Our recent paper in Laser & Photonics Reviews reports that strong dielectric anisotropy at terahertz (THz) frequencies splits the Mie magnetic dipole mode in monocrystalline titanium dioxide (TiO2) micro-spheres. The splitting is experimentally confirmed using near-field THz time-domain spectroscopy. The resonances correspond to orthogonal magnetic dipole modes with narrow linewidths of only tens of gigahertz. The anisotropy of TiO2 can be exploited in the emerging THz all-dielectric metamaterial technology.

Abstract: Monocrystalline titanium dioxide (TiO2) micro-spheres support two orthogonal magnetic dipole modes at terahertz (THz) frequencies due to strong dielectric anisotropy. For the first time, we experimentally detected the splitting of the first Mie mode in spheres of radii 10-20μm through near-field time-domain THz spectroscopy. By fitting the Fano lineshape model to the experimentally obtained spectra of the electric field detected by the sub-wavelength aperture probe, we found that the magnetic dipole resonances in TiO2 spheres have narrow linewidths of only tens of gigahertz. Anisotropic TiO2 micro-resonators can be used to enhance the interplay of magnetic and electric dipole resonances in the emerging THz all-dielectric metamaterial technology.

Early view at:
Irina Khromova et al, 'Splitting of magnetic dipole modes in anisotropic TiO2 micro-spheres', Laser & Photonics Reviews, June 2016 (DOI: 10.1002/lpor.201600084)

Surface-plasmon resonances directly visualised by near-field terahertz time-domain spectroscopy. Sub-wavelength resolution of up to 2um allowed us to map the fields excited in THz resontaors - carbon micro-fibres.

Abstract: We present the temporal evolution of the terahertz (THz) field leading to the excitation of plasmonic resonances in carbon microfibers. The field evolution is mapped in space and time for the 3/2 wavelength resonance using a subwavelength aperture THz near-field probe with an embedded THz photoconductive detector. The excitation of surface waves at the fiber tips leads to the formation of a standing wave along the fiber. Local THz time-domain spectroscopy at one of the standing wave crests shows a clear third-order resonance peak at 1.65 THz, well described by the Lorentz model. This application of the subwavelength aperture THz near-field microscopy for mode mapping and local spectroscopy demonstrates the potential of near-field methods for studies of subwavelength plasmonic THz resonators.
Oleg Mitrofanov, Irina Khromova, Thomas Siday, Robert J. Thompson, Andrey N. Ponomarev, Igal Brener, John L. Reno, 'Near-Field Spectroscopy and Imaging of Subwavelength Plasmonic Terahertz Resonators', IEEE Transactions on Terahertz Science and Technology, V.6, No. 3, pp. 382 - 388  (2016)
DOI: 10.1109/TTHZ.2016.2549367

Our paper 'Tunable beam steering enabled by graphene metamaterials' (B. Orazbayev, M. Beruete, and I. Khromova) climbing up the list of Top Downloads in Optics Express.

Our paper 'Tunable beam steering enabled by graphene metamaterials' (B. Orazbayev, M. Beruete, and I. Khromova) in the list of Top Downloads in Optics Express.

We demonstrate tunable mid-infrared (MIR) beam steering devices based on multilayer graphene-dielectric metamaterials. The effective refractive index of such metamaterials can be manipulated by changing the chemical potential of each graphene layer. This can arbitrarily tailor the spatial distribution of the phase of the transmitted beam, providing mechanisms for active beam steering. Three different beam steerer (BS) designs are discussed: a graded-index (GRIN) graphene-based metamaterial block, an array of metallic waveguides filled with graphene-dielectric metamaterial and an array of planar waveguides created in a graphene-dielectric metamaterial block with a specific spatial profile of graphene sheets doping. The performances of the BSs are numerically analyzed, showing the tunability of the proposed designs for a wide range of output angles (up to approximately 70°). The proposed graphene-based tunable beam steering can be used in tunable transmitter/receiver modules for infrared imaging and sensing.

B. Orazbayev, M. Beruete, and I. Khromova Optics Express, Vol. 24, Issue 8,  pp. 8848-8861 (2016) doi: 10.1364/OE.24.008848

I will be presenting a seminar on Near-Field Terahertz Spectroscopy at the Metamaterials Laboratory (ITMO University, St. Petersburg) at 14:00 on the 17 of February.

Audio-recording of my talk 'Near-field terahertz spectroscopy: studying terahertz resonators on a micro-scale' presented at the Royal Society  meeting: Particle, condensed matter and quantum physics: links via Max well's equations.

18-19 November 2015, Kavli Royal Society Centre, Chicheley Hall (UK)

(click to listen)

Detailed program and other recorded presentations at:

https://royalsociety.org/events/2015/11/particle-condensed-matter/

'Near-field terahertz spectroscopy: studying terahertz resonators on a micro-scale'
by I. Khromova

Terahertz radiation allows for non-destructive detection of objects and processes ’invisible’ at optical and microwave frequencies. Modern terahertz science promises break-through security, medical, and quality control techniques, as well as access to crucial astronomical observation and environmental monitoring. However, the emerging terahertz technology is held back by the scarcity of functional materials and devices required for manipulation of terahertz radiation.
This talk demonstrates opportunities and advantages of the near-field terahertz time-domain spectroscopy for direct studies of terahertz electromagnetic resonances occurring on a micrometre scale. As examples of micro-resonators, it considers conductive micro-fibres and dielectric micro-spheres. Micro-resonators are at the heart of numerous promising terahertz solutions, including the metamaterial approach – creating functional materials from artificial pre-designed resonant micrometre-sized ‘meta-atoms’. Experimental studies of micrometre-scale terahertz resonances are essential, yet inaccessible to common far-field spectroscopic techniques due to extreme sensitivity requirements.
This non-contact technique maps the field patterns of terahertz resonant modes excited in individual conductive or insulating micro-objects, and gives access to essential parameters of micro-resonators, including their resonance frequency, local field enhancement and quality factors. Depending on the underlying physics of observed terahertz resonances, it allows for material and structural characterisation of micro-objects.
This work uses the examples of carbon micro-fibres and titanium dioxide micro-spheres to show the advantages of near-field terahertz time-domain spectroscopy for non-contact terahertz conductivity probing and anisotropic material characterisation; and direct observation of versatile resonant modes, including surface-plasmon resonances in conductive dipoles, and magnetic dipole resonances in dielectric subwavelength terahertz resonators.

You are welcome to attend out invited talk titled 'Plasmonic resonances in carbon fibers observed with terahertz near-field microscopy' at 12pm on Monday, February 15,