Input Beam Research Articles

Impact of High-Power Cosh-Gaussian Beam on Second Harmonic Generation in Collisionless Magnetoplasma

The impact of high power Cosh-Gaussian (ChG) beam on Second harmonic generation (SHG) in Collisionless magnetoplasma is explored in present work. Whenever the input beam propagates along external magnetic magnetic field direction, then there are two propagation modes viz. extraordinary mode and ordinary mode. The modification in magnetic field strength causes redistribution of carriers. The density gradients get established in plasma in a normal direction to input wave due to ponderomotive force. Further, there is production of electron plasma wave (EPW) at input wave frequency due to density gradients. EPW nonlinearly interacts with pump wave causing generation of 2nd harmonics. The 2nd order differential equation (ODE) for beam width of and efficiency of 2nd harmonics are derived through well-known paraxial theory approach. RK4 method is employed for carrying out numerical calculation of nonlinear ODE along with efficiency of 2nd harmonics. Impact of change in selective laserplasma parameters and externally applied magnetic field on beam waist of input wave and efficiency of SHG are also explored.

MEDEA: A New Model for Emulating Radio Antenna Beam Patterns for 21 cm Cosmology and Antenna Design Studies

In 21 cm experimental cosmology, accurate characterization of a radio telescope’s antenna beam response is essential to measure the 21 cm signal. Computational electromagnetic (CEM) simulations estimate the antenna beam pattern and frequency response by subjecting the EM model to different dependencies, or beam hyperparameters, such as soil dielectric constant or orientation with the environment. However, it is computationally expensive to search all possible parameter spaces to optimize the antenna design or accurately represent the beam to the level required for use as a systematic model in 21 cm cosmology. We therefore present the Model for Emulating Directivities and Electric fields of Antennas (MEDEA), an emulator that rapidly and accurately generates far-field radiation patterns over a large hyperparameter space. MEDEA takes a subset of beams simulated by CEM software, spatially decomposes them into coefficients on a complete, linear basis, and then interpolates them to form new beams at arbitrary hyperparameters. We test MEDEA on an analytical dipole and two numerical beams motivated by upcoming lunar lander missions, and then employ MEDEA as a model to fit mock radio spectrometer data to extract covariances on the input beam hyperparameters. We find that the interpolated beams have rms relative errors of at most 10−2 using 20 input beams or less, and that fits to mock data are able to recover the input beam hyperparameters when the model and mock are derived from the same set of beams. When a systematic bias is introduced into the mock data, extracted beam hyperparameters exhibit bias, as expected. We propose several extensions to MEDEA to potentially account for such bias.

Controlled generation of cylindrical vector vortex beams using an optically active material.

An efficient method for the generation of tunable cylindrical vector vortex beams is proposed and demonstrated experimentally using an optically active (OA) material. The uniqueness of the proposed methodology lies in the control over the tunability of the cylindrical vector vortex beams using different concentrations of the optically active material. The efficiency of the generated beams is enhanced by using a single low oblique incidence angle of the input beam on a spatial light modulator with a single-phase profile. Two global parameters are measured experimentally to show the quantitative tunability and efficiency of the generated beams. The proposed method can find applications in the fabrication of various kinds of spiral nanostructures.

Elegant dihedral beams

Abstract Elegant Hermite-Gauss beams are a well-known family of structured beams. Although Siegman proposed them several decades ago, their properties continue to be studied and exploited. In this work, a group theory-based framework that introduces new classes of symmetric elegant beams is presented. Our formulation exploits the algebraic properties of the dihedral group of rotations and reflections to transform an input beam into a closed-form family of interference patterns with dihedral symmetry. Our approach is inspired by one of the outcomes of this work: elegant Hermite-Gauss beams can be described as a dihedral interference pattern of elegant traveling waves, a new set of solutions to the paraxial equation also developed in this paper. Taking elegant traveling waves as input beams, they are transformed into what we have called elegant dihedral beams. They have dihedral-symmetric transverse profiles and quasi-crystalline properties, including features like phase singularities, self-healing, and pseudo-nondiffracting propagation, as well as containing elegant Hermite and Laguerre-Gauss beams as special cases.

