Gaussian Envelope Research Articles

Efficient exciton generation in a semiconductor quantum-dot–metal-nanoparticle composite structure using conventional chirped pulses

We consider a nanostructure consisting of a semiconductor quantum dot coupled to a metal nanoparticle, and show with numerical simulations that the exciton state of the quantum dot can be robustly generated from the ground state even for small interparticle distances, using conventional chirped pulses with Gaussian and hyperbolic secant envelopes. The asymmetry observed in the final exciton population with respect to the chirp sign of the applied pulses is explained using the nonlinear density matrix equations describing the system, and is attributed to the real part of the parameter emerging from the interaction between excitons in the quantum dot and plasmons in the metal nanoparticle. The simplicity of the conventional chirped pulses, which can also be easily implemented in the laboratory, make the proposed robust quantum control scheme potentially useful for the implementation of ultrafast nanoswitches and quantum information processing tasks with semiconductor quantum dots.

The multitaper reassigned spectrogram for oscillating transients with Gaussian envelopes

Joint time-frequency representations are important tools when estimating the instantaneous frequency. The widely used spectrogram is known to have poor energy localisation, which the reassignment method improves. However, the reassignment method is sensitive to noise. In this paper we present a multitaper reassigned spectrogram (MTRS) that is robust to noise and tailored to short duration transient signals. The MTRS is designed for oscillating transients with Gaussian envelopes and uses the Hermite functions as windows. The novel reassignment coordinates needed for the MTRS are derived in this paper. The MTRS localises energy at the instantaneous frequencies of transients, while white Gaussian noise is made flatter by the use of multiple windows. It is shown that the MTRS is more noise robust compared to reassignment with just one window, and that it improves on the energy localisation of the spectrogram. Thus, our MTRS is sparse, easy to interpret, and well suited for instantaneous frequency estimation, even for signals with overlapping transients and low signal-to-noise ratios.

Excitation of quantum systems by short laser pulses: The effect of the pulse shape and durations

We present a simple universal approach for the description of the quantum system excitation by short laser pulses in terms of the probability during the action of a pulse. Our attention is focused on the probability dependence on the pulse duration for different pulse envelopes and detunings of the carrier from the resonance frequency. We considered Gaussian, super-Gaussian, sin2 and rectangular envelopes and a Gaussian spectral profile of the excitation cross section. It is shown that the excitation probability as a function of the pulse duration is of various characters, changing from monotonic to oscillating depending on a pulse shape, a carrier frequency detuning from the resonance, and a spectral width of the cross section, in contrast to the prediction of the standard perturbation approach based on the Fermi golden rule.

Burgers equation and convexification method: Propagation of wave packets and narrowband noise.

The Burgers equation is a standard model for the propagation of progressive finite amplitude waves in a lossy medium. This paper is mainly devoted to the one-dimensional Burgers equation and its solution deduced from a convexification method. Its starting point is the Hopf-Cole solution in the inviscid limit, then the Legendre-Fenchel transform and the convex envelope construction are employed to obtain the single-value waveform. A fractional step method is used to numerically solve the Burgers equation in a segregated manner (nonlinearity and dissipation). Numerical results are obtained for nonlinear propagation of a wave packet with a Gaussian envelope and a narrowband noise (deterministic and non-deterministic signals). The convexification method provides a tool to solve the problems of nonlinear propagation, especially in the case of noise propagation with the formation and interaction (coalescence) of a large number of discontinuities (shocks).

Spherically polarized vector Bessel vortex beams

A different type of Bessel-like vortex beams with their intensity spherically modified by a singular polarization structure is introduced. Conventional vector Bessel-Gaussian beams are either linearly or radially (and azimuthally) polarized and their transverse structure is preserved under propagation. In this work, the Bessel-like beams with a Gaussian envelope are spherically polarized: the electric fields are oriented along the radius vector or along with a combination of meridional and azimuthal vectors. For the implementation of such beams, we investigate their vector spatial spectra. The intensity distribution of the beam can be controlled by a proper choice of parameters such as the cone angles of individual plane-wave components and sizes of Gaussian apertures. In the two limiting cases of axicon angles, spherically polarized Bessel beams form either two optical needles or optical-bottle-like intensity patterns.

