Bichromatic two-photon coherent excitation of sodium to provide a dual-wavelength guidestar

A bichromatic sodium guide star based on two-photon coherent excitation followed by cascade emission is considered. Results of density matrix computations, including Doppler averaging, are presented for both a three-level model and the full hyperfine structure of the sodium 3S, 3P, and 4S levels. These results indicate that the proposed scheme for a bichromatic guide star is feasible.     

Bistability in coherent two-photon processes

Using the coupled Maxwell-Bloch equations for coherent two photon processes, we show the existence of bistable behavior of systems undergoing coherent two photon resonant processes such as two photon absorption and Raman process. The system is taken to be contained in a ring cavity. The dependence of the bistable behavior on detuning parameter and the Stark shifts is also discussed.     

The relationship of the two-photon vector model to the coherent excitation of polyatomic molecules

The relationship between multiphoton coherent excitation and the two-photon vector model of Grischkowsky, Loy and Liao is demonstrated. In particular, it is shown that the recent work of Letokhov and Makarov emphasizes the adiabatic following approximation of the vector model while that of Larsen and Bloembergen corresponds to the sudden approximation.     

Theory of coherent two-photon absorption

An exact solution for the propagation of two ultrashort pulses through a two-photon near resonant absorbing medium is obtained under the condition that the initial number of photons/s cm² in the two pulses are the same. High-power pulses develop sharp peaks as they propagate through the medium.     

Yield optimisation aspects in two-photon excitation

The question of choosing optimum laser intensities, detunings and relaxation rates in two-photon excitation is considered. It is shown that under steady state conditions the best excitation efficiency is achieved when rather modest laser powers are used and the atoms excited are not extracted too rapidly. The latter conclusion relates to coherence effects playing a non-negligible role in the excitation, which fact can be exploited to enhance the yield. On leave of absece from the Physics Department, University of Helsinki, Siltavuorenpenger 20, SF-00170, Helsinki, Finland.     

双光子激发时间分辨荧光光谱测量技术

建立了一台基于高重复频率扫描相机的双光子激发时间分辨荧光光谱测量系统，能够同时测量样品的荧光光谱和寿命. 该系统的时间分辨率为6.5—200ps，光谱分辨率为1—3nm，能够实现快速数据采集以及可靠和可重复的寿命和光谱测量. 利用标准荧光染料(若丹明6G、香豆素314)及其混合溶液对该系统进行了测试，所得到的荧光光谱分布和寿命值与文献报道一致. 实验结果表明，该系统能有效区分多组分荧光团. 这为鉴别多荧光团或多组分生物组织提供了一种独特的对比方法，可用于多光谱分辨荧光寿命成像和荧光共振能量转移成像等方面. 关键词： 荧光寿命 荧光光谱 双光子激发 高重复频率扫描相机     

Raman transients observed in Doppler free two-photon excitation

Using Doppler-free two-photon excitation, we have observed the transients produced by the sudden introduction or cut-off of the exciting light. The two-photon excitation is of a particular interest because all the atoms are excited with the same energy detuning from the resonance. We have observed the time-variation of the scattered light for several values of the detuning. The corresponding results show the transition from resonance fluorescence to Raman scattering.     

Lineshapes in collision-induced absorption

We present the classical and quantum versions of collision induced absorption spectra for three exactly solvable 1-D models, and obtain the asymptotic behaviour at high frequency. It is found that the quantum spectra fall off more slowly than the classical, and for realistic systems, they both fall off more slowly than exponential. An improved version of the Landau-Lifshitz WKB method is suggested as a numerically viable method for calculating the high frequency wing in quantum spectra and is tested on the exactly solvable models. The error is found to be negligible in two models and tolerable in the third model, even for the least classical system. Our discussion of an extension of the method using exact wavefunctions increases its scope beyond the collision induced absorption problem.     

