Substrate Lens Research Articles

A Self-adapting Pixel Antenna - Substrate Lens System for Infrared Frequencies

In this work, we propose a concentrator lens - antenna arrangement, potentially suitable for infrared sensing and energy harvesting rectenna systems. The structure consists of a pixel antenna, self-adapting for the direction of the incident infrared radiation, and a concentrator lens, both optimized for the mid-infrared spectrum. The silicon substrate lens is situated above the pixel antenna, enabling the concentration of the incident light on the antenna; silicon was chosen due to its transparency in the IR spectrum. We examine how various parameters of the lens, in conjunction with the different states of the pixel antenna, affects the performance of the system. The simulations show that the gain of the arrangement increases considerably in correlation with the radius of the lens. The results suggest that the energy conversion efficiency of infrared rectenna systems can be enhanced by several orders of magnitude with the utilization of the proposed arrangement.

Superstrate-lens integration using paraffin wax on top of semiconductor-based THz detector chips

A detector of electromagnetic radiation may benefit – if its sensor area is small – from the application of a substrate lens, which focusses the radiation onto the active sensing area of the device and thus enhances its responsivity. The use of such a lens, attached directly onto the detector backside in order to avoid reflection losses, requires that the detector substrate and backside be transparent to the radiation. However, if this is not the case, one may like to place instead a superstrate lens onto the front side of the detector. It may even be of interest to use both a substrate and a superstrate lens if the detector needs to be illuminated with two beams, e.g. for heterodyne detection, where one beam provides the local-oscillator signal. The use of a superstrate lens is, however, often hindered or impeded by an uneven surface topography or by the presence of bonding wires on the front side of the detector. Here, we address this issue and explore the use of paraffin wax to form or attach superstrate lenses. In the first case, which is the main topic of this contribution, we exploit the surface tension of liquid paraffin, brought onto the detector, to sculpt the wax itself into a lens. In the second case, only addressed conceptually here, we use paraffin to form a thin intermediate layer which also acts as an adhesive for the attachment of a plastic or silicon lens. In both cases, the application of liquid paraffin allows one to fill out an uneven detector surface and to embed wires without breaking them. We investigate the use of wax for the case of CMOS TeraFETs – detectors of terahertz radiation based on field-effect transistors – embedded into antenna structures. We describe the processing steps and analyze the performance of a TeraFET equipped with such a wax superstrate lens for front-side beam coupling.

Inkjet Printing of Pharmaceuticals.

Inkjet printing (IJP) is an additive manufacturing process that selectively deposits ink materials, layer-by-layer, to create 3D objects or 2D patterns with precise control over their structure and composition. This technology has emerged as an attractive and versatile approach to address the ever-evolving demands of personalized medicine in the healthcare industry. Although originally developed for nonhealthcare applications, IJP harnesses the potential of pharma-inks, which are meticulously formulated inks containing drugs and pharmaceutical excipients. Delving into the formulation and components of pharma-inks, the key to precise and adaptable material deposition enabled by IJP is unraveled. The review extends its focus to substrate materials, including paper, films, foams, lenses, and 3D-printed materials, showcasing their diverse advantages, while exploring a wide spectrum of therapeutic applications. Additionally, the potential benefits of hardware and software improvements, along with artificial intelligence integration, are discussed to enhance IJP's precision and efficiency. Embracing these advancements, IJP holds immense potential to reshape traditional medicine manufacturing processes, ushering in an era of medical precision. However, further exploration and optimization are needed to fully utilize IJP's healthcare capabilities. As researchers push the boundaries of IJP, the vision of patient-specific treatment is on the horizon of becoming a tangible reality.

600-GHz Fourier imaging based on heterodyne detection at the 2nd sub-harmonic.

Fourier imaging is an indirect imaging method which records the diffraction pattern of the object scene coherently in the focal plane of the imaging system and reconstructs the image using computational resources. The spatial resolution, which can be reached, depends on one hand on the wavelength of the radiation, but also on the capability to measure - in the focal plane - Fourier components with high spatial wave-vectors. This leads to a conflicting situation at THz frequencies, because choosing a shorter wavelength for better resolution usually comes at the cost of less radiation power, concomitant with a loss of dynamic range, which limits the detection of higher Fourier components. Here, aiming at maintaining a high dynamic range and limiting the system costs, we adopt heterodyne detection at the 2nd sub-harmonic, working with continuous-wave (CW) radiation for object illumination at 600 GHz and local-oscillator (LO) radiation at 300 GHz. The detector is a single-pixel broad-band Si CMOS TeraFET equipped with substrate lenses on both the front- and backside for separate in-coupling of the waves. The entire scene is illuminated by the object wave, and the Fourier spectrum is recorded by raster scanning of the single-detector unit through the focal plane. With only 56 µW of power of the 600-GHz radiation, a dynamic range of 60 dB is reached, sufficient to detect the entire accessible Fourier space spectrum in the test measurements. We present a detailed comparison between plane-to-plane imaging and Fourier imaging, and show that, with both, a lateral spatial resolution of better than 0.5 mm, at the diffraction limit, is reached.

