Single Photon Scattering in a Pair of Waveguides Coupled by a Whispering-Gallery Resonator Interacting with a Semiconductor Quantum Dot

The single photon scattering properties in a pair of waveguides coupled by a whispering-gallery resonator in- teracting with a semiconductor quantum dot are investigated theoretically. The two waveguides support four possible ports for an incident single photon. The quantum dot is considered a V-type system. The incident direction-dependent single photon scattering properties are studied and equal-output probability from the four ports for a single photon incident is discussed. The influences of backscattering between the two modes of the whispering-gallery resonator for incident direction-dependent single photon scattering properties are also pre- sented.

Optomechanical entanglement in a whispering-gallery cavity

The dynamics of the optomechanical entanglement between optical cavity field modes and a macroscopic me- chanical breathing mode in a whispering-gallery cavity as well as the continuous variable entanglement between the phase-quadrature amplitudes of the two whispering-gallery modes have been analysed. Simulated results indicate that under state-of-the^art experimental conditions, optomechanical entanglement is obvious and can occur even at temper- atures of above 40 K. Compared with the entanglement of the mechanical oscillator at the ground state temperature, optomechanical entanglement is more intense by several orders of magnitude.

Room-temperature continuous-wave InP-based 2.01 μm microcavity lasers in whispering-gallery modes with InGaAsSb quantum well

We demonstrated an electrically pumped InP-based microcavity laser operating in continuous-wave mode.The active region is designed with antimony surfactants to enhance the gain at 2 μm,and a selective electrical isolation scheme is used to secure continuous-wave operation for the microcavity laser at room temperature.The lasers were fabricated as a notched elliptical resonator,resulting in a highly unidirectional far-field profile with an in-plane beam divergence of less than 2°.Single-mode emission was obtained over the entire dynamic range,and the laser frequencies were tuned linearly with the pumping current.Overall,these directional lasers pave the way for portable and highly integrated on-chip sensing applications.

Single whispering-gallery mode lasing in polymer bottle microresonators via spatial pump engineering

Single-mode lasing in whispering-gallery mode(WGM)microresonators is challenging to achieve.In bottle microresonators,the highly non-degenerated WGMs are spatially well-separated along the long-axis direction and provide mode-selection capability.In this work,by engineering the pump intensity to modify the spatial gain profiles of bottle microresonators,we demonstrate a simple and general approach to realizing single-mode WGM lasing in polymer bottle microresonators.The pump intensity is engineered into an interference distribution on the bottle microresonator surface.By tuning the spacing between axial positions of the interference pump patterns,the mode intensity profiles of single-bottle WGMs can be spatially overlapped with the interference stripes,intrinsically enabling single-mode lasing and selection.Attractive advantages of the system,including high sidemode suppression factors 420 dB,large spectral tunability 48 nm,low-lasing threshold and reversible control,are presented.Our demonstrated approach may have a variety of promising applications,ranging from tunable single-mode lasing and sensing to nonlinear optics.

Plasmon-enhanced ZnO whispering-gallery mode lasing

Collaborative enhancements from surface plasmons （SPs） and whispering-gallery modes （WGMs） can induce intense near-field effects with high spatial localization around the surface of a semiconducting material. One can construct a highly efficient hybrid microcavity using semiconducting materials through resonant coupling between SPs and WGMs. Hexagonal ZnO micro-/nanostructures, which have been employed as natural WGM microcavities for ultraviolet （UV） lasing, can be used as ideal platforms to construct such hybrid microcavities. Here, we comprehensively review the recent efforts for improving lasing performance by resonant coupling between SPs and WGMs. Traditional SPs originating from various metals as well as novel SPs originating from atomic layers such as graphene are considered. Moreover, we discuss the mechanism of light-matter interactions beyond the improvements in lasing performance.

Whispering-gallery mode lasing from polymer microsphere for humidity sensing

Microlasers based on high quality （Q） whispering-gallery mode （WGM） resonance are pronfising low threshold laser sources for bio-sensing and imaging applications. In this Letter, dye-doped polymer microspheres were fabricated by a controlling emulsion solvent evaporation method. WGM lasing with low threshold and high Q factors was realized in an individual microsphere under femtosecond laser pumping. The slight change of environmental relative humidity （RH） can be monitored by measuring the shift of the lasing modes at tile ex- posure of water molecules, which dcmonstrates the sensitivity is as high as 6 pm/RH%. The results would offer an insight into employing microlasers as sensors.

