In this work we present results on the influence of laser pulse duration and irradiating power density
on the conversion efficiency (CE) and the ion energy of gadolinium (Gd) laser produced plasmas.
Three lasers were used with 10 ns, 150 ps and 140 fs pulse durations. By varying the lasers output
energies, experiments could be carried out for a power density range of 1011 - 1015 W/cm2. A
maximum CE of 0.4% was achieved within a 0.6% bandwidth in 2π steradians using the picosecond
laser. A faraday cup was used to calculate ion yield and time of flight measurements of each laser.
The picosecond laser also showed a reduction in the ion time of flight measurements compared with
the nanosecond pulse.
We demonstrate a table-top strong band emission water window source based on laser-produced high-Z plasmas.
Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays in the 2 to 4
nm region, extending below the carbon K edge (4.37 nm). Arrays resulting from n = 4-n = 4 transitions are overlaid
with n = 4-n = 5 emission and shift to shorter wavelength with increasing atomic number. Under spectral analysis a
guideline for microscope construction design for single-shot live cell imaging is proposed based on the use of a
bismuth plasma source, coupled with multilayer mirror optics.
We demonstrate an efficient extreme ultraviolet (EUV) source for operation at λ = 6.7 nm by optimizing the optical
thickness of gadolinium (Gd) plasmas. Using low initial density Gd targets and dual laser pulse irradiation, we
observed a maximum EUV conversion efficiency (CE) of 0.54% for 0.6% bandwidth (BW) (1.8% for 2%BW),
which is 1.6 times larger than the 0.33% (0.6%BW) CE produced from a solid density target. Enhancement of the
EUV CE by use of a low-density plasma is attributed to the reduction of self-absorption effects.
We have demonstrated a laser-produced plasma extreme ultraviolet source operating in the 6.5-6.7 nm region based
on rare-earth targets of Gd and Tb coupled with a Mo/B4C multilayer mirror. Multiply charged ions produce strong
resonance emission lines, which combine to yield an intense unresolved transition array. The spectra of these
resonant lines around 6.7 nm suggest that the in-band emission increases with increased plasma volume by
suppressing the plasma hydrodynamic expansion loss at an electron temperature of about 50 eV, resulting in
maximized emission. We also have investigated the dependence of the spectral behavior and conversion efficiencies
of rare-earth plasma extreme ultraviolet sources with peak emission at 6.7 nm on laser wavelength and the initial
target density. The maximum conversion efficiency was 1.3% at a laser intensity of 1.6 × 1012 W/cm2 at an operating
wavelength of 1064 nm, when self-absorption was reduced by use of a low initial density target.
An engineering prototype high average power 13.5-nm source has been shipped to semiconductor facilities to permit
the commencement of high volume production at a 100 W power level in 2011. In this source, UTA (unresolved
transition array) emission of highly ionized Sn is optimized for high conversion efficiency and full recovery of the
injected fuel is realized through ion deflection in a magnetic field. By use of a low-density target, satellite emission
is suppressed and full ionization attained with short pulse CO2 laser irradiation. The UTA is scalable to shorter
wavelengths, and Gd is shown to have similar conversion efficiency to Sn (13.5 nm) at a higher plasma temperature,
with a narrow spectrum centered at 6.7 nm, where a 70% reflectivity mirror is anticipated. Optimization of short
pulse CO2 laser irradiation is studied, and further extension of the same method is discussed, to realize 100 W
average power down to a wavelength of 3 nm.
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