Yuetao Li

Laser-tuned whispering gallery modes in a solid-core microstructured optical fibre integrated with magnetic fluids

A laser-assisted tuning method of whispering gallery modes (WGMs) in a cylindrical microresonator based on magnetic-fluids-infiltrated microstructured optical fibres (MFIMOFs, where MF and MOF respectively refer to magnetic fluid and microstructured optical fibre) is proposed, experimentally demonstrated and theoretically analysed in detail. The MFIMOF is prepared by infiltrating the air-hole array of the MOF using capillary action effect. A fibre-coupling system is set up for the proposed MFIMOF-based microresonator to acquire an extinction ratio up to 25 dB and a Q-factor as large as 4.0 × 104. For the MF-infiltrated MOF, the light propagating in the fibre core region would rapidly spread out and would be absorbed by the MF-rod array cladding to induce significant thermal effect. This has been exploited to achieve a WGM resonance wavelength sensitivity of 0.034 nm/mW, which is ~20 times higher than it counterpart without MF infiltration. The wavelength response of the resonance dips exhibit linear power dependence, and owing to such desirable merits as ease of fabrication, high sensitivity and laser-assisted tunability, the proposed optical tuning approach of WGMs in the MFIMOF would find promising applications in the areas of optical filtering, sensing, and signal processing, as well as future all-optical networking systems.

Angular diffraction of an optical vortex induced by the Gouy phase

We investigate the diffraction of an optical vortex, when half of the beam is blocked by a knife edge. The Gouy phase is shown to influence the diffraction pattern strongly, in particular leading to the emergence of distorted and deformed diffraction fringes in the azimuthal direction. In analogy with single-slit diffraction, this phenomenon is analyzed in terms of angular diffraction within the azimuthal degree of freedom, by considering the uncertainty principle for the angular position and the angular momentum. Numerical simulations based on the fast-Fourier algorithm and the angular momentum expansion fit well with the experimental results.

Enhanced optical angular momentum in cylinder waveguides with negative-index metamaterials

The optical angular momentum (AM) of helical electromagnetic (EM) modes in cylinder waveguides with negative-index metamaterials (NIM) is investigated. The AM density in NIM is shown to be negative valued with respect to the azimuthal mode index, in sharp contrast to that in regular dielectrics and vacuum. Furthermore, the AM flux per single-photon flux is found to be greatly enhanced in the NIM waveguides; its value depends on the form of EM momentum used in the AM. This interesting phenomenon is shown to be associated with the unique trapped EM modes in the NIM waveguides. Our investigation results could contribute to many potential applications, especially in realizing high density optical memory and buffers, and in solving the Abraham–Minkowski dilemma.

Tuning of whispering gallery modes in a magnetic-fluid-infiltrated silica capillary based on lateral pumping scheme

An all-optical tuning method of whispering gallery modes (WGMs) in a silica capillary based on lateral pumping scheme has been proposed and experimentally demonstrated by exploiting the photo-thermal effect of magnetic fluids (MFs) infiltrated into a micro-capillary. The WGMs are coupled from a tapered fiber perpendicular to the silica capillary. Experimental results indicate that the microresonator integrated with MFs shows a Q-factor of 1.2867 × 104 and its optical tunability reaches 0.0382 nm/(mW mm−2) under 532 nm laser illumination. Compared with the one infiltrated with pure water, our proposed capillary shows an optical tuning sensitivity increase of about 4270 times. The experimental results on the WGM resonance wavelength sensitivity for the capillaries with different diameters are in accordance with our simulation outcome using radially dependent heat conducting equation. Further experimental studies indicate that the proposed microresonator possesses a good spectral reversibility. Moreover, the dynamic response experiment shows that the rise and fall time of this microresonator is about 231 ms and 255 ms, respectively. The above intriguing features make the proposed WGM resonator a promising candidate for potential applications in optical filtering, microfluidic sensing, and signal processing as well as reconfigurable devices for future all-optical networks.

Excitation of Whispering Gallery Modes in Silica Capillary Using SMF-MMF Joint With Lateral Offset

Whispering gallery modes (WGMs) have been effectively excited in a silica tube by employing a single-mode-fiber/multimode-fiber (SMF/MMF) joint with the lateral offset to enhance the evanescent field over the tapered MMF region. Experimental results indicate that thanks to the lateral offset at the SMF-MMF joint, the taper diameter is about one order of magnitude larger than that of a standard fiber taper coupler. The WGMs in the silica capillary have been theoretically analyzed by using the eigen equation to deal with WGMs in cylindrical microresonators. Our proposed WGM excitation scheme is more robust than the conventional WGM systems based on fiber tapers of a few micrometers in diameter, which is anticipated to serve as micro-fluidic-channel for biological and chemical sensing applications.

WGM Micro-Fluidic-Channel Based on Reflection Type Fiber-Tip-Coupled Hollow-Core PCFs

A micro-fluidic-channel is proposed based on a fiber-tip-excited hollow-core photonic crystal fiber microresonator. The reflection conical fiber tip is fabricated based on electric arc fusion technique. A quality factor up to 8.65 × 104 has been experimentally acquired at ~1557.75 nm. Theoretical simulation results indicate that the refractive index sensitivity of the whispering-gallery-mode resonance wavelength reaches 1259.9211 nm/RIU for biocompatible low refractive index range. The proposed microfluidic detection platform has a few desirable merits, such as ease of fabrication, low-cost, and structure robustness, which ensures its applicability for practical use in chemical as well as biological sensing applications.