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Posted: Feb 07, 2018
A rewritable metacanvas for photonic applications (w/video)
(Nanowerk Spotlight) Tunable materials can change their innate optical properties on demand, rather than relying on mechanical components to focus an object such as a camera lens or telescope eyepiece. Substances like vanadium dioxide (VO2), for instance, can transition between opaque and transparent under a particular set of conditions like a certain temperature, making them difficult to incorporate into useful devices.
Previous tunable photonic devices based on VO2 are mostly lithographically made, and switchable but not fully reconfigurable. This is because they have permanent patterns and fixed functionalities. Note that reconfigurable devices allow changes of patterns and functionalities from X to Y to Z, etc, while the patterns in switchable devices are fixed and can only be switched between X-on and X-off.
"In stark contrast, our metacanvas is lithography-free and fully reconfigurable," Junqiao Wu, a Professor in the Department of Materials Science and Engineering at the University of California, Berkeley, tells Nanowerk. "Both the patterns and the functionalities of the metacanvas can be arbitrarily reconfigured, which leads to many more degrees of freedom in metasurface design and functionalities."
He adds that one piece of metacanvas is able to function as different optical components – hologram, phase-array, polarizer, modulator, etc. – at different times and on command, which has never been achieved in any of the previous VO2.
A rewritable metacanvas. a) Schematic of laser writing different photonic operator patterns on a metacanvas. b) Temperature-dependent resistance of a VO2 film, where the transition temperature (Tc) is denoted by a vertical dashed line. Insets A and B: Unpatterned VO2 film (all in I-phase). Inset D: The VO2 film (global temperature kept at Tc) is laser written with a pattern of a bear in the M-phase. c) Optical images from writing and erasing process on the metacanvas: i–iii) a pattern of a bear (M-phase) is written onto an I-phase VO2 film, iv,v) then erased by decreasing the global temperature, vi) and another pattern of the word “METACANVAS” is written subsequently. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The metacanvas is based on the phase transition of vanadium dioxide, whose conductivity, color and structure all change with temperature. Specifically, VO2 is in an insulating phase at room temperature, and changes into metallic phase at temperatures above 67 degrees Celsius.
Raising its environment's temperature to 67°C, VO2 still stays in the insulating phase due to a slight delay (so called hysteresis) in the phase transition. Then, a writing laser beam is focused onto the VO2 film, and locally heats up the material into the metallic phase.
By moving of the laser beam controlled by computer, almost arbitrary metallic patterns can be written onto the insulating film as background. These patterns could be erased by simply cooling down the environment temperature such that the entire VO2 film goes back to the insulating phase.
The metallic pattern on the insulating background is the metamaterial, and would deliver the photonic functions.
"Analogous to the revolution from pre-printed transparency slides to the current digital projectors enabled by the modern digital light processing (DLP) technology, our approach enables full dynamic control and true reconfigurability in metamaterials," notes Assistant Professor Jie Yao, who, together with Wu, led this work.
The motivation behind this research lies in the principal and experimental limitations of conventional optics by their fixed functionalities. To construct an optical system, one has to purchase a large number of optics and arrange them in a complex layout, for example, lenses with different focusing lengths, linear polarizers working at different wavelengths, etc. Researchers sometimes feel frustrated when a specific optics component is missing, and have to order and wait for its arrival. The reconfigurable photonics enabled by the metacanvas would fill this technology gap.
Now though, the researchers have successfully achieved a rewritable photonic platform on which nearly arbitrary metamaterials can be rapidly and repeatedly written and erased – just like an Etch-A-Sketch™ drawing toy to write metamaterials.
"Previously, the functionality and/or technical specifications of an optical device is fixed after fabrication, leading to fundamental limitations," says Kaichen Dong, the paper's first author. "With the metacanvas, one can realize in situ reconfiguration of optical/photonic devices, and even fully reconfigurable photonic circuitry/system. Moreover, the real-time manipulation of light enabled by metacanvases allows scientists a peek into temporal evolution of photonic phenomena. Similar examples exist in other fields, such as the field-programmable gate arrays (FPGA) in electrical engineering, which greatly improved embedded systems."
Junqiao Wu and Jie Yao, professors of materials science and members of Lawrence Berkeley Lab's Materials Sciences Division, asked themselves how it might be possible to create metamaterial devices that could be erased and written over again – like an Etch A Sketch™ drawing toy – to quickly yield a quite different device.
Such a metacanvas could benefit applications in the field of photonic metamaterials and could function as a fully reconfigurable optical/photonic device which can change its functionality or technical specifications in real time.
For example, a single device can serve as a linear polarizer in the beginning, but transform into a beam steer at a later time when needed. Immediate application might be a reconfigurable phase array, which would greatly boost the performance of radar, communication, biomedical sciences, holography, optical tweezers, etc.
"Furthermore, the metacanvas could enable dynamic transition in optics without physically replacing the optical components," notes Wu. "Hence multiple metacanvases could work together to construct a dynamic optical system without moving parts, where photonic elements can be field-programmed to deliver complex, system-level functionalities."
This work also helps scientists to investigate dynamic optical phenomena by experimentally making smooth transitions between static stages without interrupting the optics.
"Moreover, since different microstructures can be rapidly and repeatedly patterned on the metacanvas, it could help engineers test their structural design of optics in a fast and economical way – just write their designs on the metacanvas and see whether they work as expected," adds Dong. "We call this a 'physical simulator'."
Finally, the idea of metacanvas itself is inspiring, and could motivate researchers to look into other materials with phase transition for similar and new applications.
Going forward, the team is seeking opportunities to unleash the full potential of the metacanvas, such as in three dimensions. Aided with more advanced facilities, one can envision further application of the metacanvas technology for more advanced industrial and scientific applications.
"Reconfigurability will still be intensively pursued in the photonic field, because it is able to significantly improve the performance of optical systems, and lead to new findings/inventions which are difficult, or even impossible, to be experimentally achieved otherwise," says Wu. "New mechanisms to fulfill the reconfigurability will be found, and existing approaches will be technically improved – we could eventually have an 'optical FPGA' in the future.
"One of the largest challenge is the cost of reconfigurable devices," Wu concludes. "Reconfigurable photonic devices have to be more cost-effective before they could be practically utilized."