Phase Transition Of VO2 Research Articles

Infrared Transmission Characteristics of Phase Transitioning VO2 on Various Substrates.

Infrared transmission characteristics of VO2 thin films synthesized on multiple substrates, using a low-pressure direct oxidation technique, have been characterized. Material characterization of these films indicates high material quality, which resulted in large variation of electrical and optical properties at phase transition. A change in optical transmissivity greater than 80% was observed for these films utilizing infrared (IR) laser illumination at 1550 nm. Phase transition enabled by temperature change induced by a pulsed high-power laser beam resulted in modulated IR laser transmission with a low time constant in VO2 on transparent quartz and muscovite substrates. Investigation of the effect of mechanical strain on phase transition in VO2 grown on flexible muscovite substrate indicate shift in transition temperature to higher for tensile and lower for compressive strains.

Sensing enhancement of a Fabry-Perot THz cavity using switchable VO2 mirrors.

We experimentally investigate the sensing properties of an open cavity operating in the THz regime and realized by employing as mirrors two thin vanadium dioxide (VO2) films grown on silicon parallel plates and separated by a variable length. The phase transition of VO2 is used to control the behavior of the system between two different responses: a high transmission mode to the incident radiation (VO2 in the insulating state) and a high sensitivity to tiny changes in the cavity refractive index (VO2 in the conducting state). In the first state, the low loss regime enables to adjust the cavity length and easily optimize the resonances due to the Fabry-Perot (FP) effect in the Si plates and in the cavity volume. The activation of the metallic-like state instead, by damping the FP oscillations in the plates, promotes the onset of a comb-like spectrum that can be exploited as a versatile tool for accurate sensing applications. Using both an analytical model and full-wave simulations, we estimate the device response to variation in the refractive index of the cavity volume, showing that the proposed structure can achieve sensitivity values among the highest reported for THz sensors.

In situ Raman-scattering mapping and resistance study of the VO2 metal-insulator phase transition

In this study, a large-area single-phase VO2 film was successfully prepared on a Si (100) substrate by reactive magnetron sputtering deposition. The thickness of the VO2 film is approximately 220 nm. The phase transition temperature is about 67 °C, with more than three orders of magnitude of the resistance change. X-ray diffraction patterns obtained at different temperatures indicate the coexistence of two phases during the VO2 phase transition process, where a portion of the grains first transform from a monoclinic structure to a rutile structure, and then phase transition expands throughout the film. In situ Raman-scattering mapping scans further prove that the monoclinic-to-rutile phase transition occurs gradually in polycrystalline VO2 films, in which monoclinic and rutile phases coexist from 55 °C to 80 °C. Electrical resistance of the film will change dramatically when the volume fraction of monoclinic phase decreases to 43.5%, or the volume fraction of rutile phase increases to 55.5%. The phase transition temperature differs among the grains in the polycrystalline VO2 film. This study provides an important scientific approach for elucidating the mechanism of the metal-insulator phase transition.

Thermally switchable and tunable metasurface with polarization independence based on VO2 phase transition

In this paper, a thermally tunable metasurface (TTMS) based on phase material vanadium dioxide (VO2) is designed and demonstrated, which consists of VO2 patterns, metallic resonators, dielectric substrates, and a gold reflective plate. Exhibiting ultra-broadband absorption from 4.72 THz to 9.88 THz with absorptivity exceeding 90% above the phase change temperature of VO2 and realizing single-band absorption when the VO2 resonators are in the insulator phase, the proposed TTMS possesses excellent low radar cross-section (RCS) performance as well as can be tailored by controlling the temperature of the phase material VO2. Furthermore, the devised TTMS is polarization-insensitive and can work well under a wide range of incident angles. The demonstrated tunable metasurface promises potential applications in the terahertz regime, such as modulating, radar stealth, and thermal detection.

Coupling of Phase Transition and Self-Powered Photodetection of a NbSe2/VO2 Heterojunction

An endeavor to design a self-powered photodetector that can work in a broad temperature range is required for everyday applications. Herein, we present a self-powered NbSe2/VO2 bilayer photodetector with boosted performance stimulated by the phase transition of VO2 after heating. The NbSe2/VO2 heterojunction can be converted into a Schottky junction from a staggered heterojunction thanks to the semiconductor-to-metal phase transition (SMT) of VO2, resulting in a controlled built-in electric field at the bilayer junction. Its origin comes from a sharp increase in carrier transport, which results in a boosted photoresponse in the bilayer. Our photovoltaic detector has shown a fast response time of 122 μs at the VO2 metallic phase for a visible wavelength (405 nm), featuring a self-powered, high-speed, and energy-efficient device.

