Group Velocity Difference Research Articles

Diverse carrier mobilities in halide perovskites: The role of conductive network and ionized impurity scattering

In past decades, ABX3 halide perovskites have attracted great interest in solar cells due to excellent optoelectronic properties, such as high carrier mobility. However, instability and toxicity are obstacles on the commercial route for perovskites. Many studies have turned to exploring A2BX6 and A3B2X9 for better stability. Unfortunately, the carrier mobilities of these two types are inferior to ABX3, lower by an order of magnitude. Furthermore, the mobility of ABX3 is distributed over a large range of 1.78–4500 cm2 V−1 s−1 in experiments, which contributes to another diversity of mobilities. In this paper, we aim at revealing the physical origin of the above-mentioned diversities by theoretical studies on CsBX3, Cs2BX6, and Cs3B2X9 (B=Sn, Pb, Sb, Bi, X=Br, Cl). The difference in group velocities is the major reason responsible for the variation in these types. The unique three-dimensional connected conductive network of CsBX3 determines its large group velocity. As for carrier scattering, ionized impurity scattering dominates at low carrier and high ionized impurity concentrations. Detailed analysis reveals that band degeneracy is strongly related to the impurity scattering rate, while dielectric constant is almost immune. Our study provides a better understanding of the relationship between electronic structures and mobilities for potential applications in photovoltaics.

Numerical and experimental investigations on quasistatic pulse generation of ultrasonic guided waves in fiber reinforced composite pipes

The quasistatic pulse (QSP) generation of ultrasonic guided waves in composite pipes can exhibit many features that are useful for early-stage material characterization and structural health monitoring. The intrinsic relationship between the QSP generation and the weak elastic nonlinearity of solids is complex. It has yet promised for developing the nonlinear ultrasonic guided wave technique that combines its advantages of high sensitivity to microdamage and low ultrasonic attenuation in composite materials. This study presents a systematic investigation of the QSP generation in fiber reinforced composite pipes. Using a three-dimensional finite element simulation with a nonlinear material model, the temporal waveform, mode conversion effect, cumulative effect, generation efficiency, and duration effect of QSP generation are revealed. The nonlinear QSP signals in laminated composite pipes are confirmed as the fastest wave mode that only has axial displacement. The shape of QSP depends on the group velocity difference between the QSP and the primary wave. The magnitude of QSP is related to the excitation level, frequency, and tone-burst duration of the primary wave. Experiments are conducted using carbon fiber reinforced composite pipes and the signals are measured using a laser vibrometer scanning system for verifying the QSP generation. The experimental results are consistent with the numerical findings. A relative nonlinear acoustic parameter based on the QSP generation is proposed and used to evaluate the early-stage thermal fatigue damage in the pipes. The measured nonlinear acoustic parameter demonstrates high sensitivity to the damage, indicating considerable potential for industrial applications.

The energy dispersion of magnetic Rossby waves in zonally nonuniform basic states

The energy dispersion of magnetic Rossby waves was investigated by applying two-dimensional incompressible magnetohydrodynamic equations in both zonally varying basic flow and basic magnetic field. A derived cubic dispersion relation suggests that there are at most three types of magnetic Rossby wave. Two of them represent waves that gradually tend to Alfvén waves during the energy dispersion process. The energy dispersion trajectories (wave rays) finally move with the zonal group velocity that tends to be equal to the zonal phase speed after being reflected by at least one turning location at which the meridional group velocity equal to zero. Along the marching rays, both the wave action density and wave energy tend to be constant values while the wave amplitude will decrease with increasing total wavenumber. The third one represents a wave that gradually have the constant meridional wavenumber, wave action density, wave energy, and wave amplitude. However, the difference in the zonal group velocity and the zonal phase speed suggests that the wave is still dispersive. This type of wave will disappear if specifying uniform basic magnetic field. The cubic dispersion relation is then reduced to a quadratic one. Correspondingly, the remaining two dispersion relations feature a fast- and a slow-propagating magnetic Rossby wave, respectively. They finally tend to be Alfvén waves with no energy dispersion when the energy dispersion process completes.

