Population Of Upper Level Research Articles

Evolutionary Bilevel Optimization via Multiobjective Transformation-Based Lower-Level Search

Nested evolutionary algorithms (EAs) have been regarded as very promising tools for bi-level optimization. Due to the nested structure, the upper level population evaluation requires a set of complete lower level optimizations, thereby reducing the efficiency and practicability of EA methods. In this paper, a multi-objective transformation-based evolutionary algorithm (MOTEA) is proposed to perform multiple lower level optimizations in a parallel and collaborative manner. Specifically, the corresponding multiple lower level optimizations for each generation of the upper level population evaluation are transformed into locating a set of Pareto optimal solutions of a constructed multi-objective optimization problem. By utilizing the built-in implicit parallelism of evolutionary multi-objective optimization, multiple lower level problems can thus be optimized in parallel. Within one multi-objective search population, the collaboration among the parallel lower level optimization can be realized by exploiting and utilizing the implicit similarities among them for better efficiency. The effectiveness and efficiency of the proposed MOTEA are verified by comparing it with four state-of-the-art evolutionary bi-level optimization algorithms on two sets of popular bi-level optimization benchmark test problems and three application problems.

Quantum Density Matrix Theory for a Laser Without Adiabatic Elimination of the Population Inversion: Transition to Lasing in the Class‐B Limit

AbstractDespite the enormous technological interest in micro and nanolasers, surprisingly, no class‐B quantum density‐matrix model is available to date, capable of accurately describing coherence and photon correlations within a unified theory. In class‐B lasers—applicable for most solid‐state lasers at room temperature—, the macroscopic polarization decay rate is larger than the cavity damping rate which, in turn, exceeds the upper level population decay rate. Here, a density‐matrix theoretical approach for generic class‐B lasers is carried out, and closed equations are provided for the photonic and atomic reduced density matrix in the Fock basis of photons. Such a relatively simple model can be numerically integrated in a straightforward way, and exhibits all the expected phenomena, from one‐atom photon antibunching, to the well‐known S‐shaped input–output laser emission and super‐Poissonian autocorrelation for many atoms (), and from few photons (large spontaneous emission factors, ) to the thermodynamic limit ( and ). Based on the analysis of , it is concluded that super‐Poissonian fluctuations are clearly related to relaxation oscillations in the photon number. A strong damping of relaxation oscillations with an atom number as small as is predicted. This model enables the study of few‐photon bifurcations and nonclassical photon correlations in class‐B laser devices, also leveraging quantum descriptions of coherently coupled nanolaser arrays.

SUPER Scheme in Action: Experimental Demonstration of Red-Detuned Excitation of a Quantum Emitter.

The quest for the perfect single-photon source includes finding the optimal protocol for exciting the quantum emitter. Coherent optical excitation was, up until now, achieved by tuning the laser pulses to the transition frequency of the emitter, either directly or in average. Recently, it was theoretically discovered that an excitation with two red-detuned pulses is also possible where neither of which would yield a significant upper-level population individually. We show that the so-called swing-up of quantum emitter population (SUPER) scheme can be implemented experimentally with similar properties to existing schemes by precise amplitude shaping of a broadband pulse. Because of its truly off-resonant nature, this scheme has the prospect of powering high-purity photon sources with superior photon count rate.

Dynamic Stark shift in Doppler-broadened four-wave mixing

This work presents a theoretical analysis of the Autler-Townes splitting pattern in the four-wave mixing signal generated in a three-level cascade Doppler-broadened system. We employ the density matrix formalism to write the Bloch equations and solve them numerically. The solutions allow us to compare the response of the upper level population and the generated signal coherence for homogeneously and non-homogeneously broadened systems. Our results reveal an AC Stark shift in the nonlinear signal when the frequency of the strong or weak beam is scanned, in contrast to what is observed in the fluorescence. Furthermore, we present experimental data for the four-wave mixing signal in a hot rubidium vapor for the copropagating configuration of the exciting beams. The behavior of the AC Stark displacement and signal amplitude as a function of laser power indicates a good agreement between the model and the experimental results.

