Lasers in the extreme ultraviolet (EUV) and soft-x-ray wavelength range (0.2-30 nm) are expected to have an impact on applications as revolutionary as they did in the visible and infrared (IR) wavelength range. The range of these applications is wide as a result of the inherent properties of x-ray lasers: very high brilliance, short pulse duration, coherence, and short wavelength. The development of lasers in the x-ray spectral region is hence of fundamental importance in fields such as microscopy, interferometry, EUV lithography, surface analysis, photoelectron spectroscopy, atomic physics, and plasma physics.
Most of the soft-x-ray lasers demonstrated experimentally up to now have used a hot, dense plasma produced by high-power visible or near-IR lasers, in which a population inversion (the prerequisite for lasing action) is created by electron-collisional excitation. In the past, the high pumping energy required for saturated lasing output has restricted the development of x-ray lasers to a few large-scale laser facilities around the world. Reduction of the pumping energy by exploiting novel pumping schemes remains the primary goal in plasma x-ray laser research if these are to become compact laboratory tools, complementary in many respects to free-electron lasers.
Recent progress using the technique of grazing-incidence pumping (GRIP) has lead to saturated x-ray lasing in plasmas of Ni-like Ag, Pd, Sn, Sb, and Te at wavelengths down to 10.9 nm, some at repetition rates of up to 10 Hz, and for pump energies as low as 2 J. Sub-10-nm lasing has been achieved in Ni-like Ba, La, and Sm with 5-10 J of pumping energy, while simulations predict that ~50 J will be required to generate an x-ray laser in the water window (2.5-4.4 nm).
The primary goal of this project is to extend the range of saturated lasing to wavelengths well below 10 nm by exploiting and optimizing the GRIP scheme in plasmas produced from targets of Sm, Gd, etc., and possibly other elements of the lanthanide (rare-earth) group. To this end, new target fabrication techniques and/or the use of compound targets, e.g., barium fluoride and lanthanum fluoride, are being developed.
The second goal will be the construction of the compact (table-top) 2-J, few-Hz, Nd:YLF laser based on optical parametric chirped-pulse amplification (OPCPA). The increased repetition rate is imperative if applications such as EUV lithography and photoelectron spectroscopy are envisaged.