Ultrafast Detection of THz Radiation with Large Area Field Effect Transistors

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The proposed project aims for the development of a new ultrafast THz detector. The detector will be extremely broadband with an operation range covering almost the complete THz band (0.1 THz- 7 THz) with a single device. It will also be able to detect optical and THz pulses synchronously at a resolution better than 30 ps. Such detectors are key elements for aligning and calibrating optical pump-THz probe and THz pump-optical probe setups. The detector concept further provides a high damage threshold allowing for implementation in high power table-top pulsed systems and free electron lasers. The detector consists of an array of rectifying field effect transistors (large area FETs, LA-FETs) in parallel connection. The rectification effect results from simultaneous modulation of the carrier concentration in the gated region and the carrier velocity in the channel by an incident THz wave that is polarized parallel to the source-drain direction. This effect remains highly efficient at THz frequencies orders of magnitude above the current and power amplification 3 dB frequency of the respective transistor. The sensitivity to optical/NIR pulses results from the photocurrent due to an external DC source drain bias, separating optically generated carriers. First proof-of-principle experiments using (Al)GaAs high electron mobility transistors have demonstrated the high prospect of these detectors. The proof-of-principle experiments, however, have opened a series of questions that we want to answer within the proposed project: It is unclear which part of the device is responsible for the fast optical response. Furthermore, light needs to be shed into the details of the electromagnetic interaction between the THz pulses and the FETs. This proposal aims for developing a consistent model for both optical and THz detection, taking excitation of and interaction with plasmons in the two dimensional electron gas channel into account. It will be based on a transmission line model. Subsequently, we will optimize and test a set of devices for improved functionality at the free electron laser FELBE, Helmholtz-Zentrum Dresden-Rossendorf. This includes optimization with respect to ultra-high speed, high linearity range, high damage threshold, low noise equivalent power, and large operation frequency range. The results of this work will have high impact on future optical pump-THz probe and THz pump-THz probe experiments since the detectors strongly simplify the alignment and calibration of such setups.Another set of LA-FETs will be optimized for application in table top systems as fast power meters. The responsivity will be improved at the expense of device speed. We target for developing detectors that are at least 5 orders of magnitude faster than state-of-the-art thermal detectors with a comparable or even lower noise equivalent power. This allows for extremely fast data acquisition.