Spherical Microcavities Research Articles

Photoluminescence Enhancement: Over 1000‐Fold Enhancement of the Unidirectional Photoluminescence from a Microsphere‐Cavity‐Array‐Capped QD/PDMS Composite Film for Flexible Lighting and Displays (Advanced Optical Materials 24/2019)

A Tai Chi patterned microsphere-cavity array on a flexible quantum dot film to enhance and direct white-light emission is demonstrated here. Two aspects of a material, i.e. rigidity and flexibility, are implied by the symbol. The microsphere cavity is rigid with stable optical properties, while the microsphere array is flexible due to the weak connection between the microspheres. The cover image, referring to article number 1901228 by Yinzhou Yan and co-workers, combines Chinese traditional culture with modern science to motivate new ideas in flexible nanophotonics.

Tunable microlaser based on precisely formed dye-doped microsphere cavity

Background: The benefits of microresonator lasers include easy and low-cost fabrication methods and high-quality factor of microresonators, which results in low threshold current of the fabricated laser. However, the lack of tunable fabrication methods with precise size and spacing is a great challenge. Aim: Based on a microsphere fabrication method that relies on injection and surface tension effects, simultaneous demonstration of an array of cavities with precise spacing and sizes is possible where the size of cavities is widely tunable. Approach: Using a maskless lithography setup, pillars with base and tip diameters of 250 and 60μm are fabricated, which are used for demonstration of microspheres with tunable size, depending on the applied pump pressure. Microcavities composed of Rhodamine B dissolved in glycerol are injected into the polydimethylsiloxane carrier through a nozzle with a diameter of 70 μm. Results: An ∼4-μJ / mm2 lasing threshold has been obtained for a laser with a microsphere diameter of ∼350 μm. A redshift originated from the increment of dye concentration has also been observed. Conclusions: Fabrication of precise microspheres for a microresonator laser is feasible using the introduced method, which can be further generalized using different active materials.

Over 1000‐Fold Enhancement of the Unidirectional Photoluminescence from a Microsphere‐Cavity‐Array‐Capped QD/PDMS Composite Film for Flexible Lighting and Displays

AbstractThe development of quantum dot (QD) composite films for lighting and displays is of great importance because of their exceptional mechanical flexibility, chemical stability, and optical tunability. Generally, QDs are attached to plasmonic nanostructures to enhance their photoluminescence (PL) emission. However, plasmonic QD composite films suffer from deteriorated inherent quantum yields and lower enhancement ratios due to variations in the refractive index and dielectricity in the composites. Herein, the use of a dielectric microsphere cavity is reported to realize over 1000‐fold enhancement of the PL in CdSe/ZnS QD/polydimethylsiloxane films with a highly unidirectional emission angle of ≈9°. The field regulation by the optical whispering‐gallery resonance and directional antenna effect at the microscale is revealed to interpret the PL enhancement mechanism. Additionally, the microsphere cavity allows for the enhancement of white‐light emission in the RGB‐QD hybrid composite film. The present work inspires a straightforward strategy for boosting the light efficiency in QD composite films, with promising applications in flexible lighting/display devices.

Microlaser Output from Rare‐Earth Ion‐Doped Nanocrystal‐in‐Glass Microcavities

AbstractNanocrystallized glass ceramics (NGCs) are important optical materials, but few studies have focused on their laser actions. Here, by precipitation of NaYF4 nanocrystals enriched with Er3+ ions in an oxide glass matrix, great enhancement of the luminescence properties for the NGC microspheres is realized. By carefully matching the refractive index of the glass matrix with that of the NaYF4 nanocrystals and controlling the size and distribution of the precipitated nanocrystals, the absorption and Rayleigh scattering losses that are harmful to quality (Q) factors are efficiently suppressed. As a result, the NGC microsphere cavities present ultrahigh Q factors up to 106 (@ 1610 nm), which is slightly lower than that of the microsphere cavities made by precursor glass. Moreover, single mode whispering gallery modes (WGMs) lasing from the NGC microsphere cavities for the first time is successfully realized. Compared with the microsphere cavity based on glass, the NGC microsphere cavity exhibits large enhancement of the pumping efficiency by more than 7 times and a decrease in the threshold by more than 2.5 times. The results demonstrated here suggest a novel method for improving the laser output performances of microcavities, which may have potential applications in areas of photonics and optical communication.

Spherical microcavity-based membrane-free Fizeau interferometric acoustic sensor.