Propagation properties of circularly symmetric Airy beam modulated by spectral asymmetric envelope

An asymmetric envelope function for modulating the spectrum of circular Airy beam is proposed in this work. The propagation properties of the modified circular Airy beam are investigated in both theory and experiment. The three parameters of the asymmetric hyperbolic secant function can be used to adjust the ratio of the high frequency components to the low frequency components in Fourier space, and thus tuning the propagation properties of this modified circular Airy beam. The results demonstrate that the focal position is affected mainly by the high frequency components. The maximum focal intensity will not be enhanced continuously by increasing the proportion of the high frequency components. It depends on the ratio of the high frequency components to the low frequency components when the center frequency is determined. Therefore, using an asymmetric envelope in Fourier space is much more reasonable than using the high pass filtering or symmetric Gaussian envelope. The FWHM decreases significantly with the increase of center frequency. When the parameters are chosen appropriately, the size of focal spot will be reduced significantly, the maximum focal intensity, especially the abruptly autofocusing property will be enhanced greatly and the focal position can remain almost the same as the focal position of the common circular Airy beam. The maximum focal intensity of the proposed beam is 3.4 times that of the common circular Airy beam and the abruptly autofocusing property of the proposed beam is much better than that of the beam using the symmetric Gaussian envelope. The phase-only encoding method in Fourier space is used to generate the proposed beam in experiment. The experimental results are in reasonable agreement with the simulation results. It indicates that the modified beam can be generated conveniently by using the same method as that used to generate the common circular Airy beam.

Tunable ytterbium fiber laser mode-locked with a nonlinear amplifying loop mirror

We present a widely tunable ytterbium fiber laser mode-locked with a nonlinear amplifying loop mirror (NALM). By introducing a transmission grating pair and a collimator into the cavity, which work as both a wavelength selector and a dispersion compensator, continuous wavelength tunable mode-locked operation from 1020.8 nm to 1059.5 nm is achieved. At the typical wavelength of 1030 nm, the laser delivers laser pulses with a pulse duration of 1.36 ps and a time-bandwidth product of 0.71 by assuming a Gaussian pulse envelope. By altering dispersion of the grating pair, the transition between two distinct pulse-shaping regimes in a figure eight NALM laser is demonstrated for the first time, to the best of our knowledge. When the laser enters the dissipative-soliton regime, chirped pulses with 24.9-nm spectral bandwidth are generated and the pulses can be dechirped to 88.7 fs which is a new pulse-duration record for figure eight ytterbium fiber lasers.

How single-photon emission performance for a coupled hyperbolic metamaterial–plasmonic antenna varies from the visible to the infrared regime

Hyperbolic metamaterial (HMM) based resonators have emerged as a very promising tool for providing highly enhanced as well as directional emission from a quantum emitter. HMM provides high- k hyperbolic modes to the radiation emitted from a quantum emitter embedded inside the HMM. These high-momentum hyperbolic HMM modes are coupled into free space by an antenna, which overcomes the large mismatch between free-space modes and HMM modes. Recently, cylindrical metallic antennas were observed to be the most effective in overcoming this momentum mismatch. In this work, we try to understand how this free-space out-coupling of emission from a quantum emitter embedded inside the HMM, with a cylindrical plasmonic antenna over the HMM’s top surface, varies as a function of frequency (or wavelength). HMM mode hyperbolicity was observed to increase with an increase in the emission wavelength, leading to its effective coupling to a cylindrical antenna. However, for large excitation wavelengths, the absorption cross section of the plasmonic cylindrical antenna was found to be significant. This leads to increased radiation losses within the plasmonic antenna, leading to a reduction in the collection and quantum efficiency of the coupled system. The performance of the HMM coupled cylindrical plasmonic antenna was observed to have a Gaussian envelope type profile, having peak performance in the range of 1000–1100 nm, with maximum collection and quantum efficiencies reaching 52% and 70%, respectively.

