Appl Phys B (2012) 106:81–84 DOI 10.1007/s00340-011-4774-y

Versatile spectrally shapeable multi-mode terahertz system M. Scheller · M. Stecher · M. Gerhard · M. Koch

Received: 4 March 2011 / Revised version: 23 August 2011 / Published online: 11 October 2011 © Springer-Verlag 2011

Abstract We present a multimode continuous wave terahertz spectrometer driven by a compact laser diode with a tunable spectrum. An external feedback based on two optical gratings into the diode allows us to shape the laser emission spectrum, and hence the temporal shape of the terahertz signals. In particular, we show a power enhancement of a factor of ten for individual regions of the spectrum to gain a signal-to-noise ratio in the frequency range of interest.

1 Introduction The research effort for terahertz (THz) spectroscopy is steadily increasing due to the tremendous potential of this technology. A myriad of application scenarios, ranging from non-destructive testing of industrial processes [1–4] over security related areas [5, 6] to the scanning of culture heritage [7, 8] are already demonstrated. One of the most powerful techniques is THz time domain spectroscopy (TDS). THz TDS relies on ultrashort femtosecond lasers which are used to generate broadband THz pulses with a bandwidth of up to several THz. The electrical field of the THz pulses can be detected coherently using a photoconductive detector antenna [9] or an electro-optic crystal [10]. In both cases, a fraction of the laser radiation is used to gate the THz detector. Since the spectral amplitude and phase of the wave can be detected, it is possible to determine the complex refractive index of the sample under investigation within the M. Scheller () · M. Stecher · M. Gerhard · M. Koch Fachbereich Physik, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany e-mail: [email protected] Fax: +49-(0)-64212822283

bandwidth given by the THz pulse. A comprehensive review on TDS and its application is given, e.g., in [9]. Drawbacks of THz TDS are the relatively high price and complexity of the required femtosecond laser system. A less cost-intense technique, which is often discussed as alternative, is photomixing which employs two frequency stabilized continuous wave (cw) lasers [11–13]. In this scheme, the THz antennas are gated by the emission of two lasers and a narrow band cw THz wave is emitted with a frequency which corresponds to the difference frequency between the laser lines. The setup for coherent detection of this cw THz emission is similar to that in THz TDS. Instead of pulses, a sinusoidal THz signal is detected. Nevertheless, the technique allows for the determination of the complex refractive indices of samples. Yet, each measurement reveals only information at a single frequency, and thus the measured phase information suffers from a 2π ambiguity [14]. To overcome this drawback, it is necessary to measure the sample at least at two different frequencies [15]. As a kind of hybrid approach, a multimode laser can be used to drive a photomixing system. In this case, several different THz frequencies are emitted simultaneously corresponding to the various frequency differences between the multiple laser lines involved. This approach, which is discussed in [16–19], bears the advantage of being fairly inexpensive. It provides valuable information from every single measurement. Using a laser source with equidistantly spaced laser lines, e.g., conventional semiconductor laser diodes, even allows for the detection of pulse-like signals similar to that observed in a TDS measurement with high signal-to-noise ratio [18]. Consequently, this technique can be referred to as THz quasitime domain spectroscopy (QTDS). While several applications like scientific spectroscopy require high signal bandwidths, there are also cases where spe-

82

cific frequency windows are of importance, e.g., to detect absorption bands or to increase the signal to noise ratio in the case of a noisy environment. In this case, it would be favorable to have a THz system with tunable spectral characteristics. This idea has been discussed by Maki and Otani [20] for the case of TDS systems. They used spatially chirped femtosecond laser pulses to generate narrow THz band signals with a bandwidth of about 50 GHz. In a similar study [21], it was shown that a THz spectrometer driven by an external cavity laser source can operate either in a single-frequency mode with a pure sinusoidal signal or in a QTDS mode by adjusting the frequency selective mirror of the external cavity. In this paper, we present a compact cost-effective QTDS system whose spectrum can easily be shaped. The system is based on a compact laser module, emitting a comb of equidistantly spaced laser modes. While most of its radiation is used to drive an emitter and detector antenna, a part is seeded back into the laser using diffractive gratings. Thus, a frequency selective feedback is established which has direct impact on the emitted laser spectrum. We show that this setup allows for conventional QTDS operation by blocking the feedback beam as well as for frequency tuning of the cw THz spectrum by tilting the gratings.

