Upper bounds on collective light-matter coupling strength with plasmonic meta-atoms

Ultrastrong coupling between optical and material excitations is a distinct regime of electromagnetic interaction that enables a variety of intriguing physical phenomena. Traditional ways to ultrastrong light-matter coupling involve the use of some sorts of quantum emitters, such as organic dyes, quantum wells, superconducting artificial atoms, or transitions of two-dimensional electron gases. Often, reaching the ultrastrong coupling domain requires special conditions, including high vacuum, strong magnetic fields, and extremely low temperatures. Recent report indicate that a high degree of light-matter coupling can be attained at ambient conditions with plasmonic meta-atoms -- artificial metallic nanostructures that replace quantum emitters. Yet, the fundamental limits on the coupling strength imposed on such systems have not been identified. Here, using a Hamiltonian approach we theoretically analyze the formation of polaritonic states and examine the upper limits of the collective plasmon-photon coupling strength in a number of dense assemblies of plasmonic meta-atoms. Starting off with spheres, we identify the universal upper bounds on the normalized collective coupling strength $g/\omega_0$ between ensembles of plasmonic meta-atoms and free-space photons. Next, we examine spheroidal metallic meta-atoms and show that a strongly elongated meta-atom is the optimal geometry for attaining the highest value of the collective coupling strength in the array of meta-atoms. The results could be valuable for the field of polaritonics studies, quantum technology, and modifying material properties

The Microwave Absorption in Composites with Finemet Alloy Particles and Carbon Nanotubes

The absorption of waves of the centimeter and millimeter wavebands in composites with Finemet alloy particles and carbon nanotubes has been studied. It has been established that ferromagnetic resonance and antiresonance are observed in such composites. A method is proposed for calculating the effective dynamic magnetic permeability of a composite containing both a random distribution of ferromagnetic particles and a part of the particles oriented in the same way. In the approximation of effective parameters, the dependences of the transmission and reflection coefficients of microwaves are calculated. It is shown that the theoretical calculation confirms the existence of resonant features of these dependences caused by ferromagnetic resonance and antiresonance. The theory based on the introduction of effective parameters satisfactorily describes the course of the field dependence of the coefficients and the presence of resonance features in these dependences. The frequency dependence of the complex permittivity of the composite is determined. The dependence of the complex magnetic permeability on the magnetic field for millimeter-wave frequencies is calculated