Abstract-Terahertz Detection with Perfectly-Absorbing Photoconductive Metasurface

Thomas Siday,  Polina Vabishchevich, Lucy Hale, Charles Harris, Shan Ting Shan, John Reno, Igal Brener, Oleg Mitrofanov,

https://www.osti.gov/pages/biblio/1515200

Terahertz (THz) photoconductive devices are utilized for generation, detection, and modulation of THz waves, and they rely on the ability to switch electrical conductivity on a subpicosecond time scale using optical pulses. Yet, fast and efficient conductivity switching with high contrast has been a challenge, because the majority of photoexcited charge carriers in the switch do not contribute to the photocurrent due to fast recombination. Here, we improve efficiency of electrical conductivity switching using a network of electrically connected nanoscale GaAs resonators, which form a perfectly absorbing photoconductive metasurface. We achieve perfect absorption without incorporating metallic elements, by breaking the symmetry of cubic Mie resonators. As a result, the metasurface can be switched between conductive and resistive states with extremely high contrast using an unprecedentedly low level of optical excitation. We integrate this metasurface with a THz antenna to produce an efficient photoconductive THz detector. The perfectly absorbing photoconductive metasurface opens paths for developing a wide range of efficient optoelectronic devices, where required optical and electronic properties are achieved through nanostructuring the resonator network.

Abstract-Modelling the carrier dynamics of semiconductors to understand their terahertz emission (Conference Presentation)

Enrique Castro-Camus, Mariana Alfaro-Gomez, Sofia C. Corzo-Garcia, Arturo I. Hernandez-Serrano,  Oleg Mitrofanov,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/107560B/Modelling-the-carrier-dynamics-of-semiconductors-to-understand-their-terahertz/10.1117/12.2321155.short

We use a Monte-Carlo model to simulate semi-classical photo-carrier dynamics on bulk InAs, InGaAs and GaAs that leads to terahertz emission after ultrafast photoexcitation. This detailed model has allowed us to understand various aspects of the THz emission process, including the near-field distribution which has been experimentally observed, the role of the excess excitation photon energy, and the relative importance of the surface field driven, diffusive (photo-Dember) and ballistic currents. In order to understand the near-field emission we coupled a finite-difference time-domain routine to the carrier dynamics simulation, by doing this, we were able to analyse the near terahertz field emission caused by the motion of such carriers even when the excitation is performed at normal incidence. We found that both the current parallel, which has traditionally been assumed not to take part in the emission, and normal to the interface take a relevant role in the terahertz generation. We performed another set of simulations for different bandgaps and excitation-photon energies in order to compare the emission power of all three semiconductors as function of excitation photon energy finding that the carrier excess excitation energy is more relevant to explain their performance difference than their motilities. We conclude that ballistic transport after photoexcitation is the dominant mechanism for terahertz emission instead of diffusion driven or surface field driven charge separation, which were traditionally considered the most relevant mechanisms.

Abstract-Terahertz Nanoscopy of Plasmonic Resonances with a Quantum Cascade Laser

Riccardo Degl’Innocenti , Robert Wallis, Binbin Wei, Long Xiao, Stephen J. Kindness, Oleg Mitrofanov, Philipp Braeuninger-Weimer, Stephan Hofmann, Harvey E. Beere, David A. Ritchie

https://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=2366204696351202320

We present a terahertz (THz) scattering near-field optical microscope (s-SNOM) based on a quantum cascade laser implemented as both source and detector in a self-mixing scheme utilizing resonant quartz tuning forks as a sensitive nanopositioning element. The homemade s-SNOM, based on a resonant tuning fork and metallic tip, operates in tapping mode with a spatial resolution of ∼78 nm. The quantum cascade laser is realized from a bound-to-continuum active region design with a central emission of ∼2.85 THz, which has been lens-coupled in order to maximize the feedback into the laser cavity. Accordingly, the spatial resolution corresponds to >λ/1000. The s-SNOM has been used to investigate a bidimensional plasmonic photonic crystal and to observe the optical resonant modes supported by coupled plasmonic planar antennas, showing remarkable agreement with the theoretical predictions. The compactness, unique sensitivity, and fast acquisition capability of this approach make the proposed s-SNOM a unique tool for solid-state investigations and biomedical imaging.

