  We report an absolute energy response function to electronic and nuclear
recoils for germanium and liquid xenon detectors. As a result, we show that the
detection energy threshold of nuclear recoils for a dual-phase xenon detector
can be a few keV for a given number of detectable photoelectrons. We evaluate
the average energy expended per electron-hole pair to be $\sim$3.32 eV, which
sets a detection energy threshold of $\sim$2.15 keV for a germanium detector at
50 mini-Kelvin at 69 volts with a primary phonon frequency of 1 THz. The Fano
factors of nuclear and electronic recoils that constrain the capability for
discriminating nuclear recoils below 2-3 keV recoil energy for both
technologies are different.
