Optical pumping and coherence effects in fluorescence from a four level system

An effective four-level system around the D2 line of ⁸⁵Rb at room temperature, is experimentally investigated by fluorescent studies under the action of two driving fields L1 and L2. This system exhibits unique features in fluorescence as a function of frequency separation between L1 and L2. In particular, at two-photon resonance, when the Rabi frequency of L1 exceeds that of L2, signatures of Electromagnetically Induced Transperancy effect (EIT) arising from the three-level Λ sub-system is present as a sub-natural dip in fluorescence from the fourth level. At comparable strengths of L1 and L2 the fluorescence features indicate a regime, where the effects arising from optical pumping and EIT effect due to ground hyperfine level coherence coexist. We see in the coexistence regime, saturation effects arising from difference frequency crossing (DFC) resonances and optical pumping around the EIT window. At low strengths of L1, all signs of coherence vanishes from the system and the fluorescent features result from incoherent optical pumping through the Autler-Townes split states of the excited state hyperfine levels, which are split due to the stronger L2 laser. The dominant role of the L1 laser in creating a robust transparency signal even in the presence of an off-resonant excitation is brought out. The results are supported by density matrix calculations.     

Excitation of a singly charged Sr Ion by e-Sr collisions

Excitation of spectral lines of SrII, including laser lines, is studied experimentally in collisions of slow electrons with strontium atoms. Twenty-one excitation cross sections are measured at an energy of electrons of 30 eV. Five optical excitation functions are recorded at an energy ranging from 0 to 200 eV. Direct-excitation cross sections for the 5p ² P°_{1/2, 3/2} levels and the contribution to their population by cascade transitions are calculated. Excitation cross sections of two laser transitions in the IR spectral region are determined using known branching ratios. The results obtained are compared with data of previous experiments.     

Studying the possibility of deep laser cooling of <Superscript>24</Superscript>Mg atoms in an optical lattice: Two-level quantum model

The possible deep laser cooling of ²⁴Mg atoms in a deep optical lattice in the presence of an additional pumping field resonant to the narrow 3s3s¹S₀ → 3s3p³P₁ (λ = 457 nm) optical transition is studied. Two quantum models of the laser cooling of atoms in the optical trap are compared. One is based on the direct numerical solution to the kinetic quantum equation for an atomic density matrix; it considers both optical pumping and quantum recoil effects during interaction between the atoms and field photons. The second, simplified model is based on decomposing the states of the atoms over the levels of vibration in the optical trap and analyzing the evolution of these states. The comparison allows derivation of optical field parameters (pumping field intensity and detuning) that ensure cooling of the atoms to minimal energies. The conditions for fast laser cooling in an optical trap are found.     

Resonant interaction of femtosecond radiation with a cloud of cold <Superscript>87</Superscript>Rb atoms

The resonant interaction of ⁸⁷Rb atoms in a magneto-optical trap with femtosecond laser radiation in the spectral range 760–820 nm has been investigated experimentally. It has been demonstrated that femtosecond laser radiation with a spectral width of 10 nm interacts with an atomic ensemble as a set of spectrally narrow modes and as an ionizing laser field simultaneously. The dynamics of trap loading in the presence of ionization by femtosecond radiation has been studied, and the 5D _5/2 level population produced by an additional weak laser field has been measured.     

Narrow resonances of saturated absorption of the asymmetrical molecule CHFCiBr and the possibility of weak current detection in molecular physics

According to V.S. Letokhov's suggestion the neutral and charged weak currents (in weak interactions) must cause a splitting of the vibrational-rotational levels of left and right molecules by a relative value of 10^-15–10^-16. To detect this effect an experiment is proposed which consists of direct measuring the beat frequency of two frequency stabilized lasers, their references being two identical narrow spectral lines in the vibrational-rotational spectrum of two optical isomers of an asymmetrical molecule. For this purpose one may use narrow saturated absorption resonances of CHFClBr induced in the cw CO₂ laser field. The effects having influence on the reproducibility of the frequency by molecular reference and the necessity of identity conditions for cells with left and right molecules (CHFClBr) are discussed.     

