Two-photon absorption and stimulated emission in poly-crystalline Zinc Selenide with femtosecond laser excitation

The optical nonlinearity in polycrystalline zinc selenide(ZnSe),excited with 775 nm,1 kHz femtosecond laser pulses was investigated via the nonlinear transmission with material thickness and the Z scan technique.The measured two photon absorption coefficientβwas intensity dependent,inferring that reverse saturated absorption(RSA)is also relevant dur-ing high intensity excitation in ZnSe.At low peak intensity I<5 GW cm^(-2),we findβ=3.5 cm GW^(-1) at 775 nm.The spec-tral properties of the broad blueish two-photon induced fluorescence(460 nm-500 nm)was studied,displaying self-ab-sorption near the band edge while the upper state lifetime was measured to be τ_(e)~3.3 ns.Stimulated emission was ob-served when pumping a 0.5 mm thick polycrystalline ZnSe sample within an optical cavity,confirmed by significant line narrowing fromΔλ=11 nm(cavity blocked)toΔλ=2.8 nm at peak wavelength λ_(p)=475 nm while the upper state life-time also decreased.These results suggest that with more optimum pumping conditions and crystal cooling,polycrystal-line ZnSe might reach lasing threshold via two-photon pumping atλ=775 nm.

Stimulated emission and multi-peaked absorption in a four level N-type atom

Absorption and refraction of the inner transition F2 ←→F3 of the closed four level N-type atom have been investigated under a weak field. The outer transitions F1←→F3 and F2←→F4 are resonantly interacted with drive field with frequency ωc and Rabi frequency Ωc, and saturation field with ωs and Ωs, respectively. For the suitable Rabi frequencies Ωc and Ωs, we obtain the Mellow absorption spectrum of probe field. The reason is that the drive field excites the atom to the upper level F3 and simultaneously the saturation field takes the atom out of the lower level F2, leading to the stimulated emission. Meanwhile, due to the dynamic energy splitting induced by the drive and saturation fields, the two- and four-peaked absorption spectra are observed. At the zero off-resonance detuning of probe field, we also find the transfer of dispersion from negative to positive with an increment of Ωs. Finally, the refractive index enhancement is predicted for a wide spectral region.

Influence of fluorescence time characteristics on the spatial resolution of CW-stimulated emission depletion microscopy

As one of the most important realizations of stimulated emission depletion(STED)microscopy,the continuous-wave(CW)STED system,constructed by using CW lasers as the excitation and STED beams,has been investigated and developed for nearly a decade.However,a theoretical model of the suppression factors in CW STED has not been well established.In this investigation,the factors that affect the spatial resolution of a CW STED system are theoretically and numerically studied.The full-width-at-half-maximum(FWHM)of a CW STED with a doughnut-shaped STED beam is also reanalyzed.It is found that the suppression function is dominated by the ratio of the local STED and excitation beam intensities.In addition,the FWHM is highly sensitive to both the fluorescence rate(inverse of fluoresce lifetime)and the quenching rate,but insensitive to the rate of vibrational relaxation.For comparison,the suppression function in picosecond STED is only determined by the distribution of the STED beam intensity scaled with the saturation intensity.Our model is highly consistent with published experimental data for evaluating the spatial resolution.This investigation is important in guiding the development of new CW STED systems.

Stimulated emission-depletion-based point-scanning structured illumination microscopy

Wide-field linear structured illumination microscopy(LSIM)extends resolution beyond the diffraction limit by moving unresolvable high-frequency information into the passband of the microscopy in the form of moiréfringes.However,due to the diffraction limit,the spatial frequency of the structured illumination pattern cannot be larger than the microscopy cutoff frequency,which results in a twofold resolution improvement over wide-field microscopes.This Letter presents a novel approach in point-scanning LSIM,aimed at achieving higher-resolution improvement by combining stimulated emission depletion(STED)with point-scanning structured illumination microscopy(ps SIM)(STED-ps SIM).The according structured illumination pattern whose frequency exceeds the microscopy cutoff frequency is produced by scanning the focus of the sinusoidally modulated excitation beam of STED microscopy.The experimental results showed a 1.58-fold resolution improvement over conventional STED microscopy with the same depletion laser power.

Stimulated photoluminescence emission and trap states in Si/SiO_2 interface formed by irradiation of laser

Stimulated photoluminescence （PL） emission has been observed from an oxide structure of silicon when optically excited by a radiation of 514nm laser. Sharp twin peaks at 694 and 692nm are dominated by stimulated emission, which can be demonstrated by its threshold behaviour and linear transition of emission intensity as a function of pump power. The oxide structure is formed by laser irradiation on silicon and its annealing treatment. A model for explaining the stimulated emission is proposed, in which the trap states of the interface between an oxide of silicon and porous nanocrystal play an important role.

