1

Terahertz Breakthroughs Bas Zegers (3072703), University of Utrecht, June 2010

Terahertz radiation solves the hiatus in spectroscopy between microwave and optical

regimes. Many molecules exhibit specific vibrational modes, unfolding molecular spectral

fingerprints that can be probed directly by this THz radiation. In this review paper two

articles that claim to add new potential to THz technology are subjected to a discussion.

THE TERAHERTZ REGIME

Electromagnetic radiation in the terahertz frequency range from 0.1 THz to 10 THz (1 THz = 1012 Hz), and is positioned between the infrared and microwave frequency regimes (see Fig. 1). Recent attention over the past decade has triggered research and brought THz systems from the laboratory to industrial applications. THz radiation can be extensively applied in material characterization, T-ray imaging and sensing, THz tomography, and in biomedical context such as cancer detection and DNA analysis.

Fig 1 The THz gap in the electromagnetic spectrum.

MATERIALS FOR TERAHERTZ SCIENCE

AND TECHNOLOGY

Ferguson et al. from the Center for Terahertz research present a summary of key advances in both sources and detectors for terahertz spectroscopy systems. The authors claim their research extended the possible applications to the development of a quantum cascade THz laser2, mutating DNA detection3, studying quantum interactions4 and many other potential uses. First breakthroughs in the development of THz sources are crucial because of the current limitations to create low-cost, high-power THz systems that operate a room temperatures. The source techniques can be categorized into either broadband pulsed methods or

narrowband continuous wave (cw) methods, that have distinct applications. In narrowband techniques the signal bandwidth does not exceed the coherence bandwidth. All frequency components experience a somewhat equal attenuation. However, in broadband techniques the attenuation differs over a wider frequency profile.

BROADBAND THz SOURCES

Typically the pulsed broadband THz sources are based on the principle of illuminating and thereby exciting materials with ultrashort laser pulses (usually from Ti:S lasers) , creating terahertz pulses that last only a few picoseconds. Broadband THz radiation is generated by either optical rectification1 or photoconducting emission5,6. Optical rectification is a non-linear second-order process, explained as the inverse process of the electric-optic effect7. The intense laser pulse produces a DC polarization, which in turn leads to the emission of a THz pulse. Difference frequency mixing in the semiconductor crystal creates a broad spectrum of frequencies. A GaSe crystal8-10 that can be phase matched in various ways can provide broadband pulses with a tunable centre wavelength11 up to 50 THz. The other method relies on photocarrier acceleration in photoconducting antennas12. Again a fs-laser pulse is used, but now to create electron-hole pairs (the carriers) in the semiconductor material. An applied bias field accelerates the carriers which lead to a current across the antenna patterned on the semiconductor material. Because , according to Maxwell’s equations, it is the rapid rise and decay of the changing

2

current that makes radiation in the THz-regime possible. The repeated process creates short pulses 1 ps) with a bandwidth up to 4 THz. Materials with a low effective electron masses and a high breakdown field, such as InAs and InP are attractive, because radiation intensity increases with optical fluency and bias field. The photoconductive method produces output powers over 40 μW, which is higher than through optical rectification, because the latter depends directly on coupling the incident laser power to THz frequencies, which happens at relatively low efficiencies.

NARROWBAND THz SOURCES

Numerous techniques for narrowband continuous wave (cw) THz generation are presented by Ferguson et al., such as free-electron lasers (FEL), gas lasers, semiconductor lasers, upconversion of radiofrequency sources, downconversion of optical sources, non-linear photomixing, and even quantum cascade lasers. The most promising sources are reviewed below. Upconversion of long wavelength microwave radiation is in general scaling electronics down to operate in the THz regime. Basically, two or more low energy photons are combined to produce a photon with a THz frequency. Optical output powers up to 100 μW have been achieved13. In a same way downconversion can be used to split a photon at an optical frequency into emission of two or more photons of THz energies. According to S.L. Dexheimer14 material properties ultimately limit a high frequency response in up- and downconversion. A second technique uses free-electron lasers15 (FELs), in which electrons move with a very high velocity through a strong spatially varying magnetic field, thereby exhibiting strong oscillation leading to THz radiation. FELs can create ultranarrowband THz radiation with a high signal-to-noise ratio and high output powers. The main disadvantage are the cost and size of such lasers, that need a entire detailed facility to be built. Finally the technique of the quantum cascade laser (CQL) is discussed, which follows up earlier research16,17 done to simple semiconductor laser techniques as a source

