Sb2S3 is a chalcogenide material with a heat-controlled transition from amorphous to crystalline phase. Combined with significant changes in optical properties during the transition, it is an extremely interesting and applicable material for applied IR optics. In this talk, we present nanofabricated resonant Sb2S3 pillars, showing how methodically designed metasurfaces with these structures can exhibit a wide range of optical properties across the phase change. Furthermore, we show a tunable ability to control these metasurfaces with intentional design.
High-resolution white light observations of the solar corona over a large field of view (FOV) are crucial for understanding the structure and evolution of large coronal structures, including coronal mass ejections. With current telescopes for imaging the corona and inner heliosphere, there is a tradeoff between spatial resolution and the FOV; coronagraphs typically provide high-resolution (<1 arcminute) imaging over a relatively small region while heliospheric imagers are designed with a wide FOV, sacrificing spatial resolution for coverage. Incorporating a scanning system enables the conservation of high spatial resolution while adding the ability to map over a large field of regard; however, with conventional optical designs, this would require large/complex gimbaled systems, which are risky for space-based instrumentation. The Coronal and Heliospheric imaging with Achromatic Metasurfaces Pathfinder (CHAMP) aims to address this need, consisting of a visible light telescope which uses novel achromatic metasurface Risley prisms (MRPs) to create high-resolution, wide-FOV maps of the solar corona in a small form factor. With this design, optical beam steering is achieved by rotating two MRPs relative to each other using rotational stages, eliminating the need for gimbaled systems. Here we describe the CHAMP instrument concept and efforts to develop multi-layer achromatic MRPs which perform across a wide bandpass (∼100 nm) in the visible light regime.
Thermal regulation is essential for numerous applications across multiple industries such as the efficient temperature control of indoor facilities and the reliable operation of many electronic systems. Vanadium dioxide (VO2) is a phase change material that is well-suited for thermal regulation as a result of its ultrafast, reversible, solid-state transition at 68°C that produces a significant contrast in its infrared (IR) emissive properties. To meet application demands, VO2’s transition temperature can be tuned via doping with a reduction in temperature of ~22 °C per atomic percent tungsten (at. % W6+). However, historically this decrease in the transition temperature has coincided with a reduction in IR optical contrast between the two phases. In this investigation, we demonstrate that by patterning VO2 thin film composites with preoptimized thicknesses, a thermal regulation system with a tunable transition temperature and no significant degradation of contrast between the states is produced. Through carefully selected user-defined patterning of the undoped VO2 layer within the multilayer film, a 64% operating optical contrast was achieved across the 8 – 13 μm spectral region as compared to 42% in the as-deposited film. Additionally, at a doping level of 1.7%, the transition temperature in a VO2 thin film composite with micron-scaled patterning was reduced to 25°C while maintaining 58% emissive contrast in the 8 – 13 μm spectral region.
White light observations of the solar corona are crucial for understanding large-scale coronal structures and examining the evolution of solar transients, such as coronal mass ejections (CMEs), which have important space weather impacts. We seek to develop technology for producing high-resolution, wide-FOV maps of the white light corona with reduced instrument size and complexity. With conventional optical designs, scanning the corona with a small instantaneous FOV or accommodating a telescope on a spinning or non-solar centered spacecraft would require large/complex gimbaled systems. To avoid these complex systems, conventional heliospheric imagers are designed with a wide FOV, sacrificing spatial resolution and throughput for coverage. To address this need, we have designed a visible light telescope which uses novel achromatic metasurface Risley prisms (MRPs) to create high-resolution, wide-FOV maps of the solar corona in a small form factor: the Coronal and Heliospheric imaging with Achromatic Metasurfaces Pathfinder (CHAMP). MRPs enable rapid mapping of a large FOV with a small instantaneous FOV, and features of interest (e.g., CMEs, comets, etc.) can be tracked using optimized scanning patterns. Here we present the preliminary concept and optical design for CHAMP.
