J. Eur. Opt. Society-Rapid Publ. 2026, 22, 5 Ó The Author( s), published by EDP Sciences, 2026 https:// doi. org / 10.1051 / jeos / 2025056 Available online at: https:// jeos. edpsciences. org
EOSAM 2025 Guest editors: Omar El Gawhary, Stefan Witte, Ignacio Moreno
Journal of the European Optical Society-Rapid Publications
SHORT COMMUNICATION
Anomalous reflection coefficient underlying experimentally validated outstanding transmission through metal – dielectric – metal microcavities
Alejandro Doval, Lucía Suárez-Fernández, Raul de la Fuente, and Yago Arosa *
Instituto de materiais da USC- iMATUS, Grupo de Nanomateriais, Fotónica e Materia Branda, Departamento de Física Aplicada, Universidade de Santiago de Compostela, E-15782, Santiago de Compostela, Spain
Received 27 October 2025 / Accepted 29 December 2025
Abstract. Analytical modelling of light transmission through a metal-insulator-metal geometry embedded in a coupling glass surrounding medium is possible through an extended Fabry-Pérot formula. Two distinct coupled surface plasmon resonance branches are allowed inside such microcavity, where two thin metallic layers act as mirrors delimiting an inner dielectric material. In agreement with transfer-matrix method simulations, the resulting theoretical expressions predict a large and almost constant transmittance even for intracavity thicknesses greater than light’ s penetration depth. Results at k = 800 nm have been validated experimentally and show optical transmittance over 10 % until nearly 3 lm. This high transmittance under such unexpected conditions, related to an anomalously high mirror reflection coefficient, sheds light on new possibilities for the design of optical devices. The experimental setup successfully used to corroborate the validity of the transmittance formula over different angular, spectral and geometrical conditions is also presented.
Keywords: Plasmonics, Coupled surface plasmons, Microcavities, Transmittance, Resonance.
1 Introduction
Coupled Surface Plasmons( CSP) in metal-dielectric-metal( MDM) and dielectric-metal-dielectric( DMD) geometries started attracting theoretical interest since the late 20th century [ 1 ]. Initial experimental validations [ 2 ] opened the path for posterior applications of both arrangements. Specifically, referring to the case of MDM geometry that we will be addressing in this work, applications range from guiding systems [ 3 ] tospectralfiltering [ 4 ], including sensors [ 5 ] or spectroscopy [ 6 ] aswell.
Our recent studies [ 7 ] have focused on developing and validating an analytical model for transverse magnetic( TM)-polarized light transmission through an optical plasmonic microcavity( MC), involving an MDM structure. The MC is formed by two thin plane metallic mirrors( M) separated by a low-index dielectric gap( L) and surrounded by two semi-infinite higher-index dielectric media( H). An analytical expression for T – see equation( 1) – was obtained by application of Fresnel coefficients at the interfaces, and the subsequent results were calculated by implementing Fresnel formalism for coherent optical scattering at an interface and validated by comparison with simulations
* Corresponding author. yago. arosa. lobato @ usc. es using in-house developed software based on transfermatrix-method [ 8 ]. Experimental validation was also carried out, using two identical symmetrically arranged prisms playing the role of the external semi-infinite highindex media, which couple and decouple light to the possible plasmonic resonances of the inner structure.
jt LMH j 2 jt HML j 2 T ¼ h
i
4 jr LMH j 2 sinh 2 k 00 L;? d � ln jr LMHj þ sin 2 k 0 L;? d þ u LMH
where r LMH and t LMH are reflection and transmission field amplitude coefficients for each of the two three-medium mirror structures; u LMH represents the phase of r LMH; d
denotes the intracavity thickness; and k L;? ¼ k 0 L;? þ ik00 L;?
is the component of the intracavity wavevector in the direction perpendicular to the interfaces. This equation can be seen as a generalization of the common formula for transmittance in Fabry-Pérot( FP) interferometers to all incidence angles, from which the different resonant modes in the system can be obtained. On the one hand, k L,\ is real for incidence angles h below the critical angle for incidence from H to L, h cr, for which volume resonances are allowed. That case corresponds to the well-known FP regime, in which harmonic waves can
ð1Þ
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