  We have developed a microscopic theory for the generation and detection of
coherent phonons in armchair and zigzag carbon nanoribbons using an extended
tight-binding model for the electronic states and a valence force field model
for the phonons. The coherent phonon amplitudes satisfy a driven oscillator
equation with the driving term depending on photoexcited carrier density. We
examine the coherent phonon radial breathing like mode amplitudes as a function
of excitation energies and nanoribbon types. For photoexcitation near the
optical absorption edge the coherent phonon driving term for the radial
breathing like mode is much larger for zigzag nanoribbons where transitions
between localized edge states provide the dominant contribution to the coherent
phonon driving term. Using an effective mass theory, we explain how the
armchair nanoribbon width changes in response to laser excitation.