for THz radiation. A CQL consists of a series of coupled quantum wells, nanometer-thick layers of GaAs between layers of AlGaAs that act as potential barriers. The mechanism is shown in Fig. 4. First population inversion is reached the active region. Basically, a electron falls back to a lower energy level, in the process emitting a photon at THz frequency. Subsequently the electron tunnels between the quantum wells while the injector region couples it to the higher energy of the next active region. Obviously, the entire process now repeats itself, producing output powers up to 2 mWs. The downside to CQLS is that they produce relatively long wavelengths that result is a long optical mode which causes a poor coupling between the optical field and the relatively small gain medium. The relatively long wavelength of the THz radiation also leads to high optical losses due to the free electrons in the medium. Kohler et al.2 claim they overcame this problem to a significant extent. They used seven quantum wells to form one basic unit which was repeated 104 times to produce a total of 728 quantum wells. They were finally able to let this laser operate at 4.4 THz at 10 K temperature and even predict cw operation at 70 K temperatures if fabrication is optimized.

Fig 2. Quantum Cascade laser, a simplification of the conduction band. The active transition from level 2 to level 1 provides 4.4-THz emission.

THz DETECTORS

For spectroscopy measurements it is crucial to have high sensitive signal detection. The most common used technique for broadband signals is based on thermal absorption, which is the case in Ge and InSB bolometers. Due to the relatively high thermal background

3

radiation in the THz regime, extensive cooling of these systems is a prerequisite. Heterodyne sensors are even more preferable for spectroscopy applications that require very high sensor resolutions. In these superconductor sensors mixing takes place between the signal and a external source that emits at the THz frequency of interest. This results in a downconverted signal that can be measured after significant amplification. An example is the Schottky-diode mixer18, that can be used to operate at 2.5 THz. Another promising high-temperature superconducting material is YBCO, which is shown to be suitable for higher bandwidths19. Pulsed THz radiation can be detected by the technique of terahertz time-domain spectroscopy (THZ-TDS), where certain material properties are measured with a special pump-probe technique. A part (called the probe pulse) of the original laser pulse that was used to generate the THz pulse is send to the detector, where it arrives coherently with the THz pulse from the sample material (See Fig. 3). This coherence is achieved with an optical delay line. Now the most common used way of detecting these signals is the method of free-space electro-optic sampling: the THz beam induces birefringence in a electro-optic medium (usually a inorganic sensor crystal), called the Pockels effect, which in turn leads to a polarization change of the detection pulse. This polarization change is measured with the probe beam. The THz waveform can be extracted as a function of the optical delay20. With thin sensors a very high detection bandwidth up to 100 THz have been demonstrated. Finally, pulsed THz detection can also be achieved by photoconductive antennas, in a similar way to photoconductive emitters. A bias field is left out, but now the photocurrent is measured with a current amplifier and meter. Detection of high bandwidths with frequencies even over 60 THz is possible21.

Fig. 3 A THz-TDS Spectroscopy pump probe setup. The delay stage is used to shift pump and probe arrival times in the detector therefore allow the construction of a temporal profile.

APPLICATIONS

Ferguson et al. present a multitude of interesting applications for using THz radiation. In the following section the most novel and promising applications will be discussed. THz systems for instance can be applied in cancer detection3 and genetic analysis. The vibrational modes of many proteins and DNA molecules are expected to be in the THz range, and thus can be seen as spectral fingerprints if measured with the suitable THz spectroscopy (see Fig. 4). First markers are needed to detect specific DNA moleculs, which is a field that shows rapid progresses. THz spectroscopy already has demonstrated to be able to distinguish between single- and double-stranded DNA owing to changes in the complex refractive index22. The complex refractive index has already been determined for certain biomolecules, showing absorption at mostly THz frequency modes. In the future, this could even gain potential for detection of DNA mutation in cancer analysis.

