Jul 31, 2013
Terahertz emitters adapted for excitation at telecoms wavelengths (1.55 µm)
Terahertz (THz) time-domain spectroscopy (TDS) is a common technique for the investigation of many material systems. Typically, one of the most expensive parts of THz TDS systems is the laser providing ultrashort pulses, but new materials could allow cheaper lasers to be used and make systems more affordable.
In TDS systems, THz pulses are produced by generating free carriers in semiconductors with an ultrashort laser pulse. The free carriers are accelerated by the electric field of a bias voltage and thereby emit THz radiation. The most commonly used semiconductor material is GaAs that is excited by a Ti:sapphire laser. As these laser systems are quite expensive it is very interesting to develop TDS systems that can utilize cheaper laser systems such as fiber lasers emitting at the telecommunication wavelength of 1.55 µm. In this scenario, a low bandgap material like InGaAs is attractive, but there are issues to overcome. Typical InGaAs epitaxial layers are characterized by a resistivity, which is too low to apply a sufficient bias voltage.
To circumvent these restrictions, heterostructures have been designed to combine a low bandgap with a high resistivity. Trap states and Be acceptors for the carriers reduce the carrier density and the carrier lifetime. Up to now these materials have been used in antenna-based emitters. Due to the small active area in the center of the antenna, which is around 10 x 10 µm2, the saturation power of these emitters is below 10 mW, while fibre laser systems usually exceed output powers of 100 mW. To avoid this low saturation power the team, which includes scientists from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Heinrich Hertz Institute (HHI), have combined the heterostructured material with a large area interdigitated pattern with an area of up to 300 x 300 µm2.
With these new emitters the generation of strong broadband THz pulses with a spectral range from 0.1 THz to 3 THz has been demonstrated. The saturation power is increased above 100 mW so that the full power of a fibre laser can be exploited. The spectral content of the emitted THz pulse is very stable against changes in the bias voltage or the pump power which is beneficial for spectroscopy measurements.
More information can be found in the journal Nanotechnology 24 214007
About the author
Martin Mittendorff is currently working towards a PhD degree at the Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany. His cresearch interests include THz pulse generation and ultrafast spectroscopy of graphene, especially in magnetic fields.