Whispering Gallery Mode Research Articles

Multimode Emission in GaN Microdisk Lasers

AbstractQuantum well nanolasers usually show single‐mode lasing, as gain saturation suppresses emissions in other modes. In contrast, for whispering gallery mode microdisk lasers with GaN quantum wells as active material, above threshold multimode laser emission is observed. This intriguing emission feature is manifested in the fact that several modes simultaneously show the characteristic kink in the input–output curve at the onset of lasing. A quantum theory for nanolasers is used to support the experimental finding and to analyze this behavior in the presence of gain saturation. Coupling effects between neighboring modes are identified as the origin of multimode lasing, which initiate photon exchange between modes via population pulsations similar to classical wave‐mixing effects. A reduction of this type of mode coupling with increasing mode spacing is demonstrated. The results can pave the way for multimode application of nanolasers in integrated photonic circuits.

Simultaneous measurement of bidirectional magnetic field and temperature with a dual-channel sensor based on whispering gallery mode

Simultaneous measurement of bidirectional magnetic field and temperature with a dual-channel sensor based on whispering gallery mode

High-power Raman lasing and efficient anti-Stokes generation in mm-sized crystalline disk resonators

We have previously experimentally observed high-power Stokes and second-order Stokes output from a mm-sized CaF2 disk using stimulated Raman scattering. A pump laser at a wavelength of 1.06 µm was coupled via a tapered fiber to the whispering gallery modes (WGM) of the disk. In this Letter, we extend this work and demonstrate the production of the first anti-Stokes sideband at power levels as high as 60 µW in near continuous-wave (CW) operation. The result is a four-component Raman comb at the output, with a wavelength range covering from 1.023 to 1.14 µm. We discuss the threshold dependence of the observed Raman lines on the crystal orientation and provide experimental validation. These advances enable the use of such mm-sized resonators as compact, efficient sources for terahertz-level frequency modulation.

Photonic hook propagation from eccentric microcylinder

The Photonic hook (PH) is an intricately curved photonic nanojet (PNJ) or a highly intense electromagnetic beam featuring a subwavelength waist, whose principal hallmark lies in its capacity to bend light at the nanoscale. According to existing literature, the origin of PH can be attributed to symmetry breaking, whereas symmetrical microstructures predominantly contribute to PNJ formation. This study presents the novel revelation of PH emergence from an isolated eccentric core–shell dielectric microcylinder, achieved through the illumination of a paraxial Gaussian beam (PGB). The eccentrically structured core–shell microscale geometry introduces an additional degree of freedom, influencing PH formation and directly shaping its characteristic parameters. Much like PNJ, the propagation of PH depends on different parameters such as core and shell refractive indices of the micro-structures, microstructure geometry, incident light type, and direction of propagation. A fascinating outcome from our numerical simulations is the switchable occurrence of PNJ and PH from an eccentric core–shell microcylinder by a simple adjustment of eccentricity, either parallel or perpendicular to the PGB’s propagation direction. This computational investigation emphasizes the impact of eccentricity and the incident wave’s beam waist, maintaining a consistent refractive index contrast between the core and shell. The outcomes are interpreted in terms of key parameters governing PH generation characteristics, encompassing FWHM, maximum electric field enhancement, and focal plane. Notably, we have observed the coexistence of whispering gallery modes (WGM) and PH within this system and these modes exhibit high sensitivity to the excitation wavelength. The potential applications of PH are believed to be far-reaching, including areas like optical trapping, sensing, and functioning as a versatile focusing element. This study contributes to the fundamental understanding of PH and illuminates its potential as a robust tool across diverse optical applications.

