Chameleon showing red color

The Northwestern team devised a plasmonic laser whose wavelength can be tuned by changing the spacing between gold nanoparticles embedded in an elastic matrix—analogous to the way some chameleons change their color of their skin. [Image: George Lebada]

A research team from Northwestern University, USA, led by OSA Senior Member Teri Odom, has developed a flexible plasmonic nanolaser platform whose emission wavelength changes as the material is stretched (Nano Lett., doi: 10.1021/acs.nanolett.8b01774). The stretchable platform—an elastomer slab dotted with a rectangular array of large nanoparticles—achieves its wavelength tunability through an increase in the nanoparticle spacing with strain, similar to the mechanism by which a chameleon varies the color of its skin.

The team believes that the scheme could offer a route toward advances in flexible displays, wearable devices and new kinds of strain sensors.

Taking a cue from a lizard

Certain types of chameleon, such as the panther chameleon of Madagascar, perform some of their color-changing magic through guanine nanocrystals embedded in their skin. The lizards can dynamically adjust the spacing between these the nanoparticles, which behave as photonic crystals; those spacing changes, in turn, give rise to changes in the interference of scattered light that result in color shifts. This allows the critters to quickly change, for example, from their usual olive-drab appearance to a more eye-catching bright red—a useful trick for males seeking to impress potential mate.

Odom, and with co-team-leader George Schatz, saw the chameleon’s technique—nanoparticles embedded in a flexible substrate—as a potential model for a tunable nanolaser platform. Their laser, however, would leverage not interference but plasmonics, the subwavelength confinement and amplification of light energy by arrays of metal nanoparticles. Previous work had shown that such an array, with the particles surrounded by an appropriate gain medium and optically pumped, can act as an efficient laser cavity.

In principle, the wavelength for such a plasmonic laser can be tailored simply by changing the spacing of the nanoparticle array and, thus, the wavelength of the plasmon resonance. But in practice, creating a continuously tunable, stretchable nanolaser poses a problem: the plasmonic response of such nanoparticle arrays tends to rely on a strong in-plane electric dipole coupling between nanoparticles. Stretching the lattice can interfere with that coupling and thus degrade the cavity mode quality of the laser.

From dipole to quadrupole

Portrait of Teri Odom

Teri Odom. [Image: Northwestern University]

Odom and Schatz found an answer to that dilemma by looking at the plasmonic response of relatively large nanoparticles. The research team calculated that, by carefully tuning the particles’ diameter and height, they could create a nanoparticle array dominated not by in-plane dipole oscillations but by a narrow, out of-plane quadrupole resonance. Such a resonance, whose wavelength depends on the lattice spacing, could still potentially provide a nanoscale laser with a wavelength tunable by stretching. But because the resonance would no longer be dominated by in-plane coupling, the laser’s cavity mode should remain robust even as the lattice was stretched.

To test out the concept, the team used soft-process lithography to embed a rectangular lattice of cylindrical gold nanoparticles—of a comparatively large diameter of 260 nm and a height of 120 nm, and spaced 600 nm apart—into a flexible slab of the elastomer poly(dimethylsiloxane) (PDMS). They then sandwiched a droplet of the commercial laser dye IR-140-DMSO, to act as a gain medium, between the elastomer-embedded nanoparticle array and a glass cover slide. They placed the device onto a uniaxial stretching apparatus to impose uniform strains, and pumped the system with an 800-nm Ti:sapphire laser.

The team found that the nanoparticle-embedded system lased as expected—and that the researchers were able to tune the laser emission across a 31-nm wavelength range by varying the imposed strain between 2 and 5 percent. What’s more, the process was reversible: the emission wavelength tracked back down in an orderly way as the strain was released, with virtually no hysteresis experienced in repeated trials. “By stretching and releasing the elastomer substrate,” Odom said in a press release, “we could select the emission color at will.”

Expanding possibilities

The Northwestern team suggests that the platform’s tunability could be extended significantly, conceivably to range from the UV to the near-infrared, by incorporating different types of gain materials, such as a combination of dyes, quantum dots and 2-D materials, into the system. With such tunability, the researchers conclude, the system could “have wide applications in flexible photonic devices, in situ biomedical imaging and optical communication.” And, according to the authors, it could open prospects for mechanically modulating light–matter interactions in fluorescence, photocatalysis and quantum optics as well.