Non-uniform phase distribution of a tightly focused elliptically polarized vortex beam

Abstract Tight focusing of elliptically polarized vortex beams has been previously studied for optical manipulation, optical information encoding, and so on. Still, there is a lack of research on the status of the phase distribution on the focal plane. In this study, we found that the phase distribution of a tightly focused elliptically polarized vortex beam is non-uniform, i.e., the phase distribution exhibits flatter and steeper regions due to the elliptical polarization of the input vortex beam. It is mentioned that the phase non-uniformity was related to the ellipticity of the polarization of the incident beam. Furthermore, we analyzed the intensity and phase distribution of a tightly focused elliptically polarized vortex beam. We found that the spin angular momentum was converted to the orbital angular momentum because the topological charge of the output beam was greater than that of the input beam. The non-uniform phase distribution of a tightly focused elliptically polarized vortex beam enables control over light–matter interaction, leading to advancements in optical tweezers, quantum information processing, and super-resolution microscopy.

Beam dynamics design and error analysis of a high frequency RFQ for medical application

Proton therapy based on the linear accelerator is an advanced radiation treatment technology. Lanzhou University has proposed a linac scheme for proton therapy, which features a 750 MHz Radio Frequency Quadrupole (RFQ) as its first accelerator. The RFQ is capable of accelerating a 1 mA proton beam from 30 keV to 3 MeV. The article systematically introduces the beam dynamics design and error analysis of the RFQ. In the design part, a new strategy that can reduce the outlet longitudinal emittance of the RFQ by adjusting its modulation and synchronous phase was proposed. Subsequently, multi-particle simulations based on different codes consistently demonstrated that the dynamics design achieved a good output beam quality. The longitudinal emittance of the RFQ output beam was successfully controlled to below 0.1 π mm·mrad. In the error analysis part, firstly, all types of errors were analyzed separately to determine their impact patterns and to give preliminary tolerances. Then, the combined errors which considered all types of errors together were analyzed. Finally, the tolerances for input beam errors, cavity voltage errors, and mechanical errors were obtained.

Observation of nonlinear fractal higher order topological insulator

Higher-order topological insulators (HOTIs) are unique materials hosting topologically protected states, whose dimensionality is at least by 2 lower than that of the bulk. Topological states in such insulators may be strongly confined in their corners which leads to considerable enhancement of nonlinear processes involving such states. However, all nonlinear HOTIs demonstrated so far were built on periodic bulk lattice materials. Here, we demonstrate the first nonlinear photonic HOTI with the fractal origin. Despite their fractional effective dimensionality, the HOTIs constructed here on two different types of the Sierpiński gasket waveguide arrays, may support topological corner states for unexpectedly wide range of coupling strengths, even in parameter regions where conventional HOTIs become trivial. We demonstrate thresholdless spatial solitons bifurcating from corner states in nonlinear fractal HOTIs and show that their localization can be efficiently controlled by the input beam power. We observe sharp differences in nonlinear light localization on outer and multiple inner corners and edges representative for these fractal materials. Our findings not only represent a new paradigm for nonlinear topological insulators, but also open new avenues for potential applications of fractal materials to control the light flow.

The formation and stability of single-peak and multi-peak gap nematicons in a periodic potential

ABSTRACT The formation and stability of single-peak and multi-peak gap nematicons in a periodic potential are investigated using numerical methods. Both the stationary solutions and the dynamic solutions have been studied in the first band gap. Single-peak and multi-peak gap nematicons can be found in the first band gap. The single-peak gap nematicons are obtained for a wide range of propagation constants. Multi-peak gap nematicons can exist only above a certain minimum value of propagation constant. The intensity distribution among various peaks in a multi-peak gap nematicon highly depends on input beam intensity. The stability of the stationary solution against small perturbations has been studied using Bogoliubov-de Gennes equations. Single-peak gap nematicons are stable. The multi-peak gap nematicons having the same amplitudes for various peaks are stable, whereas those with unequal amplitudes among peaks are unstable.

Branched flow of light and interplay with input phase front curvature in a disordered photonic lattice

Branched flow refers to the bifurcation of waves as they propagate through customized disordered media, leading to the creation of multiple channels or branches. This dynamic wave phenomenon has been observed in various disordered systems under specific conditions. In our study, we demonstrate the branched flow of light within a disordered photonic lattice and establish that the intensity distribution of these branches follows Lévy statistics. This behavior is more pronounced when the wavefront of the input light beam is adjusted, resulting in the propagation of more stable branches through the structure. We also investigated the formation of light branches with varying input beam widths and the impact of two simultaneous input beams on the disordered photonic lattice. Our findings indicate that the distribution of branches in the disordered system can be controlled by shaping the wavefront of the input beam, which has potential applications in imaging, nano-lasing, and integrated photonic components. This research opens new avenues for fundamental studies on the manipulation of light through complex photonic systems.

Spatial mode cleaning and efficient nonlinear pulse compression to sub-50 fs in a gas-filled multipass cell.

