Despite much interest and progress in optical spatial cloaking, a three-dimensional (3D), transmitting, continuously multidirectional cloak in the visible regime has not yet been demonstrated. Here we experimentally demonstrate such a cloak using ray optics, albeit with some edge effects. Our device requires no new materials, uses isotropic off-the-shelf optics, scales easily to cloak arbitrarily large objects, and is as broadband as the choice of optical material, all of which have been challenges for current cloaking schemes. In addition, we provide a concise formalism that quantifies and produces perfect optical cloaks in the small-angle (‘paraxial’) limit.

Figures (6)

Fig. 1 Example of a practical paraxial cloak. (a)–(c) A hand is cloaked for varying directions, while the background image is transmitted properly. See ( Media 1 ) and ( Media 2 ) for videos. (d) On-axis view of the ray optics cloaking device. (e) Setup using practical, easy to obtain optics, for demonstrating paraxial cloaking principles. (Photos by J. Adam Fenster, videos by Matthew Mann / University of Rochester) Download Full Size | PPT Slide | PDF

Fig. 2 Investigating ‘perfect’ paraxial cloaking with rays. (a) A ‘perfect’ ray optics cloaking box. Rays exit the box as if the box was filled with the surrounding medium. Non-zero volume inside hides an object. Angles do not change, but the positions shift proportionally to the ray angles and box length. The image seen by the observer should match the object exactly. (b)–(d) Diagrams for a two lens (b) , three lens (c) , or four lens (d) system. f ’s are the focal lengths, t ’s are the distances between the elements. (e) All possible four lens, symmetric, perfect paraxial cloaks for rays. Plot of t 1 / f 2 (solid), t 2 / f 2 (dashed), and L/f 2 (dotted) as a function of a ≡ f 1 / f 2 . Assumed symmetric left and right halves ( f 1 = f 4 , f 2 = f 3 , and t 1 = t 3 ). L is the total length of the system. The physical feasibility and presence of a non-empty cloaking region must be checked separately. Download Full Size | PPT Slide | PDF

Fig. 3 A symmetric three lens cloak. Two diverging lenses are combined into one diverging lens, and placed in the center of two converging lenses. (a) Simulation in CODE V. Entrance pupil is 75 mm, and field-of-view is −3.5° to 3.5°. Object is placed at infinity. Ray bundles propagate from left to right, through the lenses, then are traced back to the first lens. This allows comparison of the image (dashed) rays, as seen by an observer on the right, with the original (solid) rays. We see that the angles are similar, and the transverse shifts are not large. (b) 3D rendering of (a) . The cloaking region is a 3D triangular-ring between the first and last lenses (shaded area). (c–g) Experimental demonstration of the three lens cloak. The lines seen through the lenses match those on the background wall. The inner portion of the ruler is cloaked. Images at various camera-viewing angles: (c) On-axis (0°), (d) 0.55°, (e) 0.83°, (f) 1.11°. (g) Side profile of experimental setup. Download Full Size | PPT Slide | PDF

Fig. 4 CODE V simulation of a symmetric, perfect paraxial cloak, with four lenses using rays. Four achromatic doublets are placed with separations determined from Eq. (1) . Entrance pupil is 50 mm, with −1.5° to 1.5° field-of-view. Simulations are shown with no separate optimization. Object is placed at infinity. (a) Zoomed-in region of (b) with image rays (dashed; traced back to the first lens) added to compare with the original rays (solid). We see that the angles are nearly identical, and the transverse shifts are small. (b) Full simulation using off-the-shelf optics. (c) 3D rendering. The cloaking region (shaded) is a cylindrical region between the first and last lenses. (d) Scaling of (b) by a factor of 2. The cloaking size is doubled in each dimension by doubling the optical curvatures, lengths and entrance pupil. Only the length scales distinguish (d) from (b) . Download Full Size | PPT Slide | PDF

Fig. 5 Experimental demonstration of a ‘perfect’ paraxial cloak with four lenses. Camera was focused on the wall. The grids on the wall can be seen clearly, and match the background for all colors and viewing angles. The middle of the ruler is cloaked inside the lens system for all angles shown. Images at various camera-viewing angles: (a) −0.65°, (b) on-axis (0°), (c) 0.47°, (d) 0.95°. (e) Side profile of experimental setup. See ( Media 3 ) and ( Media 4 ) for videos. (Videos by Matthew Mann / University of Rochester) Download Full Size | PPT Slide | PDF