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a renaissance of research into the terahertz range of the electromagnetic spectrum (0.1–10 THz, equivalent to

wavelengths between 3 mm and 30 μm) is being driven by the development of many new applications, including the controversial use of THz radiation for airport security checks. Because typical fabrics are transparent to THz radiation, it is possible to see through clothing to detect hidden objects. Although there are obvious security advantages to THz body scanners, an alleged problem is the invasion of privacy, leading the German public to call such machines ‘naked scanners’.

Within the scientific domain, recent advances in THz sources have led to a new technique known as ‘nonlinear THz spectroscopy’1–3. When electric field strengths reach several kilovolts per centimetre, field-induced changes of matter can occur and be monitored on timescales down to several femtoseconds. In this type of spectroscopy, the direction of the applied electric field (the polarization of incident THz radiation) is of critical importance because it determines the direction of the forces acting on the electrons and ions. In most methods of THz pulse generation (for example, using photoconductive switches or optical rectification in nonlinear optical crystals), the polarization is determined by the experimental set-up and cannot be changed easily. Although it is quite easy to rotate the polarization of visible light using passive components such as quarter- and half-wave plates, it is far more difficult in the THz range, owing in part to the lack of such components.

To solve this problem, Dai et al. have demonstrated a conceptually simple method of generating intense THz pulses with a polarization that can be rotated at will4. In this method, intense femtosecond laser pulses consisting of both the fundamental wavelength (800 nm) and the second harmonic (400 nm) are focused into a gas (argon or air at ambient pressure), generating a plasma5–10. The acceleration of the resultant charges in the electric field yields the THz pulses. Importantly, the polarization of the emitted electromagnetic waves can be controlled directly because it is inherently

related to the plasma generation process, as explained below.

The basic principle behind the scheme is shown in Fig. 1. In a neutral atom, electrons occupy energy levels with negative energies, signifying that they are bound to the nucleus. If a strong external electric field (in this case, from the femtosecond laser pulses) is applied to the atom, there is a finite probability that the electron will tunnel through the potential barrier. This results in a free electron and a positively charged ion, which form a plasma. The free electrons then accelerate in the direction of the applied field, and in doing so emit THz radiation with a polarization parallel to the electrons’ velocity. Thus, the polarization of the emitted THz pulse is related directly to the direction of the applied laser light at the time when ionization occurs.

Furthermore, the probability of field ionization depends, in a highly nonlinear way, on the strength and direction of the applied field. For example, with circularly polarized light (Fig. 2), the magnitude of the electric field is perfectly symmetric and therefore electrons will be emitted in all directions with equal probability. Thus, for every electron emitting THz radiation in a particular direction there will be another electron

emitting in exactly the opposite direction — the overall result is no THz emission. To prevent cancellation, an asymmetric electric field with a maximum in only one direction is required. One way of achieving this is to use extremely short pulses. However, it is quite difficult because the pulse length must be approximately equal to the light period (2.7 fs for 800-nm light).

Dai et al.4 circumvent this issue and achieve an asymmetric electric field by using a circularly polarized beam of both red and blue light (blue being the second harmonic of red). This beam has an electric field maximum in a direction determined by the relative delay (phase) between the red and blue laser pulses. Thus, changing the delay by less than 3 fs allows the electric field direction of the emitted THz pulses to be controlled.

The method of Dai et al.4 has potential for fast polarization changes and, using linearly polarized light, for fast intensity changes. For this purpose it is necessary to change the relative delay not by mechanically moving a quartz wedge (as in the present realization), but using an electro-optic device. In this way, it is possible to have a different polarization or intensity for each laser shot, allowing for precise measurements of the polarization

plasMa pHOtONics

Harnessing terahertz polarizationThe polarization of terahertz pulses emitted from a laser-generated plasma can be rotated at will by changing the relative delay between ultrashort red and blue excitation pulses. The result is a fast and convenient method of polarization control.

Michael woerner and Klaus Reimann

Figure 1 | Quantum mechanics causes the electron energies in an atom to be restricted to discrete levels. a, The energy level of an electron in the Coulomb potential (blue line) of a nucleus. b, Under the combined influence of the Coulomb potential of the nucleus and an external electric field, the potential energy changes (red line). Classically, the electron would still be bound to the nucleus, but quantum mechanically there is a finite probability for the electron to tunnel through the potential barrier (hatched area).

a bEnergy

0

Electron level

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or intensity dependence by reducing uncertainties caused by long-term drifts.

