Highly efficient THz emission from differently grown InN at 800 nm and 1060 nm excitation G. Mattha ¨us a, * , V. Cimalla b , B. Pradarutti c , S. Riehemann c , G. Notni c , V. Lebedev b , O. Ambacher b , S. Nolte a , A. Tu ¨ nnermann a,c a Friedrich Schiller University Jena, Institute of Applied Physics, Max-Wien-Platz 1, Jena D-07743, Germany b Technical University Ilmenau, Institute of Micro- and Nanotechnology, Kirchhofstrasse 7, Ilmenau D-98693, Germany c Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Strasse 7, Jena, D-07745, Germany Received 19 November 2007; accepted 16 March 2008 Abstract A detailed study on differently molecular-beam epitaxy (MBE) grown InN wavers as THz surface emitters is reported. The samples were excited using 120 fs and 100 fs short laser pulses delivered by a Ti:Sapphire oscillator at 800 nm and a fiber laser amplifier at 1060 nm, respectively. The InN emission properties are compared to a p-type InAs reference sample. At 800 nm, atomically smooth InN with low background electron concentration exhibits slightly stronger THz emission than the well-established p-InAs emitter. This high THz efficiency of InN is reported for the first time. The strong emission of InN is caused by the absence of any intervalley scattering, which in the case of InAs, increases the effective mass of the photogenerated electrons and, thus, reduces the photo-Dember effect, which is most responsible for THz emission. Consequently, InN is a reliable material for strong THz emission. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Terahertz; Surface emitter; InN; Ultrafast lasers 1. Introduction The terahertz (THz) region of the electromagnetic spec- trum (10 11 –10 13 Hz) has potential applications in many fields of science and technology, extending from spectros- copy, medical imaging, nondestructive materials testing through homeland security [1–5]. It was found by Zhang et al. [6] in 1990 that ultrashort THz pulses can be gener- ated by illuminating semiconductor surfaces with femtosec- ond laser pulses. Typically InAs is used as the emitter material, however, also the emission of THz radiation from InN using a common Ti:Sapphire laser has been demon- strated [7]. The potential of these narrow band gap semi- conductors (note that the band gap for InN has been revised from 1.9–2 eV to less than 0.8 eV, recently [8]) is the use of powerful fiber laser systems working at longer wavelengths, in particular at 1060 nm [9] or 1550 nm (the telecommunication regime) [10]. In contrast to conven- tional Ti:Sapphire systems, fiber lasers offer higher average power and can be designed as most compact systems [11]. InN provides a very strong absorption coefficient (10 4 cm 1 , [12]), a high saturation velocity (>1.5  10 7 cm/s, [13]) and has an extraordinary energy band struc- ture yielding high electron mobilities (l n  3500 cm/Vs, [14]) over a wide range of excitation wavelengths. Hence, a more efficient InN-based THz source compared to the commonly established arsenides like GaAs or InAs could be expected. Here, we present a detailed investigation of differently MBE grown InN layers as THz surface emitters excited at 800 and 1060 nm. We compare the InN measurements to a p-doped InAs emitter, which is known as the strongest surface emitter [15]. All InN layers show significant THz 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.03.057 * Corresponding author. Tel.: +49 3641 947824; fax: +49 3641 947802. E-mail address: matthaeus@iap.uni-jena.de (G. Mattha ¨us). URL: http://www.iap.uni-jena.de (G. Mattha ¨us). www.elsevier.com/locate/optcom Available online at www.sciencedirect.com Optics Communications 281 (2008) 3776–3780