Spontaneous Radiation Research Articles

Tunable directional emission based on graphene hyperbolic metamaterials

We have studied the tunable properties of a hyperbolic metamaterial (HMM) composed of alternative dielectric layers and graphene sheets, and proposed a highly directional emission device working in the terahertz region. By changing the chemical potential , the equifrequency contour (EFC) of the HMM is easily switched between elliptical and hyperbolic types. Especially, when a dipole source is placed inside the structure, highly directional emission is realized provided that the EFC is a flat ellipse. Interestingly, such highly directional emission can be achieved for any frequency within a broad frequency range by tuning the chemical potential . The divergence angle of such a directional beam is lower than 12°. Additionally, the influences of structural parameters ( and d) on the working bandwidth are also studied. These results have potential in many fields such as light coupling and spontaneous radiation antennas.

Analysis of the spectral properties of free electron lasers in X-RAY and other bands

Analytical formalism for the description of free electron laser (FEL) spectrum is proposed with account for the electron beam size, divergences, beam energy spread, the off-axis deflection and the periodic harmonic and non-periodic constant magnetic field components in undulators. The exact analytical expressions for the spontaneous undulator radiation (UR) are obtained in terms of generalized Bessel and Airy functions, which describe the spectrum line shape and intensities. The obtained analytical formulae distinguish the effects of each installation and beam parameter on the FEL harmonic radiation. The enhanced phenomenological model is employed for the FEL modeling; it describes the harmonic powers evolution along the undulators and gradual saturation around the saturation length with account for the saturated powers oscillations. With this model, we study the generation of FEL harmonics, including even, in several instances of FEL experiments at LCLS, LEUTL and PAL-XFEL. We analytically demonstrate the even harmonics reasons in the LCLS and LEUTL FEL experiments. The spectrum line split due to the beam size is demonstrated analytically and compared with experimentally measured spectral characteristics. The PAL-XFEL experiment is analyzed and compared with the LCLS experiment; the possibility of high harmonic radiation in it is explored. The modeling results fully agree with the measured values in the studied FELs. The analytical formalism shows physical background for each harmonic in every FEL. It allows the analysis of the radiation and beam alignment in projected, built and working FELs and it can help minimizing losses and correcting magnetic fields as well as projecting experiments with FELs.

Emission of xenon in the spectral range of 120 – 800 nm upon excitation by diffuse and spark discharges

Xenon emission in the spectral range of 120 – 800 nm upon excitation by subnanosecond voltage pulses is investigated. Excitation is performed using diffuse and spark discharges in an inhomogeneous electric field at a xenon pressure ranging from 0.3 to 3 atm. It is shown that xenon excitation by a series of successive 0.7-ns voltage pulses with a pause of 30 ns leads to a decrease in the second-continuum radiation intensity by the second and subsequent voltage pulses and an increase in the first-pulse afterglow intensity. It is confirmed that in the case of a pulsed diffuse discharge, the second continuum of xenon dimers, which is used to generate laser and spontaneous radiation in the VUV spectral region, makes the largest contribution to the radiation energy. Broadband radiation in the visible range (λ > 400 nm) is found to occur when the discharge is constricted. The obtained results indicate that the influence of the dynamic displacement current on the pin-diode caused a measurement error in the papers by V.I. Baryshnikov et al., who did not record the second-continuum radiation from xenon excited by a short homogeneous discharge.

Generation of gravity waves from thermal tides in the Venus atmosphere

Gravity waves play essential roles in the terrestrial atmosphere because they propagate far from source regions and transport momentum and energy globally. Gravity waves are also observed in the Venus atmosphere, but their characteristics have been poorly understood. Here we demonstrate activities of small-scale gravity waves using a high-resolution Venus general circulation model with less than 20 and 0.25 km in the horizontal and vertical grid intervals, respectively. We find spontaneous gravity wave radiation from nearly balanced flows. In the upper cloud layer (~70 km), the thermal tides in the super-rotation are primary sources of small-scale gravity waves in the low-latitudes. Baroclinic/barotropic waves are also essential sources in the mid- and high-latitudes. The small-scale gravity waves affect the three-dimensional structure of the super-rotation and contribute to material mixing through their breaking processes. They propagate vertically and transport momentum globally, which decelerates the super-rotation in the upper cloud layer (~70 km) and accelerates it above ~80 km.

