Pump Wave Research Articles

Terahertz wave excitation by nonlinear coupling of intense laser field with magnetized plasma

In this article, the plasma density, the laser intensity and the external magnetic field are playing vast roles firstly to generate a coherent THz wave then to enhance the stability of generated THz. Due to the relativistic increase of electron mass, the relativistic self-focusing of a right circular polarized (RCP) laser beam inside magnetized plasma will occur which leads to raising the laser power to enough limits for exciting the terahertz wave. By fulfilling the energy–momentum conservation conditions, a terahertz wave frequency at the difference between the laser pump wave frequency and plasma wave frequency is obtained. More stabilization and higher power (reaching to tens of gigawatts) of terahertz field amplitude have been observed whenever the plasma density, the laser intensity, and the external magnetic field are increased. Better results are recorded at high THz frequency (5 THz) compared with low THz frequency (1 THz).

Photonics-Based Short-Time Fourier Transform Without High-Frequency Electronic Devices and Equipment

A photonics-based short-time Fourier transform (STFT) system is proposed and experimentally demonstrated based on stimulated Brillouin scattering (SBS) without using high-frequency electronic devices and equipment. The wavelength of a distributed feedback laser diode is periodically swept by using a low-speed periodic sawtooth/triangular driving current. The periodic frequency-sweep optical signal is modulated by the signal under test (SUT) and then injected into a section of SBS medium. The optical signal from another laser diode as the pump wave is reversely injected into the SBS medium. After simply detecting the forward transmission optical signals in a low-speed photodetector, the STFT of the SUT is implemented. The system is characterized by the absence of any high-frequency electronic devices or equipment. An experiment is performed. The STFT of a variety of RF signals is carried out in a 4-GHz bandwidth. The dynamic frequency resolution is demonstrated to be around 60 MHz.

Frequency Converters for the Terahertz and Infrared Ranges

A method for solving the problem of nonlinear diffraction on two-dimensional periodic gratings of graphene ribbons has been developed. The third-order nonlinear conductivity of graphene under the action of two waves is taken into account, which is determined by the field of the pump wave, for which we use the field on graphene ribbons obtained by solving the linear diffraction problem. Numerical analysis shows the efficiency of nonlinear frequency conversion in the terahertz and infrared ranges when the frequencies of the incident pump and signal waves coincide with the resonant frequencies of the fundamental and higher order modes of surface plasmon polaritons in graphene ribbons.

Наведенный нелинейный сдвиг частоты активного кольцевого резонатора на магнонном кристалле

The nonlinear shift of the resonant frequencies of a microwave active ring resonator (ARR) induced by a pump wave has been studied for the first time. The resonator was designed according to the scheme in the form of a closed ring containing a spin-wave delay line made on the basis of a one-dimensional magnonic crystal, as well as a microwave amplifier in a feedback circuit. It is shown, that with an increase in the power of the pumping wave, the shift of the resonant frequencies of the ARR increases in different ways depending on the position of the pumping frequency. The maximum induced nonlinear frequency shift is observed if the pumping frequency is at the minimum of the ARR transmission coefficient. A model describing the studied effect is developed.

Фазовые эффекты при вынужденном рассеянии Мандельштама-Бриллюэна

In the study of stimulated Brillouin scattering (SBS) in the constant intensity approximation, the spatial behavior of the intensity of the Stokes scattering wave in a medium was investigated. It was found that as a result of nonlinear interaction of waves, the period of spatial beats changes. The intensity of the Stokes scattering component is considered as a function of the phase mismatch, pump and acoustic wave intensities. It is found that the efficiency of the scattering of the backward Stokes wave is affected by the total length of the nonlinear medium. According to the analytical expressions obtained in this work, the choice of the optimal parameters of the problem makes it possible to implement the regime of efficient generation of the Stokes scattering component in SBS. The process of generation and amplification of the Stokes scattering component is compared with experiment. By varying the pump intensity, one can control and manipulate the intensity of the output radiation of the Stokes component.

