The circulation of light within dielectric volumes enables storage of optical power near specific resonant frequencies and is important in a wide range of fields including cavity quantum electrodynamics, photonics, biosensing and nonlinear optics. Optical trajectories occur near the interface of the volume with its surroundings, making their performance strongly dependent upon interface quality. With a nearly atomic-scale surface finish, surface-tension-induced microcavities such as liquid droplets or spheres are superior to all other dielectric microresonant structures when comparing photon lifetime or, equivalently, cavity Q factor. Despite these advantageous properties, the physical characteristics of such systems are not easily controlled during fabrication. It is known that wafer-based processing of resonators can achieve parallel processing and control, as well as integration with other functions. However, such resonators-on-a-chip suffer from Q factors that are many orders of magnitude lower than for surface-tension-induced microcavities, making them unsuitable for ultra-high-Q experiments. Here we demonstrate a process for producing silica toroid-shaped microresonators-on-a-chip with Q factors in excess of 100 million using a combination of lithography, dry etching and a selective reflow process. Such a high Q value was previously attainable only by droplets or microspheres and represents an improvement of nearly four orders of magnitude over previous chip-based resonators.

Current single-molecule detection techniques require labeling the target molecule. We report a highly specific and sensitive optical sensor based on an ultrahigh quality ( Q ) factor ( Q > 10 8 ) whispering-gallery microcavity. The silica surface is functionalized to bind the target molecule; binding is detected by a resonant wavelength shift. Single-molecule detection is confirmed by observation of single-molecule binding events that shift the resonant frequency, as well as by the statistics for these shifts over many binding events. These shifts result from a thermo-optic mechanism. Additionally, label-free, single-molecule detection of interleukin-2 was demonstrated in serum. These experiments demonstrate a dynamic range of 10 12 in concentration, establishing the microcavity as a sensitive and versatile detector.

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Highly sensitive, label-free biodetection methods have applications in both the fundamental research and healthcare diagnostics arenas. Therefore, the development of new transduction methods and the improvement of the existing methods will significantly impact these areas. A brief overview of the different types of biosensors and the critical parameters governing their performance will be given. Additionally, a more in-depth discussion of optical devices, surface functionalization methods to increase device specificity, and fluidic techniques to improve sample delivery will be reviewed.

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An erbium-doped, toroid-shaped microlaser fabricated on a silicon chip is described and characterized. Erbium-doped sol-gel films are applied to the surface of a silica toroidal microresonator to create the microcavity lasers. Highly confined whispering gallery modes make possible single-mode and ultralow threshold microlasers. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1598623͔Whispering-gallery type microlasers in which the cavity boundary is defined by surface tension ͑e.g., spheres and droplets͒ have attracted much attention because the combination of their very small mode volume and high, cold-cavity Q factor enables ultralow threshold operation. [1][2][3] Recently, a class of ultrahigh-Q, surface-tension-induced microcavities fabricated on a silicon chip have been demonstrated. 4 These structures feature a toroidal-shaped cavity and enable the integration of electronics and other functions with ultrahigh-Q devices. In this letter we demonstrate surface functionalization of these devices using erbium-doped sol-gel films. In addition to being integrable with other optical or electric components, they are directly coupled to optical fiber using fiber tapers.We have previously applied the surface functionalization method using silica microsphere resonators. 5 Erbium-doped microlasers are especially interesting because their emission band falls in the important 1.5 m window used for optical communications. However, microspheres, while useful as laboratory demonstration vehicles, are not suitable for integration with other optical or electronic functions. Their properties are also difficult to control during fabrication. In contrast, microlasers on a chip can be fabricated in parallel and have characteristics that are more easily controlled using wafer-scale processing methods.Silica toroid-shaped microresonators supported by a circular silicon pillar were fabricated upon a silicon wafer containing a 2 m layer of thermal silica (SiO 2 ). 4 The process details are described in Ref. 4. The sol-gel starting solution was prepared as described in Ref. 5. After aging the sol-gel solution at room temperature for 10 h, we immersed silica microtoroids in the solution for 3-5 h. Then the wafers were heated in an oven at 160°C for another 10 h to drive off surface water. Microtoroids were then irradiated with a CO 2 laser ͑10.6 m emission͒ in order to reflow and densify the sol-gel films. As described in Ref. 4, CO 2 -laser emission is selectively absorbed by the silica layers. This and the relatively high thermal conductivity of silicon (ϳ100 times more thermally conductive than silica͒ 6,7 lead to selective reflow and densification of sol-gel at the all-important toroid periphery. Sol-gel deposited elsewhere was unaffected by this process step. Because of the large difference between the etching rate of densified and undensified silica films in buffered HF, 8 sol-gel deposited on all regions of the wafer other than the densified perimeter of the toroid could subsequently be selectively removed. Microtoroids ra...

Using standard lithographic techniques, we demonstrate fabrication of silica disk microcavities, which exhibit whispering-gallery-type modes having quality factors (Q) in excess of 1 million. Efficient coupling ͑high extinction at critical coupling and low, nonresonant insertion loss͒ to and from the disk structure is achieved by the use of tapered optical fibers. The observed high Q is attributed to the wedged-shaped edge of the disk microcavity, which is believed to isolate modes from the disk perimeter and thereby reduce scattering loss. The mode spectrum is measured and the influence of planar confinement on the mode structure is investigated. We analyze the use of these resonators for very low loss devices, such as add/drop filters.

The development of label-free biosensors with high sensitivity and specificity is of significant interest for medical diagnostics and environmental monitoring, where rapid and real-time detection of antigens, bacteria, viruses, etc., is necessary. Optical resonant devices, which have very high sensitivity resulting from their low optical loss, are uniquely suited to sensing applications. However, previous research efforts in this area have focused on the development of the sensor itself. While device sensitivity is an important feature of a sensor, specificity is an equally, if not more, important performance parameter. Therefore, it is crucial to develop a covalent surface functionalization process, which also maintains the device’s sensing capabilities or optical qualities. Here, we demonstrate a facile method to impart specificity to optical microcavities, without adversely impacting their optical performance. In this approach, we selectively functionalize the surface of the silica microtoroids with biotin, using amine-terminated silane coupling agents as linkers. The surface chemistry of these devices is demonstrated using X-ray photoelectron spectroscopy, and fluorescent and optical microscopy. The quality factors of the surface functionalized devices are also characterized to determine the impact of the chemistry methods on the device sensitivity. The resulting devices show uniform surface coverage, with no microstructural damage. This work represents one of the first examples of non-physisorption-based bioconjugation of microtoroidal optical resonators.