The emergence of metasurface technology and its accompanying design principles are enabling the development of optical components with multiple functionalities, e.g., polarization discrimination and focusing. In this report, we highlight our experimental results associated with the characterization of a reflective mid-wave infrared (mid-IR) metasurface designed for both imaging and polarization-specific beam splitting in the 4.2 through 4.8 m region of the electromagnetic (EM) spectrum. A large area metasurface (1 cm diameter), fabricated using nano-lithography, was observed to exhibit a high degree of discrimination between transverse electric (TE) and magnetic (TM) polarized light with near diffraction limited focusing.
In this paper, we report a computational and experimental study using tunable infrared (IR) metamaterial absorbers (MMAs) to demonstrate frequency tunable (7%) and amplitude modulation (61%) designs. The dynamic tuning of each structure was achieved through the addition of an active material—liquid crystals (LC) or vanadium dioxide (VO2)--within the unit cell of the MMA architecture. In both systems, an applied stimulus (electric field or temperature) induced a dielectric change in the active material and subsequent variation in the absorption and reflection properties of the MMA in the mid- to long-wavelength region of the IR (MWIR and LWIR, respectively). These changes were observed to be reversible for both systems and dynamic in the LC-based structure.
Anisotropic impedance surfaces, which include metasurfaces and high impedance surfaces (HIS), can be designed to control the amplitude and propagation direction of surface electromagnetic waves and are an effective means to enhance the isolation between antennas that share a common ground plane. To date, the majority of metastructures that have been designed for antenna isolation have relied on an isotropic distribution of unit cells that possess a stop band that inhibits the propagation of surface waves between neighboring antennas. A less common approach to isolation has been through the design of a metasurface that enables the re-direction of surface waves away from the location of the antenna structure, which effectively limits the coupling. In this paper, we discuss results from our computational investigation associated with improving antenna isolation through the use of an anisotropic metastructure. Simulated results associated with the isolation performance of two simple, but similar, anisotropic structures are compared to the corresponding results from a broadband magnetic radar absorbing materials (magRAM).
In this paper, we present a computational and experimental design of a metasurface for broadband microwave antenna isolation. Our current emphasis is on the development of a high-impedance surface (HIS) that enables broadband isolation between transmit and receive antennas. For our specific HIS, we have formed a cascade of HIS unit cells and have thus expanded the isolation to provide 56 dB/meter over one octave (7.5 to 18 GHz) relative to the bare metal ground plane. Computational models are used to design the cascaded structure to assure maximum isolation amplitude and bandwidth.
In a previous report, we have shown that the long wavelength, electromagnetic-pulsed (EMP) energy generated by ultrashort (38 fs) laser pulse ablation of a metal target is enhanced by an order of magnitude due to a preplasma generated by a different, 14-ns-long laser pulse. Here, we further investigate this EMP enhancement effect in a 2- to 16-GHz microwave region with different target materials and laser parameters. Specifically, we show a greater than two orders of magnitude enhancement to the EMP energy when the nanosecond and ultrashort laser pulses are coincident on a glass target, and greater than one order of magnitude enhancement when the pulses are coincident on a copper target.
We study the effects of the interaction of 40-fs Ti-sapphire laser radiation at 800 nm with biological materials—proteins or intact Bacillus spore, dissolved or suspended in pure water, respectively. The estimated laser intensity at the target is 10 13 W/cm 2 . On the molecular level, oxidation of solvent-accessible parts of proteins has been observed even after a single femtosecond laser pulse, as demonstrated by mass spectrometry. A remarkable morphological effect of the femtosecond laser radiation is the complete disintegration of extremely refractive cells such as bacterial spores, evidenced in scanning electron micrographs. After 500 laser pulses, all suspended spores in the irradiated volume are completely destroyed, which makes them nonviable. Characteristic spore biomolecules, e.g., small acid-soluble spore proteins, are extensively oxidized after several laser pulses. In comparative studies, no effects have been observed when irradiating the same samples with 10-ns laser pulses at the same laser wavelength and fluence. We demonstrate that the laser power density (irradiance), resulting in different amounts of total deposited energy, determines the types of effects for femtosecond laser interactions with biological matter.
