F. Raksi

Bio: F. Raksi is an academic researcher. The author has contributed to research in topics: Phase transition & Femtosecond. The author has an hindex of 1, co-authored 2 publications receiving 994 citations.

Femtosecond Structural Dynamics in VO2 during an Ultrafast Solid-Solid Phase Transition.

Abstract: Femtosecond x-ray and visible pulses were used to probe structural and electronic dynamics during an optically driven, solid-solid phase transition in VO(2). For high interband electronic excitation (approximately 5 x 10(21) cm(-3)), a subpicosecond transformation into the high-T, rutile phase of the material is observed, simultaneous with an insulator-to-metal transition. The fast time scale observed suggests that, in this regime, the structural transition may not be thermally initiated.

Femtosecond Laser Trabeculotomy in Perfused Human Cadaver Anterior Segments: A Novel, Noninvasive Approach to Glaucoma Treatment

Abstract: Purpose The purpose of this study was to investigate femtosecond laser trabeculotomy (FLT) in a clinically relevant manner (i.e., delivering the surgical laser beam through the cornea of the intact, human anterior segment to create channels from the anterior chamber into the Schlemm's canal) and to investigate the effect of this treatment on intraocular pressure in perfused human anterior segments. Methods Perfused human anterior segments (15 eyes) received either FLT treatment (n = 8) or a sham-treatment (n = 7). Intraocular pressure (IOP) in the perfused samples was recorded before and after treatment. Spectral domain optical coherence tomography, second harmonic generation imaging, and transmission electron microscopy were used to investigate the FLT channels. Results The FLT group (n = 7, 1 eye excluded) had a statistically significant reduction in mean IOP of 20.2% from baseline after treatment (5.06 ± 1.46 mm Hg to 4.04 ± 1.63 mm Hg; P < 0.0005), whereas the control group (n = 7) remained statistically unchanged (7.72 ± 3.45 mm Hg to 7.78 ± 3.51 mm Hg; P < 0.71). Imaging confirmed that the channels traversed the entire trabecular meshwork into the Schlemm's canal. Conclusions This study has provided the first direct evidence supporting the feasibility of clinically applicable, noninvasive femtosecond laser trabeculotomy for the treatment of glaucoma. Various imaging modalities revealed minimal collateral damage to adjacent issues. Translational Relevance This work demonstrates noninvasive femtosecond laser trabeculotomy in a laboratory setting that is clinically relevant.

Femtosecond optical and x-ray measurement of the semiconductor-to-metal transition in VO/sub 2/

Abstract: Summary form only given. The dynamics of phase transformations in condensed phases have been intensely investigated for decades. While the use of ultrashort visible pulses has allowed measurement of ultrafast changes in the optical properties, the correlation between atomic movement and changes in the electronic properties of materials has proven more elusive. Ultrafast X-ray diffraction provides a direct way to retrieve lattice dynamics. Here, we report on the conjunct measurement of ultrafast electronic and structural dynamics during a semiconductor-to-metal phase transition in VO/sub 2/, where rearrangement of the unit cell from monoclinic to rutile is accompanied by a sharp increase in the electrical conductivity.

Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial

Abstract: An innovative technique uses ultrafast below-bandgap electric-field pulses to induce and probe an insulator–metal transition in an oxide thin film on which a metamaterial structure has been deposited. The transition from insulating to metallic behaviour and the microscopic interactions that accompany the transition are important phenomena in electronic materials. Until now it has not been possible to observe the transition directly in a time-resolved manner. Here, Richard Averitt and colleagues use ultrafast terahertz pulses to induce a phase transition in a prototypical insulator–metal transition material (vanadium dioxide) on which a metamaterial structure has been deposited. The metamaterial serves to amplify the local terahertz field, as well as to detect macroscopic changes in vanadium dioxide. Through direct, time-resolved observations, the authors establish a detailed microscopic picture of the structural and electronic changes underlying the insulator–metal transition. They conclude that their technique is versatile and could even be used to study phase transitions in superconductors. Electron–electron interactions can render an otherwise conducting material insulating1, with the insulator–metal phase transition in correlated-electron materials being the canonical macroscopic manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator–metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field2,3,4,5,6,7. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator–metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport9. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics10,11. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.

Oxide Electronics Utilizing Ultrafast Metal-Insulator Transitions

Abstract: Although phase transitions have long been a centerpiece of condensed matter materials science studies, a number of recent efforts focus on potentially exploiting the resulting functional property changes in novel electronics and photonics as well as understanding emergent phenomena. This is quite timely, given a grand challenge in twenty-first-century physical sciences is related to enabling continued advances in information processing and storage beyond conventional CMOS scaling. In this brief review, we discuss synthesis of strongly correlated oxides, mechanisms of metal-insulator transitions, and exploratory electron devices that are being studied. Particular emphasis is placed on vanadium dioxide, which undergoes a sharp metal-insulator transition near room temperature at ultrafast timescales. The article begins with an introduction to metal-insulator transition in oxides, followed by a brief discussion on the mechanisms leading to the phase transition. The role of materials synthesis in influencing functional properties is discussed briefly. Recent efforts on realizing novel devices such as field effect switches, optical detectors, nonlinear circuit components, and solid-state sensors are reviewed. The article concludes with a brief discussion on future research directions that may be worth consideration.

Electrodynamics of correlated electron materials

Abstract: We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

Collective bulk carrier delocalization driven by electrostatic surface charge accumulation

Abstract: In the classic transistor, the number of electric charge carriers--and thus the electrical conductivity--is precisely controlled by external voltage, providing electrical switching capability. This simple but powerful feature is essential for information processing technology, and also provides a platform for fundamental physics research. As the number of charges essentially determines the electronic phase of a condensed-matter system, transistor operation enables reversible and isothermal changes in the system's state, as successfully demonstrated in electric-field-induced ferromagnetism and superconductivity. However, this effect of the electric field is limited to a channel thickness of nanometres or less, owing to the presence of Thomas-Fermi screening. Here we show that this conventional picture does not apply to a class of materials characterized by inherent collective interactions between electrons and the crystal lattice. We prepared metal-insulator-semiconductor field-effect transistors based on vanadium dioxide--a strongly correlated material with a thermally driven, first-order metal-insulator transition well above room temperature--and found that electrostatic charging at a surface drives all the previously localized charge carriers in the bulk material into motion, leading to the emergence of a three-dimensional metallic ground state. This non-local switching of the electronic state is achieved by applying a voltage of only about one volt. In a voltage-sweep measurement, the first-order nature of the metal-insulator transition provides a non-volatile memory effect, which is operable at room temperature. Our results demonstrate a conceptually new field-effect device, extending the concept of electric-field control to macroscopic phase control.