Ultrafast Optical Switching to a Metallic State by Photoinduced Mott Transition in a Halogen-Bridged Nickel-Chain Compound

We demonstrate the ultrafast photoinduced Mott transition from a charge transfer insulator to a metal in a halogen-bridged Ni-chain compound by pump-probe reflection spectroscopy. Upon the irradiation of a 130-femtosecond laser pulse, the spectral weight of the gap transition is transferred to the inner-gap region. When the photoexcitation density exceeds $0.1/\mathrm{N}\mathrm{i}$ site, the Drude-like high-reflection band appears in the infrared region, signaling the formation of a metallic state. The photogeneration of the metallic state and the subsequent recovery to the original gapped state occur within a few picoseconds.

Figure 1

(a) Crystal structure and (b) electronic structure of $[\mathrm{N}\mathrm{i}(\mathrm{c}\mathrm{h}\mathrm{x}\mathrm{n}{)}_2\mathrm{B}\mathrm{r}]{\mathrm{B}\mathrm{r}}_2$.

Figure 2

(a) Reflectivity spectra observed prior to the photoexcitation (a broken line) and at delay time $t_d$ after the photoexcitation (colored dots and solid lines) at room temperature. The excitation density $x_{ph}$ is $0.5\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{n}/\mathrm{N}\mathrm{i}$ site. Polarizations of the pump and probe lights are both parallel to the chain axis $\mathbf{b}$. (b) Spectra of the imaginary part of the dielectric constant ${ϵ}_2$ before (a broken line) and after (colored solid lines) photoexcitation, obtained by Kramers-Kronig analysis of the reflectivity data shown in (a).

Figure 3

(a) Reflectivity spectra before (a broken line) and immediately after the photoexcitation ($t_d=0.1$) (colored dots and solid lines) at room temperature. The excitation densities $x_{ph}$ are shown in the figure. (b) ${ϵ}_2$ spectra before (broken line) and immediately after the photoexcitation ($t_d=0.1$) (colored solid lines). A dotted line shows the spectrum before the photoexcitation, which is obtained by the direct measurement of the polarized absorption. (c) Photoinduced change in the effective number of electrons $N_{eff}$ ($\omega$) $[\Delta N_{eff}(\omega )]$ at $t_d=0.1\mathrm{p}\mathrm{s}$ for various $x_{ph}$.

Figure 4

(a) $\Delta N_{eff}(1.0\mathrm{e}\mathrm{V})$ and $\Delta N_{eff}(0.2\mathrm{e}\mathrm{V})$ at $t_d=0.1\mathrm{p}\mathrm{s}$ as a function of the photodoping concentration $x_{ph}$. (b) Time evolutions of $\Delta N_{eff}(0.5\mathrm{e}\mathrm{V})$ for $x_{ph}=0.5$ (open circles) and 0.012 (closed circles). Time evolutions of the photoinduced reflectivity change ($\Delta R$) observed at 0.12 eV are also shown by solid lines. The inset shows the semilogarithmic plot of the time profiles of $\Delta R$. The broken curves show the calculated decay profiles [$\Delta R(t_d)=0.4\exp [-(t_d/3)]+0.5\exp [-(t_d/8)]+0.1$ for $x_{ph}=0.012$ and $\Delta R(t_d)=0.5\exp [-(t_d/0.4)]+0.25\exp [-(t_d/3)]+0.25\exp [-(t_d/8)]$ for $x_{ph}=0.5$, respectively].