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

Thermo-optic tuning of a nematic liquid crystal-filled capillary whispering gallery mode resonator

A novel tunable whispering gallery modes (WGMs) resonator based on a nematic liquid crystal (LC)-filled capillary and magnetic nanoparticles (MNPs)-coated tapered fiber has been proposed and experimentally demonstrated. Thermo-optic tuning of the WGM resonances has been demonstrated by varying optical pump laser power injected into the MNPs-coated fiber half-taper inside the capillary. The tuning mechanism relies on the change of the effective refractive index (RI) of the nematic LC, caused by the photo-thermal effect of MNPs on the surface of the fiber half-taper inducing a temperature change inside the capillary. Tuning of the WGM resonances with sensitivities of 101.5 ± 3.5 pm/mW and 146.5 ± 3.5 pm/mW and tuning ranges of 1.96 nm and 3.28 nm respectively for the two types of liquid crystals (MLC-7012, MDA-05-2782) has been demonstrated. In addition, the relationship between the optical power of the pump laser and the local temperature of the nematic LC was investigated and the heating rate is estimated as 1.49 °C/mW. The proposed thermo-optic tuning scheme has many potential applications in tunable photonic devices and sensors.

Nanoheater-tuned whispering gallery mode lasing in liquid-filled hollow microcavities

An all-optical tunable whispering gallery mode (WGM) lasing from the liquid-filled hollow glass microsphere (LFHGM) is proposed and experimentally verified. The LFHGM-based microlaser is prepared by injecting ${{\rm NaNdF}_4}/{\rm dye}$NaNdF₄/dye co-doped liquid into the HGM, and WGM resonance is obtained under excitation of a 532 nm pulse laser. Since the high-efficiency absorption of the 793 nm continuous-wave laser by ${{\rm NaNdF}_4}$NaNdF₄ nanocrystals (NCs) can result in photothermal effect-induced effective refractive index change of the microcavity, a secondary 793 nm laser is irradiated into the LFHGM to excite the ${{\rm NaNdF}_4}$NaNdF₄ dispersed in the liquid core, thereby realizing a shift of resonant frequencies. The influence of the doping concentration of ${{\rm NaNdF}_4}$NaNdF₄ NCs on the tuning range and the sensitivity over the power intensity range of $0{-}1.{68}\;{{\rm W/mm}^2}$0-1.68W/mm² are investigated experimentally, obtaining maximum values of 4.95 and ${2}.{95}\;{\rm nm/}({\rm W}\;{{\rm mm}^{ - 2}})$2.95nm/(Wmm^-2). The ability to generate stable lasing in a LFHGM cavity highlights the practical application of the microscale lasers in future all-optical networks.

Bidirectional tuning of whispering gallery modes in a silica microbubble infiltrated with magnetic fluids

A bidirectional tuning mechanism of whispering gallery modes (WGMs) in a capillary-based microbubble microresonator infiltrated with magnetic fluids (MFs) is investigated. Owing to distinct RI responses of MFs dependent on the applied magnetic field direction with respect to the capillary axis, the RI of MFs shows different variation trends when an external magnetic field is parallel or perpendicular to the capillary axis. Experimental results indicate that WGM resonance dips exhibit wavelength shift in inverse directions for the above two cases, which is in accordance with our theoretical analysis on different refractive variation behaviors of MFs. As the applied magnetic field is perpendicular or parallel to the capillary axis, the WGM resonance wavelength tuning sensitivities tend to be $ - {15.01}\;{\rm pm/mT}$-15.01pm/mT and 6.3 pm/mT, respectively. Our proposed WGM tuning scheme has several desirable advantages, including bidirectional tunability, high Q-factor, ease of fabrication, and good compatibility with functional materials, making it a promising candidate in the field of magnetic field vector sensing and magnetically manipulated micro-optic devices.

