Abstract

We present a novel technique to generate a continuous, combless broadband Terahertz spectrum with conventional low-cost laser diodes. A standard time-domain spectroscopy system using photoconductive antennas is pumped by the output of two tunable diode lasers. Using fine tuning for one laser and fine and coarse tuning for the second laser, difference frequency generation results in a continuous broadband THz spectrum. Fast coarse-tuning is achieved by a simple spatial light modulator introduced in an external cavity. The results are compared to multi-mode operation for THz generation.

© 2011 Optical Society of America

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References

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  1. D. Grischkowsky, S. Keiding, M. v. Exter, and C. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7, 2006–2015 (1990).
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    [CrossRef] [PubMed]
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    [CrossRef]
  4. P. U. Jepsen, R. H. Jacobsen, and S. R. Keiding, “Generation and detection of terahertz pulses from biased semiconductor antennas,” J. Opt. Soc. Am. B 13, 2424–2436 (1996).
    [CrossRef]
  5. D. Molter, F. Ellrich, T. Weinland, S. George, M. Goiran, F. Keilmann, R. Beigang, and J. Léotin, “High-speed terahertz time-domain spectroscopy of cyclotron resonance in pulsed magnetic field,” Opt. Express 18, 26163–26168 (2010).
    [CrossRef] [PubMed]
  6. O. Morikawa, M. Tonouchi, and M. Hangyo, “A cross-correlation spectroscopy in subterahertz region using an incoherent light source,” Appl. Phys. Lett. 76, 1219–1521 (2000).
    [CrossRef]
  7. M. Scheller and M. Koch, “Terahertz quasi time domain spectroscopy,” Opt. Express 17, 17723–17733 (2009).
    [CrossRef] [PubMed]
  8. C. Brenner, M. Hofmann, M. Scheller, M. K. Shakfa, M. Koch, I. C. Mayorga, A. Klehr, G. Erbert, and G. Tränkle, “Compact diode-laser-based system for continuous-wave and quasi-time-domain terahertz spectroscopy,” Opt. Lett. 35, 3859–3861 (2010).
    [CrossRef] [PubMed]
  9. E. R. Brown, J. E. Bjarnason, A. M. Fedor, and T. M. Korter, “On the strong and narrow absorption signature in lactose at 0.53 THz,” Appl. Phys. Lett. 90, 061908 (2007).
    [CrossRef]

2010 (2)

2009 (1)

2007 (1)

E. R. Brown, J. E. Bjarnason, A. M. Fedor, and T. M. Korter, “On the strong and narrow absorption signature in lactose at 0.53 THz,” Appl. Phys. Lett. 90, 061908 (2007).
[CrossRef]

2000 (2)

O. Morikawa, M. Tonouchi, and M. Hangyo, “A cross-correlation spectroscopy in subterahertz region using an incoherent light source,” Appl. Phys. Lett. 76, 1219–1521 (2000).
[CrossRef]

A. G. Markelz, A. Roitberg, and E. J. Heilweil, “Pulsed terahertz spectroscopy of DNA, bovine serum albumin and collagen between 0.1 and 2.0 THz,” Chem. Phys. Lett. 320, 42–48 (2000).
[CrossRef]

1996 (1)

1995 (1)

1990 (1)

Beigang, R.

Bjarnason, J. E.

E. R. Brown, J. E. Bjarnason, A. M. Fedor, and T. M. Korter, “On the strong and narrow absorption signature in lactose at 0.53 THz,” Appl. Phys. Lett. 90, 061908 (2007).
[CrossRef]

Brenner, C.

Brown, E. R.

E. R. Brown, J. E. Bjarnason, A. M. Fedor, and T. M. Korter, “On the strong and narrow absorption signature in lactose at 0.53 THz,” Appl. Phys. Lett. 90, 061908 (2007).
[CrossRef]

Ellrich, F.

Erbert, G.

Exter, M. v.

Fattinger, C.

Fedor, A. M.

E. R. Brown, J. E. Bjarnason, A. M. Fedor, and T. M. Korter, “On the strong and narrow absorption signature in lactose at 0.53 THz,” Appl. Phys. Lett. 90, 061908 (2007).
[CrossRef]

George, S.

Goiran, M.

Grischkowsky, D.

Hangyo, M.

O. Morikawa, M. Tonouchi, and M. Hangyo, “A cross-correlation spectroscopy in subterahertz region using an incoherent light source,” Appl. Phys. Lett. 76, 1219–1521 (2000).
[CrossRef]

Heilweil, E. J.

A. G. Markelz, A. Roitberg, and E. J. Heilweil, “Pulsed terahertz spectroscopy of DNA, bovine serum albumin and collagen between 0.1 and 2.0 THz,” Chem. Phys. Lett. 320, 42–48 (2000).
[CrossRef]

Hofmann, M.

Hu, B. B.

Jacobsen, R. H.

Jepsen, P. U.

Keiding, S.

Keiding, S. R.

Keilmann, F.

Klehr, A.

Koch, M.

Korter, T. M.

