Incident Gaussian Beam Research Articles

Generation of polarization-insensitive perfect vortices by dielectric metasurfaces with diverging or converging axicon phases

Perfect vortex (PV) is a special light field with constant annular light intensity distribution while carrying orbital angular momentum. It has been widely used in the fields of optical communications and particle manipulation. However, conventional methods to produce PVs require a series of complex components, which limit their practical use in miniaturization. Here we designed models of PV generators based on highly efficient polarization-insensitive dielectric metasurfaces and smoothly generated PVs in simulation with arbitrarily polarized incident Gaussian beams. Meanwhile, by adjusting the focal length of the lens and the half fan angle of the axicon, ring-shaped focal spots with different radii could be obtained in the propagation space, rendering the system more flexible. By superimposing the phase functions of the four PVs with different ring radii, the coaxial PVs were smoothly generated in simulation. Furthermore, the multi-channel PVs were obtained in simulation by introducing the linear displacement factors. By employing polarization-insensitive dielectric metasurfaces, we believe that this method of PV generation will stimulate further applications in fields such as optical communications, particle trapping and manipulation.

An apodization method for guided-mode resonance grating with waveguide cavity

A cavity-resonator-integrated guided-mode resonance mirror (CRIGM) consisting of a small-aperture grating coupler integrated in a waveguide cavity on a high-reflection substrate reflects an incident free-space wave with reflection-phase variation of 2π in a resonance condition. CRIGM has been investigated as a key component in surface mount packaging of VCSEL for future optical interconnects but the reflectance of a fabricated CRIGM was not sufficient. Matching of electric field distribution between an incident Gaussian beam and the reflected beam from the CRIGM is discussed for improving the reflectance. A new apodization method of grating-tooth shift is proposed for the field-distribution matching. CRIGMs with the new and a well-known fill-factor apodization methods are characterized by numerical simulation. It is predicted that the previously measured reflectance will be improved by more than 1.2 dB by introducing either of the apodization methods.

Photonic spin Hall effect in a one-dimensional photonic crystal with a graphene defect film

We investigate the photonic spin Hall effect (PSHE) of a Gaussian beam passing through a one-dimensional photonic crystal with a graphene defect film. We show that an epsilon singularity point and an epsilon-near-zero point of graphene emerge by varying its chemical potential. We find that the chemical potential of graphene significantly alters the transverse shifts of PSHE, while the temperature of graphene has a slight impact on the transverse shifts of PSHE. Moreover, we find that the behavior of PSHE near the singularity point considerably differs from that near the epsilon-near-zero point. Additionally, we further explore the different wavelengths of the incident Gaussian beam on the tunability of PSHE. We show that the considerable transverse shifts can be obtained simultaneously depending on the incident wavelength of the Gaussian beam and the chemical potential of graphene.

Development of off-axis spiral phase mirrors for generating optical vortices in a range of millimeter waves.

In this paper, we report the development of off-axis spiral phase mirrors that can be used to generate optical vortices from a range of millimeter waves. An obliquely incident Gaussian beam is reflected from a spiral phase mirror and is converted into an optical vortex beam with a desired topological charge. The mirrors were fabricated by mechanical machining. The designed vortex properties of reflected waves were investigated experimentally by using a low-power test, where the designed topological charge was verified based on the interference pattern between a vortex beam and a Gaussian-like beam. The designed topological charge was also estimated by using a phase retrieval method specialized for a vortex beam. These off-axis spiral phase mirrors can be used for propagation experiments of radio frequency waves with helical wavefronts in magnetized plasma.

