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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.

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We report the first systematic femtosecond pump-probe measurements of the electron-phonon coupling constant X in thin films of Cu, Au, Cr, Ti, W, Nb, V, Pb, NbN, and V36a. The agreement between our measured X, values and those obtained by other techniques is excellent, thus confirming recent theoretical predictions of Allen. By depositing thin Cu overlayers when necessary, we can extend this technique to nearly any metallic thin film.

Ultrahigh-resolution optical coherence tomography (OCT) by use of state of the art broad-bandwidth femtosecond laser technology is demonstrated and applied to in vivo subcellular imaging. Imaging is performed with a Kerr-lens mode-locked Ti:sapphire laser with double-chirped mirrors that emits sub-two-cycle pulses with bandwidths of up to 350 nm, centered at 800 nm. Longitudinal resolutions of ~1mum and transverse resolution of 3mum, with a 110-dB dynamic range, are achieved in biological tissue. To overcome depth-of-field limitations we perform zone focusing and image fusion to construct a tomogram with high transverse resolution throughout the image depth. To our knowledge this is the highest longitudinal resolution demonstrated to date for in vivo OCT imaging.