Ultrashort laser pulses (~100 fs duration) are known to generate charge separation in solid, liquid and gas targets
through a variety of nonlinear mechanisms. This process results in the emission of a broadband electromagnetic
pulse (EMP) in the microwave and terahertz (THz) regions of the electromagnetic spectrum. Possible applications
of this phenomenon include remote RF and THz generation for material detection and diagnostics.
We investigate the energy and spectrum of the EMP emitted from copper and glass targets irradiated by single 800
nm, 38 fs duration pulses with varying energy. The detector is two feet from the target and the detection bandwidth
is 2-18 GHz. We also demonstrate our ability to enhance the emitted EMP energy from a copper target by more
than an order of magnitude by irradiating the target with a 1064 nm, 14 ns duration pulse at a specific time delay
relative to the ultrashort pulse. We attribute the increased optical to RF energy conversion to enhanced absorption
of the ultrashort pulse by the nanosecond pulse-generated plasma at the surface of the target.
Hand-held instruments capable of spectroscopic identification of chemical warfare agents (CWA) would find extensive
use in the field. Because CWA can be toxic at very low concentrations compared to typical background levels of
commonly-used compounds (flame retardants, pesticides) that are chemically similar, spectroscopic measurements have
the potential to reduce false alarms by distinguishing between dangerous and benign compounds. Unfortunately, most
true spectroscopic instruments (infrared spectrometers, mass spectrometers, and gas chromatograph-mass spectrometers)
are bench-top instruments. Surface-acoustic wave (SAW) sensors are commercially available in hand-held form, but rely
on a handful of functionalized surfaces to achieve specificity. Here, we consider the potential for a hand-held device
based on surface enhanced Raman scattering (SERS) using templated nanowires as enhancing substrates. We examine
the magnitude of enhancement generated by the nanowires and the specificity achieved in measurements of a range of
CWA simulants. We predict the ultimate sensitivity of a device based on a nanowire-based SERS core to be 1-2 orders
of magnitude greater than a comparable SAW system, with a detection limit of approximately 0.01 mg m-3.
In addition to visible and near-IR emission, recent investigations have shown that electromagnetic pulses (EMP) in the
microwave and RF regions of the spectrum are generated during femtosecond laser-matter interactions if the laser source
is sufficiently intense to ablate and ionize an illuminated solid target material. Although the mechanisms for the laserinduced
EMP pulse are not fully characterized, it is reported that this phenomenon arises from two mechanisms
associated with terawatt to petawatt level laser interactions with matter: (1) ionization via propagation in air, and (2)
plasma generation associated with the laser-excited solid material. Over the past year, our group has examined the
microwave emission profiles from a variety of femtosecond laser ablated materials, including metals, semiconductors,
and dielectrics. We have directed our measurements towards the characterization of microwave emission from ablated
surfaces in air using laser peak powers in excess of 1012 Watts (energy/pulse ~50 mJ, pulse width ~30 fs, laser diameter
at target ~200 microns). We have characterized the temporal profile of the microwave emission and determined the
emission from all samples is omni-directional. We have also observed a difference in the minimum fluence required to
generate emission from conducting and insulating materials although the peak amplitudes from these materials were
quite similar at the upper laser energy levels of our system (~50 mJ).
We report the results of scanning micro-Raman spectroscopy obtained on Au-Ag nanowires for a variety of chemical
warfare agent simulants. Rough silver segments embedded in gold nanowires showed enhancement of 105 - 107 and
allowed unique identification of 3 of 4 chemical agent simulants tested. These results suggest a promising method for
detection of compounds significant for security applications, leading to sensors that are compact and selective.
We describe a scanning Orotron Terahertz radiation (THz) source. The operational principle is as follows: a
sheet beam of electrons passes near a corrugated metal surface (Smith-Purcell grating) contained in a resonant
cavity. The periodic forces on the electrons drive the cavity on its resonances in the THz regime. We describe
theoretical predictions for the sheet beam parameters required and the likely performance of the device. We
also describe experimental progress towards sheet-beam generation using field-emitted electrons from a carbon-nanotube
array. We describe the carbon nanotube growth process and demonstrate sheet-beam current densities
which exceed the predicted turn-on current density of the Orotron cavity.
