Whispering Gallery Mode Microlasers Research Articles

GaN microdisks with a single porous optical confinement layer for whispering gallery mode lasing

This paper details the fabrication of GaN-based microdisks with a single porous n-GaN layer positioned beneath the multiple quantum wells region on a modified green light-emitting diode epiwafer. Simulations of the longitudinal light field distribution reveal effective confinement of the light field within the multiple quantum wells region due to the presence of the single porous layer. The porous layer also demonstrates sufficient conductivity as determined through calculations and serves as an effective method for thermal dispersion. Under optical pumping, all microdisks exhibit clear whispering gallery mode (WGM) lasing at room temperature, with the lowest threshold of 13.50 μJ/cm2 achieved in a 2-μm-diameter microdisk. These findings suggest that integrating the single porous layer into GaN microdisks is a highly promising approach for achieving high-efficiency WGM micro-laser diodes with effective electrical injection and heat dispersion.

Light sheet microscope scanning of biointegrated microlasers for localized refractive index sensing

Whispering gallery mode (WGM) microlasers are highly sensitive to localized refractive index changes allowing to link their emission spectrum to various chemical, mechanical, or physical stimuli. Microlasers recently found applications in biological studies within single cells, in three-dimensional samples such as multicellular spheroids, or in vivo. However, detailed studies of biological samples also need to account for the structural heterogeneity of tissues and live animals, therefore requiring a combination of high-resolution microscopy and laser spectroscopy. Here, we design and construct a light sheet fluorescence microscope with a coupled spectrometer for use in microlaser studies for combined high-resolution, high-speed imaging and WGM spectral analysis. The light sheet illumination profile and the decoupled geometry of excitation and emission hereby directly affect the lasing and sensing properties, mainly through geometric constraints and by light coupling effects. We demonstrate the basic working principle of microlaser spectroscopy under light sheet excitation and measure the absolute refractive index within agarose and in zebrafish tail muscle tissue. We further analyze the light coupling conditions that lead to the occurrence of two separate oscillation planes. These so-called cross modes can be scanned around the entire microlaser surface, which allows to estimate a surface-averaged refractive index profile of the microlaser environment.

Whispering-Gallery Mode Optoplasmonic Microcavities: From Advanced Single-Molecule Sensors and Microlasers to Applications in Synthetic Biology.

Optical microcavities, specifically, whispering-gallery mode (WGM) microcavities, with their remarkable sensitivity to environmental changes, have been extensively employed as biosensors, enabling the detection of a wide range of biomolecules and nanoparticles. To push the limits of detection down to the most sensitive single-molecule level, plasmonic nanorods are strategically introduced to enhance the evanescent fields of WGM microcavities. This advancement of optoplasmonic WGM sensors allows for the detection of single molecules of a protein, conformational changes, and even atomic ions, marking significant contributions in single-molecule sensing. This Perspective discusses the exciting research prospects in optoplasmonic WGM sensing of single molecules, including the study of enzyme thermodynamics and kinetics, the emergence of thermo-optoplasmonic sensing, the ultrasensitive single-molecule sensing on WGM microlasers, and applications in synthetic biology.

Miniaturized Laser Probe for Exosome-Based Cancer Liquid Biopsy.

Exosomes have been established as a valuable tool for clinical applications for the purpose of liquid biopsy and therapy. However, the clinical practice of exosomes as cancer biopsy markers is still to a very low extent. Active mode optical microcavity with microlaser emission has aroused as a versatile approach for chemical and biological sensing due to its benefits of larger photon population, increased effective Q-factor, decreased line width, and improved sensitivity. Herein, we report a label-free and precise quantification of exosome vesicles and surface protein profiling of breast cancer exosomes using functionalized active whispering gallery mode (WGM) microlaser probes. A detection limit of 40 exosomes per microresonator was achieved. The proposed system enabled a pilot assay of quantitative exosome analysis in cancer patients' blood with only a few microliters of sample consumption, holding good potential for large-scale cancer liquid biopsy. Multiplexed functionalization of the optical microresonator allowed us to profile cancer exosomal surface markers and distinct subclasses of breast cancer-associated exosomes and monitor drug treatment outcomes. Our findings speak volumes about the advantages of the WGM microlaser sensor, including very small sample consumption, low detection limit, high specificity, and ease of operation, offering a promising means for precious clinical sample analysis.

