Ionization and stabilization of atoms in a high-intensity,low-frequency laser field

The dynamics of a model silver atom in the strong radiation field of a Ti:sapphire laser is studied in the Keldysh parameter regions γ ⩾ 1 and γ ⩽ 1. It is found that in the entire range of Keldysh parameter variations, along with ionization, the efficient excitation of Rydberg states of the atom with principal quantum numbers n = 6−14 is observed. A Rydberg wavepacket appearing in this case proved stable with respect to ionization; i.e., the atomic system in strong low-frequency electromagnetic fields becomes stable with respect to ionization. The physical reasons behind the stabilization are discussed.     

Analysis of the unusual position of the low-frequency dispersion region of a heterogeneous mixture-spacers system

In the paper an analysis is presented of the dependence of the position of the low-frequency dispersion region of a semiconducting heterogeneous specimen-spacers system on the volume concentration of the conducting component, the thickness of the specimen, the thickness (capacity) of the spacers. It is concluded that there exists a surface capacity due to regions of the nonconducting phase of the specimen confined between the spacers and a conducting infinite cluster.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 14–18, July, 1982.     

高强度聚焦超声经颅脑肿瘤治疗焦域的仿真研究*

为了研究脑组织和脑肿瘤组织在HIFU治疗时形成焦域的特性及坏死肿瘤组织、不同治疗剂量参数以及多次治疗时焦点间距和时间间隔对HIFU形成焦域的影响,本文以脑胶质瘤患者为例,利用患者头颅CT图像数据建立HIFU经颅治疗的仿真模型,基于Westervelt声波非线性传播方程和Pennes生物热传导方程进行HIFU经颅治疗的仿真研究。结果表明,脑组织和脑肿瘤组织内形成HIFU焦域的差异较小,坏死肿瘤组织对HIFU温度场分布有较小影响;辐照声强越大,焦点温升达到同一温度所用时间越短,焦域长短轴越短,颅骨处温升越低;当焦点间距在一定范围内时,第一次辐照形成的温度场分布对第二个焦点温升达到同一温度所需时间影响较大;两次辐照时间间隔对颅骨处温升和两次聚焦形成60?C以上温度分布影响较小。     

Spatiotemporal dynamics of femtosecond pulses in nonlinear optical waveguides with material dispersion

The effect of the second-order dispersion on the spatiotemporal dynamics of ultrashort pulses in nonlinear waveguides with infinite parabolic and stepped profiles of the refractive index is studied. It is shown that the leakage of radiation from the core to the cladding of step-index waveguides prevents the formation of a steady spatiotemporal distribution of the field. The possibility of compressing light pulses in a dielectric waveguide with a positive group-velocity dispersion is discussed.     

Enhancement of nonlinear properties of superconducting films in low-frequency magnetic field

The effect of enhancement of nonlinear properties of superconducting films is found in low-frequency magnetic field. Harmonic mixing of the electromagnetic radiation is detected. These results provide an argument in support of a new approach to design the active superconducting elements. A low-frequency oscillating magnetic field directed perpendicularly to the film surface forms an unstable vortex structure in the film. The frequency multiplication is intensified in the vortex unstable state. A transition of the vortex structure into the ground state synchronized by an external resonant circuit causes the generation of the electromagnetic radiation. Main advantage of these oscillators over Josephson junctions is high power emitted into a broadband system.     

New expressions for field distribution in the focal region

In an earlier paper dealing with the flat-topped light beams [Y. Li, Opt. Lett. 27 (2002) 1007], it is shown that the flat-topped beams can be expressed as 1 − [1 − exp(−ξ²)]^M, where ξ is a dimensionless parameter and M is a non-negative number. The binomial expansion of this express contains only lowest-order Gaussian modes; this situation makes it possible to develop a new formulation of diffraction of converging spherical wave at an aperture in a plane opaque screen if the Gaussian mode expansion is employed to describe the boundary values of the screen.     

