Lasing mode regulation and single-mode realization in ZnO whispering gallery microcavities by the Vernier effect

Magnetically controlled assembly: a new approach to organic integrated photonics

Hierarchical self-assembly of organic molecules or assemblies is of great importance for organic photonics to move from fundamental research to integrated and practical applications. Magnetic fields with the advantages of high controllability, non-contact manipulation, and instantaneous response have emerged as an elegant way to prepare organic hierarchical nanostructures. In this perspective, we outline the development history of organic photonic materials and highlight the importance of organic hierarchical nanostructures for a wide range of applications, including microlasers, optical displays, information encoding, sensing, and beyond. Then, we will discuss recent advances in magnetically controlled assembly for creating organic hierarchical nanostructures, with a particular focus on their potential for enabling the development of integrated photonic devices with unprecedented functionality and performance. Finally, we present several perspectives on the further development of magnetically controlled assembly strategies from the perspective of performance optimization and functional design of organic integrated photonics.

Single-mode lasing by tailoring the excitation of localized surface plasmon resonances to whispering gallery modes in a microring laser

Cavity mode manipulation in lasers is urgent for the stable single-mode operation of a microring laser. Here, we propose and experimentally demonstrate the plasmonic whispering gallery mode microring laser for strong coupling between local plasmonic resonances and whispering gallery modes (WGM) on the microring cavity to achieve pure single-mode lasing. The proposed structure is fabricated based on integrated photonics circuits consisting of gold nanoparticles deposited on a single microring. Additionally, our numerical simulation provides deep insight into the interaction between the gold nanoparticles and WGM modes. The manufacture of microlasers for the advancement of lab-on-a-chip devices and all-optical detection of ultra-low analysts may benefit from our findings.

Studies of the transition between amplified spontaneous emission and optical lasing in ultrahigh-Q polymeric micro-pedestals

In this work, we demonstrate the properties of Rhodamine B-doped polymeric cylindrical microlasers to perform either as gain amplification devices through amplified spontaneous emission (ASE) or as optical lasing gain devices. A study based on different %wt concentrations of microcavity families with distinct geometrical features demonstrates the characteristic dependence on either gain amplification phenomena. Principal component analysis (PCA) discriminates the relationship between the main ASE and lasing properties and the geometrical aspects of the cavity families. ASE and optical lasing thresholds were found, respectively, as low as 0.2 μJcm^-2 and 0.1 μJcm^-2 passing the best-reported microlaser performances in literature for cylindrical cavities, even in comparison with those based on 2D patterns. Moreover, our microlasers showed ultrahigh Q-factor of ∼3 × 10⁶, and for the first time, to the best of our knowledge, a visible emission comb constituted by above a hundred peaks at 40 μJcm^-2 with a registered free spectral range (FSR) of 0.25 nm corroborated through the whispery gallery mode (WGM) theory.

Dynamic manipulation of WGM lasing by tailoring the coupling strength

Miniaturized lasing with dynamic manipulation is critical to the performance of compact and versatile photonic devices. However, it is still a challenge to manipulate the whispering gallery mode lasing modes dynamically. Here, we design the quasi-three-dimensional coupled cavity by a micromanipulation technique. The coupled cavity consists of two intersection polymer microfibers. The mode selection mechanism is demonstrated experimentally and theoretically in the coupled microfiber cavity. Dynamic manipulation from multiple modes to single-mode lasing is achieved by controlling the coupling strength, which can be quantitatively controlled by changing the coupling angle or the coupling distance. Our work provides a flexible alternative for the lasing mode modulation in the on-chip photonic integration.

Dual-wavelength switchable single-mode lasing from a lanthanide-doped resonator

The development of multi-wavelength lasing, particularly with the wavelength tuning in a wide spectral range, is challenging but highly desirable for integrated photonic devices due to its dynamic switching functionality, high spectral purity and contrast. Here, we propose a general strategy, that relies on the simultaneous design on the electronic states and the optical states, to demonstrate dynamically switchable single-mode lasing spanning beyond the record range (300 nm). This is achieved through integrating the reversely designed nanocrystals with two size-mismatched coupled microcavities. We show an experimental validation of a crosstalk-free violet-to-red single-mode behavior through collective control of asymmetric excitation and excitation wavelength. The single-mode action persists for a wide power range, and presents significant enhancement when compared with that in the microdisk laser. These findings enlighten the reverse design of luminescent materials. Given the remarkable doping flexibility, our results may create new opportunities in a variety of frontier applications.

