The control of light-matter interaction through the use of subwavelength structures known as metamaterials has facilitated
the ability to control electromagnetic radiation in ways not previously achievable. A plethora of passive metamaterials as
well as examples of active or tunable metamaterials have been realized in recent years. However, the development of
tunable metamaterials is still met with challenges due to lack of materials choices. To this end, materials that exhibit a
metal-insulator transition are being explored as the active element for future metamaterials because of their characteristic
abrupt change in electrical conductivity across their phase transition. The fast switching times (▵t < 100 fs) and a change
in resistivity of four orders or more make vanadium dioxide (VO2) an ideal candidate for active metamaterials. It is known
that the properties associated with thin film metal-insulator transition materials are strongly dependent on the growth
conditions. For this work, we have studied how growth conditions (such as gas partial pressure) influence the metalinsulator
transition in VO2 thin films made by pulsed laser deposition. In addition, strain engineering during the growth
process has been investigated as a method to tune the metal-insulator transition temperature. Examples of both the optical
and electrical transient dynamics facilitating the metal-insulator transition will be presented together with specific
examples of thin film metamaterial devices.
Vanadium dioxide (VO2) undergoes a metal-insulator transition (MIT) at 68°C, at which point its electrical conductivity changes by several orders of magnitude. This extremely fast transition (Δt < 100 fs) can be induced thermally, mechanically, electrically, or optically. The combination of fast switching times and response to a broad range of external stimuli make VO2 an ideal material for a variety of novel devices and sensors. While the MIT in VO2 has been exploited for a variety of microwave/terahertz applications (i.e. tunable filters and modulators), very few devices exploiting the fast switching time of VO2 have been reported. The electrical properties of thin film VO2 (conductivity, carrier concentration, switching speed, etc.) are highly dependent on growth and post-processing conditions. The optimization of these conditions is therefore critical to the design and fabrication of VO2 devices. This paper will report the effects of various pulsed laser deposition (PLD) growth conditions on the metal-insulator transition in thin film VO2. In particular, we report the effect of PLD growth conditions on the stress/strain state of the VO2 layer, and the subsequent change in electrical properties. Finally, results from fabricated VO2 devices (THz emitters and THz modulators) will be presented.
The use of laser-induced forward transfer (LIFT) techniques for the printing of functional materials has been demonstrated for numerous applications. The printing gives rise to patterns, which can be used to fabricate planar interconnects. More recently, various groups have demonstrated electrical interconnects from laser-printed 3D structures. The laser printing of these interconnects takes place through aggregation of voxels of either molten metal or of pastes containing dispersed metallic particles. However, the generated 3D structures do not posses the same metallic conductivity as a bulk metal interconnect of the same cross-section and length as those formed by wire bonding or tab welding. An alternative is to laser transfer entire 3D structures using a technique known as lase-and-place. Lase-and-place is a LIFT process whereby whole components and parts can be transferred from a donor substrate onto a desired location with one single laser pulse. This paper will describe the use of LIFT to laser print freestanding, solid metal foils or beams precisely over the contact pads of discrete devices to interconnect them into fully functional circuits. Furthermore, this paper will also show how the same laser can be used to bend or fold the bulk metal foils prior to transfer, thus forming compliant 3D structures able to provide strain relief for the circuits under flexing or during motion from thermal mismatch. These interconnect "ridges" can span wide gaps (on the order of a millimeter) and accommodate height differences of tens of microns between adjacent devices. Examples of these laser printed 3D metallic bridges and their role in the development of next generation electronics by additive manufacturing will be presented.
The emerging technique of rapid prototyping with three-dimensional (3-D) printers provides a simple yet revolutionary method for fabricating objects with arbitrary geometry. The use of 3-D printing for generating morphologically biomimetic tissue phantoms based on medical images represents a potentially major advance over existing phantom approaches. Toward the goal of image-defined phantoms, we converted a segmented fundus image of the human retina into a matrix format and edited it to achieve a geometry suitable for printing. Phantoms with vessel-simulating channels were then printed using a photoreactive resin providing biologically relevant turbidity, as determined by spectrophotometry. The morphology of printed vessels was validated by x-ray microcomputed tomography. Channels were filled with hemoglobin (Hb) solutions undergoing desaturation, and phantoms were imaged with a near-infrared hyperspectral reflectance imaging system. Additionally, a phantom was printed incorporating two disjoint vascular networks at different depths, each filled with Hb solutions at different saturation levels. Light propagation effects noted during these measurements—including the influence of vessel density and depth on Hb concentration and saturation estimates, and the effect of wavelength on vessel visualization depth—were evaluated. Overall, our findings indicated that 3-D-printed biomimetic phantoms hold significant potential as realistic and practical tools for elucidating light–tissue interactions and characterizing biophotonic system performance.