Surface states-dependent giant quantized photonic spin Hall effect in a magnetic topological insulator thin film

We theoretically investigate the in-plane and transverse photonic spin Hall effect (PSHE) in a magnetic topological ultra-thin film (MTF) in the terahertz regime in the presence of an external magnetic field. We take into account the hybridization between the top and bottom surface states (SSs) and demonstrate that both the in-plane and transverse PSHE are quantized due to the Landau levels (LLs) quantization of the magneto-optical (MO) conductivities. The interplay between the effective energy induced by the hybridization interaction and the Zeeman energy plays an important role as it results in topological and normal insulating phases. By manipulating the interplay between these two interaction energies we drive the system through several topological quantum phase transitions (TQPTs). We show that the PSHE can be greatly enhanced and the magnitude of the giant PSHE in MTF is several hundreds times of the incident gaussian beam wavelength. Furthermore, we show that the PSHE is sensitive to the SSs of the Dirac fermions which is due to its topological character. This work sheds important light both on the PSHE and on the MO transport properties in MTFs in the presence of an external magnetic field which harbor great potentials in the development of new-generation photonics spin-Hall and optoelectronic devices.

E‐Band Metasurface‐Based Orbital Angular Momentum Multiplexing and Demultiplexing

AbstractOrbital angular momentum (OAM) has received considerable attention regarding high‐capacity communication owing to its spatial orthogonality. However, it is still challenging to build a compact communication system that can generate multiple coaxial OAM beams and receive information from each. In this work, OAM multiplexing and demultiplexing at the E‐band frequency using a single metasurface structure is proposed and experimentally demonstrated. For OAM multiplexing, the metasurface used as a transceiver generates two orthogonal coaxial OAM beams for Gaussian incident beams with different incidence angles. For OAM demultiplexing, the same metasurface flipped 180° as a receiver forms a Gaussian beam in different off‐axis directions depending on the topological charge of the coaxially incident OAM beam. The Gaussian beam measured at the receiver end exhibits a high signal‐to‐noise ratio of more than 33 dB compared with the background OAM beam. OAM multiplexing and demultiplexing based on a single metasurface may provide a route for high‐capacity and compact free‐space communication systems.

Engineering a better light sheet in an axicon-based system using a flattened Gaussian beam of low order.

Lasers are fundamental tools in research and development. The shape of an incident laser beam directly affects the results, when it propagates through complex structured meso-aspheric optical elements. In conic-based systems utilizing elements such as axicons, the impact of secondary lobes is mostly overlooked, although the intensity distributions at the central spot and the side-lobes directly affect the beam properties. We investigate the interaction of two axicons (160° and 170°) with incident beams approximated by Gaussian, high-order Flattened-Gaussian, and low-order Flattened-Gaussian functions. We demonstrate that replacing an incident Gaussian beam with a low-order Flattened-Gaussian beam reduces the secondary lobes and significantly improves the uniformity of the intensity profile. We practically applied this effect in engineering a conic-aspheric-based static light-sheet microscope producing markedly improved results.

Transversal and axial modulation of axicon-generated Bessel beams using amplitude and phase masks for glass processing applications.

The control of laser-induced microcracks in the volume of transparent materials is essential for scribing processes. In this paper, we investigate the effect of various amplitude and single-level phase masks on both transverse and axial intensity distribution of the conventional axicon-generated Bessel beams. Furthermore, we demonstrate the volumetric crack control induced by an asymmetrical central core with an appropriately selected intensity level to avoid the influence of peripheral intensity maxima. Proper alignment of cracks and intra-distance between the modifications results in the reduced separation stress of the scribed samples. Furthermore, the additional amplitude modulation of the incident Gaussian beam is introduced to flatten the axial intensity distribution of the axicon-generated Bessel beam.

Compact, tunable polarization transforming reflector for quasi-optical devices used in terahertz science

We describe the design, fabrication, and characterization of a compact polarization transforming reflector (PTR). The device can be easily tuned over a broad frequency range, has very little insertion losses, and can easily be integrated into quasi-optical systems that are based on a half-cube design. By varying the distance between the wire grid and a flat mirror, the polarization state of an arbitrary polarized Gaussian incident beam can be set to an output Gaussian beam in either linear or circular polarization. In addition, by varying the orientation of the wire grid, the PTR can be used as a universal polarizer, a property that has not been discussed or demonstrated in the literature. The ability to control the electric field polarization at Terahertz (THz) frequencies is essential for many applications, such as THz spectroscopy and high-field electron paramagnetic resonance spectroscopy.

