Optical bistability and multistability in a parametric region

We study the optical bistability (OB) and optical multistability (OM) behaviors in a five-level $\Lambda $ -type parametric region atomic system by two-photon resonant transitions. We find that the intensity threshold of OB and switching from OB to OM or vice versa can be controlled via quantum interference between different two-photon transitions pathways.     

New Photoelectronic investigations of the nonlinear optical properties of n-InSb

The nonlinear optical properties of n-InSb at liquid helium temperatures are shown to be strongly related to the electron population of the shallow donor impurity levels. Solutions of nonlinear coupled rate equations predict the observed resonant behavior of (1) the number of photoexcited electrons (determined directly from photo-Hall measurements) and (2) their resonant lifetime behavior at magnetic fields where two-photon interband transitions take place.     

Two-photon laser and optical-bistability sidemode instabilities

Two-photon laser oscillation and optical bistability have been observed and studied theoretically by various groups. The studies have not considered the buildup of sidemodes. We have extended the corresponding one-photon sidemode treatments to two-photon transitions for which no resonant intermediate levels exist. For the two-photon laser, we find a wide region of single-wavelength instability for a centrally tuned strong mode. For the corresponding two-photon absorptive optical bistability, we predict no single-wavelength instabilities. For strong-mode detuning, we find that appreciable regions of sidemode gain exist for both problems allowing multiwavelength instabilities to occur.     

Enhanced Kerr Nonlinearities Caused by Cross Talk Between Optical and Microwave Transitions

We investigate the enhanced Kerr nonlinearities in four-level atomic vapors driven by a coupling, probe and microwave fields. Under the optical one-photon and two-photon resonant conditions, the linear and nonlinear responses of the weak probe field can be modified by the cross talk among optical and microwave transitions. The enhanced Kerr nonlinearity can form bright optical solitons of the probe field.     

Theoretical Investigation of Coherent Enhancement for Resonant Two-Photon Transitions

We theoretically investigate the coherent enhancement of resonant two-photon transitions （TPT） in a three-level atomic system. The TPT can be coherently enhanced by modulating spectral amplitude due to eliminating the destructive interference, though partial laser energy losses. Maximal enhancement of TPT can be achieved by modulating spectral phase due to establishing completely constructive interference. Our research provides a theoretical basis for experimental investigation and appears to have potential application on coherent control in the complicated quantum system.     

Linear circular dichroism in nonlinear absorption of light in a quantum well

The far infrared absorption spectrum caused by optical transitions of holes between size-quantization subbands is calculated for p-GaAs/AlGaAs(001) quantum-well structures. The selection rules for optical transitions at the center of the two-dimensional Brillouin zone are determined. Allowance is made for resonant saturation of one-photon electronic transitions between size-quantized subbands of light and heavy holes. The linear circular dichroism in one-photon nonlinear (resonant) and two-photon absorption of light in a size-quantized well are investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 1347–1349 (July 1998)     

Doppler-free transitions induced by strong double-frequency optical excitation

The possibilities to achieve Doppler-free transitions influenced by two strong radiations with different frequencies are discussed. Derived expressions show that two-photon absorption ceases to be Doppler-free with the intensity increase of equal-frequency counterpropagation light beams. On the contrary, absorption of distinct-frequency counterpropagating waves and moreover copropagating Stokes light scattering appears to be Doppler-free under a proper pump field intensity. Applications of these induced Doppler-free transitions to resonant nonlinear optics of gas-phase media are considered. The importance of the new features resulting from the growth of both the interacting fields is stressed.     

NOON states in entangled cavities

We show how NOON states may be generated entangling two cavities by passing atoms through them. The atoms interact with each cavity via two-photon resonant transitions. We take advantage of the fact that depending on the state the atom enter (excite or ground), it leaves or takes two photons per interaction and leaves the cavities in a pure state.     

Resonant two-photon absorption in ZnSiP2

The complex of optical investigations into the intrinsic (I) and resonant (R) two-photon absorption (TPA) inZnSiP ₂ crystals of tetragonal modifications is performed by methods of amplitude laser-modulation spectroscopy. The ITPA of allowed-forbidden type and the two Lorentz spectral lines associated with coherent resonant two-photon transitions through impurity states of allowed-allowed type are found. The electron transverse relaxation time at the RTPA is determined, and the total absorption cross section of laser-radiation photons emitted at electron transitions between the valence band v, the conduction band c, and local centers that produce real states intermediate for the RTPA in the forbidden band is estimated. M. P. Dragomanov National Pedagogical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 36–43, February, 2000.     

Transform-Limited Pulses Are Not Optimal for Resonant Multiphoton Transitions

Maximizing nonlinear light-matter interactions is a primary motive for compressing laser pulses to achieve ultrashort transform limited pulses. Here we show how, by appropriately shaping the pulses, resonant multiphoton transitions can be enhanced significantly beyond the level achieved by maximizing the pulse's peak intensity. We demonstrate the counterintuitive nature of this effect with an experiment in a resonant two-photon absorption, in which, by selectively removing certain spectral bands, the peak intensity of the pulse is reduced by a factor of 40, yet the absorption rate is doubled. Furthermore, by suitably designing the spectral phase of the pulse, we increase the absorption rate by a factor of 7.     

