Large-area Terahertz Emitters based on GaInAsN

The high price and the complexity of commonly used titanium-sapphire lasers hinder a more widespread use of pulsed THz systems. Er-doped fiber lasers can be a promising alternative. Since the band gap of GaAs is larger than the photon energy at 1.55 µm, this material – while being the standard material for photoconductive THz antennas excited with titanium-sapphire lasers – is not suitable for systems driven by fiber lasers. Dipole antennas have been successfully demonstrated on low-temperature grown InGaAs [1] and ion-irradiated InGaAs [2]. However, the resistivity of these small-gap materials is too low for microstructured large-area emitters. Such emitters consist of an interdigitated finger structure on the semiconductor substrate and a second metallization preventing destructive interference from THz wavelets excited in regions with opposite direction of the bias field [3]. Since these microstructured large-area emitters can withstand large excitation power and high bias fields, they show improved efficiency and higher THz power as compared to conventional antennas.
A 1000 nm thick Ga0.89In0.11As0.96N0.04 layer was grown lattice matched on semi-insolating GaAs by molecular beam epitaxy. On top, additional layers of 60 nm AlGaAs and 5 nm GaAs were grown, resulting in a higher resistivity of the substrate. A THz emitter with an active area of 1 mm  1 mm with a microstructure similar to the one described in ref. 3 was produced using standard optical lithography. The emitter was excited with radiation from an optical parametric oscillator (OPO; tuning range: 1.1 - 1.5 µm). The THz radiation was detected by electro-optic sampling in a ZnTe crystal gated with an 800 nm beam from a titanium sapphire laser. Strong THz emission is observed for excitation wavelength below 1.35 µm [4]. No saturation effects occur within the available range of average power (up to 50 mW) of the OPO. Furthermore the GaInAsN emitter was compared with an emitter based on semi-insolating GaAs at an excitation wavelength of 800 nm. The THz field amplitude of the GaAs emitter is eight times higher as compared to the GaInAsN emitter. Again, in the available excitation power range (up to 0.5 W), no saturation effects are observed.
In summary an efficient and easy-to-align microstructured THz emitters based on GaInAsN has been demonstrated. While further improvement of the material is necessary for excitation at 1.55 µm, the material studied here is suitable for fiber lasers operating at 1.1 µm.

References

[1] M. Suzuki and M. Tounouchi, Appl. Phys. Lett. 86, 163504, (2005).
[2] A. Takazato, et al., Appl. Phys. Lett. 91, 011102, (2007).
[3] A. Dreyhaupt, et al., Appl. Phys. Lett. 86, 121114 (2005).
[4] F. Peter et al., Appl. Phys. Lett. 93, 101102 (2008).