Significance High-order harmonics generated by ultrashort intense laser pulses interacting with matter represent a significant advance in the production of coherent extreme ultraviolet (EUV) or soft X-ray sources, and they also lead to the generation of attosecond pulse trains in the time domain. A single isolated attosecond burst could be obtained by various gating techniques. Such a promising source can be widely used in applications such as diagnosis of ultrafast dynamical processes, coherent diffractive imaging (CDI) with high resolution, probing of magnetic materials, and mask defect inspection. The exploration of more efficient harmonic generation schemes in different media has been a fascinating research topic. The employment of low-density plasma plumes subject to intense femtosecond laser pulses has allowed the extension of high-order harmonics to arbitrary solids, greatly enriching the selectivity of the materials. Due to the resonance of a specific harmonic with the ionic transition possessing strong oscillator strength, intensely monochromatic high-order harmonics can be produced in the EUV region. In addition, the resonance-enhanced harmonic can be tuned by laser wavelength, chirp, two-color field and target structure. By taking advantage of the near-field enhancement and the large recombination cross section of the nanoparticles, the conversion efficiency of the EUV harmonics can be further improved. Progress The first observation of resonance-induced enhancement of the single harmonic generated in the plasma was made in 2006, in which a very strong intensity of the H13 harmonic from the In plasmas was two orders of magnitude higher than neighboring orders. In the same year, the enhancement of the H17 harmonic in Sn plasmas was also demonstrated. The typical resonance-enhanced harmonic spectra are shown in Figs. 3, 7 and 9. The other plasmas produced by Mn, Cr, Sb and Zn are enhanced at H31, H29, H21 and H9, respectively. The four-step model can be used to interpret the peculiarly resonant harmonic, as illustrated in Fig. 5. The last step of the electrons in the three-step model of gas harmonics is replaced by the radiationless transition to the autoionizing state and relaxation to the ground state with EUV emission. The highest cut-off order H101 harmonic (7.9 nm) was obtained in the Mn plasmas. The conversion efficiency of the resonance-induced enhancement of the monochromatic harmonic has approached 10(-4). In order to improve the conversion efficiency of the harmonics in the EUV region, the nanoparticle-containing plasmas are utilized, as shown in Figs. 3 and 7. Since the generation process is dominated by neutral atoms, it is difficult to enhance the higher harmonic orders. By adopting different laser chirps, two-color field, mixed targets and multi-jet plasmas, the resonant wavelength, harmonic cut-off order and harmonic intensity can be tuned. In addition, high-order harmonics can be controlled using structured vortex and vector beams, as presented in Figs. 8. 13. The calculated attosecond pulse trains with a duration of 240 as in the laser-produced In plasmas and the experimentally measured attosecond pulse trains with a duration of 300 as in the Cr plasma plume were obtained. The metallic cylindrical rod was mounted on a rotating holder, which can achieve stable harmonic radiation in more than 106 shots under 1 kHz laser pulses (Fig. 14). Conclusions and Prospects In this article, we review the mechanism and research progress in the resonance-induced enhancement of the single harmonic from a laser-produced plasma. Many efforts have been made to tune the resonance wavelength, extend the cut-off order, increase the conversion efficiency and improve the harmonic stability. To be an applicable EUV source, the plasma harmonics must be generated with high photon flux and high repetition rate. This should be greatly improved if the nanoparticles interact with the laser pulses at high repetition rate through some techniques. We also expect that the gating techniques can be applied to produce isolated single attosecond pulse.
