The properties of light can be controlled by means of nanostructures

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A study led by the UPV/EHU-University of the Basque Country professor Ángel Rubio has simulated a new device to generate terahertz radiation using carbon nanostructures

A theoretical study based on computational simulations conducted by the UPV/EHU's Nano-bio Spectroscopy Research Group in collaboration with the Japanese research centre AIST, has shown that the intensity of ultraviolet light that is made to pass through a graphene nano-ribbon is modulated with a terahertz frequency. So we are seeing the opening up of a new field of research into obtaining terahertz radiation that has a whole host of applications. The research has been published in the prestigious journal Nanoscale.

The UPV/EHU's Nano-bio Spectroscopy Research Group led by Ángel Rubio, a UPV/EHU professor in the Department of Materials Physics and director of the Max Planck Institute for Structure and Dynamics of Matter in Hamburg, has simulated the converting of ultraviolet light into radiation in the terahertz range by making it pass through a graphene nano-ribbon, and has put forward a new compact device designed to generate radiation of this type based on the phenomenon discovered. The research, conducted in collaboration with the research group led by Yoshiyuki Miyamoto of the National Institute of Advanced Industrial Science and Technology (AIST) of Japan, has appeared in the prestigious journal Nanoscale, published by the Royal Society of Chemistry (United Kingdom).

Low-frequency terahertz radiation has a broad range of applications, such as the characterisation of molecules, materials, tissues, etc. However, right now it is difficult to manufacture small, efficient, low-cost devices to produce terahertz radiation.  This phenomenon "extends the range of applicability of radiation of this type to many other spheres in which it was not being used," explained Ángel Rubio, "owing to the fact that one would have to resort to much bigger radiation sources".

The starting point of a new field of research

To carry out this simulation, they used graphene nano-ribbons: strips cut out of sheets of graphene. In the research they concluded that UV light that exerts an effect on the nano-ribbon emits a totally different radiation (terahertz) perpendicular to the incident light. This phenomenon "opens up the possibility of generating structures that will allow the frequency range to be changed using different nanostructures," explained Prof Rubio. "A new field of research is being opened up".

Now that the existence of the phenomenon has been demonstrated, "it would be necessary to see if the same thing can be done with a different type of light source," explained Ángel Rubio. In the research they used a high-intensity laser pointer so that the simulation would be correct, but it should be possible to use "more accessible light sources", he specified. What is more, another step to be taken in this field would be "to use a set of nanostructures instead of a single one to produce an actual device."

The UPV/EHU group has worked on the proposal of the idea and its implementation in code that allows a simulation to be made on the computer, while the Japanese research centre AIST has been responsible for the numerical calculations. The researchers have used novel simulation techniques of first principles, methods in which the predictive capacity is very high: the behaviour of a material is predicted without using external parameters. "The simulation techniques have reached a point," concluded Rubio, "where systems that are later shown to actually behave in the same way experimentally can be predicted".

Additional information

The Nano-bio Spectroscopy Group is led by Ángel Rubio. The group's activity focusses on the theoretical research and modelling of electronic and structural properties of condensed matter as well as the development of new theoretical tools and computer codes to explore the electronic response of solids and nanostructures when handling external electromagnetic fields.

Ángel Rubio is a UPV/EHU professor, a member of the Department of Materials Sciences, and director of the Theory Department of the Max Planck Institute for Structure and Dynamics of Matter. He has over 300 scientific publications and over 22,000 mentions in the scientific literature. His research activity is internationally recognised and he has also received numerous distinctions and awards.

Bibliographic reference

Hong Zhang, Yoshiyuki Miyamoto, Xinlu Cheng, Angel Rubio.. Optical field terahertz amplitude modulation by graphene nanoribbons. Nanoscale, 2015,7, 19012-19017. DOI: 10.1039/C5NR05889A.

Focus on Dielectric Spectroscopy Laboratory

Spanish National Research Council's logo Español: Logo del Consejo Superior de Investigaciones Científicas (Photo credit: Wikipedia)

Dielectric Spectroscopy for Soft Matter Research

The Dielectric Spectroscopy Laboratory (DSL) is one of the laboratories of the Material Physics Center (MPC), a joint research center set up in 2001 by the University of the Basque Country (UPV/EHU) in collaboration with the Spanish National Research Council (CSIC).
Since then, the Polymers and Soft Matter Group (PSMG) laboratory has been further developed and upgraded in order to improve the quality of the different experimental set-ups and to facilitate the operation of the infrastructure by external users. The operative set-ups of DSL offer the unique possibility of performing high precision dielectric relaxation experiments over a huge range of frequencies (from 10-5 to 3-1010 Hz, i.e. nearly 16 orders of magnitude).
Depending on the frequency range involved, these experiments can be carried out in different sample environments where tem- perature can be varied over a wide range (from 20 - 600 K), and pressure can be increased up to 300 MPa. Furthermore, the existing equipment allows experiments to be performed with simultaneous access to electric/dielectric and mechanical/rheological frequency dependent properties. :
Technical details of the various instruments can be found on the ESMI website and at: www.sc.ehu.es/sqwpolim/PSMG/dielab.html
Most of the equipment uses widely-available software, and the temperature control systems are such that different set-ups do not generally involve specific training for each piece of equipment. In addition to the dielectric spectroscopy instruments, the infratructure also provides several facilities for sample preparation and standard sample characterization (such as DSC, TGA, PVT, etc.). Details can be found at: www.sc.ehu.es/sqwpolim/PSMG/infr.html
The scientists in charge of the various instruments are:
Dr. Silvina Cerveny (Micro-Wave and Terajertz spectrometers), Dr. Gustavo Schwartz (non-conventional sample environment), Dr. Silvia Arrese-Igor (Time-Domain and stanstandard Dielectric Spectrometers). ■
The current range of instruments, listed in order of frequency range, are:
- Time-Domain Dielectric Spectrometer, 10-5 - 1 Hz
- Broad-Band Dielectric Spectrometer, 10-3 - 107 Hz
- High-Frequency Dielectric Spectrometer, 106 - 109 Hz - Micro-Wave Spectrometer, 108 - 5x 1010 Hz
- Terahertz Spectrometer, 5x1010 - 4x1012 Hz
By combining several instruments, a huge dynamic range is achieved, allowing the investigation of dynamic processes over more than 16 orders of magnitude on the time/frequency scale. Several non-conventional sample environments are available over limited frequency ranges. For example, measurement temperatures can be reduced toas low as 10K. Moreover, high-pressure measurements are possible up to 300 MPa.