Transverse Mode Research Articles

Optical conductivity of multi-Weyl node semimetals due to electromagnetic and axial pseudogauge fields

We compute the optical conductivity of a multi-Weyl semimetal in the presence of electromagnetic and axial fields. In the first part of the paper we use a U(1) gauge field to compute the current of a multi-Weyl semimetal. The current is used in the computation of the optical conductivity within the Kubo–Matsubara formalism. We consider a multi-Weyl semimetal with two nodal points in the z-direction. The multi-Weyl semimetal has a width D in the transverse direction which is smaller than the length L in the longitudinal direction. Due to the width D we replace the model by a sum of transverse modes in 1+1 dimensions. We then regularize the model in 1+1 dimensions. The nodal points are responsible for a constant axial field and a static anomalous Hall effect. In the second part we study the effect of dynamic axial fields. An elastic deformation couples to the two electronic chiralities, and represents the axial pseudogauge field a→5. As a consequence, the system is excited by two kinds of field, the electromagnetic one and the axial strain field. For a Weyl system this is represented by the triangle diagram, which is responsible for the anomalous Hall effect. We can obtain the anomalous Hall effect also by a chiral transformation in 1+1 dimensions using regularization. The chiral transformation modifies the path integration measure and is the source of the induced anomalous Hall effect. The latter is controlled by a combination of strain waves and electromagnetic fields.

Anomalous thermal transport in MgSe with diamond phase under pressure

Most materials exhibit a monotonically increasing lattice thermal conductivity with pressure. However, in this work, we report anomalous thermal transport properties of MgSe with a diamond phase under high pressure by first-principles calculations. Unlike other selenides, the lattice thermal conductivity of MgSe decreases with increasing pressure. We attribute this anomalous thermal transport to both harmonic and anharmonic effects. The harmonic effect is related to the anomalous downward shift of the transverse acoustic modes, which leads to decreasing phonon group velocities. This anomalous harmonic effect can also be confirmed by anomalous soft elastic properties. The anharmonic effect, on the other hand, is related to the stronger anharmonic scattering strength under pressure. Through bonding analysis, we found that the alternating bonding and antibonding states, combined with the unique lattice structure, are the main reason behind this anomaly. Our results reveal that the abnormal soft elastic properties under pressure can be a new way to judge whether lattice thermal conductivity decreases with pressure.

A Compact Polarization MMI Combiner Using Silicon Slot-Waveguide Structures.

The study of designing a compact transverse electric (TE)/transverse magnetic (TM) polarization multimode interference (MMI) combiner based on silicon slot-waveguide technology is proposed for solving the high demands for high-speed ability alongside more energy power and minimizing the environmental impact of power consumption, achieving a balance between high-speed performance and energy efficiency has become an important consideration in an optical communication system. The MMI coupler has a significant difference in light coupling (beat-length) for TM and TE at 1550 nm wavelength. By controlling the light propagation mechanism inside the MMI coupler, a lower order of mode can be obtained which can lead to a shorter device. The polarization combiner was solved using the full-vectorial beam propagation method (FV-BPM), and the main geometrical parameters were analyzed using Matlab codes. Results show that after a short light propagation of 16.15 μm, the device can function as TM or TE combiner polarization with an excellent extinction ratio of 10.94 dB for TE mode and 13.08 dB for TM mode with low insertion losses of 0.76 dB (TE) and 0.56 dB (TM) and the combiner function well over the C-band spectrum. The polarization combiner also has a robust MMI coupler length tolerance of 400 nm. These attributes make it a good candidate for using this proposed device in photonic integrated circuits for improving power ability at the transmitter system.

