The attachment of dopant precursor molecules to depassivated areas of hydrogen-terminated silicon templated with a scanning tunneling microscope (STM) has been used to create electronic devices with subnanometer precision, typically for quantum physics experiments. This process, which we call atomic precision advanced manufacturing (APAM), dopes silicon beyond the solid-solubility limit and produces electrical and optical characteristics that may also be useful for microelectronic and plasmonic applications. However, scanned probe lithography lacks the throughput required to develop more sophisticated applications. Here, we demonstrate and characterize an APAM device workflow where scanned probe lithography of the atomic layer resist has been replaced by photolithography. An ultraviolet laser is shown to locally and controllably heat silicon above the temperature required for hydrogen depassivation on a nanosecond timescale, a process resistant to under- and overexposure. STM images indicate a narrow range of energy density where the surface is both depassivated and undamaged. Modeling that accounts for photothermal heating and the subsequent hydrogen desorption kinetics suggests that the silicon surface temperatures reached in our patterning process exceed those required for hydrogen removal in temperature-programmed desorption experiments. A phosphorus-doped van der Pauw structure made by sequentially photodepassivating a predefined area and then exposing it to phosphine is found to have a similar mobility and higher carrier density compared with devices patterned by STM. Lastly, it is also demonstrated that photodepassivation and precursor exposure steps may be performed concomitantly, a potential route to enabling APAM outside of ultrahigh vacuum.
Degraded visual environments are a serious concern for modern sensing and surveillance systems. Fog is of interest due to the frequency of its formation along our coastlines disrupting border security and surveillance. Fog presents hurdles in intelligence and reconnaissance by preventing data collection with optical systems for extended periods. We will present recent results from our work in operating optical systems in our controlled fog experimental chamber. This facility is a 180-foot-long, 10-foot-wide, and 10-foot-tall structure that has over 60 spray nozzles to achieve uniform aerosol coverage with various particle size, distributions, and densities. We will discuss the physical formation of fog in nature and how our generated fog compares. In addition, we will discuss fog distributions and characterization techniques. We will investigate the biases of different methods and discuss the different techniques that are appropriate for realistic environments. Finally, we will compare the data obtained from our characterization studies against accepted models (e.g., MODTRAN) and validate the usage of this unique capability as a controlled experimental realization of natural fog formations. By proving the capability, we will enable the testing and validation of future fog penetrating optical systems and providing a platform for performing optical propagation experimentation in a known, stable, and controlled environment.
Scattering environment conditions, such as fog, pose a challenge for many detection and surveillance active sensing operations in both ground and air platforms. For example, current autonomous vehicles rely on a range of optical sensors that are affected by degraded visual environments. Real-world fog conditions can vary widely depending on the location and environmental conditions during its creation. In our previous work we have shown benefits for increasing signal and range through scattering environments such as fog utilizing polarized light, specifically circular polarization. In this work we investigate the effect of changing fog particle sizes and distributions on polarization persistence for both circularly and linearly polarized light via simulation. We present polarization tracking Monte Carlo results for a range of realistic monodisperse particle sizes as well as varying particle size distributions as a model of scattering environments. We systematically vary the monodisperse particle size, mean particle size of a distribution, particle size distribution width, and number of distribution lobes (bi-modal), as they affect polarized light transmission through a scattering environment. We show that circular polarization signal persists better than linear polarization signal for most variations of the particle distribution parameters.
We present experimental and simulation results for a laboratory-based forward-scattering environment, where 1 μm diameter polystyrene spheres are suspended in water to model the optical scattering properties of fog. Circular polarization maintains its degree of polarization better than linear polarization as the optical thickness of the scattering environment increases. Both simulation and experiment quantify circular polarization’s superior persistence, compared to that of linear polarization, and show that it is much less affected by variations in the field of view and collection area of the optical system. Our experimental environment’s lateral extent was physically finite, causing a significant difference between measured and simulated degree of polarization values for incident linearly polarized light, but not for circularly polarized light. Through simulation we demonstrate that circular polarization is less susceptible to the finite environmental extent as well as the collection optic’s limiting configuration.
We present simulation results that show circularly polarized light persists through scattering environments better than linearly polarized light. Specifically, we show persistence is enhanced through many scattering events in an environment with a size parameter representative of advection fog at infrared wavelengths. Utilizing polarization tracking Monte Carlo simulations we show a larger persistence benefit for circular polarization versus linear polarization for both forward and backscattered photons. We show the evolution of the incident polarization states after various scattering events which highlight the mechanism leading to circular polarization’s superior persistence.