Thermal switch based on ferroelasticity VA-N binary compounds

Ferroelastic materials possess two or more equally stable orientation variants and can be effectively modulated via external fields, including stress and electronic field. In this paper, taking the VA-N ferroelastic materials as examples, we propose a thermal switch device based on their ferroelastic characteristics. The results show that the VA-N binary compound exhibits excellent ferroelasticity, high reversible elastic strain (5.5%–54.1%), and suitable switching energy barriers (0.012–0.386 eV/atom) in both δ and α phases. Utilizing the advanced on-the-fly machine learning potential, we obtain physically well-defined quadratic dispersion curves in the long-wavelength limit and further evaluate their lattice thermal conductivity of δ and α phase VA-N binary compounds. Due to the difference in phonon group velocities, the lattice thermal conductivity of VA-N binary compounds along the armchair direction is obviously smaller than that along the zigzag direction. Such remarkable anisotropy and easily switchable features based on ferroelasticity endow reversible and real-time regulation of thermal conductivity of VA-N binary compounds. The ferroelastic-based thermal switch hosts high switch ratios range from 2.08 to 5.99 and does not require additional energy to maintain the modulation state. The results presented herein provide a pavement for designing next-generation thermal switches and propose a reliable solution for eliminating the nonphysical pseudo-phenomenon of phonon dispersion curve violation of quadratic dispersion in the long-wavelength limit.

Effects of dispersion and pumping scheme on soliton-comb regimes in χ (2) microresonators

Monochromatic pumping of χ ( 2 ) microresonators is promising for frequency comb generation. Two main generation schemes refer to pumping into the phase-matched first harmonic (FH) and second harmonic (SH)—FH and SH pumping. The phase matching can be natural or artificial (employing periodic poling). Also, minimization of the FH and SH group velocity difference (temporal walk-off) is needed for effective comb generation. So far, perspective soliton-comb regimes were predicted for SH pumping in lithium niobate (LN)-based resonators at a special poling. Here we explore, analytically and numerically, the possibilities for realizing the χ ( 2 ) comb generation for the remaining options of pumping as applied to LN microresonators. This includes the FH pumping case for the pump wavelength λ p ≃ 2698 n m in radially poled resonators and also FH and SH pumping schemes at natural phase matching, λ p ≃ 1664 and 832 nm. It is found that the nonlinear regimes are substantially different and less efficient here as compared to the primary SH pumping case. This is caused by a profound difference between the FH and SH excitation schemes and also by different combinations of signs of the dispersion coefficients for FH and SH resonator modes.

Thermal transport properties of two-dimensional boron dichalcogenides from a first-principles and machine learning approach

The investigation of thermal transport is crucial to the thermal management of modern electronic devices. To obtain the thermal conductivity through solution of the Boltzmann transport equation, calculation of the anharmonic interatomic force constants has a high computational cost based on the current method of single-point density functional theory force calculation. The recent suggested machine learning interatomic potentials (MLIPs) method can avoid these huge computational demands. In this work, we study the thermal conductivity of two-dimensional MoS2-like hexagonal boron dichalcogenides (H-B2 VI 2; VI = S, Se, Te) with a combination of MLIPs and the phonon Boltzmann transport equation. The room-temperature thermal conductivity of H-B2S2 can reach up to 336 W⋅m−1⋅K−1, obviously larger than that of H-B2Se2 and H-B2Te2. This is mainly due to the difference in phonon group velocity. By substituting the different chalcogen elements in the second sublayer, H-B2 VIVI′ have lower thermal conductivity than H-B2 VI 2. The room-temperature thermal conductivity of B2STe is only 11% of that of H-B2S2. This can be explained by comparing phonon group velocity and phonon relaxation time. The MLIP method is proved to be an efficient method for studying the thermal conductivity of materials, and H-B2S2-based nanodevices have excellent thermal conduction.

Design of highly sensitive refractive index sensor based on silicon photonic Mach–Zehnder interferometer

A highly sensitive refractive index sensor based on silicon photonic Mach–Zehnder interferometer is proposed. The device consists of a subwavelength-grating slot waveguide in one arm, as a sensing region, to enhance the light-gas interaction. By optimizing the structure parameters, the device operates near the dispersion turning point when the optical mode group velocity difference between the sensing arm and the other reference arm approaches zero. The RI sensitivity is 9.6×105/RIU and gas pressure sensitivity is as high as 232 nm/MPa. The study results show that the proposed design is a potentially robust refractive index sensor and has a good performance in gas pressure sensing.

