Two-photon spectron

The propagation of a two-photon light in a transparent medium with group velocity dispersion is considered. It is shown that, even in the stationary case of two-photon light generation by cw pumping, the second-order light correlation function behaves like a short pulse: when propagating in a medium, this function smears and at large distances acquires the spectral shape of two-photon radiation.     

One- and two-photon absorption-amplification response in a four-level atomic system with vacuum-induced coherence

This paper theoretically studies the effects of the vacuum-induced coherence on one- and two-photon absorption in a four-level atomic medium. It finds that the one- and two-photon absorption and amplification properties are quite sensitive to the vacuum-induced coherence. It is also shown that the one- and two-photon absorption spectra can be dramatically affected by modulating the relative phase of the applied fields. With the proper choice of the relative phase, the amplification without inversion for the probe field can be realized.     

Influence of self-phase modulation on coherent effects in five-level system

The possibility of formation of a coherent dark state in the five-level system in the entire volume of the medium in an adiabatic pulse propagation is investigated. It is shown that the dark state formation is independent on the value of the first two-photon detuning from the resonance, which may be changed during the propagation in the medium. In the case of the M-type system with the equal strengths of oscillators at the extreme transitions it is shown that the self-phase modulation does not influence the coherent effects while in the ladder-type system it may lead to the destruction of the dark state. The estimation of the length of the medium in which the propagation effects are negligible is obtained.     

Carrier-envelope phase dependence of few-cycle ultrashort laser pulse propagation in a polar molecule medium

We theoretically investigate carrier-envelope phase dependence of few-cycle ultrashort laser pulse propagation in a polar molecule medium. Our results show that a soliton pulse can be generated during the two-photon resonant propagation of few-cycle pulse in the polar molecule medium. Moreover, the main features of the soliton pulse, such as pulse duration and intensity, depend crucially on the carrier-envelope phase of the incident pulse, which could be utilized to determine the carrier-envelope phase of a few-cycle ultrashort laser pulse from a mode-locked oscillator.     

Raman amplification of ultrashort light pulses in inhomogeneously broadened three-level systems

In the limit of pulse durations that are shorter than the characteristic times of atomic decays, the dynamics of the propagation of a probe light pulse under the action of a driving pulse in the form of a self-induced transparency soliton applied to the adjacent transition of the V-scheme is considered. For a three-level absorber, it is shown that, in the absence of two-photon inhomogeneous broadening, the effect of soliton-induced transparency (SOIT) for the probe field takes place, which consists of the probe pulse undergoing forced localization by the driving pulse and propagates further without a change in the energy or shape. This type of transparency has no threshold with respect to both the energy and the area and is observed for arbitrarily weak pulses. The SOIT effect disappears in the presence of a two-photon inhomogeneous broad-ening. The effect of Raman amplification in a three-level amplifier with the Raman population inversion is considered. It is demonstrated that, under the action of the two-photon inhomogeneous broadening, the effect of Raman amplification becomes a threshold effect. The broader the two-photon inhomogeneous contour, the larger the Raman inversion required to initiate generation.     

Formation of solitons and realization of the superluminality effect upon femtosecond pulse propagation in a medium containing gold nanoparticles

Based on computer simulation, we demonstrate the possibility of formation of solitons upon propagation of a femtosecond laser pulse in a medium containing gold nanoparticles in the presence of two-photon light absorption. The solitons are formed when the laser pulse induces a positive phase grating. The speed of solitons substantially exceeds the speed of laser radiation propagating in a linear medium.     

Generation of ultrafast pulse via combined effects of stimulated Raman scattering and non-degenerate two-photon absorption in silicon nanophotonic chip

A project of ultrafast pulse generation has been presented and demonstrated by utilizing the combined nonlinear effects of stimulated Raman scattering (SRS) and non-degenerate two-photon absorption (TPA) based on silicon nanophotonic chip, in which a continuous wave (CW) and an ultrafast dark pulse are co-propagating in the silicon chip so that the CW will be modulated inversely by the dark pulse during the propagation. As a result, an ultrafast bright pulse is achieved using the technique. Simulation results show that an ultrafast pulse with a pulsewidth (full-width at half-maximum (FWHM)) of about 50 fs is generated at the end of a 5-mm long silicon chip, when the initial conditions, including an input maximum of 0.5 W and FWHM of ∼176 fs for dark pulse, and CW with power of 5 W, are chosen.      