Investigation of Light Parameters on Image Quality and Optical Coherence Tomography

In this work, image quality and optical coherence tomography were studied. The results of the study show that there is a very significant difference between ultrasound and optical coherence tomography to produce an image with a different wave. To understand this, we studied the basic principle of optical coherence tomography in the Michelson interferometer using monochromatic and broadband sources. Time-domain and spectral-domain measurements, which exist at the detector level, are briefly described using a glass sample. The time-domain signal strength of the Michelson interferometer using a broadband source is a Gaussian envelope.

Effect of laser intensity redistribution on semiconductor plasma based THz emission

THz emission has been investigated using laser beat wave interaction with density rippled semiconductor plasmas. Two lasers with frequency ω1 and ω2 co-propagate through density rippled semiconductor plasma having wave vector α and beat laser wave imparts ponderomotive force on electrons that produce nonlinear current and allows the THz emission. Gaussian, ring shape and hollow Gaussian field envelopes have been considered introducing laser intensity distribution along the polarization direction. Redistribution of laser intensity due to varied spatial profile changes the dynamics of plasma electrons and improves the ponderomotive force. Enhancement in the doping level in semiconductor also acts as a tuning parameter for the THz emission. Combined effect of laser intensity redistribution and amplitude of density modulation (nα/n0) enhances the amplitude of the THz field. Conversion Efficiency of the order ≈ 10−6 is estimated for hollow Gaussian field envelope and density modulation of 20% in semiconductor plasma.

Модернизация эхометода ультразвуковой дефектоскопии

The modernization of the echo method of ultrasonic flaw detection is considered, which differs in that in addition to the array of transducers in contact with the working (front) surface of the object, one or more receiving transducers on the lateral surface of the object are additionally used. This method of arranging the antenna arrays can significantly improve the resolving power, and in the case of using an antenna array on the lateral surface of the object, obtain additional information for imaging the internal structure of the object. In order to obtain the potential resolution, an ultra-wideband sounding signal with a Gaussian envelope is used in combination with an original signal processing algorithm, including summation, one-way limiting and multiplication of the received signals. Some practical recommendations are given in the work. Potential resolution in various directions has been assessed.

High-dimensional vector solitons for a variable-coefficient partially nonlocal coupled Gross–Pitaevskii equation in a harmonic potential

A (3+1)-dimensional variable-coefficient partially nonlocal coupled Gross–Pitaevskii equation trapped in a harmonic potential becomes a focus of this paper. A counterpart of this variable-coefficient coupled equation is found as a (2+1)-dimensional constant-coefficient single nonlinear Schrödinger equation via the reduction procedure. By solutions of constant-coefficient single equation via the Hirota method, and from this counterpart, analytical high-dimensional vector soliton solutions with the Hermite–Gaussian envelope of the variable-coefficient coupled equation are deduced. Expanded behaviors of high-dimensional vector solitons emerge in the exponential diffraction decreasing system.

Reconstructions from randomly generated longitudinal electron bunch profiles with Gaussian envelopes using the Gerchberg-Saxton algorithm.

Knowledge of longitudinal electron bunch profiles is vital to optimize the performance of plasma wakefield accelerators and x-ray free electron laser linacs. Because of their importance to these novel applications, noninvasive frequency domain techniques are often employed to reconstruct longitudinal bunch profiles from coherent synchrotron, transition, or undulator radiation measurements. In this paper, we detail several common reconstruction techniques involving the Kramers-Kronig phase relationship and Gerchberg-Saxton algorithm. Through statistical analysis, we draw general conclusions about the accuracy of these reconstruction techniques and the most suitable candidate for reconstructing well-isolated longitudinal bunch profiles from spectroscopic data.