Vibrational spectra of a single crystal of lead germanate and the effect of RS amplification on its surface

For single crystal wafers of lead germanate cut perpendicularly to the C-axis an investigation has been made of IR transmission and RS spectra atλ _exc =632.8, 514.5, and 488 nm in the spectral range of 400–1800 cm⁻¹. An interpretation of the spectra recorded is suggested. In the RS spectra specific features of line intensity distributions are detected. The dependences of the intensity distribution in the peculiarities detected on the excitation wavelength and the angle of incidence of exciting radiation and transmission of the reflected exciting laser beam relative to the optical axis of illumination of the spectrometer inlet slit are investigated. The reason for the occurrence of these specific features is suggested: the interference of RS radiation and exciting laser radiation diffracted on the non-working narrow sides of the diffraction gratings of the spectrometer. The possibilities of amplifying the spectral lines in the RS spectra by imposing diffracted one-mode exciting radiation on them are discussed. Translated from zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 220–226, March–April, 1999.     

Multiphoton ionization of iodine atoms and CF<Subscript> 3</Subscript>I molecules by XeCl laser radiation

We report about effective ionization of iodine atoms and CF₃I molecules under the action of intense XeCl laser radiation (308 nm). The only ion fragment resulting from the irradiation of the CF₃I molecules is the I⁺ ion. We have studied the influence of the intensity, spectral composition, and polarization of the laser radiation used on the intensity of the ion signal and the shape of its time-of-flight peak. Based on the analysis of the results obtained, we have suggested the mechanism of this effect. The conclusion drawn is that the ionization of the iodine atoms by the ordinary XeCl laser with a nonselective cavity results from a three- (2 + 1)-photon REMPI process. This process is in turn due to the presence of accidental two-photon resonances between various spectral components of the laser radiation and the corresponding intermediate excited states of the iodine atom. The probability of ionization of the atoms from their ground state I(²P_3/2) by the radiation of the ordinary XeCl laser is more than two orders of magnitude higher than the probability of their ionization from the metastable state I^*(²P_1/2). The ionization of the CF₃I molecules by the XeCl laser radiation occurs as a result of a four-photon process involving the preliminary one-photon dissociation of these molecules and the subsequent (2 + 1)-photon REMPI of the resultant neutral iodine atoms.     

Photochemical xenon-oxide laser

Theoretical and experimental investigations are described, aimed at the develo pment, construction, and study of the features of a photochemical xenon-oxide laser whose action is based on the photodecomposition of nitrous oxide by vacuum ultraviolet radiation of an open hlgh-current discharge with formation of oxygen atoms followed by oxidation of the xenon. The theoretical problems connected with the determination of the mechanism and the calculation of the kinetics of the physicochemical processes in an XeO laser are considered. A numerical computation of the pumping is carried out, and the inversion and gain on the laser transition are calculated. The construction of the laser is described, as well as an experimental procedure for obtaining the optimal temperatures, composition, and pressure of the working-mixture components. The spectral composition of the laser radiation and the energy characteristics of the laser are investigated. The heat released in the course of the photolysis, the constants of the quenching rates of the upper laser state of XeO (2¹Σ⁺), and the vibrational relaxation of the lower state of XeO (1¹Σ⁺) are estimated. The internal losses in the active medium of the photochemical XeO laser are theoretically investigated. The possibility of increasing the volume of the active medium using a plane-parallel cavity is estimated.     

Observations and analysis of resonant laser ablation of GaAs

Laser ablation of solid GaAs samples has been studied using one tunable pulsed dye laser. At relatively low laser power, enhancements of up to several hundred times have been observed in the yield of resonantly ionised Ga using laser wavelengths corresponding to the atomic transition 4² P _1/2-4² D _3/2. The influences of laser power and target geometry, on the ion yield and spectral profile, are discussed. It is argued that the resonant excitation and ionisation processes occur in the gas phase of the atoms ablated from the sample surface, and the observed asymmetric spectral profile results from laser-induced collisional processes, e.g., optical collisions, under conditions of relatively high atomic density in the interaction region. Potential applications are foreseen for resonant laser ablation in trace analysis.     

