Refractive index of azobenzene holds key to tunable optical microresonators

With the application of an external optical stimulus, photoswitchable molecules can undergo reversible structural changes. This property makes photoswitchable molecules suitable for a wide range of applications, from the development of smart materials to controlled in vivo drug delivery.

Azobenzene is a well known photoswitchable molecule that changes its properties under blue light. By inducing conformational change and altering their refractive index, Kovach et al. modified the resonant wavelength of azobenzene molecules to create tunable microresonators.

In order to study a variety of effects, the researchers excited the azobenzene molecule with lasers of varying wavelengths and at different timescales and monitored its resonance peak. When exposed to 450 nanometer light, they found a 0.0023 decrease in azobenzene’s refractive index without any deterioration in its optical performance.

“This sounds really, really tiny,” said author Andrea Armani. In a system in which light only interacts with a surface a single time – like an optical fiber – a change of this size would be insignificant. “But we attached it to a high quality resonator, which means the light is confined inside and orbits around 10,000 to 100,000 times, so any small change in index is amplified.”

Because resonators are a fundamental building block of optical circuits, this broad tunability may be useful for any application that requires an all-optical system.

“Coming up with a way to manipulate the light being stored inside this fundamental building block is significant,” Armani said. “We can now do that without any kind of electrical control circuitry.”

Source: “Optically tunable microresonator using an azobenzene monolayer,” by Andre Kovach, Jinghan He, Patrick Saris, Dongyu Chen, and Andrea M. Armani, AIP Advances (2020). The article can be accessed at https://doi.org/10.1063/1.5143253 .