麻豆淫院

The iridescent blue of butterfly wings has inspired researchers to find a solution to a challenge previously considered insurmountable鈥攄ynamically tuning advanced optical processes at visible wavelengths.

The result is a patterned layer of material a fraction of the thickness of a hair, that could underpin radical new optical technology: applications of the technology are diverse, ranging from adaptive camouflage, through biosensing to quantum light engines for on-chip computing and secure communications.

The research is published in . The first author is Dr. Mudassar Nauman, from the ARC Center of Excellence for Transformative Meta-Optical Systems (TMOS) and BluGlass Ltd.

"We reimagined how light and matter interact, which enabled us to take a problem and turn it into part of the solution," said Dr. Nauman, who did the work while a student jointly enrolled at the RS麻豆淫院 Department of Electronic Materials Engineering and the ANU School of Engineering.

"By linking two processes, we've turned what was seen as a dead end and turned it into a practical reality so adaptable it can be built onto anything from glass panels to a contact lens."

The research maps a way to enable nonlinear optics with meta-surfaces, thin layers patterned with structures smaller than the wavelength of light, that generate material properties wildly different from natural materials.

Better yet, the nonlinear effects can be turned on and off by changing the polarization of the input light, and tuned by changing the temperature of the material.

The nonlinear optical processes possible with the new devices enable the up- and down- conversion of frequencies that are useful in processes such as night-vision technology (converting infrared light to visible light), or generating quantum-entangled photon pairs, said co-author Professor Andrey Miroshnichenko, from UNSW Canberra.

"The most exciting aspect of this work is that it brings us a step closer to the practical realization of fast, tunable optical elements鈥攖echnologies that can quite literally make the invisible visible," he said.

Co-author Professor Yuerui (Larry) Lu from ANU School of Engineering added, "This breakthrough paves the way for reconfigurable optical devices with tunable nonlinear responses, leveraging the unique advantages of van der Waals materials for next-generation quantum and photonic technologies."

Efficient nonlinear processes require materials with a high refractive index and strong optical qualities.

Based on these criteria, this project focused on a standout class of materials: crystals made from transition metals combined with anions from the oxygen family (Group 16, known as chalcogenides).

These transition metal dichalcogenides (TMDCs) exhibit strong semiconducting properties due to their single-crystalline quality, and wide and tunable bandgaps. They also exhibit extremely strong light-matter interactions, caused by the formation of excitons, a quasiparticle formed by an electron and a hole binding together.

TMDCs can be seamlessly integrated with silicon chip technology and so promise cheap and practical scalability.

However, in their common 2H crystal form (mirror layer crystal), TMDCs have two seemingly insurmountable problems: Firstly, although useful for telecommunication, which uses infrared wavelengths, TMDCs are opaque to the visible light needed for human-vision applications, because the excitons absorb so strongly.

Secondly, their crystal structure is symmetric about its center point, which suppresses half the nonlinear conversion processes: only frequency conversion to odd multiples is allowed (triple, quintuple, septuple frequencies and so on)鈥攗nhappily the simplest and usually most efficient process, frequency doubling (also known as second harmonic generation), is very weak.

Other research has attempted to tackle these problems but has been dogged by problems with structural fragility, absorption, or the need for cryogenic cooling.

While pondering this challenge, Dr. Nauman was inspired by the brilliant colors of the wings of the Morpho butterfly genus.

"The secret to the vibrancy is a clever two-part system. The transparent nanostructures reflect blue light, and a layer of dark melanin sits underneath absorbing any stray light," Dr. Nauman said.

"It's like a diamond on black velvet鈥攖he dark background makes the diamond's sparkle more brilliant."

"Nature taught me the best results often come from indirect solutions."

The indirect solution that Dr. Nauman hit on was to use a near-infrared wavelength pump laser, which can travel into the TMDC without absorption, and to pair this with metasurface design to engineer a resonance at 1,220 nm鈥攖wice the exciton wavelength鈥攖o harness the energy.

This resonance, of a type known as a quasi-bound state in the continuum (qBIC), was designed to be purely magnetic鈥攁voiding any electric dipole component that would lead to radiative losses. This ensured the resonance had a high Q鈥攊n other words, the pump energy was trapped efficiently, allowing it to build up to the level at which it could interact with the excitons at double the frequency (half the wavelength, 610 nm) and could generate the normally-weak second harmonic radiation.

This virtual interaction between the exciton and the qBIC resonance is the heart of the device's performance鈥攁nd, critically, it's a link that researchers can break or restore on demand.

The geometry of the metasurface needed to support qBICs comprises an array of crescent-shaped nano-structures, each smaller than the wavelength of light. Achieving such a purely magnetic qBIC in a nonmagnetic material is rare, and here it is realized in a single crystalline TMDC.

The asymmetry of the crescents gave the metasurface a polarization response鈥攂y changing the pump polarization switches the resonance鈥攁nd thus the virtual exciton link鈥攐n or off, modulating the nonlinear light intensity.

More dynamic control can be exerted by tuning the exciton resonance, which can be shifted via material strain, electric field, or, as was used in this experiment, temperature.

"It's a remarkable opportunity to use the tunability of the excitons to enable extreme tunability of the nonlinear response of the metamaterial," said TMOS Director, Professor Dragomir Neshev.

Initial experiments using tungsten disulfide successfully demonstrated the principle, achieving a two-order-of-magnitude enhancement over monolayer tungsten disulfide and four orders of magnitude enhancement over unpatterned bulk film, in the visible spectrum.

Changing the temperature between -100 degrees Celsius and 100 degrees Celsius shifted the exciton resonance by around 20 nm. The shift also changed the strength of the virtual connection, thereby modulating the nonlinear light intensity.

Teamwork was a key to the accomplishment, said Distinguished Professor Mohsen Rahmani, from Nottingham Trent University in the United Kingdom.

"It's inspiring to see how a diverse team of scientists from different corners of the globe can collaborate across borders to advance human knowledge," he said.

The success unlocks the use of TMDCs for second harmonic generation across a wide range of visible spectrum at efficiencies never achieved before.

"It's exciting to challenge conventional wisdom. One would normally expect excitons to quench the harmonic signal through absorption. Here we have shown the opposite鈥攏ot only did we achieve a greatly enhanced second harmonic generation yield but, more importantly, we gain a powerful handle for dynamically tuning it," said co-author, Associate Professor Domenico de Ceglia, from University of Brescia, in Italy.

Many benefits could flow from the successful flipping of these materials' weaknesses into strengths, Dr. Nauman said.

"We transformed a TMDC into a highly efficient nonlinear emitter. Importantly, this strategy is universal鈥攓BICs can be excited in bulk, few-layer, and even monolayer TMDCs, making them a powerful platform for tunable and highly efficient nonlinear optics," he said.

"Because it can be tuned dynamically, this approach can be used for technologies that sound like science fiction today; for example, neural interfaces where light-matter interaction can be tuned dynamically, reconfigurable ultrathin holographic AR/VR lenses, or cloaking metasurfaces controlled purely by light."