Photo-induced non-volatile VO2 phase transition for neuromorphic ultraviolet sensors

In the quest for emerging in-sensor computing, materials that respond to optical stimuli in conjunction with non-volatile phase transition are highly desired for realizing bioinspired neuromorphic vision components. Here, we report a non-volatile multi-level control of VO2 films by oxygen stoichiometry engineering under ultraviolet irradiation. Based on the reversible regulation of VO2 films using ultraviolet irradiation and electrolyte gating, we demonstrate a proof-of-principle neuromorphic ultraviolet sensor with integrated sensing, memory, and processing functions at room temperature, and also prove its silicon compatible potential through the wafer-scale integration of a neuromorphic sensor array. The device displays linear weight update with optical writing because its metallic phase proportion increases almost linearly with the light dosage. Moreover, the artificial neural network consisting of this neuromorphic sensor can extract ultraviolet information from the surrounding environment, and significantly improve the recognition accuracy from 24% to 93%. This work provides a path to design neuromorphic sensors and will facilitate the potential applications in artificial vision systems.

Li+/Al3+ Composite Solid Electrolyte Based Electro‐Thermal Dual Responsive Devices with Optimized Optical Contrast and Cycle Durability

AbstractThe limited optical contrast and band adjustment range are the main factors that restrict chromic materials. In this study, Li+/Al3+ composite solid electrolyte is incorporated between VO2 and WO3 chromic functional layers to construct a new type of VO2 based electrothermal dual responsive devices. The dual responsive devices deliver optical contrast over 50% and independent modulation of visible and near infrared band. In addition to displaying four reversible optical states in response to environmental temperature and external voltage, the devices effectively utilize the synergistic effect of Li+/Al3+ hybrid ions. Furthermore, the optimized Li+/Al3+ composite electrolyte is proved to effectively control phase transition of VO2 and significantly improve the electrochromic properties of WO3 as well. Finally, the electro‐thermal dual responsive devices show excellent service stability in simulated outdoor environment of over 10 000 cycles. Combined with the above advantages, it is expected to develop smart windows and derivative products with large size, long life, remarkable energy saving effect and pleasant comfort.

Multifield Controlled Terahertz Modulator Based on Silicon‐Vanadium Dioxide Hybrid Metasurface

AbstractDespite flourishing and deepening trend in the terahertz (THz) field, there are still challenges in high‐performance active system components, such as THz modulators. Present THz modulators are typically limited to a single external driving field and the fixed switching speed, hindering the ability to flexibly manipulate THz waves. Here, an optically and thermally controlled THz modulator based on silicon (Si) and vanadium dioxide (VO2) hybrid metasurface is proposed to overcome these limitations. The modulator enables the dynamical control of the transmitted amplitude in the range of 0.4–1.8 THz. Under thermal management, a maximum modulation depth (MD) of 97.2% at 0.9 THz can be acquired, thanks to the phase transition of VO2. Once excited with an 800 nm pump light at 1600 µJ cm−2, the device achieves a maximum MD of 91.5% at 0.87 THz accomplished with an ultrafast speed of 2 ps, resulting from photogenerated carriers in the Si substrate. Furthermore, the findings of this study propose a promising strategy for developing multifield controlled active THz components, which would be a new paradigm to design switchable metasurface devices.

