Hiding a realistic object using a broadband terahertz invisibility cloak

Abstract

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.

Figure 1. Design and simulation of 3D THz cloak.

(a) The triangle-shaped cloak structure is obtained by extruding the refractive index profile in the x-y plane along the z-axis. (b) Numerical simulation using commercial software (COMSOL Multiphysics) confirms the splitting of the wave into three waves for the bump structure with uniform refractive index. (c) The cloak structure preserves the original shape of the incoming wave. The simulation results show the normalized magnetic field component in the z direction at 0.6 THz.

Figure 2. Fabrication and characterization of 3D THz cloak.

(a) Schematic diagram illustrating projection micro-stereolithography system being used to fabricate 3D cloaking device. The grayscale of individual pixels within each 85.2 x 85.2 µm unit cell can be adjusted so the holes can be fabricated with sub-pixel precision. (b) Optical and SEM images of fabricated cloaking device. The surface of the device was metalized to enhance the contrast for better representation of the fine features in the images. The gradual change in hole size near the bump can be clearly observed.

Figure 3. Spectra maps of four experimental cases.

(I) Reflective flat, (II) exposed α-lactose monohydrate, (III) reflective bump, and (IV) cloak, are measured using reflection terahertz time-domain spectroscopy. The lactose (II) shows both a scattering effect and absorption. The reflective bump (III) avoids the absorption effect, but is still split into three peaks. The measured spot position of the cloak (IV) and the reflective flat (I) match reasonably well with each other.

Figure 4. Cross-sectional plots of experimental results.

(a) Intensity as function of detector position for four cases at 0.48 THz. The cloak and reflective flat have one similar peak, whereas the lactose and reflective bump exhibit two extra peaks (red and green arrows). (b) Intensity as function of frequency for four cases at -3 degrees (angle). Obvious absorption occurs at 0.53 THz for the exposed lactose (red arrow).