Whispering Gallery Mode Lasing Research Articles

Heterogeneous integration of an on-chip Nd:YAG whispering gallery mode laser with a lithium-niobate-on-insulator platform.

The integration of heterogeneous optical components onto an optical platform is crucial for the advancement of photonic chips. To achieve this, efficient coupling of optical signals between components and the platform is essential. Here, we have successfully integrated a Nd:YAG microdisk laser with a lithium-niobate-on-insulator (LNOI) photonic platform by modulating the propagation modes of LNOI. Ridge waveguides are fabricated on the LNOI by carefully adjusting the cross-sectional dimensions to enable the propagation of higher-order propagation modes. This ridge waveguide ensures that the effective refractive index of the higher-order mode closely matches that of the fundamental mode of the Nd:YAG microdisk, ensuring efficient waveguide-microdisk coupling. This on-chip laser, consisting of an Nd:YAG microdisk and LNOI integration, achieves a maximum output power of 23 µW, and a mode suppression ratio of 53.6 dB. This research presents an efficient approach for constructing highly functional heterogeneous integrated optical chips.

InGaN‐Based Whispering Gallery Mode Laser with Lateral Nanoporous Distributed Bragg Reflector Exhibits Superior Mode Selectivity

AbstractWhispering gallery mode (WGM) laser with single‐mode is important for optoelectronic applications because of the higher stability during the light signal transmission. But till now, for InGaN‐based WGM laser, most reports of lasing behavior are multimode lasing. To achieve single‐mode WGM laser, an InGaN‐based micro‐ring laser is proposed and demonstrated with lateral nanoporous distributed Bragg reflector (DBR), where the high‐order lasing modes are strongly suppressed. The side mode suppression ratio (SMSR) is more than 9 times larger than that of the micro‐disk laser with the same radius. Combined with FDTD simulation, the results show that the enhancements are originated from the suppression of the high‐order resonant mode because of the inner boundary of micro‐ring. This work provides a new solution to achieve stable single‐mode WGM laser for future photonic integration.

Evaporation characteristics of Er3+-doped silica fiber and its application in the preparation of whispering gallery mode lasers.

In this work, the concentration of rare-earth ions in doped silica whispering gallery lasers (WGLs) is controlled by evaporation. The fabrication of WGLs is used to experimentally evaluate the evaporation rate (mol/μm) and ratio (mol/mol) of erbium and silica lost from a doped fiber during heating. Fixed lengths of doped silica fiber are spliced to different lengths of undoped fiber and then evaporated by feeding into the focus of a CO2 laser. During evaporation, erbium ions are precipitated in the doped silica fiber to control the erbium concentration in the remaining SiO2, which is melted into a microsphere. By increasing the length of the undoped section, a critical point is reached where effectively no ions remain in the glass microsphere. The critical point is found using the spectra of the whispering gallery modes in microspheres with equal sizes. From the critical point, it is estimated that, for a given CO2 laser power, 6.36 × 10-21 mol of Er3+ is lost during the evaporation process for every cubic micron of silica fiber. This is equivalent to 1.74 × 10-7 mol of Er3+ lost per mol of SiO2 evaporated. This result facilitates the control of the doping concentration in WGLs and provides insight into the kinetics of laser-induced evaporation of doped silica.

Determination of Enantiomeric Excess by Optofluidic Microlaser near Exceptional Point.

Enantiomeric excess (ee) is an essential indicator of chiral drug purification in the pharmaceutical industry. However, to date the ee determination of unknown concentration enantiomers generally involves two separate techniques for chirality and concentration measurement. Here, a whispering-gallery mode (WGM) based optofluidic microlaser near exceptional point to achieve the ee determination under unknown concentration with a single technique is proposed. Exceptional point induces the unidirectional WGM lasing, providing the optofluidic microlaser with the novel capability to measure chirality by polarization, in addition to wavelength-based concentration detection. The dual-parameters detection of optofluidic microlaser empowers it to achieve ee determination of various unknown enantiomers without additional concentration measurements, a feat that is challenging to accomplish with other methods. Featuring the sensitivity enhancement and miniature structure of the WGM sensors, the obtained chiroptical response of the present approach is ≈30-fold higher than that of the conventional optical rotation-based polarimeter, and the reagent consumption is reduced by three orders of magnitude.