VO2-metallic hybrid metasurfaces for agile terahertz wave modulation by phase transition

The combination of VO2 and metasurfaces has opened an attractive route to dynamically control terahertz (THz) waves based on the giant conductivity change. However, the high-precision control of microfabrication and single performance of conductivity change limit the multifunctional application of VO2-based metasurfaces. Here, we proposed a VO2-metallic hybrid metasurface by in situ depositing high-quality VO2 thin films onto a metasurface composed of asymmetric Fano resonance units. It exhibits agile frequency and amplitude modulation for THz transmission across tuning the dielectric constant and conductivity of VO2. The metallic metasurface is designed as a matrix to achieve high transmission at 0.61 and 0.78 THz due to the split-ring resonance. During the thermally triggered phase transition of VO2, we found that the resonance frequency and amplitude can be tuned dominantly by the change of dielectric constant and conductivity, respectively. In particular, the increased dielectric constant enables red shift of the frequency by around 0.48 THz and the conductivity increases lead to a giant THz amplitude modulation of 88%. These results provide a route for developing VO2-based THz smart devices combined with functional metasurfaces and hold great promise for applications in THz sensor and modulation.

Periodic Arrays of 3D AuNP‐Capped VO2 Shells and Their Temperature‐Tunable SERS Performance

AbstractIn this work, periodic arrays of 3D composite structures consisting of gold nanoparticle (AuNP) capped VO2 shells on self‐assembled highly‐ordered SiO2 microspheres have been designed and fabricated, and their temperature‐tunable surface‐enhanced Raman scattering (SERS) performance is investigated. The results show that the SERS intensity enhancement factor (EF) for R6G molecules adsorbed on the AuNP‐capped VO2 shells increases from 8.3 × 105 to 3.9 × 106 when triggering the semiconductor‐to‐metal phase transition of VO2 by temperature rise, which strengthens the plasmonic coupling between the AuNPs and VO2 shells and consequently leads to a larger electric near‐field enhancement at the gaps between neighboring AuNPs. Such periodic arrays of AuNP‐capped VO2 shells with tunable plasmonic coupling can provide great opportunities for various optical applications, such as quantitative SERS detection of analytes, photothermal nano‐regulators, and biosensing in general.

Versatile polarization manipulation in vanadium dioxide-integrated terahertz metamaterial.

Broadband and switchable versatile polarization metamaterial is crucial in the applications of imaging, sensing and communication, especially in the terahertz frequency. Here, we investigated versatile polarization manipulation in a hybrid terahertz metamaterial with bilayer rectangular rods and a complementary vanadium dioxide (VO2) layer. The VO2 phase transition enables a flexible switching from dual-band asymmetric transmission to dual-band reflective half-wave plate. The full width half maximum (FWHM) bandwidths of dual-band asymmetric transmission are 0.77 and 0.21 THz, respectively. The polarization conversion ratio (PCR) of the reflective metamaterial is over 0.9 in the frequency ranges of 1.01-1.17 THz and 1.47-1.95 THz. Angular dependences of multiple polarization properties are studied. The proposed switchable polarization metamaterial is important to the development of multifunctional polarization devices and multichannel polarization detection.

Dual-broadband terahertz absorber based on phase transition characteristics of VO2

A dual-broadband absorption metamaterial based on a composite structure consisting of metal and vanadium dioxide is designed and analyzed. By controlling the temperature of vanadium dioxide, the absorption band of the device can be manipulated. At T = 300 K, before the VO2 phase transition, the designed structure can achieve broadband absorption. Results indicate that the absorption rate can exceed 90% in the broad spectral range of 1.375–2.087THz. In the case of T = 350 K, VO2 transforms into a metallic state, and the absorption band is transferred to 2.6–7.03 THz. Especially, the structure is insensitive to the polarization angle of the incident wave, which is beneficial for maintaining stable absorption performance when the incident angle is less than 45°. The proposed absorber can be applied to many other fields such as modulation devices, terahertz switching, and power harvesting fields.

Tuning Spin‐Orbit Torques Across the Phase Transition in VO2/NiFe Heterostructure

AbstractThe emergence of spin‐orbit torques as a promising approach to energy‐efficient magnetic switching has generated large interest in material systems with easily and fully tunable spin‐orbit torques. Here, current‐induced spin‐orbit torques in VO2/NiFe heterostructures are investigated using spin‐torque ferromagnetic resonance, where the VO2 layer undergoes a prominent insulator‐metal transition. A roughly twofold increase in the Gilbert damping parameter, α, with temperature is attributed to the change in the VO2/NiFe interface spin absorption across the VO2 phase transition. More remarkably, a large modulation (±100%) and a sign change of the current‐induced spin‐orbit torque across the VO2 phase transition suggest two competing spin‐orbit torque generating mechanisms. The bulk spin Hall effect in metallic VO2, corroborated by the first‐principles calculation of the spin Hall conductivity , is verified as the main source of the spin‐orbit torque in the metallic phase. The self‐induced/anomalous torque in NiFe, with opposite sign and a similar magnitude to the bulk spin Hall effect in metallic VO2, can be the other competing mechanism that dominates as temperature decreases. For applications, the strong tunability of the torque strength and direction opens a new route to tailor spin‐orbit torques of materials that undergo phase transitions for new device functionalities.