Phase effects at stimulated Brillouin scattering

In the study of stimulated Brillouin scattering (SBS) in the constant intensity approximation, the spatial behavior of the intensity of the Stokes scattering wave in a medium was investigated. It was found that as a result of nonlinear interaction of waves, the period of spatial beats changes. The intensity of the Stokes scattering component is considered as a function of the phase mismatch, pump and acoustic wave intensities. It is found that the efficiency of the scattering of the backward Stokes wave is affected by the total length of the nonlinear medium. According to the analytical expressions obtained in this work, the choice of the optimal parameters of the problem makes it possible to implement the regime of efficient generation of the Stokes scattering component in SBS. The process of generation and amplification of the Stokes scattering component is compared with experiment. By varying the pump intensity, one can control and manipulate the intensity of the output radiation of the Stokes component. Keywords: SBS, backward Stokes scattering wave, constant intensity approximation.

Quasilinear Approximation for Modeling Difference-Frequency Acoustic Wave in a Diffracting Pump-Wave Beam

A quasilinear approach is considered to simulate generation of a difference-frequency acoustic wave by the interaction of two intense high-frequency diffracting pump beams with close frequencies. The boundary condition corresponds to dual-frequency excitation of an existing parametric source used for underwater research. It is shown that the linear field of primary waves has a high directivity with a total beam divergence angle of several degrees; therefore, the nonlinear-diffraction problem is solved numerically in the parabolic approximation. The pump wave field is calculated in the linear approximation; the solutions obtained at each step of the numerical grid along the beam axis are used to calculate nonlinear sources in the equation for a three-dimensional difference-frequency beam. The one- and two-dimensional distributions of the pressure field and the directivity pattern are analyzed for three values of a difference frequency. Numerical solutions obtained with realistic boundary conditions at the source and description of diffraction effects are compared with the known approximate analytical results for the quasilinear approach.

Polarization Simultons in CARS by Polaritons

The system of shortened Maxwell’s equations simulating the processes of evolution of the stimulated Raman scattering (SRS) by polaritons in anisotropic dipole-active crystals is obtained. The theory was developed for the case of cubic crystals which become anisotropic due to the deformation of the dielectric constant by the linearly polarized pump wave. The pump field is a linearly polarized plane electromagnetic wave. We report the results of the theoretical investigation of the possibility of the existence of a regime of pulse propagation as simultaneous travel of solitary waves in coherent anti-Stokes stimulated Raman scattering by polaritons in anisotropic crystals. The emphasis was made on the existence of both Stokes and anti-Stokes pulses propagating with two stable and perpendicular to the direction of travel polarizations. We showed the theoretical possibility of simultaneous propagation of pulses not only at frequencies of Stokes and anti-Stokes waves but the pump frequency as well. We obtained the expression for the gain factor g. It is also shown that the expression for g is consistent with the experimental results for the spectra of ZnS.

Thermo-elastic gigahertz-frequency oscillator through surface acoustic wave-silicon photonics.

Opto-electronic oscillators are sources of microwave-frequency tones that may reach very low noise levels. Much effort is being dedicated to the realization of oscillators based on photonic integrated devices. In this work, we propose and demonstrate a thermo-elastic opto-electronic oscillator at 2.213 GHz frequency based on a standard silicon-photonic integrated circuit. A microwave-frequency electrical signal modulates an optical pump wave carrier. The modulated waveform launches surface acoustic waves in a silicon-on-insulator substrate, through absorption in a metallic grating and thermo-elastic actuation. The waveform is reconverted to the optical domain through photoelastic modulation of an optical probe wave carrier in a standard racetrack resonator waveguide. Both the thermo-elastic actuation and the photoelastic modulation are radio-frequency selective. The output probe wave is detected, and the receiver voltage is amplified and fed back to modulate the optical pump input. Sufficient gain drives the loop into oscillations. The oscillator does not involve piezoelectricity and can be realized on any substrate. Long acoustic delays may be implemented in compact devices. The frequency of operation is scalable to tens of GHz. The principle may be useful in integrated microwave-photonic signal processing and in the elastic analysis of surfaces and thin layers.

