# Single-Shot Temporal Contrast Enhancement Measurement of a Plasma Mirror by a Chirped Pulse
## 3 Experiment and Discussion
### Results and Discussion  
The single-shot temporal contrast enhancement measurement results of a PM are shown in Figure 3. The original spectrum \(S_{0}(\lambda)\) obtained before is shown in the inset of Figure 3a. The wavelength range is 750-850 nm, and the long wavelength corresponds to the rising edge. According to the chirp rate of 25 ps/nm,  
Besides, the intensity of the falling edge in phase 3 is slightly stronger than that of the rising edge. Excluding the possibility of stray light, we speculate that this appearance may be caused by the optical damage of the PM, as more scattered light enters the spectrometer through the damaged surface. For more details of the enhancement, we chose the partial temporal profile range from \(-300\) ps to \(+30\) ps, as shown in Figure 3b. As analyzed above, the contrast of the temporal reflectivity profile represents the temporal contrast enhancement of the PM. Thus, Figure 3b shows a temporal contrast enhancement of about two orders of magnitude for the delay time from 30 to 300 ps before the main pulse arrives. According to the fact that the temporal contrast enhancement of the PM is the ratio of efficiency (75%) to cold reflectivity (5%), the theoretical value is about 150 times, which is in good agreement with the measurement results, and verifies the validity of the method. Moreover, several groups of test experiments were carried out alternately with and without the SOCP to determine the influence of the SOCP on PM performance. We recorded the efficiency and far-field patterns of output the igniting laser with and without the SOCP. By calculation, the average PM efficiency without the SOCP, and that of with the SOCP, are both maintained at a level about 75%. The far-field patterns results without the SOCP and with the SOCP are shown in Figure 4a,b. Although the pixel numbers of the far-field patterns were limited by the spot size and pixel size, it is enough to judge whether the PM is degraded compared with the possible serious degradation phenomenon of the PM [29]. Obviously, the two figures are highly similar in the spatial profile and surrounding micro speckles, indicating that the SOCP measurement does not affect PM performance. It is easy to understand that the PM is not affected as long as the intensity of the SOCP is not enough to interact with the target of the PM in advance. All of the key parameters of the PM, including the spectrum,  
Figure 3: Single-shot temporal contrast enhancement measurement results of a plasma mirror: (**a**) Spectrum of SOCP after PM with original spectrum subtracted. The original spectrum of SOCP is in the upper left region of Figure 3a. (**b**) Temporal profile ranges from 300 ps to 30 ps. far-field pattern, and efficiency, were obtained experimentally in a single-shot measurement. Combining the SOCP with calorimeters and CCD cameras for single-shot measurement has good potential for the online experimental monitoring of PM performance. Except for the single-shot measurement of temporal contrast enhancement by the PM, SOCP has other characteristics in the PM measurement. Firstly, this method avoids the demand for sampling the main pulse after the PM, which is very helpful for reducing the energy loss and monitoring the characteristics of the PM in online high-power experiments. Secondly, this is a relative measurement method, and the required dynamic measuring range is dependent on the pulse-cleaning ability of the PM. For a single PM, the dynamic range of 10\({}^{2}\)-10\({}^{3}\) is enough for the SOCP, while that of the traditional temporal contrast measurement method needs to reach 9 orders of magnitude. At present, the temporal contrast of high-power ultrashort pulses increases with the traction of physical demand, putting forward higher requirements and challenges for the dynamic measurement range of the temporal contrast of ultrashort petawatt and even exawatt lasers in the future. It should be mentioned that SOCP has potential use in the single-shot enhancement measurement of a double plasma mirror (DPM) system.