The use of metamaterials structures has been the subject of extensive discussions given their wide range of applications. However, a large fraction of the work available to date has been limited to simulations and proof-of-principle demonstrations. One reason for the limited success inserting these structures into functioning systems and real-world applications is the high level of complexity involved in their fabrication. Direct-write processes are ideally suited for the fabrication of arbitrary periodic and aperiodic structures found in most metamaterial and plasmonic designs. For these applications, laser-based processes offer numerous advantages since they can be applied to virtually any surface over a wide range of scales. Furthermore, laser direct-write or LDW allows the precise deposition and/or removal of material thus enabling the fabrication of novel metamaterial designs. This presentation will show examples of metamaterial and plasmonic structures developed at the Naval Research Lab using LDW, and discuss the benefits of laser processing for these applications.
This work was sponsored by The Office of Naval Research.
The progressive miniaturization of electronic devices requires an ever-increasing density of interconnects attached via solder joints. As a consequence, the overall size and spacing (or pitch) of these solder joint interconnects keeps shrinking. When the pitch between interconnects decreases below 200 μm, current technologies, such as stencil printing, find themselves reaching their resolution limit. Laser direct-write (LDW) techniques based on laser-induced forward transfer (LIFT) of functional materials offer unique advantages and capabilities for the printing of solder pastes. At NRL, we have demonstrated the successful transfer, patterning, and subsequent reflow of commercial Pb-free solder pastes using LIFT. Transfers were achieved both with the donor substrate in contact with the receiving substrate and across a 25 μm gap, such that the donor substrate does not make contact with the receiving substrate. We demonstrate the transfer of solder paste features down to 25 μm in diameter and as large as a few hundred microns, although neither represents the ultimate limit of the LIFT process in terms of spatial dimensions. Solder paste was transferred onto circular copper pads as small as 30 μm and subsequently reflowed, in order to demonstrate that the solder and flux were not adversely affected by the LIFT process.
The emerging technique of three-dimensional (3D) printing provides a revolutionary way to fabricate objects with biologically realistic geometries. Previously we have performed optical and morphological characterization of basic 3D printed tissue-simulating phantoms and found them suitable for use in evaluating biophotonic imaging systems. In this study we assess the potential for printing phantoms with irregular, image-defined vascular networks that can be used to provide clinically-relevant insights into device performance. A previously acquired fundus camera image of the human retina was segmented, embedded into a 3D matrix, edited to incorporate the tubular shape of vessels and converted into a digital format suitable for printing. A polymer with biologically realistic optical properties was identified by spectrophotometer measurements of several commercially available samples. Phantoms were printed with the retinal vascular network reproduced as ~1.0 mm diameter channels at a range of depths up to ~3 mm. The morphology of the printed vessels was verified by volumetric imaging with μ-CT. Channels were filled with hemoglobin solutions at controlled oxygenation levels, and the phantoms were imaged by a near-infrared hyperspectral reflectance imaging system. The effect of vessel depth on hemoglobin saturation estimates was studied. Additionally, a phantom incorporating the vascular network at two depths was printed and filled with hemoglobin solution at two different saturation levels. Overall, results indicated that 3D printed phantoms are useful for assessing biophotonic system performance and have the potential to form the basis of clinically-relevant standardized test methods for assessment of medical imaging modalities.
