Superhydrophobic Surface by Laser Ablation of PDMS.

Superhydrophobic surfaces have important applications in generating anti-icing properties, preventing corrosion, producing anti-biofouling characteristics, and microfluidic devices. One of the most commonly used materials to make superhydrophobic surfaces is poly(dimethylsiloxane) (PDMS). Various techniques, including spin-coating, dip-coating, spray coating, surface etching, and laser-textured mold methods, have been used to make superhydrophobic surfaces. However, all these methods require several steps, the usage of multiple chemicals, and/or surface modifications. In this paper, a one-step, low-cost method to induce superhydrophobicity is described. This was done by the pulsed laser deposition of laser-ablated PDMS micro/nanoparticles, and the method applies to a variety of surfaces. This technique has been demonstrated on three important classes of material─glass, poly(methyl methacrylate) (PMMA), and aluminum. Water contact angles of greater than 150° and roll-off angles of less than 3° were obtained. Optical transmission value of as high as 90% was obtained on glass or PMMA coated with laser-ablated PDMS micro/nanoparticles. Furthermore, this method can also be used to make micron-scale patterned superhydrophobic PDMS surfaces. This would have potential applications in microfluidic microchannels and other optical devices.

Laser annealing to improve PbSe thin film photosensitivity and specific detectivity.

PbSe thin films were deposited on SiO2/Si wafers using chemical bath deposition for mid-wave infrared (MWIR) detection. To enhance the photosensitivity of PbSe thin films, oxidation, followed by iodization, was performed to create a PbI2/PbSe two-layer system for efficient MWIR detection in the spectral range from 3 µm to 5 µm. A near-infrared (IR) laser annealing was performed after sensitization with 1070 nm wavelength at an energy density of 1J/cm2 to selectively heat the PbSe thin films. After IR laser annealing, the change in resistance between dark condition and MWIR illumination improved significantly from 19.8% to 22.6%. In addition, the dark resistance increased by 32.5% after IR laser annealing. IR photoluminescence spectra after IR laser annealing shows an increase in the sub-peak intensities from iodine incorporation. The results indicate that more iodine is incorporated into Se sites at the outer regions of PbSe grains. Therefore, more donors (electrons) from iodine diffuse into PbSe and recombine with holes so that PbSe thin film after IR laser annealing shows much higher dark resistance. Test devices with NiCr electrodes at the bottom of PbSe were fabricated with feature sizes of 40 µm to investigate the effect of IR laser annealing on electrical properties and specific detectivity (D∗). I-V characteristics show dark resistance increased after IR laser annealing. The specific detectivity increases significantly after IR laser annealing at the applied bias of 10 V at 270 K from 0.55×1010cmHz1/2W-1 to 1.23×1010cmHz1/2W-1 due to dramatic noise reduction, which is originated from higher dark resistance.

Photoconductive mechanism of IR-sensitive iodized PbSe thin films via strong hole-phonon interaction and minority carrier diffusion.

Photoconductive PbSe thin films are highly important for mid-infrared imaging applications. However, the photoconductive mechanism is not well understood so far. Here we provide additional insight on the photoconductivity mechanism using transmission electron microscopy, x-ray photoelectron microscopy, and electrical characterizations. Polycrystalline PbSe thin films were deposited by a chemical bath deposition method. Potassium iodide (KI) was added during the deposition process to improve the photoresponse. Oxidation and iodization were performed to sensitize the thin films. The temperature-dependence Hall effect results show that a strong hole-phonon interaction occurs in oxidized PbSe with KI. It indicates that about half the holes are trapped by KI-induced self-trapped hole centers (Vk center), which results in increasing dark resistance. The photo Hall effect results show that the hole concentration increases significantly under light exposure in sensitized PbSe, which indicates the photogenerated electrons are compensated by trapped holes. The presence of KI in the PbSe grains was confirmed by I 3d5/2 core-level x-ray photoelectron spectra. The energy dispersive x-ray spectra obtained in the scanning transmission electron microscope show the incorporation of iodine during the iodization process on the top of PbSe grains, which can create an iodine-incorporated PbSe outer shell. The iodine-incorporated PbSe releases electrons to recombine with holes in the PbSe layer so that the resistance of sensitized PbSe is about 800 times higher than that of PbSe without the iodine-incorporated layer. In addition, oxygen found in the outer shell of PbSe can act as an electron trap. Therefore, the photoresponse of sensitized PbSe is from the difference between the high dark resistance (by KI addition and iodine incorporation) and the low resistance after IR exposure due to electron compensation (by electron traps at grain boundary and electron-hole recombination in KI hole traps).

High-efficiency flexible multilevel photon sieves by single-step laser-based fabrication and optical analysis.