Optical processes in the formation of stimulated emission from ZnO nanowires

This paper studies power dependent photoluminescence spectra, the stimulated emission occurring at ultraviolet （UV） band instead of the green emission band of ZnO nanowires, which are prepared with a chemical reduction method. The dynamics of the UV emission and green emission is given to demonstrate the reason of stimulated emission occurring at UV band but not the green emission band under high excitation, which indicates that the slow decay rate of trap state makes it easy to be fully filled and saturated, while the fast decay rate of near-band-edge exciton state makes the UV emission dominate the radiative recombination under high excitation. The UV emission, as well as the corresponding stimulated emission, occurs in competition with the green deep-trap emission. In addition, when pump fluence further increases, the multiple lasing modes appear. The dependence of these lasing modes on the pump fluence is first discussed. This diagram should be helpful to understand and design the optical nanodevices of ZnO nanowires.

Superresolution imaging of telomeres with continuous wave stimulated emission depletion (STED) microscope

The significant role of telomeres in cells has attracted much attention since they were discovered.Fluorescence imaging is an effective method to study subcellular structures like telomeres.However,the diffraction limit of traditional optical microscope hampers further investigation on them.Recent progress on superresolution fluorescence microscopy has broken this limit.In this work,we used stimulated emission depletion(STED) microscope to observe fluorescence-labeled telomeres in interphase cell nuclei.The results showed that the size of fluorescent puncta representing telomeres under the STED microscope was much smaller than that under the confocal microscope.Two adjacent telomeres were clearly separated via STED imaging,which could hardly be discriminated by confocal microscopy due to the diffraction limit.We conclude that STED microscope is a more powerful tool that enable us to obtain detailed information about telomeres.

Silicon-substituted rhodamines for stimulated emission depletion fluorescence nanoscopy

As an essential part in the toolbox of super-resolution microscopy,stimulated emission depletion(STED)nanoscopy has been widely explored in revealing the substructure and bioactivities in fluorescence imaging.Among the applied STED fluorophores,silicon-substituted rhodamines(SiRs)belong to one of the most extensively employed fluorophores.The carboxy-SiR was favored in STED bioimaging with many advantages,including reliable photostability,cell permeability,tunable fluorogenicity,feasible structural decoration and so on.We reviewed the research of carboxy-SiR in the STED nanoscopy and hopefully this can inspire more efforts in the design and application of STED fluorophores.

Fast dual-beam alignment method for stimulated emission depletion microscopy using aggregation-induced emission dye resin

A stimulated emission depletion is capable of breaking the diffraction limit by exciting fluorescent molecules with a solid Gaussian beam and quenching the excited molecules with another donut beam through stimulated emission.The coincidence degree of these two beams in three dimensions will significantly influence the spatial resolution of the microscope.However,the conventional alignment approach based on raster scanning of gold nanoparticles by the two laser beams separately suffers from a mismatch between fluorescence and scattering modes.To circumvent the above problems,we demonstrate a fast alignment design by scanning the second beam over the fabricated sample,which is made of aggregation-induced emission(AIE)dye resin.The relative positions of solid and donut laser beams can be represented by the fluorescent AIE from the labeled spots in the dye resin.This design achieves ultra-high resolutions of 22 nm in the x/y relative displacement and 27 nm in the z relative displacement for fast spatial matching of the two laser beams.This study has potential applications in scenarios that require the spatial matching of multiple laser beams,and the field of views of different objectives,for example,in a microscope with high precision.

Infrared stimulated emission with an ultralow threshold from low-dislocation-density InN films grown on a vicinal GaN substrate

Near-infrared stimulated emission from a high-quality InN layer under optical pumping was observed with a threshold excitation power density of 0.3 and 4 kW cm^(−2) at T=8 and 77 K,respectively.To achieve such a low threshold power density,vicinal GaN substrates were used to reduce the edge-component threading dislocation(ETD)density of the InN film.Cross-sectional transmission electron microscopy images reveal that the annihilation of ETDs can be divided into two steps,and the ETD density can be reduced to approximately 5×10^(8) cm^(−2) near the surface of the 5-μm-thick film.The well-resolved phonon replica of the band-to-band emission in the photoluminescence spectra at 9 K confirm the high quality of the InN film.As a result,the feasibility of InN-based photonic structures and the underlying physics of their growth and emission properties are demonstrated.

