  We demonstrate an intense broadband terahertz (THz) source based on the
interaction of relativistic-intensity femtosecond lasers with aligned copper
nanorod array targets. For copper nanorod targets with length 5 \mu m, a
maximum 13.8 times enhancement in the THz pulse energy (in $\leq$ 20 THz
spectral range) is measured as compared to that with a thick plane copper
target under the same laser conditions. A further increase in the nanorod
length leads to a decrease in the THz pulse energy at medium frequencies
($\leq$ 20THz) and increase of the electromagnetic pulse energy in the
high-frequency range (from 20 - 200 THz). For the latter, we measure a maximum
energy enhancement of 28 times for the nanorod targets of length 60 \mu m .
Particle-in-cell simulations reveal that THz pulses are mostly generated by
coherent transition radiation of laser produced hot electrons, which are
efficiently enhanced with the use of nanorod targets. Good agreement is found
between the simulation and experimental results.
