A boost for graphene-based light detectors: Photoexcited graphene puzzle solved

Light detection and control lies at the heart of many modern device applications, such as the camera you have in your phone. Using graphene as a light-sensitive material for light detectors can offer significant improvements with respect to materials being used now. For example, graphene can detect light of almost any color, and it gives an extremely fast electronic response within one millionth of a millionth of a second. Thus, in order to properly design graphene-based light detectors it is crucial to understand the processes that take place inside the graphene after it absorbs light

Schematic representation of the ultrafast optical pump - terahertz probe experiment, where the optical pump induces electron heating and the terahertz pulse is sensitive to the conductivity of graphene directly after this heating process, which occurs on a timescale faster than a millionth of a millionth of a second.

Credit: Illustration: Fabien Vialla/ICFO

https://www.sciencedaily.com/releases/2018/05/180511150442.htm

Light detection and control lies at the heart of many modern device applications, such as the camera you have in your phone. Using graphene as a light-sensitive material for light detectors can offer significant improvements with respect to materials being used nowadays. For example, graphene can detect light of almost any colour, and it gives an extremely fast electronic response within one millionth of a millionth of a second. Thus, in order to properly design graphene-based light detectors it is crucial to understand the processes that take place inside the graphene after it absorbs light.

A team of European scientists including ICFO from Barcelona (Spain), IIT from Genova (Italy), the University of Exeter from Exeter (UK) and Johannes Gutenberg University from Mainz (Germany), have now succeeded in understanding these processes. Published recently in Science Advances, their work gives a thorough explanation of why, in some cases, the graphene conductivity increases after light absorption and in other cases, it decreases. The researchers show that this behaviour correlates with the way in which energy from absorbed light flows to the graphene electrons: After light is absorbed by the graphene, the processes through which graphene electrons heat up happen extremely fast and with a very high efficiency.

For highly doped graphene (where many free electrons are present), ultrafast electron heating leads to carriers with elevated energy -- hot carriers -- which, in turn, leads to a decrease in conductivity. Interestingly enough, for weakly doped graphene (where not so many free electrons are present), electron heating leads to the creation of additional free electrons, and therefore an increase in conductivity. These additional carriers are the direct result of the gapless nature of graphene -- in gapped materials, electron heating does not lead to additional free carriers.

This simple scenario of light-induced electron heating in graphene can explain many observed effects. Aside from describing the conductive properties of the material after light absorption, it can explain carrier multiplication, where -- under specific conditions -- one absorbed light particle (photon) can indirectly generate more than one additional free electron, and thus create an efficient photoresponse within a device.

The results of the paper, in particular, understanding electron heating processes accurately, will definitely mean a great boost in the design and development of graphene-based light detection technology.

This work was funded by the E.C. under Graphene Flagship, as well as a Mineco Young Investigator grant.

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Materials provided by ICFO-The Institute of Photonic Sciences. Note: Content may be edited for style and length.

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Journal Reference:

1. Andrea Tomadin, Sam M. Hornett, Hai I. Wang, Evgeny M. Alexeev, Andrea Candini, Camilla Coletti, Dmitry Turchinovich, Mathias Kläui, Mischa Bonn, Frank H. L. Koppens, Euan Hendry, Marco Polini, Klaas-Jan Tielrooij. The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies. Science Advances, 2018; 4 (5): eaar5313 DOI: 10.1126/sciadv.aar5313

Abstract-Subwavelength hyperspectral THz studies of articular cartilage

 Rayko I. Stantchev, Jessica C. Mansfield, Ryan S. Edginton, Peter Hobson, Francesca Palombo,  Euan Hendry,

https://www.nature.com/articles/s41598-018-25057-9

Terahertz-spectroscopy probes dynamics and spectral response of collective vibrational modes in condensed phase, which can yield insight into composition and topology. However, due to the long wavelengths employed (λ = 300 μm at 1THz), diffraction limited imaging is typically restricted to spatial resolutions around a millimeter. Here, we demonstrate a new form of subwavelength hyperspectral, polarization-resolved THz imaging which employs an optical pattern projected onto a 6 μm-thin silicon wafer to achieve near-field modulation of a co-incident THz pulse. By placing near-field scatterers, one can measure the interaction of object with the evanescent THz fields. Further, by measuring the temporal evolution of the THz field a sample’s permittivity can be extracted with 65 μm spatial resolution due to the presence of evanescent fields. Here, we present the first application of this new approach to articular cartilage. We show that the THz permittivity in this material varies progressively from the superficial zone to the deep layer, and that this correlates with a change in orientation of the collagen fibrils that compose the extracellular matrix (ECM) of the tissue. Our approach enables direct interrogation of the sample’s biophysical properties, in this case concerning the structure and permittivity of collagen fibrils and their anisotropic organisation in connective tissue.