Fig 4. Vibrational modes of many molecules are in the terahertz regime. Rotational, vibrational and electronic transitions are shown along with the blackbody radiation curve at 300 K

4

Today THz systems are already known for their use in material characterization. The methods can be used to measure carrier concentrations23 and mobility in any semiconductor, but also to measure the energy gap of promising high-temperature superconductors24 such as Mg2. For the application of material characterization THz-TDS systems are used, in which the response of the material of interest is measured to reveal information about key material properties. Another promising application of THz radiation would even be in the field of security screening. Using pulsed THz-wave ‘T-ray’) phase-sensitive imaging 3D-rendered images can be produced based on changes in refractive index. It has the advantage over X-ray uses, that is reveals more information about the frequency dependent optical properties25 of materials. Discussing the article by Ferfuson et al., some very promising new potentials for THz radiation are clearly on the edge of breakthrough. After having been critically searching for other methods, I can conclude the article demonstrates a complete summary of all new methods up to publication of the article. The higher THz output powers produced by the FEL and the development of the quantum cascade laser triggered new application possibilities in THz spectroscopy. The introduction of article by Ferguson et al. claimed that terahertz sources rely heavily on new material research such as the quantum cascade laser, have been shown to be supported by the article. The only thing other physicists, for example S. L. Dexheimer14, remain sceptic about is that material properties ultimately limit a high frequency response that seem to be crucial in developing high output THz sources.

METAL WIRES FOR TERAHERTZ

WAVEGUIDING

A structure that is used to guide electromagnetic or sound waves along a material into a specific direction without significant losses is called a waveguide. Conventional metal waveguides for the microwave regime and dielectric fibers for the

visible and near-infrared, are not efficient structures to guide terahertz waves over a long distance. On one hand it is the finite conductivity of metals that leads to high losses if the electromagnetic field is exposed to metal surfaces. On the other hand it is the high absorption coefficient of dielectric materials in the THz range that leads to high losses in these conventional waveguiding techniques. In 2004 K. Wang et al.26 claimed to be able to fulfil this compelling need of a suitable THz waveguide with a completely new method of using a bare metal wire as a waveguide.

Fig 5. The THz waveform measured after 4 cm and 24 cm propagation distance in the waveguide. The red curve has been multiplied by a factor of 1.6, which amplies an attenuation coefficient of 0.03 cm

-1. The inset shows the

normalized amplitude spectra

Regardless to the importance of low attenuation, the extensive use of broadband THz pulses also requires very low dispersion in spectroscopic applications. This means different wavelengths travel at different group velocities owing to different interactions with the refractive index of the waveguide, which leads to spreading or even overlap of the optical pulses, and limits the information capacity of the waveguide. Over the years up to publication of this article (2004) guiding structures have been presented like metal tubes27, plastic ribbons28, parallel metal plates29, dielectric fibers30 and even advanced photonic crystal fibers31. But all research groups struggled with high losses and/or high group velocity dispersion.

5

Wang et al.26 claim to have found the solution in a bare stainless steel wire, that acts as the waveguiding medium. The authors used photoconductive antennas, a technique explained earlier, to generate and coherently detect pulsed broadband THz radiation in a pump-probe setup. The THz pulses are focused onto the stainless steel waveguide of 0.9 mm to create a radially polarized modes along the waveguide. A second perpendicularly orientated wire acts as an input coupler. At the end of the waveguide a receiver detects only the vertically polarized component to exclude possible interference with directly scattered radiation in the receiver. Movable stages made it possible to do intensity measurements at different propagation lengths in the waveguide.

Fig 6. Waveguiding mode measurements. a. Group velocity of the propagating mode as a function of frequency. b. The electric field amplitude attenuation coefficient as a function of frequency.

