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Radiation sources: Electrons and lasers sing THz tune, Nature Physics
date2008-06-30 08:44:47 Click No.2037

Source: Nature Physics.

Nature Physics News and Views

By Gwyn P. Williams

    In the past, the paths of laser physicists and high-energy-particle physicists rarely met. But times are changing. The extreme fields produced by emerging high-intensity lasers hold increasing promise as a means of producing high-energy particle beams. On page 390 of this issue [1], Serge Bielawski and colleagues describe the ability to coherently control the radiative behaviour of the electrons in the storage ring of a conventional particle accelerator with a pulsed laser, and in doing so demonstrate a bright, tunable source of radiation in the difficult-to-reach terahertz region of the electromagnetic spectrum. The result further widens the potential of existing accelerator facilities as sources of useful radiation.

    The principle behind the authors¨ demonstration is essentially the same as that used in a conventional radio transmitter. When an charge (q) is driven to oscillate back and forth ! such as up and down the length of a radio antenna ! it will radiate with a power given by the Larmor equation. Instead of using conventional electronics to drive the periodic oscillation of electrons in an antenna, the authors use a shaped laser pulse to modulate the charge density in an electron packet of a relativistic electron beam, so that when its path is bent by a simple dipole magnetic it radiates at a frequency determined by the modulation period (see Fig. 1).

Figure 1 : Generation of bright, tunable, narrowband, THz radiation by the interaction of a laser and a relativistic electron beam.

    Injecting an appropriately shaped pulse into the undulator region of a conventional electron accelerator storage ring periodically modulates the longitudinal distribution of charge in a relativistic electron beam packet. Subsequent bending of the path of this packet causes the emission of radiation at wavelengths determined by the period of modulation. Tuning this period enables tuning of the subsequent radiation.

    The advantages of such an approach are manifold, the most potent being the relativistic enhancement to the power emitted by a beam of particles travelling at close to the speed of light. The relativistic correction to the Larmor equation2 includes the ratio of the energy of the particles in the beam to their rest mass raised to the power of four. For the beam of 600 MeV electrons (whose rest mass is 0.511 MeV) used by Bielawski et al., this enhances the radiant power (compared with non-relativistic electrons) by more than 12 orders of magnitude. Although this correction ignores the spectral distribution, this is exactly what is being tailored here.

    In addition, the dependence of the non-relativistic Larmor equation on the square of the total charge enables further improvements in radiant power, as is exploited in conventional table-top THz sources [3, 4]. Although Bielwaski et al. did not achieve complete modulation of the electron bunch, there is no reason in principle why the entire bunch should not contribute. Future investigation should provide new insight into the collective behaviour of the electrons subject to ponderomotive forces in combined electric and magnetic fields. It is also worth remarking that the radiation that Bielawski et al. report is emitted as each electron executes approximately half a cycle, but the power radiated is independent of the acceleration time and would be a mixture of edge and dipole radiation. Further, the superposition of radiated fields from all the density modulations would change the spectral content as a function of angle in a unique way. This type of source can in principle be used with phase-detection techniques similar to those exploited by table-top sources[5].

    But perhaps the most important advantage is the relatively straightforward way in which the frequency of the emitted THz radiation can be tuned, by simply changing the modulation period of the laser beam. The terahertz region of the electromagnetic spectrum is particularly difficult to generate ! being too high in frequency to be reached by conventional electronics and too long in wavelength to be reached by most optical sources. This underexploited region offers enormous potential for discovery and for gaining an understanding of some of the fundamental mechanisms responsible for the behaviour of materials. Indeed, the evolution of terahertz techniques to study the dynamical quantum behaviour in materials has advanced very dramatically during the past decade, and will continue to do so with ultrafast table-top lasers, optical switches and phase-sensitive detectors [3, 4].

    Table-top sources are adequate for most applications, and can produce high peak power6. However, there remain some regimes that can only be reached using large-scale facilities such as particle accelerators. Higher brightness and higher average-power terahertz sources are expected to have many important applications, from fundamental studies of linear and nonlinear dynamical processes in physics, chemistry, materials science and biology to real-time imaging applications relevant to non-destructive evaluation and useful in the security and medical fields [7]. The source of bright, narrowband, tunable THz radiation demonstrated could prove useful in this endeavour.

    References

1. Bielawski, S. et al. Nature Phys. 4, 390C393 (2008).
2. Jackson, J. D. Classical Electrodynamics (Wiley, New York, 1975).
3. Orenstein, J. in Handbook of High Temperature Superconductivity: Theory and Experiment (eds Schrieffer, J. R. & Brooks. J. S.) Ch. 7 (Springer, Hamburg, 2007).
4. Cooke, D. G. et al. J. Appl. Phys. 103, 023710 (2008).
5. Shen, Y. et al. Phys. Rev. Lett. 99, 043901 (2007).
6. Yeh, K.-L., Hoffmann, M. C., Hebling, J. & Nelson, K. A. Appl. Phys. Lett. 90, 171121 (2007).
7. Tonouchi, M. Nature Photon. 1, 97C105 (2007).

 
 

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