The chameleon and the crystal maze

How a crystal structure within the skin of the famous colour-changing lizard holds the key to wearable photonic devices, next generation screens and optical communications  

In nature, chameleons can easily change colors by controlling the spacing between periodic nanocrystals on their skin.

This coloring based on surface structures is chemically stable and robust. Inspired by nature, we managed to make a laser able to change color using a similar mechanism to that seen in chameleon skin. Such mechanically tunable lasers could provide advances in responsive optical displays, wearable photonic devices, and ultra-sensitive strain sensors.

Researchers have recently discovered that panther chameleons can change their color by active tuning of spacings in a lattice of guanine nanocrystals on their skin. The periodic lattices function as photonic crystals – dielectric nanostructures with ordered refractive index variation – to control the light flow. Certain wavelengths of light associated with the lattice spacings can be reflected from the nanocrystals, while others are forbidden, which determines the colors we perceive in the eyes.

Chameleons manifest skin color change from green to yellow or orange from the resting to the excited state. At the same time, distance among guanine crystals increases by around 40%, which causes shifting of selective reflectivity from short (blue, green) to long wavelengths (orange, red). Further straining the elastic skin could enable color changes over the entire visible range. This color change occurs within a couple of minutes and is fully reversible, and can inspire diverse applications in photonic displays, optical communications and on-chip circuits.

Compared to tunable colors in chameleons, the laser emission is typically fixed at the time of fabrication, and the manipulation of output color requires complex optical device designs. Advances in nanofabrication have enabled unconventional material architectures at the nanoscale, and the engineering of nanostructures can introduce new physical properties unavailable in conventional devices.

This color change occurs within a couple of minutes and is fully reversible, and can inspire diverse applications in photonic displays, optical communications and on-chip circuits

Nature-inspired stretchable lasers Exploiting a similar mechanism, we achieved mechanical control of the lasing color by exploiting a lasing cavity based on periodic arrays of nanoparticles (cm²) in a stretchable, polymer (PDMS) matrix. Liquid dye molecules dissolved in organic solvents functioned as the gain media to ensure the dyes would surround the nanoparticles upon stretching of the substrate. In previous work, by switching the liquid dye solution, we had demonstrated the first real-time tunable nanolaser. Here, stretching the substrate provides an alternative way without micro-fluid channels.

Large metal nanoparticles (around 260 nm in diameter) arranged in a lattice (spacing 600 nm) produce high-quality cavity modes with extremely narrow resonance linewidth (< 5 nm). In cases of uneven sample surfaces and defects in the lattice – sharp and intense cavity resonance is still maintained. This new lasing mechanism enables stable high cavity mode quality upon stretching of the device, distinct from current laser designs. The cavity resonance originates from the diffractive coupling of metal nanoparticles; hence, the color of lasing is determined by the interparticle distances, and small changes in strain directly induce the emission peak shift.

When pumped with an external ultrafast laser, the nanolaser emits at around 870 nm normal to the sample surface, with a narrow beam in the far field. The lasing wavelength shifts into the near-infrared side when the device is stretched, and exhibits excellent recovery after releasing of strain. Cyclic changes in strain allow real-time modulation of the lasing output. By stretching and releasing the elastomeric substrate, we could select the lasing emission color at will.

Significantly, stretchable nanolasing from metal nanoparticle arrays induces a wavelength shift of 31 nm for a 3% increase of interparticle spacing, demonstrating a sensitivity around ten times higher than those based on photonic crystals in similar geometries. This improvement is attributed to the tuning of lasing colors directly by small changes in lattice spacing, while the microscale photonic cavities are less sensitive to nanoscale structural changes.

The device also harnesses plasmons – collective oscillations of conduction electrons ­­– on the surface of gold nanoparticles. Thanks to these plasmon resonances, light can be confined to tiny regions smaller than half its wavelength (the diffraction limit), which is typically a challenge for conventional photonic devices. The confined electromagnetic fields result in lasing action from sub-wavelength regions close to the metal nanoparticles. The generation of lasing at the nanoscale is beneficial for enhancing light-matter interactions such as fluorescence, photocatalysis and nonlinear optical processes.

Flexible nanolasers for the future The stretchable nanolaser offers new prospects for future wearable and flexible optical displays in such as televisions and mobile phones that require coherent light sources. Current electronic screens can be easily broken, while the integration with an elastic substrate enables a flexible device robust to deformation. With side-by-side patterning of these nanoparticle lattices and integrating with different gain materials, we could mimic the multi-color change in chameleons by stretching and releasing the flexible device.

Additionally, our laser system can work as an efficient and highly directional light source and achieve emissions from the ultraviolet to near-infrared by incorporating various metals, gain materials, and nanoparticle spacings. Different from gold nanoparticles, silver and aluminum nanoparticles can support nanolasing at the visible and ultraviolet wavelengths. The broadband tuning capability offers possibilities in full-color photonic displays and multi-channel optical communications.

[caption id="attachment_67579" align="alignnone" width="200"] Stretchable nanolasing based on metal nanoparticles integrated with liquid gain materials. Changing interparticle distances directly modulated the cavity resonance wavelength, and thus tunable lasing emission was achieved by stretching and releasing the substrate.[/caption]

References

1. A. Yang, T.B. Hoang, M. Dridi, C. Deeb, M.H. Mikkelsen, G.C. Schatz, and T.W. Odom, “Real-time tunable lasing from plasmonic nanocavity arrays,” Nature Communications 6, 6939 (2015)
2. D. Wang, M.R. Bourgeois, W. Lee, R. Li, D. Trivedi, M.P. Knudson, W. Wang, G.C. Schatz, T.W. Odom, “Stretchable Nanolasing from Hybrid Quadrupole Plasmons,” Nano Letters 18, 7, 4549–4555 (2018)

Authors:

 Danqing Wang is a Ph.D. candidate in the applied physics program at Northwestern University co-advised by Professor Teri W. Odom and Professor George C. Schatz. Her research focuses on structural engineering of plasmonic nanocavities and their light−matter interactions with different gain media.

 Teri W. Odom is Charles E. and Emma H. Morrison Professor of Chemistry, Professor of Materials Science and Engineering, and Associate Director of the International Institute for Nanotechnology (IIN) at Northwestern University.