WGM lasing in irregular cavities with arbitrary boundaries

Because of its limited light field mode and high Q value, the whispering-gallery-mode (WGM) cavity has been widely studied. In this study, we propose a simple, rapid, low-cost and no-manufacturing technology method that we call the drip-coating method to obtain an irregular cavity with arbitrary boundaries. By using polyvinyl alcohol (PVA) solution doped with rhodamine 6G, the irregular cavity with arbitrary boundaries was drip-coated on a high-reflection mirror, forming a WGM laser. The sample was pumped with a 532 nm pulsed laser, and the single-mode WGM and multi-WGM lasing were obtained. All WGMs are the vertical oscillation modes, which originate from both the total internal reflection of the PVA/air interface and vertical reflection of the PVA/mirror interface. The other boundaries of the cavity were not involved in the reflection and could have any shape. The mechanism of producing single-mode lasing is due to the action of the loss-gain cavity. Multi-WGM lasing is attributed to at least two groups of different WGMs existing in an irregular cavity. This can be confirmed by using a microsphere model and intensity correlation method. These results may provide an alternative for the design of WGM lasers.

Laser Diode Pumped Polymer Lasers with Tunable Emission Based on Microfluidic Channels

Tunable whispering-gallery-mode (WGM) lasers have been paid lots of attention for their potential applications in the photonic field. Here, a tunable polymer WGM laser based on laser diode pumping is realized with a threshold of 0.43 MW/cm² per pulse. The WGM laser is realized by a microfluidic microcavity, which consists of a quartz capillary and gain materials. The laser performance keeps stable for a long time (3.5 h), pumped by a 50-ns 50 Hz laser diode with a pumping peak power density of 1.08 MW/cm² per pulse. The lasing wavelength can be tuned over 15 nm by changing the gain material concentration from 3.5 mg/mL to 12.5 mg/mL in the microfluidic channel. Moreover, the lasing mode can be switched between transverse magnetic (TM) and transverse electric (TE) modes by adjusting the pump polarization. These results provide the basis for designing nanophotonic devices with laser diode pumping.

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.

Self-Aligned Emission of Distributed Feedback Lasers on Optical Fiber Sidewall

This article assembles a distributed feedback (DFB) cavity on the sidewalls of the optical fiber by using very simple fabrication techniques including two-beam interference lithography and dip-coating. The DFB laser structure comprises graduated gratings on the optical fiber sidewalls which are covered with a layer of colloidal quantum dots. Directional DFB lasing is observed from the fiber facet due to the coupling effect between the grating and the optical fiber. The directional lasing from the optical fiber facet exhibits a small solid divergence angle as compared to the conventional laser. It can be attributed to the two-dimensional light confinement in the fiber waveguide. An analytical approach based on the Bragg condition and the coupled-wave theory was developed to explain the characteristics of the laser device. The intensity of the output coupled laser is tuned by the coupling coefficient, which is determined by the angle between the grating vector and the fiber axis. These results afford opportunities to integrate different DFB lasers on the same optical fiber sidewall, achieving multi-wavelength self-aligned DFB lasers for a directional emission. The proposed technique may provide an alternative to integrating DFB lasers for applications in networking, optical sensing, and power delivery.

Large-Area Biocompatible Random Laser for Wearable Applications

Recently, wearable sensor technology has drawn attention to many health-related appliances due to its varied existing optical, electrical, and mechanical applications. Similarly, we have designed a simple and cheap lift-off fabrication technique for the realization of large-area biocompatible random lasers to customize wearable sensors. A large-area random microcavity comprises a matrix element polymethyl methacrylate (PMMA) in which rhodamine B (RhB, which acts as a gain medium) and gold nanorods (Au NRs, which offer plasmonic feedback) are incorporated via a spin-coating technique. In regards to the respective random lasing device residing on a heterogenous film (area > 100 cm2), upon optical excitation, coherent random lasing with a narrow linewidth (~0.4 nm) at a low threshold (~23 μJ/cm2 per pulse) was successfully attained. Here, we maneuvered the mechanical flexibility of the device to modify the spacing between the feedback agents (Au NRs), which tuned the average wavelength from 612.6 to 624 nm under bending while being a recoverable process. Moreover, the flexible film can potentially be used on human skin such as the finger to serve as a motion and relative-humidity sensor. This work demonstrates a designable and simple method to fabricate a large-area biocompatible random laser for wearable sensing.

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.