Field induced phase transition and magnetic phase diagram of multiferroic Ca2CoSi2O7

Magnetic phase transitions are important for the application of individual materials toward modern spintronics and sensing. Using high-resolution sound velocity and pyrocurrent measurements, we have obtained a detailed magnetic phase diagram of multiferroic Ca2CoSi2O7 for magnetic fields applied along the crystallographic [100]- and [001]-directions. The magneto-acoustic measurements were conducted for both longitudinal and transverse modes in both static and pulsed magnetic fields up to 17 and 68 T, respectively. Distinct anomalies with significant changes in sound velocity and sound attenuation were observed at the onset of the structural and magnetic phase transitions. Interestingly, results obtained for fields parallel to the [100]-direction reveal a hysteresis and step-like decrements in ultrasound indicating a transition into a different metastable structural distortion below TN and at magnetic fields between 4 and 10 T that has not been previously reported. We believe that this field-induced distorted structure phase may be the cause of the subsequent phase transition at 11 T. At low temperatures, a softening of the lattice within the ordered phases is also observed, which may be due to residual spin fluctuations.

Output characteristics’ static fluctuations versus the pump power in 1018 nm fiber oscillators

This paper investigates the static fluctuating behavior of output parameters in 1018 nm fiber lasers using 20/400 µm and 25/400 µm ytterbium-doped fibers (YDFs). It is seen that by increasing the pump power, some static fluctuations is induced in the output characteristics of the lasers, such as output power, back-reflected power, and beam quality factor (M2). The growth of these parameters fluctuates versus the pump power, without any modulation frequency in the temporal behavior of the output beam profile. This effect, which to the best of our knowledge is reported for the first time, occurs at powers much lower than the threshold for dynamic transverse mode instability (TMI). It was found that the static mode-coupling occurs between two lowest-order modes and causes these fluctuations in the lasers’ output parameters. Conducting the experiment for 1080 nm fiber lasers with different lengths of YDF, in addition to confirm the descriptions about how the static fluctuations occurs, shows that this effect occurs in other wavelengths as well.

3kW forward-pumped fiber laser via pump recycler.

We report a single-end forward-pumped fiber laser with a record high output power of 3kW. The laser is assembled exclusively from commercially widespread components such as the Yb-doped fiber with core/cladding diameter of 20/400µm, pump laser diodes at an emission wavelength of 915nm, and a signal and pump fiber combiner that serves as the pump recycler. The record high power arises from the combination of the 915nm pumping and pump recycler with an effective reflectivity of 78%, increasing simultaneously the thresholds for stimulated Raman scattering and transverse mode instability (TMI). The length of the oscillator was also varied experimentally from 20m to 5m, showing a contrast of up to 19% in the TMI threshold. This shows the importance of accurately partitioning the Yb-doped fiber length in between the oscillator and amplifier sections to minimize the impact of TMI.

Heavy quarkonium with finite three momentum near Tc

We investigate the non-trivial 3-momentum effects on the masses of heavy quarkonium states that are moving in a hot medium using QCD sum rules. For all charmonium states, we observe a negative mass shift near $T_c$ that is less than 3$\%$ at a momentum of 1$\rm{GeV}$. Specifically, we first investigate the difference between the longitudinal and transverse modes of both $J/\psi$ and $\chi_{c1}$. We find that the transverse mode of the $J/\psi$ experiences larger modification than the longitudinal mode, while the $\chi_{c1}$ has the opposite behavior. By comparing the $\eta_c$ and $\chi_{c0}$, and also the unpolarized $J/\psi$ and $\chi_{c1}$, we recognize that the P-wave particles have stronger momentum dependencies on their masses than the S-wave ones. We also find $\Upsilon$(1S) has negligible 3-momentum dependence compared to the charmonium states, e.g. less than 0.01$\%$ even at 1.4$T_c$ and at a momentum of 4$\rm{GeV}$.

Origin of SRS-induced beam quality distortion under TMI threshold.

In high power fiber lasers, the degradation of beam quality caused by Raman effect has attracted more and more attention in recent years, but its physical mechanism is still unclear. We're going to differentiate between heat effect and nonlinear effect by duty cycle operation. The evolution of beam quality at different pump duty cycles has been studied based on a quasi-continuous wave (QCW) fiber laser. It is found that even if the Stokes intensity is only -6 dB (energy proportion: 26%) lower than that of the signal light intensity, the beam quality has no obvious change with the duty cycle of 5%; on the contrary, when the duty cycle gradually approaches 100% (CW-pumped scheme), the beam quality distortion changes faster and faster with the increase of Stokes intensity. The experimental results are contrary to core-pumped Raman effect theory [IEEE Photon. Technol. Lett.34, 215 (2022)10.1109/LPT.2022.3148999], and further analysis confirms that the heat accumulation in the process of Stokes frequency shift should be responsible for this phenomenon. That is the first time, to the best of our knowledge, for intuitive reveal of the origin of stimulated Raman scattering (SRS)-induced beam quality distortion under transverse mode instability (TMI) threshold in an experiment.