The Design Methods and Experiments for a 220-GHz Quasi-Optical Cryogenic Schottky Subharmonic Mixer of High Performance

In this article, we present systematic design methods and experimental verifications of a 220-GHz quasi-optical cryogenic Schottky subharmonic mixer. To maximize device performance over a wide bandwidth and overcome the difficulties in conjugate matching designs, an impedance optimization-based cooperative design method is proposed for such quasi-optical mixer that includes the on-chip antenna and heterodyne mixing circuit. A coplanar waveguide (CPW) tuning network is utilized and optimized to offset the parasitic reactance effect of Schottky diodes. Besides, a dual-band log-periodic-slot integrated lens antenna is presented for coupling the radio frequency and local oscillator radiations with high efficiency. A reversed field tracing method is proposed to acquire the position and size of the equivalent Gaussian waists at both frequencies, thus offering the guidance for optimizing the substrate thickness and lens dimensions for improved antenna gain and radiation pattern. A 220-GHz quasi-optical mixer prototype was fabricated and experimentally verified at both room and cryogenic temperatures. At 134 K, the single-sideband noise factor and the conversion loss are around 4.2–8.5 and 6–11 dB over a wide intermediate-frequency bandwidth of 0.5–20 GHz, showing the superior device performance at the comparable bands.

High gain Vivaldi antenna from 26 up to 40 GHz for 5G applications

AbstractWe propose a directive broadband and high gain Vivaldi antenna in the Ka‐band (26–40 GHz) for 5G applications. To increase the gain of this antenna, we use a substrate lens and the quasi‐substrate integrated waveguide structure with the aim of reducing reflected waves and emphasizing the traveling waves. With this method, the gain is increased by 4.5 dBi in average and 7.5 dBi in the peak. The antenna has a good impedance matching in the whole band.

Infrared triggered smart contact lens for the treatment of presbyopia

A switchable contact lens prototype was fabricated and tested with integrated off the shelf electronic components to drive a liquid crystal (LC) active lens element. This prototype was capable of changing its focal power by an average of +1.9D, but a maximum of +3.2 ± 0.2D was also measured. Switchable focus contact lenses are intended to help restore functional near and intermediate vision accommodation to those suffering from presbyopia, an inevitable age-related eye condition. The custom poly-methyl-methacrylate contact lens substrates used in this prototype are equivalent to commercially available scleral contact lenses. It was discovered that more careful design considerations are needed when at least one of the substrates is <100 µm thick (which is needed for a final device). Without these design considerations, the switchable focal power of the lens is susceptible to change as the LC insert layer is able to flex. The prototype has an on-board electronics platform which was self-powered with a battery. Illuminating the electronics package with a 600 Hz infrared signal switched the device on, which passed an AC voltage to the electrodes of the contact lens, reorientating the director of the LC with an electric field. To our knowledge, this is the first demonstration of a battery powered and wirelessly triggered smart contact lens for the treatment of presbyopia.

Degrees of peace: universities and embodied experiences of conflict in post-war Sri Lanka

This paper examines the impacts of conflicts on universities and considers the corresponding implications for their ability to contribute to post-war recovery. Pushing against the methodological individualism associated with notions of human capital loss, I concentrate on the interaction between conflict and the social constitution of universities. I argue that the ways in which universities as social groups embody experiences of conflict can powerfully influence how they operate and how they interact with post-war peacebuilding and development. To operationalise the framing of universities as social groups, I introduce the concept of a university substrate as a means of thinking through the evolving constitution of universities. Drawing on 31 semi-structured interviews with university actors in Jaffna and elsewhere in Sri Lanka, I explore the case of the University of Jaffna and how it has been shaped by conflict. I outline the moulding of its social constitution by waves of departure during the conflict and by the environment created by the Liberation Tigers of Tamil Eelam, the main non-state actor in the war. This case study illustrates how the substrate lens can capture dynamics that are missed by human capital approaches and can help better explain the legacies of conflict for universities.

