Noraica Dávila

A micro-mechanical resonator with programmable frequency capability

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.

Programming and Projection of Near IR Images Using

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.

{\rm VO}_{2}

A fully electronic variable optical attenuator in the near-infrared region is reported. The observed attenuation varied from 0 to 19.24 dB. The device is enabled by the strong correlation between the electrical and optical properties in vanadium dioxide thin films across its phase transition. A self-sensing feedback approach is used to significantly reduce the inherently hysteretic behavior of the film. The reduction of the hysteresis, sensing the electrical resistance with respect to the optical transmission, allowed for a relatively simple approach for the modeling and control of the system.

Films

Three silicon micro-cantilevers coated with a Cr-doped vanadium dioxide (VO2:Cr) thin film were photothermally actuated with a pulsed red laser in order to study its dynamic performance in air and in water. Micro-cantilever vibration amplitudes with a magnitude similar to that of DC operation were obtained in air up to 600 Hz. In water, a decrease in amplitude started at ~100 Hz, but it did not go below -3 dB until a few hundreds of Hz. Deflection transients of all micro-cantilevers were also measured in both media. This revealed that, in water, the deflection response to the laser pulses is greatly hampered by the amount of time required for the micro-cantilevers to heat. However, in air, there was no apparent difference in the transients between heating and cooling. These results suggest the use of VO2:Cr coated micro-cantilevers as optically driven actuators with large deflections in liquid media suitable for biological based applications.

Electronically Variable Optical Attenuator Enabled by Self-Sensing in Vanadium Dioxide

This work presents a new technology suitable for localized infrared (IR) phototherapy. Specified regions of a VO2 thin film were programmed to different transmissivity values in the near IR. Such programming was done through photothermal actuation of the thin film, achieved by controlling the scanning of a red laser over its surface. The thermally-induced phase transition of the VO2 thin film comes with an abrupt change in its transmittance, which (due to its hysteretic behavior) can be stored; allowing for the programming of optical memory states in the films. Several patterns were stored and read through a simple optical system. A single film can store one pattern at a time. The nature of this material allows for a relatively fast reconfiguration of the stored pattern. After a specific pattern was programmed on the film, an IR image was projected by illumination of the entire VO2 film with an IR beam. The image was detected by a laser beam profiler (LBP). This pattern storage and projection technology proves promising in the development of applications for treatments of IR phototherapy.

Dynamics of VO 2 :Cr coated microcantilevers in aqueous media

Two types of micro-mechanical memory cells are introduced in this paper. The programmable parameters are the resonant frequency of a micro-mechanical bridge and the tip deflection of a micro-mechanical cantilever. For both cases, the actuation was done by laser pulses that locally heated the vanadium dioxide (VO2 ) thin film coating. Due to the hysteretic behavior of the mechanical properties of the VO2 film coating across the transition, a different “memory state” was programmed with every pulse. The memory state remained programmed after the pulse ended, as long as the sample was maintained at a specific temperature (programming temperature) by using a Peltier heater. The resetting of the mechanical memory cell was accomplished by driving the temperature to regions outside the hysteretic region.Copyright

Programming optical states in VO 2 film for IR phototherapy

This letter reports the resistivity variations for a VO2 thin film on silicon dioxide (SiO2 ) micromechanical bridges when the coating’s insulator-to-metal transition (IMT) is thermally induced by optical radiation with a 635 nm laser diode or by conduction using a heater. The coating’s resistivity and temperature coefficient of resistance (TCR) were calculated as a function of temperature through heating-cooling cycles. A resistivity change of nearly three orders of magnitude through the IMT was observed, and the results obtained for the two different heating methods were compared. A peak TCR of −85%/°C during the heating process and −80.8%/°C during cooling were measured.Copyright

Multifunctional micro-mechanical memory devices

The time-dependent response of a 300 μm-long VO2 -coated silicon micro-cantilevers photothermally driven over the film’s insulator-to-metal transition (IMT) by laser light pulses was studied and tip displacements of ∼ 80 μm were found for driving frequencies over 100 Hz, while displacements of ∼ 10 μm were found for frequencies close to 1.2 kHz, with no performance degradation after millions of pulses. Additionally, the deflection of a 250 μm-long VO2 -coated silicon micro-cantilever as a function of temperature heated through a Peltier heater was studied. The main mechanism limiting amplitude was found to be the time required for heat transport during heating and cooling. Hence, wireless actuation in this manner is expected to be even faster for smaller cantilevers.Copyright