Quantum beats of vibrational modes in both the low and high wavenumber regions studied by fs-CARS

We have studied the quantum beats of vibrational modes in different phase samples by employing femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) technique. The temporal chirped white-light continuum (WLC) is used for the Stokes pulse, we have achieved the selective excitation of vibrational modes in both the low and high wavenumber regions without complicated laser system, and observed quantum coherence between vibrational modes. This work is of special significance to enhance our understanding of quantum coherence and develop the applications of quantum technologies.     

Coupled-cavity passively Q-switched two-Stokes microchip laser

Experimental and theoretical studies of the coupled-cavity diode-pumped Nd:YAG/Cr:YAG microchip lasers with intracavity Raman conversion of laser pulses in a Ba(NO₃)₂ crystal into two Stokes pulses have been made. Two lasers with a different cavity length have been investigated. The minimal pulse durations at the 2nd Stokes wavelength were ??100 ps in the short-cavity laser at pulse energy of 5???J, and the pulse repetition rate reached 20?C24?kHz. The laser and Stokes pulse dynamics, as well as the spatial intensity distribution of the laser and the 1st Stokes beams at the output mirror have been recorded. A model describing such coupled-cavity microchip Raman lasers has been developed. The numerically simulated laser and Stokes pulse dynamics, and the calculated pulse energy, duration, and repetition rate are in good agreement with the experimental data.     

Temporal coherent control induced by wave packet interferences in one and two photon atomic transitions

The interaction of a sequence of two identical ultrashort laser pulses with an atomic system results in quantum interferences as in Ramsey fringes experiments. These interferences allow achievement of temporal coherent control of the excitation probability. We present the results of a temporal coherent control experiment on two different atomic systems: one-photon absorption in K (4s-4p) and two-photon absorption in Cs (6s-7d). In K, the quantum interferences between the two excitation paths associated with the laser pulses are revealed through rapid oscillations of the excitation probability as a function of the time delay between the two pulses. These oscillations take place at the transition frequency (period T = 2.56 fs). The interferences are modulated by beats (at about 580 fs) resulting from the doublet structure of the excited state (4p (² P _1/2 , ² P _3/2 )). Three complementary interpretations of this experiment are presented: in terms of beats of quantum interferences, of variation in the spectrum intensity, and of wave packet interferences. Whenever the two laser pulses are temporally overlapped, optical interferences are superimposed on to the quantum interferences. The distinction between these two types of interference is clearly revealed in the two-photon excitation scheme performed on Cs (6s-7d (² D _3/2 , ² D _5/2 )) because quantum interferences occur at twice the frequency of the optical interferences. Received: 30 December 1997 / Revised: 28 February 1998 / Accepted: 4 March 1998     

Efficient coherent population transfer among three states in NO molecules by Stark-Chirped rapid adiabatic passage

We present the experimental demonstration of a novel, efficient, and selective technique to prepare population inversion. The technique is an extension of Stark-chirped rapid adiabatic passage (SCRAP), i.e., SCRAP among three states. In this process a Lambda-type quantum system is driven by two laser pulses, the pump and Stokes pulses, which are appropriately detuned from transition frequencies. A third laser pulse induces a dynamic Stark shift in the upper energy level, and the timing of all three pulses is controlled in order to prepare population inversion between the two lower states in the Lambda-type level scheme. Our data on population transfer in nitric oxide (NO) molecules clearly show that SCRAP among three states provides an advantageous alternative to such techniques as stimulated Raman adiabatic passage.     

Generation of anti-Stokes radiation upon amplification of stokes pulses in transient stimulated Raman scattering

An analytical solution is obtained for equations of transient stimulated Raman scattering for short samples and a weak seed Stokes wave in the approximation of a given pump field, and a numerical solution for more extended media or more intense input Stokes pulses with allowance for pump depletion. The dependence of the anti-Stokes wave energy on its spatial mismatch with the Stokes and laser waves is studied. The optimum angle of the anti-Stokes generation is found as a function of the length of the system, transverse relaxation rate, laser pulse energy, and intensity ratio of the Stokes and pump pulses at the entrance of the sample.     

Stimulated raman scattering of rapidly amplified short laser pulses

The theory of transient forward stimulated Raman scattering (FSRS) of rapidly amplified short laser pulses is put forth to complement the classical theory for FSRS of stationary pulses. Quantitative conditions for FSRS suppression are identified. In particular, it is shown quantitatively how the limitation imposed by pumped pulse FSRS on the output laser intensity in plasma-based ultrapowerful backward Raman amplifiers can be overcome through a selective detuning of the Stokes resonance.     