Abstract-Sign inversion in the terahertz photoconductivity of single-walled carbon nanotube films

Peter Karlsen, Mikhail V. Shuba, Polina P. Kuzhir, Albert G. Nasibulin, Patrizia Lamberti, Euan Hendry

https://arxiv.org/abs/1804.11113

In recent years, there have been conflicting reports regarding the ultrafast photoconductive response of films of single walled carbon nanotubes (CNTs), which apparently exhibit photoconductivities that can differ even in sign. Here, we observe explicitly that the THz photoconductivity of CNT films is a highly variable quantity which correlates with the length of the CNTs, while the chirality distribution has little influence. Moreover, by comparing the photo-induced change in THz conductivity with heat-induced changes, we show that both occur primarily due to heat-generated modification of the Drude electron relaxation rate, resulting in a broadening of the plasmonic resonance present in finite-length metallic and doped semiconducting CNTs. This clarifies the nature of the photo-response of CNT films and demonstrates the need to carefully consider the geometry of the CNTs, specifically the length, when considering them for application in optoelectronic devices.

Abstract-Compressed sensing with near-field THz radiation

Rayko I. Stantchev, David B. Phillips, Peter Hobson, Samuel M. Hornett, Miles J. Padgett, and Euan Hendry

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-4-8-989&origin=search

We demonstrate a form of near-field terahertz (THz) imaging that is compatible with compressed sensing algorithms. By spatially photomodulating THz pulses using a set of shaped binary optical patterns and employing a 6-μm-thick silicon wafer, we are able to reconstruct THz images of an object placed on the exit interface of the wafer. A single-element detector is used to measure the electric field amplitude of transmitted THz radiation for each projected pattern, with the ultra-thin wafer allowing us to access the THz evanescent near fields to achieve a spatial resolution of ∼9  μm$\sim 9\mathrm{\mu m}$ (𝜆/45$\lambda /45$ at 0.75 THz). We conclude by experimentally improving the image rate by a factor of ∼3$\sim 3$ by undersampling the object with adaptive and compressed sensing algorithms.

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-Terahertz Depolarization Effects in Colloidal TiO2 Films Reveal Particle Morphology

Soeren Alkaersig Jensen , Klaas-Jan Tielrooij , Euan Hendry , Mischa Bonn , Ivan Rychetský , and Hynek Soeren 

J.Phys.Chem. C, Just Accepted Manuscript

DOI: 10.1021/jp406897y

Publication Date (Web): December 10, 2013

Copyright © 2013 American Chemical Society
http://pubs.acs.org/doi/abs/10.1021/jp406897y

Films of colloidal TiO2 nanoparticles are widely used in photovoltaic and photocatalytic applications, and the nature of electrical conductivity in such materials is therefore of both fundamental and practical interest. The conductive properties of colloid TiO2 films depend strongly on their morphology and deviate greatly from the properties of the bulk material. We report ultrafast photoconductivity studies of films consisting of sintered TiO2 particles of very different sizes performed using time-resolved Terahertz spectroscopy. Remarkably, identical photoconductivity spectra are observed for films of particles with diameters of tens and hundreds of nm respectively. The independence of photoconductivity on particle size directly demonstrates that the terahertz photoconductive response of colloidal TiO2 films is not affected by carrier backscattering at particle boundaries as has previously been concluded, but rather by depolarization fields resulting from the spatial inhomogeneities in the dielectric function inherent to these types of films. Modelling of the influence of depolarization fields on the terahertz conductivity allows us to explain the measured data and gain insights into the morphology of the film. Specifically, we show that the observed photoconductivity spectra reflect percolated pathways in the colloidal TiO2 nanoparticles films, through which charge carrier diffusion can occur over macroscopic length scales.