Dynamics of a trapped two-level and three-level atom interacting with classical electromagnetic field

A coherent trapping phenomenon for the level population of a three-level atom, which takes place on two-photon resonance, has been noted and well studied by a semi-classical approach where the classical electro-magnetic fields are regarded as given [1]. In this article we deal with a closed system of one or two quantized field modes and a three-level (∧-type) atom interacting to each other via the non-vanishing atomic dipole matrix elements d12 and d23 (see Fig. 1). We show that the condition for the coherent trapping determines not only the atomic state but also the field state for the two mode case. In the two mode case (see Fig. 1A), mode l (left) couples with the atom by d12 and mode r (right) by d23. The one mode case with its frequency can be regarded as a special case of the two modes with ωl = ωr = ω (see Fig. 1 B).

Two multi-kilowatts Nd: YAG laser beams are condensed separately and combined at surface of workpiece in order to increase penetration depth. In this system, one or two laser beams are inevitably inclined depending on size of condensing optics, thus, welding characteristics might be fairly complicated than that of a single laser beam welding. Welding characteristics are evaluated by penetration depth and penetration shape in bead-on-plate tests of stainless steel (304). This paper is intended as an investigation on the welding condition of the deepest weld penetration, especially on arrangement of condensing optics. The deepest weld penetration of 6.3 and 7.7 mm are obtained by two CW laser beams and two rectangularly modulated CW laser beams with total average power of four kilowatts at a welding speed of 5 mm/s, respectively. Deepened penetration depths of weld beads by laser beams in combination due to the increase in total laser power are obtained in comparison to penetration depths of 2.8 and 4.3 mm by a single CW laser beam and a single rectangular modulated CW laser beam with average powers of two kilowatts, respectively. Penetration depths of weld beads obtained with symmetric irradiation exceed those with asymmetric irradiation. The angle between two laser beams for the deepest penetration is 60 degrees for CW laser beams, while the angle is 30 degrees for modulated CW laser beams. The difference in the appropriate angles for the deepest penetration might be explained by different effect of keyhole bottom separation on penetration depths depending on waveform of laser beams. With asymmetric beam arrangement, a deeper penetration is obtained in backhand welding direction for the inclined laser beam than that in forehand direction both in welding using two CW laser beams in combination and two modulated CW beams in combination.

Aluminum alloy is considered to be one of the ideal materials for light-weighting on aerospace and new energy automotive. Laser welding without a filler wire is an efficient way for weight reduction. The common defects in laser welding of aluminum alloys are porosities and cracks, resulting in poor mechanical performances and short service life of joints. In order to investigate the methods to suppress defects, 1 mm thick automotive aluminum alloy AA6014 was performed for single and dual-beam laser lap welding in this study. The effects of two welding processes on microstructure, cracks, and mechanical properties were investigated. The results showed that the welding adaptability of dual laser beam lap welding was much better than single laser beam lap welding. Well-formed weld could be obtained in a suitable line energy range by both welding processes. The dual beam with an energy ratio of 1:1 was at an angle along the welding direction, which could increase the molten pool area and stir the weld pool more sufficiently. Thus, dual laser beam lap welding could significantly increase the welding speed and alleviate collapse caused by single laser beam welding. The gaps of lap welding tended to be crack sources and generate cracks, while dual-beam laser welding was advantageous for reducing such cracks. This research is of great significance to improve the laser welding quality of all-aluminum automotive bodies, increase production efficiency, and further reduce the weight of the automotive.

