Passive Lens Research Articles

The Focusing Properties of a Modular All-Metal Lens in the Near-Field Region.

This article proposes a modular and passive all-metal lens to achieve near-field focusing with adjustable focus. The proposed lens consists of all-metal units with the phase coverage range exceeding 360°, and the arrangement of units is guided by the phase compensation method. Specifically, using the strategy of module unit synthesis, the arrangement of lens units under different focuses can be assembled arbitrarily, which reduces the production costs by 39.3% and improves the freedom of lens design. The simulation and experimental results show that the lens exhibits excellent focusing properties and freely changes the position of the expected focus (0.30 m-0.75 m). Therefore, the modular all-metal lens designed in this article has the characteristics of high transparency and a high degree of freedom, which can provide low-cost and lightweight solutions for various applications in the field of antennas, such as near-field target detection, microwave imaging, biomedicine, and so on.

Single-channel acoustic vortex tweezer with attachable fan-shaped holographic lens

The utilization of acoustic vortices presents a promising technique for contactless particle manipulation. However, existing methods for generating acoustic vortices typically rely on complex active driving systems and array ultrasound transducers. Therefore, more simplified and robust approaches to inducing acoustic vortices should be considered. With inspiring the fact that acoustic vortices can be generated by the differences in the height of adjacent sectors in conventional four-panel transducers, we aim to develop a single-channel acoustic vortex tweezer with a simplified driving system by incorporating attachable fan-shaped polydimethylsiloxane (PDMS) holographic lens. This passive lens structure with different heights in adjacent sectors can generate phase-modulated ultrasonic waves. Therefore, the interaction of phase-modulated waves passing through the lens is expected to result in a vortex-like beam. The available materials of fan-shaped lens are not limited to the PDMS. This lens type can be appropriately fabricated with an understanding of the sound of speed within the desired material and predetermined thickness, inducing π/2 phase difference in the adjacent lobes. As a result, the excited acoustic waves propagated through the lens were under mutual interaction without any active electrical setup, forming a null pressure region enveloped by four distinct acoustic lobes. We systemically investigated the tweezing potential using vortex-like waves. Several types of bioparticles were utilized in these investigations: microbubbles, polystyrene (PS) beads, and cancer cells. Our findings demonstrate that the transducer with lens is sufficiently capable of spatial manipulation of bioparticles. Remarkably, the microbubbles traveling at a flow velocity of 32 cm/s in a blood vessel-mimicking phantom were feasibly trapped in the proposed transducer. Furthermore, the spatial control of PS and cells with higher acoustic impedance was also feasible in static conditions. Consequently, the proposed transducer offers an economically and practically viable solution for various biomedical applications, such as drug delivery and cell-based bioengineering.

Focusing and reduction of correlated energy spread of chirped electron beams in passive plasma lens

All-optical compact plasma focusing and transportation of electron beams, in the passive mode of a plasma lens, is studied via real geometry particle-in-cell simulations for its suitability for the laser wakefield acceleration technique. The focusing of an electron beam by a passive plasma lens is a non-linear and dynamic process, which strongly depends on the space charge induced evacuation of the plasma electrons in the vicinity of the propagating electron beam. Effects of such focusing on the energy spread, divergence, and emittance of laser-driven electron beams are analyzed numerically for different plasma densities. An initially negative chirp in electron beam energy is shown to be instrumental in suppressing the unwanted growth in the relative energy spread of the electron beam during the passive lensing. Usefulness of such a passive plasma element for a single and multi-stage laser wakefield acceleration configuration is demonstrated.

Ultrabroadband and Multifunctional Achromatic Mikaelian Lens on an Elastic Plate

The emergence and development of acoustic artificial materials stimulate a large number of applications for the manipulation of ultrasonic and elastic waves. Most of the reported metasurfaces and metamaterials only work effectively at a given frequency and target a fixed functionality. Reconfigurable designs may solve this problem, to a certain extent, but require the introduction of complex active control components. Here, by integrating multiple functionalities on a conformally mapped Mikaelian lens with a hyperbolic secant refractive-index distribution, a broadband passive lens is presented and accomplished on an elastic plate. Just by adjusting the location and type of source, different regulations of flexural waves can be switched from one sort to another. Through projecting the refractive-index profile onto the thickness of the plates, the proposed Mikaelian lens is designed and fabricated. Theoretical prediction and experimental results demonstrate beam scanning at an angle of up to 120\ifmmode^\circ\else\textdegree\fi{} from 30 to 180 kHz and achromatic subwavelength focusing with a full width at half maximum of around 0.3\ensuremath{\lambda} from 30 to 120 kHz. Without using locally resonant materials or active control elements, this work provides a feasible way to construct multifunctional flexural-wave devices.