Effect of laser temporal intensity skew on enhancing pair production in laser—electron-beam collisions

Recent high-intensity laser experiments (Cole et al 2018 Phys. Rev. X 8 011020; Poder et al 2018 Phys. Rev. X 8 031004) have shown evidence of strong radiation reaction in the quantum regime. Experimental evidence of quantum effects on radiation reaction and electron–positron pair cascades has, however, proven challenging to obtain and crucially depends on maximising the quantum parameter of the electron (defined as the ratio of the electric field it feels in its rest frame to the Schwinger field). The quantum parameter can be suppressed as the electrons lose energy by radiation reaction as they traverse the initial rise in the laser intensity. As a result the shape of the intensity temporal envelope becomes important in enhancing quantum radiation reaction effects and pair cascades. Here we show that a realistic laser pulse with a faster rise time on the leading edge, achieved by skewing the temporal envelope, results in curtailing of pair yields as the peak power is reduced. We find a reduction in pair yields by orders of magnitude in contrast to only small reductions reported previously in large-scale particle-in-cell code simulations (Hojbota et al 2018 Plasma Phys. Control. Fusion 60 064004). Maximum pairs per electron are found in colliding 1.5 GeV electrons with a laser wakefield produced envelope 7.90 × 10−2 followed by a short 50 fs Gaussian envelope, 1.90 × 10−2, while it is reduced to 8.90 × 10−5, a factor of 100, for an asymmetric envelope.

Fourier-Bessel beams of finite energy

In this paper, we consider a new type of Bessel beams having Fourier-invariance property and, therefore, called Fourier-Bessel beams. In contrast to the known Bessel beams, these beams have weak side lobes. Analytical expressions for the complex amplitude of the proposed field in the initial plane of the source and in the far field region have been obtained. It is shown that the proposed Fourier-Bessel beams have a finite energy, although they do not have a Gaussian envelope. Their complex amplitude is proportional to a fractional-order Bessel function (an odd integer divided by 6) in the initial plane and in the Fraunhofer zone. The Fourier-Bessel modes have a smaller internal dark spot compared to the Laguerre-Gauss modes with a zero radial index. The proposed beams can be generated with a spatial light modulator and may find uses in telecommunications, interferometry, and the capture of metal microparticles.

Spatially resolved x-ray detection with photonic crystal scintillators

We study the self-collimation phenomenon in photonic crystals (PhC) of wide bandgap materials for ultra-fast and high spatial resolution x-ray detection. We work on various heavy inorganic scintillators: BaF2, GaN, ZnO, CsI:Tl, NaI:Tl, LYSO, WO4 compounds, and plastic scintillators. Conventional scintillator detectors do not rely on a direct detection mechanism; hence, they require intricate design and fabrication processes. We offer a PhC design to observe self-collimation phenomena and overcome the ongoing spatial resolution challenges with these types of materials. We investigate the photonic band diagrams and iso-frequency contours. Fourier transforms based on finite-difference time-domain and frequency domain simulations are done for verifying and analyzing the self-collimation with the selected material. Light extraction efficiency at the PhC–air interface, depending on the truncation distance from the excitation point, is measured. Beam divergence values are calculated at 1 mm propagation distance. The vertical field profiles are obtained to observe the confinement. For the spatial resolution analysis, cross-sectional beam profiles have been examined. Gaussian envelopes are fitted to beam profiles for a consistent data analysis, and full-width-at-half-maximum values are considered. As a result, we theoretically prove and demonstrate the spatially resolved x-ray detection at the sub-micrometer level for a wide range of scintillator materials.