Surface enhanced Raman spectroscopy (SERS) has promise as an optical sensor for the detection of chemical and biological agents, in particular when combined with front-end processing for sample preparation prior to analysis. In this paper, we report preliminary results from a SERS analysis of Bacillus cereus T strain (BcT), which was prepared for sensor analysis via a microfluidics-based sample processor. In the microfluidics hardware, low and high molecular weight analytes from a sonicated spore sample were separated via mass-dependent diffusion into two independent microchannels. SERS analysis of the sample outputs revealed a significant separation of the low molecular spore biomarker, dipicolinic acid, from the high molecular weight protein and nucleic acid background. In addition to the processing study, measurements were performed on gold core-shell nanospheres, which are considered a potential SERS substrate for the microfluidic system. Finally, field-induced aggregation of silver nanoparticles, an alternative to chemical aggregation, was shown to be an effective approach for the production of highly enhancing SERS substrates.
We report results of scanning micro-Raman spectroscopy obtained on isolated nanowires and networks of nanowires with different geometries and surface morphologies. We measured a strong, relatively homogeneous, surface enhancement of the Raman response from nanowires with a rough surface morphology, and detected a more sporadic enhanced response detected from smooth nanowires. These results provide the first steps towards the development of selective sensors for hazardous bio- and chemical-agent detection that rely on a combination of electronic conductance measurements and Raman spectroscopic measurements from metallic nanowire networks.
In this paper, we report the preliminary results from a microfabricated substrate system that is amenable to both electromagnetic field-enhanced spectroscopy such as surface enhanced Raman scattering (SERS) and analyte separation and detection. Substrates consisting of arrays of gold post-like and pit-like features of varying pitch on gold substrates were fabricated by electron beam lithography. These substrates were characterized and tested for reproducible SERS activity, as well as evaluated for incorporation into a microfluidic system for separation and identification of components of complex matrices. Identification of analytes relevant to biodetection and biological screening is reported.
The function of a large number of MEMS and NEMS devices relies critically on the transduction method employed to convert the mechanical displacement into electrical signal. Optical transduction techniques have distinct advantages over more traditional capacitive and piezoelectric transduction methods. Optical interferometers can
provide a much higher sensitivity, about 3 orders of magnitude, but are hardly compatible with standard MEMS and microelectronics processing. In this paper, we present a scalable architecture based in silicon on sapphire (SOS) CMOS 1 for building an interferometric optical detection system. This new detection system is currently
being applied to the sense the motion of a resonating MEMS device, but can be used to detect the motion of any object to which the system is packaged. In the current hybrid approach the SOS CMOS device is packaged with both vertical cavity surface emitting lasers (VCSELs) and MEMS devices. The optical transparency of the sapphire substrate together with the ultra thin silicon PIN photodiodes available in this SOS process allows for the design of both a Michelson type and Fabry Perot type interferometer. The detectors, signal processing electronics and VCSEL drivers are built on the SOS CMOS for a complete system. We present experimental data demonstrating interferometric detection of a vibrating device.
The research center established by Army Research Office under the Multidisciplinary University Research Initiative program pursues a multidisciplinary approach to investigate and advance the use of complementary analytical techniques for sensing of explosives and/or explosive-related compounds as they occur in the environment. The techniques being investigated include Terahertz (THz) imaging and spectroscopy, Laser-Induced Breakdown Spectroscopy (LIBS), Cavity Ring Down Spectroscopy (CRDS) and Resonance Enhanced Multiphoton Ionization (REMPI). This suite of techniques encompasses a diversity of sensing approaches that can be applied to detection of explosives in condensed phases such as adsorbed species in soil or can be used for vapor phase detection above the source. Some techniques allow for remote detection while others have highly specific and sensitive analysis capabilities. This program is addressing a range of fundamental, technical issues associated with trace detection of explosive related compounds using these techniques. For example, while both LIBS and THz can be used to carry-out remote analysis of condensed phase analyte from a distance in excess several meters, the sensitivities of these techniques to surface adsorbed explosive-related compounds are not currently known. In current implementations, both CRDS and REMPI require sample collection techniques that have not been optimized for environmental applications. Early program elements will pursue the fundamental advances required for these techniques including signature identification for explosive-related compounds/interferents and trace analyte extraction. Later program tasks will explore simultaneous application of two or more techniques to assess the benefits of sensor fusion.