Analysis of trapping force exerted on multi-layered chiral sphere induced by laser sheet

Theoretical study on optical trapping of multi-layered chiral sphere has attracted more and more attention for its important applications in many frontier scientific fields such as chemical engineering, biomedicine, optical tweezers, micro/nano lithography etc. In order to trap and manipulate chiral multi-layered particles efficiently, the present paper aims at developing the theoretical research of trapping force (TF) exerted on a multi-layered chiral sphere induced by laser sheet which might have great potential to improve the light performance in optical trapping as well as capture, suspension, and high-precision delivery of chiral cells. Here, based on the Generalized Lorenz Mie theory and the completeness of spherical vector wave functions (SVWFs), the electromagnetic field of incident laser sheet are expanded in terms of SVWFs. Accordingly, by introducing the beam scattering theory and the conservation law of electromagnetic momentum (EM), the analysis of TF exerted on multi-layered chiral sphere can be analytically expressed in terms of the incident and scattering coefficients. Taking the chiral cell as an example, the TF induced by laser sheet is simulated numerically. Numerical effects of the varying chirality, polarization states, beam waist width, inner material loss and outmost size on the TF induced by laser sheet are analyzed and compared with those by circular Gaussian beam incidence in detail. It is found that the introduction of chirality parameter may reduce the axial TF exerted on chiral multi-layered cell. Thus, it is more difficult to trap and manipulate stratified chiral cells than to trap general isotropic cells. Also it is shown that the TF of chiral cells can be significantly discriminatory in nature, depending upon both the handedness of the interacting particles and the polarization of the incident light. Thus, an appropriately polarized beam should be considered in trapping chiral cells. For chiral multi-layered cells with small loss in the inner layer, when the inner refractive indices are less than the outmost refractive index, the TF of multi-layered chiral cell becomes stronger with the outmost radius decreasing. Conversely, for the inner refractive indices are greater than the outer refractive index, TF becomes weaker as the outmost radius decreases. Besides, compared with the traditional circular Gaussian beam, the strong convergence of elliptical Gaussian beam can be easier to achieve three-dimensional capture of stratified chiral cells, which may provide a recipe to understand the light interaction with more complex chiral cells with the aid of the analytical approach and could be a promising avenue for the design of optical trapping systems.

Angular Goos-Hanchen shift in subwavelength gratings enhanced by surface plasmon resonance

The angular Goos-Hanchen shift dependence on the subwavelength grating parameters and the incident Gaussian beam width is investigated theoretically. High sensitivity of the angular Goos-Hanchen shift to the incident angle of a light beam near the surface plasmon resonance is demonstrated. Splitting of the reflected beam into two angularly separated beams is shown for strongly focused beam incident at the surface plasmon resonance angle.

The effect of Rayleigh length of incident Gaussian beam on the in-plane angular spin splitting

In this paper, we established the relationship model between Rayleigh length of incident Gaussian beam and in-plane angular spin splitting (IPASS) under arbitrary linear polarization, and systematically investigated the effect of Rayleigh length on the IPASS. The results show that the IPASS shifts decrease with the increase of Rayleigh length, and the smaller the Rayleigh length, the greater its impact on IPASS. The maximum IPASS shift can almost reach half of the beam divergence angle by adjusting incident angle and polarization angle. In addition, several interesting characters of IPASS are discovered.

Generation of Flexible hyperbolic Airy-like beams using a truncated acousto-optical effect

In this paper, we demonstrate the shaping of an incident Gaussian beam into a tunable hyperbolic Airy-like beam, using an acoustooptics cell. The latter contains a transparent liquid (water), where an ultrasound wave propagating and creating a sinusoidal refractive index variation (giving rise to a sinusoidal phase transmittance). By adding an aperture, this sinusoidal phase transmittance becomes equivalent to a cubic phase (sin(x)≈x−x3/3! for x<0.2λa, λa is the ultrasound wavelength), which allows the generation of a tunable Airy-like beam. Using the Fresnel-Kirchhoff diffraction integral, we derive analytical expressions for the diffracted Gaussian beam by the acoustooptics cell in two cases; (a) with and (b) without the aperture. For the first case, the algebra is straightforward and the field has a closed analytical expression, however for the second case, the diffraction integral has no analytical expression, thus, we expand the aperture into a weighted sum of Gaussian functions, to overcome such problem. Based on the obtained mathematical expressions, we study the effect of each parameter of the acoustooptics interaction, on the propagation behavior of the generated Airy-like beam. We believe that the results of this paper could be useful for many interesting applications, involving Airy beams, such as particle manipulation, optical tweezing, and optical communication.