Analisis Kecepatan Perambatan Gelombang Internal Soliter di Selat Lombok dengan Citra Himawari-8

Internal waves are phenomena that occur below sea surface. One of the most observed types of internal waves was called a solitary wave or soliton so it is often known as an internal solitary wave (ISW). Lombok Strait was one of the straits located in the southern part of the Indonesian archipelago which has the characteristics of intensive ISW generation because this strait has complex waters characteristics. This study aim to analyzed in detail the changes in the ISW propagation speed at the first crest soliton every 10 minutes using Himawari-8 data. The detected ISW pattern is then extracted at the first crest of the soliton as a reference to measure the distance between the detected soliton every 10 minutes as input in estimating the propagation speed. The Himawari-8 image which was in this study detected 2 ISW packets in the Lombok strait area. Packet 1 which was detected in the northern part of the Lombok strait approaching Kangean island had a group speed of 2.22 m/s and Packet 2 which was detected in the area around the Lombok strait had a group speed of 2.78 m/s. The difference in group velocity of the two packets is due to the variation in depth along the ISW propagation. The analysis results of changes in ISW propagation speed in this study indicate that Himawari-8 imagery is a very effective instrument for analysing changes in ISW propagation speed in detail every 10 minutes which is difficult for satellite imagery which usually has low temporal resolution.

Stress effect on lattice thermal conductivity of anode material NiNb2O6 for lithium-ion batteries

The thermal transport properties of NiNb2O6 as anode material for lithium-ion battery and the effect of strain were studied by machine learning interatomic potential combined with Boltzmann transport equation. The results show that the lattice thermal conductivity of NiNb2O6 along the three crystal directions [100], [010], and [001] are 0.947 W⋅m−1⋅K−1, 0.727 W⋅m−1⋅K−1, and 0.465 W⋅m−1⋅K−1, respectively, indicating the anisotropy of the lattice thermal conductivity of NiNb2O6. This anisotropy of the lattice thermal conductivity stems from the significant difference of phonon group velocities in different crystal directions of NiNb2O6. When the tensile strain is applied along the [001] crystal direction, the lattice thermal conductivity in all three directions decreases. However, when the compressive strain is applied, the lattice thermal conductivity in the [100] and [010] crystal directions is increased, while the lattice thermal conductivity in the [001] crystal direction is abnormally reduced due to the significant inhibition of compressive strain on the group velocity. These indicate that the anisotropy of thermal conductivity of NiNb2O6 can be enhanced by the compressive strain, and reduced by the tensile strain.

Structural Dynamics, Phonon Spectra and Thermal Transport in the Silicon Clathrates.

The potential of thermoelectric power to reduce energy waste and mitigate climate change has led to renewed interest in “phonon-glass electron-crystal” materials, of which the inorganic clathrates are an archetypal example. In this work we present a detailed first-principles modelling study of the structural dynamics and thermal transport in bulk diamond Si and five framework structures, including the reported Si Clathrate I and II structures and the recently-synthesised oC24 phase, with a view to understanding the relationship between the structure, lattice dynamics, energetic stability and thermal transport. We predict the IR and Raman spectra, including ab initio linewidths, and identify spectral signatures that could be used to confirm the presence of the different phases in material samples. Comparison of the energetics, including the contribution of the phonons to the finite-temperature Helmholtz free energy, shows that the framework structures are metastable, with the energy differences to bulk Si dominated by differences in the lattice energy. Thermal-conductivity calculations within the single-mode relaxation-time approximation show that the framework structures have significantly lower than bulk Si, which we attribute quantitatively to differences in the phonon group velocities and lifetimes. The lifetimes vary considerably between systems, which can be largely accounted for by differences in the three-phonon interaction strengths. Notably, we predict a very low for the Clathrate-II structure, in line with previous experiments but contrary to other recent modelling studies, which motivates further exploration of this system.

Real-Time Access to Collisions between a Two-Soliton Molecule and a Soliton Singlet in an Ultrafast Fiber Laser

Optical solitons in ultrafast fiber lasers, as a result of dual balances between dispersion and nonlinearity as well as gain and loss, enable various soliton interactions. Soliton collisions are among the most intriguing soliton interactions, which fuel the understanding for particle-like properties of solitons. Here, we experimentally investigate the transient dynamics of collisions between a two-soliton molecule and a soliton singlet in a mode-locked fiber laser. By means of the dispersive Fourier transform technique, the evolving spectral interferograms of different collision scenarios are measured in real time. In particular, the “quasi-elastic” collision is observed, which shows that the soliton-molecule state remains unaltered after the collision and the group-velocity difference between the soliton molecule and the singlet is changed. It is directly demonstrated that a bond exchange occurs between the colliding solitons. By tuning the intra-cavity polarization controller, the dynamic processes of other collision outcomes, including the annihilation of a soliton in the soliton molecule as well as the formation of a stable unequally spaced soliton triplet, are also revealed. Our work facilitates a deeper understanding of soliton collision dynamics in ultrafast fiber lasers.

Quantum effects in rotating reference frames

We consider the time delay of interfering single photons oppositely traveling in the Kerr metric of a rotating massive object. Classically, the time delay shows up as a phase difference between coherent sources of light. In quantum mechanics, the loss in visibility due to the indistinguishability of interfering photons is directly related to the time delay. We can thus observe the Kerr frame dragging effect using the Hong–Ou–Mandel dip, a purely quantum mechanical effect. By Einstein's equivalence principle, we can analogously consider a rotating turntable to simulate the Kerr metric. We look at the feasibility of such an experiment using optical fiber and note a cancellation in the second order dispersion but a direction dependent difference in group velocity. However, for the chosen experimental parameters, we can effectively assume light propagating through a vacuum.

Sinusoidal Spectral Filtering-Based Soliton Dynamics in Mode-Locked Fiber Lasers

Sinusoidal spectral filtering (SSF) plays an essential role in dual- and multi-wavelength operations in mode-locked fiber lasers. However, the impact of SSF on the underlying soliton dynamics has not been systematically revealed so far. Herein, we investigate numerically the SSF-based soliton dynamics in mode-locked fiber lasers. SSF could facilitate the formation of dual-wavelength mode locking, and the generation of dual-comb solitons with slightly different group velocities. Pulse shaping based on SSF and gain filtering leads to the non-monotonic relationship between both solitons' group velocity difference and the cavity gain. In addition, dual-comb soliton mode locking exhibits threshold feature and multi-pulse instability to soliton annihilation and diverse soliton coexistences. Our investigations could shade new light on the SSF-based soliton dynamics and might bring new insights into the laser design and applications.

Anisotropic thermal and electrical transport properties induced high thermoelectric performance in an Ir2Cl2O2 monolayer.

In recent years, the energy crisis and global warming have been urgent problems that need to be solved. As is known, thermoelectric (TE) materials can transfer heat energy to electrical energy without air pollution. High-throughput calculations as a novel approach are adopted by screening promising TE materials. In this paper, we use first-principles calculations combined with the semiclassical Boltzmann transport theory to estimate the TE performance of monolayer Ir2Cl2O2 according to the prediction that Ir2Cl2O2 has potential as a good TE material via high-throughput calculations. The low thermal conductivities of 1.73 and 4.68 W mK-1 of Ir2Cl2O2 along the x- and y-axes are calculated, respectively, which exhibits the strong anisotropy caused by the difference in group velocities of low-frequency phonon modes. Then, the electronic transport properties are explored, and the figure of merit ZT is eventually obtained. The maximum ZT value reaches 2.85 (0.40) along the x-axis (y-axis) at 700 K, revealing that the TE properties of the Ir2Cl2O2 monolayer are highly anisotropic. This work reveals that the anisotropic layer Ir2Cl2O2 exhibits high TE performance, which confirms that it is feasible to screen excellent TE materials via high-throughput calculations.

Dual backgrounds and their stability during frequency comb and second harmonic generation in χ(2) microresonators

Light states relevant to the χ ( 2 ) comb generation in optical microresonators possess typically dual backgrounds for coupled first-harmonic (FH) and second-harmonic (SH) envelopes. Stability and/or instability of these backgrounds is crucial for realization of stable χ ( 2 ) combs and also for efficient SH generation. We explore the properties of the dual backgrounds and their instability for the cases of FH and SH pumping of the resonator. In contrast to optical parameteric oscillation, the instability is controlled by a fourth-degree characteristic equation for the increment. Coefficients of this equation depend not only on wavenumbers of the perturbations, pump power, and dispersion parameters, but also on FH-SH group velocity difference (temporal walk-off). Our results include characterization of the regions and conditions of stability for the FH and SH pumping cases and different spectral ranges.

Design of highly sensitive interferometric sensors based on subwavelength grating waveguides operating at the dispersion turning point

In this paper, a dual-mode interferometric sensor based on a subwavelength grating (SWG) waveguide is proposed. The design operates at the dispersion turning point (DTP) of the two dispersive TE modes in the periodic structure, exhibiting a group velocity difference of almost zero, to improve the performance of refractive index (RI) sensors. The necessary conditions for realizing DTP in SWG are analyzed. Moreover, the interference spectrum characteristics and the influence of structural parameters on gas RI sensing performance are further studied. The results show that the sensitivity value of the proposed sensor can reach ∼ 38 , 000 n m / R I U . The proposed structure can also achieve a gas pressure sensitivity of ∼ 105 n m / M P a . The proposed sensors provide a potential platform for highly sensitive RI sensors.