E. R. Brown, J. E. Bjarnason, A. M. Fedor, and T. M. Korter, “On the strong and narrow absorption signature in lactose at 0.53 THz,” Appl. Phys. Lett. 90, 061908 (2007).
[CrossRef]

Léotin, J.

Markelz, A. G.

A. G. Markelz, A. Roitberg, and E. J. Heilweil, “Pulsed terahertz spectroscopy of DNA, bovine serum albumin and collagen between 0.1 and 2.0 THz,” Chem. Phys. Lett. 320, 42–48 (2000).
[CrossRef]

Mayorga, I. C.

Molter, D.

Morikawa, O.

O. Morikawa, M. Tonouchi, and M. Hangyo, “A cross-correlation spectroscopy in subterahertz region using an incoherent light source,” Appl. Phys. Lett. 76, 1219–1521 (2000).
[CrossRef]

Nuss, M. C.

Roitberg, A.

A. G. Markelz, A. Roitberg, and E. J. Heilweil, “Pulsed terahertz spectroscopy of DNA, bovine serum albumin and collagen between 0.1 and 2.0 THz,” Chem. Phys. Lett. 320, 42–48 (2000).
[CrossRef]

Scheller, M.

Shakfa, M. K.

Tonouchi, M.

O. Morikawa, M. Tonouchi, and M. Hangyo, “A cross-correlation spectroscopy in subterahertz region using an incoherent light source,” Appl. Phys. Lett. 76, 1219–1521 (2000).
[CrossRef]

Tränkle, G.

Weinland, T.

Appl. Phys. Lett. (2)

O. Morikawa, M. Tonouchi, and M. Hangyo, “A cross-correlation spectroscopy in subterahertz region using an incoherent light source,” Appl. Phys. Lett. 76, 1219–1521 (2000).
[CrossRef]

E. R. Brown, J. E. Bjarnason, A. M. Fedor, and T. M. Korter, “On the strong and narrow absorption signature in lactose at 0.53 THz,” Appl. Phys. Lett. 90, 061908 (2007).
[CrossRef]

Chem. Phys. Lett. (1)

A. G. Markelz, A. Roitberg, and E. J. Heilweil, “Pulsed terahertz spectroscopy of DNA, bovine serum albumin and collagen between 0.1 and 2.0 THz,” Chem. Phys. Lett. 320, 42–48 (2000).
[CrossRef]

J. Opt. Soc. Am. B (2)

Opt. Express (2)

Opt. Lett. (2)

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Figures (8)

Fig. 1
Fig. 1

Scheme of the external cavity diode laser used in our experiments. The laser diode itself is not AR-coated, therefore only discrete spectral components can be fed back by the external cavity. A variable attenuator before the grating ensures the maximum available power in the experiment, while still sending enough light to the external cavity for spectral control of the laser.

Fig. 2
Fig. 2

Different types of SLM apertures. Option a) is used for generation of tunable two color operation by one laser. b) pins the lasing frequency. c) controls the laser frequency and makes it tunable by vertical displacement. d) is the scheme of the used periodical modulation. The SLM consists of repetitions of the scheme shown in c) arranged around a rotational center. The SLM used in our work was mounted on a standard chopper head.

Fig. 3
Fig. 3

Experimental setup consisting of the two fiber coupled lasers, a 50:50 fiber optic splitter, one collimator on a linear translation stage as delay line and two photoconductive dipole antennas as emitter and detector.

Fig. 4
Fig. 4

Laser frequency over time (left column) and resulting frequency mixing probabilities (right) column of different methods. a) describes the multi-mode operation of a THz spectroscopy system with and arbitrary kind of laser mode drift and modulation. Due to the nearly fixed FSR of the laser diode, the possible difference frequencies do not vary in contrast to the individual modes. The scheme for one fixed laser and a second coarsely tuned laser (mode hop modulation with a SLM) is shown in b). The resulting THz spectrum still consists of separated modes. The proposed and demonstrated principle of this work is depicted in c). The coarse fixed laser 2 is injection current modulated, resulting in continuous shifts of the lasing mode. In combination with the coarse tuned laser 1, this results in a continuous mixing spectrum.

Fig. 5
Fig. 5

Comparison between the single multi-mode laser driven system (left column) and the new approach introduced here (right column). By the combination of a coarse and a fine frequency-tuned diode laser, a continuous, combless THz spectrum is generated. A slight mismatch of the fine-tuning results in a little remaining echo about 25 ps after the main pulse.

Fig. 6
Fig. 6

Measurement of a lactose pellet with the presented setup.

Fig. 7
Fig. 7

Measurement of two Si wafers of different thickness. (Note that this measurement was performed with a different set of antennas compared to the results presented above, resulting in slightly different cross-correlation peak shapes.)

Fig. 8
Fig. 8

Measurements of the optical spectrum (left), the THz time domain signal (middle) and THz spectrum (right). By increasing the frequency separation between the laser spectra, the THz frequency is shifted towards higher values. The low efficiency of the used photoconductive antennas for higher frequencies causes the almost vanishing signal for widely separated laser spectra.

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