Theoretical Analysis of Continuous-Wave Mid-Infrared Optical Vortex Source Generated by Singly Resonant Optical Parametric Oscillator

Due to the important application in the study of vibrational circular dichroism and helical dichroism of chiral molecules, the tunable vortex beam at mid-infrared region has attracted increasing attention. Based on orbital angular momentum (OAM) conservation in nonlinear interactions, the vortex pumped singly resonant optical parametric oscillator (SRO) is recognized as a versatile source of coherent vortex radiation providing high power and broad wavelength coverage from a single device. However, the low parametric gain and high oscillation threshold under continuous wave (cw) pumping has so far been the most challenging factor in generating cw tunable vortex beams. To predict the output characteristic of vortex pumped SRO, a theoretical model describing the vortex pumped SRO is needed. In this study, the theoretical model describing the vortex pumped SRO is set up under collimated Gaussian beam approximation. Output characteristics of different SROs are simulated numerically. By proper selection of pump scheme (such as double-pass pumping scheme), the vortex pumped mid-infrared SRO can oscillate at a relatively low pump power. By controlling the gain (mode overlap ratio between the pump and resonant wave in the nonlinear crystal) and loss (employing a spot-defect mirror with different defect size as the output coupler) of the resonant signal mode in the SRO, the OAM of the pump beam can be directionally transferred to a specific down converted beam. The transfer mechanism of the OAM among the pump light and the down-converted beams and factors affecting the transfer are studied. Our study provides the guidelines for the design and optimization of vortex pumped SRO under cw operation.

СРАВНЕНИЕ СПЕКТРАЛЬНЫХ ХАРАКТЕРИСТИК УЗКОПОЛОСНОГО ИСКУССТВЕННОГО РАДИОИЗЛУЧЕНИЯ ИОНОСФЕРЫ ПРИ Х-НАГРЕВЕ ВЫСОКОШИРОТНОЙ F-ОБЛАСТИ ИОНОСФЕРЫ НА ЧАСТОТАХ НИЖЕ И ВЫШЕ КРИТИЧЕСКОЙ ЧАСТОТЫ Х-КОМПОНЕНТА СЛОЯ F2

The paper presents the results of investigating generation conditions and features of the narrowband stimulated electromagnetic emission (NSEE) induced by an extraordinary (X-mode) HF pumping into the high-latitude ionosphere F region. It was shown that NSEE spectral features were recorded at a distance of more than 1100 km from the heating facility. Distinctive features of NSEE discrete spectral lines at the pump frequencies fH below and above the F2 layer X-component critical frequency (fH < fXF2 and fH і fXF2) were analyzed. It was found that the NSEE spectrum contains the strongly pronounced discrete lines ordered by the electrostatic ion cyclotron frequency for the atomic oxygen ions (O+). When fH < fXF2, the multiple downshifted and upshifted spectral components (Stokes and anti-Stokes modes, respectively) were excited in the NSEE spectra. When fH і fXF2, the Stokes modes were only generated. The plausible mechanisms for the NSEE excitation by an X-mode HF pump waves are discussed.

Comparison of Spectral Features of Narrowband Stimulated Electromagnetic Emission Excited by an Extraordinary Pump Wave in the High-latitude Ionospheric F Region at Frequencies below and above the F2 Layer X-component Critical Frequency

Comparison of Spectral Features of Narrowband Stimulated Electromagnetic Emission Excited by an Extraordinary Pump Wave in the High-latitude Ionospheric F Region at Frequencies below and above the F2 Layer X-component Critical Frequency

Analysis of Landau–Lifshitz and neo-Hookean models for static and dynamic acoustoelastic testing

