Yongjun Bao

Hiding a Realistic Object Using a Broadband Terahertz Invisibility Cloak

The invisibility cloak has been a long-standing dream for many researchers over the decades. Using transformation optics, a three-dimensional (3D) object is perceived as having a reduced number of dimensions, making it “undetectable” judging from the scattered field12345. Despite successful experimental demonstration at microwave and optical frequencies6789101112, the spectroscopically important Terahertz (THz) domain13141516 remains unexplored due to difficulties in fabricating cloaking devices that are optically large in all three dimensions. Here, we report the first experimental demonstration of a 3D THz cloaking device fabricated using a scalable Projection Microstereolithography process. The cloak operates at a broad frequency range between 0.3 and 0.6 THz, and is placed over an α-lactose monohydrate absorber with rectangular shape. Characterized using angular-resolved reflection THz time-domain spectroscopy (THz-TDS), the results indicate that the THz invisibility cloak has successfully concealed both the geometrical and spectroscopic signatures of the absorber, making it undetectable to the observer.

Surface-plasmon-enhanced transmission through metallic film perforated with fractal-featured aperture array

A fractal-featured metallic thin film with Sierpinski Carpet pattern is fabricated on silicon wafer by microfabrication techniques. Transmission infrared spectroscopy indicates that there exists extraordinary high transmission at specific wavelengths, which can be ascribed to the effect of surface plasmon resonance, and are determined by hierarchy of apertures of different sizes in the fractal structure. This patterned film provides a unique system to achieve enhanced transmission simultaneously at different selected frequencies of electromagnetic wave.

Tailoring the resonances of surface plasmas on fractal-featured metal film by adjusting aperture configuration

Metal films perforated with two types of Sierpinski carpet patterns (SCP) are fabricated on silicon wafer, where one pattern possesses open square aperture as the building block and the other has square ring-shaped aperture as the building block. Enhanced transmissions are observed in both structures. However, for SCP made of square ring-shaped apertures, resonance of surface plasmon of both surface and localized modes has been identified. This feature does not exist in SCP made of open square apertures. This finding provides an effective way to control the extraordinary transmission through the metal film perforated with fractal-like structures.

A realistic design of three-dimensional full cloak at terahertz frequencies

This paper presents a realistic design of a three-dimensional invisibility full cloak device at Terahertz frequency, consisting of radially arranged polymer slabs covered with III-V semiconductor materials featuring Drude-like resonance. By applying high order optical transformation, it is possible to constrain the semiconductor layer of constant thickness for the convenience in practical fabrication. The anisotropic and spatially variant material constants are realized by adjusting the filling ratio of the polymer slabs. Such a structure can be readily fabricated using high-aspect ratio three-dimensional microfabrication of polymeric slabs in combine with conformal deposition of the semiconductor cladding layer.

Characterization of periodically nanostructured copper filaments self-organized by electrodeposition

We report in this paper the electric properties of nanostructured copper filament arrays self-organized by a novel electrochemical method. Due to the spontaneous oscillation of the concentration field of [Cu2+ ] in front of the growing interface, crystallites of copper and cuprous oxide appear alternately on the filaments of the electrodeposits. A conducting atomic force microscope (CAFM) and current imaging tunnelling spectroscopy (CITS) were used to characterize the electric properties of the nanostructured copper filaments. By applying a constant voltage across the conducting probe of the CAFM and the sample, an electric current mapping is achieved, in which alternating low and high current regions correspond exactly to the periodic nanostructures on the filaments. The profile of the electric current along the structured filament has been analysed, and no noticeable potential drop has been observed. A typical linear I–V curve for a metal and nonlinear I–V curve for a semiconductor were collected in the high and low current regions respectively. These results suggest that despite the periodic distribution of Cu2O crystallites on the deposited filament, there should exist a metallic core of copper crystallites inside the filament. This type of structured metal–semiconductor filament might have potential application.

Tuning the polarization of transmitted light through a double-layered gold film of U-shaped apertures by changing the chiral configuration

We propose a double-layered gold film structured with U-shaped aperture in chiral configuration to manipulate the polarization of transmitted light. Two U-shaped aperture arrays are fabricated on both gold films separated by a spacing film, where each face-to-face pair of apertures is arranged perpendicularly to each other in counterclockwise or clockwise. Due to the coupling of the pairs, surface plasmon polariton resonances in the double-layered films in phase and out of phase at two frequencies, respectively. The polarization of transmitted light is rotated 90° by switching the chiral configuration of the doubled-layered gold film between counterclockwise and clockwise. The double-layered gold film structured with apertures tailors the polarization state of the transmitted light at their resonance peaks relative to their complementary structures. These results provide us an effective way to realize an extraordinary transmission and manipulate the polarization state of a double-layered films in chiral c...

Three-dimensional Terahertz Cloak

We report the first three-dimensional invisibility cloak operates at a broad Terahertz frequency band (0.3~0.6 THz). The cloaking device is then used to conceal both the geometrical and spectroscopic signatures of α-lactose monohydrate test structure.