Two distinct technologies- metamaterials and quantum cascade structures- have been combined for the first time to create a novel and very thin form of light detector.
The delicate interactions between electrons and light make for valuable technological characteristics. Ultra-thin systems of semiconductor layers, for example, can turn electrical voltage into light. They can also be used the other way around, to serve as light detectors.
But until now, it has proved difficult to couple light into these layered semiconductor systems. At Vienna University of Technology, scientists solved this problem by using metamaterials. Metamaterials have the ability to manipulate light in the terahertz range, due to their unusual microscopic structure.
Ultrathin Layered Semiconductor Systems
“Ultra-thin layered semiconductor systems have the great advantage, that their electronic properties can be very precisely tuned”, said Professor Karl Unterrainer.
Through the selection of suitable materials, and tuning the thickness of the layers and the geometry of the device, the behaviour of the electrons in the system can be influenced. In this way, quantum cascade lasers can be built, where the electrons jump from layer to layer and emit a photon with each jump.
Light detectors can be created too, with a tunable sensitivity to one exacting wavelength.
Unfortunately, the laws of quantum physics disallow photons with a certain direction of oscillation, or polarization, from interacting with the electrons of the semiconductor system.
Light hitting the layered surface directly perpendicular to it, cannot influence the electron in the semiconductor. As a result, some method is needed to rotate the polarization of the incident light, so that it can be detected in the semiconductor layers.
This is where metamaterials come in. Metamaterials are artificial structures made up of individual sub-wavelength resonators, where the electric and magnetic resonances are defined by the geometry of its constituents.
A metamaterial has an ordered geometric structure, with a periodicity less than the wavelength of the incident light. The light is scattered according to the structure’s geometry, some wavelengths could be absorbed, while other wavelengths are reflected.
The fascinating play of colors on a butterfly’s wings derives from precisely this effect. Metamaterials on top of a semiconductor structure can rotate the incoming light’s polarization so it can interact with the electrons inside.
In this experiment, the light source used had a significantly longer wavelength than visible light. Its radiation was in the terahertz, or infrared range; a wavelength of around a tenth of a millimetre.
This kind of radiation has important technological applications, for example next-generation computer technology. It is quite challenging, however, to work with this type of wave.
The discovery of the meta-material technique opens up possibilities for integrating a light detector for terahertz radiation into a chip.
“With conventional fabrication methods, large arrays of such detectors can be built,” says Unterrainer.
The light detectors do not take up much space, since layers with a thickness in the range of nanometers are enough to detect light. The detector is more than a thousand times thinner than the wavelength of the light which is being detected.
A. Benz, M. Krall, S. Schwarz, D. Dietze, H. Detz, A. M. Andrews, W. Schrenk, G. Strasser, K. Unterrainer. Resonant metamaterial detectors based on THz quantum-cascade structures. Scientific Reports, 2014; 4 DOI:10.1038/srep04269
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