Over the past several decades, the need for high-resolution, high-efficiency, lightweight, high-contrast focusing optics has continued to increase due to their applications in fields such as astronomy, spectroscopy, free-space optical communications, defense, and remote sensing. In recent years, photon sieve planar diffractive optics, which are essentially Fresnel zone plates with the rings broken into individual "pinhole" apertures, have been developed on flexible, lightweight polyimide substrates. However, transmission efficiencies have continuously been very low (∼1%-11%) until this work, thus impeding the widespread use of photon sieves in practical applications. Here, we present flexible, lightweight, four- and eight-level phase photon sieves with 25.7% and 49.7% transmission efficiency, respectively, up to five times greater than that of any other photon sieve reported thus far. Additionally, these sieves were fabricated via a single step pulsed laser ablation method. The total time to fabricate a ∼3 cm2 photon sieve via the single-step fabrication was tens of seconds, giving the technique a significant advantage over traditional photolithography used to generate multilevel structures. Analytical analysis of the photon sieve was carried out via the finite-difference time-domain (FDTD) method and was in very good agreement with experimental results. We have also calculated via FDTD modeling the behavior of higher-level photon sieves for further enhanced efficiencies, and analytically show an estimated upper bound on photon sieve efficiency of 70% within the first focal plane null in the limit of increasing step number, and the data presented herein provide a relationship between efficiency and step number. Additionally, this process of multilevel diffractive lens fabrication can be extended to multilevel Fresnel zone plates, which have not previously been demonstrated by this process. The results presented in this work represent a new step in high-resolution diffractive optics, showing efficiencies suitable for widespread applications in addition to drastically reducing the cost and complexity of fabricating multilevel focusing elements.

Swept-frequency ultrasonic phase evaluation of adhesive bonding in tri-layer structures.

As modern aerospace and automotive designs continually strive for higher performance, and thus rely on advanced composite structures where adhesive bonding is a preferred method of joining, the need for a robust quantitative nondestructive bond strength measurement method has increased. As such, advanced nondestructive evaluation methods have been researched for increased sensitivity to weak interfacial bonding and ultimately to detect "kissing" bonds. In this work, a phase-based method for interrogating bonded joints and detecting weak adhesion is developed by using swept-frequency phase measurements of ultrasonic waves reflected from an adhesive joint and modeling adhesive interfaces as a distributed spring system. The method's sensitivity to bond strength is explored by ultrasonic phase evaluation of tri-layer joints with bond quality varied by controlling ultraviolet light exposure and extracting interfacial stiffness constants of the bonds. Mechanical tensile tests found each joint failed adhesively, allowing a linear correlation to be drawn between interfacial stiffness and tensile strength, consistent with previous theoretical research. The ultrasonic phase measurement method identifies intermediate bond strengths, rather than simply detecting good or bad bonds. This technique has the potential for the verification of bond quality in lightweight aerospace and automotive designs utilizing advanced composite structures with adhesive attachments.

Flexible binary phase photon sieves on polyimide substrates by laser ablation.

A binary phase diffractive optical element photon sieve is fabricated by direct laser ablation of a thin, flexible polyimide substrate with a nanosecond-pulsed ultraviolet laser. The binary phase photon sieve operates at 633 nm and was designed with 19 rings and a focal length of 400 mm. The total time to fabricate the photon sieves was tens of seconds. The surface properties of the laser-processed areas are examined, and the optical performance of the photon sieve is characterized and compared to FDTD simulations. By optimizing the laser fluence and travel distance between laser pulses, features with sub-wavelength surface roughness were achieved. The photon sieve showed good focusing ability with suppressed side-lobes. When the fractional area of photon sieve pinholes was made to approach 50%, the binary sieve diffraction efficiency approached 11%, matching the highest value reported in the literature for a photon sieve. Thus, this Letter demonstrates both high efficiency and lightweight diffractive optics suitable for space satellite and other applications, with capabilities for low cost and high throughput fabrication.

Fabrication of photon sieves by laser ablation and optical properties.

In this work, we demonstrate the feasibility and performance of photon sieve diffractive optical elements fabricated via a direct laser ablation process. Pulses of 50 ns width and wavelength 1064 nm from an ytterbium fiber laser were focused to a spot diameter of approximately 35 µm. Using a galvanometric scan head writing at 100 mm/s, a 30.22 mm2 photon sieve operating at 633 nm wavelength with a focal length of 400 mm was fabricated. The optical performance of the sieve was characterized and is in strong agreement with numerical simulations, producing a focal spot size full-width at half-maximum (FWHM) of 45.12 ± 0.74 µm with a photon sieve minimum pinhole diameter of 62.2 µm. The total time to write the photon sieve pattern was 28 seconds as compared to many hours using photolithography methods. We also present, for the first time to our knowledge in the literature, thorough characterization of the influence of angle of incidence, temperature, and illumination wavelength on photon sieve performance. Thus, this work demonstrates the potential for a high speed, low cost fabrication method of photon sieves that is highly customizable and capable of producing sieves with low or high numerical apertures.

Low thermal emissivity surfaces using AgNW thin films.

The properties of silver nanowire (AgNW) films in the optical and infrared spectral regime offer an interesting opportunity for a broad range of applications that require low-emissivity coatings. This work reports a method to reduce the thermal emissivity of substrates by the formation of low-emissivity AgNW coating films from solution. The spectral emissivity was characterized by thermal imaging with an FLIR camera, followed by Fourier transform infrared spectroscopy. In a combined experimental and simulation study, we provide fundamental data of the transmittance, reflectance, haze, and emissivity of AgNW thin films. Emissivity values were finely tuned by modifying the concentration of the metal nanowires in the films. The simulation models based on the transfer matrix method developed for the AgNW thin films provided optical values that show a good agreement with the measurements.

PbSe quantum dot based luminescent solar concentrators.

The results are presented for luminescent solar concentrators (LSCs) fabricated with poly(lauryl methacrylate-co-ethylene glycol dimethacrylate) (P(LMA-co-EGDMA)) and Angstrom Bond, