a1061_1.pdf
CThR2.pdf
THz Time-Domain Spectrometer Based on LT-InGaAs Photoconductive Antennas Exited by a 1.55 µm Fibre Laser Rafal Wilk1, Martin Mikulics1, Klaus Biermann2, Harald Künzel2, Ida Z. Kozma3, Ronald Holzwarth3, Bernd Sartorius2, Michael Mei3, and Martin Koch1 1
Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Schleinitzstraße 22, D-38106 Braunschweig, Germany 2 Fraunhofer-Institute for Telecommunications, Heinrich-Hertz-Institute, Einsteinufer 37, 10587 Berlin, Germany 3 Menlo SystemsGmbH, Am Klopferspitz 19, 82152 Martinsried, Germany
[email protected]
Abstract: We present a THz time-domain spectrometer based on a 1.55 µm fibre laser and LTInGaAs/InAlAs MQW photoconductive antennas. We discuss the system stability and present first spectroscopic data taken with the system. ©2007 Optical Society of America OCIS codes: 140.3510 Lasers, fiber; 140.4050 Mode-locked lasers, 300.6340 Spectroscopy, infrared
1. Introduction Terahertz (THz) time-domain spectroscopy (TDS) is already well developed. Most THz TDS systems are based on bulky and expensive femtosecond Ti:sapphire lasers emitting around 800 nm, photoconductive GaAs emitter antennas and low temperature (LT) grown GaAs receiver antennas. As an alternative to photoconductive antennas, optical rectification and electro-optic detection can be used for generation and detection, respectively. However, the wide commercial use of THz TDS systems is hindered yet by the high cost of Ti:sapphire lasers. Fibre lasers at 1.55 µm are typically less expensive as they are based on low-cost telecom components developed for long-term operation. So far only a few attempts have been reported to utilize this well developed technology for THz TDS [16]. Since GaAs and LT-GaAs cannot be excited efficiently with wavelengths above 0.87µm, other material systems have to be explored. Reported examples include Fe-implanted [2,3] and Br-irradiated [4] InGaAs. Furthermore, surface generation on InSb, InAs and InGaAs has been reported [5]. Finally, optical rectification and electro-optical detection were demonstrated using (100) GaAs and ZnTe [6]. Here, we employ photoconductive antennas produced from a novel material system comprising LTGaInAs/AlInAs quantum wells. With these antennas we efficiently generate and detect THz pulses with an excitation wavelength of 1.55 µm. 2. Experimental Setup Our THz TD spectrometer is based on a commercially available turn-key femtosecond fibre laser (provided by Menlo Systems GmbH). The erbium-doped fibre-ring laser delivers sub-150-fs, near-transform-limited pulses centred at 1.55 µm. The average output power is 130 mW at a repetition rate of 100 MHz. An embedded microcontroller monitors and regulates the passive mode-locking mechanism, resulting in a high long-term stability. The photoconductive material has to be adjusted to the laser emission spectrum. In our novel approach we apply an InGaAs/AlInAs multi quantum well structure grown by molecular beam epitaxy (MBE) at low temperature (