Efficient fabrication of high quality SU-8 photoresist based microsphere lasers via emulsion

Abstract SU-8 photoresist is a highly important material in the field of microfabrication and photonics owing to its low cost, excellent chemical and mechanical durability, high refractive index and transparency in the visible range. As a result, SU-8 photoresist has been employed as a cavity matrix for microsphere lasers. However, the current fabrication technique of SU-8 based microsphere lasers is complex and time-consuming. Here, we demonstrate a novel, cost-effective fabrication method for dye-doped SU-8 microspheres with diameters ranging from about 15 to 100 µm. These microspheres exhibit efficient lasing emission under optical pulse excitation. Lasing thresholds of 20 to 30 µJ·mm-2 and quality factors ranging from 1500 to 3000 are achieved. The size dependence of lasing characteristics indicates that the lasing mechanism is due to whispering gallery mode. Interestingly, these microsphere lasers can work in water, presenting promising application prospects in the fields of biological and chemical sensors.

Room Temperature Polariton Condensation from Whispering Gallery Modes in CsPbBr3 Microplatelets.

Room temperature (RT) polariton condensate holds exceptional promise for revolutionizing various fields of science and technology, encompassing optoelectronics devices to quantum information processing. Using perovskite materials, like all-inorganic cesium lead bromide (CsPbBr3) single crystal, provides additional advantages, such as ease of synthesis, cost-effectiveness, and compatibility with existing semiconductor technologies. In this work, the formation of whispering gallery modes (WGM) in CsPbBr3 single crystals with controlled geometry is shown, synthesized using a low-cost and efficient capillary bridge method. Through the implementation of microplatelets geometry, enhanced optical properties and performance are achieved due to the presence of sharp edges and a uniform surface, effectively avoiding non-radiative scattering losses caused by defects. This allows not only to observe strong light matter coupling and formation of whispering gallery polaritons, but also to demonstrate the onset of polariton condensation at RT. This investigation not only contributes to the advancement of the knowledge concerning the exceptional optical properties of perovskite-based polariton systems, but also unveils prospects for the exploration of WGM polariton condensation within the framework of a 3D perovskite-based platform, working at RT. The unique characteristics of polariton condensate, including low excitation thresholds and ultrafast dynamics, open up unique opportunities for advancements in photonics and optoelectronicsdevices.

Shape-Dependent Optical Waveguides and Low-Threshold Lasers from Polymorphic Two-Dimensional Organic Single Crystals.

Organic single crystals (OSCs) with uniform morphologies and highly ordered molecular aggregations are promising for high-performance optoelectronic devices, such as organic solid-state lasers (OSSLs), organic light-emitting transistors (OLETs), and organic light-emitting diodes (OLEDs). However, manipulating OSC morphologies and aggregation is challenging. In this study, we synthesized two-dimensional (2D) OSCs of 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) in hexagonal and parallelogram microplate (H-MP and P-MP) forms. Both types exhibit H-aggregation in the 2D plate plane but with different molecular transition dipole moment (TDM) orientations. This leads to different photon coupling modes with H-MP and P-MP microcavities. H-MPs enable isotropic 2D-waveguiding, forming whispering gallery mode (WGM) resonators, while P-MPs create unidirectional waveguiding, forming Fabry-Pérot mode (FP) resonators. These resonators can generate low-threshold laser emissions at 467 and 473 nm, respectively, and exhibit superior lasing stability with a half-life exceeding 2 h. Our BSBCz microplate OSCs are attractive candidates to combine controlled organic microcavities with photon transporting for realizing future integrated optoelectronic devices.

Observing Variation in Whispering Gallery Mode Resonance of a Trapped and Levitated Dye Doped Microdrop.

Whispering gallery mode (WGM) resonance was created in a spherical micro drop. A gradual shift in the resonance were observed. For a 600 nm droplet radius, the blue shift were 1.5, 0.7, 3.7 nm. It was estimated that such a shift corresponds to a reduction in optical radius of the droplet by 1.3, 0.6, 3.3 nm respectively. The droplet was created from a solution of glycerol, methanol and rhodamine 6G dye, and was trapped and levitated in a modified Paul trap. The WGMs were created by optically exciting the dye material from an external 532 nm cw laser beam. A shift in the WGM was observed during a gradual increase in power of the excitation laser, and a reason for such a shift was thought to be thermal evaporation of the liquid. For a larger droplet an initial 0.1 nm thermal expansion was also estimated, preceding the volume contraction. Such an expansion was negligible for a smaller droplet. The rate of change of the blue shift depends upon initial radius of the droplet. For the smaller droplet the estimated rate of change of WGM with a change in optical radius, was 0.771. For larger droplet, this rate is lower.