Multibeam Antenna for Ka-Band CubeSat Connectivity Using 3-D Printed Lens and Antenna Array

In this letter, the design of a passive multibeam lens antenna is proposed for the CubeSat space communication system as an alternative application of a 2-D microstrip antenna array that has originally been designed for a 39-GHz 5G multi-user, multiple-input, multiple-output (MU-MIMO) system. The half-ellipsoid lens is 3-D printed using stereolithography technology. The antenna prototype is capable of selecting the main beam between 16 different directions with a gain ranging from 14 to 16 dBi and a half-power beamwidth of 14 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> –18 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> . The measurements carried out in an anechoic chamber show good agreement with numerical simulations. The presented prototype shows that by employing 3-D printing technologies existing antennas can be easily and inexpensively converted to switched-beam or multibeam solutions.

Passive discrete lens for broadband elastic guided wave focusing

Elastic guided wave focusing is of great interest for applications such as vibroacoustic control, energy harvesting, or Structural Health Monitoring. Different strategies allow generation of this effect, GRadient-INdex devices in particular exploit medium with varying properties such as thickness to reproduce an adequate refractive index profile as in optics. The resulting continuous profiles have a curved geometry that can be hard to manufacture, and be difficult to integrate in a given design. The purpose of this paper is to propose a discrete design for a GRIN lens. It is composed of segments selected in number and thickness to give similar focusing effects as a continuous lens profile. The identified configuration is manufactured and bounded on an aluminium plate to evaluate the effective focusing performances. Numerical and experimental vibrometry results confirm that the proposed lens exhibits a fixed focal point over a broad frequency range. The discrete design overcomes fabrication issues encountered in continuous design, allowing for an easier integration in devices for elastic wave control.

Designing of active plasma lens for focusing laser-plasma-accelerated pulsed proton beams

Active plasma lenses (APLs) have been proved as an effective device for focusing charged particles. In this paper, we have theoretically studied the feasibility of using a discharged-capillary based high gradient APL to focus laser accelerated pulsed proton beams. We specify equations of motion of proton beams in both straight and curved discharge-capillary APLs, on which the regulation law of a specific APL focusing and tuning protons can be calculated. Then, we summarize several constraints of APLs. Confirmed by a 2D PIC simulation, we find that with a proper plasma state, the proton energy loss, the transverse current distribution, and the Z-pinch effect do not significantly affect the beam transport, while the passive plasma lens (PPL) effect can promote the APL focusing within an acceptable extent. The regime of working parameters, including the plasma density ${n}_{0}$, the discharge current $I$, the length $L$ and the radius $R$ of APLs are given for different aimed proton energy. The results show that APL, whose parameters can be adjusted flexibly, is suitable for focusing and transporting proton beams with energy of 1--100 MeV. It is expected to build a beam-line based on plasma with centimeter length scale for pulsed proton beams. Such compact beam-line can have significant impact on various applications.

High accuracy lens centering using edge contact mounting

We introduce a new type of drop-in technique used to passively and accurately center lenses in optical mounts. This lens mounting method is called edge contact mounting and uses the edge at the intersection of the cylindrical and optical surface of the lens as the mounting interface. By providing a spherical mounting seat for the lens on a simple standard threaded ring, it is possible to center accurately lenses of different geometries, diameters, and radius of curvatures. The method allows relaxation of some manufacturing tolerances compared with rim contact drop-in and is not subject to a minimum clamping angle as for the surface contact mounting. This innovative lens mounting method allows extension of the centering accuracy offered by passive lens centering methods to a next level without compromise on cost and complexity.

Low-divergence femtosecond X-ray pulses from a passive plasma lens

Electron and X-ray beams originating from compact laser-wakefield accelerators have very small source sizes that are typically on the micrometre scale. Therefore, the beam divergences are relatively high, which makes it difficult to preserve their high quality during transport to applications. To improve on this, tremendous efforts have been invested in controlling the divergence of the electron beams, but no mechanism for generating collimated X-ray beams has yet been demonstrated experimentally. Here we propose and realize a scheme where electron bunches undergoing focusing in a dense, passive plasma lens can emit X-ray pulses with divergences approaching the incoherent limit. Compared with conventional betatron emission, the divergence of this so-called plasma lens radiation is reduced by more than an order of magnitude in solid angle, while maintaining a similar number of emitted photons per electron. This X-ray source offers the possibility of producing brilliant and collimated few-femtosecond X-ray pulses for ultra-fast science, in particular for studies based on X-ray diffraction and absorption spectroscopy.