Coherent control of the multiple wavelength lasing of ${\rm N}_2^ +$: coherence transfer and beyond

Nitrogen molecules pumped by intense femtosecond laser pulses give rise to coherent emission in the forward direction at a series of wavelengths, coined “air lasing.” We demonstrate the coherent control of these emissions via a pair of seeding pulses at two different frequencies at low pressures, revealing a coherence transfer through vibrational motion. It is found that the injection of a 427.8 nm (391.4 nm) seeding pulse results in its amplification at the expense of the 391.4 nm (427.8 nm) signal, demonstrating a competition between the two spectral components of the emission from the upper level population. Moreover, when the delay between the seeding and pump pulses is finely tuned, the coherent control of both transitions is observed via the coherence transfer. A microscopic molecular relaxation model reproduces these observations, highlighting the crucial role of electronic and vibrational coherences, as well as their coupling, during the lasing action.

Studies of temporal characteristics of all-solid-state, gain-switched Cr:forsterite laser

A four-level rate-equation based model is presented to study the temporal dynamics of gain-switched Cr:forsterite laser. This model incorporates the effect of the excited-state absorption (ESA) present at the pump as well as the emitted wavelength from the forsterite laser. The value of the ESA cross-section at the pump wavelength (1064 nm) was experimentally obtained and utilized in the theoretical studies. The evolution of the upper laser level population and laser pulse inside the resonator cavity was numerically obtained by solving the rate-equations of the cavity photon density and population density. The Cr:forsterite laser pulse shows spiking behaviour as a consequence of the relaxation oscillation occurs during the buildup of the laser pulse inside the resonator cavity. The effect of several parameters related to pump pulse and cavity components on the output pulse parameters and the cavity buildup time were studied. The spiking behaviour in the output pulse profile of the Cr:forsterite laser was experimentally observed. The Cr:forsterite laser was pumped by the fundamental wavelength of Nd:YAG laser operated at 20 Hz pulse repletion rate in the confocal resonator cavity. The experimental results of pulse parameters were compared with the computed results and presented here.

Brightness attenuation mechanisms of Er3+ self-sensitized upconversion nanocrystals under 1.5 μm pumping

The Er3+ self-sensitized upconversion nanoparticles (UCNPs) under 1.5 μm pumping presents great promises due to their high luminescence efficiency and safe excitation wavelength. However, its practical application ability are limited due to unsatisfactory brightness induced by organic compounds. We have even observed completely quench when it dispersed in polymer. Herein, the effects of high-frequency vibration modes from the organics on luminescence properties of β-NaYF4:Er3+ excited by 1.5 μm were studied in detail. The results showed that decreased emission of β-NaYF4:Er3+ in organic compounds containing CH bonds is mainly attributed to the excitation energy attenuation induced by molecules. The CH bonds in aliphatic molecules possessing strong absorption in near infrared region will dissipate the 1.5 μm photons energy. Equally importantly, an ignored quenching pathway was considered that the Er3+ 4I13/2 state is directly quenched by the near infrared vibrations of CH at ~7000 cm−1. The de-excitation of the Er3+ first-excited state eventually affects the population of upper electronic levels. After considering to avoid the above quenching pathways, the ligand-free NaYF4:Er@NaYF4 UCNPs exhibited satisfactory brightness upon 1.5 μm excitation and could be used for anti-counterfeiting labels. Our findings will promote the developments of many advanced applications based on Er3+ doped UCNPs.

Comparison of Enhanced Absorption in He i 10830 Å in Observations and Modeling during the Early Phase of a Solar Flare

Abstract The He i 10830 Å triplet is a very informative indicator of chromospheric activities as the helium is the second most abundant element in the solar atmosphere. Taking advantage of the high resolution of the 1.6 m Goode Solar Telescope at Big Bear Solar Observatory, previous observations have shown clear evidence of the enhanced absorption, instead of typically observed emission, for two M-class flares. In this study, we analyze the evolution of the He i 10830 Å emission in numerical models and compare it with observations. The models represent the RADYN simulation results obtained from the F-CHROMA database. We consider the models with the injected electron spectra parameters close to observational estimates for the 2013 August 17 flare event (δ = 8, , ) in detail, as well as other available models. The modeling results agree well with observations, in the sense of both the maximum intensity decrease (−17.1%, compared with the observed value of −13.7%) and the trend of temporal variation (initial absorption phase followed by the emission). All models demonstrate the increased number densities and decreased ratio of the upper and lower level populations of the He i 10830 Å transition in the initial phase, which enhances the opacity and forms an absorption feature. Models suggest that the temperatures and free electron densities at heights of 1.3–1.5 Mm should be larger than ∼104 K and 6 × 1011 cm−3 thresholds for the line to start being in emission.