We demonstrate a gas-filled multipass cell (MPC) that cleaned the spatial mode of a spatial-filter-free 250 W, 100 kHz, 445 fs driven source based on an Innoslab amplifier and compressed the pulse duration to 41 fs simultaneously. The multipass cell acted as a spatial filter and benefited from its discrete waveguide nature, in which the input beam quality factor M2 was improved from 1.53 to a near-diffraction-limited value of 1.21 at 96% transmission.

RMS error analysis of spatial uniformity and energy stability of the laser beam in second harmonic generation.

The coupling of the energy stability and spatial uniformity of the laser beam before and after second harmonic generation (SHG) was analyzed. SHG experiments were performed using a Nd:YAG nanosecond laser and LBO crystals, and images, pulse shapes, and energies were measured. The relationship between energy stability and spatial uniformity uses a formula derived from the previous study to analyze changes in energy stability and spatial uniformity of the input beam and converted beam. In addition, the measured input beam shape and energy are compared with the results of applying the SHG converting equation considering pump depletion. Both methods were similar to the experimental results when corrected by empirical factors. Through SHG, it was confirmed that there is an optimal point of energy stability and spatial uniformity of the laser beam near the critical power.

Propagation dynamics of multipole solitons generated in dissipative systems

The propagation dynamics of multipole solitons generated in dissipative systems are investigated numerically based on the fractional complex cubic-quintic Ginzburg–Landau equation using the Airy beam as the input beam. The effect of different parameter values on the generation of stable solitons is explored. In addition, we observe different resultant domains of the input beam evolving in the linear loss coefficient or cubic gain coefficient and Lévy index parameter planes. The results show that the evolution can lead to the formation of stable multipole solitons. It is also demonstrated that two solitons merge to form a single soliton. And, the relation between the merger distance and the initial amplitude is given.

Neural network assisted high flexibility and high resolution Shack-Hartmann wavefront sensing for astronomical observation in darker sky areas.

Shack-Hartmann (SH) wavefront sensing is widely applied to astronomical observations with its fast and accurate measurement. However, due to the computational nature of SH that the input beam is segmented to provide local wavefront slopes, the sampling density of the sub-apertures and the calculation accuracy of each sub-focal spot's centroid have great influence on the wavefront reconstruction accuracy. Therefore, it is usually difficult to achieve high resolution wavefront reconstruction for dark stars in the astronomical observations with insufficient light intensity. We present a neural-network assisted high resolution SH wavefront sensing method to overcome the shortages and obtain results with enhanced resolution from the separated information inside each sub-aperture. With this method, high resolution wavefront sensing in darker sky area could be realized.

Subwavelength and broadband on-chip mode splitting with shifted junctions.

We design and fabricate a sub-wavelength on-chip mode splitter based on the implementation of a shifted junction between a single-mode waveguide and a multimode waveguide. A proper choice of the device parameters enables to split the input beam into a combination of different guided modes of the multimode waveguide, minimizing radiation and reflection losses that amount to ∼ 0.4 dB in our experiments. Because the splitting mechanism does not rely on phase-matching, we achieve broadband operation that could exceed 200 nm bandwidth (<0.5 dB splitting variation). This approach ensures temporal and phase synchronization among the output modes, with applications spanning from the emergent multimode photonics platform to traditional single-mode photonics operations.

Infrared imaging with nonlinear silicon resonator governed by high-Q quasi-BIC states

Nonlinear light-matter interactions have emerged as a promising platform for various applications, including imaging, nanolasing, background-free sensing, etc. Subwavelength dielectric resonators offer unique opportunities for manipulating light at the nanoscale and miniturising optical elements. Here, we explore the resonantly enhanced four-wave mixing (FWM) process from individual silicon resonators and propose an innovative FWM-enabled infrared imaging technique that leverages the capabilities of these subwavelength resonators. Specifically, we designed high-Q silicon resonators hosting dual quasi-bound states in the continuum at both the input pump and signal beams, enabling efficient conversion of infrared light to visible radiation. Moreover, by employing a point-scanning imaging technique, we achieve infrared imaging conversion while minimising the dependence on high-power input sources. This combination of resonant enhancement and point-scanning imaging opens up new possibilities for nonlinear imaging using individual resonators and shows potential in advancing infrared imaging techniques for high-resolution imaging, sensing, and optical communications.

