We investigated the photon absorption in GaAs multi-layer emitter (n(+))-intrinsic (i) structures by means of far-infrared (FIR) reflection and transmission measurements for the application of FIR detection. A Fresnel matrix method has been proposed to calculate both the FIR reflection and transmission spectra of the multi-layer structures. This method can directly present absorption in every layer through calculating the energy flux, by aid of the yielded doping concentration-dependent refractive index, extinction index and relaxation time in the FIR region. Our results show that the doping concentration and light field distribution have the identically important effect on the light absorption in the multi-layer structures. A high absorption n-GaAs FIR detector structure has been designed. (C) 2002 Elsevier Science B.V. All rights reserved.