Far-Infrared Laser Spectroscopy of Phonon Difference Band Absorption in TICI

It is well known that most crystals which show Reststrahlen behavior are nearly transparent at low temperatures for far-infrared radiation at frequencies below the infrared active eigen-frequency. The crystals are nearly opaque at room temperature due to strong absorption by phonon combination processes [1]. We report experimental results which show that for T1C1 a pronounced LA-TA two phonon difference band absorption occurs where transverse acoustic (TA) phonons and longitudinal acoustic (LA) phonons at the boundary of the Brillouin zone are involved. For other crystals for which far-infrared difference band absorption has been studied no pronounced absorption by LA-TA phonon combination processes has been found. For alkali-halides it has been concluded from the temperature dependence of the absorption at fixed frequencies that absorption due to LA-TA combination processes is very weak [2]. Difference band absorption has been found in which mainly combinations of optical and acoustic phonons are involved. Similar results were reported for GaAs [3]. From the frequency dependence of the far-infrared absorption in InSb it has also been concluded that only weak LA-TA absorption occurs [4]. In this crystal the absorption by LA-TA processes is superimposed by absorption due to phonon sum processes. We have measured the transmission of a T1C1 crystal (2 mm thick) for radiation of a discharge pumped farinfrared laser. In order to obtain a survey of the transmission behavior of TIC1 we have chosen laser radiation of frequencies at 0.89 THz (337 ~tm radiation of the HCN-laser), 1.36 THz and 2.54 THz (water vapor laser) and 1.75 THz (D20 laser). These frequencies are below and above the infrared active eigen-frequency of T1C1 at about 1.9 THz [5, 6]. The radiation was detected with a Golay cell. In case of large sample transmission the intensity was reduced by absorption filters. For sample cooling we used a temperature variable optical cryostat with crystalline quartz windows. Due to the high power of the farinfrared radiation (10 to 100 mW) it was possible to measure over a large transmission range without changing the sample. The temperature dependence of the transmission for the different far-infrared laser lines is shown in Fig. 1. The transmission for radiation at 1.36 THz and 0.89 THz decreases very strongly in the range of low temperature. The temperature dependence of the transmission at the higher frequencies is quite different. While at the lower temperatures the transmission changes slowly with temperature large changes are observed at temperatures above 100 K. Figure 2 gives a survey of the frequency dependence of the absorption coefficient obtained from an experiment in which we have determined the transmission corrigated for reflection losses [7]. It is clearly seen that a strongly temperature dependent absorption band arises at frequencies below the infrared active eigen-frequency with an absorption maximum at about 1 THz and a cut-off at about 1.5 THz. The far-infrared absorption of T1C1 is caused by the anharmonicity of the crystal vibrations. The anhar-