Rafmag Cabrera

Optoelectronic and all-optical multiple memory states in vanadium dioxide

Vanadium dioxide exhibits a well-known insulator-to-metal transition during which several of its physical properties change significantly. A hysteresis loop develops for each of them as the material is heated and then cooled through the transition. In this work VO2/SiO2 samples were maintained—by heat sinking—at a selected temperature within the heating branch of the hysteresis loops for resistance and near-infrared transmittance, while brief thermal excursions of the VO2 film were caused by either voltage pulses applied to the film or laser light pulses irradiating the film. These pulses had durations from milliseconds to a few seconds and the resulting drops in resistance or transmittance were easily and repeatably measurable without appreciably affecting their new values. A sequence of equal-duration pulses (for either equal-voltage or equal-irradiation pulses) caused the resistance and infrared transmittance to continue to drop, each time by a smaller amount, and larger energy pulses were required in ...

Phase transition behavior in microcantilevers coated with M1-phase VO2 and M2-phase VO2:Cr thin films

Silicon microcantilevers were coated by pulsed laser deposition with vanadium dioxide (VO2) (monoclinic M1 phase) and V1−xCrxO2 with x near 0.024 (monoclinic M2 phase), and their mechanical characteristics were studied as a function of temperature through the films’ insulator-to-metal transition (IMT). The undoped VO2 films grew with (011)M1 planes parallel to the substrate, while Cr-doped VO2 films grew oriented with (201)M2 and (2¯01)M2 planes parallel to the substrate. In both cases, the films transformed reversibly through the IMT to the tetragonal (rutile, R) phase, with film (110)R planes oriented parallel to the substrate. The fundamental resonant frequencies of the cantilevers were measured as the temperature was cycled from ambient temperature, through the IMT, and up to 100  °C. Very high resonant frequency changes were observed through the transition for both types of samples, with increases during heating of over 11% and over 15% for the cantilevers coated with pure and Cr-doped VO2, respectiv...

Young's modulus of VO2 thin films as a function of temperature including insulator-to-metal transition regime

Young’s modulus of VO2 thin films has been measured for the first time through the material’s insulator-to-metal transition. The resonant frequency of silicon VO2 coated cantilevers was measured in the temperature range 30–90°C. It has been found that during the semiconductor to metallic transition of VO2 thin films, which occurs at a temperature of 68°C, Young’s modulus changes most dramatically with temperature, abruptly reversing its declining trend with increasing temperature. The film is stiffened through the transition and, as the temperature is further raised, the declining trend is reasserted at a similar rate.

Performance of Electro-Thermally Driven

The integration of VO2 thin films in a MEMS actuator device is presented. The structural phase transition of VO2 was induced electro-thermally by resistive heaters monolithically integrated in the MEMS actuator. The drastic mechanical displacements generated by the large stress induced during the VO2 thin film phase transition have been characterized for static and time-dependent current pulses to the resistive heater, for air and vacuum environments. A comprehensive and simplified finite element model is developed and validated with experimental data. It was found that the cut-off frequency of the 300 μm-long VO2-based MEMS actuator operated in vacuum (f3dB=29 Hz) was mostly limited by conductive heat loss through the anchor, whereas convection losses were more dominant in air (f3dB=541 Hz). The cut-off frequency is found to be strongly dependent on the dimensions of the cantilever when operated in air but far less dependent when operated in vacuum. Total deflections of 68.7 and 28.5 μm were observed for 300 and 200 μm-long MEMS cantilevers, respectively. Full actuation in air required ~ 16 times more power than in vacuum.

{\rm VO}_{2}

The dynamic response of VO2-coated silicon microcantilevers thermally driven over the film’s insulator-to-metal transition was studied using laser light pulses directly incident on the cantilevers. The measured photothermal response revealed very high curvature changes of approximately 2500 m−1 up to pulse frequencies greater than 100 Hz and readily observable vibrations up to frequencies of a few kHz with no amplitude degradation after tens of thousands of pulses. Maximum tip amplitudes for 300-μm-long, 1-μm-thick cantilevers used in these experiments were nearly 120 μm and correspondingly less for 2-μm-thick cantilevers. The main mechanism limiting oscillation amplitude was found to be heat transport response during heating and cooling, which depends mainly on thermal conduction through the cantilever itself to the massive anchor and chip body, which acted as a heat sink at room temperature. For the laser-driven oscillations studied, damping by the surrounding air is unimportant in the range of frequencies probed. Large-curvature response is expected to extend to higher pulse frequencies for cantilevers with smaller dimensions.

-Based MEMS Actuators

Photo-thermal actuation has been used to program the resonant frequency of a VO2-coated SiO2 micro-bridge. The SiO2 micro-bridge had nominal length, width, and thickness of 300, 45, and 4.15??m, respectively. The thickness of the VO2 coating was 150?nm. The changes in resonant frequency are caused by stress changes on the bimorph structure during the coating?s insulator-to-metal-transition. A total of 13 resonant frequency memory states ranging from 215.5 to 222.7?kHz were programmed by laser pulses of increasing energy in steps of 0.7??J, focused on the micro-bridge structure. The device was maintained at 60??C during programming experiments, and the memory was reset by driving the temperature outside the hysteresis loop. After programming the device to a particular resonant frequency, the memory state was stored for more than 24?h as long as the sample was maintained at the pre-heating temperature of 60??C.

Dynamics of photothermally driven VO2-coated microcantilevers

This letter reports the use of programmed optical states in a vanadium dioxide (VO2) thin film for the projection of near infrared (NIR) images. The optical states are programmed by photothermal actuation from scanning a focused red laser on localized regions of the VO2 thin film. The abrupt change in transmissivity of NIR wavelengths across VO2s solid-to-solid phase change is used to reflect a diffused NIR beam in the regions scanned by the red laser. The transmitted NIR beam is projected on a laser beam profiler. The results of a single scan from the red laser show images with a high contrast between the two phases of the material.

A micro-mechanical resonator with programmable frequency capability

The mixed valence vanadium oxide V6O13 is an interesting material which exhibits an insulator-to-metal or semiconductor-to-semiconductor transition at low temperatures. It is also a much studied cathode material for lithium-based thin film batteries. However, there is little information available about its mechanical properties. Young’s modulus of pulsed-laser deposited V6O13 thin films has been determined by measuring the fundamental resonant frequency of silicon dioxide microcantilevers coated with V6O13. Laser deflection techniques were used to measure the cantilevers’ resonant frequencies. The films were further characterized by x-ray diffraction, atomic force microscopy, and resistivity measurements. The value of Young’s modulus associated with the direction along the material’s (001) planes was found to be approximately 100GPa. The values obtained for films ranging from 90to200nm were equal within experimental error.

Programming and Projection of Near IR Images Using

This letter reports a resonant frequency shift of approximately - 23% for a VO2-coated silicon dioxide (SiO2) buckled microbridge when the coatings insulator-to-metal transition (IMT) is thermally induced by heat conduction using a Peltier heater or by light irradiation from a laser diode. The frequency tuning is attributed to the large stress levels developed during the IMT and the large sensitivity of buckled microbridges to changes in stress.

{\rm VO}_{2}

This paper reports measurements of the electrical resistance and resonant frequency of VO2-coated silicon dioxide (SiO2) bridges when the coatings insulator-to-metal transition (IMT) is thermally induced by conduction. The measurements of these two properties were done simultaneously. The decrease in electrical resistance was close to three orders in magnitude. The resonant frequency shift across the IMT started with an increase of 1.5% for temperatures in the “cold end” of the IMT, followed by a decrease of 20% for temperatures in the “hot end” of the IMT.