Microsphere Photolithography (MPL) uses an array of self-assembled microspheres as optical elements. Flood illumination is focused to a photonic jet by each microsphere. Simulation and experiments show that photonic jet can be as small as λ/3, with collimation of more than a wavelength. This provides significant potential for pattern transfer of sub-micron patterns over large-areas and offers an inexpensive alternative to direct-write techniques such as e-beam lithography or two-photon absorption. This has applications such as SERS and SEIRA templates as well as metasurfaces to control radiation heat transfer. For these applications, the underlying substrate is important for the device performance and often presents a considerable index-contrast with the photoresist. The substrate significantly affects the behavior of the photonic jet and changes the necessary dose, minimum feature size, and morphology of the exposed area.
This paper explores the effects of the substrate on the process. Numerical models using commercial (HFSS) frequency-domain Finite Element Method (FEM) is used to simulate the interaction of light with the microsphere/photoresist/substrate. The distribution of the electric field is used to predict the exposure curve for the process. In general, metals and high index materials cause significant standing waves in the photoresist which modifies the hole morphology and ultimate feature size. These predictions are compared to i-line illuminated experiments with SEM measured hole dimensions for aluminum, germanium, and glass substrates. The objective of the paper is to establish design rules for the process which can be incorporated into the device design.
This paper investigates a filament-fed process for additive manufacturing (AM) of fused quartz. Glasses such as fused quartz have significant scientific and engineering applications, which include optics, communications, electronics, and hermetic seals. AM has several attractive benefits such as increased design freedom, faster prototyping, and lower processing costs for small production volumes. However, current research into glass AM has focused primarily on nonoptical applications. Fused quartz is studied here because of its desirability for use in high-quality optics due to its high transmissivity and thermal stability. Fused quartz filaments are fed into a CO2 laser-generated molten region, smoothly depositing material onto the workpiece. Spectroscopy and pyrometry are used to measure the thermal radiation incandescently emitted from the molten region. The effects of the laser power and scan speed are determined by measuring the morphology of single tracks. Thin walls are printed to study the effects of layer-to-layer height. This information is used to deposit solid pieces including a cylindrical-convex shape capable of focusing visible light. The transmittance and index homogeneity of the printed fused quartz are measured. These results show that the filament-fed process has the potential to print transmissive optics.
This paper describes the additive manufacturing (AM) of glass using a fiber-fed laser-heated process. Stripped SMF-28 optical fiber with a diameter of 125 μm is fed into a laser generated melt pool. A CO<sub>2</sub> laser beam is focused onto the intersection of the fiber and the work piece, which is positioned on a four-axis computer controlled stage. The laser energy at λ=10.6 μm is directly absorbed by the quartz fiber, locally heating the glass above its working point. Through the careful control of process parameters such as laser power, feed rate and scan speed, bubble free parts such as walls and lenses may be printed. These parts are assessed on the grounds of their transmissivity and refractive index homogeneity, and issues unique to the process such as the thermal breakdown of the glass and refractive index mismatch present in SMF-28 are discussed.
This paper describes the low-cost, scalable fabrication of 2D metasurface LWIR broadband polarized emitter/absorber. A Frequency Selective Surface (FSS) type design consisting of dipole antenna elements is designed for resonance in the 7.5-13 μm band. Frequency-domain Finite Element Method (FEM) is used to optimize the design with ellipsometrically measured properties. The design is synthesized to be broadband by creating a multiple cavities and by hybridizing the dipole modes with phonon resonances in a germanium/silica dielectric which separates metallic elements from a continuous ground plane. While IR metasurfaces can be readily realized using direct-write nanofabrication techniques such as E-Beam Lithography, or Focus-Ion Beam milling, or two-photon lithography, these technologies are cost-prohibitive for large areas. This paper explores the Microsphere Photolithography (MPL) technique to fabricate these devices. MPL uses arrays of self-assembled microspheres as optical elements, with each sphere focusing flood illumination to a sub-wavelength photonic jet in the photoresist. Because the illumination can be controlled over larger scales (several μm resolutions) using a conventional mask, the technique facilitates very low cost hierarchical patterning with sub-400 nm feature sizes. The paper demonstrates the fabrication of metasurfaces over 15 cm<sup>2</sup> and are measured using FTIR and imaged with a thermal camera.
