Experimental Demonstration of Efficient Harmonic Generation via Surface Plasma Compression with Lasers

The efficiency of high-order harmonic generation from a relativistic laser interacting with solid targets depends greatly on surface plasma distribution. The usual method of enhancing efficiency involves tuning the plasma scale length carefully by improving the laser contrast. Here, we experimentally demonstrate that efficient harmonics can be achieved directly by compressing large-scale surface plasma via the radiation pressure of a circularly polarized normally incident prepulse. The harmonic generation efficiency obtained by this method is comparable to that obtained with optimized plasma scale length by high-contrast lasers. Our scheme does not rely on high-contrast lasers and is robust and easy to implement. Thus, it may pave a way for the development of intense extreme ultraviolet sources and future applications with high repetition rates.

Figure 1

(a) Experimental setup for HHG and measurements. The red and violet beams represent the lasers and high-order harmonics, respectively. The harmonics are horizontally dispersed and vertically imaged by a toroidal grating onto a charge-coupled device (CCD). (b) Experimental harmonic images obtained using LC and HC lasers. (c) Preplasma profiles calculated using multi-ife for LC and HC lasers, where $n_c=1.75\times {10}^{21}{\mathrm{cm}}^{-3}$ is the critical plasma density.

Figure 2

Raw harmonic images obtained by (a) LC laser with $t_{\text{delay}}$ of NIP at $-1.6\mathrm{ps}$ and (b) HC laser with $t_{\text{delay}}$ of coaxial prepulse at $-4\mathrm{ps}$ [optimal (opt) scale length]. (c) Harmonic spectra drawn from (a), Fig. 1, and (b). (d) Dependence of the integrated intensity of harmonics (15th–20th) on the $t_{\text{delay}}$ of NIP. The red and green stripes show the experimental harmonic intensities for the HC case and the HC-opt case, respectively. Numerical simulation results are also plotted for comparison.

Figure 3

Simulated 1D (red lines) and 2D (colored images) plasma density distributions at (a) $t=0$ and (b) $t=1.6\mathrm{ps}$. The plasma density is normalized by $n_c$. (c) Simulated harmonic spectra under four different preplasma conditions. The black dashed and green dash-dotted lines are the spectral envelopes for the LC laser and the HC laser with optimum scale length ($L=0.08\lambda$), respectively. The red and blue areas show the harmonic spectra for HC laser ($L=0.05\lambda$) and with the compressed surface in (b), respectively. The harmonic intensity is normalized by $I_0=2.15\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 4

(a) Simulated 1D plasma density profiles at two different moments after the NIP compression. (b) Laser spectral evolution after passing through the low-density pedestal in (a). Harmonic spectra obtained by 2D PIC simulations with the plasma profiles at $t=0.8\mathrm{ps}$ (c) and $t=2.5\mathrm{ps}$ (d). Corresponding to the simulation results, harmonic spectral images obtained in the experiment when the $t_{\text{delay}}$ of NIP is $-0.8\mathrm{ps}$ (e) and $-2.5\mathrm{ps}$ (f).