Phase Transition Of VO2 Research Articles

Femtosecond laser-induced phase transition in VO2 films.

VO2 is a very promising material due to its semiconductor-metal phase transition, however, the research on fs laser-induced phase transition is still very controversial, which greatly limits its development in ultrafast optics. In this work, the fs laser-induced changes in the optical properties of VO2 films were studied with a variable-temperature Z-scan. At room temperature, VO2 consistently maintained nonlinear absorption properties at laser repetition frequencies below 10 kHz while laser-induced phase transition properties appeared at higher repetition frequencies. It was found by temperature variation experiments at 100 kHz that the modulation depth of the laser-induced VO2 phase transition was consistent with that of the ambient temperature-induced phase transition, which was increased linearly with thickness, further confirming that the phase transition was caused by the accumulation of thermal effects of a high-repetition-frequency laser. The phase transition process is reversible and causes substantial changes in optical properties of the film, which holds significant promise for all-optical switches and related applications.

Vanadium dioxide-based terahertz metasurfaces for manipulating wavefronts with switchable polarization

As a unique phase transition material, vanadium dioxide (VO2) has been widely applied in switchable and modulating metasurfaces. In this work, VO2-based reflective meta-atoms are designed to achieve wavefront reconfiguration in terahertz frequency. When VO2 is metal, four optimized meta-atoms with 90o phase difference provide a full phase coverage, and they achieve high polarization conversion efficiency. After phase transition of VO2, the capability in the metallic state is lost. Thus, the corresponding phase coverage is switched from 360° to 0°, and the phenomenon of polarization conversion almost vanishes. Assembled by these meta-atoms, three metasurfaces are numerically presented. When VO2 is in the state of metal, they are anomalous reflector, metalens, and vortex beam generator. The presented metasurfaces own the abilities of wavefront manipulation and polarization conversion. When VO2 is in the insulating state, three metasurfaces are switched to specular reflector. Simulations are conducted to verify their abilities of wavefront modulation and polarization control. The proposed metasurfaces may have potential applications in the fields of terahertz switching, communication, and focusing.

Bidirectional Terahertz Vortex Beam Regulator

Most of the reported vortex beam generators with orbital angular momentum (OAM) in the terahertz region only operate in either the reflection mode or the transmission mode, which greatly limits the integration and application in terahertz technology systems. Herein, we propose a full-space vortex beam regulator at two different frequencies. By changing the VO2 phase transition state, the transmission and reflection mode OAM beams can be flexibly controlled by a single metasurface. For the transmission mode, the proposed structure realizes an OAM beam at the topological charges of l = 1 and 2 at 0.6 THz and 1.4 THz. For the reflection mode, our structure generates an OAM beam at the topological charges of l = 1 and 2 at 0.9 THz and 1.5 THz. Based on the superposition theorem and convolution operation principle, the regulation of an OAM vortex beam with a specific deflection angle and a symmetrical deflection OAM vortex beam are realized. The designed metasurface integrates multiple transmitted and reflected vortex beam functions in full space and has potential application in different terahertz systems.

Thermally reconfigurable topological protection routes for the 2D phononic system

Tunable topological insulators (TIs) with active tunability of frequency or path and better adaptability to the dynamic environment than conventional TIs have attracted great interest recently. The research directions of tunable TIs are mainly in the frequency domain and spatial domain. However, the most current research is mainly based on the former. Obviously, the tunable spatial path can bring more degrees of freedom to the application of TIs. Here, we present a topological protection system realized by a thermally tunable elastic film composed of VO2 covered on a patterned substrate. The topological phase transition is induced by breaking the symmetry of the unit cell on the substrate, and we pre-define the topological protection paths in each direction on the substrate. The conduction or blocking of elastic waves is achieved by using the metal-insulator phase transition of VO2, which enables intelligent TIs with reconfigurable propagation paths and reconfigurable interfaces. In addition, the reconfigurable topological path and the thermal modulation are presented by numerical simulations. Such elastic topological materials and their corresponding devices will significantly promote the development of TIs and are expected to enable the coupling with communication band electromagnetic waves and the realization of light-induced non-contact thermal modulation.

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films.

Vanadium dioxide (VO2) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long- and short-range elastic distortions, as well as the symmetry change and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue toward mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO2 while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO2-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO2. The tunneling barrier is formed by a very thin but persistently insulating surface layer of the VO2. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO2 properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics.