M. Scheller et al.

Fig. 1 Schematic of the THz system. The beam splitters BS1 and BS2 reflect approximately 10% of the laser power to the gratings G1 and G2 that are arranged in a Littrow configuration. The remaining laser power is divided equally by the beam splitter BS3 into two parts which are guided over mirrors (M) to the detector and emitter antenna, respectively. A linear stage is employed to introduce a variable time delay. Off-axis parabolic mirrors (P) span a THz beam path

2 Experimental setup For the experiments, we use a conventional THz photomixing setup as described in [18]. The laser source is a commercial module of the size of 0.5 × 2 which includes collimating optics. The module emits around 100 mW at 662 nm and has a mode spacing between the longitudinal laser modes of 25 GHz. Please note, using a different laser diode with a longer cavity length result in a smaller laser mode spacing. Without feedback, the spectral emission bandwidth is about 2 nm. For the “feedback arms,” two reflective gratings in Littrow configuration are used. To reduce the amount of back reflected light and enable for blocking and tuning, the feedback without changing the optical path to the antennas, we use two beam splitters to reflect approximately 10% of the laser power to the individual gratings. The setup is shown in Fig. 1. The laser beam passing the two beam splitters is divided into two parts. One is focused onto an emitter antenna, the other one onto the detector. Both antennas are dipole antennas fabricated on low-temperature grown gallium arsenide (LT GaAs). This particular antenna structure leads to an emission maximum at 350 GHz [22]. To record the THz waveforms, we employ a mechanical delay stage. Changing the path length between the emitter and the detector path allows for a coherent sampling of the THz signal [18]. To increase the signal-to-noise ratio, a lockin amplifier with a time constant of 20 ms is used. As bias

Fig. 2 Signal for the case of a free running laser when both gratings are blocked: (a) waveform, (b) spectrum

voltage of the emitter antenna we apply a square wave with an amplitude of 30 V and modulation frequency of 5 kHz.

3 Measurements and results First, we measure the THz waveform for the case of the intrinsic laser emission. To do so, we simply block the optical path to the two gratings. Consequently, the diode emits a broad spectrum with a width of about 2 nm. The recorded THz signal together with its spectrum is shown in Fig 2. As can be seen in the figure, the THz signal ranges from around 50 GHz to 600 GHz with a maximum around 350 GHz. The peak at 50 GHz is due to a parasitic resonance caused by reflections from the contact pads of the antennas. The reason for the roll off at higher frequencies is the dipole characteristics induced by the metallic antenna structure and the low pass characteristics of the LT GaAs antenna substrate caused by the finite free carrier lifetime [23]. By unblocking the feedback path of only one grating, the spectral width of the lasers emission and the number of emit-

Versatile spectrally shapeable multi-mode terahertz system

83

Fig. 3 Signal for the case of one blocked grating: (a) waveform, (b) spectrum

ted modes reduces significantly. This results in a noticeably smaller bandwidth of the THz signal as shown in Fig. 3. Since the feedback is only weak, some of the multiple laser modes survive. Yet, the power in these modes is reduced. While the main THz energy is condensed in the oscillation at 50 GHz, there are also weak higher THz modes present. In contrast to these two extreme cases with either a broadband or a narrow band low frequency signal, the release of the second feedback path opens the opportunity of tuning the spectrum more freely. In this case, the relative frequency difference between the two wavelengths for which the gratings provide feedback determines the shape of the THz waveform. The absolute optical wavelengths are of minor importance. So, we kept one grating at a fixed position while slightly tilting the other. In Fig. 4, we show three typical THz signals resulting from increasing the adjusted angle of the second grating in 1 degree steps. As can be seen in the figure, the spectrum of the THz signal exhibits a maximum that moves toward higher frequencies for larger tilt angles. Since the laser does not only emit two modes but a set of modes which are separated by 25 GHz, most of the signal energy is spread over only three to four THz frequency components. If a smaller number of THz modes would be favorable, the amount of back reflection into the laser could be increased by using either beam splitters with higher reflectivity or by placing the grating into the direct beam path in a Littman configuration [24]. Due to the concentration of energy in only a few THz modes, theses components tower over the ones from the measurement without feedback and leading to a significantly enhanced signal-to-noise ratio at these frequencies. The intensity of the centered THz modes is enhanced around ten times compared to the case without feedback. However, the total integral power of the signals shows only a week variation and depends on the frequency characteristics of the employed dipole antennas. The integrated power for an enhancement of the frequencies around 350 GHz (where the antennas exhibit the maximum gain) measures 1.45 times the power of the signal resulting from the free running laser.