On the dominant role of two-photon relaxation in photonic crystals in external fields

It is shown that resonant interaction of a coherent wave with impurity atoms leads to filling of levels of an impurity atom that lie in the gap in the photon density of states and do not belong to resonant transitions, while the interaction of impurity atoms with a nonresonant coherent wave results in effective deactivation of the indicated levels. The main mechanism determining the pumping and decay of an impurity level in a gap are two-photon radiative relaxation processes previously investigated by the present author (Zh. éksp. Teor. Fiz. 102, 1126 (1992) [Sov. Phys. JETP 75, 611 (1992)]). Pis’ma Zh. éksp. Teor. Fiz. 70, No. 7, 434–438 (10 October 1999)     

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.     

Effects of Dipole Interaction on Emission Spectra of Atoms in Two-Photon Processes

We study the interaction of a pair of coupling two-level atoms with a single mode of the radiation field in an ideal cavity via two-photon resonant transition and/or degenerate Ramancoupled processes.In particular, we consider the emission spectra for both two-photon processes. The effects of atomic coupling are analyzed and the properties of emission spectra for large photon number of the cavity-mode are discussed.     

原子双光子电离截面的理论计算

我们在Hartree-Slater自洽场理论的基础上,以Cs,Xe原子为例,具体计算了其双光子电离截面。本文的理论方法可容易地应用于其它双光子过程的计算,同时还可推广到更多光子过程的计算。将计算结果和相应的实验以及其它理论结果作了比较和讨论。明确了本文理论计算的精确程度,对碱金属元素(例如Cs)本文的理论值与最近的精确实验符合得较好。 关键词：     

Resonant scattering of three-level Rydberg atoms in a microwave field

We examine the theory of potential scattering of Rydberg atoms in a microwave field. The model of a three-level atom is employed to calculate the radiative force emerging in the resonant coherent interaction with the microwave field for the case of a two-photon resonance and high intensities, using the method of quasienergies of the system consisting of the atom and the field. We determine the probabilities of Landau-Zener transitions in the spatial regions where under two-photon resonance conditions the quasienergies of the atoms approach one another by a small quantity. We also study the dynamics of the variation of the spatial profile of a beam of Rydberg atoms caused by resonant scattering. Finally, we give the results of the first experimental observation of the variation of the transverse beam profile when Rydberg atoms pass through a nonuniform microwave field formed in a rectangular waveguide and in resonance with the two-photon 36P–37P transition. Zh. éksp. Teor. Fiz. 111, 796–815 (March 1997)     

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.     

Doppler-free two-photon spectroscopy

We describe atomic transitions where two or more photons are simultaneously absorbed by an atom or a molecule (without any intermediate resonant level). We show theoretically and experimentally the possibilities of these transitions for numerous high-resolution spectroscopic studies. The principle of the method is discussed: the Doppler broadening is suppressed in two-photon transitions if the atoms are irradiated by two laser beams travelling in opposite directions, because the Doppler shift in one wave cancels that in the other. A generalization is made for multiphotonic transitions. Calculations of the multiphotonic transition probability are summarized and their hypotheses specified: one deals with line shape and orders of magnitude in practical conditions. The problem of light shifts is also discussed. A typical experimental set-up with a cw dye laser is described in detail. Typical recordings of two-photon transitions are given; they show line widths smaller than 4 MHz (on a Doppler width of 2000 MHz). Other experiments show the possibility of increasing the transition probability by using two lasers with slightly different wave lengths. One experiment with a three-photon Doppler-free transition is also described. In conclusion, we make a brief comparison with the saturated absorption technique.     

Absorption and dispersion control in a five-level M-type atomic system

We investigate the steady optical response of a coherently driven five-level M-type atomic system in three different situations.When all three coupling fields have the same zero detuning,we just find one deep transparency window accompanied by a steep normal dispersion in the probe absorption and dispersion spectra.When two coupling fields are detuned from the relevant transitions to the same extent,however,a second deep transparency window may be observed in the presence of a narrow absorption line of linewidth ～50 kHz.In this case,two single-photon far-detuned transitions can be replaced by a two-photon resonant transition,so the five-level M system in fact reduces into a four-level quasi-Λ system.Finally,we note that no deep transparency windows and no narrow absorption lines can be found when all three coupling fields have unequal detunings.     

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.     

Nonlinear absorption and refraction of light in a colloidal solution of CdSe/ZnS quantum dots upon two-photon resonant excitation

This paper reports on the results of an investigation into the nonlinear transmission of individual ultrashort pulses of a train generated by a mode-locked laser through a colloidal solution of CdSe/ZnS quantum dots in toluene upon two-photon resonant excitation of the fundamental optical transition. The specific features of the nonlinear transmission are explained in terms of the two-photon absorption and self-defocusing processes. Analysis of the experimental results makes it possible to separate the self-defocusing processes that are governed by the inertialess change in the refractive index due to the interaction of high-power optical pulses with bound electrons and those determined by the nonlinear change in the refractive index under the action of two-photon-excited carriers in a quantum dot.