Showing papers by "Lan Yang published in 2012"

Label-free detection with high-Q microcavities: a review of biosensing mechanisms for integrated devices.

Abstract: Optical microcavities that confi ne light in high-Q resonance promise all of the capabilities required for a successful nextgeneration microsystem biodetection technology. Label-free detection down to single molecules as well as operation in aqueous environments can be integrated cost-effectively on microchips, together with other photonic components, as well as electronic ones. We provide a comprehensive review of the sensing mechanisms utilized in this emerging fi eld, their physics, engineering and material science aspects, and their application to nanoparticle analysis and biomolecular detection. We survey the most recent developments such as the use of mode splitting for self-referenced measurements, plasmonic nanoantennas for signal enhancements, the use of optical force for nanoparticle manipulation as well as the design of active devices for ultra-sensitive detection. Furthermore, we provide an outlook on the exciting capabilities of functionalized high-Q microcavities in the life sciences.

Highly Unidirectional Emission and Ultralow‐Threshold Lasing from On‐Chip Ultrahigh‐Q Microcavities

Abstract: Prominent examples are whispering gallery mode (WGM) microcavities, [ 2 , 3 ] which confi ne photons by means of continuous total internal refl ection along a curved and smooth surface The long photon lifetime (described by high Q factors), strong fi eld confi nement, and in-plane emission characteristics make them promising candidates for novel light sources [ 4–9 ] and biochemical sensors with the ability of detecting few or even single nanoparticles [ 10 , 11 ] The principal disadvantage of circular WGM microcavities is their intrinsic isotropy of emission due to their rotational symmetry In addition to the photonic structures consisting of two or more perfectly spherical microcavities, [ 12 ] one of vital solutions is to use deformed microcavities by breaking the rotational symmetry, [ 13–16 ] which can provide not only the directional emission but also the effi cient and robust excitation of WGMs by a free-space optical beam [ 17–20 ] Deformed microcavities fabricated on a chip are particularly desired for high-density optoelectronic integration, but they suffer from low Q factors in experiments The Q factors are typically around or even smaller than ten thousand [ 21–27 ] limited by the large scattering losses from the involuntary surface roughness The high Q factor is of great importance in fundamental studies and on-chip photonic applications Here, with a pattern transfer technique and a refl ow process ensuring a nearly atomic-scale microcavity surface, we demonstrate experimentally on-chip undoped silica deformed microcavities which support both nearly unidirectional emission and ultrahigh Q factors exceeding 100 million Consequently, low-threshold, unidirectional microlasing in such a microcavity with Q factor about 3 million is realized by erbium doping and a convenient free-space excitation

A Robust and Tunable Add–Drop Filter Using Whispering Gallery Mode Microtoroid Resonator

Abstract: We fabricated and theoretically investigated an add-drop filter (ADF) using an on-chip whispering gallery mode (WGM) microtoroid resonator with ultrahigh-quality factor (Q) side coupled to two taper fibers, forming the bus and drop waveguides. The new device design incorporates silica side walls close to the microresonators which not only enable placing the coupling fibers on the same plane with respect to the microtoroid resonator but also provides mechanical stability, leading to an ADF with high drop efficiency and improved robustness to environmental perturbations. We show that this new device can be thermally tuned to drop desired wavelengths from the bus without significantly affecting the drop efficiency, which is around 57%.

Photonic molecules formed by coupled hybrid resonators

Abstract: We describe a method that enables free-standing whispering-gallery-mode microresonators, and report spectral tuning of photonic molecules formed by coupled free and on-chip resonators with different geometries and materials. We study direct coupling via evanescent fields of free silica microtoroids and microspheres with on-chip polymer coated silica microtoroids. We demonstrate thermal tuning of resonance modes to achieve maximal spectral overlap, mode splitting induced by direct coupling, and the effects of distance between the resonators on the splitting spectra.

Detection and size measurement of individual hemozoin nanocrystals in aquatic environment using a whispering gallery mode resonator.

Abstract: We, for the first time, report the detection and the size measurement of single nanoparticles (i.e. polystyrene) in aquatic environment using mode splitting in a whispering gallery mode (WGM) optical resonator, namely a microtoroid resonator. Using this method we achieved detecting and measuring individual synthetic hemozoin nanocrystals―a hemoglobin degradation by-product of malarial parasites―dispersed in a solution or in air. The results of size measurement in solution and in air agree with each other and with those obtained using scanning electron microscope and dynamic light scattering. Moreover, we compare the sensing capabilities of the degenerate (single resonance) and non-degenerate (split mode, doublet) operation regimes of the WGM resonator.