Additive manufacturing techniques such as 3D printing are able to generate reproductions of a part in free space without the use of molds; however, the objects produced lack electrical functionality from an applications perspective. At the same time, techniques such as inkjet and laser direct-write (LDW) can be used to print electronic components and connections onto already existing objects, but are not capable of generating a full object on their own. The approach missing to date is the combination of 3D printing processes with direct-write of electronic circuits. Among the numerous direct write techniques available, LDW offers unique advantages and capabilities given its compatibility with a wide range of materials, surface chemistries and surface morphologies. The Naval Research Laboratory (NRL) has developed various LDW processes ranging from the non-phase transformative direct printing of complex suspensions or inks to lase-and-place for embedding entire semiconductor devices. These processes have been demonstrated in digital manufacturing of a wide variety of microelectronic elements ranging from circuit components such as electrical interconnects and passives to antennas, sensors, actuators and power sources. At NRL we are investigating the combination of LDW with 3D printing to demonstrate the digital fabrication of functional parts, such as 3D circuits. Merging these techniques will make possible the development of a new generation of structures capable of detecting, processing, communicating and interacting with their surroundings in ways never imagined before. This paper shows the latest results achieved at NRL in this area, describing the various approaches developed for generating 3D printed electronics with LDW.
The field of metamaterials has expanded to include more than four orders of magnitude of the electromagnetic spectrum, ranging from the microwave to the optical. While early metamaterials operated in the microwave region of the spectrum, where standard printed circuit board techniques could be applied, modern designs operating at shorter wavelengths require alternative manufacturing methods, including advanced semiconductor processes. Semiconductor manufacturing methods have proven successful for planar 2D geometries of limited scale. However, these methods are limited by material choice and the range of possible feature sizes, thus hindering the development of metamaterials due to manufacturing challenges. Furthermore, it is difficult to achieve the wide range of scales encountered in modern metamaterial designs with these methods alone. Laser direct-write processes can overcome these challenges while enabling new and exciting fabrication techniques. Laser processes such as micromachining and laser transfer are ideally suited for the development and optimization of 2D and 3D metamaterial structures. These laser processes are advantageous in that they have the ability to both transfer and remove material as well as the capacity to pattern non-traditional surfaces. This paper will present recent advances in laser processing of various types of metamaterial designs.
Laser forward transfer of arbitrary and complex configurable structures has recently been demonstrated using a spatial light modulator (SLM). The SLM allows the spatial distribution of the laser pulse, required by the laser transfer process, to be modified for each pulse. The programmable image on the SLM spatially modulates the intensity profile of the laser beam, which is then used to transfer a thin layer of material reproducing the same spatial pattern onto a substrate. The combination of laser direct write (LDW) with a SLM is unique since it enables LDW to operate not only in serial fashion like other direct write techniques but instead reach a level in parallel processing not possible with traditional digital fabrication methods. This paper describes the use of Digital Micromirror Devices or DMDs as SLMs in combination with visible (λ = 532 nm) nanosecond lasers. The parallel laser printing of arrayed structures with a single laser shot is demonstrated together with the full capabilities of SLMs for laser printing reconfigurable patterns of silver nano-inks Finally, an overview of the unique advantages and capabilities of laser forward transfer with SLMs is presented.
The opportunities presented by the use of metamaterials have been the subject of extensive discussions. However, a large fraction of the work available to date has been limited to simulations and proof-of-principle demonstrations. One reason for the limited success inserting these structures into functioning systems and real-world applications is the high level of complexity involved in their fabrication. Most approaches to the realization of metamaterial structures utilize traditional lithographic processing techniques to pattern the required geometries and then rely on separate steps to assemble the final design. Obviously, composite structures with arbitrary and/or 3-D geometries present a challenge for their implementation with these approaches. Non-lithographic processes are ideally suited for the fabrication of arbitrary periodic and aperiodic structures needed to implement many of the metamaterial designs being proposed. Furthermore, non-lithographic techniques are true enablers for the development of conformal or 3-D metamaterial designs. This article will show examples of metamaterial structures developed at the Naval Research Laboratory using non-lithographic processes. These processes have been applied successfully to the fabrication of complex 2-D and 3-D structures comprising different types of materials.
Imaging of capillary structures and monitoring of blood flow within vasculature is becoming more common in
clinical settings. However, very few dynamic phantoms exist which mimic capillary structures. We report the
fabrication and testing of microfluidic, vascular phantoms aimed at the study of blood flow. These phantoms are
fabricated using low-cost, off-the-shelf materials and require no lithographic processing, stamping, or embossing.
