Abstract
We demonstrate experimentally the increase of optical-to-terahertz conversion efficiency for GaAs-based photoconductive terahertz emitters. This increase is achieved by preventing device breakdown through series resistors, which act as a current limiter. Pulsed photoexcitation and potential current fluctuations result in heat dissipation leading to local heating, which further increases the current and may lead to device breakdown. We manage to increase the maximum bias field before device breakdown by a factor of 3 under illuminated conditions. For a laser system with 250-kHz repetition rate, the terahertz emission amplitude increases linearly with applied bias field up to 120 kV/cm bias field, which results in 3 times higher signal as compared to the standard device. Furthermore, we have also achieved this expanded breakdown prevention at 78-MHz repetition rate, where an integrated on-chip resistance leads to an enhancement of the terahertz field amplitude by 70%. This simple technique can increase the performance of almost all photoconductive terahertz emitters by using appropriate resistances according to the emitter capacitance and laser repetition rate.
Highlights
THz radiation falls in the energy range of many interesting excitations in biological, chemical, and physical processes [1, 2]
Common sources for THz spectroscopy at room temperature using moderate optical pump energies are THz emission from photoexcited semiconductors: unbiased devices like surface field emitters and photoDember emitters, and biased devices like photoconductive emitters (PCE)
The electric field amplitude of THz pulses can be controlled by the applied bias on the PCE emitter
Summary
THz radiation falls in the energy range of many interesting excitations in biological, chemical, and physical processes [1, 2]. Common sources for THz spectroscopy at room temperature using moderate optical pump energies are THz emission from photoexcited semiconductors: unbiased devices like surface field emitters and photoDember emitters (vertical and lateral), and biased devices like photoconductive emitters (PCE). The electric field amplitude of THz pulses can be controlled by the applied bias on the PCE emitter. PCEs offer a scattering-free high-frequency electrical chopping for lock-in detection. These properties make PCEs one of the most useful emitters for THz timedomain spectroscopy using mode-locked femtosecond lasers [3, 4]. The electron-hole pairs photo-generated by a sub-ps optical pulse in the photoconductor are accelerated by the bias field and emit THz radiation. The emitted field ETHz is proportional to the time-dependent change of the photocurrent IPC, ETHz∝
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