[Paper Review] Natural Light Cloaking for Aquatic and Terrestrial Creatures

Hongsheng Chen, Bin Zheng+5 more|arXiv (Cornell University)|Jun 7, 2013

Metamaterials and Metasurfaces Applications22 references3 citations

TL;DR

This paper proposes a simplified, phase-non-preserving invisibility cloak using isotropic materials like optical glass to hide large-scale living creatures—such as a cat and a goldfish—under natural, incoherent, unpolarized light. By abandoning the need to preserve wavefront phase, the method enables broadband, polarization-insensitive cloaking in air and water, demonstrated experimentally with dynamic, real-time invisibility under human vision conditions.

ABSTRACT

A cloak that can hide living creatures from sight is a common feature of mythology but still remains unrealized as a practical device. To preserve the phase of wave, the previous cloaking solution proposed by Pendry \emph{et al.} required transforming electromagnetic space around the hidden object in such a way that the rays bending around it have to travel much faster than those passing it by. The difficult phase preservation requirement is the main obstacle for building a broadband polarization insensitive cloak for large objects. Here, we suggest a simplifying version of Pendry's cloak by abolishing the requirement for phase preservation as irrelevant for observation in incoherent natural light with human eyes that are phase and polarization insensitive. This allows the cloak design to be made in large scale using commonly available materials and we successfully report cloaking living creatures, a cat and a fish, in front of human eyes.

Motivation & Objective

• To overcome the fundamental limitation of phase preservation in transformation optics cloaking, which restricts broadband and large-scale invisibility.
• To enable practical invisibility cloaking for macroscopic living creatures under natural, incoherent, unpolarized light.
• To simplify cloak design using common isotropic materials instead of complex metamaterials or extreme anisotropic parameters.
• To demonstrate dynamic, real-time cloaking of living creatures in both aquatic and terrestrial environments under human vision conditions.
• To extend cloaking performance beyond unidirectional or single-polarization scenarios to multiple observation angles using accessible materials.

Proposed method

• Abandon the requirement of phase preservation in transformation optics cloaking, justified by the incoherent and phase-insensitive nature of natural light and human vision.
• Design a simplified cloak using isotropic optical glass (n = 1.78) with a hexagonal or four-directional geometry to guide light around the hidden object.
• Construct the cloak from six or more prismatic glass pieces arranged in a hollow transparent container to minimize optical distortion and edge effects.
• Use a dynamic background (projected scenery with incandescent light) to simulate real-world conditions and test cloaking performance in real time.
• Conduct experiments in both aquatic (fish tank with water) and terrestrial (office environment with air) settings to validate performance under natural illumination.
• Record video footage to document the cloaking effect dynamically, showing the disappearance of the creature from view while the background remains visible.

Experimental results

Research questions

• RQ1 Can phase preservation be safely abandoned in natural light cloaking without degrading visual invisibility for human observers?
• RQ2 Is it possible to achieve broadband, polarization-insensitive cloaking for large-scale living creatures using only isotropic, common materials like optical glass?
• RQ3 Can a simplified cloak design with non-superluminal phase velocities effectively hide macroscopic objects in multiple observation directions under natural illumination?
• RQ4 How does the performance of a phase-non-preserving cloak compare to traditional transformation optics cloaks in real-world, dynamic settings?
• RQ5 Can such a cloak be extended from unidirectional to multidirectional operation using accessible materials and simple geometry?

Key findings

• A goldfish was successfully hidden from view in a fish tank filled with water, with the cloak constructed from six optical glass prisms (n = 1.78) arranged in a hexagonal container, demonstrating dynamic invisibility under natural light.
• A domestic cat was rendered invisible when stepping into a unidirectional cloak made of the same glass material in an air environment, with the head and forelegs disappearing from view while the background remained visible.
• The cloak operated effectively for multiple incident directions—six directions in the aquatic setup and forward/backward in the terrestrial setup—without requiring polarization control.
• Edge shadows were observed due to divergent projector light, but the core cloaking effect remained robust, with the butterfly flitting behind the cat clearly visible through the cloak.
• The cloaking performance was validated via video recordings showing real-time disappearance of the creature from the observer’s view, confirming the method’s viability for dynamic, real-world applications.
• The method successfully bypassed the need for superluminal phase velocities and extreme material parameters by leveraging the incoherence of natural light and human visual insensitivity to phase and polarization.