Crosstalk between two-photon and two-color (two-photon) excitation in optical beam induced current generation with two confocal excitation beams

We analyze the influence of residual two-photon excitation (2PE) in two-color (two-photon) optical beam induced current (2CE-OBIC) generation in wide band gap semiconductor samples. 2CE-OBIC generation is accomplished with two confocal excitation beams of separation angle θ and wavelengths λ₁ and λ₂ where , λ_e = hc/E_b, h is the Planck’s constant, c is speed of light in vacuum, and E_b is the energy band gap. Because the conduction band of the sample is a continuum, at least one excitation beam would also contribute an undesirable 2PE-OBIC signal that degrades the signal-to-noise ratio of the measured 2CE-OBIC response and broadens the effective OBIC distribution in the sample particularly when θ ≠ 0 or π. We show that the deleterious effects of crosstalk are reduced by a careful selection of λ₁ and λ₂ and the relative excitation beam intensities. λ₁ and λ₂ should be chosen to minimize the ratio of the two-photon absorption coefficients (β₁, β₂) to the 2CE absorption coefficient β₁₂ or at least satisfy the constraint: β₁ + β₂  β₁₂. Keeping the two excitation intensities equal is beneficial only when β₁ = β₂. Otherwise, it is advantageous to bias the intensity ratio towards the wavelength with a lower 2PE absorption coefficient.     

Multistep excitation in coherent nuclear production processes

We suggest an interesting method of observing the existence of two-step excitation in nuclear production processes by analysing coherent low energy experiments with D and He targets.     

The two-photon excitation spectrum of fluorobenzene vapor

The two-photon excitation spectrum of fluorobenzene vapor has been recorded in the region of the $\text{A}\text{?}{}^1\text{B}{}_2\text{←}\text{X}\text{?}{}^1\text{A}{}_1$ transition. The spectrum shows considerable rovibronic structure with the bulk of the intensity lying in the subsystem induced by the ν₁₄(b₂) vibration. Two types of rovibronic contours, arising from ΔK_a = ±1 and from ΔK_a = 0, ±2 transitions, are identified. Major features in these contours are assigned by comparison with synthetic spectra, calculated using known upper and lower state rotational constants. The intensity distribution among the various bands in progressions of the totally symmetric vibrations is considerably different from that in the one-photon absorption spectrum, and the possible reasons for this are discussed.     

原子双光子激发截面的计算

在不可约张量法基础上,利用二次量子化方法计算了原子的双光子激发截面.作为一个实例,计算了Hg原子的双光子激发截面.理论结果与实验结果在数量级上有较好的符合.     

双光子技术在三维成像和三维存储技术中的应用

对双光子过程的空间分辨能力进行了理论分析。阐明了双光子技术在三维成像和三维存储中的独特优势。着重介绍了双光子技术与扫描共焦显微术、近场显微术相结合进行三维成像 ,以及一种多焦点多光子显微术和用连续光源激发的双光子三维成像技术的研究和进展情况。对建立在共焦显微镜基础之上的双光子三维光存储和微细加工方面的研究也作了回顾与展望     

Depolarization of radiation upon coherent excitation

The possibility of cancelling the signal of linear polarization upon coherent excitation of 4s ³ S ₁ atoms via a Λ scheme from 3p ³ P ₁ and 3p ³ P ₂ levels is demonstrated using an ensemble of sulphur atoms as an example.     

Theory of coherent,degenerate two-photon absorption and emission

The theory of the coherent, two-photon resonant interaction of a monochromatic field with N atoms is given. It is seen that the dynamics of the atom-field system can be completely determined when the field is “strong”. Two specific examples are given: (i) two-photon absorption by atoms in ground state, and (ii) stimulated two-photon emission by fully excited atoms, assuming a coherent field in both cases. In case (ii), the field shows photon-antibunching after the decay of half of the atoms. The merits of our approach are shown by comparing with other treatments. Our results can also be applied to certain degenerate four-wave mixing processes which are described by a similar Hamiltonian.     

Efficient two-photon light amplification by a coherent biexciton wave

A reversible coupling between photon pair states and a long-lived, highly coherent biexciton wave in CuCl allows efficient phase-sensitive two-photon amplification or attenuation of ultrashort light pulses. We demonstrate a gain of 350 cm(-1) for a pump intensity of 1 MW/cm(2), nearly 2 orders of magnitude higher than that achievable with conventional parametric crystal amplifiers. We develop a theoretical model that describes this new type of parametric converter where the light pump is replaced by a coherent biexciton wave and show that it is well suited for the generation of entangled photons and the squeezing of an optical beam with ultrafast time gating.     

Generalized coherent states and the statistics of two-photon lasers

Stimulated two-photon emission is proposed as a possible mechanism for the generation of “generalized coherent states”, which have useful quantum properties not available in ordinary coherent states.