The monitoring of acoustic waves is essential for a variety of applications, ranging from photoacoustic imaging to industrial non-destructive evaluation and monitoring. Optical acoustic sensors have many advantages over electrical counterparts such as passivity and immunity to electromagnetic interference, and can be quite compact with all-fiber structures. A common approach is to optically detect the acoustically induced mechanical movement of a cantilever or a reflective membrane. However, sensors based on moving mechanical parts have limitations, because they are influenced by the mechanical properties of the structures involved that behave as a coupled spring-mass system. Here a new type of optical acoustic sensing concept based on a spherical microcavity fiber Fizeau interferometer is described. The sensor is fabricated by inserting a section of single-mode fiber into a spherical microcavity to form a Fizeau interferometer. Thanks to the elasto-optic effect, the sound pressure modifies the refractive index of the microcavity; then a corresponding change in the interferometric spectrum can be observed. The sensor was successfully used to detect acoustic waves under the whole audible region, from 20Hz to 20kHz. The experimental results show that the sensitivity can be improved via altering the length of microcavity. The structure is membrane-free, and the sensitivity will not be limited by these size restrictions which makes the technology an interesting alternative to conventional transducers for acoustic wave measurement.

Ultraviolet luminescence enhancement of planar wide bandgap semiconductor film by a hybrid microsphere cavity/dual metallic nanoparticles sandwich structure.

Here we report a novel hybrid structure composing of microsphere array (MA), Al nanoparticles (Al-NPs), ZnO thin film (luminescence layer), Au nanoparticles (Au-NPs), and substrate (sapphire) for ultraviolet (UV) luminescence enhancement of planar wide bandgap semiconductor film. The plasmonic sandwich structure of Al-NPs/ZnO/Au-NPs boosts the hot electron state density in the conduction band by electron trapping from deep-defect level of ZnO and localized surface plasmon resonances (LSPRs) coupling around dual metallic NPs, enhancing UV emission and suppressing visible emission efficiently. The dielectric microsphere array capping on the plasmonic sandwich structure further increases UV emission intensity via photonic nanojets, optical whispering-gallery modes (WGMs), and directional antenna effect, by which the interaction between photon and exciton is strengthened. The contribution of microsphere cavity coupling with LSPRs to UV luminescence enhancement is therefore revealed. The maximum enhancement ratio of up to two orders of magnitude (~250-fold) is achieved by the optimized 5.06-μm-diameter-MA/Al-NPs/ZnO/Au-NPs/sapphire structure and the UV emission is highly directional with a divergent angle of ~5°. The present work provides a simple and easily-prepared structure incorporating optical WGMs and LSPRs to manipulate UV luminescence of planar wide-bandgap semiconductors for potential optoelectronic applications.

Silica-microsphere-cavity-based microwave photonic notch filter with ultra-narrow bandwidth and high peak rejection.

We propose and experimentally demonstrate a microwave photonic (MWP) notch filter based on a silica microsphere cavity. By using a high-Q-factor (∼1e7) cavity with a diameter of 132um, the filter bandwidth can be easily decreased to 15MHz in terms of simple fabrication and flexible coupling. Then we use the advanced modulation technique based on a dual parallel Mach-Zehnder modulator to further improve peak rejection (PR). The experimental results show that the MWP notch filter with its PR beyond 55dB and frequency tunability range over 8GHz has been achieved in combination with double-sideband modulation. To the best of our knowledge, this is a record for PR and bandwidth considered simultaneously for an MWP filter based on silica microcavities. Thus, the proposed MWP filter will be useful in the fields of microwave photonic signal processing, radar systems, etc.

Microcavity dispersion engineering for the visible optical frequency comb generation

Large normal material dispersion acts as an obstacle in the generation of visible optical frequency combs (OFCs) in microcavities. The trajectory separation of light propagation aggravates the dispersion unbalance in the visible region. Dispersion engineering devoted to compressing the separation reaches a bottleneck due to manufacturing difficulties. In this article, we demonstrated that the separation of the light trajectory can be used to manipulate the cavity dispersion. In a bilayer concentric spherical microcavity (CSM), the visible anomalous dispersion was achieved numerically by precisely matching the separated trajectory with the appropriate materials. Furthermore, we simulated the generation of an optical soliton in the visible region based on the proposed microcavity. Our work provides a clue for the visible dispersion control, and the proposed BK7/silica CSM will be an interesting platform for the visible OFC generation.