Vibration characteristics and load-carrying capacity of multiple steel plate girder bridges with corroded girder ends

This study investigates the effects of corrosion on the vibrational characteristics and load-carrying capacity of steel plate girder bridge ends, which are extensively used in many bridges. First, eigenvalue and static analyses were performed using the finite element method, and the following main parameters were considered: the number of main girders, corrosion locations, and corrosion levels. The correlation between the results of the eigenvalue analysis and load-carrying capacity was examined; furthermore, to serve vibration-based health monitoring purposes, the selected parameters were investigated. The results indicated that the natural frequencies of the torsion-transverse coupling mode changed considerably as the corrosion ratio increased in the case of the corroded girder end and double-sided pin-bearing conditions. Additionally, a correlation between the natural frequency and load-carrying capacity was observed regardless of the number of main girders, indicating that the natural frequency and maximum load factor were related. In conclusion, the load-carrying capacity of corroded girder bridges can be estimated by measuring the transverse mode using an acceleration sensor installed on the upper flange and slab.

Transverse Mode Instability in Raman Fiber Amplifiers

Hybrid ytterbium-Raman fiber amplifiers have reached multi-kW levels in recent years, showing great promise for kW powers at new wavelengths. Many applications need diffraction-limited output which is limited by transverse mode instability (TMI). In this work, we extended our 3D amplifier model for studying TMI in ytterbium fiber amplifiers to include Raman gain and used this new model to study TMI in two Raman fiber amplifiers: a hybrid ytterbium-Raman fiber amplifier and a passive Raman fiber amplifier. Our previous study shows that gain saturation effect in ytterbium fiber amplifiers plays a significant role in suppressing TMI in kW ytterbium fiber amplifiers. This new study shows that in a Raman fiber amplifier in the absence of similar connection between local Raman gain and local Stokes wave, TMI is not much affected by seed power, fiber length or core diameter in the absence of mode-dependent loss. Mode-dependent loss is a key factor for increasing the TMI threshold in this case.

Transverse mode instability considering bend loss and heat load.

Previously, we developed a highly efficient transverse mode instability model by integrating stimulated thermal Rayleigh scattering and quasi-3D fiber amplifier models, enabling the consideration of the 3D gain saturation effect, with its accuracy verified by reasonable fit to experimental data. Bend loss was however ignored. Higher-order-mode bend loss can be very high especially for fibers with core diameters below 25µm and is sensitive to the local heat load. By using a FEM mode solver to account for bend loss and local heat-load-induced bend loss reduction, the transverse mode instability threshold is studied in detail, resulting in some interesting new insights.

2.11 GHz all-fiber harmonic mode-locked laser based on highly nonlinear hybrid multimode interference saturable absorber

This paper proposes a harmonic mode-locked fiber laser based on a novel highly nonlinear hybrid multimode interference saturable absorber (SA). The hybrid multimode interference SA is integrated by tapered graded-index multimode fiber (TA-GIMF) and Bi2Te3. The numerical simulation results show that TA-GIMF can improve the stability of nonlinear multimode interference mode-locking by filtering higher-order transverse modes. In addition, the larger evanescent-wave fields generated by TA-GIMF are more beneficial to the light–matter interaction. By depositing Bi2Te3 on TA-GIMF, the nonlinear saturation absorption properties and nonlinear coefficients in the laser cavity are further improved. Based on the hybrid multimode interference SA, the repetition rate of the fiber laser reached 2.11 GHz, corresponding to the 30th harmonic of the fundamental repetition rate of 70.41 MHz. Our results are the highest repetition rate generated based on the NL-MMI effect and provide novel insights into implementing highly integrated, high repetition-rate mode-locked fiber lasers.