Low-Threshold Microlasers Based on Holographic Dual-Gratings

Among the efforts to improve the performances of microlasers, optimization of the gain properties and cavity parameters of these lasers has attracted significant attention recently. Distributed feedback lasers, as one of the most promising candidate technologies for electrically pumped microlasers, can be combined with dual-gratings. This combination provides additional freedom for the design of the laser cavity. Here, a holographic dual-grating is designed to improve the distributed feedback laser performance. The holographic dual-grating laser consists of a colloidal quantum dot film with two parallel gratings, comprising first-order (210 nm) and second-order (420 nm) gratings that can be fabricated easily using a combination of spin coating and interference lithography. The feedback and the output from the cavity are controlled using the first-order grating and the second-order grating, respectively. Through careful design and analysis of the dual-grating, a balance is achieved between the feedback and the cavity output such that the lasing threshold based on the dual-grating is nearly half the threshold of conventional distributed feedback lasers. Additionally, the holographic dual-grating laser shows a high level of stability because of the high stability of the colloidal quantum dots against photobleaching.

Colloidal quantum dots lasing and coupling in 2D holographic photonic quasicrystals

Global research on the solution-processable colloidal quantum dots (CQDs) constitutes outstanding model systems in nanoscience, micro-lasers, and optoelectronic devices due to tunable color, low cost, and wet chemical processing. The two-dimensional (2D) CQDs quasicrystal lasers are more efficient in providing coherent lasing due to radiation feedback, high-quality-factor optical mode, and long-range rotational symmetry. Here, we have fabricated a 2D quasicrystal exhibiting 10-fold rotational symmetry by using a specially design pentagonal prism in the optical setup of a simple and low-cost holographic lithography. We developed a general analytical model based on the cavity coupling effect, which can be used to explain the underlying mechanism responsible for the multi-wavelength lasing in the fabricated 2D CQDs holographic photonic quasicrystal. The multi-wavelength surface-emitting lasers such as λ₀ = 629.27 nm, λ₁ = 629.85 nm, λ_-1 = 629.06 nm, λ₂ = 630.17 nm, and λ_-2 = 628.76 with a coupling constant κ = 0.38 achieved from the 2D holographic photonic quasicrystal are approximately similar with the developed analytical model based on cavity coupling effect. Moreover, the lasing patterns of the 2D CQDs photonic quasicrystal laser exhibit a symmetrical polarization effect by rotating the axis of polarization with a difference of 120⁰ angle in a round trip. We expect that our findings will provide a new approach to customize the 2D CQDs holographic photonic quasicrystal lasers in the field of optoelectronic devices and miniature lasing systems.

Single-Mode Lasing in Polymer Circular Gratings

In recent years, conjugated polymers have become the materials of choice to fabricate optoelectronic devices, owing to their properties of high absorbance, high quantum efficiency, and wide luminescence tuning ranges. The efficient feedback mechanism in the concentric ring resonator and its circularly symmetric periodic geometry combined with the broadband photoluminescence spectrum of the conjugated polymer can generate a highly coherent output beam. Here, the detailed design of the ultralow-threshold single-mode circular distributed feedback polymer laser is presented with combined fabrication processes such as electron beam lithography and the spin-coating technique. We observe from the extinction spectra of the circular gratings that the transverse electric mode shows no change with the increase of incident beam angle. The strong enhancement of the conjugated polymer photoluminescence spectra with the circular periodic resonator can reduce the lasing threshold about 19 µJ/cm2. A very thin polymer film of about 110 nm is achieved with the spin-coating technique. The thickness of the gain medium can support only the zero-order transverse electric lasing mode. We expect that such a low threshold lasing device can find application in optoelectronic devices.

Multifunctional Sensing Based on an Ultrathin Transferrable Microring Laser

An ultrathin-film microring laser was fabricated using inkjet printing and a simple lift-off technique. Whispering-gallery-mode lasing was observed under optically pumped conditions in the film. The freestanding laser can be transferred to arbitrary surfaces for multifunctional applications, such as acoustic and relative humidity sensing. Using the first eigenmode of a membrane vibration, an acoustic sensor with a 0.15 Pa limit of detection was demonstrated via laser bandwidth broadening. A relative humidity sensor with a 1.1% limit of detection via wavelength shifts was demonstrated by placing the device on an optical fiber facet. These cost-effective, transferrable, multifunctional laser sensors will have many additional applications.