In-situ diagnostic of femtosecond laser probe pulses for high resolution ultrafast imaging

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variations. It relies on observing the phenomena induced by an ultrashort laser pump pulse using an ultrashort probe pulse at a later time. Recent years have seen the emergence of very successful ultrafast imaging techniques of single non-reproducible events with extremely high frame rate, based on wavelength or spatial frequency encoding. However, further progress in ultrafast imaging towards high spatial resolution is hampered by the lack of characterization of weak probe beams. For pump–probe experiments realized within solids or liquids, because of the difference in group velocities between pump and probe, the determination of the absolute pump–probe delay depends on the sample position. In addition, pulse-front tilt is a widespread issue, unacceptable for ultrafast imaging, but which is conventionally very difficult to evaluate for the low-intensity probe pulses. Here we show that a pump-induced micro-grating generated from the electronic Kerr effect provides a detailed in-situ characterization of a weak probe pulse. It allows solving the two issues of absolute pump–probe delay determination and pulse-front tilt detection. Our approach is valid whatever the transparent medium with non-negligible Kerr index, whatever the probe pulse polarization and wavelength. Because it is nondestructive and fast to perform, this in-situ probe diagnostic can be repeated to calibrate experimental conditions, particularly in the case where complex wavelength, spatial frequency or polarization encoding is used. We anticipate that this technique will enable previously inaccessible spatiotemporal imaging in a number of fields of ultrafast science at the micro- and nanoscale.

Simulation of Broadening the Second Harmonic Spectrum in KDP by Chirp Pulse Pumping

The conditions of broadening the second harmonic spectrum in KDP upon pumping by a negatively chirped pulse of fundamental frequency with central wavelength of 950 nm are analyzed numerically. It is shown that broadening of the spectrum (K = 1.4) is mainly limited by difference in group velocities of radiation pulse between first and second harmonics. The article is based on materials of the report at the first All-Russian scientific conference with international participation "YENISEI PHOTONICS – 2020"

Mechanisms of stationary converted waves and their complexes in the multi-component AB system

Under investigation in this article is a multi-component AB system which models the self-induced transparency phenomenon. By using the modified Darboux transformation, we present the breather solutions of such system. We study the subtle mechanism that converts the breathing state into the solitary and periodic ones, through which we obtain various stationary nonlinear excitations such as the multi-peak solitons, (quasi) periodic waves, (quasi) anti-dark solitons, W-shaped solitons and M-shaped solitons which exhibit stationary feature. According to the analysis of the group velocity difference, we give the corresponding conversion rule and present the explicit correspondence of phase diagram of wave numbers for various converted waves, by which we show the gradient relation among these converted waves. Further, by separating the converted waves into the solitary wave as well as the periodic wave, we classify different kinds of nonlinear waves and indicate the difference of the superposition mechanism among them. We show that the breather and various converted waves are formed by different superposition modes between the solitary wave components with different localities and periodic wave components with different frequencies. By virtue of the second-order solutions, we consider all possible superposition situations of two nonlinear waves and present the corresponding nonlinear wave complexes. In particular, for the hybrid structure made of a breather and a nonlinear wave with variable velocity, we then discover that the nonlinear wave does not change its state under the conversion condition, leading to that an additional breathing structure or a dark structure is contained in the converted waves. Finally, we unveil the underlying relationship between the conversion and modulation instability.

Slow light bimodal interferometry in one-dimensional photonic crystal waveguides

Strongly influenced by the advances in the semiconductor industry, the miniaturization and integration of optical circuits into smaller devices has stimulated considerable research efforts in recent decades. Among other structures, integrated interferometers play a prominent role in the development of photonic devices for on-chip applications ranging from optical communication networks to point-of-care analysis instruments. However, it has been a long-standing challenge to design extremely short interferometer schemes, as long interaction lengths are typically required for a complete modulation transition. Several approaches, including novel materials or sophisticated configurations, have been proposed to overcome some of these size limitations but at the expense of increasing fabrication complexity and cost. Here, we demonstrate for the first time slow light bimodal interferometric behaviour in an integrated single-channel one-dimensional photonic crystal. The proposed structure supports two electromagnetic modes of the same polarization that exhibit a large group velocity difference. Specifically, an over 20-fold reduction in the higher-order-mode group velocity is experimentally shown on a straightforward all-dielectric bimodal structure, leading to a remarkable optical path reduction compared to other conventional interferometers. Moreover, we experimentally demonstrate the significant performance improvement provided by the proposed bimodal photonic crystal interferometer in the creation of an ultra-compact optical modulator and a highly sensitive photonic sensor.