Femtosecond Pulse Driven Short -Wavelength Laser

Generation of short-wavelength laser in XUV and soft x-ray spectral region requires the delivery of enormous pump power in a controlled manner. Lawrence Livermore National Laboratory (LLNL) came up with leading effort in this regard by applying huge driving lasers (developed for studying Inertial Confinement Fusion) to produce short wavelength laser from 'laser produced plasma' by electron -ion recombination method. Researcher from Princeton University also demonstrated soft x-ray laser from laser produced plasma using same method. However, the prohibitive cost and size of pump laser with high energy made this approach unattractive.

The recent approach is to employ compact, high repitition rate and powerful laser( a table top femtosecond laser) as pump source. The intensity of the laser cause electron to the tunnel through the potential barrier to form ion. By controlling intensity of the laser one can generate ions with the closed electron shells as well as ions with the closed sub-shells. Tunneling ionization with linearly polarized light produce highly ionized ions and relatively cool electrons ( favorable for recombination laser) ,with circular polarization of light it generates ions and extremely hot electrons (favorable for the collissionally excited laser). Recently 10 Hz XUV laser in Xe^⁸⁺ was demonstrated at Standford University as an illustration to this principle. However, the main hurdle for this approach is the defocussing of femtosecond pump radiation by the plasma that it forms. It limits the length over which gain can occur. Among several approaches to overcome this problem, plasma waveguide is an attractive one. A plasma waveguide is formed by forming a long , uniform plasma that is then allowed to expend. The rapid expansion generates a density decrease on the axis and the resulting refractive index profile forms a waveguide for the main pulse which is subsequently injected into the guide structure. The group at Princeton has successfully used a plasma waveguide to demonstrate gain in a recombination laser scheme in hydrogen like lithium at 13.5 nm. The plasma waveguide was formed by ablation and ionization of material from the innerwall of a LiF capillary and was able to guide 25 degree free, pump pulse over a length of 5 mm. The prospect of truly useful high repitition rate table top XUV soft x-ray lasers has become very bright after the routine development of table top femtosecond laser.

(Optics and Photonics News, April 1997)