Theoretical and experimental investigation of broadband dispersion tailoring of high-order mode in the hybrid microsphere cavity.

The large normal dispersion of the fundamental mode (TEn=1 mode) in the whispering gallery modes (WGM) microsphere is detrimental to the visible comb generation. Herein, we demonstrate that this fundamental limitation can be removed by considering the high-order radial modes (TEn=2 mode) of the hybrid microsphere cavity (HMC). The studied HMC consists of a high-refractive-index coating (TiO2 or HfO2) and silica microsphere. The simulated electric field energy distribution and measured Q value in our experiment show that optical confinement of the coating effectively excites the TEn=2 mode and reduces the free spectral range (FSR) and modal dispersion. In addition, the observed redshift of WGM and decreased trend of FSR are in accordance with simulations. The zero-dispersion wavelength can be linearly shifted to a shorter wavelength or even into the visible region with the reduction of coating thickness or refractive index and larger microcavity, which advances the visible comb generation.

Femtosecond laser 3D printing temperature sensitive microsphere lasers

The whispering gallery mode (WGM) microcavity has been widely used for sensing and detection because of its high quality factor, small mode size, simple and diverse manufacturing process, and high sensitivity to the surrounding environment. Microsphere cavityand microdisk cavity are typical whispering gallery mode microcavities. However, the real controllable size of the on-chip three-dimensional microsphere cavity has rarely been reported because it is difficult to prepare by photolithography. At the same time, most of the current microsphere cavity are prepared by hot melting, which have the poor ability to control the size. In this article, we have mainly demonstrated the fabrication of a dye-doped polymer whispering gallery mode microsphere by femtosecond laser two-photon polymerization, which shows good surface smoothness with a fabrication spatial resolution beyond the diffraction limit. The microsphere cavity consists with commercial photoresist SU-8 as the cavity material and Rhodamine B as the gain medium. With the 532 nm pump, the RhB-doped SU-8 can emit fluorescence in the spectral range of 600–700 nm, and thus resonant whispering gallery laser modes in this spectral region can be eventually formed in the microsphere cavities. The microcavity shows excellent lasing performance with a quality factor of ~2000. Due to the special luminescence mechanism of organic dyes, the fluorescence spectrum of the dye drifts with the change of ambient temperature, and it will form a new resonance excitation with the eigenmode of the cavity. Within a certain temperature range (20 ℃－35 ℃), the wavelength of the main lasing peak is linearly related to temperature. The results shows that the organic dye doped micro-resonator has a unique laser mechanism which can be used to construct a new type of microlaser. Moreover, the tunable microsphere laser can be used as a temperature sensor after further optimized. We believe our work will provide a positive inspiration for the rational design of miniaturized lasers with ideal performance.

High-Sensitivity Temperature Sensor Based on Microsphere Cavity in Super Larger Thermo-Optic Coefficient Germanium-core Fiber

A high-sensitivity temperature sensor based on an in-fiber germanium (Ge) microsphere Fabry–Perot (FP) cavity is presented. The lower melting point of the Ge core compared to that of a silica-based fiber is utilized to fabricate the microsphere at the fiber end as a FP cavity by splicing a Ge-core borosilicate-clad fiber to a single-mode fiber. The Ge-core borosilicate-clad fiber employed is fabricated using a molten core approach with the standard optical fiber draw tower. The large thermo-optic coefficient of semiconductor Ge microsphere cavity gives rise to high temperature sensitivity. Experimental results show that the temperature sensitivity of ~221 pm/°C can be achieved as the temperature changes from −40°C to +50°C. The proposed sensor also exhibits outstanding stability, with the measurement error and standard deviation within the range ±0.184°C and 0.0049–0.0155°C, respectively. The advantages of high sensitivity, outstanding stability and in-fiber cavity make the proposed sensor a potential candidate for temperature sensing applications.

溶胶-凝胶法制备TiO2-SiO2混合微球腔的激发高阶回音壁模式

溶胶-凝胶法制备TiO₂-SiO₂混合微球腔的激发高阶回音壁模式

A Transverse Load Sensor With Ultra-Sensitivity Employing Vernier-Effect Improved Parallel-Structured Fiber-Optic Fabry-Perot Interferometer

The parallel structured fiber-optic Fabry-Perot interferometer (FPI) is theoretically proposed and experimentally demonstrated for transverse load measurement based on Vernier-effect. This kind of sensor consists of a hollow microsphere cavity fiber-optic FPI for sensing function and an air cavity for reference, both of which are parallel connected by a 3 dB optical fiber coupler with the functionality of Vernier-effect. Experimental results indicate that the proposed sensor can provide a high transverse load sensitivity of -3.75 nm/N, which is 3.4 times higher than single hollow microsphere cavity FPI. Moreover, with the excellent features of low cost, easy fabrication, and stable sensing, the parallel structured FPIs proposed will provide a new vision for high sensitivity transverse load sensing.

