  Transition metal nitrides have recently gained attention in the fields of
plasmonics, plasmon-enhanced photocatalysis, photothermal applications, and
nonlinear optics because of their suitable optical properties, refractory
nature, and large laser damage thresholds. This work reports comparative
studies of the transient response of films of titanium nitride, zirconium
nitride, and Au under femtosecond excitation. Broadband transient optical
characterization helps to adjudicate earlier, somewhat inconsistent reports
regarding hot electron lifetimes based upon single wavelength measurements.
These pump-probe experiments show sub-picosecond transient dynamics only within
the epsilon-near-zero window of the refractory metals. The dynamics are
dominated by photoinduced interband transitions resulting from ultrafast
electron energy redistribution. The enhanced reflection modulation in the
epsilon-near-zero window makes it possible to observe the ultrafast optical
response of these films at low pump fluences. These results indicate that
electron-phonon coupling in TiN and ZrN is 25-100 times greater than in Au.
Strong electron-phonon coupling drives the sub-picosecond optical response and
facilitates greater lattice heating compared to Au, making TiN and ZrN
promising for photothermal applications. The spectral response and dynamics of
TiN and ZrN are only weakly sensitive to pump fluence and pump excitation
energy. However, the magnitude of the response is much greater at higher pump
photon energies and higher fluences, reaching peak observed values of 15 % in
TiN and 50 % in ZrN in the epsilon-near-zero window.
