Kristin M. Charipar

Electrowetting Displays Utilizing Bistable, Multi-Color Pixels Via Laser Processing

Electronic paper, or e-Paper, for use in displays has seen rapid growth in the past decade because of its potential as an alternative to traditional transmissive displays. Offering several critical advantages over current display technologies, including high contrast in direct sunlight, wide viewing angles, and compatibility with flexible substrate processing, electrowetting displays (EWDs) have made it to the forefront of e-Paper research and development efforts. Here, we describe a new design for the fabrication of multi-color, bistable electrowetting displays. Using a laser-based process to pattern an in-plane electrode design, liquid can be manipulated out-of-plane. This process relies on electromechanical pressure forcing water in and out of channels, causing colored oil to be displaced. When voltage is removed, the oil remains in its current position, resulting in bistability. We have demonstrated multi-color, bistable pixels that maintain their state at V = 0 for several days, which drastically reduces the power required to drive the display.

Laser embedding electronics on 3D printed objects

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.

Laser processing of 2D and 3D metamaterial structures

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-induced forward transfer (LIFT) of 3D microstructures

The development of rapid prototyping techniques for the fabrication of microelectronic structures has seen rapid growth over the past decade. In particular, laser-induced forward transfer (LIFT) is a non-lithographic direct-write technique that offers the advantages of high speed / throughput, high resolution, materials versatility, and substrate compatibility. Because of the high degree of control over size and shape of printed material, the development of a wide range of microelectronic components, including interconnects, antennas, and sensors, has become possible using LIFT. In this paper, we explore the use of LIFT to print various 3D microstructures including high aspect ratio micro pillars using high viscosity Ag nanopastes. In addition, we demonstrate the fabrication of interconnects via LIFT on RF switches that, after printing and subsequent curing, perform similarly to an analogous wire-bonded device.

Observation of the out-of-plane magnetization in a mesoscopic ferromagnetic structure superjacent to a superconductor

The geometry of magnetic flux penetration in a high temperature superconductor at a buried interface was imaged using element-specific x-ray excited luminescence. We performed low temperature observation of the flux penetration in YBa2Cu3O7–δ (YBCO) at a buried interface by imaging of the perpendicular magnetization component in square Permalloy (Py) mesostructures patterned superjacent to a YBCO film. Element specific imaging below the critical temperature of YBCO reveals a cross-like geometry of the perpendicular magnetization component which is decorated by regions of alternating out-of-plane magnetization at the edges of the patterned Py structures. The cross structure can be attributed to the geometry of flux penetration originating from the superconductor and is reproduced using micromagnetic simulations. Our experimental method opens up possibilities for the investigation of flux penetration in superconductors at the nanoscale.The geometry of magnetic flux penetration in a high temperature superconductor at a buried interface was imaged using element-specific x-ray excited luminescence. We performed low temperature observation of the flux penetration in YBa2Cu3O7–δ (YBCO) at a buried interface by imaging of the perpendicular magnetization component in square Permalloy (Py) mesostructures patterned superjacent to a YBCO film. Element specific imaging below the critical temperature of YBCO reveals a cross-like geometry of the perpendicular magnetization component which is decorated by regions of alternating out-of-plane magnetization at the edges of the patterned Py structures. The cross structure can be attributed to the geometry of flux penetration originating from the superconductor and is reproduced using micromagnetic simulations. Our experimental method opens up possibilities for the investigation of flux penetration in superconductors at the nanoscale.

Laser processing of VO2 thin films for THz devices and metamaterials

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.

Harnessing the metal-insulator transition for tunable metamaterials

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.