(Nanowerk Spotlight) Metal nanoparticles can concentrate light near their surface through the excitation of surface plasmons, which are collective oscillations of electrons. Depending on the size and shape of the metal particles, surface plasmons can show a range of different optical responses and colors.
These phenomena are useful for plasmon-enhanced applications ranging from ultrasensitive molecular sensing to fluorescence enhancement, where high-quality (i.e., narrow) and wavelength-tunable plasmonic resonances are particularly desirable.
Although narrow optical resonances and tailoring of the plasmonic resonance have been achieved independently (see for instance our previous Nanowerk Spotlight: "Real-time tunable plasmon laser"), they have not yet been demonstrated within a single system.
The main challenge lies with the fact that either the optical responses are fixed at the time of fabrication or that intrinsic losses of the nanostructures cannot be suppressed to achieve narrow resonance linewidths.
"We have found a way to both realize narrow resonances and tune the resonance across the visible in aluminum nanoparticle arrays embedded in a PDMS slab," Teri W. Odom, Charles E. and Emma H. Morrison Professor of Chemistry and Professor of Materials Science and Engineering at Northwestern University, tells Nanowerk. "We discovered a new type of quadrupolar lattice mode with much narrower linewidth than the classic dipolar lattice mode."
She points out that both modes can be programmed and reversibly engineered by simple mechanical stretching of the substrate. Since each mode can be independently optimized depending on the stretching direction, their single system can cover a large wavelength bandwidth and meet specific application requirements at the same time.
This image shows aluminum nanoparticle arrays in flexible substrate exhibit high-quality and tunable quadrupolar lattice plasmon resonances by mechanical stretching. (Image: Ankun Yang, Odom Group, Northwestern University) (click on image to enlarge)
According to the research team, the use of hexagonal aluminum nanoparticle arrays in flexible substrates has three significant benefits:
Aluminum can show broadband high-quality dipolar or quadrupolar resonances at wavelengths not possible in traditional plasmonic materials such as gold and silver (< 530 nm).
Mechanical stretching enables modulation of the interactions between nanoparticles and tunability of the resonances over a large wavelength range.
A hexagonal array enables different lattice modes (dipolar or quadrupolar) to be accessed, tuned and optimized by stretching along different array directions.
"The strain evolution of plasmon modes offers an attractive and robust way to interrogate the physics of surface plasmons and to achieve real-time tunable substrates for plasmon-enhanced molecular sensing and plasmonic nanolasers," concludes Odom. "Moreover, these programmable plasmonic modes offer a degree of freedom, different from rigid optical systems, that can open a diverse range of possibilities by integrating with flexible electronics."