Travelling wave dynamics in a nonlinear interferometer with spatial field transformer in feedback

The properties of different spatial structures which emerge in a nonlinear resonator with a spatial transformer in a feedback are investigated. In detail, we determine the parameter regions, the amplitudes and the rotation frequency of various optical reverberators. Special emphasis is devoted to critical cases of infinite dimension which occur in a system with large geometrical size. Evolution equations obtained describe multistable travelling waves, slowly rotating step structures, or modulated waves in dependence on the spatial shift in the feedback. Sensitive dependence of the phase space construction on geometrical size of the system is demonstrated even in the case of a large-scale system.     

色散对单模激光系统非线性效应的影响

研究在强光场的作用下 ,激光系统中的腔内介质的折射率不再是常数而引起的非线性效应 ,即色散型激光系统的动力学行为。研究结果表明 ,这类系统的输出场会呈现双稳态 ,并利用极值法确定了描述这类色散型激光系统的Maxvell- bloch方程是现双稳态的参数临界点和双稳态区域     

Parametric generation of low-frequency sound in the propagation of high-intensity modulated noise

With the use of the one-dimensional Burgers equation, the evolution of a high-intensity noise with periodically modulated intensity is analyzed. The nonlinearity is shown to lead to partial suppression of the amplitude modulation and to the generation of a regular low-frequency component. The probability distributions and the power spectra of the field are studied.     

High-speed observation of cavitation bubble cloud structures in the focal region of a 1.2 MHz high-intensity focused ultrasound transducer

Cavitation bubble clouds in the focal region of HIFU play important roles in therapeutic applications of HIFU. Temporal evolution and spatial distribution of cavitation bubble clouds generated in the focal region of a 1.2 MHz single element concave HIFU transducer in water are investigated by high-speed photography. It is found that during the initial 600 micro s insonation cavitation bubble clouds organize to the "screw-like structure" or "cap-like structure". The screw-like structure is characterized by a nearly fixed tip at the geometrical focus of the HIFU transducer, and the cap-like structure is marked by a dent formed in the direction of ultrasound transmission. After 600 micro s, another two structures are recorded - "streamer structure" and "cluster structure". The streamer structure is also featured by a nearly fixed bottom position at the focus, while the cluster structure is distinguished by agglomerations of bubbles around the focus.     

Quantum field theory in curved space-time as thermo field dynamics

The strong analogy between states defined in the context of quantum field theory in curved space-time (QFT-CST) and the ones defined in the thermo field dynamics (TFD) of Takahashi and Umezawa [1] is shown. This analogy is useful in order to introduce the entropy operator in CST in the same way as in TFD. When the extremum condition in the thermodynamical potential is imposed, a family of Bogoliubov transformations that give us a planckian spectrum is found, even in pathological cases such as the minimally coupled scalar field.     

Nonlinear brain dynamics as macroscopic manifestation of underlying many-body field dynamics

Neural activity patterns related to behavior occur at many scales in time and space from the atomic and molecular to the whole brain. Patterns form through interactions in both directions, so that the impact of transmitter molecule release can be analyzed to larger scales through synapses, dendrites, neurons, populations and brain systems to behavior, and control of that release can be described step-wise through transforms to smaller scales. Here we explore the feasibility of interpreting neurophysiological data in the context of many-body physics by using tools that physicists have devised to analyze comparable hierarchies in other fields of science. We focus on a mesoscopic level that offers a multi-step pathway between the microscopic functions of neurons and the macroscopic functions of brain systems revealed by hemodynamic imaging. We use electroencephalographic (EEG) records collected from high-density electrode arrays fixed on the epidural surfaces of primary sensory and limbic areas in rabbits and cats trained to discriminate conditioned stimuli (CS) in the various modalities. High temporal resolution of EEG signals with the Hilbert transform gives evidence for diverse intermittent spatial patterns of amplitude (AM) and phase modulations (PM) of carrier waves that repeatedly re-synchronize in the beta and gamma ranges in very short time lags over very long distances. The dominant mechanism for neural interactions by axodendritic synaptic transmission should impose distance-dependent delays on the EEG oscillations owing to finite propagation velocities and sequential synaptic delays. It does not. EEGs show evidence for anomalous dispersion: neural populations have a low velocity range of information and energy transfers, and a high velocity range of the spread of phase transitions. This distinction labels the phenomenon but does not explain it. In this report we analyze these phenomena using concepts of energy dissipation, the maintenance by cortex of multiple ground states corresponding to AM patterns, and the exclusive selection by spontaneous breakdown of symmetry (SBS) of single states in sequential phase transitions.     

