Near-field probing of Mie resonances in single TiO_2 microspheres at terahertz frequencies

Abstract

We show experimentally that poly-crystalline TiO2 spheres, 20-30 μm in diameter, exhibit a magnetic dipole Mie resonance in the terahertz (THz) frequency band (1.0-1.6 THz) with a narrow line-width (<40 GHz). We detect and investigate the magnetic dipole and electric dipole resonances in single high-permittivity TiO2 microspheres, using a near-field probe with a sub-wavelength (~λ/50) size aperture and THz time-domain spectroscopy technique. The Mie resonance signatures are observed in the electric field amplitude and phase spectra, as well as in the electric field distribution near the microspheres. The narrow line-width and the sub-wavelength size (λ/10) make the TiO2 microspheres excellent candidates for realizing low-loss THz metamaterials.

Polarity of the Fano Resonance in the Near‐Field Magnetic‐Dipole Response of a Dielectric Particle Near a Conductive Surface

“…For example, refs. [], the metal screen of the aperture near‐field probe was placed within several micrometers from the studied dielectric micro‐particle.…”

“…Due to low scattering efficiency, they are barely visible in the far field. Therefore, such resonators are often studied using near‐field spectroscopy and imaging where the Fano‐like resonances with different polarities—depending on near‐field surroundings or measurement conditions—are frequently observed. For example, refs.…”

“…For example, refs. [], the near‐field probe consisted of a conductive plane with a subwavelength‐sized aperture.…”

“…For example, refs. [3,17], the near-field probe consisted of a conductive plane with a subwavelength-sized aperture. In this paper, we explore the polarity of Fano resonance observed in the near-field spectra of a high-permittivity dielectric particle near a conductive plane.…”

Full vectorial mapping of the complex electric near-fields of THz resonators

“…[3][4][5][6][7][8] Attempts to measure and characterize near-fields were done first at optical frequencies, which paved the way for near-field detection techniques at THz frequencies. [9][10][11][12][13][14][15][16][17][18][19][20] Owing to the relatively long wavelengths of THz radiation (∼100's of microns), the creation of highly confined THz local fields in small volumes is critical for enhancing the interaction of THz waves with matter. The use of resonant structures for this purpose has been the topic of many interesting studies, especially for spectroscopy of deep-subwavelength structures.…”