Beam Shaping Applications Research Articles

Graphene-based electrically reconfigurable deep-subwavelength metamaterials for active control of THz light propagation

This work studies the terahertz light propagation through graphene-based reconfigurable metasurfaces where the unit cell dimensions are much smaller than the terahertz wavelength. The proposed devices, which poses deep-subwavelength unit cell and active region dimensions can operate as amplitude and/or phase modulators in certain specific frequency bands determined by the device geometry. Reconfigurability is attained via electrostatically tuning the optical conductivity of patterned graphene layers, which are strategically located in each unit cell. The ultra-small unit cell dimensions can be advantageous for beam shaping applications.

Refractive laser beam shaping by means of a functional differential equation based design approach

Many laser applications require specific irradiance distributions to ensure optimal performance. Geometric optical design methods based on numerical calculation of two plano-aspheric lenses have been thoroughly studied in the past. In this work, we present an alternative new design approach based on functional differential equations that allows direct calculation of the rotational symmetric lens profiles described by two-point Taylor polynomials. The formalism is used to design a Gaussian to flat-top irradiance beam shaping system but also to generate a more complex dark-hollow Gaussian (donut-like) irradiance distribution with zero intensity in the on-axis region. The presented ray tracing results confirm the high accuracy of both calculated solutions and emphasize the potential of this design approach for refractive beam shaping applications.

Subwavelength beam manipulation via multiple-metal slits coupled by disk-shaped nanocavity

A novel plasmonic structure consisting of three nano-scaled slits coupled by nano-disk-shaped nanocavities is proposed to produce subwavelength focusing and beam bending at optical frequencies. The incident light passes through the metal slits in the form of surface plasmon polaritons (SPPs) and then scatters into radiation fields. Numerical simulations using finite-difference time-domain (FDTD) method show that the transmitted fields through the design example can generate light focusing and deflection by altering the refractive index of the coupled nanocavity. The simulation results indicate that the focal spot is beyond the diffraction limit. Light impinges on the surface at an angle to the optical axis will add an extra planar phase front that interferes with the asymmetric phase front of the plasmonic lens, leading to a larger bending angle off the axial direction. The advantages of the proposed plasmonic lens are smaller device size and ease of fabrication. Such geometries offer the potential to be controlled by using nano-positioning systems for applications in dynamic beam shaping and scanning on the nanoscale.

Wideband 400-Element Electronically Reconfigurable Transmitarray in X Band

A fully electronically reconfigurable 400-element transmitarray is studied numerically and experimentally in X-band. The array operates in linear polarization and consists of 20 × 20 unit-cells. A 1-bit phase resolution has been selected for the unit-cell in order to reduce the complexity of the biasing network and steering logic, the insertion loss and the overall cost of the antenna system. The unit-cell stack-up is simple and is made of four metal layers: active side, biasing lines, ground plane and passive side. Two p-i-n diodes are integrated on the active side of each cell in order to control its transmission phase. The active array contains 800 diodes in total. It demonstrates experimentally pencil beam scanning over a 140 × 80-degree window over a 15.8% fractional bandwidth, with a maximum gain of 22.7 dBi at broadside. We also show that the same antenna array can be used for beam shaping applications (flat-top beam). The experimental results presented between 8 and 12 GHz are in good agreement with the theoretical performance calculated using full-wave electromagnetic simulations and an in-house CAD tool based on analytical modeling.

Subwavelength plasmonics for graded-index optics on a chip

Planar plasmonic devices are becoming attractive for myriad applications, owing to their potential compatibility with standard microelectronics technology and the capability for densely integrating a large variety of plasmonic devices on a chip. Mitigating the challenges of using plasmonics in on-chip configurations requires precise control over the properties of plasmonic modes, in particular their shape and size. Here we achieve this goal by demonstrating a planar plasmonic graded-index lens focusing surface plasmons propagating along the device. The plasmonic mode is manipulated by carving subwavelength features into a dielectric layer positioned on top of a uniform metal film, allowing the local effective index of the plasmonic mode to be controlled using a single binary lithographic step. Focusing and divergence of surface plasmons is demonstrated experimentally. The demonstrated approach can be used for manipulating the propagation of surface plasmons, e.g., for beam steering, splitting, cloaking, mode matching, and beam shaping applications.