Phase Transition and Optical Properties of VO2 and Al: ZnO/VO2 Thin Films

Thermochromic Vanadium dioxide (VO2) has strong potential for smart window applications but its commercial scale usage is limited due to low visible light transmission. To address this issue, aluminum doped zinc oxide (AZO) anti-reflecting layer is integrated with VO2 layer in the present work. VO2 single layer and AZO/VO2 bilayer thin film samples were deposited by sputtering technique on quartz substrate. The single-phase growth of VO2 and AZO in single layer and bilayer thin film samples is confirmed by X-ray diffraction measurements. Monoclinic M1 phase of VO2 is detected in VO2 and AZO/VO2 thin film samples at room temperature. Monoclinic to rutile structural phase transition (SPT) in these samples is studied by performing temperature-dependent X-ray diffraction measurements. SPT in VO2 thin film samples is close to 68 °C and SPT temperature appears slightly lower in AZO/VO2 sample as compared to VO2 sample. Spectral transmittance measurement at room temperature showed significant improvement in the visible transmittance of AZO bilayer film than that of single layer VO2 thin film. These results demonstrate the possibility of integration of anti-reflecting layers such as AZO with VO2 layer for better visible transmittances suitable for large-scale smart window applications.

High-speed photothermally triggered bidirectional current switching in VO2/AlN/Si-based heterostructure device

The bidirectional current switching (BCS) of a two-terminal device based on a vanadium dioxide (VO2) thin film can be realized by harnessing the photothermally induced phase transition of VO2, triggered by laser pulses. Although this photothermally controlled BCS can achieve a large on-state current and high on/off current ratio compared to those obtained by other implementation approaches, a low repetition rate (< 10 Hz) and slow response time (tens of milliseconds) still restrict its practical use. To tackle these issues, we analyzed the fundamental cause of the performance limit of the BCS through simulations using COMSOL Multiphysics and found that the thermal conductivity of the substrate and the critical temperature of the VO2 film were mainly responsible for the switching speed of the BCS. On the basis of this analysis, we fabricated a VO2-based heterostructure device on a silicon substrate with a buffer layer of aluminum nitride. We measured the transient responses of the device current switched by directly irradiating the device with near-infrared laser pulses under various conditions. The highest-ever repetition rate (100 Hz) could be attained in a 50 mA BCS with 5 ms laser pulses. The response time was also remarkably improved to < 1 ms.

VOx Phase Mixture of Reduced Single Crystalline V2O5: VO2 Resistive Switching.

The strongly correlated electron material, vanadium dioxide (VO2), has seen considerable attention and research application in metal-oxide electronics due to its metal-to-insulator transition close to room temperature. Vacuum annealing a V2O5(010) single crystal results in Wadsley phases (VnO2n+1, n &gt; 1) and VO2. The resistance changes by a factor of 20 at 342 K, corresponding to the metal-to-insulator phase transition of VO2. Macroscopic voltage-current measurements with a probe separation on the millimetre scale result in Joule heating-induced resistive switching at extremely low voltages of under a volt. This can reduce the hysteresis and facilitate low temperature operation of VO2 devices, of potential benefit for switching speed and device stability. This is correlated to the low resistance of the system at temperatures below the transition. High-resolution transmission electron microscopy measurements reveal a complex structural relationship between V2O5, VO2 and V6O13 crystallites. Percolation paths incorporating both VO2 and metallic V6O13 are revealed, which can reduce the resistance below the transition and result in exceptionally low voltage resistive switching.

Embedding epitaxial VO2 film with quality metal-insulator transition to SAW devices

Epitaxial VO2 films grown by pulsed laser deposition (PLD) method with superior phase transition related switching characteristics are successfully embedded to SAW devices using concept of the “impedance loaded SAW sensor”. A resistance of VO2 sensor abruptly drops from 0.7 M[Formula: see text] to 90 [Formula: see text] when it is heated above [Formula: see text]65[Formula: see text]C and shows a narrow hysteresis loops when switching. Two designs of SAW devices are examined in which RF signal is reflected back from interdigital transducer (IDT) or a surface acoustic waves (SAW) is transferred through a coupler and the RF response is altered 2 and 3 times correspondingly upon the phase transition in VO2. In the proposed devices with external load, a SAW does not propagate via VO2 film and therefore is not attenuated which is beneficiary for wireless applications. Additionally, a SAW phase shift as great as 50[Formula: see text] is induced to the SAW transferred through the coupler due to the phase transition in VO2. The proposed approach may boost the development of remotely controlled autonomous sensors, including those based on VO2 metamaterials and hybrid plasmonic structures for near IR/middle IR and sub-THz/THz applications.