Stimulated electromagnetic emissions spectrum observed during an X-mode heating experiment at the European Incoherent Scatter Scientific Association

An extraordinary(X-mode)electromagnetic wave,injected into the ionosphere by the ground-based heating facility at Tromsφ,Norway,was utilized to modify the ionosphere on November 6,2017.The high-power high-frequency transmitter facility located at Tromsφ belongs to the European Incoherent Scatter Scientific Association.In the experiment,stimulated electromagnetic emission(SEE)spectra were observed.A narrow continuum occurred under cold-start conditions and showed an overshoot effect lasting several seconds.Cascading peaks occurred on both sides of the heating frequency only in the preconditioned ionosphere and also showed an overshoot effect.These SEE features are probably related to the ponderomotive process in the X-mode heating experiment and are helpful for understanding the physical mechanism that generated them during the X-mode heating experiment.The features observed in the X-mode heating experiments are novel and require further investigation.

Role of nitrogen and oxygen in emission of Si quantum dots formed by pulse laser

Silicon quantum dots fabricated by nanosecond pulsed laser in nitrogen, oxygen or air atmosphere have enhanced photoluminescence （PL） emission with the stimulated emission observed at about 700 nm. It is difficult to distinguish between the photoluminescence peaks emitted from samples prepared in different atmospheres. The reason for the appearance of similar peaks may be the similar distribution of the localised states in the gap for different samples when silicon dangling bonds of quantum dots are passivated by nitrogen or oxygen. It is revealed that both the kind and the density of passivated bonds on quantum dot surface prepared in oxygen or nitrogen have a strong influence on the enhancement of PL emission.

Photophysical Property of Photoactive Molecules with Multibranched Push-Pull Structures

The structure-property characteristics of a series of newly synthesized intramolecular charge- transfer （ICT） compounds, single-branch monomer with triphenylmethane as electron donor and 2,1,3-benzothiadiazole as acceptor, the corresponding two-branch dimer and three- branch trimer, have been investigated by means of steady-state and femtosecond time- resolved stimulated emission fluorescence depletion （FS TR-SEP FD） techniques in different polar solvents. The TD-DFT calculations are further performed to explain the observed ICT properties. The interpretation of the experimental results is based on the comparative studies of the series of compounds which have increased amount of identical branch moiety. The similarity of the absorption and fluorescence spectra as well as strong solvent-dependence of the spectral properties for the three compounds reveal that the excited state of the dimer and trimer are nearly the same with that of the monomer, which may localize on one branch. It is found that polar excited state emerged through multidimensional intramolecular charge transfer from the donating moiety to the acceptor upon excitation, and quickly relaxed to one branch before emission. Even so, the red-shift in the absorption and emission spectra and decreased fluorescence radiative lifetime with respect to their monomer counterpart still suggest some extent delocalization of excited state in the dimer and trimer upon excitation. The similar behavior of their excited ICT state is demonstrated by FS TR-SEP FD measurements, and shows that the trimer has the largest charge-separate extent in all studied three samples. Finally, steady-state excitation anisotropy measurements has further been carried out to estimate the nature of the optical excitation and the mechanism of energy redistribution among the branches, where no plateau through the ICT band suggests the intramolecular excitation transfer process between the branches in dimer and trimer.

Application of super-resolution fluorescence microscopy in hematologic malignancies

Hematologic malignancies are one of the most common malignant tumors caused by the clonal proliferation and differentiation of hematopoietic and lymphoid stem cells.The examination of bone marrow cells combined with immunodeficiency typing is of great significance to the diagnostic type,treatment and prognosis of hematologic malignancies.Super-resolution fluorescence microscopy(SRM)is a special kind of optical microscopy technology,which breaks the resolution limit and was awarded the Nobel Prize in Chemistry in 2014.With the development of SRM,many related technologies have been applied to the diagnosis and treatment of clinical diseases.It was reported that a major type of SRM technique,single molecule localization microscopy(SMLM),is more sensitive than flow cytometry(FC)in detecting cell membrane antigens'expression,thus enabling better chances in detecting antigens on hematopoietic cells than traditional analytic tools.Furthermore,SRM may be applied to clinical pathology and may guide precision medicine and personalized medicine for clone hematopoietic cell diseases.In this paper,we mainly discuss the application of SRM in clone hematological malignancies.

STED microscopy based on axially symmetric polarized vortex beams

A stimulated emission depletion （STED） microscopy scheme using axially symmetric polarized vortex beams is pro- posed based on unique focusing properties of such kinds of beams. The concept of axially symmetric polarized vortex beams is first introduced, and the basic principle about the scheme is described. Simulation results for several typical beams are then shown, including radially polarized vortex beams, azimuthally polarized vortex beams, and high-order axi- ally symmetric polarized vortex beams. The results indicate that sharper doughnut spots and thus higher resolutions can be achieved, showing more flexibility than previous schemes based on flexible modulation of both phase and polarization for incident beams.

