page_title: Resonant high harmonic generation from laser ablated plasma plumes
section_title: Resonant HHG
hierarchical_section_title: Resonant high harmonic generation from laser ablated plasma plumes / Resonant HHG
caption_attribution_description: English: HHG spectra from tin ablation irradiated by a femtosecond laser pulse.
context_page_description: High Harmonic Generation is a non-perturbative and extremely nonlinear optical process taking place when a highly intense ultrashort laser pulse undergoes an interaction with a nonlinear media. A typical high order harmonic spectra contains frequency combs separated by twice the laser frequency. HHG is an excellent table top source of highly coherent extreme ultraviolet and soft X-ray laser pulses.
context_section_description: In some plasma plumes, it was observed that the intensity of a certain harmonic order was exceptionally high as compared to its neighboring harmonics. For example, by using 800 nm femtosecond laser pulses, it was observed that with tin plasma, the intensity of 17th harmonic was an order of magnitude higher as compared to the intensity of its neighboring harmonics. 
   
  Similarly, it was seen that the intensity of 13th harmonic in indium plasma was much higher as compared to its neighboring harmonics. This was quite surprising because this kind of effect was never seen in gas harmonics. Upon careful investigations, it was pointed out by the researchers that this harmonic enhancement happens when the energy of a particular harmonic order matches with a strong transition present in the plasma. For example, it was observed that in tin, there exists a very strong transition  4d¹⁰ 5s² 5p ²P3/2 → 4d⁹ 5s² 5p² (¹D) ²D5/2 at 26.27 eV and this transition has a high oscillator strength (gf value) of 1.52. The energy of this transition corresponds to 17th harmonic with 800 nm excitation wavelength. Similarly, in Indium, there exists a strong transition 4d¹⁰5s² → 4d⁹ 5s² 5p at 19.92 eV with a high gf value of 1.11. The energy of this transition corresponds to 13th harmonic with 800 nm excitation wavelength. This enhancement in a particular harmonic order is most commonly known as Resonant High Harmonic Generation (RH). Apart from tin and Indium, RH has been observed in many other plasmas such as chromium, manganese, antimony etc. Presence of such resonances makes plasma makes plasma harmonics very different from gas harmonics. The enhanced harmonic efficiency of a given harmonic order can be useful for the development of narrowband tabletop XUV light sources. These type of sources can be very helpful in various types of spectroscopy techniques.
caption: HHG spectra from tin ablation irradiated by a femtosecond laser pulse.
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