Abstract
High order harmonic generation (HHG) is a unique and extreme nonlinear effect, providing coherent radiation in EUV or soft X-ray region with femto-to-attosecond duration. However, the laser-induced free electrons will induce not only the dephasing effect but also the spatial defocusing, limiting observable photon energy of HHG in low energy harmonics only. In this thesis, we realized an extension of HHG cutoff in a tight focusing geometry. A 400-fold enhancement of 65 eV harmonics in argon-filled gas cell is observed in a 0.8 mm gas cell. Furthermore, by using an iris to truncate the laser beam, the central photon energy of harmonic spectrum can be selected. We demonstrated a wide tunability of harmonics from ≈ 35 to ≈70 eV in a 1.2 mm gas cell. Based on our numerical calculations, this phase matching process in tight focusing geometry is assisted by the wavevector mismatch from the intrinsic phase. This new phase matching technique provides an opportunity to extend the phase matching cutoff. Such bright, coherent light source around 60 eV can be applied for coherent image and experiments of ultrafast, element selective dynamic.