The ultrafast optical experiments were performed using a novel setup of tunable near-to-mid UV pump and broadband UV probe, described in detail in (7). A 20-kHz Ti:sapphire regenerative amplifier (KMLabs, Halcyon + Wyvern 500), providing pulses at 1.55 eV, with 0.6-mJ energy and around 50-fs duration, pumped a noncollinear optical parametric amplifier (NOPA) (TOPAS-white, Light Conversion) to generate sub–90 fs visible pulses (range, 1.77 to 2.30 eV). The typical output energy per pulse was 13 μJ. Around 60% of the output of the NOPA was used to generate the narrowband pump pulses. The visible beam, after passing through a chopper, operating at 10 kHz and phase-locked to the laser system, was focused onto a 2-mm-thick β-barium borate crystal for nonlinear frequency doubling. The pump photon energy was controlled by the rotation of the crystal around the ordinary axis and could be tuned in a spectral range up to ∼0.9 eV (∼60 nm) wide. For our purpose, the pump photon energy was set at 4.50 eV to selectively perturb the spectral region above the c-axis excitonic peak of anatase TiO2. The typical pump bandwidth was 0.02 eV (1.5 nm), and the maximum excitation energy was about 120 nJ. The pump power was recorded on a shot-to-shot basis by a calibrated photodiode for each pump photon energy, allowing the normalization of the data for the pump power. The remaining NOPA output was used to generate the broadband UV probe pulses with ∼1.3-eV (∼100 nm) bandwidth through an achromatic doubling scheme.

To study the anatase TiO2 single crystals, the setup was used in the reflection geometry. The specimens were mounted on a rotating sample holder to explore the transient reflectivity (∆R/R) along the desired crystalline axis. Pump and probe pulses, which have the same polarization, were focused onto the sample, where they were spatially and temporally overlapped. The typical spot size of the pump and the probe were 100 and 40 μm full width at half maximum, respectively, resulting in a homogeneous illumination of the probed region. The portion of the probe beam reflected by the surface of the crystal was detected, and the time evolution of the difference in the UV probe reflection with and without the pump pulse was reconstructed. After the sample, the reflected probe was focused in a multimode optical fiber (100 μm), coupled to the entrance slit of a 0.25-m imaging spectrograph (Chromex 250is). The beam was dispersed by a holographic grating (150 g/mm) and imaged onto a multichannel detector consisting of a 512-pixel CMOS (complementary metal-oxide semiconductor) linear sensor (pixel size, 12.5 μm × 250 μm; Hamamatsu S11105) with up to 50-MHz pixel readout, so the maximum readout rate per spectrum (almost 100 kHz) allowed us to perform shot-to-shot detection easily. All the experiments were performed at RT.

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