Tongyu Wang

Increasing efficiency, speed, and responsivity of vanadium dioxide based photothermally driven actuators using single-wall carbon nanotube thin-films.

Vanadium dioxide (VO2)-based actuators have demonstrated great performance in terms of strain energy density, speed, reversible actuation, programming capabilities, and large deflection. The relative low phase transition temperature of VO2 (∼68 °C) gives this technology an additional advantage over typical thermal actuators in terms of power consumption. However, this advantage can be further improved if light absorption is enhanced. Here we report a VO2-based actuator technology that incorporates single-wall carbon nanotubes (SWNTs) as an effective light absorber to reduce the amount of photothermal energy required for actuation. It is demonstrated that the chemistry involved in the process of integrating the SWNT film with the VO2-based actuators does not alter the quality of the VO2 film, and that the addition of such film enhances the actuator performance in terms of speed and responsivity. More importantly, the results show that the combination of VO2 and SWNT thin films is an effective approach to increase the photothermal efficiency of VO2-based actuators. The integration of SWNT films in VO2 devices can be easily applied to other VO2-based phototransducers as well as to similar devices based on other phase-change materials. While adding a sufficiently thick layer of some arbitrary material with high absorption for the light used for actuation (λ = 650 nm wavelength in this case) could have improved conversion of light to heat in the device, it could also have impeded actuation by increasing its stiffness. It is noted, however, that the low effective Youngs modulus of SWNT film coating used in this work does not impair the actuation range.

Electronically-Controlled Beam-Steering through Vanadium Dioxide Metasurfaces.

Engineered metamaterials offer unique functionalities for manipulating the spectral and spatial properties of electromagnetic waves in unconventional ways. Here, we report a novel approach for making reconfigurable metasurfaces capable of deflecting electromagnetic waves in an electronically controllable fashion. This is accomplished by tilting the phase front of waves through a two-dimensional array of resonant metasurface unit-cells with electronically-controlled phase-change materials embedded inside. Such metasurfaces can be placed at the output facet of any electromagnetic radiation source to deflect electromagnetic waves at a desired frequency, ranging from millimeter-wave to far-infrared frequencies. Our design does not use any mechanical elements, external light sources, or reflectarrays, creating, for the first time, a highly robust and fully-integrated beam-steering device solution. We demonstrate a proof-of-concept beam-steering metasurface optimized for operation at 100 GHz, offering up to 44° beam deflection in both horizontal and vertical directions. Dynamic control of electromagnetic wave propagation direction through this unique platform could be transformative for various imaging, sensing, and communication applications, among others.

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

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.

Maximizing the performance of photothermal actuators by combining smart materials with supplementary advantages

This study demonstrates the use of light colors to selectively actuate micrometer-sized structures. The search for higher-performance photothermal microactuators has typically involved unavoidable trade-offs that hinder the demonstration of ubiquitous devices with high energy density, speed, flexibility, efficiency, sensitivity, and multifunctionality. Improving some of these parameters often implies deterioration of others. Photothermal actuators are driven by the conversion of absorbed optical energy into thermal energy, which, by different mechanisms, can produce mechanical displacement of a structure. We present a device that has been strategically designed to show high performance in every metric and respond to optical radiation of selected wavelength bands. The device combines the large energy densities and sensitivity of vanadium dioxide (VO2)–based actuators with the wavelength-selective absorption properties of single-walled carbon nanotube (SWNT) films of different chiralities. SWNT coatings increased the speed of VO2 actuators by a factor of 2 while decreasing the power consumption by approximately 50%. Devices coated with metallic SWNT were found to be 1.57 times more responsive to red light than to near-infrared, whereas semiconducting SWNT coatings resulted in 1.42 times higher responsivities to near-infrared light than to red light. The added functionality establishes a link between optical and mechanical domains of high-performance photoactuators and enables the future development of mechanical logic gates and electronic devices that are triggered by optical radiation from different frequency bands.

