[Paper Review] Magnetic light

Igor I. Smolyaninov, Jill Elliott+1 more|arXiv (Cornell University)|May 28, 2003

Plasmonic and Surface Plasmon Research21 references36 citations

TL;DR

This paper proposes that light confined in arrays of metallic nanoholes exhibits a novel 'magnetic light' state due to cylindrical surface plasmons (CSPs) with nonzero angular momentum, which acquire effective magnetic moments. Using magnetic force microscopy (MFM), the authors experimentally detect magnetization in the nanohole array under circularly polarized light, demonstrating that these CSPs behave like small magnetic dipoles sensitive to external magnetic fields, enabling potential control of light at the single-photon level for quantum technologies.

ABSTRACT

In this paper we report on the observation of novel and highly unusual magnetic state of light. It appears that in small holes light quanta behave as small magnets so that light propagation through such holes may be affected by magnetic field. When arrays of such holes are made, magnetic light of the individual holes forms novel and highly unusual two-dimensional magnetic light material. Magnetic light may soon become a great new tool for quantum communication and computing.

Motivation & Objective

• To investigate whether light quanta in metallic nanoholes can exhibit magnetic moments due to collective electron excitations.
• To explore the implications of cylindrical surface plasmons (CSPs) with nonzero angular momentum for magnetic interactions in 2D plasmonic lattices.
• To experimentally demonstrate the magnetic response of light in nanohole arrays using magnetic force microscopy (MFM).
• To establish a connection between CSPs and Kaluza-Klein theories, suggesting a formal analogy between chiral plasmons and electric charges in higher-dimensional field theory.
• To assess the feasibility of using external magnetic fields to control single-photon transmission through nanohole arrays for quantum information applications.

Proposed method

• Theoretical modeling of cylindrical surface plasmons (CSPs) on metallic nanoholes using a 3D Kaluza-Klein-like framework, where the compactified angular dimension corresponds to the azimuthal mode number n.
• Derivation of the effective magnetic moment of CSP quanta using the formula µ = (eħ / 2mc) × (φ / a), where φ is the number of magnetic flux quanta and a is the hole radius.
• Numerical analysis of CSP dispersion relations under magnetic fields, incorporating curvature, retardation, and Aharonov-Bohm effects to validate the magnetic moment estimate.
• Fabrication of a 30×30 µm² array of 40 nm thick gold nanoholes in a glass substrate using focused ion beam milling.
• Use of a custom magnetic force microscope (MFM) with a 50 µm radius Ni-coated tip and low force constant (~10⁻³ N/m) to enhance sensitivity for detecting weak magnetic signals.
• Illumination with 488 nm circularly polarized light and acquisition of MFM images to map local magnetization, with control measurements at 514 nm outside the CSP resonance.

Experimental results

Research questions

• RQ1 Can cylindrical surface plasmons (CSPs) with nonzero angular momentum (n ≠ 0) in metallic nanoholes exhibit effective magnetic moments?
• RQ2 How does an external magnetic field affect the radiation spectrum of CSPs in a periodic nanohole array?
• RQ3 Can the nonlinear coupling between CSPs in a 2D array be described as magnetic interactions between effective magnetic moments?
• RQ4 Is the magnetic behavior of light in nanohole arrays detectable using magnetic force microscopy (MFM)?
• RQ5 Can the magnetic state of light be controlled at the single-photon level via external magnetic fields?

Key findings

• Cylindrical surface plasmons (CSPs) with nonzero angular momentum (n ≠ 0) were found to possess effective magnetic moments on the order of the Bohr magneton (µB), while n = 0 modes have no magnetic moment.
• The magnetic force microscopy (MFM) images revealed clear bright and dark stripes indicating collective magnetic dipole behavior in the nanohole array under 488 nm circularly polarized illumination.
• A distinct magnetization signal was observed only at the CSP resonance (around 400 nm), with no signal detected at 514 nm outside the resonance, confirming spectral selectivity and direct evidence of magnetic light.
• The magnetic moment of the entire array was estimated to be up to 10⁴ µB under 100 mW continuous-wave 488 nm illumination, indicating detectable but weak magnetism.
• The nonlinear coupling between CSPs in the array was shown to resemble magnetic interactions in a 2D Ising-like lattice of magnetic moments, suggesting a new class of photonic magnetic materials.
• Theoretical modeling confirmed that CSPs in a curved 3D space-time with a compactified angular dimension exhibit behavior analogous to charged particles in Kaluza-Klein theory, with effective chiral charge proportional to angular momentum n.