Tunable single-mode laser on thin film lithium niobate

The erbium-doped lithium niobate on insulator (LNOI) laser plays an important role in the complete photonic integrated circuits (PICs). Here, we demonstrate an integrated tunable whispering gallery single-mode laser (WGSML) by making use of a coupled microdisk and microring on LNOI. A 974 nm single-mode pump light can have an excellent resonance in the designed microdisk, which is beneficial to the whispering gallery mode (WGM) laser generation. The WGSML at 1560.40 nm with a maximum 31.4 dB side mode suppression ratio (SMSR) has been achieved. By regulating the temperature, the output power of the WGSML increases, and the central wavelength can be changed from 1560.30 to 1560.40 nm. Furthermore, 1560.60 and 1565.00 nm WGSMLs have been achieved by changing the coupling gap width between the microdisk and microring. We can also use the electro-optic effect of LNOI to obtain more accurate adjustable WGSMLs in further research.

Direction-Adjustable Single-Mode Lasing via Self-Assembly 3D-Curved Microcavities for Gas Sensing

Drop-based microcavity lasers emerged as a promising tool in modern physics investigation and chemical detection owing to their cost-effective fabrication, high luminescence, and sensitive molecule sensing. However, it is of great challenge to achieve highly directional emission along with high quality (Q) factors via traditional droplet self-assembly behavior of the gain medium on a planar substrate. In this work, a single-mode microcavity laser with directional far-field emission is first proposed via droplet self-assembly 3D-curved microcavities, and simultaneously, acetic acid (AcOH) gas sensing is realized. Trichromatic single-mode lasing in 3D-curved microcavities with distinct organic polymer droplets is constructed on silica fibers via a self-assembly procedure. By regulating the curvature of the substrate, mode selection and directional emission of the lasing action are realized. The measured Q-factor of the proposed anisotropic 3D-curved active microcavity is ∼20k. Furthermore, on account of the sensitive responsiveness of liquid organic polymers, single-mode laser sensors can be realized by measuring the shift of their lasing modes on exposure to organic vapor. Benefiting from chemical reaction with rhodamine 6G, the AcOH gas sensor displays a short response time. These results may open new insights into drop-based quasi-3D-anisotropic whispering-gallery-mode microcavities to improve the development of lab-in-a-droplet, ranging from a tuneable microcavity laser to a chemical gas sensor.

Single-Mode Lasing in Plasmonic-Enhanced Woven Microfibers for Multifunctional Sensing

Single-mode plasmonic lasing has great potential for use in photonic and sensing applications. In this work, single-mode lasing is realized using a plasmonic-enhanced woven microfiber that shows ultrahigh sensitivity to the ambient environment. This plasmonic-enhanced microfiber is fabricated by spraying Ag nanospheres onto rhodamine 6G-doped polymer microfibers. Single-mode laser emission with an ultranarrow linewidth (0.1 nm) and a low threshold (18.8 kW/mm²) is achieved in the microfiber using the effects of mode selection and plasmonic enhancement provided by the Ag nanospheres. A large wavelength shift in the single-mode lasing is observed when the proposed laser is used as a sensor and exposed to a humid or acidic environment. The wavelength shift is attributed to refractive index variations in the microfiber caused by either moisture absorption or chemical reactions. In humidity sensing, the laser's sensitivity is as high as 826.6 pm/% relative humidity (RH) and the detection limit is 0.051% RH. An innovative strategy for acetic acid gas sensing is proposed that uses the chemical reaction with rhodamine 6G, and its minimum response time is 5 min. Because of the microfiber's excellent fabric compatibility, a wearable sensor is fabricated by weaving the plasmonic-enhanced microfiber into clothes, and this sensor demonstrates extreme bending stability. The results reported here provide a novel approach to the design and fabrication of ultrasensitive wearable sensors for multifunctional sensing applications.

Full-color WGM lasing in nested microcavities

A full-color whispering-gallery mode (WGM) laser has been fabricated by partitioning different light-emitting polymers in a nested microcavity. Red-green-blue WGM lasing with a high quality factor above 104 and a narrow linewidth of 0.025 nm emits from nested capillaries when excited with a nanosecond laser. The full-color WGM lasing shows a low excitation threshold for the nested microcavities, which can avoid fluorescence resonant energy transfer. We also achieve wavelength tunable lasing upon altering the different polymers in the nested microcavities. The work demonstrates a simple method to fabricate a full-color WGM laser and its potential applications in compact lighting devices and white laser sources.

