Terahertz emission from an InGaAsN large area emitter

The simple operability of stable and compact fiber lasers motivated research on photoconductive THz emitters based on substrates that allow interband excitation with wavelengths up to 1.55 μm. Low-temperature grown InGaAs and ion-irradiated InGaAs, both grown lattice-matched on InP, have been used as substrate materials for dipole emitter antennas [1,2]. However, the resistivity of these materials is still too low for large-area emitters with interdigitated electrodes [3]. These structures prevent carrier excitation in every second spacing by an additional metallization or by etching of the substrate [4]. Hereby the excited elementary THz waves interfere constructive in the far field. These emitter designs combine the advantages of high bias fields and large active areas. Furthermore detection with elements of similar electrode geometry based on GaAs substrates with sub-picosecond carrier lifetimes and resistivity in the 106 Ω cm range has been demonstrated [5]. Here we present such large-area emitters based on InGaAsN which show efficient THz emission for excitation wavelengths up to 1.35 μm.
The substrate material consists of a 1000 nm Ga1-yInyAs1-xNx (y=0.11 and x=0.04) layer grown by molecular-beam epitaxy on semi-insulating GaAs. On top there is an additional 60 nm thick Al0.3Ga0.7As barrier layer followed by a 5 nm GaAs cap layer. Transmission measurements with a Fourier spectrometer reveal a bandgap corresponding to a wavelength of 1.5 μm. The esistance of a complete device with an active area of 1 mm2 is 0.3 MΩ. This allows operation with high bias fields (30 kV/cm) without being limited by heating. For excitation an optical parametric oscillator (OPO), tunable between 1.1 μm and 1.5 μm, is used. The pulse duration is 280 fs. The THz signal is detected by electro-optical sampling using a 1 mm thick ZnTe crystal. The gating beam (λ = 820 nm) for detection is split off from a Ti:sapphire oscillator which drives the OPO.
The THz transient is measured with lock-in technique and has a signal-to-noise ratio of 400 obtained with 100 ms integration time constant. The bandwidth of the emitted radiation is 2.5 THz and is limited by the pulse duration of the OPO. Varying the OPO wavelength at fixed excitation power results in constant THz signals for wavelengths below 1.3 μm. For excitation wavelengths of 1.4 μm the signals are about 5 times smaller as compared to the shorter wavelengths. Furthermore we compare the InGaAsN emitter with an emitter based on semi-insulating (SI) GaAs when both are excited at 800 nm. Here the SI-GaAs emitter shows an 8 time higher THz field then the InGaAsN emitter at the same excited carrier density. We attribute this to the high electron obility in the SI-GaAs substrate.
In summary the THz emission indicates a high transient conductance of the InGaAsN material. The high resistivity makes it suitable for large area antennas with interdigitated electrode geometry.
References
[1] M. Suzuki and M. Tounouchi, Appl. Phys. Lett. 86, 163504, 2005
[2] A. Takazato, M. Kamakura, T. Matsui, J. Kitagawa, and Y. Kadoya, Appl. Phys. Lett. 91, 011102 ,2007
[3] A. Dreyhaupt, S. Winnerl, T. Dekorsy, and M. Helm, Appl. Phys. Lett. 86, 121114, 2005
[4] M. Awad, M. Nagel, and H. Kurz, Appl. Phys. Lett. 91, 181124, 2007
[5] F. Peter, S. Winnerl, S. Nitsche, A. Dreyhaupt, H. Schneider, and M. Helm, Appl. Phys. Lett. 91, 081109, 2007