含时电离对飞秒脉冲激光在强双光子吸收介质中传播特性和光限幅行为的影响

以具有强双光子吸收特性的4,4′-二甲氨基二苯乙烯分子体系为研究对象,通过采用时域有限差分法和预估矫正法数值求解Maxwell-Bloch方程,模拟了飞秒脉冲在该分子介质中的传播过程,研究了含时电离对双光子吸收过程和光限幅行为的影响.研究结果表明,光电离使介质与光场的非线性相互作用减弱,同时自发辐射降低, 随着入射电场的增强,光电离对主脉冲演化有明显的影响.光电离使介质的光限幅失效行为减弱.随着光电离截面增加,光限幅的动力学窗口变宽,表明了光电离有利于增强介质的光限幅效应.在不同的传播距离处,脉冲传播呈现 关键词： 含时电离 双光子吸收 光限幅效应 超短脉冲激光     

Resonant propagation of femtosecond laser pulse in DBASVP molecule：one-dimensional asymmetric organic molecule

We have investigated the resonant propagation of femtosecond laser pulse in 4-trans-[p-（N, N-Di-n-butylamino）- p-stilbenyl vinyl] pyridine medium with permanent dipole moments. The electronic structures and parameters for the compound have been calculated by using density functional theory. In the optical regime, there is one charge-transfer state, and the molecule can thus be simplified as a two-level system. Both the one- and two-photon transitions occur between the ground and charge-transfer states. The numerical results show that the permanent dipole moments have an obvious effect on the propagation of the ultrashort pulse laser. The ideal self-induced transparency disappears for 2π pulse, and second harmonic spectral components occur significantly due to the two-photon absorption process. For the 6π pulse, continuum frequency generation is produced and a shorter duration pulse in time domain with 465 as is obtained.     

Electromagnetically induced transparency; writing, storing, and reading short optical pulses

The spatiotemporal propagation dynamics of a weak probe pulse in an optically dense medium of three-level atoms is studied in the adiabatic approximation under conditions of electromagnetically induced transparency. The atomic coherence induced at the dipole-forbidden transitions is found to be spatially localized. This effect is used for the analysis of the reversible writing (reading) of short optical pulses. The method of pulse time reversal is suggested.     

Two-photon coherent propagation,transmission of 90° phase-shifted pulses,and application to isotope separation

Most single photon coherent propagation effects are shown to have their two-photon analogue. The case of zero-area pulse propagation (90° phase shifted in the two-photon situation), frequency shift with distance, and selective optical excitation for isotope separation are considered. Selective two-photon pumping is shown to occur even when the pulse spectrum overlaps the various isotope shifted lines. An application to radiation pressure isotope separation is proposed.     

Dynamic and inner-dressing control of four-wave mixing in periodically-driven atomic system

Four-wave-mixing(FWM) process is examined by using density matrix formalism in a periodically-driven atomic medium. Numerical result shows that FWM signals can be controlled by selecting different dynamic parameters of the probe field and strengths of the inner-dressing fields. It is also shown that the controllable FWM process is dominantly influenced by the evolution of atomic population difference and two-photon coherence. This dynamic and inner-dressing control of FWM is probably used for optimizing the optical nonlinear process and information processing.     

Nonstationary theory of polarized light pulse propagation through a four-level resonant medium with allowance for nonlinear complex refractive index of medium

Nonstationary theory of polarized light pulse propagation through the medium with resonant transition 1/2 -1/2 is developed with taking into account complexity of the medium nonlinear refractive index. The theory is developed based on the irreducible tensor formalism which allows diagonalizing relaxations matrix and introducing effective decay times associated with the population relaxation, as well as relaxation of coherence between magnetic sublevels of the resonant system. It is shown that as a result of interference of magnetic sublevels in the polarized radiation field the inversionless amplification of radiation occurs due to optical pumping process. The dependence of inversionless amplification on the line broadening and resonance detuning is studied.     

