Tomko John A, Aryana Kiumars, Wu Yifan, Zhou Guoqing, Zhang Qiyan, Wongwiset Pat, Wheeler Virginia, Prezhdo Oleg V, Hopkins Patrick E
Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, United States.
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
J Phys Chem Lett. 2025 Feb 6;16(5):1312-1319. doi: 10.1021/acs.jpclett.4c02951. Epub 2025 Jan 28.
Vanadium oxide (VO) is an exotic phase-change material with diverse applications ranging from thermochromic smart windows to thermal sensors, neuromorphic computing, and tunable metasurfaces. Nonetheless, the mechanism responsible for its metal-insulator phase transition remains a subject of vigorous debate. Here, we investigate the ultrafast dynamics of the photoinduced phase transition in VO under low perturbation conditions. By experimentally examining carrier relaxation dynamics at energy levels near the VO band gap (0.6-0.92 eV), we note that numerous optical features do not correspond to the first-order phase transition. Previous studies indeed induced such a phase transition, but they relied on fluences at least an order of magnitude higher, leading to temperature increases well above the transition threshold (340 K). Instead, for excitation fluences that correspond to lattice temperatures only in slight excess of the phase transition (absolute temperatures < 500 K), we find that the marked changes in optical properties are dominated by a shift in the electronic density of states/Fermi level. We find that this effect is a lattice-driven process and does not occur until sufficient energy has been transferred from the excited electrons into the phonon subsystem.
氧化钒（VO）是一种具有多种应用的奇特相变材料，其应用范围涵盖从热致变色智能窗户到热传感器、神经形态计算和可调谐超表面等领域。尽管如此，其金属 - 绝缘体相变的机制仍是激烈争论的主题。在此，我们研究了低扰动条件下VO中光致相变的超快动力学。通过实验研究VO带隙（0.6 - 0.92 eV）附近能级的载流子弛豫动力学，我们注意到许多光学特征并不对应于一级相变。先前的研究确实诱导了这种相变，但它们依赖的光通量至少高一个数量级，导致温度升高远高于转变阈值（340 K）。相反，对于仅对应于略高于相变的晶格温度（绝对温度 < 500 K）的激发光通量，我们发现光学性质的显著变化主要由态密度/费米能级的移动主导。我们发现这种效应是一个晶格驱动的过程，并且直到足够的能量从激发电子转移到声子子系统中才会发生。