Using laser micromachining, complex microfluidic structures can be fabricated in under an hour. The laser system is
capable of producing microfluidic features with sizes on the order of tens of microns, over an area of several square
centimeters. Because the laser micromachining system is computer controlled and accepts both vector and raster
files, the microfluidic structure can be simple, rectilinear patterns or complex, anatomically correct patterns. The
microfluidic devices interface with simple off the shelf syringe pumps. The microfluidic devices fabricated with this
technique were used for non-invasive monitoring of flow using speckle based techniques.
We have studied the kinetics of a congruent, pixilated laser forward transfer process known as laser decal transfer (LDT). This process allows the transfer and patterning of silver nanoparticle inks such that the transferred pixels or "voxels" maintain the shape of the laser illumination. This process is capable of creating freestanding and bridging structures with near thin-film like properties.
The ability to manufacture and assemble complex three-dimensional (3D) systems via traditional photolithographic
techniques has attracted increasing attention. However, most of the work to date still utilizes the
traditional patterning and etching processes designed for the semiconductor industry where 2D structures are
first fabricated, followed by some alternative technique for releasing these structures out-of-plane. Here we
present a novel technique called Laser Origami, which has demonstrated the ability to generate 3D microstructures
through the controlled out-of-plane folding of 2D patterns. This non-lithographic, and non silicon-based
process is capable of microfabricating 3D structures of arbitrary shape and geometric complexity on a variety
of substrates. The Laser Origami technique allows for the design and fabrication of arrays of 3D microstructures,
where each microstructure can be made to fold independently of the others. Application of these folded
micro-assemblies might make possible the development of highly complex and interconnected electrical, optical
and mechanical 3D systems. This article will describe the unique advantages and capabilities of Laser Origami,
discuss its applications and explore its role for the assembly and generation of 3D microstructures.
Digital microfabrication processes are non-lithographic techniques ideally capable of directly generating patterns and
structures of functional materials for the rapid prototyping of electronic, optical and sensor devices. Laser Direct-Write
is an example of digital microfabrication that offers unique advantages and capabilities. A key advantage of laser directwrite
techniques is their compatibility with a wide range of materials, surface chemistries and surface morphologies.
These processes have been demonstrated in the fabrication of a wide variety of microelectronic elements such as
interconnects, passives, antennas, sensors, power sources and embedded circuits. Recently, a novel laser direct-write
technique able to digitally microfabricate thin film-like structures has been developed at the Naval Research Laboratory.
This technique, known as Laser Decal Transfer, is capable of generating patterns with excellent lateral resolution and
thickness uniformity using high viscosity metallic nano-inks. The high degree of control in size and shape achievable has
been applied to the digital microfabrication of 3-dimensional stacked assemblies, MEMS-like structures and freestanding
interconnects. Overall, laser forward transfer is perhaps the most flexible digital microfabrication process
available in terms of materials versatility, substrate compatibility and range of speed, scale and resolution. This paper
will describe the unique advantages and capabilities of laser decal transfer, discuss its applications and explore its role in
the future of digital microfabrication.
An integrated array computational imaging system, dubbed PERIODIC, is presented which is capable of exploiting a
diverse variety of optical information including sub-pixel displacements, phase, polarization, intensity, and
wavelength. Several applications of this technology will be presented including digital superresolution, enhanced
dynamic range and multi-spectral imaging. Other applications include polarization based dehazing, extended depth of
field and 3D imaging. The optical hardware system and software algorithms are described, and sample results are
shown.
We describe a computational imaging technique to extend the depth-of field of a 94-GHz imaging system. The
technique uses a cubic phase element in the pupil plane of the system to render system operation relatively
insensitive to object distance. However, the cubic phase element also introduces aberrations but, since these
are fixed and known, we remove them using post-detection signal processing. We present experimental results
that validate system performance and indicate a greater than four-fold increase in depth-of-field from 17" to
greater than 68".