Feature construction as a bi-level optimization problem

Feature selection and construction are important preprocessing techniques in data mining. They allow not only dimensionality reduction but also classification accuracy and efficiency improvement. While feature selection consists in selecting a subset of relevant feature from the original feature set, feature construction corresponds to the generation of new high-level features, called constructed features, where each one of them is a combination of a subset of original features. Based on these definitions, feature construction could be seen as a bi-level optimization problem where the feature subset should be defined first and then the corresponding (near) optimal combination of the selected features should be found. Motivated by this observation, we propose, in this paper, a bi-level evolutionary approach for feature construction. The basic idea of our algorithm, named bi-level feature construction genetic algorithm (BFC-GA), is to evolve an upper-level population for the task of feature selection, while optimizing the feature combinations at the lower level by evolving a follower population. It is worth noting that for each upper-level individual (feature subset), a whole lower-level population is optimized to find the corresponding (near) optimal feature combination (constructed feature). In this way, BFC-GA would be able to output a set of optimized constructed features that could be very informative to the considered classifier. A detailed experimental study has been conducted on a set of commonly used datasets with varying dimensions. The statistical analysis of the obtained results shows the competitiveness and the outperformance of our bi-level feature construction approach with respect to many state-of-the-art algorithms.

3D atom microscopy in the presence of Doppler shift

The interaction of hot atoms with laser fields produces a Doppler shift, which can severely affect the precise spatial measurement of an atom. We suggest an experimentally realizable scheme to address this issue in the three-dimensional position measurement of a single atom in vapors of rubidium atoms. A three-level Λ-type atom–field configuration is considered where a moving atom interacts with three orthogonal standing-wave laser fields and spatial information of the atom in 3D space is obtained via an upper-level population using a weak probe laser field. The atom moves with velocity v along the probe laser field, and due to the Doppler broadening the precision of the spatial information deteriorates significantly. It is found that via a microwave field, precision in the position measurement of a single hot rubidium atom can be attained, overcoming the limitation posed by the Doppler shift.

Spontaneous emission from a V-type three-level atom in a dynamic photonic crystal

The spontaneous emission from a V-type three-level atom embedded in an isotropic photonic crystal with dynamic photonic band edge is studied. We consider the situation where the atom interacts with all possible radiation modes, and calculate numerically the evolution of atomic population without using Markov approximation. The calculation method can be used in related researches. In the present paper, we mainly discuss the effects of modulation parameters and the quantum interference on spontaneous emission when the band edge is modulated with step function or triangle function. We hope that the results can contribute to the applications in the dynamic photonic crystal environment in controlling the spontaneous emission via the quantum interference. The results show that in the step-modulated situation, the number of the photon-atom bound dressed states after the modulation has happened depends on atomic transition frequencies and the band edge frequency at that time, and is identical to the one in the unmodulated situation with the same parameters. The long-time evolution of the atomic population is affected by the time when the modulation happens. Depending on the system initial state, after the modulation has happened, the quantum interference can weaken the probability amplitude components corresponding to the photon-atom bound dressed states, and cause the upper-level population to decay quickly from a great value to a value near zero; or on the contrary, it can strengthen the bound dressed states, and make the upper levels retain a high population. In the modulated situation with trigonometric functions, after long enough time, the total upper-level population presents a decaying quasi-periodic oscillation behaviour. And the evolution of the total upper-level population tends to synchronize with the modulation, so the frequency of the quasi-periodic oscillation is approximately equal to the modulation frequency. But, the quantum interference can destroy the synchronization under some conditions. The decay rate of the total upper-level population is affected by the modulation frequency, and also by the initial state of the system and the angle between two dipole moment because of the quantum interference.

>MW peak power at 266 nm, low jitter kHz repetition rate from intense pumped microlaser.