Optical exciton Stark effect and quantum beats at exciton quasienergy levels in quantum wells

The optical Stark effect and quantum beats in a GaAs/AlGaAs quantum well are investigated theoretically for the case when the first two electron size-quantization levels are mixed dynamically by a high-intensity CO₂ laser pulse polarized perpendicular to the plane of the quantum well. The quasienergy spectrum of heavy-hole excitons and the ratio between the probabilities of exciton transition with and without a strong electromagnetic field are obtained. The time-dependent intensity of absorption of the sensing light is determined. It exhibits quantum beats at twice the electron Rabi frequency. Fiz. Tverd. Tela (St. Petersburg) 39, 1291–1294 (July 1997)     

Ba(NO3)2晶体受激拉曼散射的角度分布和脉冲压缩

报道了利用重复频率为30 Hz，波长为532 nm 的Nd∶YAG倍频激光单次通过抽运硝酸钡（Ba(NO3)2）晶体（晶体由水溶液降温法生长，长度为48 mm，横截面为10 mm×10 mm），获得高效率的一阶（563 nm），二阶（599 nm）和三阶（639 nm）斯托克斯光的实验结果.硝酸钡晶体沿着［110］晶轴方向切割.观测到一、二、三阶斯托克斯光呈锥形环分布，一、二、三阶斯托克斯光的散射外边缘与抽运光轴线间的夹角大小分别为1.7°，3.5°，5.0°.同时也观测到Ba(NO3)2的SRS角度分布与抽运光强度无关.定性分析认定,Ba(NO3)2的SRS角度分布主要是由相位匹配过程决定的.测得抽运光、一、二、三阶斯托克斯光的脉宽分别为11.6 ns，9.8 ns，8.4 ns和4.5 ns.当抽运光功率密度约为150 W/cm2时，获得一、二、三阶斯托克斯光的最大光光转换效率，分别为33.5%，8.8%和3.4%. 此外，由于晶体中的热沉积效应，观察到了Ba(NO3)2晶体的SRS转换效率饱和现象.     

Experimentelle Untersuchung von Emissionseigenschaften eines koaxialen Stickstofflasers

Inspection of the emission properties of a coaxial nitrogen laser with pulses of about 2 ns half-width and peak powers exceeding 20 kW leads to the following results: a) For the first time not only the 0 → 0 vibrational band but also the 0 → 1 and 1 → 0 bands in the second positive system of molecular nitrogen are observed in superradiant emission at gas temperatures of 300 and 400 K. Increasing the temperature in the laser has no considerable effect on the spectral intensity distribution. The theoretical results of Kaslin and Petrash for the dependence of the gain on the rotational quantum numbers of the upper laser level are verified qualitatively. b) The main reason for the decrease in power with increasing time of laser operation is found to be a growing rise time of the discharge current caused by the temperature dependence of the laser material dielectric loss factor. c) At certain working conditions laser pulses with two time separated power peaks are observed; no definitive explanation of this effect can be offered now.     

Optical experiments beyond the quantum limit: Squeezing,entanglement, and teleportation

Results of experiments recently performed are reported, in which two optical parametric amplifiers were set up to generate two independently quadrature squeezed continuous wave laser beams. The transformation of quadrature squeezed states into polarization squeezed states and into states with spatial quantum correlations is demonstrated. By utilizing two squeezed laser beams, a polarization squeezed state exhibiting three simultaneously squeezed Stokes operator variances was generated. Continuous variable polarization entanglement was generated and the Einstein-Podolsky-Rosen paradox was observed. A pair of Stokes operators satisfied both the inseparability criterion and the conditional variance criterion. Values of 0.49 and 0.77, respectively, were observed, with entanglement requiring values below unity. The inseparability measure of the observed quadrature entanglement was 0.44. This value is sufficient for a demonstration of quantum teleportation, which is the next experimental goal of the authors.     

The generation of high pulse and average power radiation in eye-safe spectral region by the third stokes generation in barium nitrate Raman laser

The generation of high pulse and average power radiation in the eye-safe region (wavelength around 1.599 μm) by the third Stokes generation in a barium nitrate Raman laser was demonstrated by pumping with 10 ns pulses of a Nd:YAG laser. Converted pulse energy was up to 93 mJ (peak power was 10 MW) at a pump energy of 300 mJ, which corresponds to a quantum efficiency of 47%. The average output power of the third Stokes radiation was 1.8 W.     

Real-time observation of phase-controlled molecular wave-packet interference

The quantum interference of two molecular wave packets has been precisely controlled in the B electronic state of the I2 molecule by using a pair of fs laser pulses whose relative phase is locked within the attosecond time scale and its real-time evolution has been observed by another fs laser pulse. It is clearly observed that the temporal evolution changes drastically as a function of the relative phase between the locked pulses, allowing us to read both amplitude and phase information stored in the wave functions of the molecular ensemble.     

非相干光时延四波混频中V型三能级原子的布居弛豫量子拍的理论研究

对简并非相干光时延四波混频中布居弛豫量子拍的物理机制及产生过程进行了理论研究,并根据真空与原子耦合观点,具体分析讨论了两种情形的三宽带光脉冲作用于V型三能级原子布居弛豫量子拍的形成过程和讯号特征.经比较发现,理论与实验结果相当吻合.     

