All-optical tuning of a magnetic-fluid-filled optofluidic ring resonator

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.

Electro-Ionic Control of Surface Plasmons in Graphene-Layered Heterostructures

Precise control of light is indispensable to modern optical communication devices especially as the size of such devices approaches the subwavelength scale. Plasmonic devices are suitable for the development of these optical devices due to the extreme field confinement and its ability to be controlled by tuning the carrier density at the metal/dielectric interface. Here, an electro-ionic controlled plasmonic device consisting of Au/graphene/ion-gel is demonstrated as an optical switch, where an external electric field modulates the real part of the electrical conductivity. The graphene layer enhances charge penetration and charge separation at the Au/graphene interface resulting in an increased photoinduced voltage. The ion-gel immobilized on the Au/graphene further enables the electrical tunability of plasmons which modulates the intensity of the reflected laser light. This work paves the way for developing novel plasmonic electro-optic switches for potential applications such as integrated optical devices.

Magnetic sensor based on serial-tilted-tapered optical fiber for weak-magnetic-field measurement

An optical fiber magnetic field sensor based on serial-tilted-tapered fiber (STTF) integrated with magnetic fluid is proposed. The compact STTF structure consists of two closely tilted-tapered fibers with a length of approximately 836 µm, which results in stronger mode coupling. The transmission characteristics of the proposed sensor under different magnetic field intensities (MFIs) have been studied. The results show that the proposed structure has an outstanding response to MFI and that the highest sensitivity is 32.67 pm/Oe in wavelength and 0.0336 dB/Oe in transmission in the range of 0-75 Oe. The minimum resolution of the proposed sensor is up to 0.6734 Oe. These types of sensors have great potential application in weak magnetic field measurements due to their compact structure and good sensing performance.

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.

Tunable Whispering Gallery Mode Photonic Device Based on Microstructured Optical Fiber with Internal Electrodes

We propose and experimentally demonstrate the first tunable whispering gallery mode (WGM) photonic device based on side-hole microstructured optical fiber (SH-MOF) with internal electrodes, in which the WGM quality factors do not decrease significantly during the tuning process. The resonant modes are redshifted simply by increasing the temperature. A description of the thermal tuning properties of the WGMs in SH-MOF with internal electrodes is performed by using a two-stage computational methodology, where the effects of metal filling process are considered. SH-MOF devices with internal electrodes are tested and the experimental results show excellent agreement with the theory. A linear relationship between the shift rate of the WGM modes and temperature is observed. The tunable SH-MOF microresonator with internal electrodes is anticipated to find potential applications in optical filtering, optical switching, and highly integrated tunable photonic devices.

Guanidine derivative polymer coated microbubble resonator for high sensitivity detection of CO2 gas concentration

A carbon dioxide (CO₂) gas sensor based on a polyhexamethylene biguanide (PHMB)-coated whispering gallery mode (WGM) microbubble resonator is proposed and verified experimentally in this work. Microbubbles were fabricated using two reverse arc discharges focused on microcapillaries. The inner wall of the microbubble was coated with a layer of PHMB using a filling and sintering process. A significant WGM resonance was observed by coupling with a tapered fiber. The experimental results show that as the concentration of carbon dioxide increases, a blue shift appears in the spectrum. Addition, a high sensitivity (0.46 pm /ppm) and a good linear relationship were obtained in the measurement range of 200-700 ppm with a detection limit of 50 ppm. The sensor features include high sensitivity, simple structure, easy manufacture, and low cost.