A comparison of three different isotropic non-linear elastic models uncovers subtle but important differences in the acoustoelastic responses of a material slab that is subjected to dynamic deformations during a pump-probe experiment. The probe wave deformations are small and are superimposed on larger underlying deformations using three different models: Landau–Lifshitz (using its fourth-order extension), compressible neo-Hookean model (properly accounting for volumetric deformations), and an alternative neo-Hookean model (fully decoupled energies due to distortional isochoric and volumetric deformations). The analyses yield elasticity tensors and respective expressions for the propagation speeds of P-wave and S-wave probes for each model. Despite having many similarities, the different models give different predictions of which probe wave types will have speeds that are perturbed by different pump wave types. The analyses also show a conceptual inconsistency in the Landau–Lifshitz model, that a simple shear deformation induces a stress and a shear wave probe speed that depend on the second-order elastic constant λ, which controls resistance to volumetric changes and thus should not be present in the expressions for shear stress and shear wave probe speeds. Thus, even though the Landau–Lifshitz model is widely used, it may not always be the best option to model experimental data.

Soliton resonances and soliton molecules of pump wave and Stokes wave for a transient stimulated Raman scattering system in optics

Soliton resonances and soliton molecules of pump wave and Stokes wave for a transient stimulated Raman scattering system in optics

Ring shaped fs supercontinuum with a thermally induced self-diffraction effect.

A broadband femtosecond supercontinuum (SC) light with a dark core has been generated in a pump-probe type geometry. It has been achieved by introducing a small thermal perturbation in a highly absorbing medium with a continuous wave pump laser. The SC was generated with the help of a photonic crystal fiber and a nJ femtosecond oscillator. Numerical simulations for the system were done by employing the Fresnel-Kirchhoff formula. The power dependence of the dark core diameter was also studied experimentally, and the results agree well with numerical simulations. The dark core size can be controlled by varying the input power of the pump beam.

Cascading parametric decay coupling between whistler and ion acoustic waves: Darwin particle-in-cell simulations

We present the results of numerical studies of the whistler wave parametric decay instability in the system with the suppressed Landau damping of ion acoustic waves (IAWs) based on the self-consistent Darwin particle-in-cell (PIC) model. It has been demonstrated that a monochromatic whistler wave launched along the background magnetic field couples to a counter-propagating whistler mode and co-propagating ion acoustic mode. The coupling of the electromagnetic mode to the electrostatic mode is guided by a ponderomotive force that forms spatio-temporal beat patterns in the longitudinal electric field generated by the counter-propagating whistler and the pump whistler wave. The threshold amplitude for the instability is determined to be δBw/B0 = 0.028 and agrees with a prediction for the ion decay instability: δBw/B0 = 0.042 based on the linear kinetic damping rates, and δBw/B0 = 0.030 based on the simulation derived damping rates. Increasing the amplitude of the pump whistler wave, the secondary and tertiary decay thresholds are reached, and cascading parametric decay from the daughter whistler modes is observed. At the largest amplitude (δBw/B0 ∼ 0.1) the primary IAW evolves into a short-lived and highly nonlinear structure. The observed dependence of the IAW growth rate on the pump wave amplitude agrees with the expected trend; however, quantitatively, the growth rate of the IAW is larger than expected from theoretical predictions. We discuss the relevant space regimes where the instability could be observed and extensions to the parametric coupling of whistler waves with the electron acoustic wave (EAW).

Topological aspects in nonlinear optical frequency conversion

Nonlinear optical frequency conversion, observed more than half a century ago, is a corner stone in modern applications of nonlinear and quantum optics. It is well known that frequency conversion processes are constrained by conservation laws, such as momentum conservation that requires phase matching conditions for efficient conversion. However, conservation laws alone could not fully capture the features of nonlinear frequency conversion. Here it is shown that topology can provide additional constraints in nonlinear multi-frequency conversion processes. Unlike conservation laws, a topological constraint concerns with the conserved properties under continuous deformation, and can be regarded as a new indispensable degree of freedom to describe multi-frequency processes. We illustrate such a paradigm by considering sum frequency generation under a multi-frequency pump wave, showing that, akin topological phases in topological insulators, topological phase transitions can be observed in the frequency conversion process both at classical and quantum level.

