  Experimental spectroscopic studies are presented, in a 5.5--25.5nm
extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma
produced by the irradiation of tin microdroplets by 5-ns-pulsed,
2-$\mu$m-wavelength laser light. Emission spectra are compared to those
obtained from plasma driven by 1-$\mu$m-wavelength Nd:YAG laser light over a
range of laser intensities spanning approximately $0.3-5 \times
10^{11}$Wcm$^{-2}$, under otherwise identical conditions. Over this range of
drive laser intensities, we find that similar spectra and underlying plasma
charge state distributions are obtained when keeping the ratio of 1-$\mu$m to
2-$\mu$m laser intensities fixed at a value of 2.1(6), which is in good
agreement with RALEF-2D radiation-hydrodynamic simulations. Our experimental
findings, supported by the simulations, indicate an approximately inversely
proportional scaling $\sim \lambda^{-1}$ of the relevant plasma electron
density, and of the aforementioned required drive laser intensities, with drive
laser wavelength $\lambda$. This scaling also extends to the optical depth that
is captured in the observed changes in spectra over a range of droplet
diameters spanning 16-51$\mu$m at a constant laser intensity that maximizes the
emission in a 2\% bandwidth around 13.5nm relative to the total spectral
energy, the bandwidth relevant for EUV lithography. The significant improvement
of the spectral performance of the 2-$\mu$m- vs 1-$\mu$m driven plasma provides
strong motivation for the development of high-power, high-energy near-infrared
lasers to enable the development of more efficient and powerful sources of EUV
light.