Development and Implementation of a Rotating Nanoimprint Lithography Tool for Orthogonal Imprinting on Edges of Curved Surfaces

Uniform molding and demolding of structures on highly curved surfaces through conformal contact is a crucial yet often-overlooked aspect of nanoimprint lithography (NIL). This study describes the development of a NIL tool and its integration into a nanopositioning and nanomeasuring machine to achieve high-precision orthogonal molding and demolding for soft ultraviolet-assisted NIL (soft UV-NIL). The process was implemented primarily on the edges of highly curved plano-convex substrates to demonstrate structure uniformity on the edges. High-resolution nanostructures of sub-200-nm lateral dimension and microstructures in the range of tens of microns were imprinted. However, the nanostructures on the edges of the large, curved substrates were difficult to characterize precisely. Therefore, microstructures were used to measure the structure fidelity and were characterized using profilometry, white light interferometry, and confocal laser scanning microscopy. Regardless of the restricted imaging capabilities at high inclinations for high-resolution nanostructures, the scanning electron microscope (SEM) imaging of the structures on top of the lens substrate and at an inclination of 45° was performed. The micro and nanostructures were successfully imprinted on the edges of the plano-convex lens at angles of 45°, 60°,and 90° from the center of rotation of the rotating NIL tool. The method enables precise imprinting at high inclinations, thereby presenting a different approach to soft UV-NIL on curved surfaces.

Point absorbers in Advanced LIGO.

Small, highly absorbing points are randomly present on the surfaces of the main interferometer optics in Advanced LIGO. The resulting nanometer scale thermo-elastic deformations and substrate lenses from these micron-scale absorbers significantly reduce the sensitivity of the interferometer directly though a reduction in the power-recycling gain and indirect interactions with the feedback control system. We review the expected surface deformation from point absorbers and provide a pedagogical description of the impact on power buildup in second generation gravitational wave detectors (dual-recycled Fabry-Perot Michelson interferometers). This analysis predicts that the power-dependent reduction in interferometer performance will significantly degrade maximum stored power by up to 50% and, hence, limit GW sensitivity, but it suggests system wide corrections that can be implemented in current and future GW detectors. This is particularly pressing given that future GW detectors call for an order of magnitude more stored power than currently used in Advanced LIGO in Observing Run 3. We briefly review strategies to mitigate the effects of point absorbers in current and future GW wave detectors to maximize the success of these enterprises.

Formation of Li2CO3 Nanostructures for Lithium-Ion Battery Anode Application by Nanotransfer Printing.

Various high-performance anode and cathode materials, such as lithium carbonate, lithium titanate, cobalt oxides, silicon, graphite, germanium, and tin, have been widely investigated in an effort to enhance the energy density storage properties of lithium-ion batteries (LIBs). However, the structural manipulation of anode materials to improve the battery performance remains a challenging issue. In LIBs, optimization of the anode material is a key technology affecting not only the power density but also the lifetime of the device. Here, we introduce a novel method by which to obtain nanostructures for LIB anode application on various surfaces via nanotransfer printing (nTP) process. We used a spark plasma sintering (SPS) process to fabricate a sputter target made of Li2CO3, which is used as an anode material for LIBs. Using the nTP process, various Li2CO3 nanoscale patterns, such as line, wave, and dot patterns on a SiO2/Si substrate, were successfully obtained. Furthermore, we show highly ordered Li2CO3 nanostructures on a variety of substrates, such as Al, Al2O3, flexible PET, and 2-Hydroxylethyl Methacrylate (HEMA) contact lens substrates. It is expected that the approach demonstrated here can provide new pathway to generate many other designable structures of various LIB anode materials.

Integrated contact lens sensor system based on multifunctional ultrathin MoS2 transistors

Smart contact lenses attract extensive interests due to their capability of directly monitoring physiological and ambient information. However, previous demonstrations usually lacked efficient sensor modalities, facile fabrication process, mechanical stability, or biocompatibility. Here, we demonstrate a flexible approach for fabrication of multifunctional smart contact lenses with an ultrathin MoS2 transistors-based serpentine mesh sensor system. The integrated sensor systems contain a photodetector for receiving optical information, a glucose sensor for monitoring glucose level directly from tear fluid, and a temperature sensor for diagnosing potential corneal disease. Unlike traditional sensors and circuit chips sandwiched in the lens substrate, this serpentine mesh sensor system can be directly mounted onto the lenses and maintain direct contact with tears, delivering high detection sensitivity, while being mechanically robust and not interfering with either blinking or vision. Furthermore, the in vitro cytotoxicity tests reveal good biocompatibility, thus holding promise as next-generation soft electronics for healthcare and medical applications.