VO 2 -Based MEMS Mirrors

This paper reports the integration of vanadium dioxide (VO2) thin films in a microelectromechanical systems (MEMS) mirror device, where the actuation is mainly due to the solid-solid phase transition of VO2. The fabrication process described in this paper provides the details that will enable the integration of VO2 thin films at any step during the fabrication of rather complex MEMS devices. The present VO2-based MEMS mirror device is operated electro-thermally through integrated resistive heaters, and its behavior is characterized across the phase transition of VO2, which occurs at a temperature of ~68 °C and spans about 10 °C. The maximum vertical displacement of the mirror platform is 75 μm and it occurs for an input voltage of 1.1 V. This translates to an average power consumption of 6.5 mW per mirror actuator and a total power consumption of 26.1 mW for the entire device. The studies included in this paper are key for future device improvements and further development of MEMS mirror actuation technology, which could include the use of the hysteresis of VO2 for programming tilting angles in MEMS mirrors.

Bolometric-Effect-Based Wavelength-Selective Photodetectors Using Sorted Single Chirality Carbon Nanotubes

This paper exploits the chirality-dependent optical properties of single-wall carbon nanotubes for applications in wavelength-selective photodetectors. We demonstrate that thin-film transistors made with networks of carbon nanotubes work effectively as light sensors under laser illumination. Such photoresponse was attributed to photothermal effect instead of photogenerated carriers and the conclusion is further supported by temperature measurements. Additionally, by using different types of carbon nanotubes, including a single chirality (9,8) nanotube, the devices exhibit wavelength-selective response, which coincides well with the absorption spectra of the corresponding carbon nanotubes. This is one of the first reports of controllable and wavelength-selective bolometric photoresponse in macroscale assemblies of chirality-sorted carbon nanotubes. The results presented here provide a viable route for achieving bolometric-effect-based photodetectors with programmable response spanning from visible to near-infrared by using carbon nanotubes with pre-selected chiralities.

Millimeter-wave phase modulator based on vanadium dioxide meta-surfaces

We present an electronically-controlled vanadium dioxide (VO2)-based millimeter-wave phase modulator. The phase modulator consists of a resonant meta-surface fabricated on a thin VO2 film integrated with a voltage-controlled heating electrode layer. By varying the applied voltage to the heating electrodes we control the electromagnetic properties of the VO2 layer and shift the resonance frequency of the mata-surface, which results in a significant phase shift near the resonance frequency. Using the presented phase modulator design, we experimentally demonstrate the highest reported phase modulation of 60° at 85 GHz through a fully-integrated voltage-controlled device platform.

Fully printed flexible carbon nanotube photodetectors

Here, we report fully printed flexible photodetectors based on single-wall carbon nanotubes and the study of their electrical characteristics under laser illumination. Due to the photothermal effect and the use of high purity semiconducting carbon nanotubes, the devices exhibit gate-voltage-dependent photoresponse with the positive photocurrent or semiconductor-like behavior (conductivity increases at elevated temperatures) under positive gate biases and the negative photocurrent or metal-like behavior (conductivity decreases at elevated temperatures) under negative gate biases. Mechanism for such photoresponse is attributed to the different temperature dependencies of carrier concentration and carrier mobility, which are two competing factors that ultimately determine the photothermal effect-based photoresponse. The photodetectors built on the polyimide substrate also exhibit superior mechanical compliance and stable photoresponse after thousands of bending cycles down to a curvature radius as small as 3...

Fully-integrated and electronically-controlled millimeter-wave beam-scanning

We present a fully-integrated and electronically-controlled millimeter-wave beam-scanner based on a reconfigurable meta-surface. The meta-surface consists of a two-dimensional array of sub-wavelength cross-shape structures fabricated on a vanadium dioxide (VO2) film. Joule heating electrodes are integrated with the meta-surface structure to control the temperature of the VO2 layer and its dielectric properties. By controlling the applied voltage to the heating electrodes of each across-shape element, phase of a transmitted millimeter-wave beam through the cross-shape element is controlled and phase front of the transmitted millimeter-wave beam through the meta-surface is modified, accordingly. We demonstrate 44° of continuous beam-scanning at 95 GHz through an electronically-controlled and fully-integrated device platform.

Polycrystalline VO2 film characterization by quantum capacitance measurement

Capacitance measurement is performed using a home-built bridge on quasi two-dimensional vanadium dioxide films grown on silicon-dioxide/p-doped silicon substrates. Correlated effects appearing in the quantum capacitance are obtained as a function of temperature at low frequencies. The thermodynamic density of states reveals the opening band gap in the insulating monoclinic phase.