Micro-resonator Probe for THz Near-field Imaging Beyond the Diffraction Limit

Lead Research Organisation: University College London
Department Name: Electronic and Electrical Engineering


Terahertz (THz) device research and studies of THz phenomena in solid state systems require detection of THz waves and signals on the scale of few microns. These measurements present a major technological problem caused by diffraction of THz waves. The diffraction limit prevents the use of the recently developed THz spectroscopic instrumentation for studies of objects smaller than approximately a wavelength. Near-field surface probing methods have shown potential solutions in overcoming the diffraction limit. However all the existing THz near-field techniques exhibit another fundamental limitation due to significant perturbations in the electric field caused by the near-field probe. The probe invasiveness and a non-uniform frequency response across the THz spectrum prevent the use of the existing near-field probes for mapping of electric field distribution in THz devices. In addition, THz near-field imaging systems with spatial resolution better than ~1/20 of a wavelength suffer from a severe reduction in sensitivity.To mitigate these problems and to allow high spatial resolution studies with THz waves we propose to develop a THz imaging and spectroscopy system with a novel near-field probe. The probe concept exploits the non-invasive nature of the electro-optic detection method and utilizes an optical micro-resonator to enhance the detection sensitivity. The proposed electro-optic micro-resonator will be integrated into a fibre-coupled near-field probe. It will allow THz wave and signal probing with a spatial resolution of ~5 microns (~1/100 of the wavelength) and it will offer full spectroscopic capabilities in the THz range (0.1-2.0 THz). The novelty of this approach is in exploiting the optical cavity resonance for electro-optic detection of THz waves by an extremely small near-field probe. The goal of this research programme is to develop and build the THz near-field probing system and apply it in device research on the sub-wavelength scale. The proposed technology will expand the spectrum of THz studies to micrometre-scale objects. It will aid in the progress of THz device research and will facilitate studies of THz phenomena in physics, materials science and other disciplines.


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Description Development of Terahertz (THz) near-field microscopy probe technology: This grant allowed us to make significant progress in the underpinning THz near-field probe technology, which is now enabling novel applications for THz imaging, including THz surface wave imaging, THz waveguide mode analysis, and THz spectroscopy of sub-wavelength size objects, such as THz subwavelength size plasmonic and dielectric resonators.
Surface waves (SWs), also called as surface plasmon polaritons, are electro-magnetic excitations supported (and guided) by conductive surfaces. SWs play a key role in many THz devices, including waveguides, antennas, plasmonic filters, metamaterials and metasurfaces. SWs typically remain undetected due to their localised nature. Our investigations demonstrated that THz SWs can be detected and visualised with the integrated sub-wavelenght aperture probe that we have developed. We explained the coupling mechanism and developed a high-spatial resolution method for mapping THz SWs. We demonstrated application of this method for studies of planar metallic antennas (commonly used for improving efficiency of THz sources and detectors). We also developed and demonstrated a novel THz near-field microscopy method exploiting THz surface waves.

Waveguides are essential for enabling applications of THz technologies. Development of efficient waveguides for THz frequencies remain a challenging problem due to high signal transmission losses at THz frequencies. We developed a method for characterising modal properties for THz waveguides (transmission losses, dispersion and spatial mode profile). The method was instrumental in the development and demonstration of dielectric-lined hollow cylindrical metallic waveguides, which exhibit some of the best THz transmission properties reported to date (in collaboration with Rutgers University (USA)).

Near-field microscopy and spectroscopy technology is essential for applications in fundamental scientific and engineering research as well as for practical applications. Building blocks of THz technology are often of sub-wavelength size (resonators for metamaterials and metasurfaces, Josephson Junction oscillators, high-speed electronic elements, such as the transistor); many potential applications of THz technology also require probing sub-wavelength size samples (biomedical applications, sensor applications) To enable applications of THz spectroscopy for sub-wavelength objects, we developed and demonstrated broadband (non-resonant) concentration of THz waves to deeply sub-wavelength volumes. We have achieved one of the highest THz wave confinement demonstrated experimentally at that time.
Exploitation Route Research collaborations: the unique capabilities of THz high-resolution microscopy for broader scientific and engineering applications already allowed us to establish fruitful research collaboration with universities and industrial research laboratories in the UK and abroad (USA and Europe). The developed technology, knowledge and methods are being transferred to other universities and research labs to enable studies which were not possible before (U of Leeds, U of Cambridge, CNR).
Sectors Electronics,Pharmaceuticals and Medical Biotechnology,Other

URL http://www.ee.ucl.ac.uk/~olegm/research/
Description Our research on terahertz (THz) near-field microscopy is making the UK more competitive in the world arena in the rapidly developing area of THz technologies, where USA, Germany, France, Spain, S. Korea and Japan are making rapid progress. There are two commercial companies in Germany with products for THz near-field microscopy and spectroscopy. Our research helped to establish collaborative contacts with both companies. We also collaborated with a commercial company (SWISSto12, Switzerland) to develop THz waveguides. A number of collaborations with universities and research labs in the UK, Europe and USA have been developed with numerous joint scientific publications. An EPSRC Programme Grant (COherent Terahertz Systems (COTS)-opening up the terahertz spectrum for widespread application) with one Workpackage dedicated to further develop THz microscopy, was award in 2012; and a new EPSRC Programme Grant (HyperTHz), also with one Workpackage dedicated to THz near-field microscopy, was awarded in 2017.
First Year Of Impact 2011
Sector Education,Electronics
Description EPSRC Programme Grant
Amount £6,570,000 (GBP)
Funding ID EP/J017671/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 05/2012 
End 04/2017
Description Royal Society University Research Fellowship
Amount £530,000 (GBP)
Funding ID UF080745 
Organisation The Royal Society 
Sector Academic/University
Country United Kingdom
Start 10/2009 
End 03/2015
Title Integrated sub-wavelength aperture near-field THz probe 
Description High-spatial resolution near-field THz probe (to be used with THz time-domain spectroscopy) 
Type Of Material Improvements to research infrastructure 
Year Produced 2014 
Provided To Others? Yes  
Impact Further development of this technology in collaboration with the National Research Laboratory in USA (Sandia National Lab) 
Description CINT - Sandia National Lab 
Organisation Sandia Laboratories
Country United States 
Sector Private 
PI Contribution Development and fabrication of integrated THz near-field probes based on THz photoconductive detectors
Collaborator Contribution Access to semiconductor device fabrication facility with specialised instruments (at no-cost) Sample fabrication and material growth (AlGaAs/GaAs heterostructure, graphene) Access to testing equipment Clean-room training Clean-room consumables
Impact Novel THz near-field probes, which we presently use for research Several research articles and conference presentations
Start Year 2012
Description Prof. J.A. Harrington, Rutgers, USA 
Organisation Rutgers University
Country United States 
Sector Academic/University 
PI Contribution Design, modelling and experimental testing of THz waveguides
Collaborator Contribution Development and fabrication of THz waveguides (dielectric-lined hollow metallic waveguides)
Impact several research articles and conference presentations (incl. two invited presentations)
Start Year 2009
Description Scuola Normale Superiore 
Organisation Scuola Normale Superiore di Pisa
Country Italy 
Sector Academic/University 
PI Contribution Development of coupling waveguide coupling techniques for Quantum Cascade Lasers
Collaborator Contribution Experimental testing of THz waveguides using Quantum Cascade Lasers
Impact - several joint research articles and conference presentations - waveguide-coupled quantum cascade lasers
Start Year 2010