Beam shaping system design using double freeform optical surfaces

A numerical double-freeform-optical-surface design method is proposed for beam shaping applications. In this method, both the irradiance distribution and the wavefront of the output beam are taken into account. After numerically obtaining the input-output ray mapping based on Energy conservation using the variable separation method, the two freeform optical surfaces can be constructed simultaneously and point by point corresponding to the ray mapping based on Snell's law and the constancy of the optical path length. The method is only applicable for separable irradiance distributions. However, such a restriction is fulfilled by many practical laser beam shaping examples. Moreover, the restriction can simplify the computation considerably. Therefore, the method may be quite useful in practice, although it is not applicable to more general cases. As an example, the method was applied to design a two-plano-freeform-lens system for transforming a collimated 20 mm Gaussian laser beam (beam waist: 5mm) into a uniform 10 × 40 mm(2) rectangular one without changing the wavefront. Simulation results show that we can obtain a dual lens beam shaping system with the relative root mean square deviation of the irradiance ranging from 0.0652 to 0.326 and the power ratio concentrated on the desired region ranging from 97.5% to 88.3% as the output beam transfers from 0mm to 1000mm.

Research and Development on Phase-Shifting Surfaces (PSSs)

Phase-shifting surfaces (PSSs) developed in recent years are reported. Research and development on these phase-shifting surfaces are set in the historical context of prior-art free-standing lensing devices, as well as in the context of recent studies performed by other research groups. In addition, initial work on a phase- and amplitude-shifting surface (PASS), an extension of the phase-shifting surface, is demonstrated in a beam-shaping application.

Electrically thin free‐standing phase and amplitude shifting surface for beam shaping applications

AbstractA free‐standing phase and amplitude shifting surface (PASS) composed of four conductive layers separated by electrically thin dielectric sheets is presented in this article.This surface can be used for applications in which one must control both the phase and amplitude of electromagnetic waves propagating through it at every location over the aperture. The PASS provides a simple and efficient technique for manipulating the radiation pattern from a feed antenna for beam shaping applications. As a proof of concept, the technique is applied in the design of a flat‐topped shaped beam antenna. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1566–1571, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26911

Matched-filtering generalized phase contrast using LCoS pico-projectors for beam-forming

We report on a new beam-forming system for generating high intensity programmable optical spikes using so-called matched-filtering Generalized Phase Contrast (mGPC) applying two consumer handheld pico-projectors. Such a system presents a low-cost alternative for optical trapping and manipulation, optical lattices and other beam-shaping applications usually implemented with high-end spatial light modulators. Portable pico-projectors based on liquid crystal on silicon (LCoS) devices are used as binary phase-only spatial light modulators by carefully setting the appropriate polarization of the laser illumination. The devices are subsequently placed into the object and Fourier plane of a standard 4f-setup according to the mGPC spatial filtering configuration. Having a reconfigurable spatial phase filter, instead of a fixed and fabricated one, allows the beam shaper to adapt to different input phase patterns suited for different requirements. Despite imperfections in these consumer pico-projectors, the mGPC approach tolerates phase aberrations that would have otherwise been hard to overcome by standard phase projection.

Freeform manufacturing of a microoptical lens array on a steep curved substrate by use of a voice coil fast tool servo

We report what is to our knowledge the first approach to diamond turn microoptical lens array on a steep curved substrate by use of a voice coil fast tool servo. In recent years ultraprecision machining has been employed to manufacture accurate optical components with 3D structure for beam shaping, imaging and nonimaging applications. As a result, geometries that are difficult or impossible to manufacture using lithographic techniques might be fabricated using small diamond tools with well defined cutting edges. These 3D structures show no rotational symmetry, but rather high frequency asymmetric features thus can be treated as freeform geometries. To transfer the 3D surface data with the high frequency freeform features into a numerical control code for machining, the commonly piecewise differentiable surfaces are represented as a cloud of individual points. Based on this numeric data, the tool radius correction is calculated to account for the cutting-edge geometry. Discontinuities of the cutting tool locations due to abrupt slope changes on the substrate surface are bridged using cubic spline interpolation.When superimposed with the trajectory of the rotationally symmetric substrate the complete microoptical geometry in 3D space is established. Details of the fabrication process and performance evaluation are described.

Propagation characteristics of partially coherent beams with spatially varying correlations

We introduce a class of partially coherent beams with spatially varying correlation properties. It is shown that mathematically simple modifications in the coherence function of conventional Gaussian Schell-model beams lead to partially coherent fields with extraordinary free-space propagation characteristics, such as locally sharpened and laterally shifted intensity maxima. We study the properties of such fields based on an elementary-mode interpretation and by numerical simulations. The results demonstrate the potential of coherence modulation for beam shaping applications.