Ultrarelativistic electrons in counterpropagating laser beams

The dynamics and radiation of ultrarelativistic electrons in strong counterpropagating laser beams are investigated. Assuming that the particle energy is the dominant scale in the problem, an approximate solution of classical equations of motion is derived and the characteristic features of the motion are examined. A specific regime is found with comparable strong field quantum parameters of the beams, when the electron trajectory exhibits ultrashort spike-like features, which bears great significance to the corresponding radiation properties. An analytical expression for the spectral distribution of spontaneous radiation is derived in the framework of the Baier–Katkov semiclassical approximation based on the classical trajectory. All the analytical results are further validated by exact numerical calculations. We consider a non-resonant regime of interaction, when the laser frequencies in the electron rest frame are far from each other, avoiding stimulated emission. Special attention is devoted to settings when the description of radiation via the local constant field approximation fails and to corresponding spectral features. Periodic and non-periodic regimes are considered, when lab frequencies of the laser waves are always commensurate. The sensitivity of spectra with respect to the electron beam spread, focusing and finite duration of the laser beams is explored.

Integrated superconducting circuit for qubit and resonator protection* *Project supported by the National Key R&D Progarm of China (Grant No. 2016YFA0301802), the National Natural Science Foundation of China (Grant Nos. 61521001, 12074179, and 11890704), and the Key R&D Program of Guangdong Province, China (Grant No. 2018B030326001).

A semi-infinite waveguide acts as a mirror and helps protect the qubit in front of it from decoherence. Here, we investigate the interference effect in an open waveguide consisting of resonators with different decay rates. We find that a lossy resonator works as a mirror and changes the effective decay rate of the other. The spontaneous radiation of qubit is related to its environment, and we can control it by arranging the lossy resonator’s position or frequency. Our approach helps improving the qubit performance, as well as the quantum gate fidelities.

Human hand as a powerless and multiplexed infrared light source for information decryption and complex signal generation

With the increasing pursuit of intelligent systems, the integration of human components into functional systems provides a promising route to the ultimate human-compatible intelligent systems. In this work, we explored the integration of the human hand as the powerless and multiplexed infrared (IR) light source in different functional systems. With the spontaneous IR radiation, the human hand provides a different option as an IR light source. Compared to engineered IR light sources, the human hand brings sustainability with no need of external power and also additional level of controllability to the functional systems. Besides the whole hand, each finger of the hand can also independently provide IR radiation, and the IR radiation from each finger can be selectively diffracted by specific gratings, which helps the hand serve as a multiplexed IR light source. Considering these advantages, we show that the human hand can be integrated into various engineered functional systems. The integration of hand in an encryption/decryption system enables both unclonable and multilevel information encryption/decryption. We also demonstrate the use of the hand in complex signal generation systems and its potential application in sign language recognition, which shows a simplified recognition process with a high level of accuracy and robustness. The use of the human hand as the IR light source provides an alternative sustainable solution that will not only reduce the power used but also help move forward the effort in the integration of human components into functional systems to increase the level of intelligence and achieve ultimate control of these systems.

Transverse Beam Emittance Measurement by Undulator Radiation Power Noise.

Generally, turn-to-turn power fluctuations of incoherent spontaneous synchrotron radiation in a storage ring depend on the 6D phase-space distribution of the electron bunch. In some cases, if only one parameter of the distribution is unknown, this parameter can be determined from the measured magnitude of these power fluctuations. In this Letter, we report an absolute measurement (no free parameters or calibration) of a small vertical emittance (5-15nm rms) of a flat beam by this method, under conditions, when it is unresolvable by a conventional synchrotron light beam size monitor.

Self-Induced Dark States in Two-Dimensional Excitons

We have obtained an ultralong lifetime exciton emission in InAs/GaAs single quantum dots (QDs) when the QD films are transferred onto the Si substrate covered by Ag nanoparticles. It is found that when the separation distance from the QD layer (also the wetting layer) to the Ag nanoparticles is around 19 nm, the QD emission lifetime changes from approximately 1 to 2000 ns. A classical dipole oscillator model is used to quantitatively calculate the spontaneous radiation decay rate of the excitons in the wetting layer (WL), and the simulated calculation result is in good agreement with the experimental one, revealing that the long lifetime exciton emission is due to the existence of the dark state in the WL. The self-induced dark state stems from the destructive interference between the exciton emission field and the induced dipole field of the Ag nanoparticles.

