Figure 1: Reflection from relativistic electron mirrors.
(a) Electron density ne (in units of nc, logarithmic scale) showing the periodic emission of dense electron bunches at every half cycle of the driving laser field. (b) Frequency-filtered probe field intensity (filter: ω/ωL>5). The counter-propagating probe pulse (not shown here) reflects off the created relativistic electron mirrors, causing the periodic emission of intense, attosecond short radiation. (c) Lineout of the electron mirror γ-factor γz=(1−βz2)−1/2 versus z (averaged in transverse dimension within ±1 μm around x=0 μm, logarithmic scale) superimposed with the normalized intensity lineout of the incident and reflected pulses. The mirror structure is sharply located in space, reflects off the probe pulse in the vacuum region behind the target and causes a frequency upshift governed by (1+βz)2γz2ωL. (d) Reflectivity of the generated electron mirrors analysed for a backreflected wavelength of 80 nm (band width: 20%) as a function of the number of electrons involved in the reflection process, that is all electrons within the mirror structure with (1+βz)2γz2=10 (band width: 20%). The reflectivity obtained from PIC simulation (dots) follows a quadratic scaling, as expected from coherent scattering theory (analytic curve). The best estimate of the experimentally observed reflectivity is 5 × 10−5 (green line), in fair agreement with the expected value. The estimate for the electron mirror reflectivity is discussed in detail in the Methods section. (e) Spectrum of the drive and probe field recorded behind the target normalized to the continuous background level. Dashed curve: spectrum (rescaled) of one isolated backscattered pulse.