Investigations of Yb-doped optical fiber using selective laser excitation

The method of tunable site-selective laser excitation was used for a study of the spectral and fluorescence characteristics of Yb³⁺ optical centers in laser optical fiber. Decay curves and positions of fluorescence spectra maxima at different wavelengths of selective laser excitation and positions of excitation spectra maxima at different fluorescence selective registration provide new information on lifetimes and Stark energy distribution among inhomogeneously broadened Yb³⁺ lines. The obtained Stark splitting energy distribution diagram demonstrates the tuneability of Yb³⁺ laser oscillation wavelength under tunable excitation of a pump laser.     

Laser locking to the ¹⁹⁹Hg ¹S₀ - ³P₀ clock transition with 5.4 × 10^-15/✓τ fractional frequency instability

With Hg199 atoms confined in an optical lattice trap in the Lamb-Dicke regime, we obtain a spectral line at 265.6 nm for which the FWHM is ?15??Hz. Here we lock an ultrastable laser to this ultranarrow S₀1?P₀3 clock transition and achieve a fractional frequency instability of 5.4×10^?15/? for ??400??s. The highly stable laser light used for the atom probing is derived from a 1062.6 nm fiber laser locked to an ultrastable optical cavity that exhibits a mean drift rate of ?6.0×10^?17??s^?1 (?16.9??mHz?s^?1 at 282 THz) over a six month period. A comparison between two such lasers locked to independent optical cavities shows a flicker noise limited fractional frequency instability of 4×10^?16 per cavity.     

光梳主动滤波放大实现锶原子光钟二级冷却光源

提出一种结合注入锁定技术的主动滤波放大方法,将光梳直接注入锁定至光栅外腔半导体激光器,产生窄线宽激光光源,该光源可以用于锶原子光钟二级冷却.实验中,将中心波长为689 nm,带宽为10 nm的光梳种子光源注入689 nm光栅式外腔半导体激光器,通过半导体增益光谱与半导体光栅外腔,从飞秒光梳的多个纵模梳齿中挑选出一个纵模模式来进行增益放大,再通过模式竞争,实现单纵模连续光输出;同时,光梳的重复频率锁定在线宽为赫兹量级的698 nm超稳激光光源上,因此,注入锁定后输出的窄线宽激光也继承了超稳激光光源的光谱特性.利用得到的输出功率为12 mW的689 nm窄线宽激光光源实现了88Sr原子光钟的二级冷却过程,最终获得温度为3μK,原子数约为5×10~6的冷原子团.该方法可拓展至原子光钟其他光源的获得,从而实现原子光钟的集成化和小型化.     

Sub-Doppler spectral resolution in a resonantly driven four-level coherent medium

Sub-Doppler spectral resolution has been studied in Doppler-broadened multi-level ⁸⁷Rb atoms coherently coupled by two strong laser fields. Narrow spectral features of absorption or gain are observed in the center or sides of the Doppler-broadened absorption profile. Analytical and numerical calculations based on a four-level N type model are presented to explain the experimental results.     

Study of 2<Emphasis Type="Italic">ω</Emphasis> and 3/2<Emphasis Type="Italic">ω</Emphasis> harmonics in ultrashort high-intensity laser plasma interaction

An experimental study is presented on measurements of optical spectrum of the laser light scattered from solid surface irradiated by Ti:sapphire laser pulses up to an intensity of 1.2 × 10¹⁸ W cm⁻². The spectrum has well-defined peaks at wavelengths corresponding to 2ω and 3/2ω radiations. The spectral features vary with the laser intensity and show blue-shift with increasing laser intensity. At a constant laser fluence, the spectrum is red-shifted with increasing laser pulse duration. The observed results are explained in terms of the density scale length variation of the plasma and laser chirp.     

Excimer laser-induced ablation of clean and CO-covered polycrystalline copper surfaces: Generation and characterization of high energy copper species

Copper atoms, clusters and ions were generated by ablation of clean and CO-covered polycrystalline copper surfaces under UHV conditions, using a high peak power, pulsed excimer laser as the energy source. The species in the vapor phase were characterized by time-resolved mass spectrometry and optical detection of laser induced fluorescence. The laser power density threshold of vaporization for the clean copper (300(30) MW/cm²) was significantly lower than the threshold in the case of CO-covered copper surfaces (> 400 MW/cm²).     

