No abstract

Individual nanoparticles in aqueous solution are observed to be attracted to and orbit within the evanescent sensing ring of a Whispering Gallery Mode micro-sensor with only microwatts of driving power. This Carousel trap, caused by attractive optical gradient forces, interfacial interactions, and the circulating momentum flux, considerably enhances the rate of transport to the sensing region, thereby overcoming limitations posed by diffusion on such small area detectors. Resonance frequency fluctuations, caused by the radial Brownian motion of the nanoparticle, reveal the radial trapping potential and the nanoparticle size. Since the attractive forces draw particles to the highest evanescent intensity at the surface, binding steps are found to be uniform.

We describe and demonstrate a physical mechanism that substantially enhances the label-free sensitivity of a whispering-gallery-mode biosensor for the detection of individual nanoparticles in aqueous solution. It involves the interaction of dielectric nanoparticle in an equatorial carousel orbit with a plasmonic nanoparticle bound at the microparticle's equator. As the dielectric particle parks to hot spots on the plasmonic particle we observe frequency shifts that are enhanced by a factor of 4, consistent with a simple reactive model. Once optimized the enhancement by this mechanism should exceed several orders of magnitude, putting individual protein within reach.

Optofluidic dye lasers hold great promise for adaptive photonic devices, compact and wavelength-tunable light sources, and micro total analysis systems. To date, however, nearly all those lasers are directly excited by tuning the pump laser into the gain medium absorption band. Here we demonstrate bioinspired optofluidic dye lasers excited by FRET, in which the donor-acceptor distance, ratio, and spatial configuration can be precisely controlled by DNA scaffolds. The characteristics of the FRET lasers such as spectrum, threshold, and energy conversion efficiency are reported. Through DNA scaffolds, nearly 100% energy transfer can be maintained regardless of the donor and acceptor concentration. As a result, efficient FRET lasing is achieved at an unusually low acceptor concentration of micromolar, over 1,000 times lower than that in conventional optofluidic dye lasers. The lasing threshold is on the order of μJ∕mm 2 . Various DNA scaffold FRET lasers are demonstrated to illustrate vast possibilities in optofluidic laser designs. Our work opens a door to many researches and applications such as intracavity bio/chemical sensing, biocontrolled photonic devices, and biophysics. O ptofluidic lasers are an emerging technology that combines the advantages of compactness and easy liquid manipulation of microfluidics, and dynamic wavelength tunability and broad spectral coverage of dye lasers (1-3). Optical feedback in those optofluidic lasers has been achieved using high-Q ring resonators [e.g., microdroplets (4, 5), microspheres (6), microcylinders (7), microcapillaries (8, 9), and microfiber knots (10)], Fabry-Pérot cavities (11, 12), and distributed feedback gratings (3, 13). In nearly all those lasers, the gain medium is directly excited by tuning the pump laser into the dye absorption band, which requires that the pump laser wavelength match the particular dye absorption. An alternative excitation scheme is through energy transfer, in which dye mixtures, composed of the donor and the acceptor, are used. Donors are directly excited and subsequently transfer energy to acceptors for lasing. The energy transfer significantly extends the laser emission wavelength range without the need to change the pump wavelength. Moreover, dye lasers based on energy transfer have a much higher pump efficiency and lower lasing threshold than the corresponding single dye lasers due to the low donor absorption loss at the acceptor lasing wavelength (14, 15).Generally, there are two transfer mechanisms between the donor and the acceptor in an optical cavity: nonradiative FRET (14-17), in which the transfer is mediated by short-ranged resonant dipole-dipole interaction, and cavity-assisted radiative transfer (18)(19)(20), in which the emission from the donor is first coupled into the cavity, which stores photons for an extended amount of time before they are reabsorbed by the acceptor. The FRET efficiency between a donor and acceptor pair is R 0 6 ∕ðR 0 6 þ r 6 Þ, where R 0 and r are the Förster distance and the donor-acceptor distance, r...