Intense pulse pumped microlaser is proposed for high peak power and low timing jitter at high repetition rate. It is based on Intense and Fast Pulse Pump (IFPP) technique, in which fast pulse pumps up the upper-level population and then dumps it rapidly by Q-switching. That could come close to complete pumping efficiency to reduce thermal problems and contribute to suppress the timing jitter of passively Q-switched laser. In this work, linearly polarized 1064 nm beam from [100]-cut YAG/Nd3+:YAG and [110]-cut Cr4+:YAG passively Q-switched microlaser is directly guided into nonlinear crystals to obtain 532 nm and 266 nm output. By implementing IFPP concept, over 1 MW peak power, 215 ps pulse duration, 1 kHz pulses at 266 nm with reduced standard deviation timing jitter of 37 ns were obtained.

FORMATION OF FIELD INDUCED ABSORPTION IN THE PROBE RESPONSE SIGNAL OF A FOUR-LEVEL ‘V’ TYPE ATOMIC SYSTEM: A THEORETICAL STUDY

A density matrix based analytical model is developed to study the coherent probe field propagation through a four-level V type system in presence of a coherent control field. The model allows coupling of the probe field from the upper ground level to both of the excited levels keeping the control field locked to a particular transition. The addition of an extra ground level to a conventional three-level V type system creates extra decay paths to the ground levels for the upper level population. A set of sixteen density matrix based equations are formed and then solved analytically under rotating wave approximation to study the probe response under steady state condition. The simulated probe absorption spectra shows absorption dip at the centre of a transparency window only under Doppler broadened condition although the conventional EIT window appears under Doppler free condition. The dependence of the field induced absorption signal on the Rabi frequency of the control field, population transfer rate among the ground levels and temperature of the vapour medium has been studied in details.

The study of burst pulses envelope in Ytterbium-doped fiber amplifier modulating by pulsed pump source

The characteristics of fiber amplifiers of burst pulses envelope during the propagation are comprehensively studied under the conditions of pulsed pump with low repetition rate. The variation of signal envelopes, pump power and upper-level population distribution are discussed in this paper. The impacts of linear varied and exponential varied pump to the raising edge and tailing edge of the output pulses' envelope, are mainly emphasized. Finally, the genetic algorithm is employed to reshape the pulses' envelopes.

Effect of thermal noise on random lasers in diffusion regime

In this paper, we study the effects of thermal noise on the time evolution of a weak light pulse (probe) in the presence of a strong light pulse (pump) within a gain medium which includes random scatterer particles. Suitable thermal noise term is added to a set of four coupled equations including three diffusion equations for energy densities and a rate equation for the upper level population in a four-level gain medium. These equations have been solved simultaneously by Crank–Nicholson numerical method. The main result is that the back-scattered output probe light is increased as the thermal noise strength is increased and simultaneously, with the same rate, the amplified spontaneous emission is decreased. Therefore, the amplified response of the random laser in diffusion regime for the input probe pulse is enhanced due to effect of the thermal noise.

Ecosystem characteristics and processes facilitating persistent macrobenthic biomass hotspots and associated benthivory in the Pacific Arctic

The northern Bering and Chukchi Seas are areas in the Pacific Arctic characterized by high northward advection of Pacific Ocean water, with seasonal variability in sea ice cover, water mass characteristics, and benthic processes. In this review, we evaluate the biological and environmental factors that support communities of benthic prey on the continental shelves, with a focus on four macrofaunal biomass “hotspots.” For the purpose of this study, we define hotspots as macrofaunal benthic communities with high biomass that support a corresponding ecological guild of benthivorous seabird and marine mammal populations. These four benthic hotspots are regions within the influence of the St. Lawrence Island Polynya (SLIP), the Chirikov Basin between St. Lawrence Island and Bering Strait (Chirikov), north of Bering Strait in the southeast Chukchi Sea (SECS), and in the northeast Chukchi Sea (NECS). Detailed benthic macrofaunal sampling indicates that these hotspot regions have been persistent over four decades of sampling due to annual reoccurrence of seasonally consistent, moderate-to-high water column production with significant export of carbon to the underlying sediments. We also evaluate the usage of the four benthic hotspot regions by benthic prey consumers to illuminate predator–prey connectivity. In the SLIP hotspot, spectacled eiders and walruses are important winter consumers of infaunal bivalves and polychaetes, along with epibenthic gastropods and crabs. In the Chirikov hotspot, gray whales have historically been the largest summer consumers of benthic macrofauna, primarily feeding on ampeliscid amphipods in the summer, but they are also foraging further northward in the SECS and NECS hotspots. Areas of concentrated walrus foraging occur in the SLIP hotspot in winter and early spring, the NECS hotspot in summer, and the SECS hotspot in fall. Bottom up forcing by hydrography and food supply to the benthos influences persistence and composition of benthic prey that then influences the distributions of benthivorous upper trophic level populations.