Биофизическая модель процесса распознавания картин ответной реакции кожного покрова на оптическую активацию

This paper presents a biophysical model of the recognition process of glow patterns (response) of the skin in the area of biologically active zones to external optical activation. The model was developed to increase the efficiency of the process of identifying the causes of differences in glow patterns. The structures of glow patterns are described and their differences are described. A rationale for the use of forced radiation of skin cells in current studies is given (recognition of weak signals of spontaneous radiation is technically difficult). Forced radiation of biological objects appears as a result of excitation of the biological environment by an external action and is much higher than spontaneous radiation. Spontaneous optical radiation of skin cells under certain conditions is one of the characteristic features of the skin. This radiation can be caused by a field form of intercellular interaction, which causes biochemiluminescence in the form of spontaneous weak signals ensuring intercellular communication. The model uses a biotechnical system related to the human physical state, its characteristics, type of meridian and external conditions. We have identified subsystems (biological and technical), the main elements of the system, determined the links between them, necessary and sufficient for drawing conclusions, formulated the main requirements for the system, determined the initial data. The results of the preliminary experimental investigations with the use of the developed model are presented in which some dependence of the luminescence patterns on the meridian type is revealed. It is noted that final conclusions on the causes of skin glow patterns differences in certain points of biologically active zones will be made when more extensive statistical data is collected using the developed model.

Design and Research of Enhanced LED Performance Based on Graphical Substrate with Multilayer Hyperbolic Metamaterials

This paper mainly studies the influence of multilayer hyperbolic metamaterials (HMMs) with different structural parameters on the intensity of spontaneous radiation of quantum wells, thereby improving the coupling efficiency of incident electromagnetic waves and free electrons on metal nano-surfaces. In this paper, numerical simulations of visible light bands of 450–700 nm of Ag, Au, and Cu thin films are performed. The local field enhancements of multilayer HMMs with different shapes are compared, and it is found that circle Ag/Si multilayer HMMs have stronger field enhancement effects than other structures. At the same time, Purcell analysis was performed by changing various parameters of multilayer HMMs. It is found that the thickness of the metal/dielectric layer, the distance between the dipole and the HMMs, and the length of the multilayer HMMs change the intensity of the plasmon resonance radiation and have a great impact on the position of the resonance wavelength.

One‐Pot Synthesis of CsPbX3 (X = Cl, Br, I)@Zeolite: A Potential Material for Wide‐Color‐Gamut Backlit Displays and Upconversion Emission

AbstractAll‐inorganic perovskite (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) with promising optical properties are recognized as a rising star among photoelectric materials, but their inevitable instabilities limit extensive application. Herein, a new one‐step strategy for encapsulating CsPbX3 NCs with porous zeolite via the hot injection method is explored. By altering the mole ratio of PbX2, the emission wavelength of CsPbX3@zeolite is found to be regularly modulated from blue‐emission (CsPbCl2Br@zeolite, 436 nm) to red‐emission (CsPbBrI2@zeolite, 648 nm), thus enabling full spectrum adjustment. Due to the surface passivation of zeolite to CsPbX3 NCs, CsPbX3@zeolite possess an outstanding quantum yield and display excellent thermal/photo/air stability. Subsequently, white light‐emitting diodes based on green‐CsPbBr3@zeolite and red‐CsPbBrI2@zeolite are constructed and applied to the backlit display, achieving a wide color gamut of 134.2% for NTSC 1953 and 99.8% for Rec. 2020. The CsPbX3@zeolite implanted with a polymethyl methacrylate film is used to realize upconversion emission under an 800 nm laser at room temperature. Furthermore, amplified spontaneous radiation based on CsPbBr3@zeolite films is achieved. These results convincingly indicate broad application prospects for CsPbX3@zeolite prepared via this simple method.

The forming mechanism of spontaneous emission noise flux radiated from hydrogen-like atoms by means of vibrational Hamiltonian

The main goal of this article is to investigate the origin of spontaneous emission radiation by applying a new method. In this method, the vibrational Hamiltonian of hydrogen-like atoms (HLAs) is first constructed in accordance with the Hamiltonian of a simple harmonic oscillator. The Hamiltonian is then used to derive the vibrational motion equations of HLAs in a Heisenberg picture. The Langevin equation will ultimately be formed by adding the dissipative term and fluctuating (Langevin) force according to the fluctuation–dissipation theorem. The solution of the Langevin equation provides the exact data on the fluctuations that occur for the electron position during its rather fast vibrational oscillations. On the other hand, the fluctuations in the position of the oscillating electron, in turn, lead to the fluctuations in its potential and kinetic energies so that the appearance of the potential and kinetic noise fluxes is vulnerable. It is finally demonstrated that the potential and kinetic noise fluxes commonly act as an internal source of producing the external noise flux emitted from HLAs in the form of spontaneous emission with a Lorentzian profile. In contrast with all previous procedures, no ambient effect has been involved to describe the forming mechanism of spontaneous emission for the first time.

