Ultrafast Photoinduced Phase Change in SnSe

Time-resolved multiterahertz (THz) spectroscopy is used to observe an ultrafast, nonthermal electronic phase change in SnSe driven by interband photoexcitation with 1.55 eV pump photons. The transient THz photoconductivity spectrum is found to be Lorentzian-like, indicating charge localization and phase segregation. The rise of photoconductivity is bimodal in nature, with both a fast and slow component due to excitation into multiple bands and subsequent intervalley scattering. The THz conductivity magnitude, dynamics, and spectra show a drastic change in character at a critical excitation fluence of approximately $6\mathrm{mJ}/{\mathrm{cm}}^2$ due to a photoinduced phase segregation and a macroscopic collapse of the band gap.

Figure 1

(a),(b) SnSe layered atomic structure along the in-plane zigzag $b$ axis and armchair $c$ axis in the room temperature $Pnma$ phase. (c) $Pnma$-phase SnSe electronic band structure from ab initio calculations using data provided from Ref. [5] with permission from the authors. Red arrows show carrier excitation from the 1.55 eV pump, blue arrows show postexcitation relaxation pathways to band extrema.

Figure 2

(a),(b) Two-dimensional real conductivity $\mathrm{\Delta }{\sigma }_1(\omega ,\tau )$ maps for 1.8 and $12.0\mathrm{mJ}/{\mathrm{cm}}^2$. (c) Complex THz differential photoconductivity spectra $\mathrm{\Delta }\tilde{\sigma }(\omega ,\tau )=\mathrm{\Delta }{\sigma }_1(\omega ,\tau )+i\mathrm{\Delta }{\sigma }_2(\omega ,\tau )$ for various fluences. $\mathrm{\Delta }{\sigma }_1(\omega ,\tau =1\mathrm{ps})$ shown in dark colors, $\mathrm{\Delta }{\sigma }_2(\omega ,\tau =1\mathrm{ps})$ shown in light colors, and Lorentzian fits shown in black solid and dashed lines. Note the significant difference in vertical axis scales. Fluence dependence of the (d) Lorentzian oscillator strength $N{e}^2/m^{*}$, (e) center frequency ${\omega }_0$, and (f) linewidth $\gamma$. ${\omega }_0$ is fitted with a scaled $\sqrt{F}$ function, shown in red and discussed in the text.

Figure 3

(a)–(e) $\mathrm{\Delta }{\sigma }_1(\omega ,\tau )$ dynamics at $\omega /2\pi =10\mathrm{THz}$ for indicated pump fluences $F$. Rate equation model fits [Eq. (3)] are shown as black lines. Note the differences in vertical axis scaling. (f) $\mathrm{\Delta }{\sigma }_1$ (10 THz, 1 ps) as a function of pump fluence, showing approximately linear scaling below $F_c$ and a sharp drop with little scaling above $F_c$. (g) Fitted proportion of carriers deposited in band extrema $\eta$ [Eq. (3)] as a function of fluence showing sharp drop near $F_c$.