Analogue of Electromagnetically Induced Transparency in a Terahertz Metamaterial

Abstract: We experimentally demonstrate at terahertz frequencies that a planar metamaterial exhibits a spectral response resembling electromagnetically induced transparency. The metamaterial unit cell consists of a split ring surrounded by another closed ring where their dimensions are such that their excitable lowest order modes have identical resonance frequencies but very different life times. Terahertz time-domain spectroscopy verifies that the interference of these two resonances results in a narrow transparency window located within a broad opaque region. In contrast to previous studies this enhanced transmission is achieved by independently exciting two resonances in which their coupling to the radiation field, and thus their linewidth, differs strongly. Rigorous numerical simulations prove that the transparency window is associated with a large group index and low losses, making the design potentially useful for slow light applications. This experiment opens an avenue to explore quantum mechanical phenomena using localized resonances in metallic structures,

• The paper introduces a novel split ring resonator (SRR) design that mimics EIT by matching resonant frequencies with distinct Q factors.
• Experimental THz-TDS measurements reveal a narrow transparency window near 1.25 THz, with a peak group index exceeding 75 and minimal losses.
• This work provides a robust classical analogue to quantum EIT, opening avenues for slow light applications and advanced photonic device designs.

Analogue of Electromagnetically Induced Transparency in a Terahertz Metamaterial

This paper presents a detailed experimental and theoretical analysis of a planar metamaterial exhibiting characteristics analogous to electromagnetically induced transparency (EIT) at terahertz (THz) frequencies. The metamaterial consists of a novel split ring resonator (SRR) configuration: a split ring enclosed within a larger, closed metallic ring. The design ensures that the resonances of these structures have matching frequencies but distinct quality factors (Q factors), leading to a marked interference effect that creates a transparency window amidst a broad opaque band.

Key numerical results highlight this metamaterial's unique capability. Terahertz Time-Domain Spectroscopy (THz-TDS) confirms the resemblance to EIT phenomena. The study provides evidence of a narrow transparency window occurring at around 1.25 THz for certain polarizations, notably when the resonances of the split and closed rings interact. This transparency window is associated with a high group index and significantly low losses, pointing towards substantial potential for slow light applications. In more quantitative terms, the paper reports the peak group index exceeding 75, a crucial property for practical slow light applications, alongside a lowest imaginary refractive index value of just 5 within the window.

The paper moves beyond conventional studies where the dark mode in metamaterials is only indirectly excitable. Here, the dark eigenmode of the split ring remains independently excitable without the outer closed ring’s presence. This experimental setup provides a more robust analogy to quantum EIT. In standard quantum mechanics, EIT is a result of destructive interference between quantum pathways induced by two laser beams. In this metamaterial, analogous behavior is achieved by cleverly designed optical interference of independently excitable resonances.

This work's theoretical implications suggest a potential shift in exploring classical analogues of quantum phenomenon within optical systems. It attempts a more precise mapping of terms and phenomena from the quantum EIT to classical EIT-like behavior. Notably, the ability to tune individual Q factors through structural design, rather than mutual coupling strength, offers a more controlled and comprehensive approach to manipulating light in metamaterial structures.

Looking forward, these insights could lay the groundwork for further exploration into metamaterial designs for slowing down light without the stringent requirements of low-temperature environments necessitated in quantum EIT systems. This holds promise for developing optical components that can be implemented in a range of photonic and quantum devices under standard operational conditions.

In summary, the paper provides an important experimental milestone in the journey from quantum-to-classical analogies, specifically in the metamaterials context. This work could act as a bridge, facilitating deeper comprehension of classical mappings of complex quantum phenomena, and inspire new experimental designs for advanced optical system functionalities.