Dynamically controlled asymmetric transmission of linearly polarized waves in VO2-integrated Dirac semimetal metamaterials

We numerically demonstrated a novel chiral metamaterial to achieve broadband asymmetric transmission (AT) of linearly polarized electromagnetic waves in terahertz (THz) band. The proposed metamaterial unit cell exhibits no rotational symmetry with vanadium dioxide (VO2) inclusion embedded between Dirac semimetals (DSMs) pattern. The resonant frequency of AT can be dynamically tunable by varying the Fermi energy (E F) of the DSMs. The insulator-to-metal phase transition of VO2 enables the amplitude of the AT to be dynamically tailored. The transmission coefficient |Tyx | can be adjusted from 0.756 to nearly 0 by modifying the conductivity of VO2. Meanwhile, the AT parameter intensity of linearly polarized incidence can be actively controlled from 0.55 to almost 0, leading to a switch for AT. When VO2 is in its insulator state, the proposed device achieves broadband AT parameter greater than 0.5 from 1.21 THz to 1.80 THz with a bandwidth of 0.59 THz. When the incident wave propagates along the backward (–z) direction, the cross-polarized transmission | Tyx | reaches a peak value 0.756 at 1.32 THz, whereas the value of | Txy | well below 0.157 in the concerned frequency. On the other hand, the co-polarized transmission |Txx | and |Tyy | remained equal in the whole frequency range. This work provides a novel approach in developing broadband, tunable, as well as switchable AT electromagnetic devices.

Active beam manipulation and convolution operation in VO2-integrated coding terahertz metasurfaces.

Coding metasurfaces have received tremendous interest due to their unprecedented control of beams through the flexible design of coding sequences. However, realizing tunable coding metasurfaces with scattering-pattern shifts in the terahertz range is still challenging. Here, we propose a VO2-integrated coding metasurface to realize a thermally controlled scattering-pattern shift by convolution operation. The required phase profiles and high amplitudes of 1-bit and 2-bit coding metasurfaces are easily obtained only by changing the length of the VO2 cut-wires. The insulator-metal phase transition of the VO2 cut-wires leads to an ultrafast switching effect between multiple deflected scattering beams and one normally reflected beam. In particular, the VO2 phase transition contributes to dynamical convolution operations of the 2-bit coding metasurface. The proposed VO2-integrated coding metasurfaces are important for realizing tunable terahertz beam manipulation as well as arbitrary required scattering beams.

Large‐Area Plasmonic Switches Based on Crystalline Au Nanoparticles Between VO2 Layers for Enhanced Switching in Visible–Near‐Infrared Regime

Vanadium dioxide (VO2) undergoes a reversible phase change, from semiconducting to metallic, with a change in temperature. Herein, plasmonic switches–based on gold nanoparticles (Au NPs) embedded between layers of VO2 films–that exhibit enhanced switching of the optical transmission as the temperature is increased above the VO2 phase transition threshold are presented. The operating wavelength of the switching device can be tuned between 650 and 1000 nm using the localized surface plasmon resonance properties of the Au NPs. This is for the first time that plasmonic switches have been demonstrated in the visible spectral regime (i.e., for wavelengths shorter than 700 nm) when VO2 is employed as the phase change material. A higher switchability, i.e., a higher extinction ratio upon phase transition of VO2, than what has been shown earlier in the spectral regions less than 1000 nm is demonstrated experimentally and theoretically. All the device layers are grown at the same substrate temperature and laser intensity via a single‐vacuum pulsed laser deposition process, leading to a simplified large‐area fabrication process. It is also shown for the first time that the Au NPs embedded in the proposed Au‐VO2 nanocomposites are crystalline.

