Yun-Feng Xiao

Fabrication of high-Q polydimethylsiloxane optical microspheres for thermal sensing

Polydimethylsiloxane (PDMS) optical microspheres are fabricated and whispering gallery modes with quality factors of 106 in the 1480 nm band are demonstrated. The dependence of the resonance shifts on the input power is investigated in both the transient (blueshift) and the steady-state (redshift) regimes. Moreover, we demonstrate that such high-Q PDMS optical resonators can be used as highly sensitive thermal sensors with temperature sensitivity of 0.245 nm/°C, which is one order of magnitude higher than conventional silica microsphere resonators. The estimated thermal resolution of the sensor is 2×10−4 °C.

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.

Visible-Frequency Dielectric Metasurfaces for Multiwavelength Achromatic and Highly Dispersive Holograms

Dielectric metasurfaces built up with nanostructures of high refractive index represent a powerful platform for highly efficient flat optical devices due to their easy-tuning electromagnetic scattering properties and relatively high transmission efficiencies. Here we show visible-frequency silicon metasurfaces formed by three kinds of nanoblocks multiplexed in a subwavelength unit to constitute a metamolecule, which are capable of wavefront manipulation for red, green, and blue light simultaneously. Full phase control is achieved for each wavelength by independently changing the in-plane orientations of the corresponding nanoblocks to induce the required geometric phases. Achromatic and highly dispersive meta-holograms are fabricated to demonstrate the wavefront manipulation with high resolution. This technique could be viable for various practical holographic applications and flat achromatic devices.

Analog to multiple electromagnetically induced transparency in all-optical drop-filter systems

We theoretically study a parallel optical configuration which includes N periodically coupled whispering-gallery-mode resonators. The model shows an obvious effect which has a direct analogy with the phenomenon of multiple electromagnetically induced transparency in quantum systems. The numerical simulations illuminate that the frequency transparency windows are sharp and highly transparent. We also briefly discuss the experimental feasibility of the current scheme in two practical systems, microrings and microdisks.

Plasmon modes of silver nanowire on a silica substrate

Plasmon mode in a silver nanowire is theoretically studied when the nanowire is placed on or near a silica substrate. It is found that the substrate has much influence on the plasmon mode. For the nanowire on the substrate, the plasmon (hybrid) mode possesses not only a long propagation length but also an ultrasmall mode area. From the experimental point of view, this cavity-free structure holds a great potential to study a strong coherent interaction between the plasmon mode and single quantum system (for example, quantum dots) embedded in the substrate.

Compensation of thermal refraction effect in high-Q toroidal microresonator by polydimethylsiloxane coating

We experimentally and theoretically characterize the thermal refraction effect in a silica microtoroid and demonstrate that such effect can be reduced or even eliminated by applying a thin layer of polydimethylsiloxane (PDMS) to the surface of the silica resonator. By increasing the coating thickness, the whispering gallery modes (WGMs) experience a transition from redshift to blueshift induced by thermal absorption. Experiment results demonstrate that at the thickness of 0.52 μm, the fundamental WGM with observed Q factor of 1.5×106 shows no shift with the input optical power since the thermal refraction of the silica for this mode is compensated completely by the PDMS layer, which has an opposite thermal refraction effect. This work shows that the PDMS layer could be used to reduce thermal noise in high-Q silica microcavities for applications in sensing, lasing, and nonlinear optics.

Experimental realization of optomechanically induced non-reciprocity

Non-magnetic non-reciprocal transparency and amplification is experimentally achieved by optomechanics using a whispering-gallery microresonator. The idea may lead to integrated all-optical isolators or non-reciprocal phase shifters.

Realizing quantum controlled phase flip through cavity QED

We propose a scheme to realize quantum controlled phase flip (CPF) between two rare-earth ions embedded in the respective microsphere cavity via interacting with a single-photon pulse in sequence. The numerical simulations illuminate that the CPF gate between ions is robust and scalable with extremely high fidelity and low error rate. Our scheme is more applicable than other schemes presented before based on current laboratory cavity-QED technology, and it is possible to be used as an applied unit gate in future quantum computation and quantum communication.