We present both simulation and experimental results showing that circularly polarized light maintains its degree of
polarization better than linearly polarized light in scattering environments. This is specifically true in turbid
environments like fog and clouds. In contrast to previous studies that propagate single wavelengths through broad
particle-size distributions, this work identifies regions where circular polarization persists further than linear by
systematically surveying different wavelengths through monodisperse particle diameters. For monodisperse polystyrene
microspheres in water, for particle diameters of 0.99 and 1.925 microns and varying optical depths, we show that circular
polarization’s ability to persist through multiple scattering events is enhanced by as much as a factor of four, when
compared to that of linear polarization. These particle diameters correspond to size parameters found for infrared
wavelengths and marine and continental fog particle distributions. The experimental results are compared to Monte
Carlo simulations for all scattering environments investigated.
We find for infrared wavelengths there are clear particle size ranges and indices representative of fog and rain where the use of circular polarization imaging can penetrate to larger optical depths than linear polarization. Using polarization tracking Monte Carlo simulations for varying particle size, wavelength, and index systematically, we show that for specific scene parameters circular polarization vastly outperforms linear polarization in maintaining degree of polarization for large optical depths in transmission and reflection. This enhancement in circular polarization can be exploited to improve imaging in obscurant environments that are important in many critical imaging applications. Specifically, circular polarization performs better than linear for radiation fog in the SWIR and MWIR regime, advection fog in the LWIR regime, and small sized particles of Sahara dust in the MWIR regime.
Conductive polymers with high solids loading (> 40wt.%) are challenging to pattern to single micron feature sizes and require unique techniques or templates to mold the material. The development of a conductive polymer optical tag is discussed for identifying the presence of hydrofluoric acid (HF) and leverages free standing silicon fins as a template utilizing deep reactive ion etching (DRIE) techniques will be discussed. This work is aimed towards a future flexible conductive polymer tag to be transferred via adhesive or epoxy for a novel sensor surface. The advantage to this technique over wafer thinning is a higher throughput of device manufacture without damage to the silicon fins or polymer due to chemical-mechanical interactions or added protective layers. The gratings consist of a high spatial frequency (1.15 μm pitch) grating consisting of lines of conductive polymer and lines of silicon which are free standing. A novel running bond pattern aims to minimize the intrinsic stress and allows the conductive polymer to infiltrate without distorting the template. The polymer conductivity mechanism has been designed to break down under a chemical binding to fluorine; changing its conductivity upon exposure, and results in a change in the polarization response. The use of the polarization response makes the signal more robust to intensity fluctuations in the background or interrogation system. Additionally, the use of optical interrogation allows for standoff detection in instances where hazardous conditions may be present. Examples of material and device responses will be shown and directions for further investigation are discussed.
Designing and integrating micro-optical components into atom and ion traps are enabling steps toward miniaturizing
trap dimensions in quantum computation applications. The micro-optic must have a high numerical aperture for precise
illumination of the ion and should not introduce scatter. Due to the extreme optical efficiency requirements in trapped
ion and atom-based quantum information processing, even slight losses from integrated micro-optics are detrimental.
We have designed and fabricated aspheric micro-lenses through grayscale transfer into a fused silica in an effort to
realize increased transmissive efficiency and decreased scatter compared to an equivalent diffractive optical element.
The fabricated grayscale lens profile matched the desired lens profile well, and the measured and predicted optical
performances were in good agreement. The pattern was transferred via coupled plasma reactive-ion etching smoothly
into the fused silica with a RMS roughness ~ 35 nm. The micro-lens had a diameter of 88 um and 14.2 um sag, with an
as-designed focal length of 149 um and spot diameter of 2.6 um. The maximum measured efficiency was ~80% (86% of
theoretical, possibly due to rms roughness). This realized efficiency is superior to the equivalent diffractive lens
efficiency, designed to the same use parameters. The grayscale approach demonstrated an increase in collection
efficiency, at the desired optical focal length, providing the potential for further refinement.
We explore the spectral and angular selectivity of near surface normal transmission of grating modified metallic surfaces
and their ultimate potential for application as narrow-band spectro-polarimetric planar filter components in the
development of advanced infrared focal plane arrays. The developed photonic microstructures exhibit tailored spectral
transmission characteristics in the long wavelength infrared, and can be fabricated to preferentially transmit a given
linear polarization within the design band. Modification of the material and structural properties of the diffractive optical
element enables sub-pixel tuning of the spectro-polarimetric response of the device allowing for intelligent engineering
of planar filter components for development of advanced focal plane arrays in the long wavelength infrared. The planar
nature of the developed components leaves them immune to fabrication issues that typically plague thin film interference
filters used for similar applications in the infrared, namely, deposition of multiple low-stress quarter-wavelength films
and modification of the film thicknesses for each pixel. The solution developed here presents the opportunity for subpixel
modification of the spectral response leading to an efficient, versatile filter component suitable for direct
integration with commercially available focal plane array technologies via standard fabrication techniques. We will
discuss the theoretical development and analysis of the described components and compare the results to the current
state-of-the-art.