Physical Investigations on Bias-Free, Photo-Induced Hall Sensors Based on Pt/GaAs and Pt/Si Schottky Junctions

Hall-effect in semiconductors has wide applications for magnetic field sensing. Yet, a standard Hall sensor retains two problems: its linearity is affected by the non-uniformity of the current distribution; the sensitivity is bias-dependent, with linearity decreasing with increasing bias current. In order to improve the performance, we here propose a novel structure which realizes bias-free, photo-induced Hall sensors. The system consists of a semi-transparent metal Pt and a semiconductor Si or GaAs to form a Schottky contact. We systematically compared the photo-induced Schottky behaviors and Hall effects without net current flowing, depending on various magnetic fields, light intensities and wavelengths of Pt/GaAs and Pt/Si junctions. The electrical characteristics of the Schottky photo-diodes were fitted to obtain the barrier height as a function of light intensity. We show that the open-circuit Hall voltage of Pt/GaAs junction is orders of magnitude lower than that of Pt/Si, and the barrier height of GaAs is smaller. It should be attributed to the surface states in GaAs which block the carrier drifting. This work not only realizes the physical investigations of photo-induced Hall effects in Pt/GaAs and Pt/Si Schottky junctions, but also opens a new pathway for bias-free magnetic sensing with high linearity and sensitivity comparing to commercial Hall-sensors.

Robust anomalous Hall effect and temperature-driven Lifshitz transition in Weyl semimetal Mn3Ge

Topological Weyl semimetals have attracted considerable interest because they manifest underlying physics and device potential in spintronics. Large anomalous Hall effect (AHE) in non-collinear antiferromagnets (AFMs) represents a striking Weyl phase, which is associated with Bloch-band topological features. In this work, we report robust AHE and Lifshitz transition in high-quality Weyl semimetal Mn3Ge thin film, comprising stacked Kagome lattice and chiral antiferromagnetism. We successfully achieved giant AHE in our Mn3Ge film, with a strong Berry curvature enhanced by the Weyl phase. The enormous coercive field HC in our AHE curve at 5 K reached an unprecedented 5.3 T among hexagonal Mn3X systems. Our results provide direct experimental evidence of an electronic topological transition in the chiral AFMs. The temperature was demonstrated to play an efficient role in tuning the carrier concentration, which could be quantitatively determined by the two-band model. The electronic band structure crosses the Fermi energy level and leads to the reversal of carrier type around 50 K. The results not only offer new functionality for effectively modulating the Fermi level location in topological Weyl semimetals but also present a promising route of manipulating the carrier concentration in antiferromagnetic spintronic devices.

Tunable WGM Laser Based on the Polymer Thermo-Optic Effect

In this work, the thermo-optic effect in polymers was used to realize a temperature-tunable whispering-gallery-mode laser. The laser was fabricated using a capillary tube filled with a light-emitting conjugated polymer solution via the capillary effect. In the whispering-gallery-mode laser emission wavelength can be continuously tuned to about 19.5 nm using thermo-optic effect of polymer. The influence of different organic solvents on the tuning rate was studied. For a typical lasing mode with a bandwidth of 0.08 nm, a temperature-resolved tuning rate of ~1.55 nm/°C was obtained. The two-ring coupling effect is responsible for the suppression of the WGM in the micro-cavity laser. The proposed laser exhibited good reversibility and repeatability as well as a sensitive response to temperature, which could be applied to the design of photothermic and sensing devices.

Dual-color plasmonic random lasers for speckle-free imaging

A dual-color plasmonic random laser under single-excitation is achieved in an ultrathin membrane doped with binary quantum dots and gold nanorods. The gold nanorods tune the luminescence lifetime and emission efficiency of quantum dots. Under single excitation, low-threshold random lasing is observed. Green random lasing at 547 nm is 'turned on' and red random lasing at 630 nm is greatly enhanced by the transversal and longitudinal surface plasmon resonance of the gold nanorods, respectively. Speckle-free color imaging is achieved by using the proposed dual-color random laser source. These properties would facilitate the development of random lasers in fields of illumination and imaging.