Free space whispering gallery mode microlasers as highly sensitive biosensors

High-precision biosensors for single or few molecules detection play a central role in numerous key fields, such as environmental monitoring and healthcare for early-stage disease diagnosis. In the last decade, laser biosensors have been investigated as proofs of concept, and several technologies have been proposed. Here we propose a demonstration of polymeric whispering gallery microlasers as biosensors for detecting proteins at low concentrations. Free space microlasers have the great advantage of working without any need for waveguiding for input excitation or output signal detection. The photonic microsensors can be easily patterned on microscope slides and operate in air and solution. We could detect down to 400 pg of protein without specific binding, and few tens of pg/mL with specific binding.

Plasmon‐Coupled GaN Microcavity for WGM Lasing and Label‐Free SERS Sensing of Biofluids

AbstractA signal amplification strategy is always required to improve sensing performance, especially for mass‐prepared, miniature and highly sensitive testing equipment. Herein, an on‐chip integrated biosensor combining microcavity lasing and surface enhanced Raman scattering (SERS) for label‐free biochemical analysis is developed. The gallium nitride (GaN) microrings array is fabricated as a whispering gallery mode microlaser based on totally internal wall reflection, and gold nanoparticles (AuNPs) are further assembled on the cavity surface to confine the optical field synergistically. The improvement of lasing capability indicates the strong light‐matter interaction that is conducive to superior response of biomolecules. As a functional example, a GaN/AuNPs substrate is employed in a urine assay, which avoids the shortcomings of requiring specific reactive reagents on different display modules. Ultimately, the chip not only realizes a wide range of pH (3.6–7.8) identification depending on the ultra‐sensitivity feedback of lasing to the refractive index of liquids, but also significantly enhances SERS signals to enable real‐time determination of the biomolecules in human urine. Even the lowest levels of creatinine (0.02%) can be quantitatively detected in less than a minute; thus this work with dual‐mode spectral analyses realizes rapid diagnosis for urinary systems, as well as provides a more accurate reference for the screening of other diseases.

Er3+ doped tellurite whispering gallery mode microlasers in 1.5 μm–1.61 μm wavelength region generated by 0.98 μm and 1.48 μm pump lasers

In this paper, we demonstrate Er3+ doped tellurite single and multi-mode microlasers in 1.5 μm–1.6 μm wavelength range fabricated via the plasma torch method. It is a simple and cost-effective method to produce microspheres with diameter ranges from 11 to 88 μm. Single mode laser output were observed with 0.98 μm and 1.48 μm pump, respectively. In addition, the quality factor of the fabricated microsphere is measured.

Blue perovskite single-mode lasing in a rubidium lead bromide microcubic cavity

Lead halide perovskite microlasers have shown impressive performance in the green and red wavebands. However, there has been limited progress in achieving blue-emitting perovskite microlasers. Here, blue-emitting perovskite-phase rubidium lead bromide (RbPbBr3) microcubes were successfully prepared by using a one-step chemical vapor deposition process, which can be utilized to construct optically pumped whispering gallery mode microlasers. By regulating the growth temperature, we found that a high-temperature environment can facilitate the formation of the perovskite phase and microcubic morphology of RbPbBr3. Notably, blue single-mode lasing in a RbPbBr3 microcubic cavity with a narrow linewidth of 0.21 nm and a high-quality factor (∼2200) was achieved. The obtained lasing from RbPbBr3 microlasers also exhibited an excellent polarization state factor (∼0.77). By modulating the mixed-monovalent cation composition, the wavelength of the microlaser could be tuned from green (536 nm) to pure blue (468 nm). Additionally, the heat stability of the mix-cation perovskite was better than that of conventional CsPbBr3. The stable and high-performance blue single-mode microlasers may thus facilitate the application of perovskite lasers in blue laser fields.

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.

Magnetic nanoparticles embedded in microlasers for controlled transport in different sensing media

In recent years, whispering gallery mode microlasers have attracted tremendous interest in sensing due to their ultra-high sensitivity at atomic levels. However, due to the non-magnetic properties, it is difficult to locate the microlasers at hard-to-reach positions, thus, limiting their sensing potential in many in-vitro and in-vivo applications. In this work, we report magnetic microlasers fabricated by encapsulating Ni0.2Zn0.8Fe2O4 magnetic nanoparticles (MNPs) within their cavity made of bovine serum albumin. The presence of MNPs allows the transportable actuation of the magnetic microlasers while maintaining lasing emission characteristics. Microlasers with various concentrations of MNPs are investigated to identify the optimum concentration that can balance a good magnetization, a low lasing threshold, and a high quality (Q) factor. These magnetic microlasers can be employed for sensing applications where sensors need to be navigated through different sensing media. As a proof of concept, we observed a clear shift of lasing wavelength of a magnetic microlaser while dragging it through different adjacent media by magnetic navigation. This result demonstrates the potential applications of magnetic microlasers for future biological and chemical applications.

