I am aware of many femtosecond laser based approaches to excite phonon modes e.g. in crystals and minerals. Quantum Cascade Lasers (QCLs) seem like an interesting alternative, especially since they can be tuned by frequency and can be operated continuously. Has someone tried to use QCLs to excite phonon modes? Or is there a catch?
What do you want to do with the "excited phonons"? If you just want to excite them, of course this application exists and is used in infrared spectrometers:
https://www.laserfocusworld.com/test-measurement/spectroscopy/article/16548279/spectrometers-midir-spectrometer-uses-quantumcascade-laser-frequency-combs
However, since you talk about femtosecond lasers, you probably mean some sort of time dependency. That of course can only be monitored with a laser pulse that has a width in the timescale of a vibration - and that's the femtosecond range.
This paper help you
https://www.researchgate.net/publication/232088360_Investigation_of_coherent_acoustic_phonons_in_terahertz_quantum_cascade_laser_structures_using_femtosecond_pump-probe_spectroscopy
Yes QCL lasers have been used to excite phonons in solid state wide gap semiconductors as GaN.
This book chapter may help you:
1. Coherent phonon excitation in wide gap semiconductors
https://books.google.dz/books?id=6-Y8YnOfK7QC&pg=PA149&lpg=PA149&dq=quantum+cascade+laser+phonon+excitation&source=bl&ots=BHZmWt-F2G&sig=ACfU3U1B36YD6JkebqhFOUI1oRH2rZqInw&hl=fr&sa=X&ved=2ahUKEwip7b2M_-npAhUb5uAKHfcYB4EQ6AEwCXoECAoQAQ#v=onepage&q=quantum%20cascade%20laser%20phonon%20excitation&f=false
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