[Paper Review] Free-Space Wide-Aperture Sheet-Isolator Based on a Multilayered Resonant Cavity

Rodion Kononchuk, Carl Pfeiffer+4 more|arXiv (Cornell University)|Feb 6, 2019

Magneto-Optical Properties and Applications23 references3 citations

TL;DR

This paper presents the first conceptual design of a free-space, wide-aperture, thin-sheet optical isolator using a multilayered resonant cavity with subwavelength magnetic layers and dichroic nanolayers. The cavity enhances Faraday rotation and enables nearly total resonant absorption of backward-propagating light, while an optional metallic nanolayer suppresses oblique incidence, achieving broadband omnidirectional isolation with 0.04 dB insertion loss and 33 dB return loss at 94.20 GHz.

ABSTRACT

We introduce the first conceptual design of a free-space thin-sheet isolator with unlimited aperture and the possibility of a broadband omnidirectional rejection of the backward propagating light. The proposed design involves a multilayered resonant cavity incorporating subwavelength magnetic layers, dichroic nanolayers, and an optional metallic nanolayer. The cavity resonance enhances the Faraday rotation produced by the subwavelength magnetic layers, while providing nearly total absorption of the backward-propagating light by the dichroic nanolayers. The latter is a necessary and the most challenging condition for a thin-sheet isolator with unlimited aperture to function. The (optional) metallic nanolayer provides rejection of the obliquely incident light, which otherwise would be partially transmitted in either direction. Our numerical simulations and quasi-optical measurements at millimeter-wave frequencies illustrate how the key elements of the layered-sheet isolator work. Our approach can be scaled down to long- and mid-infrared wavelengths.

Motivation & Objective

• To address the fundamental challenge of backward light reflection in thin-sheet optical isolators, which prevents isolation in free-space configurations.
• To design a wide-aperture isolator that enables complete absorption of backward-propagating light without relying on beam-splitting or tilted polarizers.
• To overcome the limitation of conventional sheet polarizers, which are partially reflective and cause isolation failure due to multiple reflections.
• To achieve broadband, omnidirectional isolation using a single resonant cavity structure with integrated magnetic and dichroic layers.
• To demonstrate feasibility at millimeter-wave frequencies and enable scalability to long- and mid-infrared wavelengths.

Proposed method

• The isolator is based on a multilayered dielectric resonant cavity hosting subwavelength magnetic layers for Faraday rotation and dichroic nanolayers for polarization-selective absorption.
• Cavity resonance enhances the Faraday rotation in magnetic layers, enabling a thin, low-loss isolator structure with total thickness under a few wavelengths.
• Dichroic nanolayers are engineered to achieve nearly total absorption of backward-propagating light under resonant conditions, while remaining transparent to forward-propagating light.
• An optional metallic nanolayer is placed at a nodal plane of the electric field distribution to reflect obliquely incident light, preventing isolation degradation.
• The design uses high-Q resonant cavities to tailor electromagnetic field distributions, enabling selective absorption and nonreciprocal behavior.
• Numerical simulations and quasi-optical measurements at 94.20 GHz validate the resonant absorption, insertion loss, and return loss performance.

Experimental results

Research questions

• RQ1 Can a wide-aperture, thin-sheet optical isolator achieve complete absorption of backward-propagating light without relying on reflective polarizers?
• RQ2 How can resonant cavity engineering enhance Faraday rotation and enable broadband, omnidirectional isolation in a single-layered structure?
• RQ3 What role does the placement of a metallic nanolayer at a nodal plane of the electric field play in suppressing oblique incidence?
• RQ4 How does the integration of magnetic and dichroic layers within a single cavity improve isolation performance compared to cascaded components?
• RQ5 Can the design be scaled to long- and mid-infrared wavelengths while maintaining high isolation and low insertion loss?

Key findings

• The integrated sheet isolator achieved an insertion loss of 0.04 dB and a return loss of 33 dB at the resonance frequency of 94.20 GHz.
• Backward-propagating light was nearly completely absorbed due to resonant enhancement of dichroic nanolayer absorption, enabling effective isolation.
• The isolator exhibited high reflectivity for both forward and backward waves away from resonance, preventing spurious transmission.
• The addition of a metallic nanolayer at a nodal plane suppressed oblique incidence, improving omnidirectional performance.
• The design demonstrated scalability to long- and mid-infrared wavelengths through the use of appropriate resonant materials and layer structures.
• Quasi-optical measurements at millimeter-wave frequencies confirmed the predicted resonant behavior and isolation performance.