Temperature dependence of InSb reflectivity in the far infrared: Determination of the electron effective mass

We measured the dependence of the reflectivity of InSb crystals upon temper ature in the submillimeter region using monochromatic radiation from an opt ically pumped far infrared (FIR) laser. The measures allowed us to determin e the value of the electron effective mass at low temperatures with radiati ons of different frequencies. Our measurements extend the results obtained recently on pure crystals with magneto-optical methods to the low temperatu res region where only old measures were available.