Role of dispersion in multiple-collapse dynamics

The multiple-collapse dynamics of ultrashort pulses along the propagation direction are investigated under conditions of both normal and anomalous group-velocity dispersion (GVD). In the anomalous-GVD regime we find that collapse events can occur at locations in the medium many diffraction lengths beyond the initial collapse point, in contrast with the normal-GVD regime in which multiple collapse occurs within a diffraction length. Numerical simulations of a modified nonlinear envelope equation are found to be in good qualitative agreement with the observed lengths of the filaments.     

Treatable focal region modulated by double excitation signal superimposition to realize platform temperature distribution during transcranial brain tumor therapy with high-intensity focused ultrasound

Recently, the phase compensation technique has allowed the ultrasound to propagate through the skull and focus into the brain. However, the temperature evolution during treatment is hard to control to achieve effective treatment and avoid over-high temperature. Proposed in this paper is a method to modulate the temperature distribution in the focal region.It superimposes two signals which focus on two preset different targets with a certain distance. Then the temperature distribution is modulated by changing triggering time delay and amplitudes of the two signals. The simulation model is established based on an 82-element transducer and computed tomography(CT) data of a volunteer's head. A finitedifference time-domain(FDTD) method is used to calculate the temperature distributions. The results show that when the distances between the two targets respectively are 7.5–12.5 mm on the acoustic axis and 2.0–3.0 mm in the direction perpendicular to the acoustic axis, a focal region with a uniform temperature distribution(64–65℃) can be created.Moreover, the volume of the focal region formed by one irradiation can be adjusted(26.8–266.7 mm~3) along with the uniform temperature distribution. This method may ensure the safety and efficacy of HIFU brain tumor therapy.     

Ionization of atoms in strong low-frequency electromagnetic field

The ionization of atoms in a low-frequency linearly polarized electromagnetic field (the photon energy is much lower than the ionization potential of an atom) is considered under new conditions, in which the Coulomb interaction of an electron with the atomic core in the final state of the continuum cannot be considered in perturbation theory in the interaction of the electron with the electromagnetic field. The field is assumed to be much weaker that the atomic field. In these conditions, the classical motion of the electron in the final state of the continuum becomes chaotic (so-called dynamic chaos). Using the well-known Chirikov method of averaging over chaotic variations of the phase of motion, the problem can be reduced to non-linear diffusion on the energy scale. We calculate the classical electron energy in the final state, which is averaged over fast chaotic oscillations and takes into account both the Coulomb field and the electromagnetic field. This energy is used to calculate the probability of ionization from the ground state of the atom to a lower-lying state in the continuum using the Landau-Dykhne approximation (to exponential accuracy). This ionization probability noticeably depends on the field frequency. Upon a decrease in frequency, a transition to the well-known tunnel ionization limit with a probability independent of the field frequency is considered.     

高非线性光纤正常色散区利用皮秒脉冲产生超连续谱的相干特性

使用复互相干度的定义对时域光波分裂前后以及不同输入噪声、不同初始啁啾和波形下抽运脉冲在高非线性光纤中产生的超连续谱的相干度进行了数值计算,得到了光波分裂前后和不同输入噪声下生成的超连续谱的演化和相干性变化,结果表明:皮秒脉冲在高非线性光纤正常色散区产生超连续谱的相干性主要受到系统中噪声占比的影响,其中由光波分裂生成的频谱旁瓣的相干度低于由自相位调制生成的中心频谱的相干度:抽运脉冲啁啾和波形对在高非线性光纤正常色散区产生超连续谱的相干性的影响不明显,若想获得高相干的超连续谱,需要采用低噪声的脉冲进行抽运;若获得大谱宽高相干度的超连续谱,则需要合理选择皮秒脉冲的功率。