Optical Microsphere Resonators Research Articles

Single and dual optical microsphere resonator as Humidity Sensor

This study explores into the influence of whispering gallery mode (WGMs) on single and dual microsphere resonator (MSR) as a humidity sensor which can confine light by continuous internal reflection. The construction of the device is simple, and it has a low cost as MSR is made of silica material with a high refractive index. MSR coupling tapered optical fibers with size 5µm to the surfaces of single and two microspheres is demonstrated as a relative humidity (RH) sensor. The Q-factor was measured 7.754 × 105 dBm for single MSR while for dual MSR, Q-factor is 7.765 × 105 dBm. When sensitivity, linearity, and repeatability were analyzed based on transmitted power, the results were conclusively excellent for the performance of the double MSR.

Batch Fabrication of High-Quality Infrared Chalcogenide Microsphere Resonators.

Optical microsphere resonators working in the near- and mid-infrared regions are highly required for a variety of applications, such as optical sensors, filters, modulators, and microlasers. Here, a simple and low-cost approach is reported for batch fabrication of high-quality chalcogenide glass (ChG) microsphere resonators by melting high-purity ChG powders in an oil environment. Q factors as high as 1.2 × 106 (7.4 × 105 ) are observed in As2 S3 (As2 Se3 ) microspheres (≈30µm in diameter) around 1550-nm wavelength. Smaller microspheres with sizes around 10µm also show excellent resonant responses (Q≈ 2.5 × 105 ). Based on the mode splitting of an azimuthal mode in a microsphere resonator, eccentricities as low as ≈0.13% (≈0.17%) for As2 S3 (As2 Se3 ) microspheres are measured. Moreover, by coupling ChG microspheres with a biconical As2 S3 fiber taper, Q factors of ≈1.7 × 104 (≈1.6× 104 ) are obtained in As2 S3 (As2 Se3 ) microspheres in the mid-infrared region (around 4.5µm). The high-quality ChG microspheres demonstrated here are highly attractive for near- and mid-infrared optics, including optical sensing, optical nonlinearity, cavity quantum electrodynamics, microlasers, nanofocusing, and microscopic imaging.

Defect engineered ZnO whispering gallery modes via doping with alkali metal ions for label-free optical sensors

A systematic investigation on the proper utilization of defect levels present in ZnO is very much in demand to avail many applications of photonics in visible and near infrared (NIR) regions. In this paper, we have engineered intrinsic defects of zinc oxide (ZnO) to achieve high-quality intense whispering gallery modes (WGMs) in a single ZnO microsphere optical resonator by doping with alkali metal ions. Here, a single microsphere of undoped and doped ZnO was considered to investigate WGMs by recording luminescence spectra using a microphotoluminescence system under green laser excitation having a central wavelength of 532 nm and a fixed power of 55 mW/cm2. We have found that there is a significant enhancement in the intensity of WGMs in the case of doped ZnO in comparison to undoped ones. Among all the doped ZnO microspheres, 2 mol. % Li-doped ZnO yields the strongest and intense WGMs, which are accompanied by high-quality (Q)-factors. Furthermore, the pump power dependence measurement performed in 2 mol. % Li-doped ZnO reveals the lasing action in the visible optical window by explicitly exploiting the defect levels present in the material. Thus, our proposed defect engineered ZnO microsphere may represent a promising optical microresonator for developing highly sensitive WGMs based optical sensors.

Investigation of the Effect of Notch and Particle Geometries on Performance of WGM Microsphere Sensor

Optical microsphere resonators that are based on whispering gallery modes (WGMs) are emerging as highly sensitive devices for label-free detection of nanoparticles. In this paper, the change in resonant spectrum due to the presence of a notch on the surface of the microsphere resonator is investigated. The notch causes strong coupling between the WGMs field and nanoparticle; also, a broad range of nanoparticles will be detected with the change of the notch size. When a nanoparticle having a diameter close to the notch size enters the notch, the detection limit will occur, and consequently, the nanoparticle will not be detected. To overcome this detection limit, a chalcogenide glass (As2S3) layer is used at the bottom of the notch. This layer causes a shift of the symmetric mode to the left of the asymmetric mode. Therefore, the detection ranges of nanoparticles compared with the notch without layer are increased. At a frequency close to 106.7 THz, high- $Q$ modes, up to $7.6\times 10^{3}$ can be excited. The results have been achieved with a numerical simulation and finite-difference method.