Function switchable broadband wave plate based on the Au-VO2 hybrid metasurface.

In recent years, the integration of active materials into a metasurface to achieve tunable devices has attracted much attention. Here, we design an Au-VO2 hybrid metasurface, which can switch between quarter-wave plate and half-wave plate due to the phase transition of VO2. At 298 K, the proposed structure acts as a quarter-wave plate in the 0.87-1.2 THz band, achieving the mutual conversion between linear polarization and circular polarization. Raising the temperature to 358 K, it works as a broadband half-wave plate in the range of 0.65-1.45 THz, with the reflective chirality preservation of circular polarization and the cross-polarization conversion of linear polarization. In the above cases, the response efficiencies are both above 90%. The switchable multifunction results from the tunable geometric phase of the metasurface, where the elaborately designed Au and VO2 blocks separately bring the phase of π/2. Furthermore, the electric field and current density distributions are employed to explain the physical mechanisms leading to the different functions. Such an active broadband metasurface is expected to find applications in tunable and multifunction devices manipulating the polarization and phase of terahertz waves.

Tunable plasmonics with Au nanoparticles coupled to thin film vanadium dioxide

The development of active devices featuring dynamic tunable plasmonic resonances is vital for wide implementation in many optoelectronic devices. By coupling plasmonic nanoparticles to an underlying thin film of phase change material vanadium dioxide (VO2), dynamic tuning of the plasmon resonance can be achieved within the visible and near IR spectral regions. It is shown through selection of single nanoparticle or dimer structures, the plasmon resonances can be tuned over a large spectral range, the scattering cross-section can be increased, and the E-field enhancement and spatial profile can be controlled via the VO2 phase transition. Hybrid Au nanocuboid–VO­2 structures exhibit larger reversible wavelength shifts than rounded nanoparticles, such as rods and discs, of similar dimensions. A plasmon resonance shift of over 600 nm is observed in the near-IR after the semiconducting to metallic VO2 phase transition. The largest increases in the scattering cross-section are achieved with a VO2 thin film thickness of 30–50 nm. Disc, rod, bowtie and cuboid dimers show larger increases of the scattering cross-section at lower wavelengths, even extending into the visible spectral range. On VO2 phase transition the bowtie dimers can provide an increase in the scattering cross-section of over 70% and 3.6-fold increase in the E-field intensity within the dimer gap. Additionally, the near-field enhancement spreads over the entire height of the dimer gap, and in particular, there is a large enhancement at the surface of the dimers. The increased scattering cross-section and modification of the spatial profile of the E-field enhancement provides mechanisms for tunable metasurfaces.

Piezoelectric Lateral-Extensional Mode Resonators With Reconfigurable Electrode and Resonance Mode-Switching Behavior Enabled by a VO₂ Thin-Film.

This article presents the first two-port lateral-extensional mode zinc oxide (ZnO) piezoelectric resonator with a reconfigurable bottom electrode that is enabled by embedding a vanadium dioxide (VO2) thin film. The insulator-to-metal phase transition of VO2 is triggered by substrate heating that translates to abrupt changes in electric field patterns and piezoelectrically transduced modal vibrations, thus allowing mode-switching of piezoelectric resonators at specific frequencies. Finite element method (FEM) analysis was used to model the broadband frequency response, while frequency characteristics of the corresponding two-port resonator were measured over a temperature range between 20 °C and 95 °C with a specific focus on two resonances at 88 and 148 MHz. By leveraging the hysteretic behavior of VO2 thin film during a heating/cooling cycle, a change in both the capacitive feedthrough and resonance signal levels was observed, due to the abrupt change in the conductivity of VO2 during its phase transition. The unique switch- ON behavior of the resonance at 88 MHz starts at 70 °C during the heating cycle, while the switch- OFF transition begins at 60 °C during the cooling cycle. On the other hand, when the temperature is increased from 20 °C to 60 °C, a decrease in the insertion loss and resonance frequency of 12 dB and 0.28 MHz, respectively, were observed for the resonance at 148 MHz. Meanwhile, a resonance frequency increase of 0.42 MHz was observed during a temperature increase from 60 °C to 95 °C, which can be ascribed to VO2 phase transition from monoclinic to rutile phase. The hysteresis loops for insertion loss and resonance frequency indicate a different critical temperature for phase transition from the monoclinic (insulator) phase and rutile (metallic) phase and vice versa. The substantial variation in the temperature coefficient of frequency can be largely ascribed to electrode reconfiguration enabled by VO2 phase transition.

