Kerker’s condition of simultaneous backward scattering suppression and forward scattering enhancement. We show that such scattering pattern shaping can be obtained in both all-dielectric and plasmonic multilayered cavities at different spectral positions, and believe that the mechanism we have revealed provides extra freedom for scattering shaping, which may play a significant role in many scattering related applications and also in optoelectronic devices made up of intrinsically anisotropic two dimensional materials.