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Research

Ultrafast insulator-metal phase transition in V3O5

Vanadium oxides is very broad class of materials with exotic physical and chemical properties. The metal-insulator phase transition phenomena in these oxides have drawn considerable attention over the past decades. Many of these oxides can undergo phase transition by applying a pressure, strain or heat, and only two of them (VO2 and V3O5) show metal-insulator phase transition above room temperature. Another possibility to induce the phase transition is to illuminate the material by light. This makes vanadium oxides extremely promising for novel ultrafast optoelectronic applications. However to date, the light-induced transition was discovered for a few oxides only. While the first observation of photoinduced phase transition and optical nonlinearity of VO2 was made in 70s, for V3O5 these features were not discovered until now. First results on strong optical nonlinearity and nonequilibrium excited-state dynamics of V3O5 were recently obtained in our lab. In series of measurements it was confirmed that the optical nonlinearity is originated from light-induced insulator-metal phase transition. It was found that the formation of metallic phase occurs within an ultrafast timescale of ~1 picosecond. The nature of stable insulating phase of V3O5 can be understood in terms of Wigner crystallization of small polarons due to strong correlations between carriers. However this stability of insulating phase can be destroyed by light. A photogeneration of dense electron-hole plasma can produce a screening of electron correlations to the level when electrostatic interaction cannot provide segregation and localization of charges. As a result, the polaronic crystal melts into non-correlated polaronic states. Photoinduced oscillations of the optical signal were observed due to excitation of coherent acoustical phonons in the material. Also we provided first quantum calculation of electronic band structure and density of states for V3O5 .
Phys. Rev. Lett. 119, 057602 (2017)      J. Appl. Phys. 121, 235302 (2017)


Ultrafast Diffraction Conoscopy: optical tracking the lattice distortion of phase-change electronic materials

In the case of light-induced solid-to-solid phase transition, it was always an immense problem to monitor by optical techniques only lattice relaxation or only relaxation of electronic subsystem. Conventional optical pump-probe methods provide information about the collective response of lattice plus electronic subsystems, and usually it is impossible to separate these two contributions in optical signal. In order to monitor only the lattice distortion (neglecting electronic dynamics), we developed ultrafast diffraction conoscopy (UDC) technique. This optical method reliably detects structural dynamics separately from electronic dynamics in epitaxial films of phase-change materials: VO2 and V2O3. 4D ultrafast scatterometer equipped by polarization optics allows performing UDC measurements with extraordinary detail even in films with a thickness much less than the optical wavelength. Using this method, we monitor transient polarization state of scattered light by recording distinctive conoscopy patterns at different time delays. For epitaxial VO2 films deposited on different single-crystal sapphire substrates, UDC patterns show two-stage evolution of light polarization. This specific optical response reveals two components in photoinduced structural phase transition of VO2 on subpicosecond timescale.
Phys. Rev. B. 95, 235157 (2017)     Proc. SPIE. 10345, 103451F (2017)

      


Pathways of light-induced structural dynamics in phase-change materials

A new approach to analyze different pathways of photoinduced structural dynamics in phase-change materials was proposed. Angle-resolved ultrafast light diffraction technique enables tracking complex mesoscale phase transition. 4D light diffraction along with conventional pump-probe reflection and transmission techniques reveals distinct contributions of optical and acoustical phonons, material morphology and strain in phase trajectories.
Semiclassical computation of molecular dynamics reveals significant instability of the monoclinic phase in the absence of electron-electron correlations. Experimental data of ultrafast VO2 dynamics enable the reconstruction/estimation of the potential barrier between two phases of photoexcited material. The analysis of relaxation rate vs. laser pump fluence F on picosecond time scale yields the equation for the potential barrier which separates different structural phases: ΔG(F)=-NkBTln[F/Fmax]. This result enables the reconstruction/estimation of free energy landscape (thermodynamic potential Φ of photoexcited VO2) from experimental data. The realistic modeling of ultrafast structural dynamics can be performed quantitatively in terms of Ginzburg-Landau formalism. This model provides a reliable explanation of experimentally observed nonequilibrium dynamics and metastability of VO2, where the phase trajectories depend on the excitation level.
Phys. Rev. B. 96, 075128 (2017)



4D Light Scattering: Diffractive Imaging of Surface Dynamics

4D scatterometer was built to monitor ultrafast angle-resolved light scattering. Ultrafast optical diffraction allows detecting transient changes in crystal symmetry, surface morphology, domain size and statistical properties of phase-change materials with high temporal and spatial resolution. The real-time numerical analysis and post-processing of ultrafast light diffraction data provides rigorous approach to diffractive imaging and ultrafast statistical analysis of photoinduced surface dynamics on mesoscale. The reconstruction of surface profile and computation of autocorrelation functions for nonequilibrium processes not only shows how fast these phenomena occur but also what really is happening on the surface. New algorithms for multiprocessing data analysis, including GPU and CUDA technology, facilitate obtaining important information about correlations between photoexcited state dynamics, grain size and transient structural disorder of stochastic surfaces. A subwavelength super-resolution can be achieved in diffractive imaging.
News release: "Luz esparcida, femtosegundos y aplicaciones del dióxido de vanadio"
Appl. Opt. 54, 2141, (2015)     J. Appl. Phys. 117, 184304 (2015)     J. Appl. Phys. 114, 153514 (2013)     OSA Ultr. Ph. 2016, UW4A.15     OSA Ultr. Ph. 2016, UTu4A.48     MRS Advances 2, 1231 (2017)
                         



Super-resolution in diffractive imaging

This work has the capability to provide answers to some ultimate questions related to diffractive imaging in light scattering metrology. In this work we use lab-built optical scatterometer to reconstruct surface profile from 3D diffraction patterns of light scattered by photonic crystals. New approach to numerical analysis of 3D light diffraction allows obtaining a super-resolution of submicron objects. Metrological precision, submicron and subwavelength resolution were achieved by applying phase-retrieval algorithms and filtering of noisy experimental data which contains significant amount of diffuse scattering. Proposed technique is very promising for the monitoring of ultrafast photoinduced surface dynamics, where all scattered field can be recorded in single optical shot. Sophisticated but robust methods of diffractive imaging can be very useful for modern optics and condensed matter physics.
Opt. Lett. 42, 2263 (2017)



Advanced Materials

Our materials fabrication and characterization work centers on thin film growth by pulsed laser deposition (PLD) and pulsed-dc magnetron sputtering. Main materials of current interest include several vanadium oxide phases, particularly those which exhibit phase transitions, as well as other oxides. Attention is directed also to variation of properties caused by structural differences, including those in the nanoscale, and by stress. Film characterization extends to determination of crystal structure by x-ray diffraction as well as study of electronic, optical, and mechanical properties.
J. Appl. Phys. 121, 235302 (2017)      J. Appl. Phys. 118, 125308 (2015)      J. Appl. Phys. 107, 074506 (2010)      J. Appl. Phys. 105, 113504 (2009)