Exploiting Phase Transitions in Catalysis: Adsorption of CO on doped VO2 -Polymorphs.

VO2 is well known for its low-temperature metal-insulator transition between two phases with tetragonal rutile and monoclinic structure. The adsorption of CO on the two polymorphs of Mo-doped VO2 is calculated to investigate the effect of a substrate phase change on the adsorption energy. The system is investigated theoretically at density-functional theory level using a hybrid functional with London dispersion correction. We establish a computational protocol applicable for the study of physisorption on open-shell transition metal oxides. The main task is to control the spin state of open-shell slab models used to model adsorption of closed-shell molecules in order to obtain numerically stable adsorption energies and to reduce spin contamination within the broken-symmetry unrestricted Kohn-Sham approximation. Applying this procedure, it is possible to identify the most stable adsorption positions of CO on both phases of VO2 . CO adsorbs vertically with the C atom on a surface V atom in the monoclinic phase with an adsorption energy of -56 kJ/mol. The same adsorption position has an adsorption energy of only -46 kJ/mol on the rutile phase. Similar differences were obtained with multireference methods using an embedded cluster model. This effect may inspire experimental strategies exploiting the rutile monoclinic VO2 phase transition in catalytic processes where CO is formed as product or as an intermediate.

Anisotropic coding metasurfaces and their active manipulation based on vanadium dioxide for multifunctional applications in the terahertz region.

Various kinds of metasurfaces have been proposed because they can be tailored to achieve the desired modulations on electromagnetic wave that do not occur in nature. Compared to conventional metamaterials, coding metasurfaces integrated with information science theory possess numerous distinctive advantages - simple design, time-saving and compatibility with digital devices. Here we propose terahertz multifunctional anisotropic reflective metasurfaces with a metal-insulator-metal cavity structure whose top constructional layer consists of a pair of gold arc-rings and a gold cut-wire located between them. Two different functions of narrow-band absorption and broadband polarization conversion are realized based on different coding matrices using the binary codes '0' and '1'. Furthermore, we integrate a specific coding metasurface with vanadium dioxide (VO2) to realize a temperature-controlled active metasurface. Through the temperature change, dynamic functionalities switching between a narrow-band polarization converter with a polarization conversion ratio over 94% and an efficient low-pass filter are achieved under the phase transition of VO2, and the active metasurface is polarization independent. The proposed coding metasurfaces are verified numerically and experimentally, and have promising applications in terahertz modulation and functional devices.

Polarization-sensitive tunable extraordinary terahertz transmission based on a hybrid metal-vanadium dioxide metasurface.

Thermally tunable extraordinary terahertz transmission in a hybrid metal-vanadium dioxide (VO2) metasurface is numerically demonstrated. The metasurface consists of a metal sheet perforated by square loops, while the loops are connected with strips of VO2. The frequency and amplitude of the transmission resonance are modulated by controlling the conductivity of VO2. For a y-polarized incident field, the resonance transmission peak redshifts from 0.88 to 0.81 THz upon insulator-to-metallic phase transition of VO2. For an x-polarized incident field, the transmission resonance at 0.81 THz is observed in the insulator phase. However, in the metallic phase of VO2, the electromagnetic field is effectively reflected in the 0.5-1.1 THz range with a transmission level lower than 0.14. The proposed metasurface can be utilized as a terahertz modulator, reconfigurable filter, or switch.

A design strategy of wide angle, broadband tunable near-infrared absorber

Near infrared absorbers have many applications in the optoelectronic filed. In this study, we design and fabricate two near-infrared broadband tunable absorbers composed of V1-xWxO2/HfO2/Ag layers (the single-layer VO2 absorber) and V1-xWxO2/VO2/HfO2/Ag layer (the double-layer VO2 absorber). The absorption bands with above 90% of the single-layer VO2 absorber and the double-layer VO2 absorber can be tuned from 1366–1766 nm to 834–980 nm and 931–2000 nm to 845–1056 nm by thermally induced VO2 phase transition, respectively. And the two absorbers we fabricated maintained high absorption over a wide range of incident angles. Furthermore, this paper verified that the introduction of a double-layer absorption layer into the absorber structure can effectively broaden the absorption bandwidth of the absorber and increase the absorption strength.

Nonvolatile Control of Metal-Insulator Transition in VO2 by Ferroelectric Gating.