Glasses including have significant scientific and engineering applications including optics, communications, electronics, and hermetic seals. This paper investigates a filament fed process for Additive Manufacturing (AM) of borosilicate glasses. Compared to soda-lime glasses, borosilicate glasses have improved coefficient of thermal expansion (CTE) and are widely used because of thermal shock resistance. In this work, borosilicate glass filaments are fed into a CO2 laser generated melt pool, smoothly depositing material onto the workpiece. Single tracks are printed to explore the effects that different process parameters have on the morphology of printed glass as well as the residual stress trapped in the glass. The transparency of glass allows residual stress to be measured using a polariscope. The effect of the substrate as well and substrate temperature are analyzed. We show that if fracture due to thermal shock can be avoided during deposition, then the residual stress can be relieved with an annealing step, removing birefringence. When combined with progress toward avoiding bubble entrapment in printed glass, we show the AM approach has the potential to produce high quality optically transparent glass for optical applications.
Frequency Selective Surfaces (FSS) are periodic array of sub-wavelength antenna elements. They allow the absorptance
and reflectance of a surface to be engineered with respect to wavelength, polarization and angle-of-incidence. This paper
applies this technique to microbolometers for uncooled infrared sensing applications. Both narrowband and broadband
near perfect absorbing surfaces are synthesized and applied engineer the response of microbolometers. The paper
focuses on simple FSS geometries (hexagonal close packed disk arrays) that can be fabricated using conventional
lithographic tools for use at thermal infrared wavelengths (feature sizes > 1 μm). The affects of geometry and material
selection for this geometry is described in detail. In the microbolometer application, the FSS controls the absorption
rather than a conventional Fabry-Perot cavity and this permits an improved thermal design. A coupled full wave
electromagnetic/transient thermal model of the entire microbolometer is presented and analyzed using the finite element
method. The absence of the cavity also permits more flexibility in the design of the support arms/contacts. This
combined modeling permits prediction of the overall device sensitivity, time-constant and the specific detectivity.
Bubble formation is a common problem in glass manufacturing. The spatial density of bubbles in a piece of glass is a
key limiting factor to the optical quality of the glass. Bubble formation is also a common problem in additive
manufacturing, leading to anisotropic material properties. In glass Additive Manufacturing (AM) two separate types of
bubbles have been observed: a foam layer caused by the reboil of the glass melt and a periodic pattern of bubbles which
appears to be unique to glass additive manufacturing. This paper presents a series of studies to relate the periodicity of
bubble formation to part scan speed, laser power, and filament feed rate. These experiments suggest that bubbles are
formed by the reboil phenomena why periodic bubbles result from air being trapped between the glass filament and the
substrate. Reboil can be detected using spectroscopy and avoided by minimizing the laser power while periodic bubbles
can be avoided by a two-step laser melting process to first establish good contact between the filament and substrate
before reflowing the track with higher laser power.
This paper reports on a system using a Digital Micromirror Device (DMD) to modulate a near-infrared laser source spatially and temporally. The DMD can produce an arbitrary heat source varying both spatially and temporally over the target. When the thermal response of the target surface is recorded using a thermal imager, this provides new possibilities in subsurface defect detection, partially with regard to features whose orientation does not allow them to be resolved using conventional thermographic inspection techniques. In this respect it is similar to conventional focused spot detection approaches; however, the DMD allows the signal to be frequency/phase multiplexed which provides for simultaneous interrogation over a large area. The parallel nature of the process permits a longer inspection time at each point which has signal-to-noise benefits. Preliminary experiments demonstrating the multiplexing approach are presented using a low-cost thermal imager. A NIR laser is spatially and temporary modulated to generated multiple thermal line sources on the surface of a composite circuit board. The infrared response is demodulated point-by-point at each drive frequency. This permits the thermal response from each line source to be resolved individually. Beyond damage detection the approach also has applications to system identification. Initial limitations due to the test setup are discussed along with future system improvements.