We describe a novel technique, called laser decal transfer, for the laser forward transfer of electronic inks that allows the
non-contact direct writing of thin film-like patterns and structures on glass and plastic substrates. This technique allows
the direct printing of materials such as metallic nano-inks from a donor substrate to the receiving substrate while
maintaining the size and shape of the area illuminated by the laser transfer pulse. That is, the area of the donor substrate
or ribbon exposed to the laser pulse releases an identical area of nano-ink material which retains its shape while it
travels across the gap between the ribbon and the receiving substrate forming a deposited pattern of the same
dimensions. As a result, this technique does not exhibit the limited resolution, non-uniform thickness, irregular edge
features and surrounding debris associated with earlier laser forward transfer techniques. Continuous and uniform
metallic lines typically 5 micrometers or less in width, and a few hundred nanometers in thickness were fabricated by
laser decal transfer. These lines are of similar scale as patterns generated by lithographic techniques. Once transferred,
the lines are laser-cured in-situ using a CW laser beam, becoming electrically conductive with resistivities as low as 3.4
μΩ cm. This novel laser direct-write technique is a significant improvement in terms of quality and fidelity for directwrite
processes and offers great promise for electronic applications such as in the development, customization,
modification, and/or repair of microelectronic circuits.
The use of direct-write techniques might revolutionize the way microelectronic devices such as interconnects, passives,
IC's, antennas, sensors and power sources are designed and fabricated. The Naval Research Laboratory has developed a
laser-based microfabrication process for direct-writing the materials and components required for the assembly and
interconnection of the above devices. This laser direct-write (LDW) technique is capable of operating in subtractive,
additive, and transfer mode. In subtractive mode, the system operates as a laser micromachining workstation capable of
achieving precise depth and surface roughness control. In additive mode, the system utilizes a laser-forward transfer
process for the deposition of metals, oxides, polymers and composites under ambient conditions onto virtually any type
of surface, thus functioning as a laser printer for patterns of electronic materials. Furthermore, in transfer mode, the
system is capable of transferring individual devices, such as semiconductor bare die or surface mount devices, inside a
trench or recess in a substrate, thus performing the same function of the pick-and-place machines used in circuit board
manufacture. The use of this technique is ideally suited for the rapid prototyping of embedded microelectronic
components and systems while allowing the overall circuit design and layout to be easily modified or adapted to any
specific application or form factor. This paper describes the laser direct-write process as applied to the forward transfer
of microelectronic devices.
Laser-based direct-write (LDW) processes offer unique advantages for the transfer of unpackaged semiconductor bare die for microelectronics assembly applications. Using LDW it is possible to release individual devices from a carrier substrate and transfer them inside a pocket or recess in a receiving substrate using a single UV laser pulse, thus per-forming the same function as pick-and-place machines currently employed in microelectronics assembly. However, conventional pick-and-place systems have difficulty handling small (< 1mm2) and thin (< 100 μm) components. At the Naval Research Laboratory, we have demonstrated the laser release and transfer of intact 1 mm2 wafers with thicknesses down to 10 microns and with high placement accuracy using LDW techniques. Furthermore, given the gentle nature of the laser forward transfer process it is possible to transfer semiconductor bare die of sizes ranging from 0.5 to 10 mm2 without causing any damage to their circuits. Once the devices have been transferred, the same LDW system can then be used to print the metal patterns required to interconnect each device. The implementation of this technique is ideally suited for the assembly of microelectronic components and systems while allowing the overall circuit design and layout to be easily modified or adapted to any specific application or form factor including 3-D architectures. This paper describes how the LDW process can be used as an effective laser die transfer tool and will present analysis of the laser-driven release process as applied to various types of silicon bare dies.
In this work we describe the use of laser direct-write for the rapid prototyping of frequency selective surfaces. Frequency selective surfaces are generally described by a periodic array of conducting or dielectric features (i.e. crosses, loops, grids, etc.) that when properly designed can pass or reject specific frequency bands of incoming electromagnetic radiation. While simple frequency selective surfaces are relatively straight forward to design and fabricate, operational demands, particularly military, have motivated the design and fabrication of much more complicated patterns. These new designs combine features of significantly different length scales, randomly dithered patterns and combinations of passive and active elements. We will demonstrate how laser direct-write is an ideal tool for the rapid prototyping of these new more complicated frequency selective surface designs. We will present experimental results for devices fabricated using several different laser direct-write processes.
Diabetic retinopathy (DR) is a complication of diabetes affecting up to 80% of all diabetic patients. DR can lead to
blindness and reduced quality of life. Some authors have hypothesized that changes in the flow dynamics associated
with DR as well as changes in retinal oxygenation can lead to macular edema. Measurements of oxygen saturation in
the retina could help understand the real mechanisms behind this condition. We present a novel spectroscopic imaging
device to measure oxygen saturation in the retina. Our system uses a lenslet array to spatially and spectrocopically
divide a fundus image. A three wavelengths algorithm is used to calculate oxygen saturation in small vessels. Only
wavelengths in the 500 - 580 nm range are considered in order to minimize the wavelength dependence of the scattering
from erythrocytes. Preliminary testing on healthy subjects showed values of oxygen saturation comparable to the one
reported in the literature.