Tunability of Hi-Bi photonic crystal fiber integrated with selectively filled magnetic fluid and microfluidic manipulation

Optical fiber microfluidics technology can implement the mutual tune of the light field and fluid in micro-nano scale. In this paper, one core of high-birefringence photonic crystal fiber (Hi-Bi PCF) is used as a microfluidic channel. The birefringence of Fe₃O₄ nanofluid is experimentally and theoretically investigated by selectively infiltrating the magnetic fluid into the core of the Hi-Bi PCF. The presence of magnetic fluid alters the birefringence of the original Hi-Bi PCF and can be modulated by the intensity of the external magnetic field. The optical field distribution is simulated, and the birefringence of the Hi-Bi PCF with selective filling is approximately 6.672×10^-4. The experimental results show that the structure has a highly linear response to the external magnetic field from 0 Oe to 300 Oe, and the sensitivity is 16.8 pm/Oe with a high resolution of 1.19 Oe. Due to several advantages such as all-fiber compact structure, low transmission loss, and high linear response, this device can find various applications, including weak magnetic field measurement with high accuracy, optical fiber gyroscopes, and magneto-optic modulators. Particularly, it also has important significance to realize the all-fiber microfluidic chip laboratory.

Tunable sub-kHz single-mode fiber laser based on a hybrid microbottle resonator

We experimentally demonstrated an all-optical tunable sub-kHz single-mode fiber laser based on an ultrahigh-quality (Q)-factor hybrid microbottle resonator. The wavelength tunability is a very important function for fiber lasers, and the all-optical tuning method has rarely been proposed. Here, we use the iron-oxide-nanoparticle-coated silica microbottle resonator with a Q factor of 1.8×10⁸ as the feedback element of the fiber ring laser and suppress the higher-order modes of the microresonator to achieve single-mode lasing with a linewidth of ∼500  Hz and a signal-to-noise ratio of 49 dB. Iron oxide nanoparticles are coated on the tapered area of the microbottle resonator and the control light is fed through the axial direction of the microbottle. The lasing wavelength of the fiber laser can be all-optically and linearly tuned with a range of 2.7 nm due to the strong photothermal effect of iron oxide nanoparticles. With such an excellent tunability and a narrow linewidth, this single-mode fiber laser has great potential in applications, such as optical spectroscopy, sensing, and signal processing.

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.

Whispering gallery modes in a single silica microparticle attached to an optical microfiber and their application for highly sensitive displacement sensing

A compact and relatively stable structure is experimentally demonstrated to excite whispering gallery modes (WGMs) in a single chemically fabricated silica microparticles with a diameter of around 10.6 μm attached to an optical microfiber. The resonance dip with an extinction ratio of 14 dB and Q factor of around 300 has been achieved. Based on the WGMs in this structure, an in-line fiber-optic displacement sensor is presented with a high sensitivity of 33 dB/mm and a measurement range of over 400 μm. The measurement resolution of this displacement sensor can reach to ~10 μm. The good reversibility and repeatability are also verified. This work offers a scheme to observe the WGMs in a single silica microparticles and demonstrates their application for in-line highly-sensitive displacement sensing.

All-optical control of ultrahigh-Q silica microcavities with iron oxide nanoparticles

We propose and experimentally demonstrate, to the best of our knowledge, the first all-optical control scheme of ultrahigh-quality (Q)-factor silica microcavities, which can maintain their Q factors over 10⁸ during the tuning process. For silica microcavities, the resonance tunability is very important and is also challenging for many applications. However, almost all previous works on resonance tuning deteriorate the Q factors of silica microcavities at different levels, and evidently these schemes are not suitable for applications in which ultrahigh Q factors are required. In this work, based on the proposed silica microbottle cavity and iron oxide nanoparticles, we realize all-optical control of the silica microcavity and maintain its Q factor of around 1.2×10⁸ during the tuning process. A tuning range of 85.9 GHz (0.68 nm) and a tuning sensitivity of 13.6 GHz/mW are obtained, and it is possible to realize full tunability by bridging the azimuthal free spectral range using six adjacent q-series modes. Moreover, all-optical control of the reflection spectrum is also carried out. This work will broaden the applications of ultrahigh-Q silica microcavities in nonlinear optics, microwave photonics, cavity optomechanics, and cavity quantum electrodynamics.