Ultralow Threshold, Single-Mode InGaAs/GaAs Multiquantum Disk Nanowire Lasers

We present single-mode nanowire (NW) lasers with an ultralow threshold in the near-infrared spectral range. To ensure the single-mode operation, the NW diameter and length are reduced specifically to minimize the longitudinal and transverse modes of the NW cavity. Increased optical losses and reduced gain volume by the dimension reduction are compensated by an excellent NW morphology and InGaAs/GaAs multiquantum disks. At 5 K, a threshold low as 1.6 μJ/cm² per pulse is achieved with a resulting quality factor exceeding 6400. By further passivating the NW with an AlGaAs shell to suppress surface nonradiative recombination, single-mode lasing operation is obtained with a threshold of only 48 μJ/cm² per pulse at room temperature with a high characteristic temperature of 223 K and power output of ∼0.9 μW. These single-mode, ultralow threshold, high power output NW lasers are promising for the development of near-infrared nanoscale coherent light sources for integrated photonic circuits, sensing, and spectroscopy.

Single-mode lasing of CsPbBr3 perovskite NWs enabled by the Vernier effect

Inorganic lead halide perovskite (CsPbX3, X = Cl, Br, I) NWs (NWs) have been employed in lasers due to their intriguing attributes of tunable wavelength, low threshold, superior stability, and easy preparation. However, current CsPbX3 NW lasers usually work in a multi-mode modal, impeding their practical applications in optical communication due to the associated false signaling. In this work, high-performance single-mode lasing has been demonstrated by designing and fabricating coupled cavities in the high-quality single-crystal CsPbBr3 NWs via the focused ion beam (FIB) milling approach. The single-mode laser shows a threshold of 20.1 μJ cm-2 and a high quality factor of ∼2800 profiting from the Vernier effect, as demonstrated by the experiments and finite-different time-domain (FDTD) simulations. These results demonstrate the promising potentials of the CsPbX3 NW lasers in optical communication and integrated optoelectronic devices.

Linearly polarized lasing based on coupled perovskite microspheres

The polarization of lasing, as a fundamental property related to the emission coherence of microlasers, is one of the criteria for a high quality laser beam but has been rarely investigated. Unlike conventional lasers which can be induced by highly polarized seed light or by using a polarizing film, the microlaser for on-chip integrated photonic circuits generally possesses a low degree of polarization due to unpolarized spontaneous emission. Here, we firstly demonstrate that the Vernier effect can be used to improve the degree of polarization from ∼0.2 to 0.78 based on metal halide perovskite microspheres. After coupling, linearly polarized single-mode lasing with a low threshold and high quality can be achieved. In addition, by using the finite element method, the mode distributions of CsPbBr₃ microspheres before and after coupling are analyzed systematically and a clear physical diagram of coupled resonances is obtained. Our work clearly suggests that a coupled linearly polarized single-mode microlaser by the Vernier effect would offer a real step closer to the platform for on-chip integrated photonics.

Controllable Growth of Aligned Monocrystalline CsPbBr3 Microwire Arrays for Piezoelectric-Induced Dynamic Modulation of Single-Mode Lasing

CsPbBr₃ shows great potential in laser applications due to its superior optoelectronic characteristics. The growth of CsPbBr₃ wire arrays with well-controlled sizes and locations is beneficial for cost-effective and largely scalable integration into on-chip devices. Besides, dynamic modulation of perovskite lasers is vital for practical applications. Here, monocrystalline CsPbBr₃ microwire (MW) arrays with tunable widths, lengths, and locations are successfully synthesized. These MWs could serve as high-quality whispering-gallery-mode lasers with high quality factors (>1500), low thresholds (<3 µJ cm^-2 ), and long stability (>2 h). An increase of the width results in an increase of the laser quality and the resonant mode number. The dynamic modulation of lasing modes is achieved by a piezoelectric polarization-induced refractive index change. Single-mode lasing can be obtained by applying strain to CsPbBr₃ MWs with widths between 2.3 and 3.5 µm, and the mode positions can be modulated dynamically up to ≈9 nm by changing the applied strain. Piezoelectric-induced dynamic modulation of single-mode lasing is convenient and repeatable. This method opens new horizons in understanding and utilizing the piezoelectric properties of lead halide perovskites in lasing applications and shows potential in other applications, such as on-chip strain sensing.

Controlled Outcoupling of Whispering-Gallery-Mode Lasers Based on Self-Assembled Organic Single-Crystalline Microrings

The outcoupling of whispering-gallery-mode (WGM) lasers is crucial for the realization of various photonic functionalities, yet present material structures still suffered the unexpected surface damages or contaminations in the multistep micro/nanofabrications. Here, we propose a strategy to achieve controlled outcoupling of WGM lasers in self-assembled organic single-crystalline microrings. The microrings with molecular-smooth surfaces functioned as organic crystalline whispering-gallery-mode microlasers with a lasing threshold of ∼14.2 μJ cm^-2 and a quality factor on the order of 10³ to 10⁴. The circular self-assembly allowed us to design different derived ring-based structures toward desired outcoupling of the WGM lasers, including unidirectional laser output from wire-ring coupled structures, and single-mode lasing in double-ring coupled systems. The results would provide an alternative avenue to construct versatile organic nanoscale lasers and related components with specific photonic applications.