Blue micro-lasing from dye-doped hollow polymer optical fiber with an Ag nanoparticle-doped microring

We report the modified and improved whispering gallery mode laser emission from a hollow polymer optical fiber doped with 1,4-Bis(2-methyl styryl) benzene (Bis-MSB), a high-quantum yield dye with a blue emission, with the introduction of an Ag nanoparticle-doped microring. In the presence of the Ag-doped microring, the laser emission was enhanced, and a reduction in the laser threshold from 40 to 15 µJ was attained, along with a small redshift in the emission spectrum. The quality factor of the microring embedded hollow polymer optical fiber was found to be 1.3×103. Moreover, the introduction of an Ag-doped microring inside the hollow polymer optical fiber helps enhance certain lasing modes. The observations open the way for the development of low-threshold polymer optical fiber lasers in the blue region.

Two wavelength band emission WGM lasers via photo-isomerization

Abstract Wavelength switchable microcavity is indispensable component for various integrated photonic devices. However, achieving two wavelength band emission of the whispering gallery mode (WGM) laser is challenging. Here, we propose a strategy to realize two wavelength band emission WGM lasers activated by photo-isomerization based on excited-state intramolecular proton transfer (ESIPT) process in isolated/coupled polymer microfiber cavities. The WGM microcavity is built by highly polarized organic intramolecular charge-transfer (ICT) dye molecules. The two cooperative gain states of ICT dye molecules can be controlled by optimizing energy levels. Thereby, the lasing wavelength can be reversibly switched under photo-isomerization activated in the ESIPT energy-level progress. The photonic bar code can be generated by following the strategy of proposed design. This work provides a promising route to achieve switchable WGM laser in on-chip photonic integration.

Switchable whispering gallery mode lasing via phase transition.

Combining phase-transition materials with optical microcavities may advance the applications of whispering-gallery mode (WGM) lasing in performance customization, sensing, and optical switching. In this study, switchable WGM lasing based on phase transition is reported. The device is designed by introducing the phase-transition hydrogel into the capillary microcavity. After approaching the phase-transition point in hydrogel, the number of WGM lasing modes decreases sharply with a significant blueshift in the wavelength. The phenomenon is caused by the increase in light scattering and decrease in effective refractive index of the device. Furthermore, single-mode lasing is obtained by manipulating the phase transition, which exhibits superior reversibility. This study may pave the way for designing and multifunctioning of novel WGM lasing in photonic devices.

Low-threshold InGaN-based whispering gallery mode laser with lateral nanoporous distributed Bragg reflector

Low-threshold InGaN-based whispering gallery mode laser with lateral nanoporous distributed Bragg reflector

Excitonic processes and lasing in ZnO thin films and micro/nanostructures

Low dimensional ZnO-based materials have drawn much attention for the past few decades due to their unique electronic and optical properties and potential applications in optoelectronic devices. In this Tutorial, we will cover the past and the latest developments in ZnO thin films and micro/nanostructures in terms of excitonic and related lasing processes. First, we give a brief overview of structural and band properties of ZnO along with the linear optical and excitonic properties. Second, we introduce a feedback mechanism for lasing in various forms of ZnO, ranging from nanoparticles to nanowires, nanodisks, and thin films. As for the feedback mechanism, detailed descriptions are given to random lasing, Fabry–Pérot lasing, and whispering gallery mode lasing. Third, we discuss possible gain mechanisms, i.e., excitonic gain and electron–hole plasma (EHP) gain, in ZnO. A special interest is also devoted to the Mott carrier density, which is a crucial parameter to distinguish between excitonic and EHP contributions to lasing. Lastly, recent developments on exciton–polariton lasers based on ZnO microcavities are introduced.

Molecular and Supramolecular Designs of Organic/Polymeric Micro-photoemitters for Advanced Optical and Laser Applications.