Abstract-Terahertz s-SNOM with > λ/1000 resolution based on self-mixing in quantum cascade lasers

 Binbin Wei, Robert Wallis,  Stephen Kindness,  Oleg Mitrofanov,   Harvey E. Beere,  David A. Ritchie,   Riccardo Degl'Innocenti

http://ieeexplore.ieee.org/document/8086433/


Near-field imaging techniques have great potential in many applications, ranging from the investigation of the optical properties of solid state and 2D materials to the excitation and direct retrieval of plasmonic resonant modes, to the mapping of carrier concentrations in semiconductor devices. Further to this, the capability of performing imaging with non-ionizing terahertz (THz) radiation on a subwavelength scale is of fundamental importance in biological applications and healthcare. The implementation of stable, compact solid state sources such as quantum cascade lasers (QCLs) in apertureless scanning near field optical microscopes (s-SNOM), instead of bulkier gas lasers, has been already reported with a resolution ≥ 1 μm [1] based on metallic tips. Here we report on the realization of an s-SNOM, based on tuning fork sensors [2], to maintain a constant sample/tip distance in tapping mode, and using quantum cascade lasers emitting around 3 THz as both source and detector in a self-mixing scheme [3]. The implementation of a fast and efficient feedback mechanism allowed the achievement of a spatial resolution lower than 100 nm, as shown in Fig. 1, thus achieving the record resolution with a QCL better than λ/1000. The self-mixing approach allows an extremely sensitive and fast detection scheme, which overcomes the slow response of traditional THz detectors, by monitoring the scattered signal fed back into the QCL cavity, modulating the power or the bias. In order to enhance the sensitivity of the whole apparatus, as well as the collection of the scattered light, silicon lenses have been attached to the QCLs with an antireflection parylene coating which was thick enough to strongly reduce the laser emission, but still allowed enough power for alignment. Figure 1 reports the topography a) and the THz voltage signal on the QCL b) of Au square features (top-left square corner) over a Si substrate, exhibiting an enhanced scattering. As the reference voltage used for subtraction from the QCL voltage was placed lower than the QCL voltage, the THz signal dropped on the Au square.

Abstract-Resonant terahertz probes for near-field scattering microscopy

Thomas Siday, Michele Natrella, Jiang Wu, Huiyun Liu, and Oleg Mitrofanov

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-22-27874


We propose and characterize a scattering probe for terahertz (THz) near-field microscopy, fabricated from indium, where the scattering efficiency is enhanced by the dipolar resonance supported by the indium probe. The scattering properties of the probe were evaluated experimentally using THz time-domain spectroscopy (TDS), and numerically using the finite-difference time-domain (FDTD) method in order to identify resonant enhancement. Numerical measurements show that the indium probes exhibit enhanced scattering across the THz frequency range due to dipolar resonance, with a fractional bandwidth of 0.65 at 1.24 THz. We experimentally observe the resonant enhancement of the scattered field with a peak at 0.3 THz. To enable practical THz microscopy applications of these resonant probes, we also demonstrate a simple excitation scheme utilizing a THz source with radial polarization, which excites a radial mode along the length of the tip. Strong field confinement at the apex of the tip, as required for THz near-field microscopy, was observed experimentally.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract-Near-Field Characterization of Conductive Micro-resonators for Terahertz Sensing

Irina Khromova, Oleg Mitrofanov,

https://link.springer.com/chapter/10.1007/978-94-024-1093-8_3

Near-field (NF) terahertz (THz) time-domain spectroscopy (TDS) is an excellent tool for direct studies of THz electromagnetic resonances occurring on a micrometre scale. Micro-resonators are at the heart of numerous promising THz sensing and detecting solutions. Experimental studies of individual micrometre-scale THz resonances are essential, yet extremely challenging for the common far-field spectroscopic methods due to extreme sensitivity requirements. NF THz spectroscopy and microscopy are non-contact techniques for spectroscopic studies of individual micro-resonators and mapping the field patterns of THz resonant modes excited in individual conductive or insulating micro-objects. They give access to essential parameters of micro-resonators, including their resonance frequency, local field enhancement and quality factors. It allows for material and structural characterisation of micro-objects. Using the example of carbon micro-fibres, we show the advantages of NF THz TDS for non-contact THz conductivity probing and direct observation of the fundamental and the third-order surface-plasmon resonance modes in conductive THz micro-resonators.