Adjustable spin Hall effect of light in surface plasmon resonance structure with consideration of angular Imbert–Fedorov shift

For a horizontally polarized Gaussian beam incident on surface plasmon resonance structure composed of prism, silver film, and absorptive liquid, the left- and right-handed circularly polarized components of the reflected beam undergo spin-dependent spatial and angular Imbert–Fedorov (IF) shift. We investigate spin Hall effect of light (SHEL) after the reflected beam travels a distance of Rayleigh range. It is shown that the maximum total transverse shift (TTS) of one spin component at Rayleigh range is always larger than the maximum spatial IF shift, and TTS is more sensitive to variations in the parameters of the structure than spatial IF shift. Furthermore, this SHEL is adjustable by changing the complex refractive index of the absorptive liquid. This research provides a simple way to enhance SHEL and may have potential application of improving the accuracy of sensors based on SHEL.

Analysing the impact of non-parallelism in Fabry-Perot etalons through optical modelling.

Fabry-Perot (FP) etalons, composed of two parallel mirrors, are used widely as optical filters and sensors. In certain applications, however, such as when FP etalons with polymer cavities are used to detect ultrasound, the mirrors may not be perfectly parallel due to manufacturing limitations. As little is known about how the mirrors being non-parallel impacts upon FP etalon performance, it is challenging to optimize the design of such devices. To address this challenge, we developed a model of light propagation in non-parallel FP etalons. The model is valid for arbitrary monochromatic beams and calculates both the reflected and transmitted beams, assuming full-wave description of light. Wavelength resolved transmissivity simulations were computed to predict the effect that non-parallel mirrors have on the sensitivity, spectral bandwidth and peak transmissivity of FP etalons. Theoretical predictions show that the impact of the non-parallel mirrors increases with both mirror reflectivity and incident Gaussian beam waist. Guidelines regarding the maximum angle allowed between FP mirrors whilst maintaining the sensitivity and peak transmissivity of a parallel mirror FP etalon are provided as a function of mirror reflectivity, cavity thickness and Gaussian beam waist. This information, and the model, could be useful for guiding the design of FP etalons suffering a known degree of non-parallelism, for example, to optimize the sensitivity of polymer based FP ultrasound sensors.

Terahertz focusing metalens of reflectionless meta-atoms with negative refractive indices.

Terahertz continuous-wave (CW) sources oscillating around the 1.0 THz band at room temperature have rapidly been developed to bridge the terahertz gap. However, reflectionless metasurfaces suitable for integration with terahertz CW sources as optical components have yet to be developed in the terahertz gap. Here, we propose a terahertz-focusing metalens consisting of reflectionless meta-atoms with a discrete distribution of negative refractive indices from ${-}{1.1}$ to ${-}{2.8}$. The proposed 2D gradient-refractive-index metalens converts an incident terahertz Gaussian beam to a line focus. We also experimentally demonstrate a metasurface of reflectionless meta-atoms with a negative refractive index of ${-}{2.8}$ adopted in the periphery of the metalens. The reflectionless metasurface in the terahertz gap would be a welcome contribution to the rapid growth of terahertz industrial applications with terahertz CW sources. Further, the design approach based on reflectionless meta-atoms with negative refractive indices could be applied to various 2D planar optical components with attractive functionalities such as collimating, arbitrary wavefront shaping, and light vortices.

Diffraction of a Gaussian Beam with Limited cross Section by a Volume Phase Grating under Waveguide Mode Resonance.