Research Progress of Gallium Nitride Microdisk Cavity Laser

Whispering gallery mode (WGM) cavities provide resonance configurations for light propagation through internal reflection, achieving high Q factors, low thresholds, and small mode volumes. GaN-based materials exhibit high freedom in band engineering and are highly compatible with contemporary semiconductor processing technology. Recently, lasers from artificial GaN microdisks, obtained by combining the excellent material properties of GaN with the advantages of WGM, have attracted considerable research attention. These have a wide application scope in optical communication, display, and optoelectronic integration. In this review, we summarize the recent advances in GaN-based WGM microlasers, including the fabrication methods for GaN microcavities, observations of optical pumped GaN microdisk lasing, lasing mechanisms, comparison of Q factors, lasing modes, and threshold properties, commonly used light field control techniques, and mode clipping methods. Furthermore, we introduce the recent advances in electrically driven GaN-based laser diodes, followed by research challenges and strategies for promising applications, such as electrically pumped lasers and optoelectronic chip integration.

Microdroplet lasers and their applications

Bio-integrated lasers, that are lasers made of biological and biocompatible materials and implanted into cells and tissues, are gaining interest from the research community. Here we show how whispering gallery mode microlasers and microcavities made of solid beads or droplets can be used for sensing different processes in biological materials including inside cells. By making microcavities of a predefined size they can also be used to encode some information and for cell tracking. Sensing and tracking can be applied to highly scattering tissues.

On the Performance of Intracavity Gas Sensors Based on Whispering-Gallery Microlasers

We analytically investigate the performance of whispering gallery mode (WGM) spherical microlasers for gas sensing based on intra-cavity absorption. Using a simple model we estimate the response of a compact optical gas sensor based on these microlasers. We use detection of carbon dioxide by an Er:ZBLAN mid-IR microlaser with a relative intensity noise (RIN) of - 110 dB/Hz as an example, and show that using a commercial mid-IR photodetector, the limit of detection (LOD) for the resulting sensing system is about 5 ppm. We also investigate the impact of using a fiber optical amplifier on the LOD for different values of RIN. The numerical calculations and the presented approach can be applied to analyze the performance of any intracavity sensor based on WGM microlaser.

Tunable laser emissions in freestanding high-Q polymethylmethacrylate microbubbles

A freestanding dye-doped polymethylmethacrylate microbubble laser based on high-Q whispering gallery mode (WGM) is demonstrated. Low threshold tunable laser emissions are observed in microbubbles with different sizes fabricated by a volumetric pipette. The whispering gallery mode laser emissions in the 944 nm diameter microbubble have a low laser threshold of about 15 μJ pulse−1 and a 0.083 nm free spectral range. The smooth wall of the microbubble ensures a high quality factor of approximately 24 120. In addition, 590 nm–610 nm laser emission is achieved by the reabsorption effect of the dye. This work provides an efficient way to implement a freestanding,easily doped and low cost tunable WGM micro-laser.

Ultrasensitive optofluidic enzyme-linked immunosorbent assay by on-chip integrated polymer whispering-gallery-mode microlaser sensors.

Optical whispering-gallery-mode (WGM) microcavities offer great promise in ultrasensitive biosensors because of their unique ability to enable resonant recirculation of light to achieve strong light-matter interactions in microscale volumes. However, it remains a challenge to develop cost-effective, high-performance WGM microcavity-based biosensing devices for practical disease diagnosis applications. In this paper, we present an optofluidic chip that is integrated with directly-printed, high-quality-factor (Q) polymer WGM microlaser sensors for ultrasensitive enzyme-linked immunosorbent assay (ELISA). Optical 3D μ-printing technology based on maskless ultraviolet lithography is developed to rapidly fabricate high-Q suspended-disk WGM microcavities. After deposition with a thin layer of optical gain material, low-threshold WGM microlasers are fabricated and then integrated together with optical fibres upon a microfluidic chip to achieve an optofluidic device. With flexible microfluidic technology, on-chip, integrated, WGM microlasers are further modified in situ with biomolecules on surface for highly selective biomarker detection. It is demonstrated that such an optofluidic biochip can measure horseradish peroxidase (HRP)-streptavidin, which is a widely used catalytic molecule in ELISA, via chromogenic reaction at the concentration level of 0.3 ng mL-1. Moreover, it enables on-chip optofluidic ELISA of the disease biomarker vascular endothelial growth factor (VEGF) at the extremely low concentration level of 17.8 fg mL-1, which is over 2 orders of magnitude better than the ability of current commercial ELISA kits.