Flattop focusing with full Poincaré beams under low numerical aperture illumination

In this Letter, we propose and experimentally demonstrate the generation of second-order full Poincaré beams and its applications in two-dimensional flattop beam shaping with spatially variant polarization under low NA focusing condition. High-quality flattop profiles with steep edge roll-off can be obtained with this technique. The experiment results also demonstrate that flattop profile can be maintained for different input beam sizes by conveniently rotating a half-wave plate.

Experimental demonstration of Generalized Phase Contrast based Gaussian beam-shaper

We report the first experimental demonstration of Gaussian beam-shaping based on the Generalized Phase Contrast (GPC) approach. We show that, when using a dynamic spatial light modulator (SLM), this approach can rapidly generate arbitrarily shaped beams. Moreover, we demonstrate that low-cost binary-phase optics fabricated using photolithography and chemical etching techniques can replace the SLM in static and high power beam shaping applications. The design parameters for the binary-phase elements of the module are chosen according to the results of our previously conducted analysis and numerical demonstrations [Opt. Express 15, 11971 (2007)]. Beams with a variety of cross-sections such as circular, rectangular and square, with near flat-top intensity distributions are demonstrated. GPC-based beam shaping is inherently speckle-free and the shaped beams maintain a flat output phase. The non-absorbing components used in this beam-shaping approach have a high-damage-threshold and are thus ideally suited for high power applications.

Diffraction properties of partially coherent electromagnetic four-petal Gaussian beams

The concept of existing scalar coherent four-petal Gaussian (FPG) beam is extended to the more general partially coherent electromagnetic four-petal Gaussian (PCFPG) beam, and the cross-spectral density matrix of the latter is derived in analytical expressions. Intensity distributions and degree of polarization in free space are investigated by numerical examples. Results show that propagation properties of the PCFPG beam are different from their scalar coherent and scalar partially coherent counterparts. Both intensity distributions and polarization strongly depend on transversal coherence lengths. When the propagation distance increases to a large value, intensity distributions of PCFPG beams convert into the Gauss profile. Polarization of PCFPG beams shows an oscillating characteristic upon propagation, and the oscillation would enhance with the increase of coherence lengths. These results may find potential applications in beam shaping and free space optical communications.

Nanoscale plasmonic devices for dynamically controllable beam focusing and scanning

We have performed simulations to investigate the variable focusing and scanning capability of metallic nano-slit configurations. In a symmetric nanorod configuration inside an aperture with adjustable offset of the center rod, the focal position is found to be variable in the 0.5–3.5 μm range. In a ladder configuration of the rods, the transmitted beam is found to be deflected up to 23°. Horizontal displacement of rods allows for finer control of angular scanning up to 4°. Such slit geometries offer the potential to be controlled by using nano-positioning systems for applications in dynamic beam shaping and scanning on the nanoscale.

Lens With Transformable-Type and Tunable-Focal-Length Characteristics

A versatile lens with transformable-type and tunable-focal-length characteristics has been presented. The conventional punching technology is adopted to fabricate the lens structure due to its inherently high direction-holding capability. As a result, the alignment between two ends of the lens can be easily achieved without requiring complicated process. At the same time, soft lithography is used to realize structure similar to liquid-filled lens except the working medium being air rather than liquid acting as a tuning mechanism. These two parts are eventually combined together with bonding under oxygen plasma activation. With this design, large flexibility in performance tuning will be added to the lens, which has been experimentally demonstrated in beam shaping and imaging applications.

F2-laser microfabrication of multilevel diffractive optical elements

The 157nm F2-laser drives strong and precisely controllable interactions with fused silica, the most widely used material for bulk optics, optical fibers, and planar optical circuits. A diffractive optical element (DOE) breaks apart wavefronts and redirects segmented beamlets through phase control for novel beam steering and shaping applications. This paper shows that precise excisions of 10-30 nm depth are available from the F2 laser for the generation of efficient DOEs for the visible and ultraviolet spectrum. F2-laser radiation was applied with beam homogenization optics and high-precision computer-controlled motion stages to shape up to 16-level, 256 × 256 pixel DOE devices on bulk glasses, with distinguishable level-tolevel spacing of ∼100 nm and surface roughness of ∼13 nm. The 1st order diffraction efficiency was ∼35%. The farfield pattern when illuminated with a HeNe laser was found to agree with simulations based on an iterative Fourier transform algorithm. Future improvements in the laser micromachining process and possible directions are also offered.

