The nature of photoinduced phase transition and metastable states in vanadium dioxide

Abstract

Photoinduced threshold switching processes that lead to bistability and the formation of metastable phases in photoinduced phase transition of VO2 are elucidated through ultrafast electron diffraction and diffusive scattering techniques with varying excitation wavelengths. We uncover two distinct regimes of the dynamical phase change: a nearly instantaneous crossover into an intermediate state and its decay led by lattice instabilities over 10 ps timescales. The structure of this intermediate state is identified to be monoclinic, but more akin to M2 rather than M1 based on structure refinements. The extinction of all major monoclinic features within just a few picoseconds at the above-threshold-level (~20%) photoexcitations and the distinct dynamics in diffusive scattering that represents medium-range atomic fluctuations at two photon wavelengths strongly suggest a density-driven and nonthermal pathway for the initial process of the photoinduced phase transition. These results highlight the critical roles of electron correlations and lattice instabilities in driving and controlling phase transformations far from equilibrium.

Ultrafast diffraction conoscopy of the structural phase transition in VO2 : Evidence of two lattice distortions

“…However, these techniques only partially resolve lattice relaxation dynamics, and many features remain hidden. After rigorous analysis of experimental data, several works [16,28,29,56,58] reported the presence of at least two stages of phase transition. Here, applying UDC technique the two stages of SPT process can be clearly resolved on a femtosecond time scale as shown in Fig.…”

“…Recently, several studies using tempearature [17,[67][68][69][70], pressure [71][72][73][74], light [29,35,58,66] and voltage [27] demonstrate that VO 2 phase transition takes place through an intermediate phase with monoclinic symmtery. Bai et al suggest the presence of several new phases (e.g.…”

Active control of bright electron beams with RF optics for femtosecond microscopy

“…For example, on the granular nanoscale VO 2 films, the weak signals from key UED reflections relevant to the structural phase transition stand on a large diffusive scattering background. Furthermore, a low repetition rate is frequently required for the thermal relaxation of the isolated samples (>1 ms) 56 . All of these demand a high instantaneous dose, and to clearly track individual reflections from complex structural changes, a large coherence length is also typically required.…”

“…1(a)] is further employed, resulting in a beam waist of ∼65  μ m at the specimen. Figure 8(a) shows the diffraction image obtained with over 4000 shots from a 40 nm VO 2 film, which is deposited on a 100  μ m × 100  μ m TEM nano-membrane window 56 . From electron counting, we determine D e  = 6.8 e/ μ m 2 .…”

“…Meanwhile, the coherence length is ≥15 nm, which is determined based on L C =λ e /(2 α s ), where α s , the half width at half maximum of the beam divergence angle at the specimen, is estimated to be no more than 0.007°. We note that α s is retrieved based on σ B recorded on the camera (at s  =   7 Å −1 ) without excluding the finite inherent diffraction width from the VO 2 film (which is appreciable due to inhomogeneity 56 ) and energy-spread-led broadening, representing an upper limit of beam divergence. Correspondingly, we may estimate the transverse emittance for the incident beam contributing to the observed diffraction pattern.…”