Some Results of Photometric Measurements of Ionospheric Artificial Airglow at 557.7 and 630 nm Lines of Atomic Oxygen Caused by High-Frequency Radio Emission of the SURA Facility during Development of Sporadic E Layer

The results of analysis of the experimental data collected on 5 September 2021 on 557.7 and 630 nm artificial airglow of the ionosphere induced by powerful HF radio waves at the SURA facility are presented. For optical measurements, a photometric suite located directly next to the SURA facility was used. Fast variations in the atmospheric emission intensity at 557.7 nm, 630 nm, and 391.4 nm with a three-channel photometer and spatial–temporal variations in the 557.7 nm with a CCD camera were measured. An ionospherically reflected pump wave and the stimulated electromagnetic emission (SEE) were recorded. Background ionospheric conditions were registered with ionosonde. For the first time, an increase in the 557.7 nm emission intensity induced by the SURA facility radiation was found concurrently with a partial blocking ionosphere in the F-region and suppression HF-induced phenomena in the F-region (the 630 nm airglow increase and SEE generation, powerful radio wave anomalous absorption) during the sporadic E-layer (Es) development. Additionally, we managed to obtain images showing moving spots of the SURA-induced 557.7 nm emission increased intensity at the Es layer heights.

Interannual variability of sea surface chlorophyll a in the southern tropical Indian Ocean: Local versus remote forcing

Phytoplankton are crucial to marine ecosystems and the global carbon cycle. This study investigates the characteristics and causes of the interannual variation of the surface chlorophyll a (chl a) in the southern tropical Indian Ocean (STIO). We find that in addition to the Seychelles-Chagos thermocline ridge (SCTR) upwelling zone in the southwestern basin, large interannual variability also shows up in the southeastern tropical Indian Ocean (SETIO; 6°–18°S, 70°–92°E). The chl a in the SCTR shows two dominant periods of four years and two years, whereas it shows a dominant period of six years and a weaker period of three years in the SETIO. Surface wind forcing anomalies within the STIO, but not the Indonesian throughflow, cause the interannual chl a anomaly. In the SETIO region, the thermocline dynamics control the chl a variation via a combination of equal contributions from local Ekman pumping and remote Rossby waves generated by the wind stress curl from the east. In the SCTR region, the thermocline dynamics explain only about 67% of the chl a variation, with a larger contribution from the remotely generated Rossby waves than local Ekman pumping. The circulation in the western basin also contributes to the chl a variation by transporting western equatorial chl a to this region during the boreal summer-fall. Both the Indian Ocean Dipole and the El Niño–Southern Oscillation act to modulate the chl a variation in the STIO. They make comparable contributions in the SETIO, while the Indian Ocean Dipole plays a more important role in the SCTR.

Two-Dimensional Nonlinear Thouless Pumping of Matter Waves.

We consider theoretically the nonlinear quantized Thouless pumping of a Bose-Einstein condensate loaded in two-dimensional dynamical optical lattices. We encountered three different scenarios of the pumping: a quasilinear one occurring for gradually dispersing wave packets, transport carried by a single two-dimensional soliton, and a multisoliton regime when the initial wave packet splits into several solitons. The scenario to be realized depends on the number of atoms in the initial wave packet and on the strength of the two-body interactions. The magnitude and direction of the displacement of a wave packet are determined by Chern numbers of the populated energy bands and by the interband transitions induced by two-body interactions. As a case example we explore a separable potential created by optical lattices whose constitutive sublattices undergo relative motion in the orthogonal directions. For such potentials, obeying parity-time symmetry, fractional Chern numbers, computed over half period of the evolution, acquire relevance. We focus mainly on solitonic scenarios, showing that one-soliton pumping occurs at relatively small as well as at sufficiently large amplitudes of the initial wave packet, while at intermediate amplitudes the transport is multisolitonic. We also describe peculiarities of the pumping characterized by two different commensurate periods of the modulations of the lattices in the orthogonal directions.