Whispering-gallery modes in a triple-layer-coated microsphere resonator for refractive index sensors

In this study, a highly sensitive refractive index (RI) sensor is designed by functionally coating three layers of high, low, and high RI from inside to outside on the external surface of an optical microsphere resonator. The properties of whispering-gallery modes, confined in normal microspheres, as well as triple-layer-coated microspheres are presented theoretically and numerically through electromagnetic analyses. The whispering-gallery modes in such a triple-layer-coated microsphere resonator are found to be efficiently excited when optimizing the gap between the microsphere and waveguide. Two cladding modes, referred to as the inner mode (IM) and outer mode (OM), are exhibited in two high-RI layers. Potential RI-sensing applications stemming from the coupling for the IM and OM are investigated thereafter using the perturbation theory and the finite difference time domain method. It is found that the sensing properties of the OM can exceed single-layer-coated microspheres in terms of sensitivity. Specifically, an increased sensitivity of 49.5 nm/RIU (Refractive Index Unit) can be achieved by tailoring the film thickness of the middle low-RI layer. When applying a thick coating in the middle layer, there is a mode bonding in the sensitivity for the IM and OM. The detection limit, or quality factor, is also investigated by using an approximate theory. Results demonstrate that it is possible to reach a detection limit with a variation of ∼10−6. The study’s theoretical and numerical predictions provide guidelines for the improved design and fabrication of microsphere RI sensors.

Continuous spectrum of modes for optical micro-sphere resonators

This paper presents an improved modal analysis for the spherical dielectric resonator. This is commonly carried out by assuming an outgoing spherical Hankel function for the region surrounding the dielectric sphere. It will be shown that this assumption is incomplete and cannot lead to the entire spectrum of resonance frequencies. Following an analytical formulation, we prove that, like cylindrical resonators, the only choice for the outer region of the dielectric sphere can be a proper linear combination of an inward and an outward traveling wave. Starting from this formulation, we determine the complete spectrum of the resonance frequencies and the associated mode fields. In this analysis, the continuous spectrum of resonance frequencies is introduced and the properties of radiation modes are studied in detail. The proposed analytical formulation is thereafter employed to calculate the quality factor of the resonator due to radiation and dielectric loss.

Integrated chemical vapor sensor based on thin wall capillary coupled porous glass microsphere optical resonator

A miniaturized chemical vapor sensor probe was developed using a porous glass microsphere (PGM) as the alignment-free optical microresonator. The porous microsphere was placed inside a thin wall silica capillary tube that was fusion-spliced to an optical fiber. The whispering gallery modes (WGMs) of the microsphere were excited by the evanescent field of the light propagating inside the capillary thin wall. Adsorption of chemical vapor molecules into the pores led to a refractive index change of the PGM and thus the resonance wavelength shift of the WGMs. To facilitate the in-taking of chemical vapor molecules into the PGM, a micro window was opened at the backend of the capillary tube using femtosecond laser micromachining. Ethanol vapor was used to demonstrate the probe for chemical vapor sensing. With a miniaturized size, integrated structure and reflection mode of operation, the proposed probe may find useful in many practical applications such as environmental monitoring and biomedical sensing.

