| Selective laser excitation of gallium arsenide (GaAs) leads to different power flows (JP, JS) in the semiconductor. The associated radiated electromagnetic fields overlap, and the sum total field E propagates at an angle (theta) from the surface normal. A change in the polarization of the excitation laser changes the angle of the total field due to a change in the direction of the current JP. (Image: PTB) |
Braunschweig, Germany--Researchers at the Physikalisch-Technische Bundesanstalt (PTB) have invented a way to easily steer a terahertz beam generated by a laser exciting a semiconductor. They demonstrated that exciting different directions of current flow in the semiconductor steers the generated terahertz beam. Rapid steering of terahertz radiation will be useful in, for example, body scanners and other security-based applications.
In the PTB experiments, ultrafast laser pulses produce two different currents flowing parallel and perpendicular to the surface of the semiconductor. The radiated terahertz field results from both currents, because of their polarization properties and areas of mutual gain or attenuation. From these interference effects, directional emission is produced. Because the direction of flow parallel to the surface current can be reversed by changing the polarization of the laser pulse (the vertical flow component is not polarization dependent and remains unchanged), the regions of gain and attenuation of the electromagnetic fields can be reversed, and radiation direction changed.
In initial experiments, a change in direction of the maximum terahertz field strength of up to 8° was measured. The angle of deflection depended on different excitation parameters and could be tailored. The dominant influences were determined, allowing optimization via a theoretical model. As a result, the maximum deflection angle can be increased, for example by using other semiconductor material systems, special silicon lenses, or external magnetic fields.
The researchers' findings are published in Applied Physics Letters.
Source: http://www.ptb.de/de/aktuelles/archiv/presseinfos/pi2013/pitext/pi130627.html