Lanthanide-based microlasers:Synthesis,structures,and biomedical applications

The large size of lasers limits their applications in confined spaces,such as in biosensing and in vivo brain tissue imaging.In this regard,micron-sized lasers have been developed.They exhibit great potential for biological detecting,remote sensing,and depth tracking due to their small sizes,sensitive properties of their spectral fingerprints,and flexible positional modulation in the microenvironment.Lanthanide-based luminescent materials that possess long excited-state lifetime,narrow emission bandwidth,and upconversion behaviors are promising as gain mediums for novel microlasers.In addition,lanthanide-based microlasers could be generated under natural ambient conditions with pumped or continuous light sources,which significantly promotes the practical applications of microlasers.Recent progress in the design,synthesis,and biomedical applications of lanthanide-based microlasers has been outlined in this review.Lanthanide ions doped and upconverted lanthanide-based microlasers are highlighted,which exhibit advantageous structures,miniaturized dimensions,and high lasing performance.The applications of lanthanide-based microlasers are further discussed,the upconverted microlasers show great advantages for biological applications owing to their tunable excitation and emission characteristics and excellent environmental stability.Moreover,perspectives and challenges in the field of lanthanide-based microlasers are presented.

Study of Mode Characteristics for Equilateral Triangle Semiconductor Microlasers

InGaAsP semiconductor ETR microlasers with side length of 5 and 10 um are fabricated by ICP etching. The peaks in photoluminenscent spectra corresponding to longitudinal modes are observed with the interval consisting with the theoretical formulae.

AlGaInAs/InP coupled-circular microlasers

AlGalnAs/InP coupled-circular microlasers with radius of 10-and 2-μm width middle bus waveguide are fabricated by photolithography and inductively coupled plasma etching techniques. Room-temperature continuous-wave single-mode operation is realized with an output power of 0.17 mW and a side-mode suppression ratio of 23 dB at 45 mA. A longitudinal mode interval of 11 nm is obtained from the lasing spectra, and mode Q factor of 6.2×10^3 is estimated from 3-dB width of a minor peak near the threshold current. The mode characteristics are simulated by finite-difference time-domain technique for coupled- circular resonators. The results show that, in addition to the coupled modes, high-radial-order whispering gallery modes with travelling wave behaviors can also have high Q factors in the coupled-circular resonators with a middle bus waveguide.

Demonstration of ultralow-threshold 2 micrometer microlasers on chip

We demonstrate ultralow-threshold thulium-doped, as well as thulium-holmium-codoped, microtoroid lasers on silicon chips, operating at the wavelength of around 2 ?m. High quality factor whispering gallery mode(WGM) microtoroid cavities with proper thulium and holmium concentrations are fabricated from the silica sol-gel films. The highly confined WGMs make the microcavity lasers operate with ultralow thresholds, approximately 2.8 ?W and 2.7 ?W for the thulium-doped and the thulium-holmium-codoped microlasers, respectively.

Laser patterning of large-scale perovskite single-crystal-based arrays for single-mode laser displays

Lead halide perovskites have attracted considerable attention as potential candidates for high-performance nano/microlasers,owing to their outstanding optical properties.However,the further development of perovskite microlaser arrays(especially based on polycrystalline thin films)produced by the conventional processing techniques is hindered by the chemical instability and surface roughness of the perovskite structures.Herein,we demonstrate a laser patterning of large-scale,highly crystalline perovskite single-crystal films to fabricate reproducible perovskite single-crystal-based microlaser arrays.Perovskite thin films were directly ablated by femtosecond-laser in multiple low-power cycles at a minimum machining line width of approximately 300 nm to realize high-precision,chemically clean,and repeatable fabrication of microdisk arrays.The surface impurities generated during the process can be washed away to avoid external optical loss due to the robustness of the single-crystal film.Moreover,the high-quality,large-sized perovskite single-crystal films can significantly improve the quality of microcavities,thereby realizing a perovskite microdisk laser with narrow linewidth(0.09 nm)and low threshold(5.1µJ/cm2).Benefiting from the novel laser patterning method and the large-sized perovskite single-crystal films,a high power and high color purity laser display with single-mode microlasers as pixels was successfully fabricated.Thus,this study may offer a potential platform for mass-scale and reproducible fabrication of microlaser arrays,and further facilitate the development of highly integrated applications based on perovskite materials.

