As a result of their ability to amplify input light, ultra-high quality factor (Q) whispering gallery mode optical resonators fabricated from silica have demonstrated extremely low threshold nonlinear behaviors (eg FWM, Raman). However, while the cavity Q may reduce the threshold, it is not able to improve the efficiency. By coating optical resonators with gold nanorods functionalized with small molecule coatings or magnetic nanoparticles, we are able to increase the nonlinearity of the material system and demonstrate an efficient frequency comb generator in the near-IR. Additional nonlinear behaviors, e.g. Anti-Stokes/Stokes generation, are also observed with low thresholds.
From multi-photon to single molecule, the past several decades have witnessed a revolution in fluorescent microscopy. These techniques have revealed the inner working of cells and tissue and have relied on symbiotic advances in advanced molecular probes, light emitting molecules and particles, and novel instrumentation. More recently, researchers have begun to develop functional nanomaterials or materials that can response to their environment. In this talk, I will discuss some of our recent work in developing functional imaging agents for multi-wavelength and multi-photon live-cell imaging, focusing on recent molecular designs performed using density functional theory as well as in vitro studies.
Due to their high circulating intensities, ultra-high quality factor dielectric whispering-gallery mode resonators have enabled the development of low threshold Raman microlasers. Subsequently, other Raman-related phenomena, such as cascaded stimulated Raman scattering (CSRS) and stimulated anti-Stokes Raman scattering (SARS), were observed. While low threshold frequency conversion and generation have clear applications, CSRS and SARS have been limited by the low Raman gain. In this work, the surface of a silica resonator is modified with an organic monolayer, increasing the Raman gain. Up to four orders of CSRS is observed with sub-mW input power, and the SARS efficiency is improved by three orders of magnitude compared to previous studies with hybrid resonators.
Azobenzene is capable of reversibly switching its conformation upon the UV/Visible optical exposure due to its reversible trans/cis photoisomerization. By merging this organic material with conventional photonic devices, new architectures can be developed. In our study, we developed hybrid organic/inorganic whispering gallery mode microcavities consisting of a self-assembled 4-(4-diethylaminophenylazo)pyridine (Aazo) monolayer anchored on an integrated SiO2 optical microtoroid. As the Aazo monolayer changed conformations, the resonant wavelength was tuned. The surface density of Aazo was modified by introducing CH3 spacer molecules providing control over the magnitude of the shift. Owing to the uniformity of Aazo monolayer, cavity quality factors reached above 1 million in the near-IR range. Two optical lasers were simultaneously coupled into the Aazo-coated devices with a single waveguide. The 1300 nm laser is used to excite and monitor a single resonant wavelength of the cavity, and the 410 nm laser triggers the thermodynamically stable trans-Aazo to photoswitch to the thermodynamically unfavored cis-Aazo. When the Aazo photoswitches, the cavity resonant wavelength at near-IR wavelength shifts due to a change of refractive index in the Aazo layer. To revert the molecule back to trans-Aazo, a CO2 laser is used to heat the device system. Even after storage in air, the switching behavior is unchanged. Theoretical analyses are conducted based on density functional theory of the Aazo isomers combined with finite element method simulations of the optical mode. The theoretical results agree with the experimental findings.
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.
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
special molecules have found uses in the development of “smart”polymers, optical writing of grating films, and even
controllable in-vivo drug release. Traditional photoswitchable small molecules include azobenzenes, spiropyrans,
diarylethenes, and a whole host of their derivatives. These classes of molecules can either photoisomerize or undergo
reversible ring opening, respectively. Being the simplest class of photoswitches in terms of structure, azobenzenes have
become the most ubiquitous, well-characterized, and implemented organic molecular switch. In this work, an
azobenzene derivative is utilized and covalently attached to the surface of a silica microtoroidal optical resonator and is
used to tune the resonance around fifty percent of the cavity’s free spectral range. An evanescently coupled 1300nm
laser is used as the probe wavelength to monitor the trans-cis isomerization initiated by a 450nm laser source which is
also coupled into the device. Results and kinetics are compared to UV-Vis spectroscopy and ellipsometry, and the tuning
sensitivity is compared to other established methods in the literature.
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