Using a pulsed YAG Laser, meltability of Al-Mg and Al-Mg-Si alloys were investigated by a bead-on-plate method. In order to improve the quality in laser beam welding, the effectiveness of Ultrasonic Vibration Laser Beam Welding (ULBW) method proposed in this paper was investigated experimentally. And, this proposed method was compared with traditional welding method of Normal Laser Beam Welding (NLBW) and Preheating Laser Beam Welding (HLBW). These welding methods were evaluated from the geometry in fusion zone generated by a single laser beam. It was suggested as a fact that ultrasonic vibration was useful for welding defects and fusion characterstics with cavitation effect and dispersion action of particles in the molten pool during laser beam welding. Moreover, the effects of active stirring in the molten pool by preheating were investigated also.In these experiments, ULBW was the most useful laser welding method from a view-point to improve the laser welding-quality in Al alloys.Using a pulsed YAG Laser, meltability of Al-Mg and Al-Mg-Si alloys were investigated by a bead-on-plate method. In order to improve the quality in laser beam welding, the effectiveness of Ultrasonic Vibration Laser Beam Welding (ULBW) method proposed in this paper was investigated experimentally. And, this proposed method was compared with traditional welding method of Normal Laser Beam Welding (NLBW) and Preheating Laser Beam Welding (HLBW). These welding methods were evaluated from the geometry in fusion zone generated by a single laser beam. It was suggested as a fact that ultrasonic vibration was useful for welding defects and fusion characterstics with cavitation effect and dispersion action of particles in the molten pool during laser beam welding. Moreover, the effects of active stirring in the molten pool by preheating were investigated also.In these experiments, ULBW was the most useful laser welding method from a view-point to improve the laser welding-quality in Al alloys.

This paper describes the development work on single and multi beam laser grooving technology for 40nm node low-k/ULK semiconductor device. A Nd:YAG ultraviolet (UV) laser diode operating at a wavelength of 355 nm was used in this study. The effects of single and multi beam laser micromachining parameters, i.e. laser power, laser frequency, feed speed, and defocus amount were investigated. The laser processed die samples were thoroughly inspected and characterized. This includes the die edge and die sidewall grooving quality, the grooving shape/profile and the laser grooving depth analysis. Die strength is important and critical. Die damage from thermal and ablation caused by the laser around the die peripheral weakens the mechanical strength within the die, causing a reduction in die strength. The strength of a laser grooved die was improved by optimizing the laser process parameter. High power optical microscopy, Scanning Electron Microscopy (SEM), and focused ion beam (FIB) were the inspection tools/methods used in this study. Package reliability and stressing were carried out to confirm the robustness of the multi beam laser grooving process parameter and condition in a mass production environment. The dicing defects caused by the laser were validated by failure analysis. The advantages and limitations of conventional single beam compared to multi beam laser grooving process were also discussed. It was concluded that, multi beam laser grooving is possibly one of the best solutions to consider for dicing quality and throughput improvements for low-k/ULK wafer dicing. The multi beam laser process is a feasible, efficient, and cost effective process compared to the conventional single beam laser ablation process.

Noise-free phase conjugation of high power single mode laser beams by stimulated Brillouin scattering

We study a nonlinear semiclassical and fully quantum-mechanical theory of a laser with an active medium of three-level atoms that may be initially prepared in a coherent superposition and interact with a classical external electromagnetic field. In particular, we determine the intracavity and output photon statistics of the laser light in a broad range of parameters. We find an inner sub-Poissonian statistics with a Mandel parameter close to the minimal value -1 when the laser is operating with initially prepared atoms, off resonant, and without inversion of the populations. In another regime, with, however, less favorable intracavity statistics, we find squeezing in the intensity output fluctuations of 80%, which is well above the present state of art noise reduction for this kind of coherently driven laser system.

Strong interaction of a system of quantum emitters (e.g., two-level atoms) with electromagnetic field induces specific correlations in the system accompanied by a drastic increase of emitted radiation (superradiation or superfluorescence). Despite the fact that since its prediction this phenomenon was subject to a vigorous experimental and theoretical research, there remain open question, in particular, concerning the possibility of a first order phase transition to the superradiant state from the vacuum state. In systems of natural and charge-based artificial atom this transition is prohibited by “no-go” theorems. Here we demonstrate numerically and confirm analytically a similar transition in a one-dimensional quantum metamaterial – a chain of artificial atoms (qubits) strongly interacting with classical electromagnetic fields in a transmission line. The system switches from vacuum state to the quasi-superradiant (QS) phase with one or several magnetic solitons and finite average occupation of qubit excited states along the transmission line. A quantum metamaterial in the QS phase circumvents the “no-go” restrictions by considerably decreasing its total energy relative to the vacuum state by exciting nonlinear electromagnetic solitons.

