It is well known that a pair of oscillating electric charges of opposite signs, oscillating electric dipole, produces electromagnetic radiation at a frequency of the oscillations1
. Although, distinct “magnetic charges”, or monopoles, have not been observed so far, magnetic dipoles are very common sources of magnetic field in nature. The field of the magnetic dipole is usually calculated as the limit of a current loop shrinking to a point. The field profile in this case is similar to that of an electric dipole with one important difference that electric and magnetic field are exchanged. The most common example of a magnetic dipole radiation is an electromagnetic wave produced by an excited metal split-ring resonator (SRR), which is a basic constituting element of metamaterials ()2,3,4,5,6,7,8,9,10,11,12,13,14,15,16
. The real currents excited by external electromagnetic radiation and running inside the SRR produce a transverse oscillating up and down magnetic field in the center of the ring, which simulates an oscillating magnetic dipole. The major interest to these artificial systems is due to their ability to response to a magnetic component of incoming radiation and thus to have a non-unity or even negative magnetic permeability (µ) at optical frequencies, which does not exist in nature. This provides possibilities to design unusual material properties such as negative refraction2,3,4,5,6,7,8,9,10,11,12,13,14,15,16
, or superlensing19
. The SRR concept works very well for gigahertz8,9,10
and even near-infrared (few hundreds THz)13,14,15,16
frequencies. However, for shorter wavelengths and in particular for visible spectral range this concept fails due to increasing losses and technological difficulties to fabricate smaller and smaller constituting split-ring elements14,20
. Several other designs based on metal nanostructures have been proposed to shift the magnetic resonance wavelength to the visible spectral range2,3,4,5,6,21,22,23,24,25
. However, all of them are suffering from losses inherent to metals at visible frequencies.
Schematic representation of electric and magnetic field distribution inside a metallic split-ring resonator (a) and a high-refractive index dielectric nanoparticle (b) at magnetic resonance wavelength.
An alternative approach to achieve strong magnetic response with low losses is to use nanoparticles made of high-refractive index dielectric materials6,26
. As it follows from the exact Mie solution of light scattering by a spherical particle, there is a particular parameter range where strong magnetic dipole resonance can be achieved. Remarkably, for the refractive indices above a certain value there is a well-established hierarchy of magnetic and electric resonances (see Supplementary Figure 1
). In contrast to plasmonic particles the first resonance of dielectric nanoparticles is a magnetic dipole resonance, and takes place when the wavelength of light inside the particle equals to the diameter
. Under this condition the polarization of the electric field is anti-parallel at opposite boundaries of the sphere, which gives rise to strong coupling to circulation displacement currents while magnetic field oscillates up and down in the middle ().
index dielectric-based metamaterials has been done at gigahertz frequencies, where materials with extremely high refractive index up to several tens of refractive index units (RIU) exist27,28
. Several theoretical works have also discussed various realistic materials to achieve magnetic response of dielectric particles at terahertz29,30,31,32,33
, and what is more important for us, at visible frequencies33,36,37,38,39
. Recent experimental demonstrations also include magnetic response of single silicon carbide microrods40
and arrays of tellurium microcubes41
, both in the midinfrared spectral range. However, the experimental proof of this concept at visible frequencies is still lacking.
“Seeing is believing” is an idiom supported by people since ancient times. To see a medium response to the magnetic field component of light, “magnetic light”, by naked eyes would be an additional proof of the concept of metamaterials, which people can design to control light at the new level beyond nature.
In this paper, we experimentally demonstrate for the first time that spherical silicon nanoparticles with sizes in the range from 100 nm to 200 nm have strong magnetic dipole response in the visible spectral range. The scattered “magnetic” light by these nanoparticles is so strong that it can be easily seen under a dark-field optical microscope. The wavelength of this magnetic resonance can be tuned throughout the whole visible spectral range from violet to red by just changing the nanoparticle size.