Terahertz-range intracenter silicon lasers have demonstrated significant progress over past decade in expanding the frequency operation range, in manipulation of a silicon matrix as well as in lowering of optical pump threshold. While terahertz gain realized in silicon lasers has been already defined, optical losses in the active media have not been yet measured experimentally. We applied continuous wave terahertz quantum cascade lasers, operating at 2.5 THz and 3.1 THz for direct measurements of the light losses at conditions of optical pumping with pump intensities exceeding laser thresholds for corresponding media. These measurements allow estimates of THz losses as well as determination of physical mechanisms mainly contributing for total THz absorption in optically excited silicon. It was found that absorption by so called D¯-centers, dynamically induced in optically excited silicon, cause major loss of THz emission. These losses, however, can be significantly reduced by a proper procedure of crystal growth and doping.
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