A wearable contact lens sensor for noninvasive in-situ monitoring of intraocular pressure

Glaucoma is the second leading cause of blindness in the world. Intraocular pressure (IOP) is a primary indicator of glaucoma which can be measured for the treatment of the disease. This paper presents a piezo-resistive principle pressure sensor to monitor IOP continuously and non-invasively. The sensor is designed based on the Wheatstone bridge circuit and fabricated by the spray-coating method. The hybrid nanomaterials of graphene and carbon nanotubes are introduced as sensing layers which are embedded inside the soft contact lens substrate composed of flexible polydimethyl siloxane (PDMS) and parylene. The sensing performance is discussed followed by a brief description of our sensor design and fabrication. Tests on a PDMS eyeball model indicate that it has a high sensitivity of 36.01 μV mmHg−1. Also, the frequency response and the ability to track dynamic pressure change cycles are demonstrated in normal IOP variation range from 9 to 34 mmHg. It shows good repeatability and linearity, and can accurately track fluctuating IOP. Thus, this sensor, with its ease of fabrication and simple design, as well as allowance for continuous pressure measurement, offers a promising approach for IOP monitoring in clinical diagnosis of glaucoma.

Novel e-SIL with embedded cone structure for the enhancement of photon collection efficiency from nitrogen–vacancy center in diamond

We present the design and theoretical simulation of a nano-range optical structure with an elliptical solid immersion lens (e-SIL) in a rectangular diamond substrate containing nitrogen–vacancy (NV) center in diamond. We have varied the numerical aperture of the objective lens and collected photons with a maximum collection efficiency of ∼88% for NA=1. The far field variations have been studied over a wide range of emission spectra of nitrogen–vacancy center in diamond. This highly efficient single-photon emitter has a wide range of applications in the field of nanoscale magnetic sensing and quantum information devices.

A Comparative Study of Adhesion Evaluation Methods on Ophthalmic AR Coating Lens

Ophthalmic resin lenses are widely used to correct myopia and defend harmful light waves. Ophthalmic lens with anti-reflective (AR) coating has become the mainstream product in the lens market. The AR coating is composed by inorganic metal oxides, which is very different to the organic lens substrate in thermal expansion coefficients. In a normal wearing environment, coating delaminating often occurs resulting that AR function is disabled. How to evaluate adhesion of the AR coating is important. In this paper, a specially designed cutting tool was used to scratch two grids on each surface of the lens. The peel off operation was carried out with the tape within specified adhesion range. The coating detachment was evaluated by visual inspection and microscopy based on the methods defined in ISO 2409 and GB 10810.4, the applicability was compared and discussed.

Thermal lensing effects and nonlinear refractive indices of fluoride crystals induced by high-power ultrafast lasers.

Thermo-optical and nonlinear property characterization of refractive optical components is essential for endoscopic instrumentation that utilizes high-power, high-repetition-rate ultrafast lasers. For example, ytterbium-doped fiber lasers are well suited for ultrafast laser microsurgery applications; however, the thermo-optical responses of many common lens substrates are not well understood at 1035 nm wavelength. Using a z-scan technique, we first measured the nonlinear refractive indices of CaF2, MgF2, and BaF2 at 1035 nm and found values that match well with those from the literature at 1064 nm. To elucidate effects of thermal lensing, we performed z-scans at multiple laser repetition rates and multiple average powers. The results showed negligible thermal effects up to an average power of 1 W and at 10 W material-specific thermal lensing significantly altered z-scan measurements. Using a 2D temperature model, we could determine the source of the observed thermal lensing effects. Linear absorption was determined as the main source of heating in these crystals. On the other hand, inclusion of nonlinear absorption as an additional heat source in the simulations showed that thermal lensing in borosilicate glass was strongly influenced by nonlinear absorption. This method can potentially provide a sensitive method to measure small nonlinear absorption coefficients of transparent optical materials. These results can guide design of miniaturized optical systems for ultrafast laser surgery and deep-tissue imaging probes.

Electronic Contact Lens: A Platform for Wireless Health Monitoring Applications

Electronic contact lenses are used for noninvasively monitoring vital human signs and medical parameters. However, maintaining a secure communications connection and a self‐sustainable power source are still looming challenges. Herein, a proof‐of‐concept electronic contact lens is demonstrated that includes a spiral antenna with its wireless circuit unit for data telemetry, a rectifier circuit for power conditioning, and a micro‐light‐emitting diode (μLED) as a load. The spiral antenna with its rectifying circuit is designed considering operation in the industrial, scientific, and medical (ISM) band of 2.4 GHz. The spiral coil with an inner diameter of 10 mm, an outer diameter of 12 mm, and a wire width of 0.2 mm is fabricated on a donut‐shaped flexible polyimide substrate. For biocompatibility purposes, polyimide is used as the contact lens substrate and polydimethylsiloxane (PDMS) is used for encapsulation. A 3D‐printed eye model is developed for accurately shaping the curvature of the PDMS‐encapsulated contact lens. The reflection coefficient (S11) of the fabricated antenna is tested in different conditions and on an eye model to mimic the liquid condition of the human eye. In a wide range of conditions, a minimum of −20 dB reflection coefficient (S11) is obtained.