Fig. 4 Signals for the case of feedback from both gratings. One of the gratings was kept at a constant angle while the angle of the other was titled in one degree steps. The resulting waveforms are shown on the left (a, c, e) and the corresponding frequency spectra on the right-hand side (b, d, f)

The ratio for an enhancement around 410 GHz and 500 GHz is 1.24 and 0.76, respectively. Thus, the accuracy of measurements in the case of a noisy environment can be enhanced in the frequency interval of interest. Since the signal still contains multiple THz frequencies, the approach does not suffer from the phase ambiguity known from single frequency cw systems as discussed in [15].

4 Conclusion In conclusion, we proposed a THz QTDS system with tunable spectral characteristics. The system is driven by a compact laser diode that’s emission spectrum is controlled by an

84

optical feedback based on two gratings. Titling one of the gratings moves the spectral maximum of the THz signal to the frequency interval of interest. Thus, the spectral signalto-noise ratio can be increased for the selected frequencies compared to the case of a free running laser. Using a weak feedback, the laser diode emits a low number of modes and THz signals with multiple frequency components result.

References 1. K. Yamamoto, M. Yamaguchi, M. Tani, M. Hangyo, S. Teramura, T. Isu, N. Tomita, Appl. Phys. Lett. 85, 5194 (2004) 2. F.M. Al-Douseri, Y. Chen, X.-C. Zhang, Int. J. Infrared Millim. Waves 27, 481 (2006) 3. T. Yasui, T. Yasuda, K. Sawanaka, T. Araki, Appl. Opt. 44, 6849 (2005) 4. C.D. Stoik, M.J. Bohn, J.L. Blackshire, Opt. Express 16, 17039 (2008) 5. H. Liu, Y. Chen, G. Bastiaans, X.-C. Zhang, Opt. Express 14, 415 (2006) 6. J. F Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, D. Zimdars, Semicond. Sci. Technol. 20, 266 (2005) 7. K. Fukunaga, Y. Ogawa, S. Hayashi, I. Hosako, IEICE Electron. Express 4, 258 (2007) 8. A.J.L. Adam, P.C.M. Planken, S. Meloni, J. Dik, Opt. Express 17, 3407 (2009) 9. P. Jepsen, D.G. Cooke, M. Koch, Laser & Photonics Rev. 5, 124 (2011)

M. Scheller et al. 10. Q. Wu, X.-C. Zhang, Appl. Phys. Lett. 67, 3523 (1995) 11. S. Matsuura, M. Tani, K. Sakai, Appl. Phys. Lett. 70, 559 (1997) 12. K.J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H.G. Roskos, S. Czasch, Appl. Phys. Lett. 80, 3003 (2002) 13. A.J. Deninger, T. Göbel, D. Schonherr, T. Kinder, A. Roggenbuck, M. Koberle, F. Lison, T. Müller-Wirts, P. Meissner, Rev. Sci. Instrum. 79, 044702 (2008) 14. R. Wilk, F. Breitfeld, M. Mikulics, M. Koch, Appl. Opt. 47, 3023 (2008) 15. M. Scheller, K. Baaske, M. Koch, Appl. Phys. Lett. 96, 151112 (2010) 16. O. Morikawa, M. Tonouchi, M. Hangyo, Appl. Phys. Lett. 76, 1519 (2000) 17. M. Tani, O. Morikawa, S. Matsuura, M. Hangyo, Semicond. Sci. Technol. 20, 151 (2005) 18. M. Scheller, M. Koch, Opt. Express 17, 17723 (2009) 19. M. Scheller, M. Stecher, M. Gerhard, M. Koch, Opt. Express 18, 15887 (2010) 20. K.-i. Maki, C. Otani, Opt. Express 16, 10158 (2008) 21. C. Brenner, M. Hofmann, M. Scheller, M.K. Shakfa, M. Koch, I.C. Mayorga, A. Klehr, G. Erbert, G. Tränkle, Opt. Lett. 35, 3859 (2010) 22. N. Vieweg, M. Mikulics, M. Scheller, K. Ezdi, R. Wilk, H.W. Hübers, M. Koch, Opt. Express 16, 19695 (2008) 23. E.R. Brown, F.W. Smith, K.A. McIntosh, J. Appl. Phys. 73, 1480 (1993) 24. J. Struckmeier, A. Euteneuer, B. Smarsly, M. Breede, M. Born, M. Hofmann, L. Hildebrand, J. Sacher, Opt. Lett. 24, 1573 (1999)