The spatial, temporal, and spectroscopic characteristics associated with pulsed THz (100 GHz - 70 THz) radiation provide this emerging technology with the potential for reliable identification of buried objects such as non-metallic landmines. With a suitable integration of these attributes, one can envision a THz detection platform that provides: (1) accurate identification of buried objects, and (2) a source-to-sample working distance that is sufficient for remote sensing applications. In our preliminary laboratory studies, we have demonstrated the detection capabilities of THz radiation by imaging a small rubber object embedded in a moist, sand-like soil. Despite the significant attenuation of the THz radiation via water absorption and particle scattering, the initial transmission results showed that pulsed THz imaging could identify the non-metallic object when buried in a few inches of soil. The sub-millimeter resolution observed in our THz images illustrates the potential to discriminate landmines from other buried objects. Finally, THz calculations and measurements determined that our current THz source and detector has sufficient SNR to detect a buried object to a depth of 6 inches in moist sand.
Neutron irradiation of sapphire with 1 x 1022 neutrons(<EQ MeV)/m2 increases the c-axis compressive strength by a factor of 3 at 600 degree(s)C. The mechanism of strength enhancement is the retardation of r-plane twin propagation by radiation-induced defects. 1-B and Cd shielding was employed during irradiation to filter our thermal neutrons (<EQ1 eV), thereby reducing residual radioactivity in the sapphire to background levels in a month. Yellow-brown irradiated sapphire is nearly decolorized to pale yellow by annealing at 600 degree(s)C with no loss of mechanical strength. Annealing at sufficiently high temperature (such as 1200 degree(s)C for 24 h) reduces the compressive strength back to its baseline value. Neutron irradiation decreases the flexure strength of sapphire at 600 degree(s)C by 0-20% in some experiments. However, the c- plane ring-on-ring flexure strength at 600 degree(s)C is doubled by irradiation. Elastic constants of irradiated sapphire are only slightly changed by irradiation. Infrared absorption and emission and thermal conductivity of sapphire are not affected by irradiation at the neutron fluence used in this study. Defects that might be correlated with strengthening were characterized by electron paramagnetic resonance spectroscopy. Color centers observed in the ultraviolet absorption spectrum were not clearly correlated with mechanical response. No radiation-induced changes could be detected by x-ray topography or x-ray diffraction.
An optical technique for measuring surface stress in chromium-doped sapphire windows is reported. The approach utilizes the well-known effects of temperature and stress on the spectral profile of chromium ion fluorescence in crystalline sapphire. In this study, the sapphire samples were selectively doped with a surface concentration of chromium ions, which provided a direct measure of the stress and temperature in the surface region of the window. A series of fluorescence measurements were performed to calibrate the effects of temperature and mechanical stress on the spectral characteristics of the surface fluorescence. The results of this laboratory study are currently being developed into a dynamic, non-contact probe of stress in infrared seeker windows while under simulated conditions of flight.
A unique solid-state optical sensor configuration has been invented that can serve as a development platform for a host of chemical and biochemical sensors in either gaseous or liquid environments. We present results from measurements from the first adaptation of the device to oxygen sensing via fluorescence quenching and note the distinct advantages over existing electrochemical and more recent fiber-optic methods. The platform technology itself features greatly enhanced energy efficiency, high sensitivity, low-power consumption, ease of miniaturization, low cost, high-volume manufacturability using standard methods, very fast response/recovery profiles, and high reliability. The oxygen sensor embodiment has been demonstrated to operate well over the temperature range from -20 to 50 degrees C, not to be interfered with by other common gases including water vapor at high levels, and capable of response times less than 100 milliseconds.
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