Optimization of microsphere’s DQ product based on resonant micro-optical gyro

Based on the development of high sensitivity, low cost, high integration and miniaturization demand of the resonant micro-optical gyro(R-MOG), and in order to achieve a resonant micro-optical-mechano-electrical integrative gyro having high sensitivity, a microsphere optical resonator key sensitive element for producting a cavity with high quality value (Q value) and large diameter in the field of integrated optical micro resonator is proposed, for making a resonant micro optical gyro. Microsphere optical resonator is made by means of water-hydrogen flame melting, and the SiO2 microspherical cavity is formed under the natural cooling and contraction surface tension. Microsphere optical resonator with its diameter D ranging from 300 μm to 2200 μm is fabricated by melting method with hydrogen flame as a heat source through controlling the hydrogen flame’s area by regulating the flow of hydrogen gas. The resonator serves as the key unit of the resonant optical gyro sensitive parts, its Q value and diameter D have direct effect on the performance of the resonant angular velocity sensor. Affect parameters on the performance of the microsphere optical resonator with different diameters is tested and processed to obtain the result. The corresponding relationship among Q value, DQ product, resonant micro-optical gyro’s sensitivity and microspherical cavity diameter D is analyzed, and the reason for them is given. With the increase of microspherical cavity diameter D, the Q value and DQ product reduce after rising first, while the gyro sensitivity goes to rise and fall. Based on the microsphere optical resonator DQ product optimization research, the resonant micro-optical gyro’s key sensitive unit with best parameters is obtained. When the microspherical cavity diameter D varies from 600 to 200 μm, the gyro sensitivity can meet the condition that δΩ D is 1260 μm, the Q value of microsphere optical resonator is 7.18×107 and the corresponding optimal limited sensitivity of the resonant micro-optical gyro is almost 10°/h, and this result adequately meets the requirement of business level gyro applications. This work can serve as an experimental foundation in the research of new type resonant micro optical gyro at chip level, high accuracy and low cost, and will also provide a technical reference for further study of high integrated and high precision resonant micro optical gyro.

Evaluation of single virus detection through optical biosensor based on microsphere resonator

Shift of resonance frequency in microsphere optical resonator due to attachment of a desirable particle is obtained. Our 3-D finite element numerical method (FEM) simulations’ results show the path of light through microsphere and its variation due to attachment of particle. It is apparent that after attachment of particle to microsphere's surface, light is inclined to pass through the particle. Subsequently, the path of light becomes longer than previous. Because of this phenomenon, the resonance wavelength shifts to longer wavelengths. It is shown that microsphere optical resonator is a prominent biosensor for single virus detection since we applied characteristics of virus for particle in our simulations. Response of this biosensor depends on the characteristics of particle like its radius as we show in this article. Transmission spectrum of fiber which reveals a selected resonance frequency, have been studied in the frequency range of 106.3 to 107THz under three different sizes of particles. The results show that the amount of frequency shift rises by enhancement of particle's size.

Human cardiac troponin I sensor based on silver nanoparticle doped microsphere resonator

Human cardiac troponin I (cTnI) is a specific biomarker for diagnosis of acute myocardial infarction (AMI). In this paper, a composite sensing system of an optical microsphere resonator and silver nanoparticles based on surface enhanced Raman scattering (SERS) and stimulated Raman scattering (SRS) techniques towards a point of care diagnostic system for AMI using the cTnI biomarker in HEPES buffered solution (HBS) is proposed. Pump and Raman signals enter the optical fiber coupling into the microsphere, and then SRS occurs in the microsphere. The presence of silver nanoparticles on the microsphere surface provides a tremendous enhancement of the resulting Raman signal through an electromagnetic enhancement of both the laser excitation and Stokes-shifted light of the order of 1010. This enhancement occurs in metals as surface plasmon resonance (SPR), which increases the Raman gain through the SERS effect. Our simulation results show that this sensor presents a linear response for cTnI detection. The calculated enhanced Raman signal can be employed to detect the cTnI molecules around the microsphere.

MEMS-actuated wavelength drop filter based on microsphere whispering gallery modes

MEMS-enabled tuneable optical coupling between optical microsphere resonators and optical fibre waveguides is reported. We describe the design, fabrication and experimental characterization of a MEMS platform, based on electrothermal actuators, which controls the resonator-to-waveguide separation. We compare the simulated and experimental displacements of the actuators in an unloaded and loaded state, where the load is a 1 mm optical spherical resonator. We then demonstrate the proof of concept application of selective wavelength dropping using the MEMS platform by modulating the coupling between the spherical resonator and a tapered optical fibre waveguide.

Photonic Quantum Information Science and Surface Science

Recent works of the authors on quantum information science using photons are reviewed. After the general introduction of photonic quantum information science, unique nano photonic devices such as tapered optical fibers, microsphere resonators, and nano diamond crystals are explained and their relationship with surface science is discussed. In the latter half, the realization of optical quantum circuits, and the application of such quantum circuits for quantum metrology are explained.