We present a broadband, all-dielectric, diffractive optical element (DOE) for spectral beam combining with
optimized efficiency. We achieve maximal efficiency and polarization insensitivity for the sum of incident
wavelengths by varying grating etch depth and duty cycle of a rectangular profile grating realized with the precision
of ebeam mask definition. Design and fabrication considerations that maximize efficiency are quantified, including
material options, e-beam defined lithographic parameters such as grating periods and aspect ratios, tailored
wavelength dispersion, and polarization independence. These results are compared to published efficiency values of
>95% diffraction efficiency for a single polarization and single wavelength and polarization-independent efficiency
values of >98% also for a single wavelength.
We report on the progress of an optical tag designed to indicate the presence of HF. The approach we followed uses a
high spatial frequency grating consisting of lines of conductive polymer. The conductive polymer has been designed to
be sensitive to HF; changing its conductivity upon exposure. This material change results in a change in the polarization
response of the grating which can be read out remotely using optical techniques. The use of a polarization response
makes the signal more robust to intensity fluctuations in the background or interrogation system. Additionally, the use
of optical interrogation allows for standoff detection in instances where hazardous conditions may be present. A review
of the material development work will be presented as well as the device fabrication efforts. Examples of material and
device responses will be shown and directions for further investigation discussed.
Polarimetric imaging captures the polarization state of light from all the points of a scene.
Snapshot polarimetric imaging collects the Stokes' parameters spatial distribution
simultaneously. We will discuss state-of-the-art achievements and some fundamental
diffraction limitations in polarimetric imaging with an array of micro-components. We will
also look at the natural vision system of the mantis shrimp, with many of these same sensing
abilities. The evolved and exquisite vision system possesses a recently-discovered circular
polarization capability. This comprehensive polarization vision may enable
imaging/communicating advantages in the underwater environment as well as more general
turbid environments such as smoke and fog.
In this paper, we describe our efforts to control the thermal emission from a surface utilizing structured surfaces with
metal/dielectric interfaces. The goal was not to eliminate the emission, but to control the output direction and spectrum.
We focus on methods that lead to high emissivity at grazing angles, with low emission near normal. We describe the
fabrication and measurement of large passive devices (15×15 mm) and arrays of smaller chips for thermal emission
control in the longwave infrared (8 to 12 micron) spectral region. All the devices consist of a metal base layer covered
with dielectric/metal posts or lines, 0.5 microns tall. The posts (0.9×0.9 micron) and lines (0.3 micron wide) are subwavelength.
One-dimensional and two-dimensional devices with a 3 micron pitch will be shown. The devices are
measured with both a hemispherical directional reflectometer and a variable angle directional emissometer. Both
simulated and experimental results show the thermal emission effectively limited to a small spectral region and grazing
angles from the surface (≥ 80°) in stark contrast to the typical Lambertian radiation seen from unstructured material.
Finally, the effect of this thermal emission control is illustrated using an infrared camera.
We report on several recently demonstrated microphotonic devices based in ferroelectric single crystals. The device concept is to micro-pattern ferroelectric domains in the shape of lenses and prisms whose refractive index is electric field tunable through the electro-optic effect. Two different devices have been fabricated and tested. The performance of an integrated lens and scanner device is demonstrated with electric field tunable focusing power and a scanning range of 14.6°. A 2-stage cascaded horn-shaped scanner is demonstrated with a scanning range of 25.4° at 5 kHz. Ferroelectric materials offer a unique platform for integrating several optical functions, like focusing, scanning, and frequency conversion, all in the same device.
We report theoretical and experimental studies of 1-D and 2-d tunable nonlinear photonic crystals made of liquid crystal or liquid crystal infiltrated periodic structures. Theoretical modeling shows that such structures exhibit tunable bandgap, and sugar-prism effect. Experimentally, we have demonstrated the possibility of writing dynamic or permanent [but switchable] index gratings to dye-doped LC films that act as planar waveguides.
Dynamic scanning and focusing of a laser beam using a robust integrated solid state platform is desirable for many space-based applications including free-space optical communications, target tracking, and optical data storage/processing. Electro-optic devices offer promise as an ideal platform for such applications. However, the relatively low focal powers and scan angles of existing electro-optic devices have limited their use in these areas. Recently, we have applied state of the art fabrication techniques and design optimization to achieve the first integrated lens and scanner device capable of collimating and focusing beams with input diameters of a few microns, then scanning the beam through a large angular range (nearly 20 degrees). The device, fabricated in a bulk LiTaO3 crystal, is deal for coupling light into or out of channel waveguide or fiber optic systems. We will discuss the expected performance of the existing device in a radiation environment for space-based applications. In addition, we will discuss the expected performance of similarly optimized devices fabricated in other ferroelectric materials, radiation hardened and otherwise.
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