Subwavelength-scale metal and dielectric nanostructures have served as important building blocks for electromagnetic metamaterials, providing unprecedented opportunities for manipulating the optical response of the matter. Recently, hyperbolic metamaterials have been drawing particular interest because of their unusual optical properties and functionalities, such as negative refraction and hyperlensing of light. Here, as a promising application of a hyperbolic metamaterial at visible frequency, we propose an invisible nanotube that consists of metal and dielectric alternating thin layers. The theoretical study of the light scattering of the layered nanotube reveals that almost-zero scattering can be achieved at a specific wavelength when the transverse-electric- or transverse-magnetic-polarized light is incident to the nanotube. In addition, the layered nanotube can be described as a radial-anisotropic hyperbolic metamaterial nanotube. The low scattering occurs when the effective permittivity of the hyperbolic nanotube in the angular direction is near zero, and thus the invisibility of the layered nanotube can be efficiently obtained by analyzing the equivalent hyperbolic nanotube. Our new method to design and tune an invisible nanostructure represents a significant step toward the practical implementation of unique nanophotonic devices such as invisible photodetectors and low-scattering near-field optical microscopes.

Photolithography is the prevalent microfabrication technology. It needs to meet resolution and yield demands at a cost that makes it economically viable. However, conventional farfield photolithography has reached the diffraction limit, which imposes complex optics and short-wavelength beam source to achieve high resolution at the expense of cost efficiency. Here, we present a cost-effective near-field optical printing approach that uses metal patterns embedded in a flexible elastomer photomask with mechanical robustness. This technique generates sub-diffraction patterns that are smaller than 1/10 th of the wavelength of the incoming light. It can be integrated into existing hardware and standard mercury lamp, and used for a variety of surfaces, such as curved, rough and defect surfaces. This method offers a higher resolution than common light-based printing systems, while enabling parallel-writing. We anticipate that it will be widely used in academic and industrial productions.

We demonstrate selective pump focusing for highly isolated single-mode lasers in microdisk and microring cavities, and achieve lasing action from a microdisk cavity underneath a scattering medium. The spatial profile of the pumping light evolves by an iterative feedback process and is optimized to maximize the field overlap with a selected cavity mode. The high order of mode selectivity and high resolving power are obtained in a multimode cavity in the presence of significant modal overlaps. As a result of the adaptive optical pumping, we successfully achieve the efficient energy transfer to a microdisk underneath a random scattering medium and observe lasing action through the scattering medium. We believe that our selective pumping procedure will pave the way for the development of low-threshold, single-mode nanolasers embedded in various materials.

Despite recent advances in three-dimensional (3D) fabrication, the epitaxial growth of finely controlled, single crystalline 3D structures is still constrained. In this study, we demonstrate the step-by-step growth of hierarchical 3D architectures composed of single crystalline semiconductors. To achieve this goal, we first established control of the preferential growth direction of ZnO crystals by polarity-selective crystallization during the low-temperature solution-phase synthesis. The time-dependent analysis showed that the binding of a citrate additive to the positively charged, top (0001) surface of ZnO crystals led to an inversion of the growth anisotropy from predominantly the vertical direction to the lateral direction with more than a 10 times increase in the width-to-height ratio. With further fine optimization of the charge balance, we minimized the interruption of citrate ions for epitaxial and iterative stacking of Zn and O ions, and achieved single-crystalline hexaplates. This feature was further combined with multistage epitaxial growth of the nanocrystal constituents, producing a 3D, single crystalline semiconductor with excellent luminescent characteristics.

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