ConspectusFor optical and electronic applications of supramolecular assemblies, control of the hierarchical structure from nano- to micro- and millimeter scale is crucial. Supramolecular chemistry controls intermolecular interactions to build up molecular components with sizes ranging from several to several hundreds of nanometers using bottom-up self-assembly process. However, extending the supramolecular approach up to a scale of several tens of micrometers to construct objects with precisely controlled size, morphology, and orientation is challenging. Especially for microphotonics applications such as optical resonators and lasers, integrated optical devices, and sensors, a precise design of a micrometer-scale object is required. In this Account, we review the recent progress on precise control of microstructures from π-conjugated organic molecules and polymers, which work as micro-photoemitters and are suitable for optical applications.After the introduction on the importance of the control of the hierarchical structures from molecular assembly, we review supramolecular methodology for assembling molecules and supramolecules to form microstructures such as spheres and polygons with precisely controlled morphology and molecular orientations. The resultant microstructures act as anisotropic emitters of circularly polarized luminescence. We report that synchronous crystallization of π-conjugated chiral cyclophanes forms concave hexagonal pyramidal microcrystals with homogeneous size, morphology, and orientation, which clearly paves the way for the precise control of skeletal crystallization under kinetic control. Furthermore, we show microcavity functions of the self-assembled micro-objects. The self-assembled π-conjugated polymer microspheres work as whispering gallery mode (WGM) optical resonators, where the photoluminescence exhibits sharp and periodic emission lines. The spherical resonators with molecular functions act as long-distance photon energy transporters, convertors, and full-color microlasers. Fabrication of microarrays with photoswitchable WGM microresonators by the surface self-assembly technique realizes optical memory with physically unclonable functions of WGM fingerprints. All-optical logic operations are demonstrated by arranging the WGM microresonators on synthetic and natural optical fibers, where the photoswitchable WGM microresonators act as a gate for light propagation via a cavity-mediated energy transfer cascade. Meanwhile, the sharp WGM emission line is appropriate for utilization as optical sensors for monitoring the mode shift and mode splitting. The resonant peaks sensitively respond to humidity change, absorption of volatile organic compounds, microairflow, and polymer decomposition by utilizing structurally flexible polymers, microporous polymers, nonvolatile liquid droplets, and natural biopolymers as media of the resonators. We further construct microcrystals from π-conjugated molecules with rods and rhombic plates, which act as WGM laser resonators with light-harvesting function. Our developments, precise design and control of organic/polymeric microstructures, form a bridge between nanometer-scale supramolecular chemistry and bulk materials and pave the way toward flexible micro-optics applications.

Whispering gallery mode micro-lasing in CsPbI3 quantum dots coated on TiO2 microspherical resonating cavities.

Whispering gallery mode (WGM) lasing in CsPbI3 quantum dots (QDs) coated on TiO2 spherical microcavities is demonstrated. The photoluminescence emission from a CsPbI3-QDs gain medium strongly couples with a TiO2 microspherical resonating optical cavity. Spontaneous emission in these microcavities switches to a stimulated emission above a distinct threshold point of 708.7 W/cm2. Lasing intensity increases three to four times as the power density increases by one order of magnitude beyond the threshold point when the microcavities are excited with a 632-nm laser. WGM microlasing with quality factors as high as Q∼1195 is demonstrated at room temperature. Quality factors are found to be higher for smaller TiO2 microcavities (∼2 µm). CsPbI3-QDs/TiO2 microcavities are also found to be photostable even after continuous laser excitation for 75 minutes. The CsPbI3-QDs/TiO2 microspheres are promising as WGM-based tunable microlasers.

Excitonic Mechanisms of Stimulated Emission in Low-Threshold ZnO Microrod Lasers with Whispering Gallery Modes.

Whispering gallery mode (WGM) ZnO microlasers gain attention due to their high Q-factors and ability to provide low-threshold near-UV lasing. However, a detailed understanding of the optical gain mechanisms in such structures has not yet been achieved. In this work, we study the mechanisms of stimulated emission (SE) in hexagonal ZnO microrods, demonstrating high-performance WGM lasing with thresholds down to 10-20 kW/cm2 and Q-factors up to ~3500. The observed SE with a maximum in the range of 3.11-3.17 eV at room temperature exhibits a characteristic redshift upon increasing photoexcitation intensity, which is often attributed to direct recombination in the inverted electron-hole plasma (EHP). We show that the main contribution to room-temperature SE in the microrods studied, at least for near-threshold excitation intensities, is made by inelastic exciton-electron scattering rather than EHP. The shape and perfection of crystals play an important role in the excitation of this emission. At lower temperatures, two competing gain mechanisms take place: exciton-electron scattering and two-phonon assisted exciton recombination. The latter forms emission with a maximum in the region near ~3.17 eV at room temperature without a significant spectral shift, which was observed only from weakly faceted ZnO microcrystals in this study.