In this work, the diffraction of a Gaussian beam on a volume phase grating was researched theoretically and numerically. The proposed method is based on rigorous coupled-wave analysis (RCWA) and Fourier transform. The Gaussian beam is decomposed into plane waves using the Fourier transform. The number of plane waves is determined using the sampling theorem. The complex reflected and transmitted amplitudes are calculated for each RCWA plane wave. The distribution of the fields along the grating for the reflected and transmitted waves is determined using inverse Fourier transform. The powers of the reflected and transmitted waves are determined based on these distributions. Our method shows that the energy conservation law is satisfied for the phase grating. That is, the power of the incident Gaussian beam is equal to the sum of the powers of the reflected and transmitted beams. It is demonstration of our approach correctness. The numerous studies have shown that the spatial shapes of the reflected and transmitted beams differ from the Gaussian beam under resonance. In additional, the waveguide mode appears also in the grating. The spatial forms of the reflected and transmitted beams are Gaussian in the absence of resonance. It was found that the width of the resonance curves is wider for the Gaussian beam than for the plane wave. However, the spectral and angular sensitivities are the same as for the plane wave. The resonant wavelengths are slightly different for the plane wave and the Gaussian beam. Numerical calculations for four refractive index modulation coefficients of the grating medium were carried out by the proposed method. The widths of the resonance curves decrease with the increasing in the refractive index modulation. Moreover, the reflection coefficient also increases.

A crossed focused vortex beam with application to cold molecules* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11834003 and 91836103).

We report the generation of a crossed, focused, optical vortex beam by using a pair of hybrid holograms, which combine the vortex phase and lens phase onto a spatial light modulator. We study the intensity distributions of the vortex beam in free propagation space, and the relationship of its dark spot size with the incident Gaussian beam’s waist, the lens’s focal length, and its orbital angular momentum. Our results show that the crossed, focused, vortex beam’s dark spot size can be as small as 16.3 μm and adjustable by the quantum number of the orbital angular momentum, and can be used to increase the density of trapped molecules. Furthermore, we calculate the optical potential of the blue-detuned, crossed vortex beam for MgF molecules. It is applicable to cool and trap neutral molecules by intensity-gradient-induced Sisyphus cooling, as the intensity gradient of such vortex beam is extremely high near the focal point.

Transmission and rainbow trapping of acoustic waves in a fluid medium using gradient-index superlattices

Recently, rainbow trapping of acoustic/elastic waves in gradient artificial structures has shown promising applications in energy harvesting and frequency separation. However, the novel phenomenon of acoustic superlattices for underwater acoustic waves has not been explored. In this work, we establish the theoretical model of underwater acoustic waves propagating through the gradient-index superlattice (GISL) by using the transfer matrix method. We show that the combined band structures of infinite sub-superlattices and the transmission curves of finite GISL exhibit Bragg forbidden bands that are wider than those of periodic ones, which implies the occurrence of rainbow trapping. On this basis, the effects of gradient factor, period number, and water thickness are discussed, and the tunability of the trapping range is also explained. Furthermore, simulations with Gaussian beam incidence are performed through finite element methods. The results in frequency-domain and time-domain both indicate perfect performances of energy localization and frequency separation for acoustic waves of different frequencies. Our findings show possibilities for the design of underwater acoustic devices, such as acoustic sensors and frequency dividers.

Nonlinear propagation dynamics of Gaussian beams in fractional Schrödinger equation

We investigated theoretically the nonlinear propagation dynamics of Gaussian beams in the fractional Schrödinger equation (FSE). When the nonlinearity is introduced into FSE without invoking an external potential, the evolution behaviors of incident Gaussian beams are modulated regularly and some novel phenomena arise. In the one-dimensional case, by changing the values of Kerr or saturated nonlinear coefficient, specific localized or diffracted phenomena appear in the corresponding intensity region, where the splitting angle of one-dimensional incident beam will be modulated flexibly to become larger or smaller in weak nonlinear region, besides, when the self-focusing strength is moderate, the energy of the beam is highly concentrated to form a breathing soliton structure. For two-dimensional case, Kerr or saturated nonlinearity will modulate the energy to the middle or edge in a certain nonlinear region, corresponding to the decrease or increase of the conical diffraction radius, it should be noted that there are two evolution periods under the saturated self-focusing nonlinearity. The work may provide more possibilities for beam modulation in FSE from a nonlinear perspective.