Unlocking the secrets of high-order harmonic generation (HHG) in intense laser fields unveils a pathway to coherent collimated XUV beams of femtosecond or attosecond time durations. While noble gas jets dominate experimental studies, exploration extends to diverse media like atomic clusters, tin films and solid crystals. Laser plumes present an innovative avenue, where a femtosecond laser induces plasma formation on solid surfaces, yielding remarkable efficiency enhancements. Leveraging resonant harmonic generation, particularly with ions featuring strong dipole transitions, propels the conversion efficiency to new maxima. These resonant effects, notably observed near autoionizing states, promise efficient short-wavelength radiation generation.
 
The four-step model elucidates resonant HHG mechanisms, showcasing coherent emission behaviors near resonance. Yet, challenges persist in understanding HHG under varying ionization regimes. Computational simulations, employing the time-dependent Schrödinger equation and semiclassical approaches, underscore resonant enhancement phenomena, albeit underestimating experimental observations. Refinement efforts targeting continuum states hold promise for bridging theoretical-experimental disparities. The present investigation with Indium ions reveals substantial enhancements, hinting at the potential for future advancements in attosecond pulse generation.

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