Polarization Shaping of Free‐Electron Radiation by Gradient Bianisotropic Metasurfaces

AbstractFree‐electron radiation phenomena facilitate enticing potential to create light emission with highly tunable spectra, covering hard‐to‐reach frequencies ranging from microwave to X‐ray. Consequently, they take part in many applications such as on‐chip light sources, particle accelerators, and medical imaging. While their spectral tunability is extremely high, their polarizability is usually much harder to control. Such limitations are especially apparent in all free electron based spontaneous radiation sources, such as the Smith−Purcell (SP) radiation. Here, anomalous free‐electron radiation phenomenon is demonstrated at the microwave regime from gradient bianisotropic metasurfaces, by using a phased dipole array to mimics moving charged particles. The phase gradient and the bianisotropy in metasurfaces provide new degrees of freedom for the polarization shaping of free‐electron radiation, going beyond the common spectral and angular shaping. Remarkably, the observed anomalous free‐electron radiation obeys a generalized SP formula derived from Fermat's principle.

Universal lasing condition

Usually, the cavity is considered an intrinsic part of laser design to enable coherent emission. For different types of cavities, it is assumed that the light coherence is achieved by different ways. We show that regardless of the type of cavity, the lasing condition is universal and is determined by the ratio of the width of the atomic spectrum to the product of the number of atoms and the spontaneous radiation rate in the laser structure. We demonstrate that cavity does not play a crucial role in lasing since it merely decreases the threshold by increasing the photon emission rate thanks to the Purcell effect. A threshold reduction can be achieved in a cavity-free structure by tuning the local density of states of the electromagnetic field. This paves the way for the design of laser devices based on cavity-free systems.

Polydisperse inter-electrode plasma of Pd nanoclusters as a random cavity for x-ray spontaneous emission bursts

It is demonstrated experimentally that polydisperse Pd plasma of low energy (∼1 J) nanosecond vacuum discharge operating in a virtual cathode regime becomes an effective stochastic cavity in hard x-ray domain. A diffusion model of stochastic propagation of spontaneous x-ray radiation in the volume of randomly located reflecting clusters is developed also. The model provides qualitative explanation of both the experimentally observed effects of high-intensity bursts and partial ‘trapping’ of x-ray radiation. The x-ray burst emerges when photons are stored inside the plasma volume followed by instant release at the end of the discharge while the trapping regime corresponds to the slow developing x-ray diffusion with the characteristic time larger than the discharge duration. The results obtained are compared qualitatively with well-known concept of Letokhov’s random laser.

How Do the Purcell Factor, the Q‐Factor, and the Beta Factor Affect the Laser Threshold?

AbstractAs lasers get more and more miniaturized and their dimensions become comparable to the wavelength, two interconnected phenomena take place: the fraction of spontaneous radiation going into a specific laser mode (β‐factor) increases and can ultimately reach unity, while the radiative lifetime gets shortened by the Purcell factor Fp. Often it is assumed that an increase of these two factors, along with the quality factor (Q‐factor), almost invariably causes reduction of the lasing threshold. This assumption is tested on various photonic and plasmonic lasers, demonstrating that, while there is obvious correlation between the aforementioned factors and the laser threshold, the dependence is far from being straightforward and omnipresent. Depending on specific laser material and geometry, the threshold can decrease, increase, or stay unchanged when β‐factor, Q‐factor, and Fp increase. For the most part, the reduction of threshold is achieved simply by reducing the laser volume and this volume reduction can concurrently cause the increase in β‐factor and/or Purcell factor, but it would be imprudent to say that the increase in either of these factors is the cause of the threshold reduction.

Spontaneous Radiation Amplification in a Microsphere‐Coupled CsPbBr3 Perovskite Vertical Structure

AbstractMicrosphere (MS) cavities hold great potential in nanophotonics due to their extraordinary optical properties. All‐inorganic trihalide perovskites with high environmental stability and easy‐for‐preparation have shown great promise for the next‐generation optoelectronic nanodevices. Unfortunately, the nonradiative recombination of exciton is inevitably aggravating the photoluminescence quantum yields (PLQYs). Here, a microsphere‐coupled CsPbBr3 perovskite vertical structure is proposed for amplification of spontaneous radiation (SR) up to 190‐fold. The mechanisms of SR amplification by an MS cavity are completely revealed, for the first time, including Mie focusing (MF) for energy harvesting, directional antenna (DA) for emission convergence, and whispering‐gallery modes (WGMs) for regulation of exciton dynamics. The temperature‐dependent photoluminescence (PL) spectra clarify the contributions of WGMs for the boosting of SR decay and suppression of exciton–phonon interactions for giant PL enhancement. Furthermore, a straightforward approach is presented for a selective MS array capped on a perovskite microcrystal (PeMC) film as a vertical structure for patterned PL enhancement. The excitation‐power‐ and excitation‐observation‐angle‐dependent luminescence contrast inspire a novel anticounterfeiting strategy. The vertical architecture not only advances the understanding of microcavity‐based SR amplification from PeMCs, but also offers opportunities to facilitate the synergistic manipulation of optical fields and exciton–phonon interactions for developing high‐efficient optoelectronic devices.

