A Modified $8f$ Geometry With Reduced Optical Aberrations For Improved   Time Domain Terahertz Spectroscopy
  We present a modified $8f$ geometry for time domain terahertz (THz) spectroscopy (TDTS) experiments. We show, through simulations and data, that a simple rearranging of the off-axis parabolic mirrors, which are typically used to focus and direct THz radiation in TDTS experiments, results in a nearly 40% reduction in the THz focal spot diameter. This effect stems from significant reduction of the principle optical aberrations which are enhanced in the conventional $8f$ geometry but partially compensated in the modified $8f$ experimental setup. We compare data from our home-built TDTS spectrometer in the modified $8f$ geometry to that of previous iterations that were designed in the conventional $8f$ geometry to demonstrate the effect. 
N. J. Laurita, Bing Cheng, R. Barkhouser, V. A. Neumann, and N. P.   Armitage
physics.optics cond-mat.mtrl-sci physics.ins-det