Uniform and bright light emission from a 3D organic light-emitting device fabricated on a bi-convex lens by a vortex-flow-assisted solution-coating method

We herein present the results of a study on the novel fabrication process of uniform and homogeneous semiconducting polymer layers, in this case hole-injecting and fluorescent light-emitting layers that were produced by a simple solution-coating process for 3D conformal organic light-emitting diodes (3D OLEDs) on curvilinear surfaces. The solution-coating process used was a newly developed method of vortex-flow-assisted solution-coating with the support of spinning of the coating solution. It is shown that the vortex-flow-assisted spin-coating process can produce high-quality thin films at nanoscale thicknesses by controlling the liquid surface of the coating solutions, which can easily be adjusted by changing the spinning speed, even on complex curvilinear surfaces, i.e., a quasi-omnidirectional coating. This excellent film-forming ability without any serious film defects is mainly due to the reduction of line tension among the solution, air, and the substrate at the contact line due to vortex flows of the coating solution on the substrate during the vortex-spin-coating process. As a proof of concept, we present vortex-spin-coated 3D OLEDs fabricated on bi-convex lens substrates which exhibit excellent device performance with high brightness and current efficiency levels comparable to those of a conventional spin-coated 2D planar OLED on a flat substrate. It is also shown that the EL emission from the 3D OLED on the bi-convex lens substrate exhibits a diffusive Lambertian radiation pattern. The results here demonstrate that the vortex-flow-assisted spin-coating process is a promising approach for producing efficient and reliable next-generation OLEDs for 3D conformal opto-electronics.

Study on Infrared Transmittance of Si-Polymer Hybrid Structure Press Molded Using a Coupling Agent

Hybrid structures of single-crystal silicon and high-density polyethylene (HDPE) with high transmittance in the mid-to-far infrared region are used as infrared lens substrates. The hybrids are usually fabricated by high-precision press molding. The Si-HDPE hybrid lens previously fabricated had a low transmittance in the 9–10 μm wavelength region, thereby limiting its application for human body detection. In this study, a Si-polymer hybrid structure was fabricated using a new polymer without any silane coupling agent. Interfacial adhesion between the polymer and the Si substrate was realized with an extremely thin (a few micron thick) layer of an interfacial silane coupling agent. The press molding conditions that led to improved bonding strength and infrared transmittance of the hybrid substrate were investigated. A transmittance similar to that of a single-crystal Si substrate was achieved in the 9–10 μm wavelength range.

A High-Sensitivity AlGaN/GaN HEMT Terahertz Detector With Integrated Broadband Bow-Tie Antenna

Many emerging applications in the terahertz (THz) frequency range demand highly sensitive, broadband detectors for room-temperature operation. Field-effect transistors with integrated antennas for THz detection (TeraFETs) have proven to meet these requirements, at the same time offering great potential for scalability, high-speed operation, and functional integrability. In this contribution, we report on an optimized field-effect transistor with integrated broadband bow-tie antenna for THz detection (bow-tie TeraFET) and compare the detector's performance to other state-of-the-art broadband THz detector technologies. Implemented in a recently developed AlGaN/GaN MMIC process, the presented TeraFET shows a more than twice performance improvement compared to previously fabricated AlGaN/GaN-HEMT-based TeraFETs. The detector design is the result of detailed modeling of the plasma-wave-based detection principle embedded in a full-device detector model to account for power coupling of the THz radiation to the intrinsic gated FET channel. The model considers parasitic circuit elements as well as the high-frequency impedance of the integrated broadband antenna, and also includes optical losses from a silicon substrate lens. Calibrated characterization measurements have been performed at room temperature between 490 and 645 GHz, where we find values of the optical (total beam power referenced) noise-equivalent power of 25 and ${\text{31 pW}}/\surd{\text{Hz}}$ at 504 and 600 GHz, respectively, in good agreement with simulation results. We then show the broadband detection capability of our AlGaN/GaN detectors in the range from 0.2 to 1.2 THz and compare the TeraFETs’ signal-to-noise ratio to that of a Golay cell and a photomixer. Finally, we demonstrate an imaging application in reflection geometry at 504 GHz and determine a dynamic range of >40 dB.