Solution dynamics of 5-fluorouracil entrapped in poly lactic-co-glycolic acid (PLGA) microsphere-A study with 1D selective NMR methods.

In this report, our main focus is to introduce a set of one-dimensional (1D) NMR methods based on chemical shift, relaxation, and magnetization transfer, namely, NOE and chemical exchange involving selective pulse excitation to study the solution dynamics of drug in free and encapsulated state within polymeric microsphere. In this regard 5-fluorouracil (5-FU) loaded poly lactic-co-glycolic acid (PLGA) microspheres are prepared as model system via standard water-in-oil-in-water emulsification method. One-dimensional 1 H and 19 F nuclear magnetic resonance (NMR) spectra of 5-FU in presence of PLGA microspheres presented a significant change in linewidth and relaxation rates compared with free 5-FU confirming encapsulation. Furthermore, loss of coupling pattern in 1 H and 19 F NMR of PLGA encapsulated 5-FU as compared with free 5-FU suggests an enhanced -NH and -H2 O protons exchange dynamics in the interior of the microsphere indicating hydrated microsphere cavity. Quantification of exchange dynamics in case of free and PLGA-encapsulated 5-FU was attempted employing 1D selective NOESY and 1D multiply selective inversion recovery experiments. Analysis of the exchange rates confirmed existence of more than one kind of water population within the cavity as mentioned in an earlier solid state NMR report.

Ultrahigh Quality Upconverted Single‐Mode Lasing in Cesium Lead Bromide Spherical Microcavity

AbstractThe realization of ultrahigh quality upconverted single‐mode lasing in wavelength‐scale cavity is still a great challenge, because of limitation of microcavity quality, gain medium, and low spontaneous emission coupling efficiency β. In this work, it is demonstrated that inorganic cesium lead bromide (CsPbBr3) perovskite spheres with size of ≈780 nm serve as a gain medium and whispering‐gallery cavity simultaneously for two‐photon excited single‐mode laser. Lasing mode with a very narrow linewidth (≈0.037 nm) is realized, corresponding to an ultrahigh Q factor ≈1.5 × 104. Surprisingly, the lasing performance of such small microcavity is best among reported naturally formed semiconductor microcavities. Furthermore, room‐temperature single‐mode lasing at same wavelength can be obtained at any angle between −30° and 30°, showing the nearly uniform emission output of the spherical microcavity. This work paves a simple and straight way to obtain ultrahigh Q two‐photon excited single‐mode lasing in micro/nanostructures, holding great promise for practical applications in frequency upconversion, high sensitive sensing in nano‐optics.

Room temperature WGM resonances in the red spectral range from Ho3+ activated ZnO micro-spherical cavities

Highly crystalline and smooth 1% Ho3+ doped ZnO microspheres with diameters ranging from 0.5 to 15 μm were synthesized using laser ablation technique. Near band edge whispering gallery mode (WGM) resonances from Ho:ZnO microspheres with a single oscillation route in the UV range are observed with 355 nm excitation. Apart from the significantly enhanced visible WGM resonances associated with intrinsic oxygen vacancy defects by Ho3+ doping, the visible range WGM resonances associated with the Ho3+ 5F5 → 5I8 emission at 629–690 nm are also observed with a 488 nm excitation. The WGMs of Ho3+ f-f emission possess lower threshold and high Q-factor values.

Effects of elastic strain and structural defects on slow light modes in a one-dimensional array of microcavities

We calculate the dispersion of exciton-polariton modes in one-dimensional arrays of microcavities. We consider a two-sublattice array and a one-sublattice array of unevenly spaced spherical microcavities some of which contain embedded quantum dots. In both cases the dispersion of polariton eigen modes is shown to be efficiently controlled by a weak applied uniform strain. The structures we consider sustain slow-light modes that are of a specific importance for quantum optics applications. The high sensitivity of the optical response of microcavity arrays to the applied strain may be used in classical and quantum optical switches and the polariton based integrated photonics.