Influence of patterned TiO_2/SiO_2 dielectric multilayers for back and front mirror facets on GaN-based laser diodes

Ridge InGaN multi-quantum-well-structure （MQW） edge-emitting laser diodes （LDs） were grown on （0001） sapphire substrates by low-pressure metal-organic chemical vapour deposition （MOCVD）. The dielectric TiO2/SiO2 front and back facet coatings as cavity mirror facets of the LDs have been deposited with electron-beam evaporation method. The reflectivity of the designed front coating is about 50% and that of the back high reflective coating is as high as 99.9%. Under pulsed current injection at room temperature, the influences of the dielectric facets were discussed. The threshold current of the ridge GaN-based LDs was decreased after the deposition of the back high reflective dielectric mirrors and decreased again after the front facets were deposited. Above the threshold, the slope efficiency of the LDs with both reflective facets was larger than those with only back facets and without any reflective facets. It is important to design the reflectivity of the front facets for improving the performance of GaN-based LDs.

Spectral Parameter Study on Tm^(3+) in Na_5Tm(WO_4)_4 Crystal

On the basis of the obtained intensity parameters Ω λ(λ =2,4,6) of Tm 3+ in the Na 5Tm(WO 4) 4 crystal, we performed calculations on the radiative transition probability, radiative lifetime, branching ratios and stimulated emission integrated cross sections between the two excited J manifolds of Tm 3+ , and discussed potentiality for the usage of this crystal as a laser material.

E®ect of light polariztion on pattern illumination super-resolution imaging

Far-¯eld°uorescence microscopy has made great progress in the spatial resolution,limited by light diffraction,since the super-resolution imaging technology appeared.And stimulated emission depletion(STED)microscopy and structured illumination microscopy(SIM)can be grouped into one class of the super-resolution imaging technology,which use pattern illumination strategy to circumvent the di®raction limit.We simulated the images of the beads of SIM imaging,the intensity distribution of STED excitation light and depletion light in order to observe effects of the polarized light on imaging quality.Compared to¯xed linear polarization,circularly polarized light is more suitable for SIM on reconstructed image.And right-handed circular polarization(CP)light is more appropriate for both the excitation and depletion light in STED system.Therefore the right-handed CP light would be the best candidate when the SIM and STED are combined into one microscope.Good understanding of the polarization will provide a reference for the patterned illumination experiment to achieve better resolution and better image quality.

Advances in metal halide perovskite lasers: synthetic strategies, morphology control, and lasing emission

.In the past decade,lead halide perovskites have emerged as potential optoelectronic materials in the fields of light-emitting diode,solar cell,photodetector,and laser,due to their low-cost synthesis method,tunable bandgap,high quantum yield,large absorption,gain coefficient,and low trap-state densities.In this review,we present a comprehensive discussion of lead halide perovskite applications,with an emphasis on recent advances in synthetic strategies,morphology control,and lasing performance.In particular,the synthetic strategies of solution and vapor progress and the morphology control of perovskite nanocrystals are reviewed.Furthermore,we systematically discuss the latest development of perovskite laser with various fundamental performances,which are highly dependent on the dimension and size of nanocrystals.Finally,considering current challenges and perspectives on the development of lead halide perovskite nanocrystals,we provide an outlook on achieving high-quality lead perovskite lasers and expanding their practical applications.

Study on forward stimulated Brillouin scattering in a backward pumped fiber Raman amplifier

Strong multi-order forward stimulated Brillouin scattering (SBS) has been observed in the backward pumped S-band distributed fiber Raman amplifier (PRA) with tunable narrow signal source (less than 100 MHz) when the pump power of FRA reached the SBS threshold. This does not obey the theory that only weak backward SBS lines exist according to the conservation of energy and momentum and the wave vector selected rule. This is because the sound waveguide characteristic weakens the wave vector rule, and the forward transmitted sound waveguide Brillouin scattering lines are generated and amplified in FRA. When the pump power is further increased, 11 orders of SBS lines and comb-like profile are observed. For the excited line, the frequency is 197.2296 THz and the power is 0 dBm. The even order SBS lines are stronger than odd order SBS lines, the power of the 2nd and 4th order SBS lines is 1.75 dBm, which is 16 dB higher than that of the 1st and 3rd order SBS lines. The odd order SBS lines are named Brillouin Rayleigh scattering lines.