Theoretical investigation of F-P cavity mode manipulation by single gold nanoparticles

The ability to manipulate microlaser is highly desirable towards high-performance optoelectronic devices.Here we demonstrate feasible mode manipulation of Fabry-Perot type microlasers of a perovskite nanowire via incorporation of single gold nanoparticles.The influences of resonant wavelength,quality factor and emission directions are successively investigated using a two-dimensional finite-difference time-domain method.It is found that blueshift of resonant wavelength could be achieved together with either promoted or degraded quality factor of the microlaser via single Au NPs with varied sizes.Unidirectional emission could also be realized which is favorable for on-chip integration.Our results provide useful reference for feasible manipulation of light-matter interactions and mode selection.

Sensing and lasing applications of whispering gallery mode microresonators

Optical whispering gallery mode （WGM） microresonators have attracted great attention due to their remarkable proper- ties such as extremely high quality factor, small mode volume, tight confinement of modes, and strong evanescent field. All these properties of WGM microresonators have ensured their great potentials for applications, such as physical sen- sors, bio/chemical sensors and microlasers. In this mini-review, the key parameters and coupling conditions of WGM microresonators are firstly introduced. The geometries of WGM optical microcavities are presented based on their fabri- cation methods. This is followed by the discussion on the state-of-the-art applications of WGM microresonators in sen- sors and microlasers.

Research progress of low-dimensional metal halide perovskites for lasing applications

Metal halide perovskites have been regarded as remarkable materials for next-generation light-harvesting and light emission devices.Due to their unique optical properties,such as high absorption coefficient,high optical gain,low trappingstate density,and ease of band gap engineering,perovskites promise to be used in lasing devices.In this article,the recent progresses of microlasers based on reduced-dimensional structures including nanoplatelets,nanowires,and quantum dots are reviewed from both fundamental photophysics and device applications.Furthermore,perovskite-based plasmonic nanolasers and polariton lasers are summarized.Perspectives on perovskite-based small lasers are also discussed.This review can serve as an overview and evaluation of state-of-the-art micro/nanolaser science.

Advancing mid-infrared microdisk laser emission with Tm:YAG

Mid-infrared microcavity lasers have important applications in biosensing,mid-infrared spectroscopy,and environmental monitoring.However,the low output power of existing mid-infrared microcavity lasers hinders their practical use.This drawback is attributed to the insufficient laser gain medium,which limits the development of mid-infrared whispering-gallerymode(WGM)lasers.To address this issue,we have employed ion implantation-enhanced etching to fabricate Tm:YAG thin films as effective gain media for mid-infrared WGM lasers.The Tm:YAG thin film,with a thickness of 2μm,exhibits excellent fluorescence characteristics.Subsequently,the Tm:YAG thin film is processed using focused ion beam to form microdisks with a diameter of 30μm.Under 785 nm laser pumping,the maximum output power of the Tm:YAG microdisk at 2023.1 nm is229μW,with a slope efficiency of 2.9%.This work demonstrates the outstanding potential of Tm:YAG as a mid-infrared laser gain medium,providing a new option for the development of mid-infrared lasers.

Plasmon-assisted mode selection lasing in a lanthanide-based microcavity

Lanthanide-based microlasers have attracted considerable attention owing to their large anti-Stokes shifts,multiple emission bands,and narrow linewidths.Various applications of microlasers,such as optical communication,optical storage,and polarization imaging,require selecting the appropriate laser polarization mode and remote control of the laser properties.Here,we propose a unique plasmon-assisted method for the mode selection and remote control of microlasing using a lanthanide-based microcavity coupled with surface plasmon polaritons(SPPs)that propagate on a silver microplate.With this method,the transverse electrical(TE)mode of microlasers can be easily separated from the transverse magnetic(TM)mode.Because the SPPs excited on the silver microplate only support TM mode propagation,the reserved TE mode is resonance-enhanced in the microcavity and amplified by the local electromagnetic field.Meanwhile,lasingmode splitting can be observed under the near-field excitation of SPPs due to the coherent coupling between the microcavity and mirror microcavity modes.Benefiting from the long-distance propagation characteristics of tens of micrometers of SPPs on a silver microplate,remote excitation and control of upconversion microlasing can also be realized.These plasmon-assisted polarization mode-optional and remote-controllable upconversion microlasers have promising prospects in on-chip optoelectronic devices,encrypted optical information transmission,and high-precision sensors.