Mode splitting induced by an arbitrarily shaped Rayleigh scatterer in a whispering-gallery microcavity

We investigate theoretically the mode splitting induced by an arbitrarily shaped Rayleigh scatterer attached to a whispering-gallery microcavity. The information including polar position, orientation, and polarizability tensor of the nanoparticle can be obtained through the mode-splitting signal. It is found that when the electric field of a pair of counterpropagating modes have different orientations and are not linearly polarized, both of the split modes experience frequency shift and linewidth broadening. The polar angle, at which a nanoparticle binds on the microsphere cavity, can be extracted by comparing frequency splittings of two fundamental TE modes. Moreover, the orientation and polarizability of the particle can be acquired by combining frequency splittings and linewidth broadenings of three modes which polarize the particle in different orientations. This work provides a comprehensive theory base for microcavity sensing of nanoparticles of arbitrary shape instead of nanospheres and could push nanoparticle detection towards quantitative characterization.

Gelatin/Nanohyroxyapatite Cryogel Embedded Poly(lactic-co-glycolic Acid)/Nanohydroxyapatite Microsphere Hybrid Scaffolds for Simultaneous Bone Regeneration and Load-Bearing.

It is desirable to combine load-bearing and bone regeneration capabilities in a single bone tissue engineering scaffold. For this purpose, we developed a high strength hybrid scaffold using a sintered poly(lactic-co-glycolic acid) (PLGA)/nanohydroxyapatite (nHAP) microsphere cavity fitted with gelatin/nHAP cryogel disks in the center. Osteo-conductive/osteo-inductive nHAP was incorporated in 250–500 μm PLGA microspheres at 40% (w/w) as the base matrix for the high strength cavity-shaped microsphere scaffold, while 20% (w/w) nHAP was incorporated into gelatin cryogels as an embedded core for bone regeneration purposes. The physico-chemical properties of the microsphere, cryogel, and hybrid scaffolds were characterized in detail. The ultimate stress and Young’s modulus of the hybrid scaffold showed 25- and 21-fold increases from the cryogel scaffold. In vitro studies using rabbit bone marrow-derived stem cells (rBMSCs) in cryogel and hybrid scaffolds through DNA content, alkaline phosphatase activity, and mineral deposition by SEM/EDS, showed the prominence of both scaffolds in cell proliferation and osteogenic differentiation of rBMSCs in a normal medium. Calcium contents analysis, immunofluorescent staining of collagen I (COL I), and osteocalcin (OCN) and relative mRNA expression of COL I, OCN and osteopontin (OPN) confirmed in vitro differentiation of rBMSCs in the hybrid scaffold toward the bone lineage. From compression testing, the cell/hybrid scaffold construct showed a 1.93 times increase of Young’s modulus from day 14 to day 28, due to mineral deposition. The relative mRNA expression of osteogenic marker genes COL I, OCN, and OPN showed 5.5, 18.7, and 7.2 folds increase from day 14 to day 28, respectively, confirming bone regeneration. From animal studies, the rBMSCs-seeded hybrid constructs could repair mid-diaphyseal tibia defects in rabbits, as evaluated by micro-computed tomography (μ-CT) and histological analyses. The hybrid scaffold will be useful for bone regeneration in load-bearing areas.

Optomechanical Self-Regulated Coupling of a Suspended Microsphere Cavity and a Waveguide in the Aqueous Medium

Optomechanics of colloidal microparticles has wide applications in biological analysis and sensing. For colloidal microspheres or microdroplets, the optomechanical force can be magnified through the cavity enhancement effect. However, it is difficult to analyze the force since the colloidal microspheres are suspended in liquid, and addressing a movable microsphere at a specific position in three-dimensional space is also challenging. An on-chip integrated operating platform comprising waveguides and microelectromechanical systems is employed to study the cavity-enhanced optical gradient force on colloidal microspheres, owing to the ability to precisely control the distance between a suspended microsphere and a waveguide through dielectrophoretic force. We introduce two kinds of optomechanical coupling mechanisms at resonance, depending on the initial coupling gap without inclusion of the optical gradient force. One is self-adjusted coupling, where the coupling gap of a suspended microsphere continuously varies with the optical input power, and the other is bistable coupling, where the coupling gap hops from one state to the other as the input power exceeds over a threshold value, which is caused by the nature of nonlinear gap-dependent optical gradient force.