Controlling phase transitions in correlated materials yields emergent functional properties, providing new aspects to future electronics and a fundamental understanding of condensed matter systems. With vanadium dioxide (VO2 ), a representative correlated material, an approach to control a metal-insulator transition (MIT) behavior is developed by employing a heteroepitaxial structure with a ferroelectric BiFeO3 (BFO) layer to modulate the interaction of correlated electrons. Owing to the defect-alleviated interfaces, the enhanced coupling between the correlated electrons and ferroelectric polarization is successfully demonstrated by showing a nonvolatile control of MIT of VO2 at room temperature. The ferroelectrically-tunable MIT can be realized through the Mott transistor (VO2 /BFO/SrRuO3 ) with a remanent polarization of 80 µC cm-2 , leading to a nonvolatile MIT behavior through the reversible electrical conductance with a large on/off ratio (≈102 ), long retention time (≈104 s), and high endurance (≈103 cycles). Furthermore, the structural phase transition of VO2 is corroborated by ferroelectric polarization through in situ Raman mapping analysis. This study provides novel design principles for heteroepitaxial correlated materials and innovative insight to modulate multifunctional properties.

On/Off Ultra‐Short Spin Current for Single Pulse Magnetization Reversal in a Magnetic Memory Using VO2 Phase Transition

AbstractUltrafast magnetization reversal of magnetic thin films induced by one single femtosecond laser light pulse, without any applied magnetic nor electrical field, known as all‐optical switching has become one of the most attractive technologies for prospective ultrafast and low‐energy magnetic devices. A single femtosecond laser pulse switching of a ferromagnet in specific spin valves, driven by electron pulse originating from ultrafast demagnetization of a ferrimagnetic layer, has been recently demonstrated. This opens the way to ultrafast and energy‐efficient writing for magnetic memory. Here, a new functionality is presented through introducing a thin VO2 into the specific spin valve, where a switchable ultra‐short polarized electron pulse by thermally triggering the phase transition of VO2 to dynamically control the switching of the ferromagnet is experimentally achieved. It is demonstrated that the generated ultra‐short electron pulse can be free to pass through the VO2 in its metallic state while it is blocked in its insulating state. Additionally, the change on electrical and optical properties of VO2 during the phase transition leads to various possible magnetic states. On/off ultra‐short spin current controlled by phase transition paves the way to address specific ultra‐fast spintronic devices in large array devices.

Reconfigurable high-Q terahertz filtering of VO2-based metamaterials using optical tunneling

Up to date, reconfigurable terahertz (THz) band-pass filters based on the design of metamaterials with vanadium dioxide (VO2) have been investigated extensively. However, VO2-based THz metamaterials with high-quality tunable narrowband transmission and good stop-band rejection have yet to be reported, which are essential for THz wave filtering components or systems. Here, we propose a reconfigurable filter in combination with a dielectric metastructure and a VO2 thin film. Utilizing the optical tunneling effects of a cavity layer, the filter has a high band-pass transmission up to 98% at 0.246 THz with a narrow bandwidth of 0.0009 THz, an ultra-high Q-factor of 273, and two broad nearly-suppressed sidebands. The transmission intensity can be modulated by the phase transition of VO2, and the modulation depth is as high as 96.4%. By efficiently tuning the cavity layer thickness, the filter can be designed for a series of transmission wavelengths and further accomplish the switch between the narrow band-pass filtering and wideband reflection. Moreover, the manipulation of the incident angle and polarization can be adopted as an additional degree to achieve a similar switching function. Our study implies a potential for developing various THz applications, where reconfigurable high-Q THz filtering is required.

Photothermal bistability in the molecular energy transfer near a VO2@Au nanoshell triggered by the continuous-wave laser

The thermally triggered semiconductor-metal phase transition of vanadium dioxide (VO2) is a frequent subject in the study of nanostructure responses due to the high speed of the transition. Here we report on the molecular energy transfer near a hybrid VO2@Au nanoshell during the VO2 phase transition when induced by a continuous-wave (CW) laser. The presence of VO2 causes a bistable and reversible change in the optical response of the nanoshell through the thermo-optical process at the resonance wavelength of the VO2@ Au nanoshell. This behavior is achieved by controlling the laser intensity during the heating and cooling processes. In this paper we couple the thermodynamics with the Forster-Dexter theory of energy transfer between molecules which is generalized to use a nearby VO2@Au nanoshell. The bistable and reversible change in the response of the nanoshell causes the molecular energy transfer in the same manner over an intensity range of 1.52 (GW/m2) <I< 4.39 (GW/m2). This work also provides general guidelines for designing switchable surface plasmon based biosensors, switchable molecular junction devices and switching the energy exchange between proteins.