A Taper-Fused Microspherical Laser Source

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report on the realization of an integrated lasing device consisting of a microsphere optical resonator fused to a tapered optical fiber. A microsphere fabricated from Er : Yb-codoped phosphate glass is heated above its glass transition temperature of 375 <emphasis><formula formulatype="inline"> <tex>$^\circ$</tex></formula></emphasis>C by pumping it at 977 nm with 70 mW via a tapered optical fiber. The onset of thermal stress in the glass at a maximum pumping power results in the sphere melting and fusing to the taper coupler, without inhibition of whispering gallery mode lasing. A taper-fused microsphere laser with <emphasis><formula formulatype="inline"><tex>$\sim$</tex> </formula></emphasis>4.5 <emphasis><formula formulatype="inline"> <tex>$\mu$</tex></formula></emphasis>W of lasing power at 1593 nm is demonstrated. </para>

Aptamer Based Microsphere Biosensor for Thrombin Detection

We have developed an optical microsphere resonator biosensor using aptamer asreceptor for the measurement of the important biomolecule thrombin. The sphere surface ismodified with anti-thrombin aptamer, which has excellent binding affinity and selectivityfor thrombin. Binding of the thrombin at the sphere surface is monitored by the spectralposition of the microsphere’s whispering gallery mode resonances. A detection limit on theorder of 1 NIH Unit/mL is demonstrated. Control experiments with non-aptameroligonucleotide and BSA are also carried out to confirm the specific binding betweenaptamer and thrombin. We expect that this demonstration will lead to the development ofhighly sensitive biomarker sensors based on aptamer with lower cost and higher throughputthan current technology.

Special Issue of Photonic Sensors for Chemical, Biological, and Physical Parameter Detection

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Aqueous mercuric ion detection with microsphere optical ring resonator sensors

We have developed a novel optical sensor for mercuric ion Hg(II) detection in an aqueous environment by utilizing a fused silica microsphere ring resonator. Due to the high Q-factor associated with the whispering gallery modes (WGM) (>10 5) at the surface of the microsphere, the light interaction with the analytes on the sphere surface is significantly increased, resulting in enhanced sensitivity. The spectral position of the WGM of a microsphere shifts in response to the binding of Hg(II) to the surface of the microsphere, which is activated by the thiol group. Our experimental results show that the Hg(II) detection limit of the microsphere sensor is approximately 50 ppb (w/w) and that mercuric ions bind to thiol groups with 1:3 stoichiometry. Control experiments with zinc ions and 2-mercaptobenzothiazole (2-MBT) were also performed to ensure that Hg(II) binds selectively to thiol groups.

Label-Free Protease Sensors Based on Optical Microsphere Resonators

Simple and effective sensors for proteolytic activity are of interest to clinical diagnosis, homeland security, and nanofabrication. We demonstrate a novel label-free sensor for proteolytic activity utilizing an optical microsphere resonator that relies on circulation of the light to enhance the interaction with analytes. As a model, we employ the protease trypsin and a monolayer of BSA as a substrate on the sphere surface. We show a trypsin detection limit of approximately 10−4 Units/ml, corresponding to removal of 0.6% of each BSA molecule on the sphere surface. The detection limit obtained in this work indicates that a microsphere sensor with specificity for a protease of interest can be designed using synthesized peptides and a functionalized microsphere.

Tuning whispering gallery modes in optical microspheres with chemical etching

We demonstrate a new method to tune the resonance of whispering gallery modes in a fused silica optical microsphere resonator by removing atomic layers from the sphere surface with low concentrations of hydrofluoric acid. Our results show that the WGMs can be tuned over 660 pm (430 GHz), more than one free spectral range of the microsphere resonator, with a tuning precision better than 0.2 pm (130 MHz). Both atomic force microscope images and a Q-factor measurement performed in air suggest that no additional degradation in Q-factor due to surface roughness is introduced during this etching process.

Morphology-dependent resonances of optical microsphere resonators for the realization of passive wavelength-division multiplexing components

Morphology-dependent resonances of microspheres can provide the necessary optical feedback for applications in spectroscopy, laser science, and optical communications. The elastic scattering of focused light from dielectric microspheres is understood by the localization principle and the generalized Lorenz-Mie theory. We excited the morphology-dependent resonances of glass microspheres by a tunable distributed-feedback laser and detected the elastically scattered signal. Efficient coupling to morphology-dependent resonances is achieved using an optical fiber half coupler. Resonance peaks in the elastic scattering spectra and associated dips in the transmission spectra are observed experimentally. Simulation results of elastic scattering spectra of glass microspheres in the C-band are presented.

Focus section on Cavity QED

Focus section on Cavity QED