Two-photon lasers based on intersubband transitions in semiconductor quantum wells

We propose to make a two-photon laser based on intersubband (sublevel) transitions in semiconductor nanostructures. The advantages and feasibility of such a two-photon laser are analyzed in detail using the density matrix approach. Both one-photon and two-photon gains in a three subband quantum well structure are studied on the same footing to show how the two-photon gain can be maximized, while the competing one-photon gain is minimized. The results show that a sufficient two-photon gain can be achieved to overcome one-photon competition and the loss of a conventional semiconductor cavity, making intersubband transitions one of the very few feasible approaches to two-photon lasing.     

76500～81120cm~(-1)范围内碘甲烷分子的双光子电离光谱

在离子速度成像的实验装置中,将波长为492~523 nm范围内的激光倍频,以经过倍频的激光作光源,将碘甲烷分子(CH3I)电离,获得了碘甲烷母体分子离子(CH3I+)在76 500~81 120 cm-1范围内的高分辨双光子电离光谱;介绍了CH3I的双光子电离机制,根据Rydberg公式和量子数亏损成功对实验所得到的CH3I+谱中的46个谱峰做了标识,并观察到了CH3I+谱中p,d,f系列的能级分裂。标识结果表明,在CH3I的双光子电离谱中除了可以观察到单光子电离谱的特征,还可以观察到在单光子电离中禁阻的f系列跃迁。     

Narrowing of a two-photon resonance due to elastic collisions

The lineshape of two-photon absorption in a standing-wave field during elastic collisions is studied. It has been found that the transit broadening of the two-photon resonance on vibrational-rotational molecular transitions is sharply decreased because of an increase in the time of interaction of particles with the electromagnetic field due to elastic collisions. The effect considered allows one to obtain supernarrow optical resonances of two-photon absorption in a low-pressure gas with high intensity.     

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：     

Excitonic molecule in semiconductors by two-photon absorption

Direct creation of bi-exciton states by two-photon absorption in direct gap semiconductors is investigated theoretically. A numerical application to the case of CuCl shows that the two-photon absorption coefficient for bi-excitonic transitions is larger than that for two-photon interband transitions by three orders of magnitude. It becomes comparable to that for one-photon excitonic transitions for available laser intensities. The main contribution to this enhancement of the absorption coefficient for the transitions to the bi-exciton states is found to be from the resonance effect.     

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.     

Observation of cascaded two-photon-induced transitions between fluxoid states of a SQUID

We present evidence for transitions between fluxoid wells of a SQUID due to cascaded, two-photon processes. Such transitions are evidenced by an anomalous dependence on the transition rate from the one-photon resonant level within the initial well, which cannot be explained by previously observed macroscopic resonant tunneling. These two-photon processes may be a significant source of decoherence in SQUID qubits subject to microwave radiation.     

强光场下非共振双光子跃迁相干控制

利用飞秒激光脉冲相位整形技术,从理论和实验两方面,对强激光场下分子体系苝溶液中非共振双光子跃迁的相干控制进行了研究。通过对飞秒激光脉冲进行相位扫描和余弦相位调制,发现分子体系中非共振双光子的跃迁几率能够被削弱,但不能完全消除。分子体系中非共振双光子跃迁的相干控制受到激光光场的形状和分子吸收谱谱宽的影响;与原子体系相比,分子体系中非共振双光子跃迁相干控制的调制效率降低。     

Multiphoton ionization spectra in lithium vapour

The two-photon absorption spectrum to high-lying states of Li vapour has been obtained using ion-cell detection. ²S states are observed to n = 32 and the ²D series to n = 49. Simple heating techniques circumvented the problem of absorption by the blue-green band system of Li₂ which overlaps the series limit at 4600 Å. The order of the ionization process was examined and found to vary as the square of the laser intensity for two-photon transitions. A conflict with data obtained with a space-charge-limited diode is examined and possible ionization mechanisms are discussed.     

Isomeric orientation by two-photon excitation: a theoretical study

I develop the theory of molecular orientation induced by two-photon isomerization. Phenomenologically, two-photon isomeric orientational hole burning and reorientation induce molecular orientation while rotational diffusion tends to randomize it. Using the formalism of Legendre polynomials solves the general equations of the theory, and analytical solutions are derived for actual physical cases at the early time evolution and the steady state of two-photon excitation. The theory explains the observed results of two-photon molecular orientation, and predicts additional experiments in this direction. The isomerization parameters and the movement of the isomers during polarized two-photon excitation can be quantified by the analytical theory. A theoretical study on the creation of molecular orientation by competing orientational Kerr effect and two-photon orientation is also presented.     

空场诱导相干性对倒转Y型四能级原子系统中单光子和双光子吸收性质的影响

当倒转Y-型四能级原子的两基态能级邻近简并时,连接基态的两个跃迁路径与同一个真空场辐射场相互作用导致的量子干涉效应便产生了空场诱导相干性.空场诱导相干性能使倒转Y-型四能级原子在共振点的单光子和双光子电磁诱导透明现象减弱,而在其他失谐量处产生单光子增益现象。同时,外加相干光场的相对相位对原子的吸收性质有重要的影响.     

