Graphene Brings Photodetectors to the Brink of Terahertz Speeds

By Dexter Johnson

Illustration: Achim Woessner

Researchers at the Institute of Photonic Sciences (ICFO) in Barcelona, Spain have been at the forefront of exploiting graphene’s optoelectronic capabilities.

The latest research out of ICFO has demonstrated a graphene-based ultrafast photodetector that can convert absorbed light into an electrical voltage at speeds of less than 50 femtoseconds. How fast is that? A femtosecond is a thousandth of a millionth of a millionth of a second. So fast—ultrafast.

In research published in the journal Nature Nanotechnology, the ICFO team addressed the niggling issue in graphene-based photothermoelectric devices, specifically charge carrier cooling times, which has limited their switching speeds.

The photothermoelectric effect occurs in graphene when light is focused on the interface between graphene layers that have been doped differently. Light causes an excitation leading to the generation of electron-hole pairs and a photovoltage.

After the electron-hole pairs are generated, the electrons heat up from being scattered between the charge carriers. Finally, the electrons cool. Typically in photothermoelectric devices this cooling time for the conversion of light to an electric voltage occurs in picoseconds, limiting their switching rates to a few hundred gigahertz.  

In fact, last year, with great fan fare, the fastest photon switch was measured at 500 gigahertz. Now with this latest research having shortened the cooling times from picoseconds to femtoseconds, the potential for terahertz switching speeds appears to be within the grasp of graphene-based photodetectors.

The key property of graphene that allows this faster electron cooling is the efficient interaction between all its conduction band carriers. This results in the very rapid creation of an electron distribution in the material, resulting in a higher electron temperature. This higher electron temperature means that when light hits graphene it is very rapidly changed into electron heat. That “electron heat is converted into a voltage” according to the press release, and the whole process is nearly instantaneous in graphene.

“Graphene photodetectors show fascinating performance and properties, enabling a wide range of applications,” said Frank Koppens of ICFO in an article published by the Graphene Flagship. “Ranging from multi-spectral imaging to ultra-fast communications, such applications are being actively developed within the Graphene Flagship programme.”

World-record electric motor for aircraft

To slim down the end shield, Siemens developed a special optimization algorithm and integrated it into the Siemens CAE-Program NX Nastran.


Siemens researchers have developed a new type of electric motor that, with a weight of just 50 kilograms, delivers a continuous output of about 260 kilowatts – five times more than comparable drive systems. The motor has been specially designed for use in aircraft. Thanks to its record-setting power-to-weight ratio, larger aircraft with takeoff weights of up to two tons will now be able to use electric drives for the first time.

To implement the world-record motor, Siemens' experts scrutinized all the components of previous motors and optimized them up to their technical limits. New simulation techniques and sophisticated lightweight construction enabled the drive system to achieve a unique weight-to-performance ratio of five kilowatts (kW) per kilogram (kg). The electric motors of comparable strength that are used in industrial applications deliver less than one kW per kg. The performance of the drive systems used in electric vehicles is about two kW per kg. Since the new motor delivers its record-setting performance at rotational speeds of just 2,500 revolutions per minute, it can drive propellers directly, without the use of a transmission. "This innovation will make it possible to build series hybrid-electric aircraft with four or more seats," said Frank Anton, Head of eAircraft at Siemens Corporate Technology, the company's central research unit. The motor is scheduled to begin flight-testing before the end of 2015. In the next step, the Siemens researchers will boost output further. "We're convinced that the use of hybrid-electric drives in regional airliners with 50 to 100 passengers is a real medium-term possibility," said Anton.

The development of this motor was supported by the German Aviation Research Program LuFo in a project of Grob Aircraft and Siemens.

In 2013, Siemens, Airbus and Diamond Aircraft successfully flight-tested a series hybrid-electric drive in a DA36 E-Star 2 motor glider for the first time. The test aircraft had a power output of 60 kW.

Since the new motor delivers its record-setting performance at rotational speeds of just 2,500 revolutions per minute, it can drive propellers directly, without the use of a transmission. "This innovation will make it possible to build series hybrid-electric aircraft with four or more seats," said Frank Anton, Head of eAircraft at Siemens Corporate Technology, the company's central research unit.

Thanks to its record-setting power-to-weight ratio, larger aircraft with takeoff weights of up to two tons will now be able to use electric drives for the first time.