Polarization singularities for shaping of vector flat-top beams in utilization for high-power laser micromachining of various materials

Structured light is desired in the micromachining of various materials. As the latest ultrashort laser sources increase the average power by increasing the repetition rate, the efficiency of the process becomes crucial in laser micromachining. The standard Gaussian beam shape, because of its slow ascending/descending slope, is not the most efficient beam, neither in precise energy deposition nor in heat management. Flat-top beams are required for precise and even energy distribution and are used in laser ablation or thin-film removal. We look at a rather abstract concept of polarization singularities and investigate them to create a vector flat-top beam. For this purpose, we devise a beam-shaping method based on the volumetric birefringent nano gratings which allow the generation of high-energy polarization singularities. By tuning the input beam waist, we have generated three most common laser micromachining beam profiles (Gaussian, Flat-top, and ring- or doughnut-shaped). The resulting high-quality and high-energy polarization singularities allow us to systematically demonstrate their use in transparent and opaque material micromachining, as well as the usual application for flat-top beams of thin-film removal.

Polarimetric measurement of temporal coherence in electromagnetic light beams

We present a method to determine the degree of temporal coherence of a quasimonochromatic vectorial light beam by polarimetric measurements. More specifically, we employ Michelson’s interferometer in which the polarization Stokes parameters of the output (interference) beam are measured as a function of the time delay. Such a measurement enables us to deduce the magnitudes of the coherence (two-time) Stokes parameters, and hence the degree of coherence, of the input beam. Compared to existing methods the current technique has the benefits that neither optical elements in the arms of the interferometer nor visibility measurements are needed. The method is demonstrated with a laser diode and a filtered halogen source of various degrees of polarization.

Specular transformation of electromagnetic partially coherent beams with a wave-folding interferometer

We consider a class of specular or anti-specular vector beams, by illuminating stochastic vector beams into a prism-based wavefront-folding interferometer. Such transform is applied to various genuine model input beams, and then the properties of the resulting fields are discussed. Numerical results show that the specular nature of these vector fields not only creates sharp internal spectral density distribution, but also produces novel polarization patterns with oscillations or a central dip on the degree of polarization-profile. Such optical characteristics can be flexibly modulated by the correlation structure of the source. We also suggest that the specular transform could be efficiently employed in developing novel partially coherent vector beams.

Plasmonic tuning of nano-antennas for super-gain light amplification

Nanoscale conductive materials are often used for inducing localized free electron oscillations known as plasmons. This is due to their high electronic excitability under optical irradiation owing to their super-small volume. Recently, plasmons have been of interest for enhancing the gain-bandwidth product of optical amplifiers. There are currently two well-established mechanisms for light amplification. The first one is via stimulated emission of radiation (lasers) using a given energy source and often an optical feedback mechanism. The second one is based on the nonlinear coupling of a low-intensity input wave and a high-intensity pump wave for energy exchange (parametric amplifiers). Both techniques have shortcomings. Lasers have a small operation bandwidth and offer a limited gain, but require moderate energy pumping to operate. Whereas optical parametric amplifiers (OPAs) offer a high operation bandwidth along with a much higher optical gain, with the drawback of requiring intense pumping to be functional. The aim of this paper is to introduce a technique that combines the advantages and eliminates the drawbacks of both techniques in the nanoscale to allow for a better amplification performance in integrated optical devices. This is achieved by inducing a plasmonic chirp in conductive nanomaterials a.k.a nano-antennas, which enables the confinement of an enormous electric energy density that can be coupled to an input beam for amplification. Using the Finite Difference Time Domain numerical-method with the material parameters of well-known semiconductors, intramaterial condensation of electric energy density is observed in semiconductor nano-antennas for certain plasmonic chirp-frequencies which enables broadband high-gain optical amplification based on free-electron oscillations that is promising for small-scale optical devices requiring a high gain-bandwidth product. The results are in good agreement with semiempirical data.

Dispersion‐Engineered Super‐Gain Parametric Amplification in Nanoscale Optical Cavities

The gain factor of the optical parametric amplification (OPA) process is known to be negligible in the small scale due to low‐interaction‐medium length. Hence, in the nanoscale, OPA is deemed as infeasible. Therefore, in small‐scale‐integrated optical devices, stimulated‐emission‐based amplifiers (lasers) are employed instead of OPAs. In contrast, the major advantage of OPAs over lasers is that unlike lasers which only provide amplification over a narrow spectral band, OPAs provide high‐gain amplification over a very large, user‐controlled spectral band. In this article, it is shown that OPA can yield wideband high‐gain amplification over a nanoscale beam propagation distance through dispersion engineering. This is achieved by a proper tuning of the pump (source) wave frequency, which can maximize the effective medium nonlinearity by a few orders of magnitude, while concurrently maximizing the intracavity energy density, thereby compensating for the small co‐propagation distance for the input (signal) and pump beams. In this study, it is shown that an input wave can be amplified by a factor in a nanoscale cavity via precise dispersion engineering. Both empirical and computational formulations are used for the investigation, which display a reasonable agreement.