Dynamics of cooperative emissions in a cascade three-level molecular system driven by an ultrashort laser pulse

This paper investigates the dynamics of cooperative emissions in a cascade three-level system driven by an ultra, short laser pulse by solving numerically the full-wave Maxwell-Bloch equations. The 4, 4＇-bis（dimethylamino） stilbene molecule is used as the model molecule because of its strong two-photon absorption property. The two-colour cooperative emissions are studied as functions of molecular number density and dephasing rate of the dipole coherence. The propagation effects on the evolution of the cooperative radiations are also taken into account. The cooperative radiations are enhanced for large number density of the molecule, while the fast dephasing of the dipole coherence reduces the intensity of the cooperative radiations and delays the emission times or even inhibits the formation of the emissions. The delay time of the radiation decreases with the increase of the molecular number density and the propagation distance.     

Two-photon and two-photon-assisted slow light

We show that light pulses propagating in two-photon absorbing systems may present time delays like slow light produced via coherent population oscillations in one-photon interactions. Two regimes are numerically studied for a simplified two-level system: (a) a light pulse at frequency ω/2 undergoes two-photon absorption (TPA) and is delayed by the absorbing system (two-photon slow light) and (b) a light pulse at frequency ω is delayed in a system prepared by TPA of a light pulse at frequency ω/2 (two-photon-assisted slow light). The study carried out in solutions of dyes and dendrites shows significant delays, low distortion, and good transmission for easily reachable experimental conditions. The working principle can be applied to other media and can be used in telecommunications technology.     

Raman self-focusing at maximum coherence

We demonstrate a type of Raman self-focusing and -defocusing that is inherent in operation at maximum coherence. In this regime the two-photon detuning from the Raman resonance controls the refractive index of the medium.     

Dissipative soliton-like plasmon-polariton pulses in extended medium

The propagation of the coupled state of the electron density perturbation in an extended metallic medium and the excitation of a two-level resonant medium are analyzed. The one- and two-photon transitions in the resonant medium are considered. The electron density perturbation is described using the hydrodynamic approximation. The formation of plasmon-polariton pulses is analyzed in the case when losses in the extended medium are compensated for by the pumping of the two-level dielectric medium. Numerical analysis carried out for the two models revealed that the losses in a soliton-like pulse in a thin metallic medium can be compensated for due to the energy transfer from the amplifying medium to electron density waves. It is shown that the dispersion of a medium containing a two-level component may considerably affect the characteristics of the pulses. The possibility of effectively controlling the evolution of soliton-like pulses by varying an external electromagnetic field and the characteristics of the matrix is demonstrated.     

超短脉冲激光在有机分子材料中的动力学过程研究

利用从头计算方法，在密度泛函理论上，计算了分子的电子结构和电偶极矩.通过求解麦克斯韦-布洛赫方程，研究了超短脉冲激光与硝基苯胺分子材料的相互作用，着重分析了分子的固有偶极矩对脉冲激光波形、频谱成分以及分子能级占有率产生的影响.研究结果表明，慢变幅近似和旋波近似不能很好地描述超短脉冲在PNA分子介质中传播.分子的固有偶极矩进一步使脉冲传播背离面积定理，引起了脉冲更快地分裂.当脉冲激光在PNA分子介质中以电荷转移态的激发能共振传播时，脉冲激光频谱中明显地出现了二次谐波成分，显示了该分子具有较强的双光子吸收性质. 关键词： 超短脉冲激光 硝基苯胺分子 麦克斯韦-布洛赫方程     

Femtosecond pulse propagation in silicon waveguides: Variational approach and its advantages

We investigate the propagation characteristics of ultrafast pulses inside silicon waveguides considering frequency chirp associated with each input pulse. Effects of linear losses, two-photon absorption, and free-carrier dynamics are included analytically within the framework of a modified variational formalism and the results are validated by comparing them with full numerical simulations. It is found that an initial chirp helps in maintaining the pulse shape and spectrum in the anomalous-dispersion regime, thereby resulting in soliton-like propagation of ultrashort optical pulses.     

绝热条件下通常的脉冲序列和反直觉的脉冲序列对布居数转移和相干的影响

本文研究了绝热条件下，三能级Λ系统实现布居数转移和相干的两种方法，并比较了激光脉冲序列对布居数转移效率和相干的影响。运用自洽方程和密度矩阵方程，得出了相干控制量子态和布居数的数值模拟图。我们可以看到运用反直觉的脉冲序列比通常的脉冲序列在实现布居数转移和相干上更容易更优越。