Amplification of whispering gallery microlaser emission using dye-doped graded-index polymer optical fiber

PMMA-based Rhodamine-640 perchlorate doped microring embedded hollow polymer optical fiber (MEHPOF), as well as graded-index polymer optical fiber (GIPOF), were fabricated. At a threshold pump pulse energy (PPE) of 0.23 mJ, a whispering gallery mode lasing was observed from the MEHPOF. The quality factor and free spectral range of the fabricated microcavity were found to be 3.69×103 and 0.25 nm respectively. The lasing modes from the MEHPOF were amplified using a GI fiber doped with the same gain material. The maximum gain achieved from the experiment is 17.5 dB. The optimum length of the amplifier fiber for maximum gain was found to be 8 cm.

Whispering Gallery Mode Lasing Performance’s Evolution of Floating GaN Microdisks Varying with Their Thickness

Optical gain and loss of microcavity greatly affect the quality of lasing, how to improve optical gain and decrease optical loss is of great significance for the preparation of laser. In this study, four types standard microdisks with different thicknesses of 2.2 μm, 1.9 μm, 1.7 μm, and 1.45 μm were fabricated by micromachining technology process to modulate optical gain and loss of microdisk lasing. The whispering gallery mode lasing in the ultraviolet range of GaN microdisk devices was investigated for these devices in order to clarify the effect of microdisk thickness on device characteristics. The quality factor Q and lasing mode number for different thicknesses are calculated from the stimulated spectra. The lifetimes of the exciton combination properties of the devices were observed using time-resolved PL spectroscopy. The lasing modes are modulated, and the lifetime decreases, while the Q factor of the devices first increases and then decreases with decreasing thickness. All these results are induced by optical gain and loss competition.

Glassy behavior of modes in lasing systems with varying openness: random and whispering gallery mode lasers

We elucidate lasing characteristics, intensity statistics, and glassy behavior of lasing systems with varying degrees of openness and intrinsic disorder. In this pursuit, we consider three distinct dye-doped polymer based lasing systems, i.e., a planar waveguide random laser (PWRL) and two different geometries of whispering gallery mode resonators (WGMRs), viz., spherical and bottle microresonators. In WGMRs, improved optical confinement enhances the quality factor of resonances and provides a low threshold and fast saturation of the number of lasing modes with pump strength. Additionally, covariance across the lasing mode intensities reveals the presence of strong mode competition. In PWRL, the statistical study reveals bimodal probability distribution (PD) of modal intensity fluctuations near-above threshold with their gradual transition to heavy tail PD far above the threshold, while the PDs for WGMRs were found to remain unimodal throughout the pumped regimes with transition to positively skewed unimodal distribution above the threshold. We also report correspondence between the onset of bimodal PD and the spin glass phase near-above threshold in PWRL. Such observations are absent for WGMRs, wherein the spin glass behavior at and above the threshold are relatively suppressed. The non-zero value of the spectral intensity fluctuation overlap parameter in WGMRs manifests the inherent disorder held by the system. We also discuss the requirement of strong mode coupling achieved by virtue of openness for the realization of glassy behavior in lasing systems.

Narrow linewidth CsPbBr3 perovskite quantum dots microsphere lasers

The whispering gallery mode (WGM) microcavity has a high-quality factor and small mode volume, for the property of confining photons in a small volume for a long time, which enhances the light-matter interaction. Combining with quantum dots that act as gain medium, it can realize low-threshold and narrow-linewidth microlasers with excellent performance. However, achieving conventional whispering gallery mode lasers still requires complex preparation conditions. Here, we report a narrow linewidth whispering gallery mode lasing from all-inorganic perovskite quantum dots (CsPbBr3 QDs), illustrating facile and cost-effective properties. CsPbBr3 QDs with excellent properties were synthesized by hot injection method and used as gain media for lasers, and microspheres as optical resonators. The whispering gallery mode laser with a quality factor Q of 4791 and a narrow linewidth of 0.11 nm was obtained at room temperature. The WGM microlaser achieved by this paper could be a great application prospects in biotechnology.