Spontaneous and Stimulated Undulator Radiation in Symmetric and Asymmetric Multi-Periodic Magnetic Fields

In this work, the radiation from electrons in multi-periodic undulator fields with symmetric and asymmetric harmonics was analyzed using generalized Bessel functions formalism. The asymmetric, symmetric, and anti-symmetric periodic magnetic fields with harmonics were studied in order to get the enhanced radiation of the high harmonics of undulator radiation (UR). The effect on the spontaneous and stimulated UR was explored. The exact integral forms for the Bessel coefficients were obtained for undulators with general symmetric and asymmetric field harmonics. Spectral properties of the radiation from several configurations of the undulator fields with harmonics were compared with each other. The resulting spontaneous UR spectrum and harmonic intensities were obtained analytically in the form of integrals and compared with the respective results that were obtained numerically with SPECTRA program. The dimensionless scaling parameter of a free electron laser (FEL)—the Pierce parameter (ρ)—was computed and compared for the different considered undulators. We studied the differences in the behavior of the high-gain single pass FEL harmonics and the spontaneous UR harmonics in the same undulators. The undulators with variable deflection parameter (k) were considered. The effect of the k parameter (deflection parameter for a common planar undulator) on the spontaneous UR and on the high-gain FEL radiation was explored. In this context, an experiment with variable strength undulators at FLASH 2 FEL was analyzed; the shorter saturated length in high harmonic self-seeding (HHSS) regime vs. self-amplified spontaneous emission (SASE) is explained.

Narrow-linewidth coherent population oscillation spectroscopy of room-temperature cesium atomic ensemble

Coherent population oscillations spectroscopy, which is based on the interaction between atoms and the phase locked laser, is a kind of atomic population modulation spectroscopy. When the laser frequency difference is less than natural width of energy level, the coherent oscillation of atomic population will be induced by laser intensity modulation so that the probe laser transmission with narrow bandwidth can be realized. For a closed two-level system (TLS), the spectral line-width is limited mainly by the spontaneous emission lifetime of the upper atomic energy level. As for a three-level atomic system of Λ configuration, the two linearly polarized beams with both σ+ and σ- polarization component, the laser-atom interaction satisfies the selection rule. The spectral line-width mainly depends on the ground-state relaxation time, and the dependence on the line-width of spontaneous radiation is eliminated. In this paper, the laser from a external-cavity diode laser has its frequency locked to Cesium <inline-formula><tex-math id="M1">\begin{document}$6{{\rm{S}}_{1/2}}\left( {F = 3} \right) \to 6{{\rm{P}}_{3/2}}\left( {F' = 3} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M1.png"/></alternatives></inline-formula> transition. The frequencies of the two beams are shifted down by two independent double-passed acousto-optic modulators (AOM) to nearly resonate to Cesium <inline-formula><tex-math id="M2">\begin{document}$6{{\rm{S}}_{1/2}}\left( {F = 3} \right) \to 6{{\rm{P}}_{3/2}}\left( {F' = 2} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M2.png"/></alternatives></inline-formula> transition. The probe beam and the coupling beam are superposed at polarization beam splitter (PBS) cube and transmitted through the magnetically shielded cesium vapor cell in the same direction. The two beams have approximately the same Gaussian diameter of 6.6 mm. The beams are separated by another PBS behind the vapor cell, and the probe beam is detected by a photodiode. We realize the coherent population oscillation spectroscopy through the Cesium vapor cell at room temperature without buffer gas. The spectral linewidth is typically less than 50 kHz which is far below the spontaneous radiation linewidth(~5.2 MHz). The linewidth of coherent population oscillation spectroscopy of the Λ-type atomic energy level structure depends only on the population associated with the oscillation of multiple degenerate level systems except phase correlations of atomic states. Coherent population oscillation is beneficial to the obtaining of the narrow linewidth spectroscopy through the Rydberg atomic system with long excited state lifetime. Considering the importance of electric field measurement using Rydberg atoms, the method of coherent population oscillation can be used to improve the sensitivity of precise measurements based on Rydberg atoms.