Femtosecond laser direct writing of perovskite patterns with whispering gallery mode lasing

Whispering gallery mode microlasers based on evaporated perovskite films were fabricated by femtosecond laser direct writing with ultra-low threshold.

Whispering gallery mode microlaser based on a single polymer fiber fabricated by electrospinning

Electrospinning is a very simple method of preparing polymer solutions into high performance fibers. In this work, PM597-doped polymer fibers with different diameters have been fabricated by electrospinning technology. Whispering gallery mode (WGM) lasing emission was observed when a 532 nm pulse laser beam radially excited the single electrospun polymer fiber (EPF) created by this method. The distribution of WGMs is also quantitatively confirmed through theoretical calculation. Additionally, the number of WGM laser peaks decrease as the diameter of the EPF decreases. Single longitudinal mode laser emission can be obtained when the EPF diameter decreases to 8.8 µm. It has also been found that the WGM laser from the single EPF has relatively low thresholds and good lifetime compared to droplet lasers or other fiber lasers. Moreover, EPF networks have unique research prospects in photonic devices, optical pump lasers and biosensing due to their very large surface to volume ratio.

Dynamical Analysis of Modal Coupling in Rare-Earth Whispering-Gallery-Mode Microlasers

We report on an experimental study of laser regime in erbium-doped whispering gallery mode (WGM) microspheres under modal-coupling between the co- and counter-propagating modes. The evidence of modal coupling has been observed in the relative intensity noise spectrum of several WGM lasers. Cross-correlation measurements have been carried out in order to determine precisely the emission regimes. It is shown that depending on the material constituting the WGM resonator, frequency locked bidirectional emission or self-modulated regime can be reached. The control of the laser emission regime of WGM micro-lasers is of great importance in the aim of applications in microwave optics or optical sensor miniaturization.

Flying particle microlaser and temperature sensor in hollow-core photonic crystal fiber.

Whispering-gallery mode (WGM) resonators combine small optical mode volumes with narrow resonance linewidths, making them exciting platforms for a variety of applications. Here we report a flying WGM microlaser, realized by optically trapping a dye-doped microparticle within a liquid-filled hollow-core photonic crystal fiber (HC-PCF) using a CW laser and then pumping it with a pulsed excitation laser whose wavelength matches the absorption band of the dye. The laser emits into core-guided modes that can be detected at the endfaces of the HC-PCF. Using radiation forces, the microlaser can be freely propelled along the HC-PCF over multi-centimeter distances-orders of magnitude farther than in previous experiments where tweezers and fiber traps were used. The system can be used to measure temperature with high spatial resolution, by exploiting the temperature-dependent frequency shift of the lasing modes, and may also permit precise delivery of light to remote locations.

Comparison of various excitation and detection schemes for dye-doped polymeric whispering gallery mode micro-lasers.

We compare different excitation and collection configurations based on free-space optics and evanescently coupled tapered fibers for both lasing and fluorescence emission from dye-doped doped polymeric whispering gallery mode (WGM) micro-disk lasers. The focus of the comparison is on the lasing threshold and efficiency of light collection. With the aid of optical fibers, we localize the pump energy to the cavity-mode volume and reduce the necessary pump energy to achieve lasing by two orders of magnitude. When using fibers for detection, the collection efficiency is enhanced by four orders of magnitude compared to a free-space read-out perpendicular to the resonator plane. By enhancing the collection efficiency we are able to record a pronounced modulation of the dye fluorescence under continuous wave (cw) pumping conditions evoked by coupling to the WGMs. Alternatively to fibers as a collection tool, we present a read-out technique based on the detection of in-plane radiated light. We show that this method is especially beneficial in an aqueous environment as well as for size-reduced micro-lasers where radiation is strongly pronounced. Furthermore, we show that this technique allows for the assignment of transverse electric (TE) and transverse magnetic (TM) polarization to the observed fundamental modes in a water environment by performing polarization-dependent photoluminescence (PL) spectroscopy. We emphasize the importance of the polarization determination for sensing applications and verify expected differences in the bulk refractive index sensitivity for TE and TM WGMs experimentally.