[Paper Review] One Way Light Transmission through metal membranes

Hai Wang, Yun-Song Zhou+6 more|arXiv (Cornell University)|Aug 19, 2011

Electrochemical Analysis and Applications4 citations

TL;DR

This paper proposes a mechanism for one-way light transmission through asymmetric metal membranes using surface plasmon polaritons (SPPs), where differing SPP-to-radiation field conversion efficiencies on opposite sides of a silver/AAO membrane enable unidirectional transmission. In experiments, a 15 nm Ag film on anodic aluminum oxide (AAO) showed up to 50% difference in transmittance between forward and backward directions at terahertz frequencies, demonstrating intrinsic unidirectional light propagation due to asymmetric SPP coupling.

ABSTRACT

We propose a new mechanism for irreversibility of light assisted by surface plasmon polaritons (SPPs). By achieving the different mutual conversion efficiency between the radiation field and the SPPs on the opposite surfaces of metal membrane, the unidirectional transmission is observed in a quite simple asymmetric metallic sub-wavelength structures Ag(15nm)/AAO, in which the difference of the two-way transmittance is large than 50%.

Motivation & Objective

• To investigate whether asymmetric sub-wavelength metallic structures can break optical reciprocity and enable one-way light transmission.
• To explore the role of surface plasmon polaritons (SPPs) in mediating unidirectional energy transfer across metal membranes.
• To experimentally verify that differing SPP excitation and radiation conversion efficiencies on opposite sides of a membrane lead to non-reciprocal transmission.
• To demonstrate a simple, scalable platform for one-way optical transmission based on SPP asymmetry, enabling potential applications in optical diodes and integrated photonic circuits.

Proposed method

• Fabricated an asymmetric Ag(15 nm)/AAO membrane with a single-side roughened Ag surface to induce differential SPP coupling.
• Used field-emission SEM to characterize the morphology of AAO templates and Ag-coated surfaces, confirming nanostructured features on the Ag side.
• Employed terahertz time-domain spectroscopy (THz-TDS) to measure transmittance from both sides of the same sample, ensuring direct comparison.
• Theoretical modeling considered the non-reciprocal conversion efficiency: I = I₀ξ_R-S t ξ_S-R for forward transmission and I′ = I₀ξ′_R-S t′ ξ′_S-R for reverse, with ξ_R-S ≠ ξ′_R-S and ξ_S-R ≠ ξ′_S-R.
• Measured transmittance ratios T₁ and T₂ for incidence from opposite sides of the Ag/AAO membrane to quantify non-reciprocity.
• Constructed a symmetric Ag/AAO/Ag structure to test the product of forward and reverse transmittance, confirming T₁₂ = T₂₁ = T₁T₂ as predicted.

Experimental results

Research questions

• RQ1 Can asymmetric SPP coupling in a metal membrane lead to non-reciprocal light transmission?
• RQ2 What is the role of SPP-to-radiation field conversion efficiency asymmetry in enabling one-way transmission?
• RQ3 How does the morphology of the metal surface affect SPP excitation and far-field transmission efficiency?
• RQ4 Can a simple, single-layer metallic structure exhibit significant unidirectional transmission without external bias or magnetic fields?
• RQ5 To what extent can the transmittance difference be enhanced through structural optimization of SPP coupling?

Key findings

• The Ag/AAO membrane exhibited a transmittance difference of up to 50% between forward and backward incidence in the terahertz frequency range, demonstrating non-reciprocal transmission.
• The transmittance ratio T₁/T₂ reached approximately 50% in a specific frequency band, with T₁ and T₂ measured from opposite sides of the same sample.
• The observed discrepancy in transmittance was attributed to asymmetric SPP excitation and radiation conversion efficiencies on the two surfaces of the Ag film.
• The measured transmittance spectra showed negligible differences between the two directions for Si and Ag/Si samples, confirming that the effect is specific to the Ag/AAO structure with SPP-mediated coupling.
• In the Ag/AAO/Ag configuration, the product of forward and reverse transmittance (T₁₂ = T₂₁ = T₁T₂) matched theoretical predictions, validating the non-reciprocal mechanism.
• The results indicate that intrinsic unidirectional transmission can be achieved in a simple, all-dielectric-metal structure without external control, relying solely on surface asymmetry.