Multiple Fano resonances by MIM waveguide coupling whispering gallery mode resonator and application in Refractive index sensing

In this study, a baffled metal–insulator–metal (MIM) waveguide coupled with a whispering gallery mode resonator (WGMR) is proposed. This structure could excite quadruple Fano resonances. The asymmetric Fano resonance transmittance spectrum and the electric field at the resonance peak were numerically simulated using the finite difference time domain method. The obtained data were fitted using the multimode interference coupled mode theory. The number of Fano resonance peaks was tuned by the outer radius of the WGMR. With other geometric parameters unchanged, the number of Fano resonance peaks increased with increasing outer radius of the WGMR. Thus, it achieved multiple Fano resonances by adjusting solely the radius of the WGMR. When the ring width was fixed, the structure could excite multiple Fano resonances within a certain outer radius range. This structure was used for refractive index sensing and its sensitivity and figure of merit (FOM) were 3004 nm/RIU and 3.14 × 104 in a gas environment, respectively. Therefore, the proposed structure can excite multiple Fano resonances and achieve high sensitivity and FOM, providing a theoretical basis for micro and nano applications.

Hierarchical pore structure with a confined resonant mode for improving the solar energy utilizing efficiency of ultra-thin perovskite solar cells

The perovskite solar cell (PSC) has the benefits of flexibility, inexpensiveness, and high efficiency, and has important prospective applications. However, serious optical losing and low solar energy-utilizing efficiency remain a challenge for the ultra-thin PSCs because of the interface reflection of traditional planar structure. In this study, a hierarchical pore structure with a confined resonant mode is introduced and optimized by electromagnetic theory to improve the solar energy absorbing and utilizing efficiency of ultra-thin PSCs. The large pores in the top layer that support a whispering gallery mode can focus and guide the incident light into the solar cell. The small pores in the bottom layer enable backward scattering of the unabsorbed light and can improve the effective absorption of active layer. The finite-difference time-domain method is employed to optimize the geometric parameters of hierarchical pore structure to improve the light absorption of PSCs. The proposed resonant hierarchical pore structure can greatly improve sunlight absorption of ultra-thin PSCs, and the effective light absorption and photocurrent of PSCs with a hierarchical pore structure is 20.7% higher than that of PSCs with traditional planar structure. This work can offer a beneficial guideline for improving solar energy utilizing efficiency of various thin-film solar cells.

Bioinspired Carbonized Polymer Microspheres for Full-Color Whispering Gallery Mode Emission for White Light Emission, Unclonable Anticounterfeiting, and Chemical Sensing Applications.

Light-element-based fluorescent materials, colloidal graphene quantum dots, and carbon dots (CDs) have sparked an immense amount of scientific interest in the past decade. However, a significant challenge in practical applications has emerged concerning the development of solid-state fluorescence (SSF) materials. This study addresses this knowledge gap by exploring the unexplored photonic facets of C-based solid-state microphotonic emitters. The proposed synthesis approach focuses on carbonized polymer microspheres (CPMs) instead of conventional nanodots. These microspheres exhibit remarkable SSF spanning the entire visible spectrum from blue to red. The highly spherical shape of CPMs imparts built-in photonic properties in addition to its intrinsic CD-based attributes. Leveraging their excitation-dependent photoluminescence property, these microspheres exhibit amplified spontaneous emission, assisted by the whispering gallery mode resonance across the visible spectral region. Remarkably, unlike conventional semiconductor quantum dots or dye-doped microresonators, this single microstructure showcases adaptable resonant emission without structural/chemical modifications. This distinctive attribute enables a plethora of applications, including microcavity-assisted energy transfer for white light emission, highly sensitive chemical sensing, and secure encrypted anticounterfeiting measures. This interdisciplinary approach, integrating photonics and chemistry, provides a robust solution for light-element-based SSF with inherent photonic functionality and wide-ranging applications.