Integrated Hybrid VO2–Silicon Optical Memory

Vanadium dioxide (VO2) is an interesting material for hybrid photonic integrated devices due to its insulator–metal phase transition. Utilizing the hysteresis of the phase transition in voltage-biased VO2, we demonstrate a compact hybrid VO2–silicon optical memory element integrated into a silicon waveguide. An optical pulse writes the VO2 memory, leading to an optical attenuation that can be read out by the optical transmission in a silicon waveguide. Our on-chip memory cell can be optically written with energy as low as 23.5 pJ per pulse and with a 10–90% rise time of ∼100 ns. This approach is promising for optical data storage in silicon photonic integrated circuits.

Narrowband-to-broadband switchable and polarization-insensitive terahertz metasurface absorber enabled by phase-change material

A terahertz absorber with controllable and switchable bandwidth that is insensitive to polarization is of great interest. Here, we propose and demonstrate a metasurface absorber with switchable bandwidth based on a phase-change material of vanadium dioxide (VO2) and verify its performance by finite element method simulations. The metasurface absorber is composed of a hybrid cross fractal as a resonator separated from a gold ground plane by a polyimide spacer. Switching from narrowband to broadband absorber is achieved via connecting VO2 patches to the gold first-order cross fractal converting the resonator to a third-order cross fractal. In the insulator phase of VO2, the main narrowband absorption occurs at the frequency of 6.05 THz with a 0.99 absorption and a full-width half-maximum (FWHM) of 0.35 THz. Upon insulator-to-metal transition of VO2, the metasurface achieves a broadband absorption with FWHM of 6.17 THz. The simulations indicate that by controlling the partial phase transition of VO2, we can tune the bandwidth and absorption level of the absorber. Moreover, the designed absorber is insensitive to polarization due to symmetry and works well for a very wide range of incident angles. In the metallic state of VO2, the absorber has an absorption exceeding 0.5 in the 3.57–8.45 THz frequency range with incident angles up to 65°.

Tunable and reflective polarization converter based on single-layer vanadium dioxide-integrated metasurface in terahertz region

A single-layer and actively tunable reflective polarization converter is proposed, which is composed of periodical two-dimensional unit cell of inclined split circular ring filled into by vanadium oxide (VO2) and central rod. Through numerical simulation obtained by the CST Microwave Studio, we find that the broad bandwidth of reflective cross-polarization conversion above 90% is normally obtained from 2.03 to 5 THz at the temperature about 25 °C for both the linearly and circularly polarized wave incidence. At the same time, the polarization conversion ratio (PCR) and energy conversion ratio (ECR) have been furtherly investigated, which reflect high cross-polarization conversion efficiency with nearly no energy loss at room temperature of 25 °C. Thereafter, namely, the cross-polarization conversion can be achieved by theoretical analysis from the views on qualitative variables of polarization azimuth angle (θ), ellipticity (η) and symmetrical polarization decomposition. What is more, the device is sensitive to the incident angles which result into the split reflected frequency spectrum. Afterwards, the actively dynamic responses for reflective frequency spectrum depending on the insulator-to-metal phase transition of VO2 have been investigated. To be demonstrated, the physical mechanism of changeable polarization conversion has been discussed by the distributions of current densities. Lastly, also, the influences of structural parameter variation can be further studied. According to the interesting results, the method provides a strategy that manipulate active and passive metadevices for polarization converter, wireless communication in THz wavelength domain.

Photothermal polydopamine coated VO2 nanoparticle thin film with enhanced optical property and stability

Vanadium dioxide (VO2) exhibits great potential in smart windows due to its phase-transition property. Herein, polydopamine (PDA), coated on VO2 nanoparticles, is employed to replace doping to trigger the phase transition of VO2 at ambient temperature based on its photothermal effect. The VO2@PDA thin films not only can avoid optical property degradation caused by doping, but also exhibit better optical property than VO2 films due to the improved dispersion after coating PDA that the solar energy modulation (ΔTsol) is improved from 11.2 to 14.5% when the luminous transmittance (Tlum) remains unchanged. In addition, VO2@PDA nanoparticles remain stable under acidic and oxygenic condition, which is attributed to the stable PDA shell and the interfacial interaction between VO2 core and PDA shell. This work provides an novel method to replace doping treatment to promote the phase transition of VO2.