Multiphoton satellites in the saturated-absorption Spectrum of SiF<Subscript>4</Subscript> gas

A saturated-absorption spectrum of SiF₄ gas, free of Doppler broadening, has been recorded in the lasing range of a low-pressure CO₂ laser. Multiplets of superfine structure are observed, which are due to the tunnel transitions between the states of molecular rotation around equivalent symmetry axes. Multiplet satellites are revealed, whose origin is explained by two-photon processes. Splitting of the energy levels of the superfine structure multiplet is derived from satellite interpretation. A satellite is found, which can be caused by fourphoton processes.     

Optoacoustic detection of Doppler-free two-photon resonances in the v2 bands of NH3

A new approach to infrared two-photon spectroscopy is described. An optoacoustic cell operating at low pressure is used to detect Doppler-free transitions excited by a conventional and a waveguide CO₂ laser. Beams from the two lasers are passed through the sample in opposite directions and an electric field tunes the transitions into resonance by molecular Stark effect. A few examples are discussed to illustrate the features and the sensitivity of the method.     

The molecular properties of the positronium negative ion

The dynamics of the positronium negative ion is described in a molecular adiabatic approximation. It is shown, how the classification of the spectrum, propensity rules for radiative and non-radiative transitions and conditions for the occurrence of shape resonances can easily be derived within this approach. Propensity rules for two-photon processes are also derived and used to demonstrate the possibility of an absorption experiment from the ground state to excite a¹S^e shape resonance, which is unique to Ps^–.     

Synchronization of atomic quantum transitions by light pulses

The synchronization of atomic quantum transitions with natural Raman oscillations by ultrashort light pulses has been investigated. This phenomenon may be observed if the duration of the perturbation pulse is less than the period of oscillations of the forbidden atomic transitions. The accuracy of the direct measurements of the quantum transition times for trapped particles can be of the same order as the ratio of the two-photon transition frequency to the homogeneous line width.     

Optical Bistability and Multistability in an EIT Medium with Two-Photon Resonance Transitions

Intensity threshold of optical bistability (OB) and optical multistability (OM) can be controlled by amplitude and phase control of microwave driven field in the two-photon resonance transitions in a parametric region. It is found that in two-photon resonance case, the weak microwave field can reduce the threshold of optical bistability and strong microwave field can lead to optical multistability. The effect of relative phase between applied fields is also discussed. It is shown that for weak and strong microwave field, intensity threshold of OB and OM can be modified. Moreover, it is found that for strong microwave field, relative phase has an essential role for switching between OB and OM or vice versa.     

On the self-shift and broadening of doppler-free Rydberg 2S spectral lines in alkali atoms

The shift and broadening of Doppler-free, two-photon transitions to Rydberg states of alkali atoms are analyzed for the case of Rb in the framework of a molecular approach to collisions of ground state and Rydberg atoms. The overall features of the observed spectra are determined by the electrostatic interactions between the two atoms and are calculated using the impact theory of spectral line-shapes. The superimposed oscillations in the widths and shifts as functions of the Rydberg quantum number can be understood in terms of resonance electron-ground state atom scattering. The corresponding contributions to the spectra are determined by the energy and lifetime of a ³P autoionizing state of the negative alakali ion.     

Cascade avalanche production of electron-hole pairs in type II quantum wells

It is shown that the photon avalanche mechanism can be used for producing nonequilibrium electron-hole pairs by low-intensity IR light with a photon energy smaller than the energy gap of a semiconductor by a factor of 3–5. A type II heterostructure with deep quantum wells is proposed to be employed for this purpose. In the model under investigation, the photon avalanche effect is due to a combination of a cascade of one-and two-photon transitions and Auger-type transitions.     

Polarization control of multi-photon absorption under intermediate femtosecond laser field

It has been shown that the femtosecond laser polarization modulation is a very simple and well-established method to control the multi-photon absorption process by the light–matter interaction. Previous studies mainly focused on the multiphoton absorption control in the weak field. In this paper, we further explore the polarization control behavior of multiphoton absorption process in the intermediate femtosecond laser field. In the weak femtosecond laser field, the secondorder perturbation theory can well describe the non-resonant two-photon absorption process. However, the higher order nonlinear effect(e.g., four-photon absorption) can occur in the intermediate femtosecond laser field, and thus it is necessary to establish new theoretical model to describe the multi-photon absorption process, which includes the two-photon and four-photon transitions. Here, we construct a fourth-order perturbation theory to study the polarization control behavior of this multi-photon absorption under the intermediate femtosecond laser field excitation, and our theoretical results show that the two-photon and four-photon excitation pathways can induce a coherent interference, while the coherent interference is constructive or destructive that depends on the femtosecond laser center frequency. Moreover, the two-photon and fourphoton transitions have the different polarization control efficiency, and the four-photon absorption can obtain the higher polarization control efficiency. Thus, the polarization control efficiency of the whole excitation process can be increased or decreased by properly designing the femtosecond laser field intensity and laser center frequency. These studies can provide a clear physical picture for understanding and controlling the multi-photon absorption process in the intermediate femtosecond laser field, and also can provide a theoretical guidance for the future experimental realization.