Optical resonant cavities form the foundation for a wide range of integrated optical components. While a high performance laser requires a high quality factor (Q) cavity, other types of devices, like modulators, rely on the cavity resonant wavelength being tunable. Numerous mechanisms based on the thermo-optic and electro-optic effects have been leveraged to create switchable or tunable devices; however, these are very power hungry and/or require complex control machinery. In the present work, we graft an air-stable, optically triggerable functional group to the surface of an ultra-high-Q optical cavity. The Aazobenzene functional group switches from trans to cis upon exposure to blue light, and it can be thermally triggered to revert to the initial trans state. Using a single tapered optical fiber waveguide, blue and near-IR light can be coupled into the device simultaneously. When the blue light interacts with the Aazo group, the resonant wavelength blue shifts. Upon exposure to a CO2 laser, the resonant wavelength returns to its initial position. Several different aspects of the device operation were investigated, including the kinetics of the switching, the effect of switching via a resonant or non-resonant optical field, and sterics of the switching. Notably, by tuning the surface density of the Aazo groups using a multi-material surface chemistry, it is possible to control the magnitude of the shift.
Photoswitchable organic molecules can undergo reversible structural changes with an external light stimulus. These optically controlled molecules have been used in the development of “smart” polymers, optical writing of grating films, and even controllable in-vivo drug release. Being the simplest class of photoswitches in terms of structure, azobenzenes have become the most ubiquitous, well-characterized, and implemented organic molecular switch. Given their predictable response, they are ideally suited to create an all-optically controlled switch. However, fabricating a monolithic optical device comprised solely from azobenzene while maintaining the photoswitching functionality is challenging. In this work, we combine integrated photonics with optically switchable organic molecules to create an optically controlled integrated device. A silica toroidal resonant cavity is functionalized with a monolayer of an azobenzene derivative. After functionalization, the loaded cavity Q is above 105 . When 450 nm light is coupled into cavity resonance, the azobenzene isomerizes from trans-isomer to cis-isomer, inducing a refractive index change. Because the resonant wavelength of the cavity is governed by the index, the resonant wavelength changes in parallel. At the probe wavelength of 1300 nm, the wavelength shift is determined by the duration and intensity of the 450 nm light and the density of azobenzene functional groups on the device surface, providing multiple control mechanisms. Using this photoswitchable device, resonance frequency tuning as far as sixty percent of the cavity’s free spectral range in the near-IR is demonstrated. The kinetics of the tuning agree with spectroscopic and ellipsometry measurements coupled with finite element method calculations.