Lasing-Mode Switch of a Hexagonal ZnO Pyramid Driven by Pressure within a Diamond Anvil Cell

Nanolasers are expected to be integrated on chips as miniaturized coherent light sources, and their application is strongly dependent on their lasing behavior. In this work, the lasing behavior of a single hexagonal ZnO pyramid (HZOP) is tailored by tuning the electronic bandgap with pressure. The lasing of the HZOP nanolaser is dominated by a helical whispering-gallery-like mode, and the lasing threshold varies little with increasing pressure. All lasing peaks of HZOP are limited in a spectral prescreen window on the right shoulder of the fluorescence emission and gradually blue-shift accompanied by several abrupt hops with increasing pressure. This feature of a spectral prescreen window originates from the strong coupling between excitons, and the coupling is described by a dispersive complex refractive index. These results provide a new perspective to tune and switch the lasing mode of a nanolaser with precision by the pressure-induced bandgap broadening of a semiconductor.

Configurable Integration of On-Chip Quantum Dot Lasers and Subwavelength Plasmonic Waveguides

The integration of on-chip dielectric lasers and subwavelength plasmonic waveguides has attracted enormous attention because of the combination of both the advantages of the high performances of the small dielectric lasers and the subwavelength plasmonic waveguides. However, the configurable integration is still a challenge owing to the complexity of the hybrid structures and the damageability of the gain media in the multistep micro/nanofabrications. By employing the dark-field optical imaging technique with a position uncertainty of about 21 nm and combining the high-resolution electron beam lithography, the small colloidal quantum dot (CQD) lasers without any damages are accurately aligned with the silver nanowires. As a result, the integration of the CQD lasers and the silver nanowires can be flexibly configured on chips. In the experiment, the tangential coupling, radial coupling, and complex coupling between the high-performance CQD lasers and the subwavelength silver nanowires are demonstrated. Because of the subwavelength field confinements of the silver nanowires, the deep-subwavelength coherent sources (multimode, one-color single-mode, or two-color single-mode) with a mode area of only 0.008λ² are output from these hybrid structures. This configurable on-chip integration with high flexibility and controllability will greatly facilitate the developments of the complex functional hybrid photonic-plasmonic circuits.

Reverse-bias-driven whispering gallery mode lasing from individual ZnO microwire/p-Si heterojunction

In this paper, electrically driven whispering gallery mode (WGM) lasing was observed from ZnO single microwire (SMW)/p-Si heterojunctions operated at reverse bias. Current-voltage curve exhibits a non-ideal rectification characteristic with a turn-on voltage of about 0.8 V. When the reverse current of 20 mA was applied, several sharp lasing peaks with FWHM as narrow as ∼2 nm appeared in the spectra, which demonstrated that the gain was now large enough to enable the cavity resonant in ZnO SMW. The resonant process, lasing mode and quality factor (Q) were investigated via experiments and theory. The observed discrete lasing peak positions effectively matched the simulated lasing modes. The carrier transport process and light emission mechanism in heterojunctions are also discussed by energy band theory and interface defect.

Single-Mode Lasers Based on Cesium Lead Halide Perovskite Submicron Spheres

Single-mode laser is realized in a cesium lead halide perovskite submicron sphere at room temperature. All-inorganic cesium lead halide (CsPbX₃, X = Cl, Br, I) microspheres with tunable sizes (0.2-10 μm) are first fabricated by a dual-source chemical vapor deposition method. Due to smooth surface and regular geometry structure of microspheres, whispering gallery resonant modes make a single-mode laser realized in a submicron sphere. Surprisingly, a single-mode laser with a very narrow line width (∼0.09 nm) was achieved successfully in the CsPbX₃ spherical cavity at low threshold (∼0.42 μJ cm^-2) with a high cavity quality factor (∼6100), which are the best specifications of lasing modes in all natural nano/microcavities ever reported. By modulating the halide composition and sizes of the microspheres, the wavelength of a single-mode laser can be continuously tuned from red to violet (425-715 nm). This work illustrates that the well-controlled synthesis of metal cesium lead halide perovskite nano/microspheres may offer an alternative route to produce a widely tunable and greatly miniaturized single-mode laser.