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

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

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.

Ultrasensitive terahertz/infrared waveguide modulators based on multilayer graphene metamaterial
Irina Khromova, Andrei Andryieuski and Andrei Lavrinenko
Laser & Photonics Reviews, Volume 8, Issue 6, pages 916–923, November 2014
DOI: 10.1002/lpor.201400075

http://onlinelibrary.wiley.com/doi/10.1002/lpor.201400075/abstract

This paper studies and classifies the electromagnetic regimes of multilayer graphene-dielectric artificial metamaterials in the terahertz/infrared range. The employment of such composites for waveguide-integrated modulators is analysed and three examples of novel tunable devices are presented. The first one is a modulator with excellent ON-state transmission and very high modulation depth: >38 dB at 70 meV graphene's electrochemical potential (Fermi energy) change. The second one is a modulator with extreme sensitivity towards graphene's Fermi energy - a minute 1 meV variation of the latter leads to >13.2 dB modulation depth. The third one is a tunable waveguide-based passband filter. The narrow-band cut-off conditions around the ON-state allow the latter to shift its central frequency by 1.25% per every meV graphene's Fermi energy change.