We investigate the use of a novel multi-lens imaging system in the context of biometric identification, and more
specifically, for iris recognition. Multi-lenslet cameras offer a number of significant advantages over standard
single-lens camera systems, including thin form-factor and wide angle of view. By using appropriate lenslet spacing
relative to the detector pixel pitch, the resulting ensemble of images implicitly contains subject information
at higher spatial frequencies than those present in a single image. Additionally, a multi-lenslet approach enables
the use of observational diversity, including phase, polarization, neutral density, and wavelength diversities. For
example, post-processing multiple observations taken with differing neutral density filters yields an image having
an extended dynamic range. Our research group has developed several multi-lens camera prototypes for the
investigation of such diversities.
In this paper, we present techniques for computing a high-resolution reconstructed image from an ensemble of
low-resolution images containing sub-pixel level displacements. The quality of a reconstructed image is measured
by computing the Hamming distance between the Daugman4 iris code of a conventional reference iris image,
and the iris code of a corresponding reconstructed image. We present numerical results concerning the effect of
noise and defocus blur in the reconstruction process using simulated data and report preliminary work on the
reconstruction of actual iris data obtained with our camera prototypes.
The development of embedded surface mount devices, IC's, interconnects and power source elements offers the ability to achieve levels of miniaturization beyond the capabilities of current manufacturing techniques. By burying or embedding the whole circuit under the surface, significant reduction in weight and volume can be achieved for a given circuit board design. In addition, embedded structures allow for improved electrical performance and enhanced function integration within traditional circuit board substrates. Laser-based direct-write (LDW) techniques offer an alternative for the fabrication of such embedded structures at a fraction of the cost and in less time that it would take to develop system-on-chip designs such as ASIC’s. Laser micromachining has been used in the past to machine vias and trenches on circuit board substrates with great precision, while laser forward transfer has been used to deposit patterns and multilayers of various electronic materials. At NRL, we have been exploring the use of these LDW techniques to both machine and deposit the various materials required to embed and connect individual components inside a given surface. This paper describes the materials and processes being developed for the fabrication of embedded microelectronic circuit structures using direct-write techniques alongside with an example of a totally embedded circuit demonstrated to date.
Significant reduction in weight and volume for a given circuit design can be obtained by embedding the required surface mount devices, bare die and power source elements into the circuit board. In addition, embedded structures allow for improved electrical performance and enhanced function integration within traditional circuit board substrates and non-traditional surfaces such as the external case. Laser-based direct-write techniques can be used for developing such embedded structures at a fraction of the cost and in less time that it would take to develop system-on-chip alternatives such as ASIC's. Laser micromachining has been used in the past to machine vias and trenches on circuit board substrates with great precision, while laser forward transfer has been used to deposit patterns and multilayers of various electronic materials. This paper describes recent work performed at the Naval Research Laboratory using the above laser direct-write techniques to machine the surface and deposit the materials required to embed, connect and encapsulate individual electronic components and microbatteries inside a plastic substrate.
Failure analysis has come to play a key role in ensuring quality and reliability in semiconductor devices, associated packaging and printed wiring boards. Tools are increasingly available to those investigating high-density integrated circuits at the die level, particularly for edit and repair operations. Until recently however, this capability has been limited by the inherent low-resolution mechanical/manual processes used for destructive analysis on electronics packaging. A laser-based tool has been developed to selectively and locally enable access to traces and layers within packages and provide a way to perform edits to an area of interest.
A prototype workstation has been developed that allows the fabrication of passive electronic components at low temperatures using a laser direct-write process. The work station combines a variety of laser processing techniques onto a single, integrated platform. These techniques include material deposition, laser micromachining, laser sintering, and laser trimming. One particular process, referred to as 'mill and fill', combines the laser micromachining ability of the tool with 'off-the-shelf' conductor pastes to allow the fabrication of high density metalization at very low temperatures. The present work describes the details of the 'mill and fill' process and shows examples of prototype devices fabricated using this technique.
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