Bei-Bei Li

Detection of Single Nanoparticles and Lentiviruses Using Microcavity Resonance Broadening

A new label-free sensing mechanism is demonstrated experimentally by monitoring the whispering-gallery mode broadening in microcavities. It is immune to both noise from the probe laser and environmental disturbances, and is able to remove the strict requirement for ultra-high-Q mode cavities for sensitive nanoparticle detection. This ability to sense nanoscale objects and biological analytes is particularly crucial for wide applications.

Single nanoparticle detection using split-mode microcavity Raman lasers

Significance Optical sensing with ultrahigh sensitivity of single nanoscale objects is strongly desirable for applications in various fields, such as in early-stage diagnosis of human diseases and in environmental monitoring, as well as in homeland security. In this article, we report an optical technique for single nanoparticle detection in both air and an aqueous environment, with an ultralow detection limit. Ultrasensitive nanoparticle detection holds great potential for early-stage diagnosis of human diseases and for environmental monitoring. In this work, we report for the first time, to our knowledge, single nanoparticle detection by monitoring the beat frequency of split-mode Raman lasers in high-Q optical microcavities. We first demonstrate this method by controllably transferring single 50-nm–radius nanoparticles to and from the cavity surface using a fiber taper. We then realize real-time detection of single nanoparticles in an aqueous environment, with a record low detection limit of 20 nm in radius, without using additional techniques for laser noise suppression. Because Raman scattering occurs in most materials under practically any pump wavelength, this Raman laser-based sensing method not only removes the need for doping the microcavity with a gain medium but also loosens the requirement of specific wavelength bands for the pump lasers, thus representing a significant step toward practical microlaser sensors.

High-Q Exterior Whispering-Gallery Modes in a Metal-Coated Microresonator

We propose a kind of plasmonic whispering-gallery mode highly localized on the exterior surface of a metal-coated microresonator. This exterior (EX) surface mode possesses high quality factors at room temperature, and can be efficiently excited by a tapered fiber. The EX mode can couple to an interior (IN) mode and this coupling produces a strong anticrossing behavior, which not only allows conversion of IN to EX modes, but also forms a long-lived antisymmetric mode. As a potential application, the EX mode could be used for a biosensor with a sensitivity high of up to 500 nm per refraction index unit, a large figure of merit, and a wide detection range.

Strongly enhanced light-matter interaction in a hybrid photonic-plasmonic resonator

State Key Lab for Mesoscopic Physics, Department of Physics, Peking University, P. R. China(Dated: June 13, 2012)We propose a hybrid photonic-plasmonic resonant structure which consists of a metal nanoparticle(MNP) and a whispering gallery mode (WGM) microcavity. It is found that the hybrid mode enablesa strong interaction between the light and matter, and the single-atom cooperativity is enhanced bymore than two orders of magnitude compared to that in a bare WGM microcavity. This remarkableimprovement originates from two aspects: (1) the MNP offers a highly enhanced local field in thevicinity of an emitter, and (2), surprisingly, the high-Q property of WGMs can be maintained inthe presence of the MNP. Thus the present system has great advantages over a single microcavityor a single MNP, and holds great potential in quantum optics, nonlinear optics and highly sensitivebiosening.

Experimental observation of Fano resonance in a single whispering-gallery microresonator

We experimentally observe Fano resonance in a single silica toroidal microresonator, in which two whispering-gallery modes (WGMs) are excited simultaneously through a fiber taper. By adjusting the fiber-cavity coupling strength and the polarization of incident light, the Fano-like resonance line shape can be engineered and further convert to the electromagnetically induced transparency (EIT) like line shape. Our theoretical analysis reveals that both the Fano and EIT resonances originate from an indirect-coupling of two originally orthogonal WGMs, which is mediated by the common fiber taper waveguide. The sharp Fano line shape holds great potential in optical switching and sensitivity-enhanced biochemical sensing.

On chip, high-sensitivity thermal sensor based on high-Q polydimethylsiloxane-coated microresonator

A high-sensitivity thermal sensing is demonstrated by coating a layer of polydimethylsiloxane (PDMS) on the surface of a silica toroidal microresonator on a silicon wafer. Possessing high-Q whispering gallery modes (WGMs), the PDMS-coated microresonator is highly sensitive to the temperature change in the surroundings. We find that, when the PDMS layer becomes thicker, the WGM experiences a transition from redshift to blueshift with temperature increasing due to the negative thermal-optic coefficient of PDMS. The measured sensitivity (0.151 nm/K) is one order of magnitude higher than pure silica microcavity sensors. The ultrahigh resolution of the thermal sensor is also analyzed to reach 10−4 K.

Experimental controlling of Fano resonance in indirectly coupled whispering-gallery microresonators

We experimentally studied the transmission spectrum of a coupled resonator structure in which a low-Q microdisk and a high-Q microtoroid indirectly interact with each other mediated by a fiber taper. Asymmetric Fano resonances were observed and could be controlled to change periodically by adjusting the distance between the two microresonators. It is revealed that the Fano resonance originates from the coupling of the two modes belonging to the two microresonators. The observed period of distance change is around 8 μm, which shows good agreement with the theoretical prediction by the beat of multiple propagating modes in the fiber taper.

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

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.

Direct laser writing of whispering gallery microcavities by two-photon polymerization

We demonstrate that high-Q polymer whispering gallery microcavities can be directly written by the two-photon polymerization of zirconium/silicon hybrid sol-gel, benefiting from the high spatial resolution and three-dimensional nature of this direct laser writing technique. The quality factors of the fabricated whispering gallery microcavities are up to 1.48×105 limited by the material absorption. The surface roughness is less than 12 nm. This opens the way to fabricate intricate three-dimensional microcavities for the fundamental and applied physics research based on optical resonators.

High quality factor, small mode volume, ring-type plasmonic microresonator on a silver chip

We theoretically propose a microresonator on a silver chip. The microresonator supports hybrid plasmonic modes with highly localized electromagnetic fields (ultra-small mode volumes ~0.1 µm^3) and long photon lifetime (high quality factors >400) at room temperature, both of which exceed the traditional plasmonic microresonators. The total quality factor is found to be limited by two factors, the metal absorption and the radiation loss, and exhibit a tradeoff with the mode volume. This type of hybrid plasmonic microresonators holds great potential in fundamental physics and applications, for instance, cavity quantum electrodynamics in weak coupling regime and low-threshold microlasers.