| Abstract [eng] |
Photonic structures such as photonic crystals and dielectric gratings control light propagation through geometry, enabling photonic band gaps, guided modes, and narrow resonant spectral features. When combined with optically active thin-film coatings, pump-induced changes in refractive index, absorption, or carrier concentration can modify these resonances. Such hybrid systems are therefore promising for ultrafast optical modulation and wavelength-selective switching in the near- and short-wave infrared range. The aim of this work was to investigate the ultrafast optical dynamics of coated microscale photonic structures using transient absorption spectroscopy. Two systems were studied: aluminum-doped zinc oxide (AZO) coated photonic crystals and a tantalum pentoxide (Ta2O5) coated silicon dioxide (SiO2) grating. A microscopic pump-probe setup was used to measure small photonic crystal samples, with the probe focused to approximately 9 µm. White-light continuum generation in YAG was optimized to provide a broadband probe from approximately 1.2 µm to 2.08 µm, enabling transient transmission and reflection measurements with sub-mOD sensitivity. For AZO coated photonic crystals, linear transmission spectra showed distinct near-infrared spectral features associated with photonic resonances or band-gap-like behavior. Measurements confirmed that standalone AZO films exhibit a broadband transient absorption response, while bare photonic crystals produce only a weak pump–probe signal. In the hybrid structures, the transient response combined the broadband character of AZO with additional spectral modulation near the photonic resonances. This indicates that the photonic crystal influences how the pump-induced AZO response appears spectrally, while the relaxation dynamics remain mainly governed by intrinsic carrier relaxation in the AZO coating. The Ta2O5 coated SiO2 grating showed a sharper and more spectrally defined response. Fano-type resonance near 1400 nm for p-polarized incident light was strongly modulated under 257 nm excitation. The transient transmission signal evolved from an initial positive absorption feature into a bipolar spectral response, consistent with a pump-induced redshift of the resonance. Transient reflectance measurements showed a corresponding sign-changing response, supporting this interpretation. The grating response was slower, with relaxation on hundreds of picoseconds, suggesting contributions from longer-lived photoinduced states in Ta2O5. Overall, AZO coated photonic crystals showed faster carrier-driven dynamics but greater sample-to-sample variability, while the Ta2O5 coated SiO2 grating provided more precise and reproducible resonant modulation. These results demonstrate that coated microscale photonic structures can shape ultrafast optical responses in the short-wave infrared and may be useful for wavelength-selective photonic modulation. |
