Confined electrons live longer

Aug 18, 2009
TEM picture of a quantum dot on a gallium arsenide layer. On top is a glue layer due to TEM preparation only. Image: University of Sheffield

Electrons that are trapped in very small structures of only a few nanometer, demonstrate fascinating features. These could be useful for novel computers or semiconductor lasers. Researchers from the University of Sheffield, the Ecole Normale Supérieure in Paris, and the Forschungszentrum Dresden-Rossendorf research center measured for the first time the exact lifetime of excited electrons and published their findings in the journal Nature Materials.

For many applications it is highly desirable that electrons, excited to a higher state, take a long time until they relax back to the ground state. This is a key ingredient for any kind of laser, but also would be desirable for modern applications in quantum information processing (where also the phase coherence should be conserved).

Starting about 20 years ago, researchers have been able to grow so-called on standard semiconductor substrates, such as gallium arsenide (the material used e.g. in CD players). These dots are tiny pyramids, containing typically between 1,000 and 10,000 atoms of a different semiconductor material than the substrate in which they are embedded. As the volumes of the dots are extremely small, the electrons follow quantum-mechanical rules and are supposed to enter only sharply defined energetic states. Furthermore, the electrons are confined in all three directions, and thus they represent a kind of artificial atom, which could become a building block of revolutionary future (opto-)electronic devices.

At that time it was predicted that excited electrons should live for a very long time in these quantum dots, since they hardly find any ways in which to lose their energy. For many years it has remained a puzzle why such long lifetimes, also called the “phonon bottleneck” at that time, were never observed. Further work a few years back has shed new light on this issue: Due to the strong confinement of the electrons, the well known theory describing the loss of energy of electrons to lattice vibrations (phonons) is not applicable, since the form entities which are strongly coupled with phonons, so-called polarons.

Now, taking seriously the predictions of this new theory, researchers from University of Sheffield, UK, Ecole Normale Superieure in Paris, France, and Forschungszentrum Dresden-Rossendorf in Germany have designed quantum dots which allow a rigid test of the theory over a wide parameter range. By making the separation of the energy levels in the quantum dots significantly smaller than the energy of the most important lattice vibration, they were able to observe lifetimes which differed by a factor of thousand for an energy separation which only varied by a factor of two. In numbers, the relaxation time increased from few picoseconds (a millionth of a millionth of a second) to nanoseconds (a thousandth of a millionth of a second), when reducing the electron energy only by half. These long lifetimes, although being of different origin than the originally proposed “phonon bottleneck”, could open a wealth of applications, in particular for terahertz (THz) devices based on quantum dots. The reason for this lies in the fact that the relevant energy level separation is of the order of 10-20 milli-electronvolt (meV), which can be expressed as a frequency of a few THz.

In order to accurately measure these lifetimes, the researchers used a unique type of short-pulse terahertz laser, a so-called free-electron laser (FEL), located at the Forschungszentrum Dresden-Rossendorf. In this free-electron laser high-intensity infrared and terahertz pulses can be generated at a wide range of wavelengths (or frequencies) to fit many kinds of scientific problems in physics, chemistry and biology. In this collaboration, the access of the UK researchers to this FEL facility was supported by the EU through a transnational access programme.

More information: “Long lifetimes of quantum-dot intersublevel transitions in the terahertz range”, E. A. Zibik(1), T. Grange(2), B. A. Carpenter(1), N. E. Porter(1), R. Ferreira(2), G. Bastard(2), D. Stehr(3), S.Winnerl(3), M. Helm(3), H. Y. Liu(4), M. S. Skolnick(1), L. R.Wilson(1), in: Nature Materials, Advance Online Publication (AOP), 16 August 2009, DOI: 10.1038/NMAT2511

Source: Forschungszentrum Dresden Rossendorf

Explore further: New absorber will lead to better biosensors

add to favorites email to friend print save as pdf

Related Stories

Laser light in the deep infrared

Aug 23, 2006

Free-electron lasers (FEL) are large and expensive, but they can deliver unique light for research and applications. On August 21, 2006, at the Forschungszentrum Rossendorf (FZR) in Dresden, Germany, the second ...

Fast quantum computer building block created

Aug 20, 2008

(PhysOrg.com) -- The fastest quantum computer bit that exploits the main advantage of the qubit over the conventional bit has been demonstrated by researchers at University of Michigan, U.S. Naval Research Laboratory and ...

Quantum dot lasers -- 1 dot makes all the difference

Apr 12, 2007

Physicists at the National Institute of Standards and Technology and Stanford and Northwestern Universities have built micrometer-sized solid-state lasers in which a single quantum dot can play a dominant ...

Quantum electronics: Two photons and chips

Jan 20, 2006

Scientists at Toshiba Research Europe Limited (Cambridge, UK) believe they are on to a way of producing entangled twins of photons using a simple semiconductor electronic device. Such a chip-based source of entangled photons ...

Recommended for you

New absorber will lead to better biosensors

1 hour ago

Biological sensors, or biosensors, are like technological canaries in the coalmine. By converting a biological response into an optical or electrical signal, they can alert us to dangers in our external and internal environments. ...

Ultrafast remote switching of light emission

22 hours ago

Researchers from Eindhoven University of Technology can now for the first time remotely control a miniature light source at timescales of 200 trillionth of a second. They published the results on Sept. 2014 ...

Nanotube cathode beats large, pricey laser

Sep 30, 2014

Scientists are a step closer to building an intense electron beam source without a laser. Using the High-Brightness Electron Source Lab at DOE's Fermi National Accelerator Laboratory, a team led by scientist ...

User comments : 0