Abstract-Near-field terahertz probes with room-temperature nanodetectors for subwavelength resolution imaging

Oleg Mitrofanov, Leonardo Viti, Enrico Dardanis, Maria Caterina Giordano, Daniele Ercolani, Antonio Politano, Lucia Sorba,  Miriam S. Vitiello

http://www.nature.com/articles/srep44240

Near-field imaging with terahertz (THz) waves is emerging as a powerful technique for fundamental research in photonics and across physical and life sciences. Spatial resolution beyond the diffraction limit can be achieved by collecting THz waves from an object through a small aperture placed in the near-field. However, light transmission through a sub-wavelength size aperture is fundamentally limited by the wave nature of light. Here, we conceive a novel architecture that exploits inherently strong evanescent THz field arising within the aperture to mitigate the problem of vanishing transmission. The sub-wavelength aperture is originally coupled to asymmetric electrodes, which activate the thermo-electric THz detection mechanism in a transistor channel made of flakes of black-phosphorus or InAs nanowires. The proposed novel THz near-field probes enable room-temperature sub-wavelength resolution coherent imaging with a 3.4 THz quantum cascade laser, paving the way to compact and versatile THz imaging systems and promising to bridge the gap in spatial resolution from the nanoscale to the diffraction limit.

Abstract-Detection of internal fields in double-metal terahertz resonators

Oleg Mitrofanov1,2,a), Zhanghua Han3,a), Fei Ding4, Sergey I. Bozhevolnyi4, Igal Brener2,5, and John L. Reno

http://aip.scitation.org/doi/10.1063/1.4975802

Terahertz (THz) double-metal plasmonic resonators enable enhanced light-matter coupling by exploiting strong field confinement. The double-metal design however restricts access to the internal fields. We propose and demonstrate a method for spatial mapping and spectroscopic analysis of the internal electromagnetic fields in double-metal plasmonic resonators. We use the concept of image charges and aperture-type scanning near-field THz time-domain microscopy to probe the fields confined within the closed resonator. The experimental method opens doors to studies of light-matter coupling in deeply sub-wavelength volumes at THz frequencies.

Abstract-Near-field terahertz imaging using sub-wavelength apertures without cutoff

Shuchang Liu, Oleg Mitrofanov, and Ajay Nahata

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-3-2728

We demonstrate near-field imaging capabilities of a conical waveguide without cutoff using broadband terahertz (THz) radiation. In contrast to conventional conically tapered waveguides, which are characterized by strong suppression of transmission below the cutoff frequency, the proposed structure consists of two pieces, such that there is an adjustable gap along the length of the waveguide. We also ensure that the sidewalls are thin in the vicinity of the gap. The combination of these geometrical features allow for significantly enhanced transmission at frequencies below the cutoff frequency, without compromising the mode confinement and, consequently, the spatial resolution when used for imaging applications. We demonstrate near-field imaging with this probe simultaneously at several frequencies, corresponding to three regimes: above, near and below the cutoff frequency. We observe only mild degradation in the image quality as the frequency is reduced below the cutoff frequency. These results suggest that further refinements in the probe structure will allow for improved imaging capabilities at frequencies well below the cutoff frequency.

© 2016 Optical Society of America

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Terahertz microscopy breakthrough revealed

Image: nanostructured optical materials boost photonic device efficiencies for THz microscopy
By: Rebecca Pool
http://www.microscopy-analysis.com/editorials/editorial-listings/terahertz-microscopy-breakthrough-revealed

Researchers from University College London, UK, and Sandia National Laboratories, US, have demonstrated optoelectronic probes for high spatial resolution terahertz microscopy.

 

Similar to infra-red waves and x-rays, terahertz waves allow observing objects and phenomena invisible to the human eye, but the clarity of THz images is limited by diffraction.

 

To advance resolution capabilities of near-field scanning probe microscopy with terahertz waves, Dr Mitrofanov and colleagues have developed a nanostructured terahertz detector and integrated it into the near-field microscopy probe.

 

The detector structure contains an array of optical nanoantennas and a distributed Bragg reflector.

 

When illuminated by a short optical pulse, this structure traps optical photons and activates a small terahertz detector, which allows sampling terahertz waves on the scale over 100 times smaller than the terahertz wavelength.

 

The researches anticipate that applications of these probes with terahertz time-domain spectroscopy will enable further scientific investigations of terahertz phenomena.

 

The technique of near-field scanning probe microscopy was pioneered using radio waves by Professor Sir Eric Ash in the Department of Electronic and Electrical Engineering at UCL, more than 40 years ago. This technique opened doors to investigations of sub-wavelength scale objects.

 

Research is published in ACS Photonics.