Systems and methods for particle detection

Abstract: A particle detection system is provided. The particle detection system includes at least one tapered optical fiber, a light source configured to transmit light through the at least one tapered optical fiber, a photodetector configured to measure a characteristic of the light being transmitted through the at least one optical fiber, and a computing device coupled to the photodetector and configured to determine whether a nanoparticle is present within an evanescent field of the at least one tapered optical fiber based on the measured light characteristic.

Fiber taper based Raman spectroscopic sensing

Abstract: A method for performing Raman spectroscopic analysis of micro-particles adhered to a tapered optical fiber as they interact with the taper-guided evanescent pump wave is explored as a chemically selective, label-free taper-particle sensing technique.

Encapsulation of a microtoroid resonator side-coupled to a fiber taper into a polymer matrix

Abstract: We demonstrate that a whispering gallery mode (WGM) microtoroidal optical resonator retains its high quality factor Q > 107 when encapsulated into a low index polymer matrix. We tested the packaged system in water with various salt concentrations and temperatures, and in air under various room conditions, showing that the polymer package keeps the resonator intact from degrading effects of environment, providing a robust and stable resonator-fiber coupling. The developed packaging technique provides portability to on-chip WGM resonators, enabling operations in the field.

An on-chip tunable add-drop filter using a microtoroid resonator

Abstract: We present an add-drop filter (ADF) with a drop efficiency of 57% and quality factor of 4.5 × 106 composed of an on-chip whispering gallery mode (WGM) ultra-high quality factor (Q) microtoroid resonator side coupled to two taper fibers. Different than the exisiting microtoroid based ADF designs, we fabricated reflowed silica side walls along the microtoroid resonators to support the fiber-taper waveguides, not only making the alignment process easier but also providing mechanical stability. As an additional benefit, we observe that the drop-wavelength can be tuned without compromising drop efficiency.

Mode splitting based single particle size measurement in water

Abstract: We report the first demonstration of single nanoparticle detection and size measurement in water using mode splitting in high-Q optical resonators. We achieved detecting and measuring polystyrene individual nanoparticles down to 75 nm in radius.

Quantum internet using code division multiple access

Abstract: A crucial open problem in large-scale quantum networks is how to efficiently transmit quantum data among many pairs of users via a common data-transmission medium. We propose a solution by developing a quantum code division multiple access (q-CDMA) approach in which quantum information is chaotically encoded to spread its spectral content, and then decoded via chaos synchronization to separate different sender-receiver pairs. In comparison to other existing approaches, such as frequency division multiple access (FDMA), the proposed q-CDMA can greatly increase the information rates per channel used, especially for very noisy quantum channels.

On-chip whispering-gallery-mode lasers for sensing applications

Abstract: Whispering-Gallery-Mode (WGM) microresonators have shown great promise for ultra-sensitive and label-free chemical and biological sensing. The linewidth of a resonant mode determines the smallest resolvable changes in the WGM spectrum, which, in turn, affects the detection limit. The fundamental limit is set by the linewidth of the resonant mode due to material absorption induced photon loss. We report a real-time detection method with single nanoparticle resolution that surpasses the detection limit of most passive micro/nano photonic resonant devices. This is achieved by using an on-chip WGM microcavity laser as the sensing element, whose linewidth is much narrower than its passive counterpart due to optical gain in the resonant lasing mode. In this microlaser based sensing platform, the first binding nanoparticle induces splitting of the lasing line, and the subsequent particles alter the amount of splitting, which can be monitored by measuring the beat frequency of the split modes. We demonstrate detection of polystyrene and gold nanoparticles as small as 15 nm and 10 nm in radius, respectively, and Influenza A virions. The built-in self-heterodyne interferometric method achieved in the monolithic microlaser provides a self-referencing scheme with extraordinary sensitivity, and paves the way for detection and spectroscopy of nano-scale objects using micro/nano lasers.

Hybrid photonic molecules

Abstract: We report spectral tuning of coupled whispering-gallery-mode microresonators formed with different geometries and materials. Thermal tuning is utilized to achieve spectral overlap and mode splitting is studied as a function of coupling distance.