Periodic grating structures self-formed on a metal surface under the irradiation of a femtosecond laser pulse are characterized by grating spaces which are shorter than the laser wavelength, as well as by dependence on the laser fluence. This Brief Report presents a different interpretation of these features in terms of the process of parametric decay of laser light to surface plasma waves. Depending on the electron density, grating spaces with lengths of 680 nm to as short as 400 nm can be produced for 800 nm laser wavelength as a result of the interaction of laser pulses with laser-produced surface plasma.

Periodic structures self-formed on the surface of several metals under femtosecond laser-pulse irradiation are investigated by electron microscopy. For the self-formation of periodic gratings on metal surfaces, the interspaces of the periodic structures depend on laser fluence. This dependence is the same for all metals, although the range of laser fluence in which the structures are formed differs between metals. The laser fluence dependence can be explained by the generation of a plasma wave through the parametric decay of laser light ͓S. Sakabe, M. Hashida, S. Tokita, S. Namba, and K. Okamuro, Phys. Rev. B 79, 033409 ͑2009͔͒. This indicates that the formation of periodic structures depends not only on metal properties but also on the electron density of plasma produced on a surface by femtosecond laser pulses.

A 24 W liquid-cooled CW 3 microm fiber laser with a multimode-core Er-doped ZBLAN fiber has been developed. The output power of 24 W and an optical-to-optical efficiency of 14.5% (with respect to incident pump power) were obtained with 975 nm diode pumping. Efficient cooling was implemented by a combination of fluid cooling over the entire length of the fiber and conductive cooling at both end faces of the fiber. Consequently, stable high-power operation was demonstrated. To our knowledge, this is the highest output power obtained by a 3 microm fiber laser. Furthermore, the high power can be further scaled up, since the output power in the present work is limited only by the available pump power.

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