Abstract

We present a detailed experimental and theoretical study of terahertz (THz) generation and beam propagation in an optoelectronic THz system consisting of a large-area (ZnTe) electro-optic emitter and a standard electro-optic detector, and provide a comparison to typical biased GaAs emitters. As predicted by theory, in the absence of saturation the generated THz pulse energy is inversely proportional to the area of the optical pump beam incident on the emitter, although the detected on-axis electric field amplitude of the subsequently focused THz beam is practically independent of this area. This latter result promotes the use of larger emitter crystals in amplifier-laser-based THz systems in order to minimize saturation effects. Moreover, the generation of an initially larger THz beam also provides improved spatial resolution at intermediate foci between emitter and detector.

© 2005 Optical Society of America

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References

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Appl. Phys. A

C. Luo, K. Reimann, M. Woerner, and T. Elsaesser, �??Nonlinear terahertz spectroscopy of semiconductor nanostructures,�?? Appl. Phys. A 78, 435-440 (2004).
[CrossRef]

Appl. Phys. Lett.

J. T. Darrow, X.-C. Zhang, and D. H. Auston, �??Power scaling of large-aperture photoconducting antennas,�?? Appl. Phys. Lett. 58, 25-27 (1991).
[CrossRef]

N. Hasegawa, T. Löffler, M. Thomson, and H. G. Roskos, �??Remote identification of protrusions and dents on surfaces by THz reflectometry with spatial beam filtering and out-of-focus detection,�?? Appl. Phys. Lett. 83, 3996-3998 (2003).
[CrossRef]

T. J. Carrig, G. Rodriguez, T. S. Clement, A. J. Taylor, and K. R. Stewart, �??Generation of terahertz radiation using electro-optic crystal mosaics,�?? Appl. Phys. Lett. 66, 10-12 (1995).
[CrossRef]

CLEO 2000

F.G. Sun, W. Ji, and X.-C. Zhang, �??Two-photon absorption induced saturation of THz radiation in ZeTe,�?? in Conference on Lasers and Electro-Optics, OSA Technichal Digest (Optical Society of America, Washington DC, 2000), 479-480.

J. Opt. Soc. Am. A

J. Opt. Soc. Am. B

Jpn. J. Appl. Phys.

T. Hattori, R. Rungsawang, K. Ohta, and K. Tukamoto, �??Gaussian beam analysis of temporal waveform of focused terahertz pulses,�?? Jpn. J. Appl. Phys. 41, 5198-5204 (2002).
[CrossRef]

Nature Materials

B. Ferguson, and X.-C. Zhang, �??Materials for terahertz science and technology,�?? Nature Materials 1, 26-33 (2002).
[CrossRef]

Opt. Lett.

Optics Express

T. Löffler, T. Bauer, K. J. Siebert, H. G. Roskos, A. Fitzgerald, and S. Czasch, �??Terahertz dark-field imaging of biomedical tissue,�?? Optics Express 9, 616-621 (2001).
[CrossRef] [PubMed]

Phys. Rev. B

G. Segschneider, F. Jacob, T. Löffler, H. G. Roskos, S. Tautz, P. Kiesel, and G. Döhler, �??Free-carrier dynamics in low-temperature-grown GaAs at high excitation densities investigated by time-domain terahertz spectroscopy,�?? Phys. Rev. B 65, 125205-1 (2002).
[CrossRef]

Semiconductor Science and Technology

T. Löffler, M. Kress, M. Thomson, T. Hahn, N. Hasegawa, and H. Roskos, �??Comparative performance of terahertz emitters in amplifier-laser-based systems,�?? Semiconductor Science and technology, (in press).

Other

A. Yariv, Quantum Electronics (John Wiley, New York, 1998).

N. Hasegawa, �??A fundamental work on THz measurement techniques for application to steel manufacturing processes.�?? (Dissertation, University of Frankfurt, 2004), <a href=�??http://deposit.ddb.de/cgi-bin/dokserv?idn=975373056�??> http://deposit.ddb.de/cgi-bin/dokserv?idn=975373056</a>.

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Figures (5)

Fig. 1.
Fig. 1.

Schematic of experimental setup with two possible terahertz emitters.

Fig. 2.
Fig. 2.

Time-domain THz pulse signals emitted from (a) a large-area ZnTe crystal and (b) a large-area GaAs antenna with 1-kV/cm external DC bias, and (c) corresponding power spectra. Both emitters were illuminated with 360-µJ, 150-fs optical pump pulses.

Fig. 3.
Fig. 3.

(a) Electro-optically measured peak amplitude of THz pulses generated using ZnTe and GaAs emitters vs optical pump pulse energy, with two optical pump beam diameters corresponding to ropt=4 mm (“small beam”) and 12 mm (“large beam”). Inset shows magnified views of low pulse-energy region. (b) Corresponding data with both optical pump pulse energy and THz peak amplitudes normalized to respective pump beam 1/e2-area Aopt (Common x-axis range only).

Fig. 4.
Fig. 4.

(a) Measured THz pulse energies for both ZnTe and GaAs emitters and each optical pump beam size ropt=4 mm (“small beam”) and 12 mm (“large beam”), using bolometric detection. (b) Graph of the corresponding THz energy conversion efficiency (within the detection bandwidth) vs the effective fluence (optical pump pulse energy normalized to the respective pump beam 1/e2-area, Aopt). Inset: Magnified view for low pump fluence (note the different vertical scaling).

Fig. 5.
Fig. 5.

Measured THz beam intensity profiles measured (a) directly after the ZnTe emitter and (b) at a subsequent focal plane, for three characteristic frequency components (0.62, 1.25 and 2.19 THz), for the two optical pump beam sizes, ropt=4 mm (“small beam”) and 12 mm (“large beam”), as indicated. Red solid line 1/e2-beam region. Yellow dashed line: Expected 1/e2-beam region according to Eq. (4).

Equations (6)

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E THz ( t ) = K eff W opt T ( t ) .
W opt ( r ) = 2 J opt A opt exp ( 2 r 2 r opt 2 ) ,
J THz = c ε 0 [ E THz ( r , t ) ] 2 r d r d φ d t = c ε 0 K eff 2 J opt 2 A opt .
r det = r THz 2 π c f det ω THz A THz = 2 c f det ω THz r THz
E det ( r = 0 , t ) = E det 0 d d t T ( t )
with E det 0 = K eff J opt 2 π c f det

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