Bamboo-inspired composite metastructure for broadband microwave absorption and load bearing

Microwave absorbing structures that can simultaneously cope with the complex electromagnetic and physical environments have become indispensable in contemporary stealth technology. Herein, a bamboo-inspired composite metastructure with integrated functions of broadband electromagnetic microwave absorption and mechanical load bearing is designed and investigated by 3D printing technology with lossy magnetic composite. By integrating the benefits of gradient structure design in both microwave absorption and mechanical load bearing areas, structural optimization and experimental demonstration are conducted to simultaneously achieve a range of -10 dB microwave absorption from 3.2 GHz to 40 GHz, a -15 dB absorption bandwidth in 4.2–5.23 GHz and 6.9–40 GHz, and an average compressive yield stress of 13.27 MPa. Furthermore, this metastructure also exhibits broadband microwave absorption at wide incident angles for different polarizations of transverse electric and transverse magnetic modes. These results indicate that the integrated functional design facilitates practical engineering applications in severe service environments for stealth technology.

Broadband multifunctional plasmonic polarization converter based on multimode interference coupler

We propose a multifunctional integrated plasmonic–photonic polarization converter for polarization demultiplexing in an indium-phosphide membrane on silicon platform. By using a compact 1 × 4 multimode interference coupler, this device can provide simultaneous half-wave plate (HWP) and quarter-wave plate (QWP) functionalities. Our device employs a two-section HWP to obtain a very large conversion efficiency of ≥ 91% over the entire C to U telecom bands, while it offers a conversion efficiency of ≥ 95% over ∼ 86% of the C to U bands. Our device also illustrates QWP functionality, where the transmission contrast between the transverse electric and transverse magnetic modes is ≈ 0 dB over the whole C band and 55% of the C to U bands. Using this functionality, our device generates two quasi-circular polarized beams with opposite spins and topological charges of l=±1, based on only one input polarization, and one incoming light beam direction. We expect that this device can be a promising building block for the realization of ultracompact on-chip polarization demultiplexing and lab-on-a-chip biosensing platforms. Finally, our proposed device allows the use of the polarization and angular momentum degrees of freedom, which makes it attractive for quantum information processing.

Multimode topological interface states in a one-dimensional elastic-wave phononic crystal

The one-dimensional (1D) Su-Schrieffer-Heeger (SSH) model is the simplest way to realize the topological phase transition. The topological interface state (TIS) induced by the Zak phase in the SSH model helps researchers to understand and develop the topological band theory, which has been widely promoted and applied in optics and phononics. In this work, we introduce the impedance matching method into the 1D elastic-wave system with three modes (one pure transverse mode and two quasi-longitudinal/transverse) to realize multimode TISs in the same structure. Superior to Zak phase analysis which only can qualitatively determine the presence of one TIS in the band gaps, the proposed impedance matching method allows us quantitatively to design the frequencies of multimode TISs in our elastic-wave model. Numerical band structures, transmission spectra, and field distributions verify these results. We not only extend the calculation of TIS to more complex multimode systems but also obtain multiband TISs with broader application value.

Oblique incidence-insensitive metamaterial absorber using strong magnetic coupling resonance for low-frequency microwave absorption

In this work, we presented an oblique incidence-insensitive metamaterial absorber (MA) based on the strong magnetic coupling resonance using the printed circuit board process, and the spacer is produced by a typical magnetic absorber material (MAM). As for reflection loss (RL) at oblique angles, it can achieve high-efficiency absorption for both transverse-electric (TE) and transverse-magnetic (TM) polarizations with a wide angular range from 0° to 60° in the low frequency of 2.33 GHz to 3.44 GHz. In addition, the magnetic coupling mechanism of the metamaterial absorber was deeply analyzed. Comparing the results of the simulation and experiment, it displays excellent absorption performance of the proposed absorber in TE and TM modes under large oblique incidence angles.