LED-pumped whispering-gallery laser

A low-cost light-emitting diode(LED) is sufficient to pump a quasi-continuous-wave bidirectional high-Q whispering-gallery resonator laser made of Nd:YVO_4. This is remarkable because of the very limited spatial and spectral coherence of an LED. The LED, delivering up to 3.5 W, centered around 810 nm, is turned on in intervals of 100 μs duration, and for these periods a laser output exceeding 0.8 mW has been verified.Furthermore, 0.1-s-long laser pulses are demonstrated. To the best of our knowledge, this is the first demonstration of an LED-pumped high-Q whispering-gallery laser. The concept can be extended easily to other laser active materials. A prospect is also to pump several of such lasers with a single LED.

Parity-time-symmetric whispering-gallery mode nanoparticle sensor[Invited]

We present a study of single nanoparticle detection using parity-time （PT） symmetric whispering-gaUery mode （WGM） resonators. Our theoretical model and numerical simulations show that, with balanced gain and loss, the PT-symmetric WGM nanoparticle sensor, tailored to operate at PT phase transition points （also called exceptional points）, exhibits significant enhancement in frequency splitting when compared with a single WGM nanopartide sensor subject to the same perturbation. The presence of gain in the PT-symmetric system leads to narrower linewidth, which helps to resolve smaller changes in frequency splitting and improve the detection limit of nanoparticle sensing. Furthermore, we also provide a general method for detecting multiple nanopartides entering the mode volume of a PT-symmetric WGM sensor one by one. Our study shows the feasibility of PT-symmetric WGM resonators for ultrasensitive single nanoparticle and biomolecule sensing.

Modulated Photon Emission of Eu^3＋ in Microsphere Cavity

Fused silica microsphere with a few Eu^3＋ ions on the equator is fabricated. The photon emission sharply modulated by whispering gallery （WG） modes is observed under excitation of 395 nm laser, which is in agreement with the prediction in theory. The quality factor of the WG modes in microsphere is estimated larger than 3000 from the emission spectrum, which is limited by our monochrometer. It proves that pumping Eu^3＋ in microsphere cavity in free space is feasible, and this system seems suitable for realizing strong coupling in future quantum computation purposes.

Low fabrication cost wavelength tunable WG-FP hybrid-cavity laser working over 1.7μm

A wide wavelength tuning range and single-mode hybrid cavity laser consists of a square Whispering-Gallery(WG)microcavity and a Fabry–Pérot(FP)was introduced and demonstrated.A wavelength tuning range over 12.5 nm from 1760.87 to 1773.39 nm which was single-mode emitting was obtained with the side-mode suppression ratio over 30 dB.The hybrid cavity laser does not need grating etching and special epitaxial structure,which reduces the fabrication difficulty and cost,and shows the potential for gas sensing with absorption lines in this range.

Square microcavity semiconductor lasers

Square microcavities, which support whispering-gallery modes with total internal reflections, can be employed as high-quality laser resonators for fabricating compact, low-threshold semiconductor lasers. In this paper, we review the recent progress of square microcavity semiconductor lasers. The characteristics of confined optical modes in the square microcavities are introduced briefly. Based on the mode properties of the square microcavities, dual-mode lasers with tunable wavelength intervals are realized for generating microwave signals. Furthermore, deformed square microcavity lasers with the sidewalls replaced by circular sides are proposed and experimentally demonstrated to enhance the mode confinement and increase the dual-mode interval to the THz range. In order to further reduce the device size, metal-confined wavelength-scale square cavity lasers are also demonstrated.

Broadband second-harmonic generation in thin-film lithium niobate microdisk via cyclic quasi-phase matching

Whispering-gallery-mode(WGM)microresonators can greatly enhance light-matter interaction,making them indispensable units for frequency conversion in nonlinear optics.Efficient nonlinear wave mixing in microresonators requires stringent simultaneous optical resonance and phase-matching conditions.Thus,it is challenging to achieve efficient frequency conversion over a broad bandwidth.Here,we demonstrate broadband second-harmonic generation(SHG)in the x-cut thinfilm lithium niobate(TFLN)microdisk with a quality factor above 107by applying the cyclic quasi-phase-matching(CQPM)mechanism,which is intrinsically applicable for broadband operation.Broadband SHG of continuous-wave laser with a maximum normalized conversion efficiency of~15%/m W is achieved with a bandwidth spanning over 100 nm in the telecommunication band.Furthermore,broadband SHG of femtosecond lasers,supercontinuum lasers,and amplified spontaneous emission in the telecommunication band is also experimentally observed.The work is beneficial for integrated nonlinear photonics devices like frequency converters and optical frequency comb generator based on second-order nonlinearity on the TFLN platform.

