J. Mu, T. Zh. Esirkepov, P. Valenta, Y. Gu, T. M. Jeong, A. S. Pirozhkov, J. K. Koga, M. Kando, G. Korn, and S. V. Bulanov, Physical Review E 102, 053202 (2020); doi:10.1103/PhysRevE.102.053202.
With the help of analytical modeling and large-scale computer simulations, the authors have found novel physical realization of the relativistic mirror, the relativistic flying forcibly oscillating mirror (RFFOM). It combines the properties of the relativistic flying mirror (RFM) and relativistic oscillating mirror (ROM). In addition, due to strong periodic modulations of the mirror surface, it acts as a reflective diffraction grating.
Snapshot of the 3D particle-in-cell simulation. Wake wave cavity, bow wave, and cusp seen in the electron density distribution are shown in grayscale, the reflected electric field normalized to that of the driver (Ed) is shown by the blue-red color scale.
Scheme of relativistic flying forcibly oscillating mirror (RFFOM). The blue-red color scale is for the electric field and grayscale is for the electron density. On the bottom, the driver laser pulse (not shown but revealed via density modulations) creates a cavity and bow wave in underdense plasma (2D particle-in-cell simulation). Overshooting electrons cause transverse wave breaking. Shown on top is a close-up of the area near the density cusp. A counter-propagating source pulse is reflected off the sides of the cusped mirror, forcibly oscillating under the action of the driver.
A relativistic mirror may be defined as an object that reflects incoming radiation while moving at relativistic velocity. The electromagnetic wave reflected from a relativistic mirror in a counter-propagating configuration is compressed, amplified and its frequency is upshifted due to the double Doppler effect. Albert Einstein used the concept of relativistic mirrors to illustrate the theory of special relativity in his seminal paper in 1905. Later, it turned out that relativistic mirrors can be realized by irradiating plasma targets by intense laser pulses. Relativistic mirrors are currently studied in many different contexts because of their great potential for both fundamental science (e.g. light intensification towards the Schwinger limit, investigation of photon-photon and Delbruck scattering, detection of Hawking radiation and the information loss paradox) and practical applications (e.g. attosecond spectroscopy, molecular imaging, plasma diagnostics).
The RFFOM appears as a cusped mirror, the low-dimensional region of highest electron density at the joining of the electron cavity wall and the bow wave excited by the intense laser pulse (driver) propagating in plasma. The RFFOM undergoes forced oscillations imposed by the driver pulse and moves with the group velocity of the driver. It efficiently reflects a counter-propagating laser pulse (source). The reflected radiation spectrum exhibits well-pronounced diffractive orders and harmonic orders. The base frequency of the reflected radiation and correspondingly all harmonic orders are strongly upshifted with respect to the base frequency of the incident source pulse, due to the double Doppler effect. The described scheme can be used for the generation of bright sources of high-frequency radiation. Its realization is easier than the RFM, where one should maintain a sufficiently slow approach to longitudinal wave breaking.