We report femtosecond time-resolved pump-probe reliection experiments in semimetals and semiconductors that show large-amplitude oscillations with periods characteristic of lattice vibrations. Only A& modes are detected, although modes with other symmetries are observed with comparable intensity in Raman scattering. We present a theory of the excitation process in this class of materials, which we refer to as displacive excitation of coherent phonons (DECP). In DECP, after excitation by a pump pulse, the electronically excited system rapidly comes to quasiequilibrium in a time short compared to nuclear response times. In materials with Aq vibrational modes, the quasiequilibrium nuclear A& coordinates are displaced with no change in lattice symmetry, giving rise to a coherent vibration of A& symmetry about the displaced quasiequilibrium coordinates. One important prediction of the DECP mechanism is the excitation of only modes with A& symmetry. Furthermore, the oscillations in the reflectivity R are excited with a cos(uot) dependence, where t = 0 is the time of arrival of the pump pulse peak, and uo is the vibrational frequency of the A& mode. These predictions agree well with our observations in Bi, Sb, Te, and Ti203. The fit of the experimental AR(t)/R(0) data to the theory is excellent.
A type of device is described which can be used as a microwave amplifier, spectrometer, or oscillator. Experimental results are given. When operated as a spectrometer, the device has good sensitivity, and, by eliminating the usual Doppler broadening, a resolution of 7 kc/sec has been achieved. Operated as an oscillator, the device produced a frequency stable to at least 4 parts in 10 12 in times of the order of a second, and stable over periods of an hour or more to at least a part in 10 10 . The device is examined theoretically, and results are given for the expected sensitivity of the spectrometer, the stability and purity of the oscillation, and the noise figure of the amplifier. Under certain conditions a noise figure approaching the theoretical limit of unity, along with reasonably high gain, should be attainable.of N 14 H3 with this apparatus. Certain of its properties which are necessary for an understanding of the relative intensities of the hyperfine structure components are also discussed there.
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