Di-Hu Xu

Freely Tunable Broadband Polarization Rotator for Terahertz Waves

A freely tunable polarization rotator for broadband terahertz waves is demonstrated using a three-rotating-layer metallic grating structure, which can conveniently rotate the polarization of a linearly polarized terahertz wave to any desired direction with nearly perfect conversion efficiency. This low-cost, high-efficiency, and freely tunable device has potential applications as material analysis, wireless communication, and THz imaging.

Position-sensitive spectral splitting with a plasmonic nanowire on silicon chip

On-chip nanophotonics serves as the foundation for the new generation of information technology, but it is challenged by the diffraction limit of light. With the capabilities of confining light into (deep) subwavelength volumes, plasmonics makes it possible to dramatically miniaturize optical devices so as to integrate them into silicon chips. Here we demonstrate that by cascading nano-corrugation gratings with different periodicities on silver nanowires atop silicon, different colors can be spatially separated and chronologically released at different grating junctions. The released light frequency depends on the grating arrangement and corrugation periodicities. Hence the nanowire acts as a spectral splitter for sorting/demultiplexing photons at different nano-scale positions with a ten-femtosecond-level interval. Such nanowires can be constructed further into compact 2D networks or circuits. We believe that this study provides a new and promising approach for realizing spatiotemporal-sensitive spectral splitting and optical signal processing on nanoscales, and for general integration of nanophotonics with microelectronics.

Oblique metal gratings transparent for broadband terahertz waves

In this work, we experimentally and theoretically demonstrate that oblique metal gratings with optimal tilt angles can become transparent for broadband terahertz waves under normal incidence. Direct imaging is applied to intuitively prove this broadband transparency phenomenon of structured metals. The transparency is insensitive to the grating thickness due to the non-resonance mechanism, and the optimal tilt angle is determined only by the strip width and the grating period. The oblique metal gratings with broadband transparence may have many potential applications, such as transparent conducting panels, white-beam polarizers, and stealth objects.

Van der Waals epitaxy of ultrathin α-MoO3 sheets on mica substrate with single-unit-cell thickness

We report on van der Waals epitaxy of single-crystalline α-MoO3 sheets with single-unit-cell thickness on the mica substrate. The crystalline lattice structure, growth habits, and Raman spectra of the grown α-MoO3 sheets are analyzed. The anisotropic growth of α-MoO3 sheets can be understood by period bond chains theory. Unlike monolayer MoS2 or graphene, Raman spectra of α-MoO3 do not possess frequency shift from bulk crystal to single-unit-cell layer. The relative intensities of two Raman modes (Ag) at 159 and 818 cm−1 are sensitive to the polarization of incident light. This scenario provides a quick approach to determine the lattice orientation of α-MoO3 crystals. Our studies indicate that van der Waals epitaxial growth is a simple and effective way to fabricate high-quality ultrathin α-MoO3 sheets for physical property investigations and potential applications.

Tuning the dispersion relation of a plasmonic waveguide via graphene contact

In this work, we have investigated experimentally and theoretically the dispersion relation of a plasmonic slab waveguide, where the thin gold film with nano-aperature arrays is sandwiched by graphene and a silica layer on a silicon chip. It is shown that the plasmonic slab waveguides are compatible with silicon technology. We have found that when the light waves irradiate the nanostructured waveguides with or without graphene, surface plasmon polaritons are always excited at the metal-dielectric interface due to the interaction between the surface charge oscillation and the electromagnetic field of the light. But in the slab waveguide with graphene, the resonant dips definitely shift in the reflection spectra, which indicates that the contact of graphene can tune the dispersion relation of the waveguide in the visible regime. Experimental measurements on optical reflections are in good agreement with calculated plasmonic band structures. Further calculations show that the dispersion relation of plasmonic slab waveguides can be tuned by electron doping and the nonlinear effect of graphene. The investigations provide a way to actively control the dispersion relation of plasmonic waveguides on silicon chips and benefit the development graphene-related active optical devices.

Broadband light trapping and absorption of thin-film silicon sandwiched by trapezoidal surface and silver grating

In this work, we demonstrate the high optical absorption efficiency of a thin-film silicon solar cell. In thin-film solar cells, the efficiency is strongly dependent on light trapping by structures capable of exciting different resonance modes. Here, we consider a trapezoidal surface design that not only reduces reflection with a gradient index of refraction but also excites multiple cavity modes. The absorption can be enhanced further by combining a plasmonic structure, i.e., a silver grating. For comparison, we have separately simulated the silver grating structure, trapezoidal surface structure, and the combined structure. The combined structure retains all absorption effects shown by the individual components, achieving broadband absorption with a high efficiency. The investigations provide a unique design for high-performance solar cells of thin-film silicon.

Broadband enhanced transmission of acoustic waves through serrated metal gratings

In this letter, we have demonstrated that serrated metal gratings, which introduce gradient coatings, can give rise to broadband transmission enhancement of acoustic waves. Here, we have experimentally and theoretically studied the acoustic transmission properties of metal gratings with or without serrated boundaries. The average transmission is obviously enhanced for serrated metal gratings within a wide frequency range, while the Fabry-Perot resonance is significantly suppressed. An effective medium hypothesis with varying acoustic impedance is proposed to analyze the mechanism, which was verified through comparison with finite-element simulation. The serrated boundary supplies gradient mass distribution and gradient normal acoustic impedance, which could efficiently reduce the boundary reflection. Further, by increasing the region of the serrated boundary, we present a broadband high-transmission grating for wide range of incident angle. Our results may have potential applications to broadband acoustic...

Strong Localization of Surface Plasmon Polaritons with Engineered Disorder

In this work, we experimentally demonstrate for the first time strong localization of surface plasmon polaritons (SPPs) at visible regime in metallic nanogratings with short-range correlated disorder. By increasing the degree of disorder, the confinement of SPPs is significantly enhanced, and the effective SPP propagation length dramatically shrinks. Strong localization of SPPs eventually emerges at visible regime, which is verified by the exponentially decayed fields and the vanishing autocorrelation function of the SPPs. Physically, the short-range correlated disorder induces strong interference among multiple scattered SPPs and provides an adequate fluctuation to effective permittivity, which leads to the localization effect. Our study demonstrates a unique opportunity for disorder engineering to manipulate light on nanoscale and may achieve various applications in random nanolasing, solar energy, and strong light-matter interactions.