Glasses including fused quartz have significant scientific and engineering applications including optics, communications, electronics, and hermetic seals. This paper investigates a filament fed process for Additive Manufacturing (AM) of fused quartz. Additive manufacturing has several potential benefits including increased design freedom, faster prototyping, and lower processing costs for small production volumes. However, current research in AM of glasses is limited and has focused on non-optical applications. Fused quartz is studied here because of its desirability for high-quality optics due to its high transmissivity and thermal stability. Fused quartz also has a higher working temperature than soda lime glass which poses a challenge for AM. In this work, fused quartz filaments are fed into a CO<sub>2</sub> laser generated melt pool, smoothly depositing material onto the work piece. Single tracks are printed to explore the effects that different process parameters have on the morphology of printed fused quartz. A spectrometer is used to measure the thermal radiation incandescently emitted from the melt pool. Thin-walls are printed to study the effects of layer-to-layer height. Finally, a 3D fused quartz cube is printed using the newly acquired layer height and polished on each surface. The transmittance and index homogeneity of the polished cube are both measured. These results show that the filament fed process has the potential to print fused quartz with optical transparency and of index of refraction uniformity approaching bulk processed glass.
We design, fabricate, and characterize a frequency-selective surface (FSS) with directional thermal emission and absorption for long-wave infrared wavelengths. The FSS consists of an array of patch antennas connected by microstrips, the ensemble of which supports leaky-wave-type modes with forward and backward propagating branches. The branches are designed to intersect at 9.8 μm and have a broadside beam with 20-deg full width at half maximum at this wavelength. The absorption along these branches is near unity. Measurement of the hemispherical directional reflectometer shows good agreement with simulation. The ability to control the spectral and directional emittance/absorptance profiles of surfaces has significant applications for radiation heat transfer and sensing.
In this work, we investigate the formation of interference patterns appearing in s-NSOM results. A single nanoslit is used to demonstrate the mechanism of formation of these interference patterns experimentaly: the interaction between the in-plane component of the incident light and SPP launched by the nanoslit. This is in contrast to some other explanations that the SPP is launched from the NSOM probe. We also use an analytical model and numerical simulations to compute the formation of interference patters. This study will help to understand s-NSOM results from plasmonic nanostructures.
A circular polarized (CP) infrared (IR) leaky wave surface design is presented. The metasurface consists of an array of
rectangular patches connected by microstrip and operating over the long-wave infrared (LWIR) spectrum with
directional wave emission and absorption. The surface is composed of periodically aligned arrays of sub-wavelength
metal patches separated from a ground plane by a dielectric slab. The design combines the features of the conventional
patch and leaky wave antenna leading to a metasurface that preferentially emits CP IR radiation by use of axial
asymmetrical unit cells. This is a deviation from reported structures that mainly employ a phase shifter to combine
linearly polarized waves in order to attain circular polarization. The performance of this leaky wave surface is verified
through full-wave simulation using the ANSYS HFSS finite element analysis tool. The leaky wave phenomenon is
demonstrated by the frequency and angular dependence of the absorption while circular polarization is characterized via
stokes parameters. The main beam of this surface can be steered continuously by varying the frequency while
maintaining circular polarization within the main beam direction. A CP leaky wave at 10.6 μm with a scanning angle of
30° is demonstrated. Metasurfaces exhibiting spectral and polarization selectivity in absorption/emission hold the
potential for impact in IR applications including detection, imaging, thermal management, energy harvesting and
Apertureless scattering-type Scanning Near-field Optical Microscopy (s-SNOM) has been used to study the electromagnetic response of infrared antennas below the diffraction limit. The ability to simultaneously resolve the phase and amplitude of the evanescent field relies on the implementation of several experimentally established background suppression techniques. We model the interaction of the probe with a patch antenna using the Finite Element Method (FEM). Green's theorem is used to predict the far-field, cross-polarized scattering and to construct the homodyne amplified signal. This approach allows study of important experimental phenomena, specifically the effects of the reference strength, demodulation harmonic, and detector location.
We design, fabricate, and characterize a Frequency Selective Surface (FSS) with directional thermal emission and
absorption for long-wave infrared wavelengths (LWIR). The FSS consists of an array of patch antennas connected by
microstrips, the ensemble of which supports leaky-wave type modes with forward and backward propagating branches.
The branches are designed to intersect at 9.8 μm, and have a broadside beam with 20° FWHM at this wavelength. The absorption along these branches is near-unity. Measurement of the hemispherical directional reflectometer (HDR)
shows good agreement with simulation. The ability to control the spectral and directional emittance/absortpance profiles
of surfaces has significant applications for radiation heat transfer and sensing.