Analysis and constrained optimal impulsive control of a Holling-II type trophic system with two sources

In this paper, we study a multi-trophic prey–predator system composed by top- and intermediate-level consumers and two food sources. Biomass fluxes across trophic levels are modeled by means of Holling-II type functional responses and one of the sources is subject to periodic impulsive events consisting in biomass injections. Referring to this system, we analyze the local and global stability properties of periodic eradicated solutions and establish permanence properties for the populations in terms of relations between the model parameters and the intensity of the impulses. Secondly, we formulate an optimal control problem for this system as well as an Iterative Dynamic Programming (IDP) scheme for its solution. By means of the proposed algorithm we specify the intensity of the impulsive controls to be applied periodically in order to regulate the biomass levels along time. In our formulation, the intensity of the periodic impulsive events depends on the biomass of the upper-level consumer population, as we can often see in an agricultural framework wherein the quantity of human workforce can significantly affect the size of crop periodically collected.

Efficient saturation of an ion in free space

We report on the demonstration of a light-matter interface coupling light to a single $^{174}\textrm{Yb}^+$ ion in free space. The interface is realized through a parabolic mirror partially surrounding the ion. It transforms a Laguerre-Gaussian beam into a linear dipole wave converging at the mirror's focus. By measuring the non-linear response of the atomic transition we deduce the power required for reaching an upper-level population of $1/4$ to be $692\pm20~\textrm{pW}$ at half linewidth detuning from the atomic resonance. Performing this measurement while scanning the ion through the focus provides a map of the focal intensity distribution. From the measured power we infer a coupling efficiency of $7.2\pm0.2~\%$ on the linear dipole transition when illuminating from half solid angle, being among the best coupling efficiencies reported for a single atom in free space.

Role of nonlinear refraction in the generation of terahertz field pulses by light fields

The generation of microwave (terahertz) pulses without any envelope in a four-level quasi-resonant medium is considered. Two intense quasi-monochromatic laser fields lead to a partial upper-level population. Microwave field pulses cause the transition between these levels. For appropriately chosen scales, the evolution of the fields is shown to be described by the pseudo-spin evolution equations in a microwave field with the inclusion of nonlinear refraction caused by an adiabatic upper-level population. The evolution of terahertz field pulses is described outside the scope of the slow-envelope approximation. When a number of standard approximations are taken into account, this system of equations is shown to be equivalent to an integrable version of the generalized reduced Maxwell-Bloch equations or to the generalized three-wave mixing equations. The soliton solution found by the inverse scattering transform method is used as an example to show that nonlinear refraction leads to a strong compression of the microwave (terahertz) field soliton.

Study on self-pulsing dynamics in Yb-doped photonic crystal fiber laser

In this paper, we present the experimental and analytical study of self-pulsing dynamics in Yb-doped photonic crystal fiber laser in single-end and double-end pumping configurations. It has been found that the intensity of self-pulses is considerably higher in the single-end pumping configuration as compared to that for double-end pumping configuration. In both the cases, it was noted that when the pump power was sufficiently high, intensity and occurrence of the self-pulses were reduced. However, the reduction in the intensity of self-pulses was significant in the double-end pumping. Experimental study shows that occurrence of self-pulses is due to the initial relaxation oscillations and re-absorption and there is no role of nonlinear phenomena in the generation of self-pulses. The rate equation analysis shows that the non-uniformity in single pass distribution of upper level population along the fiber length is responsible for re-absorption and hence self-pulsing. In the case of single-end pumping, single pass distribution of upper level population along the fiber length has more non-uniformity as compared to the double-end pumping, and hence stronger self-pulsing is observed.