The results shows a propagation of the THz waves that has very low dispersion (see Fig. 5). At two different propagation distances (4 and 24 cm), the waveform profiele are shown to be very similar. The red curve corresponding to the longer waveguide shows a slightly

narrowing of the signal, which indicates that the low frequency components in the broadband signal are attenuated slightly more than the high ones. From spectrum analysis the group velocity can be determined and the attenuation coefficient, both as a function of frequency. Figure 7 again shows a almost dispersionless propagation. Figure 8 provides a attenuation coefficient ( 0.3 cm-1), weighted by the pulse power spectrum, that is according to the authors the lowest reported to date32 for THz waveguiding. This significant reduction of attenuation can be attributed to the fact that a metal wire has much smaller surface area interacting with the field33. The authors do note a increase with lower frequencies (Fig. 8), that are claimed to be the result of diffractive spreading due to superposition of the guided modes. The authors also found a way to direct THz pulses around corners without substantial increase in propagation losses. A small metal plate fixed to the end of two metal wires in an Y-structure (see Fig. 9) makes it possible to direct THz waves in an almost 180-degree angle, and provide a detectable signal. The discovered the radial mode profile is even conserved for several centimetres as it propagates of the waveguide. So it even possible to place the small metal plate of the two metal wires and still maintain a detectable signal.

Fig 7. The configuration for a THz radiation experiment for waveguiding around corners. A Y-splitter structure of two metal wires is used with the red bar indicating the metal plate than can be fixed to the glass flask up to several cms from the waveguide while still maintaining a detectable signal.

Reviewing the article by Wang et al.26 substantial advances have been presented in the field of terahertz waveguiding. The use of a bare stainless steel wire shows to have a very low attenuation and dispersion. In addition it is shown that THz pulses can be bend around corners, where line-of-sight

6

optical access is not possible. This opens up potential for new applications for terahertz sensing and imaging such as endoscopy. However, the authors claim in the introduction to have a fully operating THz endoscope, which is not entirely true. Only a small part of the article is spent on the diffractive losses of the waveguide that are still too large. Furthermore, the authors should also address more extensively the loss problems concerning the coupling of the free-space THz beam to the guided mode. Further progress in more effective mode-matching is still crucial to optimize both the input coupling and propagation losses. If done, a scientific breakthrough in THz waveguiding is really on the way. 1. Ferguson B., Z. X. (2002). Materials for terahertz science and tecnology. Nature Materials 1 (1) , 26-33. 2. Kohler, R. et al. Terahertz semiconductor-heterostructure laser. Nature 417,156–159 (2002). 3. Woodward, R. M. et al. in OSA Trends in Optics and

Photonics (TOPS),Vol 56,Conference on Lasers and Electro-

optics. (OSA,Washington DC, 2001). 4. Huber, R. et al.How many-particle interactions develop after ultrafast excitation of an electron-hole plasma. Nature 414,

286–289 (2001). 5. Chang, Q.et al. Broadband THz Generation from Photoconductive Antenna, Progress in Electromagnetic Research Symposion 2005, (Hangzhou, China) 6. Mourou, G. A., Stancampiano, C. V.,Antonetti,A.& Orszag,A. Picosecond microwave pulses generated with a subpicosecond laser driven semiconductor switch.Appl. Phys. Lett. 39, 295–296 (1981). 7. Mourou, G. A., Stancampiano, C. V.,Antonetti,A.& Orszag,A. Picosecond microwave pulses generated with a subpicosecond laser driven semiconductor switch.Appl. Phys. Lett. 39, 295–296 (1981). 8. Gornik, E. & Kersting,R. in Semiconductors and Semimetals

(ed. Tsen,K. T.) (Academic, San Diego, 2001). 9. Leitenstorfer, A.,Hunsche, S., Shah, J.,Nuss, M. C. & Knox,W. H. Femtosecond charge transport in polar semiconductors.Phys.

Rev. Lett. 82, 5140–5142 (1999). 10. Leitenstorfer, A.,Hunsche, S., Shah, J.,Nuss, M. C. & Knox,W. H. Femtosecond high-field transport in compound semiconductors.Phys. Rev. B. 61, 16642–16652 (1999). 11. Katzenellenbogen,N.& Grischkowsky,D. Efficient generation of 380 fs pulses of THz radiation by ultrafast laser pulse excitation of a biased metalsemiconductor interface.Appl. Phys.