At present, lasers with the high intensities required for this technique are limited to repetition rates of a few kilohertz and it

is not possible to extend this method of polarization control to other THz generation schemes that are suitable for lower intensity (and subsequently higher repetition rate) lasers. The comparison of experimental

results with theory by Dai et al.4 confirms and strongly supports the field ionization model for THz generation in a plasma and will likely stimulate discussions of the theories describing field-induced phenomena. ❐

Michael Woerner and Klaus Reimann are at the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Straße 2A, D-12489 Berlin, Germany. e-mail: woerner@mbi-berlin.de; reimann@mbi-berlin.de

References1. Gaal, P. et al. Phys. Rev. Lett. 96, 187402 (2006).2. Gaal, P. et al. Nature 450, 1210–1213 (2007).3. Günter, G. et al. Nature 458, 178–181 (2009).4. Dai, J., Karpowicz, N. & Zhang, X.-C. Phys. Rev. Lett.

103, 023001 (2009).5. Cook, D. J. & Hochstrasser, R. M. Opt. Lett. 25, 1210–1212 (2000).6. Bartel, T., Gaal, P., Reimann, K., Woerner, M. & Elsaesser, T.

Opt. Lett. 30, 2805–2807 (2005).7. Kreß, M. et al. Nature Phys. 2, 327–331 (2006).8. Karpowicz, N. & Zhang, X.-C. Phys. Rev. Lett. 102, 093001 (2009).9. Xie, X., Xu, J., Dai, J. & Zhang, X.-C. Appl. Phys. Lett.

90, 141104 (2007).10. Kim, K. Y., Taylor, A. J., Glownia, J. H. & Rodriguez, G. Nature

Photon. 2, 605–609 (2008).

Electrons

Figure 2 | The electric field strength of circularly polarized monochromatic light has the same magnitude in all directions, both for the fundamental wavelength (red) and the second harmonic (blue). This is not the case, however, for light containing both wavelengths (violet). Here, the electric field has a maximum with a position that depends on the relative phase between the fundamental and second harmonic. Because field-induced ionization of atoms is a highly nonlinear process, it occurs only near the field maximum. Ionization yields free electrons, which the electric field accelerates (green). This causes THz emission with a polarization parallel to the electric field maximum.

The simple act of unpeeling a roll of ‘off-the-shelf’ adhesive tape can generate terahertz radiation with a broad peak at approximately 2 THz, according to researchers in Australia (Opt. Lett. 34, 2195–2197; 2009). The news follows earlier reports that peeling adhesive tape can generate radiation in other parts of the electromagnetic spectrum, including X-rays, visible light and radio waves.

Josip Horvat and Roger Lewis from the Institute of Superconducting and Electronic Materials at the University of Wollongong, Australia, used a motor to unpeel various forms of tape, including electrical tape, Scotch Magic 810 and double-sided Scotch 665. A cryogenically cooled silicon bolometer, a lock-in amplifier and digital oscilloscope recorded the intensity of the signal generated during the unpeeling process as a function of the tape unwinding speed. Optical filters and a Fourier-transform spectrometer were used to analyse the spectra of the emitted radiation.

Measurements indicate that the tapes emitted broadband terahertz radiation with a broad peak at approximately 2 THz, above which the intensity fell

sticky tape surpriseteRaHeRtz tecHNOlOgy

OSA

and then slowly rose with increasing frequency. The radiation was unpolarized and its power (<1 μW) rose with faster unwinding speeds.

The emission mechanism is suggested to be due to triboelectric charging — contact-induced charge separation. When the tape is peeled, its adhesive side becomes positively charged and the tape below becomes negatively charged. The resulting electric field leads to dielectric breakdown of the surrounding air, causing the accelerated charges to emit broadband Bremsstrahlung radiation. The emission peaks are a result

of absorption from the surroundings. Although a vacuum is necessary for the accelerated charges to reach sufficient energy to emit X-rays, Horvat and Lewis indicate that much lower-frequency terahertz radiation can be generated in air. All measurements were made at ambient temperature and pressure.

The researchers suggest that optimization of the scheme may lead to higher output powers and result in an inexpensive source of 1–20 THz radiation.

OliveR gRaydON

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