December 1, 2010 / Vol. 35, No. 23 / OPTICS LETTERS
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Compact diode-laser-based system for continuous-wave and quasi-time-domain terahertz spectroscopy Carsten Brenner,1,* Martin Hofmann,1 Maik Scheller,2 Mohammad Khaled Shakfa,2 Martin Koch,2 Iván Cámara Mayorga,3 Andreas Klehr,4 Götz Erbert,4 and Günther Tränkle4 1
Lehrstuhl für Photonik und Terahertztechnologie, Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany 2
4
Fachbereich Physik, Philipps Universität Marburg, Renthof 5, 35032 Marburg, Germany 3 Max-Planck-Institut für Radio Astronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Strasse 4, 12489 Berlin, Germany *Corresponding author:
[email protected] Received June 7, 2010; revised October 21, 2010; accepted October 22, 2010; posted October 27, 2010 (Doc. ID 129334); published November 17, 2010 We present a multimodal diode-laser-based terahertz (THz) spectroscopy system. In contrast to other laser-based THz setups that provide either cw or broadband THz generation, our configuration combines the advantages of both approaches. Our low complexity setup enables fast switching from cw difference frequency generation to broadband THz emission, enabling sophisticated data analysis like much more complex time domain spectroscopy systems. © 2010 Optical Society of America OCIS codes: 070.6020, 140.5960, 170.6795, 300.6495.
Terahertz (THz) radiation sources range from synchrotron facilities with great performance and enormous cost down to millimeter-sized quantum cascade lasers [1–3]. Apart from established techniques based on Fourier transform interferometry on the short wavelength side and electronic devices in the higher gigahertz regime, most groups use either time-domain spectroscopy (TDS) based on femtosecond lasers [4–8] or cw systems based on photomixing with different underlying systems [9–13]. While TDS systems provide spectrally resolved amplitude and phase information, cw systems deliver only single frequency information at a time. Yet TDS systems are complex and expensive owing to the femtosecond laser source, while diode-laser-based cw systems are compact and cost effective. Recently, Scheller and Koch demonstrated a quasi-TDS (QTDS) system based on a cw multimode laser diode that provides information comparable to that of TDS systems [14]. But so far, to our knowledge, no system has been reported that is able to combine TDS and cw options for fast adaptation to a specific application. In this Letter, we introduce such a THz system based on an external cavity diode laser. Our approach is based on a Fourier-transform external-cavity laser (FTECAL), as shown in Fig. 1. The laser has a double-quantum-well active region with a symmetric large optical cavity waveguide of 3:2 μm thickness to reduce the vertical far-field angle. The length of the diode laser is 2 mm with an antireflection-coated front facet (R < 10−4 ). The specific feature of this external cavity setup is that the different emission frequencies of the diode laser are spatially separated by a grating and focused with a lens onto an end mirror with a patterned reflective coating. This enables shaping of the emission spectrum of the laser by selecting only certain optical wavelengths depending on the perpendicular position of the patterned mirror that is hit by the beam. More details about the used laser geometry are given in [15]. A tapered amplifier (TA) is implemented to boost the output power to the optimal level for driving the photoconducting antennas (PA). As we introduce no mode locking, the output of the FTECAL, and therefore of 0146-9592/10/233859-03$15.00/0
the TA, is not pulsed. Because of the mode beating of the high number of external resonator modes with a spacing of 300 MHz, the output appears to be continuous. For THz measurements, the output of the TA is split into two parts (probe arm and reference arm) and fed onto two PAs, as described in [16]. To measure amplitude and phase information, the reference arm of the setup contains a delay stage. The PAs consist of a high-efficiency photoconductive material fabricated by defect engineering in GaAs [17,18]. The properties of the material were tuned to achieve trade-offs among quantum efficiency, ultrafast trapping time, and durability. The metallization consists of a smallarea Ti/Au metal–semiconductor–metal structure feeding a planar logarithmic periodic antenna patterned by electron-beam lithography. The optical excitation power for the emitter and receiver antenna was 30 mW, while a square wave of amplitude 10 V was applied to the transmitter to enable lock-in amplification of the photocurrent at the receiver. A similar setup has already been shown to enable twocolor operation for THz generation with tunable difference frequency [19]. The emission spectrum of our laser, the THz waveform measured at the receiver PA, and the corresponding THz spectrum for two-color operation are shown in Fig. 2. The THz spectrum shows the absolute value of the fast Fourier transformation of the THz transient [jXðjωÞj]. As the spectral selectivity of the external cavity is not enough to select pure two-color operation (two external cavity modes only), the laser is operating in the so-called semi-coherent regime [20], which leads to a beating signal in the THz transient at the receiver and thus a broadening of the THz spectrum owing to externalcavity mode competition, as can be seen in Fig. 2. Thus, this setup can be used for THz spectroscopy applications where moderate spectral resolution is sufficient. By changing the vertical position of the end mirror in the external cavity, the operation of the FTECAL can be switched to a broadband operation with an optical bandwidth in the THz regime. This operation yields the superposition of many beat frequencies of the optical modes in © 2010 Optical Society of America
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Fig. 1. (Color online) FTECAL and scheme for THz homodyne detection.