Numerical simulation of efficient solar absorbers and thermal emitters based on multilayer nanodisk arrays

In this work, we design a solar absorber and thermal emitter with ultra-broadband perfect absorption and high thermal radiation efficiency. The solar absorber has a high absorption efficiency of 91.5 % in the full wavelength range (280–4000 nm), and the weighted average absorption efficiency (AM1.5) is as high as 99 % by the finite difference time domain method (FDTD) simulation calculation. And the absorption bandwidth with absorption efficiency greater than 90 % reaches 2929 nm (280–3209 nm), and the average absorption efficiency in this band is as high as 97.4 %. Such strong absorption is due to the plasmon resonance and near-field coupling of the multilayer nanodisks. In addition to being used in solar absorbers, the structure also has potential applications in thermal emitters. The novelty of this work is to explore the application of the structure in the field of thermal radiation, and the optimal working temperature of the structure for thermal radiation is obtained. Through calculation, we approximately consider that the optimal working temperature of the structure as a thermal emitter is 2000 K, and the thermal radiation efficiency at this temperature is 94.8 %. The structure proposed by us is polarization-independent, insensitive to incident angle changes, the absorption spectra of transverse electric (TE) mode and transverse magnetic (TM) mode are the same, and the absorption efficiency remains 80 % even when the incident angle is increased to 60°. The excellent performance makes the structure widely used in the fields of solar energy absorption and emission.

Improvement of Optical Confinement for Terahertz Vertical-Cavity Surface-Emitting Laser with Square-Lattice Photonic Crystal Structure

A new method proposed to enhance the optical confinement of the terahertz band in a vertical cavity surface emitting laser involves introducing a square-lattice photonic crystal structure. This structure’s filling factor was optimized by computing the energy band structure and optical band values of the photonic crystal. The optimal optical band value is 0.436–0.528 a/λ. At a specific carrier concentration, the real part of dielectric constant of GaAs/AlGaAs materials will gradually increase with the increase of Al elements. By adjusting the length of the resonant cavity, a vertical cavity surface emitting laser with two wavelengths can be created without utilizing current injection. Additionally, the photonic crystal structure’s control effect on the transverse mode of the vertical cavity surface emitting laser and the release effect of the PN junction light confinement were analyzed. Numerical calculations indicated that incorporating a cubic photonic crystal structure in the vertical cavity surface emitting laser resulted in a 2× increase in the difference frequency intensity and a 6.33× increase in the optical field intensity.

Sound characteristics of disordered granular disks: effects of contact damping

We investigate numerically the sound properties of disordered dense granular packings in two dimensions. Employing linear equations of motion and excluding contact changes from our simulations, we demonstrate time evolution of sinusoidal standing waves of granular disks. We varied the strength of normal and tangential viscous forces between the disks in contact to explore the dependence of sound characteristics such as dispersion relations, attenuation coefficients, and sound speeds on the contact damping. For small wave numbers, the dispersion relations and sound speeds of acoustic modes are quite insensitive to the damping. However, a small dip in the phase speed of the transverse mode decreases as the viscous force in normal direction increases. In addition, the dispersion relation of the rotational mode differs qualitatively from the theoretical prediction for granular crystals. Therefore, disordered configurations with energy dissipation play a prominent role in sound properties of granular materials. Furthermore, we report how attenuation coefficients depend on the contact damping and quantify how they differ from the prediction of lattice theory. These improved relations, based on our numerical results, can in future be compared to advanced theories and experiments.

Suppressing transverse mode instability through multimode excitation in a fiber amplifier

High-power fiber laser amplifiers have enabled an increasing range of applications in industry, science, and defense. The power scaling for fiber amplifiers is currently limited by transverse mode instability. Most techniques for suppressing the instability are based on single- or few-mode fibers in order to output a clean collimated beam. Here, we study theoretically using a highly multimode fiber amplifier with many-mode excitation for efficient suppression of thermo-optical nonlinearity and instability. We find that the mismatch of characteristic length scales between temperature and optical intensity variations across the fiber generically leads to weaker thermo-optical coupling between fiber modes. Consequently, the transverse mode instability (TMI) threshold power increases linearly with the number of equally excited modes. When the frequency bandwidth of a coherent seed laser is narrower than the spectral correlation width of the multimode fiber, the amplified light maintains high spatial coherence and can be transformed to any target pattern or focused to a diffraction-limited spot by a spatial mask at either the input or output end of the amplifier. Our method simultaneously achieves high average power, narrow spectral width, and good beam quality, which are required for fiber amplifiers in various applications.