Metal-Oxide-Metal diodes offer the possibility of directly rectifying infrared radiation. To be effective for sensing or energy harvesting they must be coupled to an antenna which produces intense fields at the diode. While antennas significantly increase the effective capture area of the MOM diode, it is still limited and maximizing the captured energy is still a challenging goal. In this work we investigate integrating MOM diodes with a slot antenna Frequency Selective Surface (FSS). This maximizes the electromagnetic capture area while minimizing the transmission line length which helps reduce losses because metal losses are much lower at DC than at infrared frequencies. Our design takes advantage of a single self-aligned patterning step using shadow evaporation. The structure is optimized at 10.6 µm to have less than 2% reflection (polarization sensitive) and simulations predict that 70% of the incident energy is dissipated into the oxide layer. Initial experimental results fabricated with e-beam lithography are presented and the diode coupled FSS is shown to produce a polarization sensitive unbiased DC short circuit current. This work is promising for both infrared sensing and imaging as well as direct conversion of thermal energy.
Weak absorption of light near the absorption band edge of a photovoltaic material is one limiting factor on the efficiency of photovoltaics. This is particularly true for silicon thin-film solar cells because of the short optical path lengths and limited options for texturing the front and back surfaces. Directing light laterally is one way to increase the optical path length and absorption. We investigate the use of a periodic array of apertures originated from bowtie aperture antennas to couple incident light into guided modes supported within a thin silicon film. We show the presence of the aperture array can increase the efficiency of a solar cell by as much as 39%.
Weak absorption of light near the band gap is one limiting factor on the efficiency of photovoltaics. This is particularly
true for thin-film solar cells because the short optical path lengths and limited options for texturing the front and back
surfaces. Scattering light laterally is one way to increase the optical path length to increase the chance that a given low
energy photon is absorbed. We investigate the use of a periodic array of bowtie apertures to couple incident light to
parallel plate waveguide modes supported between two conductors. We show that this increases the efficiency of solar
cells by 39% and explain the physical mechanisms. This architecture has potential for thin film photovolatics or for
forming an intermediate conductor in multi-junction solar cells.
This work investigates fabrication of functional conductive carbon paste onto a plastic substrate using a laser. The
method allows simultaneous sintering, patterning, and functionalization of the carbon paste. Experiments are carried out
to optimize the laser processing parameters. It is shown that sheet resistance values obtained by laser sintering are close
to the one specified by the manufacturer using conventional sintering method. Additionally, a heat transfer analysis
using numerical methods is conducted to understand the relationship between the temperature during sintering and the
sheet resistance values of sintered carbon wires. The process developed in this work has the potential of producing
carbon-based electronic components on low cost plastic substrates.
Efficiently coupling far field light into a nano structure is a challenge. This limits the utility of many of the
nanophotonics devices that have been developed recently, such as waveguides carrying surface plasmon polariton modes
with subwavelength confinement and long propagation length. End fire coupling is an option but requires careful
alignment and may not be suitable for exciting densely packed arrays of waveguides. This paper investigates the use of
a nanoscale bowtie aperture as an antenna for directly coupling far field light into a nanoscale plasmonic waveguide.
The paper shows that when the aperture is designed to be resonant the coupling coefficient can be as high as 600%
relevant to the energy incident on the open area of the aperture.
We investigate fabrication of functional conductive carbon paste onto a plastic substrate using a laser. The method allows simultaneous sintering, patterning, and functionalization of the carbon paste. Experiments are carried out to optimize the laser-processing parameters. It is shown that sheet resistance values obtained by laser sintering are close to the one specified by the manufacturer using the conventional sintering method. Additionally, a heat transfer analysis using numerical methods is conducted to understand the relationship between the temperature during sintering and the sheet resistance values of sintered carbon wires. The process developed has the potential of producing carbon-based electronic components on low-cost plastic substrates.
Nanoscale apertures, especially high transmission nanoantennas, have been shown to have great potential for
nanolithography as well as near-field measurements. This paper describes a method for mapping the near-field
distribution as a function of distance from the aperture surface. The measurements are performed using a home-built
near-field optical microscopy (NSOM) with home-made aperture probes. The force distance curve is used to determine
the tip-sample distance. The calibrated NSOM system is then used to correlate the collected near-field optical
distribution data with the distance from the surface. The in-plane optical images are obtained at a constant height by
turning off the vertical position feedback. The experiment results show that this is a potential method to obtain three
dimensional optical measurements of nano structures.