ESTABLISHMENT OF COHERENT PHONON EXCITATION --as a new method for phase transition study--

A new experimental method COHERENT PHONON EXCITATION，which is quite adequate for the study of phase transition has been established during the period of the present research project for two years from 1998 to 2000。This method provides a direct observation of the soft excitations closely related to the phase transition dynamics.The time dependence of the motion of the soft phonon mode and/or a relaxational mode are precisely distinguishable by their time dependence because the@equation of motion of thoes excitation is usually driven in the time domain。As a result of the present project，a special improvement of the experimental system has been completed using an optical interference plate(Phase Mask)and heterodyne detection for the light diffracted from the excited phonons.The project is summarized as follows；1.The soft ferroelectric B I D 22个D 2 mode of KDP has been excited and observed its time dependence as a function of temperature in a range from RT to Tc.A clear single exponential time dependence of the B I D22 ii D2 mode even30K far above Tc indicates an order-disorder type phase transition dynamics in KDP。2.A precise observation of the dispersion relation of the phonon-polariton of the ferroelectric LiNbO锡D23个D 2has been done at room temperature.Because the present method has advantage in a small wave vector region，the dispersion relation of the A I D21文件D2 phonon-polariton is determined with high accuracy elucidating a very simple curve in contrast to the several resonance points below7 THz reported in the previous studies.3.An ultra-slow relaxational motion in the glass-liquid transition has been studied with wave vectors defined precisely。

A new experimental method COHERENT PHONON EXCITATION, which is quite adequate for the study of phase transition has been established during the period of the present research project for two years from 1998 to 2000. This method provides a direct observation of the soft excitations closely related to the phase transition dynamics. The time dependence of the motion of the soft phonon mode and/or a relaxational mode are precisely distinguishable by their time dependence because the equation of motion of thoes excitation is usually driven in the time domain. As a result of the present project, a special improvement of the experimental system has been completed using an optical interference plate (phase mask) and heterodyne detection for the light diffracted from the excited phonons. The project is summarized as follows; 1. The soft ferroelectric BィイD22ィエD2 mode of KDP has been excited and observed its time dependence as a function of temperature in a range from RT to Tc. A clear single exponential time dependence of the BィイD22ィエD2 mode even 30 K far above Tc indicates an order-disorder type phase transition dynamics in KDP. 2. A precise observation of the dispersion relation of the phonon-polariton of the ferroelectric LiNbOィイD23ィエD2 has been done at room temperature. Because the present method has advantage in a small wave vector region, the dispersion relation of the AィイD21ィエD2 phonon-polariton is determined with high accuracy elucidating a very simple curve in contrast to the several resonance points below 7THz reported in the previous studies. 3. An ultra-slow relaxational motion in the glass-liquid transition has been studied with wave vectors defined precisely.