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.
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.
On-chip Kerr frequency combs have attracted significant attention because of their compact footprint and numerous applications. While many integrated material systems are being investigated for generating the on-chip Kerr frequency combs, so far only silica devices have achieved quality factors above 100 million, which is important for decreasing the threshold and power consumption of the system. However, as an intrinsic property of silica, the hydroxyl groups present on the surface of the devices will attract water molecules in the air, which decreases the quality factor of the devices. To maintain the performance of the frequency combs, methods like putting the devices in nitrogen purged boxes or building covers for the system are proposed, which would largely increase the complexity of the system. Here we studied another material system, silicon oxynitride microtoroids, whose quality factors can achieve and stay constant at more than 100 million because of the lack of the hydroxyl groups on the surface. Kerr frequency combs are generated from the SiOxNy microtoroids with normal dispersion with avoided mode crossing. Thresholds as low as 280 μW are achieved as a result of the high quality factor. The comb spectrum remains the same for the same pump power over the nine day period after fabrication, which indicates that the performance of the frequency combs remains constant despite the silicon oxynitride devices being stored in ambient atmosphere without any special treatment the whole time.
Malaria remains a significant global health problem with nearly half of the world’s population living in malaria-endemic regions and more than 500,000 deaths from malaria and its complications each year. Although significant success has been achieved in malaria therapeutic development, accurate early-stage diagnosis of the disease remains a barrier to eradication, especially in low-resource areas. Optical microscopy and antibody-based diagnostic tests are commonly used for identifying the infected population. However, the cost and reliability of these methods in low-resource environments limit the efficacy and accuracy of malaria screening. In this work, we designed, built, and validated a portable optical diagnostic system for malaria detection. The system is based on the detection of Hemozoin, which is a magnetic nanoparticle byproduct of the parasite. Therefore, the presence of Hemozoin is indicative of malarial infection. Unlike all other naturally occurring materials in the blood, hemozoin is paramagnetic. This property is the foundation of our magneto-optic detection system. In our experiments, β-hematin (a mimic for hemozoin) is used to allow for the verification of our device without the need to handle malaria-infected samples. The system is optimized and tested with spherical iron oxide magnetic nanoparticles and β-hematin in different concentrations of PEG solutions. Finally, β-hematin in whole rabbit blood is detected with this system. Detection limits of <8.1 ng/mL (corresponding to <26 parasites/μL) in 500μL of blood are demonstrated. The threshold for early stage malaria infection is 100 parasites/μL. Therefore, the present system is easily able to detect within a clinically relevant range.
Recent advances in optical materials have enabled the development of a wide range of integrated photonic devices from high speed modulators to frequency combs. With low optical loss over a wide wavelength range and environmental stability in ambient environments for several weeks, silicon oxynitride (SiOxNy) shows potential in many of these applications. However, unlike many classic optical materials, the thermo-optic response (dn/dT) in both the visible and near-IR is poorly characterized, limiting researcher’s ability to accurately model device performance. Here, we leverage the intrinsic thermal response of resonant cavities to measure the dn/dT of SiOxNy with a 12.7:1 and 4:1 oxygen to nitrogen ratio based on EDX measurements. The thermo-optic coefficient is measured in the visible and near-IR and compared with SiO2. The refractive indices of the silicon oxynitride films were also measured using spectroscopic ellipsometry. Based on an analysis of the O:N ratio and a comparison with both SiO2 and Si3N4, an expression for the dependence of the dn/dT on the stoichiometric ratio is developed.
Whispering gallery mode optical resonators integrated on silicon have demonstrated low threshold Raman lasers. One of the primary reasons for their success is their ultra-high quality factors (Q) which result in an amplification of the circulating optical field. Therefore, to date, the key research focus has been on maintaining high Q factors, as that determines the lasing threshold and linewidth. However, equally important criteria are lasing efficiency and wavelength. These parameters are governed by the material, not the cavity Q. Therefore, to fully address this challenge, it is necessary to develop new materials. We have synthesized a suite of metal-doped silica and small molecules to enable the development of higher performance Raman lasers. The efficiencies and thresholds of many of these devices surpass the previous work. Specifically, the silica sol-gel lasers are doped with metal nanoparticles (eg Ti, Zr) and are fabricated using conventional micro/nanofabrication methods. The intercalation of the metal in the silica matrix increases the silica Raman gain coefficient by changing the polarizability of the material. We have also made a new suite of small molecules that intrinsically have increased Raman gain values. By grafting the materials to the device surface, the overall Raman gain of the device is increased. These approaches enable two different strategies of improving the Raman efficiency and threshold of microcavity-based lasers.
As a result of their ability to amplify input light, ultra-high quality factor (Q) whispering gallery mode optical resonators have found numerous applications spanning from basic science through applied technology. Because the Q is critical to the device’s utility, an ever-present challenge revolves around maintaining the Q factor over long timescales in ambient environments. The counter-approach is to increase the nonlinear coefficient of relevance to compensate for Q degradation. In the present work, we strive to accomplish both, in parallel. For example, one of the primary routes for Q degradation in silica cavities is the formation of water monolayers. By changing the surface functional groups, we can inhibit this process, thus stabilizing the Q above 100 million in ambient environments. In parallel, using a machine learning strategy, we have intelligently designed, synthesized, and verified the next generation of small molecules to enable ultra-low threshold and high efficiency Raman lasing. The molecules are verified using the silica microcavity as a testbed cavity. However, the fundamental design strategy is translatable to other whispering gallery mode cavities.
High quality whispering gallery mode resonators can greatly enhance the optical field by trapping the light through total internal reflection, which makes these resonators a promising platform for many areas of research, including optical sensing, frequency combs, Raman lasing and cavity QED. Among these resonators, silica microtoroidal resonators are widely used because of their ability to be integrated and to achieve ultrahigh quality factors (above 100 million). However, quality factors of traditional silica toroids gradually decrease over time because there is an intrinsic layer of hydroxyl groups on the silica surface. This layer of hydroxyl groups attracts water molecules in the atmosphere and results in high optical losses. This property of silica degrades the behavior and limits the applications of the integrated silica toroids. In this work, we address this limitation by fabricating integrated microtoroids from silicon oxynitride. The surface of silicon oxynitride has a mixture of hydroxyl groups and fluorine groups. This mixture prevents the formation of a layer of water molecules that causes the optical losses. Our experiments demonstrate that the quality factors of the silicon oxynitride toroids exceed 100 million, and these values are maintained for over two weeks without controlling the storage conditions. As a comparison, quality factors of traditional silica toroids fabricated and stored under same conditions decayed by approximately an order of magnitude over the same duration.
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