Observation of upconversion fluorescence and stimulated emission based on three-photon absorption

The observations of three-photon-induced frequency-upconversion fluorescence and the highly directional stimulated visible emission in two dyes, 4-[p-(dicyanoethylamino) styryl]-N-methylpyridinium iodide (abbreviated as CEASP) and the complex of CEASP and Ce(NO₃) (abbreviated as CEASP-Ce), are reported. The photographs of the forward amplified spontaneous emissions spots, pumped by an optical parametric oscillator idler with a pulse width of 8 ns and a wavelength of 1.3 μ m, are shown. The upconversion fluorescence produced both in dimethyl formamide solution and 2-hydroxyethyl methacrylate (HEMA) polymer spans from green to red, with a cubic dependence on the pump light intensity. The experimental results imply that the existence of the lanthanide ion Ce^{3 +} sensitizes the nonlinear absorption and emission.     

原子的吸收光谱和发射光谱的演示

介绍了利用光学多道分析仪和投影仪在课堂上实时演示原子的吸收光谱和发射光谱的实验方法。     

Picosecond buildup and relaxation of intense stimulated emission in GaAs

In support of the idea developed previously based on circumstantial evidence, we have found that stimulated emission emerges in GaAs and its intensity increases with a picosecond delay relative to the front of powerful picosecond optical pumping that produced a dense electron-hole plasma. The emission intensity relaxes with decreasing pumping with a characteristic time of ～10 ps. We have derived the dependences of the delay time, the relaxation time, and the duration of the picosecond emission pulse on its photon energy. The estimates based on the fact that the relaxation of emission is determined by electron-hole plasma cooling correspond to the measured relaxation time.     

Transfer-of-coherence-enhanced stimulated emission and electromagnetically induced absorption in Zeeman split Fg-->Fe=Fg-1 atomic transitions

The probe absorption spectra in single and multiple tripod systems formed when a weak sigma polarized pump and a tunable pi polarized probe interact with a Zeeman split F(g)-->F(e)=F(g)-1 atomic transition are characterized by two interfering stimulated Raman features separated by an electromagnetically induced absorption (EIA) peak at the line center. These Raman features can appear as either sharp stimulated emission peaks or electromagnetically induced transparency windows. In the multitripod systems, the EIA and stimulated emission peaks derive from the combined effects of interference between the stimulated Raman features and transfer of coherence from the excited to ground states.     

Low-frequency stimulated emission and induced absorption in a three-level atom

Investigations of the motion of conventional negative ions (electron bubbles) in He II under pressures above 11 bar have provided the only means of measuring the Landau critical velocity for roton creation,v _L, and for studying supercritical dissipation at higher velocities. Earlier work on roton creation is reviewed and it is pointed out that there is still no generally agreed explanation of the fact that the rotons seem to be emitted from the moving ionin pairs; nor is it known why the matrix element characterising the pair emission process should decrease rapidly with pressure. The possibility of studying these phenomena through use either of the fast ion (whose nature remains unknown), or of selected ions from the large variety of species that can be injected into He II by the recently developed technique of laser ablation, is discussed.     

Low-frequency stimulated emission and induced absorption in a three-level atom

We consider the excitation spectra of a bichromatically driven three-level atom in the lambda configuration and in the low-frequency domain. These dynamically induced spectra occur when the excitation fields operate at low intensities and are neither one- nor two-photon resonant with the various transitions of the system.     

Experimental assessment of fluorescence microscopy signal enhancement by stimulated emission

The quantity of photons generated during fluorescence microscopy is principally determined by the quantum yield of the fluorescence dyes and the optical power of the excitation beam. However, even though low quantum yields can produce poor images, it is challenging to tune this parameter, while increasing the power of the excitation beam often results in photodamage. Here, we propose the use of stimulated emission (SE) as a means of enhancing both the signal intensity and signal-to-noise ratio during confocal fluorescence microscopy. This work experimentally confirmed that both these factors can be enhanced by SE radiation, through generating a greater number of photons than are associated with the standard fluorescence signal. We also propose the concept of stimulated emission enhancing fluorescence (SEEF) microscopy, which employs both the SE and fluorescence signals, and demonstrate that the intensity of an SEEF signal is greater than those of the individual SE and fluorescence signals.     

