Experimental Observation of Electromagnetically Induced Transparency-Like Effect in a Hybrid Plasmonic Multilayer System

Electromagnetically induced transparency (EIT) is a quantum interference phenomenon originally observed in atomic systems, where an otherwise opaque medium becomes transparent within a specific spectral range. EIT has important applications in data storage, nonlinear optics, and spectral modulation. Recently, optical analogs of EIT effects have gained traction especially in plasmonic systems because of their ability to break diffraction limits and support strong light confinement in subwavelength regions. Two promising candidates for realizing EIT-like effects in photonic systems are the surface plasmons (SPs) which arise at metal-dielectric interfaces and enable extreme light confinement and the Tamm plasmons (TPs) which are localized modes formed at the interface between a conductor and a Bragg mirror. The interaction between SPs and TPs has been theoretically explored, with potential applications in optical biosensing, slow-light propagation, and tunable photonic devices. However, the direct experimental observation of an EIT-like effect emerging from SP-TP coupling has not been achieved to date. The main challenge lies in precisely controlling the coupling conditions to achieve the desired interference effects because it requires careful structural engineering to align the resonance conditions of SPs and TPs and at the same time minimize optical losses inherent in metal-based systems. Additionally, maintaining the structural integrity and uniformity of fabricated nanostructures is technically difficult. Addressing these challenges, new research paper published in Optics Letters and conducted by Professor Hua Lu from the Northwestern Polytechnical University and his colleagues Jianxu Zhao, Shouhao Shi, Chunyu Wang, Dikun Li and Jianlin Zhao performed an experimental demonstration of the EIT-like effect in a multilayer plasmonic system comprising a silver grating, a silver film, a SiO₂ spacer, and a Bragg mirror. The structure is designed to enable strong SP-TP coupling, leading to a sharp reflection peak within a broad reflection dip, indicative of EIT-like behavior.

The researchers fabricated a multilayer structure consisting of a Bragg mirror, a silver thin film, a SiO₂ spacer, and a nanofabricated silver grating as the top layer. The grating was introduced to satisfy the wave vector matching condition necessary for SP excitation. The Bragg mirror, composed of alternating layers of SiO₂ and Ta₂O₅, provided the required high reflectivity and confined the TPs at the silver interface. The structure was designed with specific parameters optimized for near-infrared operation, ensuring resonance alignment between SPs and TPs. The authors also performed numerical simulations to predict the spectral response of the system. The results from the simulated reflection spectra showed a narrow peak appearing within the broad SP-induced reflection dip, indicative of destructive interference between coupled SP and TP modes. The simulations further demonstrated a redshift of the spectral dip with increasing silver grating width, confirming the tunability of the EIT-like effect by structural modifications. Moreover, they validated these simulations by fabricating the multilayer structures using electron beam deposition for the Bragg mirror and thermal evaporation for the silver films. The SiO₂ spacer was deposited via magnetron sputtering, and the silver grating was patterned using focused ion beam lithography. The structural integrity and dimensions were verified through scanning electron microscopy and atomic force microscopy which ensured consistency with the designed parameters. Furthermore, the team measured the reflection spectra of the fabricated samples using a custom-built optical microscopy system and found the experimental results closely matched the simulation predictions which confirms the emergence of a sharp reflection peak within the SP reflection dip. The spectral peak position remained nearly unchanged with varying grating widths, while the reflection dip exhibited a systematic redshift. According to the authors, this behavior was attributed to the differential sensitivity of SP and TP modes to structural variations, with SPs being more affected by the grating width than TPs. Further analysis of the field distributions at the resonance wavelengths confirmed the coupling mechanism. The SP mode, excited in the silver grating, exhibited strong field confinement at the metal-dielectric interface. The evanescent wave from the SPs penetrated through the SiO₂ spacer, exciting TPs at the interface between the silver film and Bragg mirror. The observed interference between these modes was analogous to a three-level quantum system, where the SP mode acted as a bright state, and the TP mode functioned as a dark state, leading to the observed EIT-like effect.

In conclusion, the research work of Professor Hua Lu and his colleagues will advance photonic devices with tunable spectral properties. They successfully demonstrated EIT-like effect can provide a pathway for controlling light propagation in multilayer plasmonic systems which will open the door for new applications in optical signal processing, high-resolution sensing, and slow-light devices. Moreover, the new work presented a scalable approach for engineering EIT-like effects in solid-state systems without the need for complex atomic coherence conditions. The use of nanofabrication techniques ensures compatibility with existing photonic device architectures, paving the way for on-chip implementation in optical computing and quantum photonics. Additionally, the tunability of the spectral response through structural modifications offers flexibility in designing application-specific plasmonic devices. Furthermore, the findings also lay the groundwork for further exploration of hybrid plasmonic structures that incorporate additional resonant elements, such as dielectric metasurfaces or active materials, to enhance tunability and functionality. Future research may focus on dynamic control of SP-TP coupling through external stimuli such as electrical or thermal modulation, to achieve active spectral tuning in real-time applications.