Lett. 58, 222–224 (1991). 12. Bass, M., Franken, P. A.,Ward, J. F.& Weinreich,G.Optical rectification. Phys.Rev. Lett. 9, 446–448 (1962). 13.Maiwald, F. et al. in IEEE Microwave Theory and Techniques

Society International Symposium Digest (ed. Sigmon, B.) Vol. 3 1637–1640 (IEEE,Piscataway,New Jersey, 2001). 14. Dexheimer, S.L. Terahertz Spectroscopy. Principles and Applications. Boca Raton: CRC Press (2008) 15.Williams, G. P. Far-IR/THz radiation from the Jefferson Laboratory, energy recovered linac, free electron laser. Rev. Sci.

Instrum. 73, 1461–1463 (2002). 16. Komiyama, S. Far-infrared emission from population-inverted hot-carrier system in p-Ge. Phys. Rev. Lett. 48, 271 (1982).

17. Gousev,Yu. P. et al.Widely tunable continuous wave THz laser. Appl. Phys. Lett. 75, 757–759 (1999). 18. Gaidis, M. C. et al. A 2.5 THz receiver front-end for spaceborne applications. IEEE Trans.Microwave Theory

Technol. 48, 733–739 (2000). 19. Carlstrom, J. E. & Zmuidzinas, J. in Reviews of Radio Science

(ed. Stone,W. R.) 1193–1995 (Oxford Univ. Press, Oxford, UK, 1996). 20 Mueller, E.R., Terahertz radiation: applications and sources, The Industrial Physicist,27-29 (2003) 21. Kono, S., Tani, M.,Gu P. & Sakai, K. Detection of up to 20 THz with a lowtemperature-grown GaAs photoconductive antenna gated with 15 fs light pulses.Appl. Phys. Lett. 77, 4104–4106 (2001). 22. Brucherseifer, M. et al. Label-free probing of the binding state of DNA by timedomain terahertz sensing.Appl. Phys. Lett.

77, 4049 –4051 (2000). 23. van Exter, M., Fattinger,C.& Grischkowsky,D. Terahertz time-domain spectroscopy of water vapor. Opt. Lett. 14, 1128–1130 (1989). 24.Kaindl, R. A. et al. Far-infrared optical conductivity gap in superconducting MgB2 films.Phys. Rev. Lett. 88, 027003 (2002). 25. Ferguson, B.,Wang, S.,Gray,D.,Abbott,D. & Zhang, X.-C. Towards functional 3D THz imaging. Phys.Med. Biol. (in the press). 26. Wang K.L.,Mittleman M. D. Metal wires for terahertz wave guiding. Nature 432 (7015) , 376-379 (2004) 27. McGowan, R. W., Gallot, G. & Grischkowsky, D. Propagation of ultrawideband short pulses of THz

radiation through submillimeter-diameter circular waveguides.

Opt. Lett. 24, 1431–1433 (1999).

28. Mendis, R. & Grischkowsky, D. Plastic ribbon THz waveguides. J. Appl. Phys. 88, 4449–4451 (2000).

29. Mendis, R. & Grischkowsky, D. Undistorted guided-wave

propagation of subpicosecond terahertz pulses. Opt. Lett. 26,

846–848 (2001). 30. Jamison, S. P., McGown, R. W. & Grischkowsky, D. Single-

mode waveguide propagation and reshaping of sub-ps terahertz

pulses in sapphire fiber. Appl. Phys. Lett. 76, 1987–1989

(2000). 31. Han, H., Park, H., Cho, M. & Kim, J. Terahertz pulse

propagation in a plastic photonic crystal fiber. Appl. Phys. Lett. 80, 2634–2636 (2002).

32. Coleman, S. & Grischkowsky, D. A THz transverse electromagnetic mode two-dimensional interconnect layer

incorporating quasi-optics. Appl. Phys. Lett. 83, 3656–3658

(2003).

33. Marcuvitz, N. Waveguide Handbook (McGraw-Hill, New

York, 1951).