Advances in lithium niobate thin-film lasers and amplifiers: a review

Lithium niobate(LN)thin film has received much attention as an integrated photonic platform,due to its rich and great photoelectric characteristics,based on which various functional photonic devices,such as electro-optic modulators and nonlinear wavelength converters,have been demonstrated with impressive performance.As an important part of the integrated photonic system,the long-awaited laser and amplifier on the LN thin-film platform have made a series of breakthroughs and important progress recently.In this review paper,the research progress of lasers and amplifiers realized on lithium niobate thin film platforms is reviewed comprehensively.Specifically,the research progress on optically pumped lasers and amplifiers based on rare-earth ions doping of LN thin films is introduced.Some important parameters and existing limitations of the current development are discussed.In addition,the implementation scheme and research progress of electrically pumped lasers and amplifiers on LN thin-film platforms are summarized.The advantages and disadvantages of optically and electrically pumped LN thin film light sources are analyzed.Finally,the applications of LN thin film lasers and amplifiers and other on-chip functional devices are envisaged.

Simultaneous ultraviolet,visible,and near-infrared continuous-wave lasing in a rare-earth-doped microcavity

Microlaser with multiple lasing bands is critical in various applications,such as full-color display,optical communications,and computing.Here,we propose a simple and efficient method for homogeneously doping rare earth elements into a silica whispering-gallery microcavity.By this method,an Er-Yb co-doped silica microsphere cavity with the highest quality(Q)factor(exceeding 108)among the rare-earth-doped microcavities is fabricated to demonstrate simultaneous and stable lasing covering ultraviolet,visible,and near-infrared bands under room temperature and a continuous-wave pump.The thresholds of all the lasing bands are estimated to be at the submilliwatt level,where both the ultraviolet and violet continuous wave upconversion lasing from rare earth elements has not been separately demonstrated under room temperature until this work.This ultrahigh-Q doped microcavity is an excellent platform for highperformance multiband microlasers,ultrahigh-precision sensors,optical memories,and cavity-enhanced light–matter interaction studies.

Emission from quantum-dot high-β microcavities: transition from spontaneous emission to lasing and the effects of superradiant emitter coupling

Measured and calculated results are presented for the emission properties of a new class of emitters operating in the cavity quantum electrodynamics regime.The structures are based on high-finesse GaAs/AlAs micropillar cavities,each with an active medium consisting of a layer of InGaAs quantum dots(QDs)and the distinguishing feature of having a substantial fraction of spontaneous emission channeled into one cavity mode(highβ-factor).This paper demonstrates that the usual criterion for lasing with a conventional(lowβ-factor)cavity,that is,a sharp non-linearity in the input–output curve accompanied by noticeable linewidth narrowing,has to be reinforced by the equal-time second-order photon autocorrelation function to confirm lasing.The paper also shows that the equal-time second-order photon autocorrelation function is useful for recognizing superradiance,a manifestation of the correlations possible in high-βmicrocavities operating with QDs.In terms of consolidating the collected data and identifying the physics underlying laser action,both theory and experiment suggest a sole dependence on intracavity photon number.Evidence for this assertion comes from all our measured and calculated data on emission coherence and fluctuation,for devices ranging from light-emitting diodes(LEDs)and cavity-enhanced LEDs to lasers,lying on the same two curves:one for linewidth narrowing versus intracavity photon number and the other for g(2)(0)versus intracavity photon number.

Analysis of mode characteristics for microcircular resonators confined by different metallic materials

Mode characteristics of metallically confined microcircular resonators are theoretically studied by solv- ing eigenvalue equations for two-dimensional multilayer structures. The influences of conventional metals includ- ing Au, Ag, Cu, A1, and Ti, on the mode wavelengths and Q factors of whispering gallery modes （WGMs） are an- alyzed and compared. The results show silver has the best optical confinement among these metals, and aluminum presents similar behavior to Au. However, Ti, which is usually applied to enhance the adhesion of p-electrode to semiconductors, results in a great dissipation for confined modes. Furthermore, circular microlasers with A1 as both p-electrode and optical confinement medium are fabricated, and continuous-wave operations are realized at room temperature for the microlasers with a radius of 15μm.