the THz path. Since all competing frequencies are multiples of the round-trip frequency of the external cavity, the obtained THz signal at the receiver antenna exhibits pulselike features, although the laser is operating cw (see [14] for a mathematical description). Figure 3 shows the diode laser emission spectrum, the THz waveform, and the THz spectrum for this case. It can be clearly seen that a broadband THz spectrum is obtained that extends up to more than 0:5 THz. The chirp of the signal we attribute to the beam characteristics of the PA, resulting in different beam paths for the high- and low-frequency parts of the spectrum. The acquisition time of one waveform is around 20 s and is limited by the speed of the mechanical delay line. The advantage of our setup over the use of a multimode diode laser without an external cavity is the much higher flexibility, since we can shape the optical spectrum nearly arbitrarily. This has a direct consequence on the emitted THz spectrum, as the emission contains all possible beating frequencies of the different optical modes. A broad spectrum with an FWHM of 2 nm can be seen in Fig. 3. This shape yields a very homogeneous THz spectrum and a TDS-like behavior in our setup. It should be noted that, for QTDS operation, it is not necessary that the laser emits all different modes simultaneously. The random emission of several colors at one moment leads to the same signal as a phase synchronized emission in a pulsed system, since the measurement of the photocurrent at the receiver antenna is an average
Fig. 2. (Color online) Two-color operation of the FTECAL with (a) experimental setup, (b) optical spectrum, (c) THz transient, and (d) THz spectrum.
Fig. 3. (Color online) Multimode operation of the FTECAL. (a) Experimental setup, (b) optical spectrum, (c) THz transient, and (d) corresponding THz spectrum.
over the competing modes in the entire spectrum, as was shown in detail in [14]. To demonstrate a spectroscopic measurement, we investigate a dielectric THz mirror, the transmission spectrum of which is well structured (see [21] for details). The comparison between our QTDS system and a standard TDS is shown in Fig. 4. While the signal-to-noise ratio is significantly higher in the TDS system owing to the higher peak intensity, the major spectral features of the mirror can be reproduced, except for the frequency shift between the two measurements, which we attribute to the inhomogeneities of the sample. Because of the comparable shape of TDS and QTDS signals, many algorithms from the time-domain system can be applied to our system for extraction of parameters such as refractive indices or sample thickness [22]. Next, we compare the THz power levels for cw and QTDS operation at the emitted frequencies. The corresponding data are shown in Fig. 5. The overall THz power of the system is limited by the damage threshold of the PAs. Therefore, the spectral intensity of the individual frequency components in the broadband spectrum in QTDS operation is significantly reduced, as compared to the narrow spectrum in cw operation. The signal-to-
Fig. 4. (Color online) Comparison of standard TDS (red dashed curve) and QTDS (black solid curve).
December 1, 2010 / Vol. 35, No. 23 / OPTICS LETTERS
Fig. 5. Comparison of two-color cw operation and QTDS operation.
noise ratio of the QTDS amplitude can be estimated to be 3 orders of magnitude. However, since the optical spectrum can be shaped in our system, it is possible to suppress parts of the spectrum that are not needed by utilizing an appropriate mirror structure. Thus, the THz bandwidth could be extended by reallocating energy from the lower THz frequency components to the higher ones. In general, the optical spectrum can be shaped exactly for the needs of a specific application, thus providing optimum power levels at the required THz frequencies. In conclusion, we have presented a diode laser system that is able to bridge the gap between cw operation and pulsed THz generation for TDS applications. The change between the operation regimes is a matter of seconds and standard algorithms for parameter extraction can be applied. Even further optimization for different applications can be accomplished by adapting the optical spectrum to specific needs. References 1. W. B. Colson, Nucl. Instrum. Methods Phys. Res. A 475, 397 (2001). 2. R. Köhler, A. Tredicucci, F. Beltram, H. Beere, E. Linfield, A. Davies, D. Ritchie, R. Iotti, and F. Rossi, Nature 417, 156 (2002). 3. A. Barkan, F. Tittel, D. Mittleman, R. Dengler, P. Siegel, G. Scalari, P. Siegel, G. Scalari, L. Ajili, J. Faist, H. Beere, E. Linfield, A. Davies, and D. Ritchie, Opt. Lett. 29, 575 (2004).
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