Nonlinear interaction of a three-level atom with a two-mode field in an optomechanical cavity: Field and mechanical mode dissipations

Circularly polarized laser beams propagating through sodium vapour and tuned to the buffer-gas-broadened atomic transition can optically pump sodium atoms into a non-absorbing ground state. This causes an intensity-dependent refractive index gradient along as well as transverse to the laser beam propagation direction, giving rise to a number of nonlinear spatio-temporal intensity and polarization pattern creating processes. In the case of a single circularly polarized laser beam we have observed self-focusing and defocusing, the transformation of the incident Gaussian beam intensity profiles into ring profiles, a large shift of about 5 GHz of the maximum of the absorption profile when suitable magnetic fields are applied and the deflection of a beam by the inhomogeneous transverse magnetic field of a current-carrying wire. When two beams of opposite circular polarization are superimposed, astonishing effects such as the mutual deflection of both beams (beam bouncing), the mutual extinction of both beams (beam switching), the separation of initially overlapping beams (beam splitting) and the mutual attraction of both beams (beam attraction) can be observed. While most of the effects can be well described for the stationary state by a to atomic transition model, the correct description of the dynamics requires the consideration of all hyperfine states.

A single femtosecond laser beam can be focused to process material surfaces. However, the low processing efficiency by a single laser beam prevents broad applications of femtosecond laser surface processing. Here we have demonstrated an array of high-intensity femtosecond laser beams that self-focus in a piece of glass medium, enabling high-efficiency, millimeter scale parallel laser surface processing. Experimental and simulation results suggest that the laser fluence of each focused laser beamlet in the array is more than 1.2 J/cm2, which is sufficient for micro- or nano-structure laser processing. To prove the concept, nanoscale structures on silicon surface are generated by the self-focused laser beam array, which may improve the efficiency of black silicon fabrication for photovoltaic devices.

As usual cylindrical hohlraum with double laser ring cones may lead to serious laser-plasma interaction, such as the simulated Raman scatter and cross-beam energy transfer effect, spherical hohlraum with octahedral 6 Laser Entrance Holes (LEHs) and single cone laser beams, was investigated and reported to have a consistent high radiation symmetry during the whole implosion process. However, it has several potential challenges such as the smaller space left for diagnosis and the assembly of centrally located capsule. In this paper, based on the view-factor model, we investigate the radiation symmetry and the drive temperature on the capsule located in the spherical hohlraum with tetrahedral 4 LEHs and single cone laser beams, since there is more available space for laser disposition and diagnosis. Then, such target is optimized on the laser beam pointing direction to achieve a high radiation performance, i.e., the radiation symmetry and drive temperature on the capsule. Finally, an optimal spherical hohlraum with optimal laser beam pointing has been demonstrated and compared with the spherical hohlraum with octahedral 6 LEHs. The resulting radiation symmetry and the drive temperature shows that it has almost a similar radiation symmetry (the radiation asymmetry variation is no more than 0.2%), and higher drive temperature (the temperature has been increased by 1.73%, and an additional 133 kJ energy of 2 MJ energy for fusion can be saved).

The interaction of an atomic ensemble localized in a microcavity with external electromagnetic fields under Raman resonance conditions with an optically forbidden atomic transition involving photons of the microcavity mode has been described in terms of third-order polynomial algebra. It has been shown that atoms and photons localized in the microcavity under these conditions form a united object, an atom-photon cluster, on the states of which the irreducible representations of polynomial algebra are implemented. Classical coherent and quantum broadband electromagnetic fields are considered as external fields. The effective Hamiltonian, effective dipole moment operator, and relaxation operator of the atom-photon cluster are expressed in terms of the generators of polynomial algebra, which is the algebra of the dynamical symmetry of the problem. The developed mathematical technique has been applied to describe the main radiative processes—spontaneous emission and nutation effect—on atom-photon clusters. All of these effects are peculiar and differ from similar phenomena on two-level atoms, but only simple cases of the mentioned radiative processes have been considered.

We present the first experimental demonstration of full-duplex communication on a single laser beam. The forward link beam simultaneously transmits the forward link data and serves as the carrier wave for the return link. A fraction of incident forward link beam is modulated and retro-reflected back to the receiver location. The forward link data format must be designed so that it is invisible to the return link’s data detection system. We have named this architecture the “Lightwire. In addition there was no system trade-off required by this architecture. Thus we have demonstrated full-duplex on a single optical beam without any performance penalty. We will present the experimental system and discuss the experimental results.

Tomography vs quantum control for a three-level atom