Investigation of optical force on magnetic nanoparticles with magnetic-fluid-filled Fabry-Perot interferometer

The optical force acting on the magnetic nanoparticles (MNPs) is investigated with the magnetic-fluid-filled fiber-optic Fabry-Perot interferometer. The shift of interference spectra is related with the local refractive index variation in the light path, which is assigned to the optical-force-induced outward movement of MNPs. The influence of magnetic fluid’s viscosity, ambient temperature, strength and orientation of the externally applied magnetic field on the optical-force-induced MNPs’ movement is studied in details. The results of this work provide a further understanding of interaction between light and MNPs and clarify the dynamic micro-processes of MNPs within magnetic fluid under external stimuli. It may have the potentials in the fields of light-controllable magnetic-fluid-based devices and vector magnetic field detection.

Optical bio-chemical sensors based on whispering gallery mode resonators

Whispering gallery mode (WGM) resonators have attracted extensive attention and their unique characteristics have led to some remarkable achievements. In particular, when combined with optical sensing technology, the WGM reonator-based sensor offers the advantages of small size, high sensitivity and a real-time dynamic response. At present, this type of sensor is widely applied in the bio-chemical sensing field. In this paper, we briefly review the sensing principle, the structures and the sensing applications of optical bio-chemical sensors based on the WGM resonator, with particular focuses on their sensing properties and their advantages and disadvantages. In addition, the existing problems and future development trends of WGM resonator-based optical bio-chemical sensors are discussed.

Broadband laser-tuned whispering gallery mode in a micro-structured fiber embedded with iron oxide nanoparticles

A grapefruit microstructured fiber-based resonator embedded with iron oxide nanoparticles is demonstrated in this paper. Due to efficient photon-to-heat conversion and transfer of the magnetic nanoparticles, such a device possesses broadband all-optical wavelength tuning with high sensitivity. Experimental results show that the tuning range and sensitivity can be up to ∼5.32  nm and 0.106 nm/mW, respectively, when pump laser with a wavelength of 1550 nm is injected into the resonator. Moreover, it exhibits other excellent features such as ease of fabrication and excellent repeatability, making it a good candidate for potential applications in the area of optical filtering.

Microstructured optical fiber for multichannel sensing based on Fano resonance of the whispering gallery modes

We present the design and theoretical demonstration of a microstructured optical fiber (MOF) for multichannel sensing applications based on the Fano resonance among the different whispering-gallery modes (WGMs) propagating in the MOF. The proposed MOF consists of a number of capillary channels with different diameters inside a tubular frame. When the phases of the WGMs in the capillary channels and the frame are matched, the Fano resonance will occur and the resonant peaks can be observed in the output spectrum of the tubular frame resonator. Sensing signals from the individual channels can be detected by measuring the central wavelengths of the corresponding Fano resonant peaks. To demonstrate the practicality, we study a dual-channel MOF for bio-sensing applications, i.e., detecting the refractive index variation in biological samples. In the analysis, we have shown that channel 1 and 2 achieve a sensitivity of 29.0557 nm/RIU (refractive index unit) and 22.9160 nm/RIU in the TE mode; and 16.0694 nm/RIU and 13.3181 nm/RIU in the TM mode respectively, when the refractive index of the biological samples varies between 1.330 and 1.345. The new MOF can be a compact, flexible, and low-cost solution for a variety of applications including multichannel bio/chemical sensing, multi-microcavity laser, and tunable photonics devices.

Ringing phenomenon in chaotic microcavity for high-speed ultra-sensitive sensing

The ringing phenomenon in whispering-gallery-mode (WGM) microcavities has demonstrated its great potential for highly-sensitive and high-speed sensing. However, traditional symmetric WGM microcavities have suffered from an extremely low coupling efficiency via free-space coupling because the emission of symmetric WGMs is non-directional. Here we report a new approach for high-speed ultra-sensitive sensing using the ringing phenomenon in a chaotic regime. By breaking the rotational symmetry of a WGM microcavity and introducing chaotic behaviors, we show that the ringing phenomenon in chaotic WGM microcavities extends over both the positive and the negative frequency detune, allowing the ringing phenomenon to interact with analytes over a much broader bandwidth with a reduced dead time. Because the coupling of the chaotic microcavity is directional, it produces a significantly higher signal output, which improves its sensitivity without the need of a fiber coupler.