In-Fiber Liquid Microsphere Whispering Gallery Mode Resonator Fabricated by Femtosecond Laser

In-Fiber Liquid Microsphere Whispering Gallery Mode Resonator Fabricated by Femtosecond Laser

Crystalline germanium high-Q microresonators for mid-IR

High-quality-factor optical microresonators have become an appealing object for numerous applications. However, the mid-infrared band experiences a lack of applicable materials for nonlinear photonics. Crystalline germanium demonstrates attractive material properties such as high nonlinear refractive index, large transparency window including the mid-IR band, particularly long wave multiphonon absorption limit. Nevertheless, the reported optical losses in germanium microresonators might not allow the potential of the Ge-based devices to be revealed. In this study, we report the fabrication of germanium microresonators with radii of 1.35 and 1.5 mm, exhibiting exceptional quality factors (Q-factors) exceeding 20 million, approaching the absorption-limited values at a wavelength of 2.68 µm. These Q-factors are a hundred times higher than previously reported, to the best of our knowledge. We measured the two-photon absorption coefficient combined with free-carrier absorption leveraging the high-Q of the resonators (obtained βTPA = (0.71 ± 0.12) · 10−8 m/W at 2.68 µm). This research underscores the potential of whispering gallery mode microresonators as valuable tools for measuring absorption coefficients at different wavelengths, providing a comprehensive analysis of various loss mechanisms. Furthermore, the exceptional Q-factors observed in germanium microresonators open intriguing opportunities for the advancement of germanium-based photonics within the mid-infrared spectral band.

Boosted Second Harmonic Generation and Cascaded Sum Frequency Generation from a Surface Crystallized Glass Ceramic Microcavity.

The development of novel materials and structures for efficient second-order nonlinear micro/nano devices remains a significant challenge. In this study, we demonstrate the remarkable enhancement of second-harmonic generation (SHG) and even cascaded sum frequency generation (CSFG) in whispering gallery mode microspheres made of surface-crystallized glass with a 6-μm Ba2TiSi2O8 crystal layer. Attributed to the core-shell design, the Ba2TiSi2O8 located on the surface can be efficiently coupled with whispering gallery modes, resulting in a highly efficient micron-scale cavity-enhanced second-order optical nonlinearity. Greatly enhanced SHG of the microcavity was observed, which is up to 80 times stronger than that of non-resonant bulk sample. Furthermore, owing to the wavelength non-selectivity of random quasi-phase matching, we demonstrated ultra-wideband SHG with a strong response ranging from 860nm to 1600nm and high-contrast polarization characteristics. The glass-ceramic based microsphere cavity also boosts the cascading optical nonlinearity, manifested by a two-magnitude enhancement of CSFG. This work delineates an efficient strategy for boosting nonlinear optical response in glass-ceramics, which will open up new opportunities for applications in photonics and optical communications. This article is protected by copyright. All rights reserved.

Compact high-Q Ka-band sapphire distributed Bragg resonator

In a class of high quality (Q-) factor dielectric resonators with low radiative losses, including popular whispering-gallery mode (WGM) resonators with high azimuthal mode numbers, due to high confinement of modal field in dielectric, the Q-factor is limited by the value of inverse dielectric loss tangent of dielectric material. Metal enclosures necessary for device integration only marginally affect the Q-factor while eliminating the residual radiative loss and allowing the optimization of input and output coupling. While very high Q-factors ∼200000 are available in sapphire WGM resonators in X-band, at millimeter wave frequencies increasing dielectric loss limits the Q-factor to much smaller values, e.g. ∼50000 and ∼25000 for quasi-TE and quasi-TM modes, correspondingly, at 36 GHz. The use of distributed Bragg reflection (DBR) principle allows to push modal energy outside dielectric while also isolating it from Joule losses in metallic enclosure walls. Very high Q∼600000>tgδ has been demonstrated in X-band [C. A. Flory and R. C. Taber, IEEE Trans. Ultrason., Ferroelectr., Freq. Control 44, 486–495 (1997).] at the expense of impractically large dimensions. In this work, we report on the assembly and testing of a compact Ka-band sapphire distributed Bragg reflector cavity characterized with Q-factor seven times larger than one predicted by the material’s dielectric loss at the frequency of interest. An intrinsic Q-factor of ∼200000 is demonstrated at 36 GHz for the lowest order TM-mode of a sapphire DBR. The resonator has 50 cm3 volume, smaller than previously demonstrated DBRs.