Alteration of the profile of ink-jet-deposited UV-cured lenses using applied electric fields

Microlens arrays and microoptical components in general are integral components in a wide range of high-tech products. The ability to fabricate such elements cheaply and with a high degree of accuracy is vital for the development of the next generation of optics-based technologies. There are currently a wide range of microoptical element fabrication technologies. These techniques all have advantages and disadvantages and no one technique is yet sufficient to meet all possible application criteria. One method that has been examined recently is the ink-jet deposition method. This method applies well-established ink-jet printer technology and is thus one possible candidate for large-scale fabrication of inexpensive components. The lenses fabricated using this method are normally found to have spherical profiles. In this paper, we examine the possibility of modifying the profile of these spherical lenses using an applied electric field (E-field). We note that the resulting aspheric lenses have a wide number of applications. These include beam shaping and power transfer applications including fiber coupling. In this paper we describe initial experiments involving single lenses. The single lenses produced using applied E-Fields differ significantly from lenses produced with no applied E-field.

Review of iterative Fourier-transform algorithms for beam shaping applications

We present a comparison of some of the most used iterative Fourier transform algorithms (IFTA) for the design of continuous and multilevel diffractive optical elements (DOE). Our aim is to provide optical engineers with advice for choosing the most suited algorithm with respect to the task. We tackle mainly the beam-shaping and the beam-splitting problems, where the desired light distributions are almost binary. We compare four recent algorithms, together with the historical error-reduction and input-output methods. We conclude that three of these algorithms are interesting for continuous-phase kinoforms, and two, namely the three-step method proposed by Wyrowski and the over-compensation of Prongué, still perform well with multilevel- and binary-phase DOE.

Radiosurgery and stereotactic radiation therapy for vestibular schwannoma in neurofibromatosis type 2 patients

Neurofibromatosis type 2 (NF2) patients typically suffer from bilateral vestibular schwannoma (VS) and are at risk for developing bilateral deafness, bilateral trigeminal and bilateral facial nerve function loss. Previous reports suggest that treatment outcomes in these patients are worse compared to patients with sporadic solitary VS. Especially in NF2 patients who acquired unilateral hearing loss already, avoidance of hearing loss on the opposite side poses a challenge for stereotactic radiosurgery. We studied our treatment results in NF2 patients treated for VS with linac based radiosurgery (RS) and stereotactic radiation therapy (SRT). In 204 VS patients treated with RS or SRT in Amsterdam starting from 1992, we identified 25 NF2 patients with bilateral tumors. Indications for treatment were either tumor progression on sequential MRI and/or progressive hearing loss. Stereotactic irradiation was administered to all patients using 5 non-coplanar arcs with a single isocenter to a dose of 10–12.5 Gy in a single fraction or 20–25 Gy in 5 fractions in 1 week prescribed to the 80% isodose encompassing the tumor. In the first patients a single round collimator was used for every treatment. In subsequent patients conformal arcs were used. Dynamic conformal arcs application, resulting in superior dose distributions, is used currently. Five patients were followed less than 1 year. Of the remaining 20 patients, 5 were ipsilateral deaf prior to treatment. Consequently 15 patients were at risk for treatment related hearing loss. They showed a mean Pure Tone Average (PTA) of 51 dB (8–112) prior to treatment. On the untreated side, all patients showed hearing loss and 8 (53%) were deaf. After treatment all patients were assessed at yearly intervals including MRI and pure tone audiometry. Median follow-up time was 51 months (12–109). Local tumor control was obtained in all 20 patients and no treatment related trigeminal or facial nerve toxicity was observed. Hearing status was assessed yearly after treatment. This revealed that the mean PTA in the 15 hearing patients dropped from 51 to 77 dB (40–120). In 6 of these patients (40%) the additional PTA loss ranged from 0–15 dB, in another 6 patients (40%) it ranged from 15–45 dB and in 3 of these patients (20%) it was more than 45 dB. No additional hearing loss was observed beyond 36 months after treatment. Excellent local control rates can be obtained with RS and SRT for VS in NF2 patients, with minimal facial and trigeminal nerve toxicity. Hearing preservation, however, remains disappointing with 60% of the patients losing 15 dB or more of PTA on the irradiated side. This seems worse than our previously reported outcomes of SRS and RS in patients with solitary sporadic tumors. Improvements may be expected by application of dynamic conformal beam shaping