Effective stimulated emission and excited-state absorption cross-section spectra of para-phenylene-ethynylene polymers

The effective stimulated emission cross-section spectra, em,eff(), the stimulated emission cross-section spectra, em(), and the excited-state absorption cross-section spectra, ex()=em()–em,eff(), of the conjugated polymers poly(2,5-dioctadecyloxy-paraphenylene-ethynylene-co-2,5-thienyl) (OPT) and poly(2,5-dioctadecyloxy-paraphenylene-ethynylene-co-2,5-pyridinyl) (OPP) in tetrahydrofuran are determined in the fluorescence region. em,eff() determined by a fluorescence amplification technique is found to be small compared to em() indicating that ex() is nearly as large as em(). The laser excitation to the wide inhomogeneously broadened S1-absorption band leads to the formation of spectrally structured inhomogeneously broadened lattice relaxed exciton conformations extending on average over two monomer units which are thought to be responsible for upper-to-lower-state stimulated emission and lower-to-upper-state (excited-state) absorption.     

共轭聚合物中受激吸收与受激辐射的量子动力学研究

基于扩展的一维SSH紧束缚模型结合非绝热的分子动力学方法,理论研究了共轭聚合物分子(PPV)在光脉冲作用下受激吸收和受激辐射的量子动力学过程.首先,设定分子初始处于基态,讨论了受激吸收过程中不同的电子受激跃迁模式与光激发脉冲的关系.通过对终态的分析,发现分子受激后只能产生电子-空穴的束缚态,包括:激子、双激子和高能激子.计算了各种激发态的产率,特别是,给出了各种激发态产率与光激发能量的定量关系.此外,基于实验,分别讨论了光激发强度对高能激子和双激子产率的影响,并与实验结果进行了比较.最后,设定分子初始分别处于激子和双激子态,研究了分子内定域能级之间的受激辐射过程,并简单讨论了激子和双激子受激辐射与光激发能量及强度的关系.     

Calculation of the amplification coefficient of stimulated emission from fluorescence measurements

A formula for calculating the amplification coefficient for a four-level system from luminescence data is derived. Measurement of the absolute value of the quantum yield of photoluminescence is described and the amplification coefficient is calculated for four different sorts of glass.This work was performed in the Institute of Physics, Czechoslovak Academy of Sciences, Prague.Discussions on the problem with K. Pátek and preparation of the samples by J. Walter are gratefully acknowledged.     

Blue and infrared stimulated emission from alkali vapors pumped through two-photon absorption

The 5 ²D_3/2, 5 ²D_5/2, and 7 ²S_1/2 states of rubidium and the 7 ²D_3/2, 7 ²D_5/2, and 9 ²S_1/2 states of cesium were populated at low pressure by two photon excitation using a pulsed dye laser. Blue beams from the Rb 6 ²P_{3/2, 1/2}–5 ²S_1/2 and Cs 7 ²P_{3/2, 1/2}–6 ²S_1/2 transitions were observed. In addition, infrared beams were observed arising directly from the pumped ²D states, establishing a collisionless cascade mechanism. Threshold is modest at about 0.3 mJ/pulse or 2×10⁵ W/cm². Slope efficiencies increase dramatically with alkali concentration and peak at 0.4%, with considerable opportunity for improvement. This versatile system might find applications in both underwater communications and for infrared counter measures.     

The generation of stimulated resonance Raman scattering using external dye fluorescence

Different dyes in locally separated cells have been investigated in order to obtain information about the mechanisms responsible for the generation of stimulated resonance Raman scattering. The experiments show in agreement with the theoretical results that the appearance of initial signals as well as the effect of dye amplification influences the behaviour of stimulated resonance Raman scattering.     

Ultrashort 1.5-microm laser excited upconverted stimulated emission based on simultaneous three-photon absorption

Simultaneous three-photon excited stimulated emission at (cavityless lasing) approximately 610 nm has been achieved in an organic chromophore solution pumped by approximately 1.5-microm ultrashort coherent radiation. The stimulated emission can be observed only in the forward and backward directions and is characterized by its high directionality and spectral narrowing. The divergence angle for the backward stimulated emission is considerably smaller than that for the forward stimulated emission, which resembles optical phase conjugation.