The features of nonlinear absorption during resonance one- and two-photon excitation of the basic exciton transition in colloidal CdSe/ZnS quantum dots

The features of the nonlinear absorption of CdSe/ZnS quantum dots (colloidal solution) in the case of resonant one- and two-photon excitation of the basic exciton transition by powerful ultra-short laser pulses were determined. In one-photon excitation, with an increasing intensity of impulses, a decrease in absorption (bleaching) is relayed by an increase in absorption, which is associated with the process of the filling of the states (saturation) of a two-level system with the lifetime of the excited state depending on the light intensity. The arising Fresnel or Fraunhofer diffraction of the laser ray that pass through a colloidal solution with a high concentration of quantum dots is associated with the formation of the transparency channel and self-diffraction of laser ray on an induced diaphragm. In two-photon excitation, the features of the nonlinear absorption and luminescence tracks (the dependence of luminescence intensity on distance) were explained by the influence, in addition to the two-photon absorption, of the processes that are responsible for the slower growth of nonlinear absorption and luminescence quenching at high intensities of laser pulses.     

Time-resolved coherent anti-Stokes Raman scattering with a femtosecond soliton output of a photonic-crystal fiber

We demonstrate time-resolved coherent anti-Stokes Raman scattering (CARS) by using a frequency-tunable femtosecond soliton output of a silica photonic-crystal fiber (PCF) as a Stokes field. This approach allows quantum beats originating from two close Raman modes to be resolved in the time-domain CARS response. The nonresonant CARS background is efficiently suppressed by introducing a delay time between the probe pulse and the pump-Stokes pulse dyad, suggesting a convenient fiber-optic format for the Stokes source in time-resolved CARS and allowing sensitivity improvement in PCF-based CARS spectroscopes and microscopes.     

High-intensity laser-field amplification between two foils

Interaction of two oppositely directed ultraintense laser pulses with two closely placed thin foils is modeled analytically and investigated by particle-in-cell simulation. It is shown that laser energy can be trapped and accumulated between the foils. The intensity could reach a 100-fold that of the pump lasers. The trapping is found to be bistable and the parameters for stable energy confinement and enhancement are given. The ultrahigh fields that can be produced have many potential applications, including that of verifying nonlinear quantum electrodynamics effects.     

Insulator–metal transition in CaTiO_3 quantum dots induced by ultrafast laser pulses

According to time-dependent density functional theory(TDDFT), we study the interactions between ultra-fast laser pulses and two kinds of calcium titanate quantum dots(PCTO-QDs and MCTO-QDs). Under the action of localized field effect, ultrafast laser can induce quantum dots to make the transition from insulator to metal. The PCTO-QDs are ultimately metallic, while the MCTO-QDs are still insulator after experiencing metal state. This is bacause the stability of the unsaturated atoms in the outermost layer of PCTO-QDs is weak and the geometric configuration of MCTO-QDs as a potential well will also reduce the damage of laser. Moreover, laser waveforms approaching to the intrinsic frequency of quantum dots tend to cause the highest electron levels to cross the Fermi surface. In this paper, it is reported that the insulating quantum dots can be transformed into metal by adjusting the intensity and frequency of laser. The importance of local morphology is emphasized by comparing two kinds of CTO-QDs. More importantly, it is an important step to identify the potential properties of perovskite materials.     

Interference of dissociating wave packets in the I2 molecule driven by femtosecond laser pulses

The interference between two dissociating wave packets of the I2 molecule driven by femtosecond laser pulses is theoretically studied by using the time-dependent quantum wave packet method.Both the internuclear distanceand velocity-dependent density functions are calculated and discussed.It is demonstrated that the interference pattern is determined by the phase difference and the delay time between two pump pulses.With two identical pulses with a delay time of 305 fs and a FWHM of 20 fs,more interference fringes can be observed,while with two pump pulses with a delay time of 80 fs and a FWHM of 20 fs,only a few interference fringes can be observed.     

Picosecond pulse generation in a benzene raman generator amplifier system

Picosecond ruby laser pulses generate simultaneously Stokes pulses of the 992 cm^–1 ring breathing mode and of the 3063 cm^–1 CH-stretching mode of benzene in a Raman generator cell by stimulated Raman scattering under self-focusing conditions. The Raman generator light is spectrally filtered, collimated, and amplified in three Raman amplifier cells pumped by ruby laser pulses. Both first Stokes lines have been selectively amplified to short, low divergence, intense light pulses.     

中红外组合激光场产生低于50阿秒的单个脉冲

研究了氦离子在中红外组合激光场作用下的高次谐波辐射和孤立阿秒脉冲的产生. 研究结果表明,当主脉冲强度相对低时,谐波截止在657阶次处,形成一个宽度为287 eV的连续谱. 当主脉冲强度相对高时,可使谐波截止拓展到1795阶次,连续谱加宽到834 eV. 在两种情况下,我们实现了长量子路径选取,并且产生转换效率较高的连续谱. 特别是对于相对高的激光强度,叠加450—1590阶次内任意87 eV的谐波都可以直接得到50 as以内的单个脉冲.