High power diode pumped solid-state femtosecond laser systems

We demonstrate high power direct diode-pumped femtosecond laser based on a commercially available configuration and pumped by multi-diode laser modules delivering high brightness and average power. The multi-diode modules employ beam-combining scheme preserving linear polarization and focusing properties of individual laser diodes. Stable passive Kerr lens modelocked (KLM) laser operation was demonstrated, producing 30 fs pulses at ∼300 mW average output power. A simple approach to numerical modelling of KLM laser operation in cases when longitudinal pump beam exhibits strong astigmatism is also discussed.

Thermal lens with passively tuned focal length formed in a photochromic material.

A new, to the best of our knowledge, way of tuning the focal length of a thermal lens is presented. The thermal lens is formed in a photochromic thermo-optical material, through illumination of the material by a heating laser beam of constant power. The tunability of the lens is achieved by changing the absorption coefficient of the thermo-optical material at the heating laser beam wavelength. This change is obtained by modifying the intensity of the additional, flat in profile, light of a wavelength different than that of the heating laser beam. The change in intensity of this light may follow from the effects of external factors, as, e.g., decrease in daylight intensity. Therefore, the lens may be considered as a passive auto-tuning lens, with focal length auto-adjusting to the external illumination.

Experimental characterization of the effects induced by passive plasma lens on high brightness electron bunches

We report on the experimental characterization of the effect that a passive plasma lens in the overdense regime has on high-brightness bunch quality by means of 6D phase-space analysis. The passive lens is generated by confining hydrogen gas with a capillary tube pre-ionized with a high-voltage discharge. We observed that the optimum condition is retrieved at the end of the overdense regime with almost no effect on bunch brightness. The presence of gas jets, leaking from the hollow capillary end-points, extends the lens effects also outside of the capillary, resulting in longer focusing channels. Experimental results are supported with numerical simulations of the complete accelerator line together with the plasma channel section.

Power drainage and energy dissipation in lossy but perfect lenses

It has recently been shown that passive lens designs can retain perfect lensing despite intrinsic loss in the comprising left-handed materials. Here we show that energy conservation is not at odds with the operation of such lossy perfect lenses: The irreversible transfer of electromagnetic power to the lossy material dictates that a smaller portion of the power emitted by the source arrives at the image, but image fidelity remains absolute. Moreover, the power that does arrive at the perfect image must drain from the system there. This is shown to apply to any layered perfect lens based on left-handed media, including the well-known Veselago lens. Combined with what is known about Maxwell's fisheye lens, our results suggest that power drainage is universal to all perfect lenses and is intrinsic to any perfect image. Properly addressing this unique power dynamics is therefore essential to any successful realization of practical perfect lenses with existing metamaterials.

Design and fabrication of a passive droplet dispenser for portable high resolution imaging system

Moldless lens manufacturing techniques using standard droplet dispensing technology often require precise control over pressure to initiate fluid flow and control droplet formation. We have determined a series of interfacial fluid parameters optimised using standard 3D printed tools to extract, dispense and capture a single silicone droplet that is then cured to obtain high quality lenses. The dispensing process relies on the recapitulation of liquid dripping action (Rayleigh-Plateau instability) and the capturing method uses the interplay of gravitational force, capillary forces and liquid pinning to control the droplet shape. The key advantage of the passive lens fabrication approach is rapid scale-up using 3D printing by avoiding complex dispensing tools. We characterise the quality of the lenses fabricated using the passive approach by measuring wavefront aberration and high resolution imaging. The fabricated lenses are then integrated into a portable imaging system; a wearable thimble imaging device with a detachable camera housing, that is constructed for field imaging. This paper provides the full exposition of steps, from lens fabrication to imaging platform, necessary to construct a standalone high resolution imaging system. The simplicity of our methodology can be implemented using a regular desktop 3D printer and commercially available digital imaging systems.

Demonstration of passive plasma lensing of a laser wakefield accelerated electron bunch

We report on the first demonstration of passive all-optical plasma lensing using a two-stage setup. An intense femtosecond laser accelerates electrons in a laser wakefield accelerator (LWFA) to 100 MeV over millimeter length scales. By adding a second gas target behind the initial LWFA stage we introduce a robust and independently tunable plasma lens. We observe a density dependent reduction of the LWFA electron beam divergence from an initial value of 2.3 mrad, down to 1.4 mrad (rms), when the plasma lens is in operation. Such a plasma lens provides a simple and compact approach for divergence reduction well matched to the mm-scale length of the LWFA accelerator. The focusing forces are provided solely by the plasma and driven by the bunch itself only, making this a highly useful and conceptually new approach to electron beam focusing. Possible applications of this lens are not limited to laser plasma accelerators. Since no active driver is needed the passive plasma lens is also suited for high repetition rate focusing of electron bunches. Its understanding is also required for modeling the evolution of the driving particle bunch in particle driven wake field acceleration.

