Ultrafast dynamics of photoinduced semiconductor-to-metal transition in the optical switching nano-oxide ${\mathrm{Ti}}_3{\mathrm{O}}_5$

Nanogranular trititanium pentaoxide $\left({\mathrm{Ti}}_3{\mathrm{O}}_5\right)$, an oxide showing a photoreversible phase transition at room temperature, has recently attracted considerable attention because of its applicability in optical data storage. We report the dynamics of the photoinduced semiconductor-to-metal transition (β phase to λ phase) at room temperature. With time-resolved diffuse reflection spectroscopy conducted over a wide range of time scales, from femtoseconds to microseconds, the overall relaxation behavior was characterized. We found that a transient phase transition occurs within a few hundreds of femtoseconds and that 40% of the converted λ-phase fractions revert to β phase within 1 ps. Carriers are generated instantaneously and disappear within 20 ns. The λ phase increases from 1 to 10 ps, indicating growth of the metallic domain. A slow recovery process, with a duration ranging from ${10}^{-9}$ to ${10}^{-4}$ s, was found and is ascribed to the shrinking and annihilation of the size-distributed metastable λ-phase domains. The fast onset, with a duration within a few hundreds of femtoseconds and the subsequent stabilization process of less than 10 ps, signal the applicability of this material to ultrafast photoswitching.

Figure 1

Schematics of the experimental apparatus, (a) femtosecond pump-probe measurements (100 fs–1 ns) and (b) long duration measurement (>1 ns) with a streak camera. The inset shows a timing chart for the experiment. Regen.: Ti:sapphire regenerative amplifier system, OPA: optical parametric amplifier, BS: beam splitter, DR: diffuse reflection light, PD: Si photodetector, PC: personal computer, and AOM: acoustic optic modulator.

Figure 2

(a) Diffuse reflectance (DR) spectra of β phase (red thick curve) and λ phase (black thin curve). The λ phase was created after irradiation with 5000 pulses (800 nm, 120 fs, 500 Hz, 8.5 × ${10}^{-7}$$\text{mJ}/\mu {\mathrm{m}}^2$); the power of these pulses is indicated by the arrow in the inset. The inset shows the fraction of the surviving β phase versus the excitation density per pulse, which suggests the threshold behavior of the persistent phase transition. The fraction was evaluated from the intensity at 633 nm of the spectrum. (b) The difference in DR spectrum between the β phase and the λ phase, which is normalized by the DR spectrum of the β phase.

Figure 3

(a) Temporal evolution of the transient diffuse reflectance (ΔDR) probed at 633 nm after pumping at 800 nm. The tip of the sharp decline is referred to as the time origin (0 ps) throughout the article, though the true excitation time may be slightly earlier within temporal resolution. The inset shows the ΔDR curves probed at 405, 540, and 600 nm. The dotted lines in the inset are visual guides to show the variation of the periods in the oscillations. (b) The ΔDR curve probed at 633 nm in the range from 10 ps to 200 μs in a logarithmic scale, which is obtained by pump-probe measurement in the earlier stage (red) and by streak camera in the later stage (green). The blue dotted line is a fitting curve, which is a superposition of two decay components: an exponential function with a time constant of 20 ns and a stretched exponential function with an averaged time constant of ∼20 μs and a β factor of ∼0.3. See the text for details of this function. The inset shows the ΔDR curve probed by the streak camera up to 100 ns in a linear scale. The Roman indices (i)–(v) represent the characteristic five processes in the transition, while the capital letters, A (0 ps), B (1.3 ps), C (50 ps), and D (50 ns), represent the typical relaxation stages between these processes.

Figure 4

ΔDR spectra at 0 ps (red filled circles), 1.3 ps (green open triangles), 50 ps (blue crosses), and 50 ns (orange filled rectangles). Behind the spectrum at 0 ps, the difference DR spectrum (${\mathrm{DR}}_{\beta \to \lambda }$) shown in Fig. 1 is replotted with a coefficient of 0.3 and an offset for comparison (gray dotted curve). The capital letters A–D correspond to those defined in Fig. 3.

Figure 5

Excitation power dependence of the ΔDR probed at 633 nm at 0 ps (red filled circles), 1.3 ps (green open triangles), 50 ps (blue crosses), and 50 ns (orange filled rectangles). The capital letters A–D correspond to those in Figs. 3 and 4. The lines are visual guides for easier discrimination of the threshold behaviors.

Figure 6

Schematic drawing of the relaxation dynamics in the case that β-${\mathrm{Ti}}_3{\mathrm{O}}_5$ is excited below the threshold for the persistent transition. The yellow-masked background represents the excited free carriers.