Scattering-Assisted and Logic-Controllable WGM Laser in Liquid Crystal Micropillar

Whispering gallery mode (WGM) microcavities can efficiently store and manipulate light with strong light confinement and long photon lifetime, while coupling light into and from WGMs is intrinsically hindered by their unique feature of rotational symmetry. Here, a scattering-assisted liquid crystal (LC) micropillar WGM laser is proposed. WGM lasing at the surface of the micropillar is obviously enhanced by fluorescence scattering in the core of the micropillar. Besides, weak scattering of LC molecules also builds efficient coupling channels between the laser modes and the axial transmission modes of the micropillar-based waveguide, providing an all-in-one liquid WGM laser with functions of self-seeding and self-guiding. Furthermore, based on the hysteresis characteristics of the electrically anchored LC molecules under the interaction of thermal force, an erasable read-write liquid memory device is proposed, paving the way for the application of logic-controllable WGM lasers in optical storage and optical control.

III–V Semiconductor Whispering-Gallery Mode Micro-Cavity Lasers: Advances and Prospects

III-V semiconductor lasers are ubiquitous in modern optoelectronic devices, with applications ranging from telecommunication to general lighting. Among the different kinds of laser cavity designs, whispering-gallery mode (WGM) micro-cavity lasers boast outstanding optical performance due to advantages such as ultra-high Q-factor, compact mode volume, and narrow emission linewidth. Over the past decades, research in III-V WGM micro-cavity lasers has progressed rapidly in various aspects, including the fabrication techniques, emission outcoupling methods, and practical applications. In this paper, a comprehensive review is performed on the advances in these aspects. Although III-V WGM lasers are conventionally fabricated with top-down approaches, recent reports have demonstrated the potential of highly-scalable bottom-up methods in fabricating low-loss WGM lasers. Despite the strong optical confinement in III-V WGM cavities, various techniques have been developed to either outcouple WGM emission into waveguides or direct it into free-space with small beam divergence. Finally, recent developments in different applications of III-V WGM micro-cavity lasers are discussed. Other than serving as integrated photonic components, III-V WGM lasers have also displayed exciting potential in other applications such as label-free sensing and the study of cavity quantum electrodynamics (cQED).

Microfluidic flowmeter based on tunable polymer WGM laser

We demonstrate a microfluidic flowmeter based on a polymer whispering gallery mode (WGM) resonant microcavity. The microcavity is fabricated by coating the dye-doped polymer on the capillary tube. The flowmeter measures the flow velocity based on the temperature variation by the thermo-optic effect. The refractive index of the microcavity changes when the temperature decreases, which is caused by air flow cooling down the microcavity. Therefore, the flow of the fluid can be estimated through calculating the wavelength shift of the resonant microcavity. We achieve a sensitivity of 17 p.m./sccm, and the dynamic range up to 50 sccm. This work will provide a platform for microfluidic flowmeter and miniaturized lasers sensing. The flowmeter measures the flow velocity based on the temperature variation by the thermo-optic effect. The refractive index of the microcavity changes when the temperature decreases, which is caused by air flow cooling down the microcavity. Therefore, the flow of the fluid can be estimated through calculating the wavelength shift of the resonant microcavity. • The flowmeter measures the flow velocity based on the thermo-optic effect. • The changes of fluid flow can be read out optically using a microfluidic flowmeter through calculating the wavelength shift in the resonant microcavity.

Single-Mode Emission in InP Microdisks on Si Using Au Antenna.

An important building block for on-chip photonic applications is a scaled emitter. Whispering gallery mode cavities based on III–Vs on Si allow for small device footprints and lasing with low thresholds. However, multimodal emission and wavelength stability over a wider range of temperature can be challenging. Here, we explore the use of Au nanorod antennae on InP whispering gallery mode lasers on Si for single-mode emission. We show that by proper choice of the antenna size and positioning, we can suppress the side modes of a cavity and achieve single-mode emission over a wide excitation range. We establish emission trends by varying the size of the antenna and show that the far-field radiation pattern differs significantly for devices with and without antenna. Furthermore, the antenna-induced single-mode emission is dominant from room temperature (300 K) down to 200 K, whereas the cavity without an antenna is multimodal and its dominant emission wavelength is highly temperature-dependent.