Photon Acceleration and Relativistic Engineering
In this chapter we describe laser-plasma interaction schemes in which the plasma has the role of an active optical medium that increases the frequency of the incident laser pulse, allowing its compression in time and space. The two main schemes we describ
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Photon Acceleration and Relativistic Engineering
Abstract In this chapter we describe laser-plasma interaction schemes in which the plasma has the role of an active optical medium that increases the frequency of the incident laser pulse, allowing its compression in time and space. The two main schemes we describe, namely “flying mirrors” in underdense plasmas and high harmonic generation in overdense plasmas, are broadly related to the concept of the relativistic moving mirror.
6.1 Towards Higher Frequencies and Higher Fields Efficient conversion of laser light into higher frequencies (i.e. “photon acceleration”) up to the X-ray range and beyond is a long-term effort oriented to the development to coherent sources of “hard” radiation, which may have a number of applications in imaging and spectroscopy. Frequency upshift is also a necessary way to shorten the pulse duration down in the attosecond regime and to obtain higher intensities, as a high frequency pulse might be focused in an extremely small volume. The ultimate frontier of the race towards higher and higher intensities may be considered the Schwinger field E s = m 2e c3 /e that has been already introduced at the end of Sect. 2.1.5. Nonlinear optical effects in plasmas may be exploited to manipulate intense laser pulses. In a broad sense the use of plasma as an optically active and controllable medium is unavoidable at high intensity, since any material is ionized instantaneously by the laser field already at intensities two orders of magnitude lower than the relativistic threshold a0 = 1. Exploiting nonlinear optical properties in the relativistic regime a0 1 has been named as “relativistic engineering”. The moving mirror, already described in Sect. 5.7.1, offers a paradigm for extreme spectral and intensity modulation. For instance, if a laser pulse impinges on a counterpropagating mirror with β → 1, then both the frequency and the intensity of the reflected pulses are increased by a factor (1+|β|)/(1−|β|) 4γ 2 1. Since the number of cycles in the pulse is a Lorentz invariant, the reflected pulse is also shortened by the same factor. A. Macchi, A Superintense Laser-Plasma Interaction Theory Primer, SpringerBriefs in Physics, DOI: 10.1007/978-94-007-6125-4_6, © The Author(s) 2013
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6 Photon Acceleration and Relativistic Engineering
In the next two Sections we describe two schemes for frequency upshift and amplification of intense laser pulses which are both based on the moving mirror concept. First we describe the flying mirror concept which is based, in a broad sense, on the generation of thin and relativistically fast plasma sheets for back-reflection of an external source pulse. We focus on the realization of flying mirrors in an underdense plasma, exploiting the reflection and focusing from nonlinear wake waves (Sects. 3.6.1, 3.6.2). Then we discuss high harmonic generation in an overdense plasma, where the reflecting surface performs oscillations driven by the Lorentz force of the laser pulse (see Sect. 4.2) resulting in the modifi
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