Abstract—Surface THz pulse emission from samples containing 40 layers of InAs quantum dots grown on a AlAs/GaAs Bragg reflector as well as the picosecond photoconductivity of devices manufactured from this material was investigated as a function of the optical wavelength of femtosecond pulses generated by an optical parametric amplifier. I. INTRODUCTION HZ time-domain-spectroscopy (TDS) systems are predominantly based on optoelectronic components manufactured from various semiconductor materials. Photoconductors made from materials with short carrier lifetimes serve as efficient THz radiation emitters and detectors; THz radiation can be generated relatively efficiently also using unbiased surfaces of semiconductor crystals illuminated by femtosecond laser pulses [1]. All these applications stimulate research into the THz response of various semiconductors, especially so since the advent of new ultrafast lasers operating at longer wavelengths. Recently, novel structures with multiple layers of semiconductor quantum dots (QD) are finding important applications in optoelectronic devices. Although three- dimensional confinement of charge carriers in QDs results in discrete electronic states, a large number of dots with different energy levels leads to carrier dynamics in such an ensemble that are strongly influenced by contributions from inter-dot diffusion and carrier trapping. Ultrafast processes in QD systems were investigated by THz spectroscopy in [2, 3]. Here we present the first results of spectral measurements of surface THz emission and picoseconds photoconductivity in QD layers. II. EXPERIMENTAL Samples were grown by molecular-beam-epitaxy on a (100) GaAs substrate. It had 40 layers of InAs QDs overgrown by InGaAs quantum wells and GaAs spacers, grown on top of a AlAs/GaAs Bragg reflector. The total thickness of QD and spacer region was 1400 nm. Our experimental system was based on an amplified Yb-doped potassium gadolinium tungstate (Yb:KGW) laser system (Light Conversion PHAROS) operating at 1030 nm with a pulse duration of 160 fs and pulse repetition rate of 200 kHz. This laser was used to drive a cavity-tuned optical parametric oscillator (OPO, Light Conversion ORPHEUS) generating 140-160 fs duration pulses with the central wavelength tunable from 640 nm to 2600 nm. For THz surface emission measurement, the sample was illuminated by different wavelength pulses generated by the OPO and the emitted THz signal was sampled by a THz detector made from low-temperature-grown GaAsBi, which was activated by part of the Yb:KGW laser beam. For the picosecond photoconductivity experiment, a Hertzian dipole antenna structure with a length of 90 μm and a photoconductive gap width of 5 μm was made on top of the QD layer. In this case, the gap was illuminated by the OPO beam at different wavelengths, whereas the conductivity of the gap was measured using THz pulse radiated by a p-InAs crystal excited by Yb:KGW laser pulses. Measured dependences are presented on the Figure 1. The spectra of both THz emission (triangles) and photoconductivity (squares) show peaks in the vicinity of the interdot electron transitions. The photoluminescence spectrum (gray line) is shown for reference. The origins of these dependences we will discuss at the conference. Figure 1. Measured spectra of THz emission (triangles) and photoconductivity (squares) from QD structure. III. CONCLUSION In conclusion, THz excitation spectra of samples containing InAs QDs were measured for the first time. Those measurements give insight into the possibilities of using this material in the emitters and detectors of optoelectronic THz radiation systems. G. Molis*, A. Arlauskas**, A. Krotkus**, R. Leyman***, N.Bazieva*** and E. Rafailov*** *Teravil Ltd, A.Goštauto 11, Vilnius, LT-01108, Lithuania **Center for Physical Science and Technology, Savanorių 231, LT-02300, Vilnius, Lithuania ***Photonics and Nanoscience Group, School of Engineering, Physics and Mathematics, University of Dundee, Dundee, DD1 4HN, UK THz emission spectroscopy of self-organized InAs quantum dot ensembles T 978-1-4673-1597-5/12/$31.00 ©2012 IEEE