(Nanowerk Spotlight) Manipulating light is crucial for modern technologies, from the optical fibers transmitting internet data to the lasers in our smartphones. Despite significant advancements, our progress has been limited by the optical properties of natural materials, particularly in harnessing near-infrared (NIR) light – a part of the electromagnetic spectrum vital for medical imaging, telecommunications, and emerging technologies like autonomous vehicles.
NIR light occupies a unique position between visible light and longer-wavelength radiation, enabling deeper penetration into materials than visible light and allowing non-invasive imaging of biological tissues or sensing through fog and smoke. At the same time, NIR can be focused into tight beams for high-bandwidth communication or precise industrial processing. This combination of properties makes NIR invaluable for various applications, from detecting cancer to facilitating high-speed satellite internet.
However, fully exploiting NIR has been hampered by the challenge of precisely controlling its interaction with matter. Natural materials lack the necessary optical properties to manipulate NIR light with high precision, largely due to their atomic structures.
Metamaterials – artificially engineered structures – offer a solution by interacting with light in ways natural materials cannot. Researchers design these materials with nanoscale patterns to achieve tailored optical properties. While promising, creating metamaterials for the NIR range has been particularly challenging due to the precise nanoengineering required. Effective NIR metamaterials must have structures large enough to interact strongly with NIR wavelengths but small and uniform enough to act as a homogeneous material, a difficult feat to achieve over large areas.
Recent advances in nanotechnology have brought us closer to overcoming this challenge. Improved techniques for synthesizing metal nanoparticles with controlled shapes and sizes have opened new possibilities for plasmonic metamaterials, which leverage interactions between light and the collective oscillations of electrons in metals (plasmons) to produce extraordinary optical effects. Simultaneously, methods for assembling nanoparticles into ordered structures have improved, enabling the creation of large-area arrays with precise control over spacing and orientation.
In this context, a research team from South Korea has made a significant breakthrough, as detailed in their publication in the journal Advanced Materials (“Proximal High-Index Metamaterials based on a Superlattice of Gold Nanohexagons Targeting the Near-Infrared Band”). The team developed a novel approach to creating large-area plasmonic metamaterials specifically designed for the NIR range. By precisely engineering the shape, size, and arrangement of gold nanoparticles, they achieved optical properties previously thought unattainable in this spectral region.
The researchers’ innovation centers on synthesizing and assembling gold nanohexagons (AuNHs) into highly ordered planar superlattices. These hexagonal nanoparticles were chosen for their ability to efficiently fill space in a two-dimensional array, crucial for creating a uniform optical response over large areas.
Shape engineering of the plasmonic polygonal nanoplates into nanohexagons (NHs) via bottom-up synthesis: The ternary phase diagram of three quantitative metrics (triangularity (fT), circularity (fC), and hexagonality (fH)) for the evaluation of the morphological transformation from Au nanotriangles (AuNTs) to AuNHs. (Image: Adapted from DOI:10.1002/adma.202405650 with permission by Wiley-VCH Verlag)
The team used a multi-step process to create uniform AuNHs with carefully controlled dimensions. Starting with gold nanotriangles, they employed etching and regrowth steps to form nearly perfect hexagons, a shape critical for maintaining uniform optical properties. Small variations in shape or size could significantly impact the metamaterial’s optical properties.
A key advancement was the surface modification of AuNHs with two types of organic molecules, creating “amphiphilic” nanoparticles that assembled at the interface between two immiscible liquids. By carefully controlling the evaporation of the top liquid layer, the researchers induced the AuNHs to pack tightly together, forming a large-area planar superlattice.
The resulting superlattice exhibited extraordinary optical properties, with refractive indices exceeding 10 at certain NIR wavelengths—far higher than any natural material and surpassing previous records for metamaterials in this spectral range. Even exotic materials like silicon rarely have refractive indices above 4 in the NIR. This dramatic increase in refractive index allows for unprecedented control over NIR light.
Importantly, the researchers demonstrated they could systematically tune the optical properties of their metamaterial by adjusting the gap between neighboring nanohexagons. This precise tuning was achieved using a plasmonic percolation model, varying the length of organic molecules coating the nanoparticles to control the interparticle gap.
This approach offers several advantages over previous efforts to create NIR metamaterials. It allows for large-area, uniform structures essential for practical applications. Additionally, the wet-chemistry methods employed are potentially scalable for industrial production, unlike more exotic fabrication techniques. The planar nature of the superlattice also makes it compatible with existing semiconductor manufacturing processes, which could simplify integration into devices.
To demonstrate the potential of their metamaterial, the researchers constructed a distributed Bragg reflector (DBR), an optical component used in lasers, filters, and sensors. By alternating layers of their high-index AuNH superlattice with low-index polymer layers, they created a DBR that showed strong and selective reflectivity in the NIR range. This proof-of-concept device illustrates potential applications in optical communications and sensing.
Distributed Bragg reflector (DBR) composed of 1D photonic crystal containing the planar AuNH superlattices. a) A Schematic illustration of the fabrication method of the DBR composed of alternatively deposited AuNHs superlattices (monolayer) and polyurethane acrylate (PUA) thin film. b) Cross-sectional SEM images of the fabricated AuNH/PUA DBRs with different numbers of the multilayers (i.e., 3, 5, 7, 9, and 11 layers) (scale bar = 1 µm). c) Vis-NIR reflectance spectra of the AuNH/PUA DBR with the different numbers of the multilayers. d) A comparison of photoluminescence (PL) spectra of upconverting nanoparticles (UCNPs) on glass, gold film, and the AuNH/PUA DBR (excited at 980 nm NIR laser with power density of 0.8 W cm−2). (Image: Reproduced from DOI:10.1002/adma.202405650 with permission by Wiley-VCH Verlag) (click on image to enlarge)
The significance of this work extends beyond the specific metamaterial created. It showcases a new approach to engineering plasmonic nanostructures that could be adapted to other wavelength ranges and material systems. The ability to produce large-area, uniform metamaterials with precisely controlled optical properties opens new avenues for manipulating light in ways previously considered impossible.
This research could enable a new generation of NIR optical devices. Improved medical imaging systems could use the high refractive index to create sharper, more detailed images of tissues. Telecommunications networks might benefit from more efficient optical switches and modulators. In sensing, the strong light-matter interactions enabled by these metamaterials could lead to more sensitive detectors for applications ranging from environmental monitoring to security screening.
While this work represents a significant advance, challenges remain before these metamaterials can be widely adopted. Scaling up production while maintaining precise nanostructures will be crucial. Further research is needed to fully understand and optimize the optical properties for specific applications.
Nonetheless, this research marks an important step forward in controlling near-infrared light. By bridging the gap between nanoscale engineering and large-area fabrication, it brings us closer to harnessing the full potential of this critical part of the electromagnetic spectrum. As the field progresses, we may see new technologies that leverage these extraordinary optical properties, potentially revolutionizing sectors from healthcare to information technology.
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