Abstract-Plasmonic enhancement of sensitivity in terahertz (THz) photo-conductive detectors

Oleg Mitrofanov; Ting Shan Luk; Igal Brener; John L. Reno

http://spie.org/Publications/Proceedings/Paper/10.1117/12.2188177

We demonstrate enhancement of sensitivity in terahertz photoconductive detectors achieved by incorporation of plasmonic structures into the photo-conductive region of the detector. Auston switches based on lowtemperature grown GaAs (LT GaAs) have been reliably used for detection of THz pulses over two decades. This material exhibits high electron mobility with sub-picosecond carrier lifetimes and high dark resistivity. This combination is difficult to achieve in other materials. Application of LT GaAs in THz devices is nevertheless limited due to absorption characteristics of this material. Plasmonic structures can be employed to modify the distribution of the optical field in the photoconductive region and hence modify the response of the THz photoconductive detectors. We will discuss design of plasmonic structures to enhance the response of THz detectors based on LT GaAs and demonstrate incorporation of such structures into THz detectors. We also apply the developed design in integrated photo-conductive probes for THz near-field microscopy, where the enhancement of the material absorption translates into an increase of the detector sensitivity and an improvement in spatial resolution. Performance on these near-field probes that provide a spatial resolution of 3- 5 micrometers (~1/100 of the wavelength) will be discussed and demonstrated.

Abstract-Terahertz near-field spectroscopy through sub-wavelength apertures

Oleg Mitrofanov and Irina Khromova

https://www.osapublishing.org/abstract.cfm?uri=Sensors-2015-SeM4D.5

We demonstrate THz near-field spectroscopy of resonances in sub-wavelength size dielectric and conductive structures using the effect of enhanced transmission through a sub-wavelength aperture in the presence on a resonator.

© 2015 OSA

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Abstract-Terahertz wave transmission in flexible polystyrene-lined hollow metallic waveguides for the 2.5-5 THz band

Miguel Navarro-Cía, Miriam S. Vitiello, Carlos M. Bledt, Jeffrey E. Melzer, James A. Harrington, and Oleg Mitrofanov  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-21-20-23748

A low-loss and low-dispersive optical-fiber-like hybrid HE₁₁ mode is developed within a wide band in metallic hollow waveguides if their inner walls are coated with a thin dielectric layer. We investigate terahertz (THz) transmission losses from 0.5 to 5.5 THz and bending losses at 2.85 THz in a polystyrene-lined silver waveguides with core diameters small enough (1 mm) to minimize the number of undesired modes and to make the waveguide flexible, while keeping the transmission loss of the HE₁₁ mode low. The experimentally measured loss is below 10 dB/m for 2 < ν < 2.85 THz (~4-4.5 dB/m at 2.85 THz) and it is estimated to be below 3 dB/m for 3 < ν < 5 THz according to the numerical calculations. At ~1.25 THz, the waveguide shows an absorption peak of ~75 dB/m related to the transition between the TM₁₁-like mode and the HE₁₁ mode. Numerical modeling reproduces the measured absorption spectrum but underestimates the losses at the absorption peak, suggesting imperfections in the waveguide walls and that the losses can be reduced further.

© 2013 Optical Society of America

 

Abstract-Coherent terahertz photonics

 

 

 

Alwyn J. Seeds, Martyn J. Fice, Katarzyna Balakier, Michele Natrella, Oleg Mitrofanov, Marco Lamponi, Mourad Chtioui, Frederic van Dijk, Michael Pepper, Gabriel Aeppli, A. Giles Davies, Paul Dean, Edmund Linfield, and Cyril C. Renaud

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-21-19-22988

We present a review of recent developments in THz coherent systems based on photonic local oscillators. We show that such techniques can enable the creation of highly coherent, thus highly sensitive, systems for frequencies ranging from 100 GHz to 5 THz, within an energy efficient integrated platform. We suggest that such systems could enable the THz spectrum to realize its full applications potential. To demonstrate how photonics-enabled THz systems can be realized, we review the performance of key components, show recent demonstrations of integrated platforms, and give examples of applications.

© 2013 OSA

Abstract-Probing terahertz surface plasmon waves in graphene structures

Oleg Mitrofanov^1,2, Wenlong Yu³, Robert J. Thompson¹, Yuxuan Jiang³, Igal Brener^2,4, Wei Pan⁴, Claire Berger^3,5, Walter A. de Heer³, and Zhigang Jiang³

¹Electronic and Electrical Engineering, University College London, London WC1E 7JE, United Kingdom
²Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
³School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
⁴Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
⁵CNRS/Institut Néel, BP166, 38042 Grenoble, France

Epitaxial graphene mesas and ribbons are investigated using terahertz (THz) near-field microscopy to probe surface plasmon excitation and THz transmission properties on the sub-wavelength scale. The THz near-field images show variation of graphene properties on a scale smaller than the wavelength, and excitation of THz surface waves occurring at graphene edges, similar to that observed at metallic edges. The Fresnel reflection at the substrate SiC/air interface is also found to be altered by the presence of graphene ribbon arrays, leading to either reduced or enhanced transmission of the THz wave depending on the wave polarization and the ribbon width.

© 2013 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License