Paraxial propagation and focusing of higher-order cosh-Gaussian beams

Based on the Huygens-Fresnel diffraction integral, a concise formula for the higher-order cosh-Gaussian beams (HOChGB) propagating through a paraxial ABCD optical system is derived. The propagation formula of the HOChGB is expressed as a superposition of cosh-Gaussian modes and in terms of the Gaussian beam-parameter q. The profile and the inter-lobe space of the beam at the initial plane are analyzed with the help of the maximum intensity criterion as a function of the decentered parameter and the beam order. It is shown that during propagation the HOChGB with appropriate parameters may progressively change profile, and in far-field the amplitude of the beam is expressed as a superposition of cosine functions modulated by a Gaussian envelope. Furthermore, it is found that the focal shift of the focused HOChGB increases with the decrease of the Fresnel number, the decentered parameter and the beam order.

Comparison of parameters of the electric strength of air in a surface antenna during the radiation of single microwave pulses of various shapes

When substantiating the requirements for the radiation parameters of high-power microwave generators it becomes necessary to assess their maximum permissible values, conditioned to air breakdown in the radiating surface antenna. To predict the parameters of the dielectric strength of air, a theory based on a model of microwave pulses with a rectangular envelope is widely used. However, in practice, there are cases when the shape of the pulse envelope of the generator differs significantly from the rectangular. The aim of this work is to evaluate and compare the amplitude and energy parameters of the electric strength of air in the surface antenna of a high-power relativistic microwave generator when pulses are emitted with envelopes of various shapes. Pulses with rectangular, trapezoidal, parabolic and Gaussian envelopes were selected as the initial ones. The breakdown parameters were found based on the criterion of pulsed breakdown of gases and the regularity connecting the electron density with the pulse amplitude. The problem is solved numerically. In the range of pulse durations typical for high-power relativistic microwave generators, the dependences of the breakdown field and the maximum allowable energy on the pulse duration under normal atmospheric conditions are determined. The regularities connecting the maximum permissible energy and the breakdown field for the considered pulses are determined. With the same breakdown fields, a pulse with a Gaussian envelope has the highest maximum permissible energy, and a pulse with a rectangular envelope has the lowest.

Aberrations in (3+1)-dimensional Bragg diffraction using pulsed Laguerre-Gaussian laser beams

We analyze the transfer function of a three-dimensional atomic Bragg beamsplitter formed by two counterpropagating pulsed Gaussian laser beams. Even for ultracold atomic ensembles, the transfer efficiency depends significantly on the residual velocity of the particles as well as on losses into higher diffraction orders. Additional aberrations are caused by the spatial intensity variation and wavefront curvature of the Gaussian beam envelope, studied with (3+1)D numerical simulations. The temporal pulse shape also affects the transfer efficiency significantly. Thus, we consider the practically important rectangular-, Gaussian-, Blackman- and hyperbolic secant pulses. For the latter, we can describe the time-dependent response analytically with the Demkov-Kunike method. The experimentally observed stretching of the $\pi$-pulse time is explained from a renormalization of the simple Pendell\"osung frequency. Finally, we compare the analytical predictions for the velocity-dependent transfer function with effective (1+1)D numerical simulations for pulsed Gaussian beams, as well as experimental data and find very good agreement, considering a mixture of Bose-Einstein condensate and thermal cloud.

Fractional-order-Bessel Fourier-invariant optical vortices

A new set of vortex rotationally symmetric Bessel beams is investigated, which are Fourier-invariant and have low sidelobes, in contrast to all the other known Bessel beams. Complex amplitudes in the source plane and in the far field are obtained analytically. These beams are of finite energy (and thus physically realizable for the topological charge exceeding 4), but they do not have the Gaussian envelope. In the initial plane and in the Fraunhofer diffraction area, their complex amplitude is proportional to the Bessel function of a fractional order (odd integer number divided by 6), although the vortex itself is of an integer order. Compared to the zero-radial-index Laguerre–Gaussian modes, such beams have smaller inner dark spot. Such beams can be generated by a spatial light modulator and applied in data transmission, interferometry, trapping of metallic particles.