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.

Polymeric photonic molecule super-mode lasers on silicon

Optically coupled microcavities have emerged as photonic structures with promising properties for investigation of fundamental science as well as for applications.We report on the fabrication and spatially resolved spectroscopy of on-chip photonic molecule(PM)lasers consisting of two coupled,dye-doped polymeric microdisks on a silicon substrate.We investigate the fundamental lasing properties with focus on the spatial distribution of modes,the coupling dependent suppression of lasing modes,and in particular the application-oriented operation of these devices in aqueous environments.By depositing an additional polymer layer onto the lithographically structured cavities made of dye-doped poly(methyl methacrylate),coupling-gap widths below 150 nm with aspect ratios of the micro-/nanostructure exceeding 9:1 are achieved.This enables strong optical coupling at visible wavelengths despite relatively small resonator radii of 25 μm.The lasing properties of dye-doped PMs are investigated using spatially resolved micro-photoluminescence(μ-PL)spectroscopy.This technique allows for the direct imaging of whispering-gallery modes(WGMs)in the photonics molecules.For subwavelength coupling gaps,we observe lasing from delocalized eigenstates of the PMs(termed in the following as super-modes).Using size-mismatched cavities,the lasing mode suppression for different coupling-gap widths is investigated.We further demonstrate single-mode lasing operation in aqueous environments with PMs,which are realized on a low-cost,polymer-on-silicon platform.

Quantum wells micro-ring resonator laser emitting at 1746 nm for gas sensing

Gas sensing for measurement of gas components, concentrations, and other parameters plays an important role in many fields. In this Letter, a micro-ring resonator laser used for gas sensing is experimentally demonstrated. The multi-quantumwells micro-ring laser based on whispering-gallery modes with an annular resonator and an output waveguide was fabricated. A single-mode laser with a wavelength of 1746.4 nm was fabricated for the first time, to the best of our knowledge,experimentally. The output power of 1.65 m W under 40 m A injection current was obtained with a side-mode suppression ratio over 33 d B.

Dynamic photonic barcodes for molecular detection based on cavity-enhanced energy transfer

Optical barcodes have demonstrated a great potential in multiplexed bioassays and cell tracking for their distinctive spectral fingerprints.The vast majority of optical barcodes were designed to identify a specific target by fluorescence emission spectra,without being able to characterize dynamic changes in response to analytes through time.To overcome these limitations,the concept of the bioresponsive dynamic photonic barcode was proposed by exploiting interfacial energy transfer between a microdroplet cavity and binding molecules.Whispering-gallery modes resulting from cavity-enhanced energy transfer were therefore converted into photonic barcodes to identify binding activities,in which more than trillions of distinctive barcodes could be generated by a single droplet.Dynamic spectral barcoding was achieved by a significant improvement in terms of signal-to-noise ratio upon binding to target molecules.Theoretical studies and experiments were conducted to elucidate the effect of different cavity sizes and analyte concentrations.Timeresolved fluorescence lifetime was implemented to investigate the role of radiative and non-radiative energy transfer.Finally,microdroplet photonic barcodes were employed in biodetection to exhibit great potential in fulfilling biomedical applications.

Continuous-wave laser operation of a dipole antenna terahertz microresonator

Resonators and the way they couple to external radiation rely on very different concepts if one considers devices belonging to the photonic and electronic worlds.The terahertz frequency range,however,provides intriguing possibilities for the development of hybrid technologies that merge ideas from both fields in novel functional designs.In this paper,we show that high-quality,subwavelength,whispering-gallery lasers can be combined to form a linear dipole antenna,which creates a very efficient,lowthreshold laser emission in a collimated beam pattern.For this purpose,we employ a terahertz quantum-cascade active region patterned into two 19-μm-radius microdisks coupled by a suspended metallic bridge,which simultaneously acts as an inductive antenna and produces the dipole symmetry of the lasing mode.Continuous-wave vertical emission is demonstrated at approximately 3.5 THz in a very regular,low-divergence(±10°)beam,with a high slope efficiency of at least 160 mWA^(−1) and a mere 6 mA of threshold current,which is ensured by the ultra-small resonator size(VRES/λ^(3)≈10^(−2)).The extremely low power consumption and the superior beam brightness make this concept very promising for the development of miniaturized and portable THz sources to be used in the field for imaging and sensing applications as well as for exploring novel optomechanical intracavity effects.