Yanyan Zhi

Single Nanoparticle Detection Using Optical Microcavities

Detection of nanoscale objects is highly desirable in various fields such as early-stage disease diagnosis, environmental monitoring and homeland security. Optical microcavity sensors are renowned for ultrahigh sensitivities due to strongly enhanced light-matter interaction. This review focuses on single nanoparticle detection using optical whispering gallery microcavities and photonic crystal microcavities, both of which have been developing rapidly over the past few years. The reactive and dissipative sensing methods, characterized by light-analyte interactions, are explained explicitly. The sensitivity and the detection limit are essentially determined by the cavity properties, and are limited by the various noise sources in the measurements. On the one hand, recent advances include significant sensitivity enhancement using techniques to construct novel microcavity structures with reduced mode volumes, to localize the mode field, or to introduce optical gain. On the other hand, researchers attempt to lower the detection limit by improving the spectral resolution, which can be implemented by suppressing the experimental noises. We also review the methods of achieving a better temporal resolution by employing mode locking techniques or cavity ring up spectroscopy. In conclusion, outlooks on the possible ways to implement microcavity-based sensing devices and potential applications are provided.

Detection of Single Nanoparticles Using the Dissipative Interaction in a High- Q Microcavity

Ultrasensitive optical detection of nanometer-scaled particles is highly desirable for applications in early-stage diagnosis of human diseases, environmental monitoring, and homeland security, but remains extremely difficult due to ultralow polarizabilities of small-sized, low-index particles. Optical whispering-gallery-mode microcavities, which can enhance significantly the light-matter interaction, have emerged as promising platforms for label-free detection of nanoscale objects. Different from the conventional whispering-gallery-mode sensing relying on the reactive (i.e., dispersive) interaction, here we propose and demonstrate to detect single lossy nanoparticles using the dissipative interaction in a high-

Whispering Gallery Mode Devices for Sensing and Biosensing

Q

Optically sizing single atmospheric particulates with a 10-nm resolution using a strong evanescent field

toroidal microcavity. In the experiment, detection of single gold nanorods in an aqueous environment is realized by monitoring simultaneously the linewidth change and shift of the cavity mode. The experimental result falls within the theoretical prediction. Remarkably, the reactive and dissipative sensing methods are evaluated by setting the probe wavelength on and off the surface plasmon resonance to tune the absorption of nanorods, which demonstrates clearly the great potential of the dissipative sensing method to detect lossy nanoparticles. Future applications could also combine the dissipative and reactive sensing methods, which may provide better characterizations of nanoparticles.

Whispering gallery mode structure in polymer-coated lasing microspheres

Optical sensors based on resonant whispering gallery mode (WGM) optical cavities are discussed in the context of recent research developments. WGM-based sensors are among the most sensitive optical sensing devices currently known, especially when combined with surface plasmon effects. The basic theory of WGM “reactive” sensing is first discussed, along with the basic parameters that are used to characterize such devices (i.e., sensitivity, signal-to-noise ratio, and detection limit). Surface chemistry methods are of critical importance for targeted sensing applications and the fundamentals of this aspect are examined. The performance of fluorescent WGM-based devices is compared to state-of-the-art “evanescently-coupled” structures. Finally, the performance of WGM-based optical sensors is compared and contrasted to a range of alternative optical microsensor technologies currently under development.

Optical microcavity sensing: from reactive to dissipative interactions (Conference Presentation)

Although an accurate evaluation of the distribution of ultrafine particulate matter in air is of utmost significance to public health, the usually used PM2.5 index fails to provide size distribution information. Here we demonstrate a low-profile and cavity-free size spectrometer for probing fine and ultrafine particulate matter by using the enhanced particle-perturbed scattering in strong optical evanescent fields of a nanofiber array. The unprecedented size resolution reaches 10 nm for detecting single 100-nm-diameter nanoparticles by employing uniform nanofibers and controlling the polarizations of the probe light. This size spectrometry was tested and used to retrieve the size distribution of particulate matter in the air of Beijing, yielding mass concentrations of nanoparticles, as a secondary exercise, consistent with the officially released data. This nanofiber-array probe shows potential for the full monitoring of air pollution and for studying early-stage haze evolution and can be further extended to explore nanoparticle interactions.

Optical microcavity sensing: From dispersive to dissipative interactions

Whispering gallery modes (WGMs) are frequently observed in the emission spectrum of a fluorophore coupled to a dielectric microsphere. If the fluorophore is excited strongly enough the WGMs can become lasing modes, producing a much higher signal-to-noise ratio and higher Q-factors. These favorable properties have led to recent demonstrations of biochemical sensing with lasing WGMs; however, as we show here, the underlying cavity structure that leads to lasing can be highly complicated and the lasing spectrum can result from a large number of closely spaced or overlapping modes. In silica spheres coated with a dye-doped polymer bilayer, hints of underlying complexity are already indicated by the skewed shape of the fluorescence WGMs. Under lasing conditions, a more complicated structure was observed with Q-factors over 10 times higher than observed in fluorescence, while transmission measurements showed a dense forest of resonances due to m-order degeneracy breaking from a lack of perfect spherical symmetry. Lasing preferentially occurs at wavelengths where these modes tended to be most densely spaced. The tapered part of the fiber to which the microsphere is attached can also have a strong effect on the lasing spectrum, leading to significant differences between the fluorescence and lasing resonances.

Whispering-gallery-type Sensor for Single Nanoparticle Detection Using the Dissipative Interaction

Single nanoparticle detection is demonstrated using reactive and dissipative interactions in a high-Q optical microcavity. The combination of those two methods adds new dimensions in microcavity sensing.

Cooling of macroscopic mechanical resonators in hybrid atom-optomechanical systems

Detection of single nanoscale objects is highly desirable for applications in various fields, such as in early-stage diagnosis of human diseases and in environmental monitoring. High-Q microcavities, which can significantly enhance light-matter interactions, provide ideal platforms for label-free, ultrasensitive detection of single nano- objects. In the first part of this talk, we report the experimental demonstration of single nanoparticle detection using either resonance mode broadening or microcavity Raman laser splitting. For the latter, by monitoring the step changes in beat frequency of the two split Raman mode lasing, detection of single 20-nm-radius nanoparticles is realized in an aqueous environment. In the second part, we propose and demonstrate to detect single lossy nanoparticles using the dissipative interaction in a high-Q toroidal microcavity. In experiment, detection of single gold nanorods in aqueous environment is realized by monitoring simultaneously the linewidth change and shift of cavity mode. The experimental result shows a good agreement with the theoretical prediction.

Size spectrometry of environmental particulate matter using optical evanescent fielda nanofiber array

Different from the conventional whispering-gallery-mode sensing using reactive interaction, the dissipative interaction is proposed and demonstrated to detect single nanoparticles which are lossy and with an ultra-small real part of the polarizability.