Modeling of self-injection locking bandwidth of a whispering gallery mode microresonator

Modeling of self-injection locking bandwidth of a whispering gallery mode microresonator

Photolithographed Whispering Gallery Mode Microdisk Cavities Coupled to Semiconductor Quantum Dots

Photolithographed Whispering Gallery Mode Microdisk Cavities Coupled to Semiconductor Quantum Dots

Underwater Low-Frequency Acoustic Wave Detection Based on a High-Q CaF2 Resonator

Whispering gallery mode (WGM) resonators with an ultra-high quality (Q) factor provide a new idea for high-precision underwater acoustic sensing. However, acoustic energy loss due to watertight encapsulation has become an urgent problem for its underwater application. In order to solve this problem, this paper proposes a hollowed-out array structure. The finite element simulation shows that the acoustic wave transmission loss is improved by 30 dB compared with that of the flat plate encapsulation structure. Using a calcium fluoride (CaF2) resonator with a Q factor of 1.2 × 108 as an acoustic sensitive unit, the amplitude and frequency of the loaded acoustic wave are retrieved by means of the dispersion coupling response mechanism. The resonator’s underwater experimental test range is 100 Hz–1 kHz, its acoustic sensing sensitivity level reaches −176.3 dB re 1 V/µPa @ 300 Hz, and its minimum detectable pressure can be up to 0.87 mPa/Hz1/2, which corresponds to a noise-equivalent pressure (NEP) of up to 58 dB re 1 µPa/Hz1/2.

Enhanced upconversion luminescence of NaYF4:Er nanocrystals based on the synergy between whispering gallery mode excitation and förster resonant energy transfer

The upconversion luminescence at the visible band was excited by the high power density whispering gallery mode (WGM) on the surface of the microsphere. Under the WGM excitation, Er3+ of NaYF4:Er3+ directly absorbed three photons at 1525 nm, followed by the generation of upconversion luminescence. We used WS2 quantum dots (QDs) as donors and NaYF4:Er3+ upconversion nanoparticles (UCNPs) as acceptors to enhance upconversion luminescence by Förster resonant energy transfer (FRET). The physical mixture composed of monolayer WS2 and NaYF4:Er3+ nanoparticles was coated on SiO2 microsphere by immersion method. Excited by high power density WGM, three photons were absorbed by WS2-QDs based on the quantum size effect to generate excited electrons as donors and UCNPs as acceptors based on FRET, subsequently, the energy of the electrons were transferred to the luminescence center Er3+ to improve the upconversion luminescence efficiency. The mechanism model of upconversion luminescence enhancement based on FRET of WS2–NaYF4:Er3+ was proposed, and the enhancement effect was experimentally studied and demonstrated. To study the distance dependence of energy transfer between QD and UCNP pairs, we covered the surface of the silica microsphere with a controlled concentration of WS2 mixed with NaYF4:Er3+ nano-particles. When the mixed concentration of WS2-QDs was 0.15%, the upconversion luminescence reached the highest efficiency. The results showed that a twofold upconversion luminescence enhancement can be experimentally achieved. The proposed approach for enhancing the upconversion luminescence could benefit the improvement of imaging signal-to-noise ratio (SNR) using UCNPs for fluorescent labels in life sciences.

Weak coupling strength measurement between two whispering-gallery-mode microresonators near an exceptional point

Sensing is one of the important applications of the whispering-gallery-mode (WGM) microresonator. Here we consider a model consisting of two WGM microresonators and one fiber waveguide. We show that the system can be near an exceptional point when the two tapers in the fiber waveguide are coupled to the two microresonators in a way that can lead to an unidirectional coupling. We propose a method to measure the weak coupling strength between the two microresonators. Compared to the conventional method, our method operates near an exceptional point and has the ability to measure smaller coupling strength.