Advanced lanthanide doped upconversion nanomaterials for lasing emission

Microlasers are narrow-band and coherent light from small cavities,which have been widely applied in biomedicine,optical interconnection,integration devices,etc.Lanthanide doped upconversion materials are potential gain media for microlasers from near infrared(NIR)to visible and UV regimes due to their multi ladder-like metastable energy levels and superior optical frequency conversion capability.The optical feedback from photon scattering of the porous upconversion nanoparticles clusters has been reported to produce upconversion random lasers.The light bouncing back and forth between two reflective surfaces or internal surface has been utilized to achieve modulated upconversion lasing emission.In addition,photon lattices and plasmonic cavities with enhanced electromagnetic fields can amplify the upconversion process within the sub-diffraction volumes and produce highly efficient upconverting lasers.In this review,the recent advances on using lanthanide doped upconversion materials for random,whispering gallery mode(WGM)/Fabry-Perot(FP)cavity and photon lattice/plasmonic cavity modulated upconversion microlasers are overviewed.Current challenges and future directions of the upconverting lasers are also discussed.

Electro-optically tunable microdisk laser on Er^(3+)-doped lithium niobate thin film

We report an electro-optically(EO)tunable microdisk laser fabricated on the erbium(Er^(3+))-doped lithium niobate on insulator(LNOI) substrate.By applying a variable voltage on a pair of integrated chromium(Cr) microelectrodes fabricated near the LNOI microdisk,electro-optic modulation with an effective resonance-frequency tuning rate of 2.6 GHz/100 V has been achieved.This gives rise to a tuning range of 45 pm when the electric voltage is varied between-200 V and 200 V.

Full-color flexible laser displays based on random laser arrays

Flexible laser display is a critical component for an information output port in next-generation wearable devices.So far,the lack of appropriate display panels capable of providing sustained operation under rigorous mechanical conditions impedes the development of flexible laser displays with high reliability.Owing to the multiple scattering feedback mechanism,random lasers render high mechanical flexibility to withstand deformation,thus making them promising candidates for flexible display planes.However,the inability to obtain pixelated random laser arrays with highly ordered emissive geometries hinders the application of flexible laser displays in the wearable device.Here,for the first time,we demonstrate a mass fabrication strategy of full-color random laser arrays for flexible display panels.The feedback closed loops can be easily fulfilled in the pixels by multiple scatterings to generate durative random lasing.Due to the sustained operation of random laser,the display performance was well-maintained under mechanical deformations,and as a result,a flexible laser display panel was achieved.Our finding will provide a guidance for the development of flexible laser displays and laser illumination devices.

Whispering-gallery microcavity semiconductor lasers suitable for photonic integrated circuits and optical interconnects

The characteristics of whispering-gallery-like modes in the equilateral triangle and square microresonators are introduced,including directional emission triangle and square microlasers connected to an output waveguide.We propose a photonic interconnect scheme by connecting two directional emission microlasers with an optical waveguide on silicon integrated circuit chip.The measurement indicates that the triangle microlasers can work as a resonance enhanced photodetector for optical interconnect.

Single-mode hybrid Al Ga In As/Si octagonal-ring microlaser with stable output

Hybrid octagonal-ring microlasers are investigated for realizing a stable output from a silicon waveguide based on a two-dimensional simulation. The inner radius of the ring is optimized to achieve single-mode and low-threshold operation. Using the divinylsiloxane-benzocyclobutene （DVS-BCB） bonding technique, a hybrid A1GaInAs/Si octagonal-ring microlaser vertically coupled to a silicon waveguide is fabricated with a side length of 21.6 pm and an inner radius of 15 pm. A single transverse-mode operation is achieved with a threshold current density of 0.8 kA/cm2 and a side-mode suppression ratio above 30 dB, and a stable output from the lower silicon waveguide is obtained.

Impact of cavity symmetry on mode suppression and increase of free spectral range in solid-state dye microlaser

We describe modeling the solid-state dye laser with the microcavity size comparable to light wavelength. Certain symmetry in the allocation of gain material leads to depletion of odd longitudinal modes that, in turn, increases the tunability range of the microlaser. We provide simple physical explanation for the modeling results.