Perfect Lensing by a Single Interface: Defying Loss and Bandwidth Limitations of Metamaterials.

Loss is known to be detrimental for achieving perfect focusing with the passive perfect lens designs suggested thus far, and it is believed to pose a fundamental barrier. We show that perfect lensing can be achieved with actual lossy left-handed metamaterials, without a need for gain or nonlinearity. The proposed loss-immune perfect lens is composed of a single interface between a conventional dielectric material on the source side and a lossy left-handed material on the image side. Its immunity to material loss was derived analytically using three complementary methodologies, confirming perfect lensing with point-to-point accuracy and shedding light on the underlying focusing mechanism. This result provides a new road map for practical realization of a near-field camera based on the single-interface lens design.

Scattering effects in passive foil focusing of ion beams

A stack of thin, closely spaced conducting foils has been investigated by Lund et al. [Phys. Rev. ST Accel. Beams 16, 044202 (2013)] as a passive focusing lens for intense ion beams. The foils mitigate space-charge defocusing forces to enable the beam self-magnetic field to focus. In this study, we analyze possible degradation of focusing due to scattering of beam ions resulting from finite foil thickness using an envelope model and numerical simulations with the particle-in-cell code WARP. Ranges of kinetic energy where scattering effects are sufficient to destroy passive focusing are quantified. The scheme may be utilized to focus protons produced in intense laser-solid accelerator schemes. As an example, the spot size of an initially collimated 30 MeV proton beam with initial rms radius $200\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, perveance $Q=1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$, and initial transverse emittance ${ϵ}_{x,\mathrm{rms}}=0.87\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}$ propagating through a stack of $6.4\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ thick foils, spaced $100\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ apart, gives a $127.5\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ spot with scattering and a $81.0\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ spot without scattering, illustrating the importance of including scattering effects.

Non-Invasive Wireless and Passive MEMS Intraocular Pressure Sensor Based on Flexible Substrate

The intraocular pressure (IOP) is an indicator for the diagnosis and treatment of glaucoma. The traditional measurement of IOP cannot provide a continuous measurement within 24 hours with compromised functionality in glaucoma diagnosis. This paper presents a wireless, passive and non-invasive contact lens sensor (CLS) for continuous monitoring of IOP based on the flexible substrate. This curvature-sensitive sensor includes an inductor and two capacitors to form a C-L-C resonant circuit, which were embedded in parylene C using PDMS as the intermediate medium layer. The C-L-C resonant circuit is used for passive and wireless sensing and simplifying the wireless bonding step. Compared to parylene C, PDMS has a much lower Young Modulus, enabling the quantification of the sensor curvatures, as an indicator of IOP. To fit human eyeballs, the size of the sensor was designed as follows: the outer diameter: 12 mm, the radius of curvature: 8.5 mm. The sensor was characterizedin vitroto obtain the frequency response, and the results show that the sensor has a linearity R&gt;0.97 and a sensitivity&gt;9.7 kHz/kPa. This sensor can be used to monitor IOP together with the reading circuit continuously, enabling the diagnosis of glaucoma without impacting the daily life of users.

An Active Reconfigurable Antenna at 60 GHz Based on Plate Inhomogeneous Lens and Feeders

The conception and performances of a 60 GHz active antenna, reconfigurable in terms of radiation pattern is described in this paper. This antenna is based on a plate inhomogeneous Luneburg lens fed by several ridged waveguide sources. The manufacturing technique of the plate lens is briefly described and its measured radiation patterns are given, showing the good performances of this lens. Simulated and measured radiation patterns are given for a passive lens antenna fed by several sources. The results show the beam scanning capability of this antenna. The active antenna is described and measurements of radiation patterns for several beams are given and demonstrate the beam scanning and beam shaping reconfigurability.

Simple and versatile heterodyne whole-field interferometer for phase optics characterization

A wavefront sensor for thermally induced lens and passive lens characterization based on low-coherence digital speckle interferometry was developed and studied. By illuminating the optical setup with two slightly detuned red diode lasers, whole-field contour interference fringes were generated according to the resulting synthetic wavelength. For fringe pattern visualization the optical setup used the light transmitted through a ground glass plate as object beam. The performance of the sensor was investigated and its versatility was demonstrated by measuring the thermal lens induced in an Er-doped glass sample pumped by a 1.76-W diode laser emitting at 976 nm and by evaluating the wavefront distortion introduced by an ophthalmic progressive lens.