Magnetic field sensing using whispering-gallery modes in a cylindrical microresonator infiltrated with ferronematic liquid crystal

An all-fiber magnetic field sensor based on whispering-gallery modes (WGM) in a fiber micro-resonator infiltrated with ferronematic liquid crystal is proposed and experimentally demonstrated. The cylindrical microresonator is formed by a 1 cm-long section of a photonic crystal fiber infiltrated with ferronematic materials. Both ferronematics suspensions are prepared based on the nematic liquid crystal 1-(trans-4-Hexylcyclohexyl)-4-isothiocyanatobenzene (6CHBT) doped with rod-like magnetic particles in the first case and with spherical magnetic particles in the second case. WGMs are excited in the fiber microresonator by evanescent light coupling using a tapered fiber with a micron-size diameter. The Q-factor of the microresonator determined from the experimentaly measured transmission spectrum of the tapered fiber was 1.975 × 10³. Under the influence of an applied magnetic field the WGM resonances experience spectral shift towards shorter wavelengths. The experimentally demonstrated sensitivity of the proposed sensor was -39.6 pm/mT and -37.3 pm/mT for samples infiltrated with rod like and spherical like ferromagnetic suspensions respectively for a magnetic field range (0-47) mT. Reducing the diameter of the cylindrical micro-resonator by tapering leads to enhancement of the magnetic field sensitivity up to -61.86 pm/mT and -49.88 pm/mT for samples infiltrated with rod like and spherical like ferromagnetic suspensions respectively for the magnetic field range (0-44.7) mT.

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.

Tunable polarization beam splitter based on optofluidic ring resonator

An efficient polarization beam splitter (PBS) based on an optofluidic ring resonator (OFRR) is proposed and experimentally demonstrated. The PBS relies on the large effective refractive index difference between transverse-electric (TE) and transverse-magnetic (TM) polarization states, since the silica-microcapillary-based OFRR possesses a slab-like geometry configuration in the cross section through which the circulating light travels. To the best of our knowledge, this is the first OFRR-based PBS. In our work, the maximum polarization splitting ratio of up to 30 dB is achieved. Besides, water and ethanol are pumped into the core of the silica microcapillary respectively, and the maximum wavelength tuning range of 7.02 nm is realized when ethanol flows through the core, verifing the tuning principle of the PBS effectively. With such a good performance and simple scheme, this OFRR-based PBS is promising for applications such as tunable optical filters, demultiplexers, and routers.

Microfluidic fabrication of cholesteric liquid crystal core-shell structures toward magnetically transportable microlasers

We report a magnetically transportable microlaser with cholesteric liquid crystal (CLC) core-shell structure, operating in band-edge mode. The dye doped CLC shells as a water-in-oil-in-water (W/O/W) double emulsion were fabricated by microfluidics. Water-dispersible Fe3O4 magnetic nanoparticles were incorporated in the inner aqueous phase by taking advantage of the immiscibility with the middle CLC oil phase. The influence of temperature and shell thickness on laser properties was discussed in detail. The non-invasive manipulation of microlasers was realized under a magnetic field. The dependence of velocity on the viscosity of the carrying fluid and size of the core-shell structure was theoretically analyzed and experimentally investigated using a prototype electromagnetic platform. We also discussed the design principles for this type of DDCLC core-shell structure. Such magnetically transportable microlasers offer promise in in-channel illumination applications requiring active control inside micro-channels.

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.

Magnetic-field sensor based on whispering-gallery modes in a photonic crystal fiber infiltrated with magnetic fluid

In this work, a magnetic-field sensor was designed to take advantage of the tunability of the resonance wavelengths of a cylindrical whispering-gallery-mode microresonator. The microresonator is based on a 1.3 cm length of photonic crystal fiber infiltrated with a magnetic fluid containing nanoparticles with diameters of either 5 or 10 nm. The Q-factor achieved for the microresonators was 4.24×10(3) or higher. When a magnetic field is applied, the whispering-gallery-mode resonances shift toward longer wavelengths. The experimentally demonstrated sensitivity of the proposed sensor was as high as 110 pm/mT in the magnetic field range from 0 to 38.7 mT.