A diode-laser optical frequency standard based on laser-cooled Ca atoms: Sub-kilohertz spectroscopy by optical shelving detection

We report an optical frequency standard at 657 nm based on laser-cooled/trapped Ca atoms. The system consists of a novel, compact magneto-optic trap which uses 50 mW of frequency-doubled diode laser light at 423 nm and can trap >10⁷ Ca atoms in 20 ms. High resolution spectroscopy on this atomic sample using the narrow 657 nm intercombination line resolves linewidths (FWHM) as narrow as 400 Hz, the natural linewidth of the transition. The spectroscopic signal-to-noise ratio is enhanced by an order of magnitude with the implementation of a “shelving" detection scheme on the 423 nm transition. Our present apparatus achieves a fractional frequency instability of in 1 s with a potential atom shot-noise-limited performance of and excellent prospects for high accuracy. Received 2 November 1998     

High repetition-rate wavelength tuning of an extended cavity diode laser for gas phase sensing

A method for rapid wavelength tuning of an extended cavity diode laser (ECDL) is presented providing for high resolution, narrow bandwidth output over limited spectral regions. The method permits tuning over isolated spectroscopic features at repetition rates of tens of kHz, greatly exceeding conventional ECDL tuning speeds. In this paper we present high repetition rate laser induced fluorescence (LIF) spectroscopy of the 5²P_1/2 to 6²S_1/2 transition in indium at 410 nm, to demonstrate the technique. The presented ECDL design is very easy to implement, cheap and robust, as it employs no moving parts and can be used over all wavelength regions where FP diode lasers are available. This extends the usefulness of standard FP diode lasers to high speed sensing applications. Advantages and disadvantages of the technique are discussed. PACS 42.55.Px; 42.60.Fc; 42.62.Fi; 32.50.+d     

Molecule formation and energy transfer processes in a vapor with high density of 3P-excited sodium atoms

A rich fluorescence spectrum extending between 4000 and 8200 Å has been observed whenever sodium vapor is excited by dye laser light tuned to the 3²S → 3²P transition. Molecule formation due to collisions between excited and unexcited atoms is manifested by the presence of an emission band of sodium in the spectral range 4160–4570 Å.     

Discovery of dumbbell-shaped Cs*Hen exciplexes in solid 4He

We have observed several new spectral features in the fluorescence of cesium atoms implanted in the hcp phase of solid helium following laser excitation to the 62P states. Based on calculations of the emission spectra using semiempirical Cs-He pair potentials the newly discovered lines can be assigned to the decay of specific Cs*Hen exciplexes: an apple-shaped Cs(APi3/2)He2 and a dumbbell-shaped Cs(APi1/2)Hen exciplex with a well-defined number n of bound helium atoms. While the former has been observed in other environments, it was commonly believed that exciplexes with n>2 might not exist. The calculations suggest Cs(APi1/2)He7 to be the most probable candidate for that exciplex, in which the helium atoms are arranged on a ring around the waist of the dumbbell-shaped electronic density distribution of the cesium atom.     

Comparative study of spectral narrowing of a pulsed Ti:Sapphire laser using pulsed and CW injection seeding

We report a comparative study of a pulsed as well as continuous-wave (cw) injection seeding of a Ti:Sapphire laser pumped by aQ-switched frequency-doubled Nd³⁺:YAG laser for achieving narrow spectral bandwidth. The results have indicated that the Ti:Sapphire laser using either a pulsed or a cw injection seeding could achieve efficient energy extraction in a narrow spectral bandwidth. In the case of pulsed injection seeding, the injection energy required for the complete injection seeding critically depended upon the timing of the Ti:Sapphire laser with respect to the delayed onset of the slave laser. On the other hand, in the case of cw injection seeding, the spectral bandwidth of the Ti:Sapphire laser was efficiently narrowed down to approximately 0.01 cm^–1 with an injection power of less than 1 mW. In both types of injection seeding, characteristics observed experimentally were compared with those obtained by a numerical simulation code based on the one-dimensional rate-equation model.