Genome-wide identification and characterization of microRNAs responding to ABA and GA in maize embryos during seed germination

MicroRNAs (miRNAs) are an important class of non-coding small RNAs that regulate the expression of target genes through mRNA cleavage or translational inhibition. Previous studies have revealed their roles in regulating seed dormancy and germination in model plants such as Arabidopsis thaliana, rice (Oryza sativa) and maize (Zea mays). However, the miRNA response to exogenous gibberellic acid (GA) and abscisic acid (ABA) during seed germination in maize has yet to be explored. In this study, small RNA libraries were generated and sequenced from maize embryos treated with GA, ABA or double-distilled water as control. A total of 247 miRNAs (104 known and 143 novel) were identified, of which 45 known and 53 novel miRNAs were differentially expressed in embryos in the different treatment groups. In total, 74 (37 up-regulated and 37 down-regulated) and 55 (23 up-regulated and 32 down-regulated) miRNAs were expressed in response to GA and to ABA, respectively, and a total of 18 known and 38 novel miRNAs displayed differential expression between the GA- and ABA-treated groups. Using bioinformatics tools, we predicted the target genes of the differentially expressed miRNAs. Using GO enrichment and KEGG pathway analysis of these targets, we showed that miRNAs differentially expressed in our samples affect genes encoding proteins involved in the peroxisome, ribosome and plant hormonal signalling pathways. Our results indicate that miRNA-mediated gene expression influences the GA and ABA signalling pathways during seed germination.

Low-cost biosensors based on a plasmonic random laser on fiber facet

Low-cost and miniaturized biosensors are key factors leading to the possibility of portable and integrated biomedical system, which play an important role in clinical medicine and life sciences. Random lasers with simple structures provide opportunities for detecting biomolecules. Here, low-cost biosensors on fiber facet for label-free detecting biomolecules are demonstrated based on a plasmonic random laser. The random laser is achieved resorting to a self-assembled plasmonic scattering structure of Ag nanoparticles and polymer film on fiber facet. Refractive index sensitivity and near-surface sensitivity of the biosensor are systematically studied. Furthermore, the biosensor is used to detect IgG through specific binding to protein A, exhibiting the detecting limit of 0.68 nM. It is believed that this work may promote the applications of a plasmonic random laser bio-probe in portable or integrated medical diagnostic platforms, and provide fundamental understanding for the life science.

Tailoring Whispering Gallery Lasing and Random Lasing in A Compound Cavity

A compound cavity was proposed to achieve both whispering gallery mode (WGM) lasing and random lasing. The WGM-random compound cavity consisted of a random structure with an annular boundary, which was fabricated by a method combining both inkjet printing and metal-assisted chemical etching methods. An ultrathin polymer membrane was attached to the WGM-random compound cavity, forming a polymer laser device. A transformation from WGM lasing to random lasing was observed under optical pumping conditions. The laser performance could be easily tailored by changing the parameter of the WGM-random compound cavity. These results provide a new avenue for the design of integrated light sources for sensing applications.

Distributed feedback organic lasing in photonic crystals

Considerable research efforts have been devoted to the investigation of distributed feedback (DFB) organic lasing in photonic crystals in recent decades. It is still a big challenge to realize DFB lasing in complex photonic crystals. This review discusses the recent progress on the DFB organic laser based on one-, two-, and three-dimensional photonic crystals. The photophysics of gain materials and the fabrication of laser cavities are also introduced. At last, future development trends of the lasers are prospected.

Tunable polymer lasing in chirped cavities

Continuously tunable polymer lasing was achieved in one-dimensional, two-dimensional, and compound chirped cavities. The chirped cavity was simply fabricated by using interference lithography and spin coating. Two-dimensional and compound chirped cavities were obtained by employing oblique exposure and double exposure, respectively. The tunability range of two-dimensional chirped cavities was much wider than that of one-dimensional chirped cavities, which varied from 557 nm to 582 nm. The interaction between lasing modes was studied in the compound cavity by introducing an additional nanostructure into the two-dimensional chirped cavities. The threshold of the compound chirped cavities changed with the coupling strength between lasing modes. These results may be helpful for designing compact polymer laser sources.

A silicon-based quantum dot random laser

Herein, a quantum dot random laser was achieved using a silicon nanowire array. The silicon nanowire array was grown by a metal-assisted chemical etching method. A colloidal quantum dot solution was spin-coated on silicon nanowires to form the random laser. The performance of the random laser was controlled by the resistivity of silicon wafers and the length of silicon nanowires. A transition from incoherent random lasing to coherent random lasing was obtained by increasing the resistivity of the silicon wafers. The random lasing threshold increased with an increase in the length of the silicon nanowires. These results may be useful to explore high-performance silicon-based random lasers.

A silicon-based quantum dot random laser fabricated by a metal-assisted chemical etching method.