  We demonstrate the highest efficiency ($\sim$80%) second harmonic generation
(SHG) of Joule level, 27 femtosecond, high contrast pulses in a type-I lithium
triborate (LBO) crystal. In comparison, potassium dihydrogen phosphate (KDP)
gives a maximum efficiency of 26%. LBO thus offers high intensity ($>$10$^{19}$
W/cm$^{2}$), ultra-high contrast femtosecond pulses, which have great potential
for high energy density science particularly with nanostructured targets as
well as technological applications.
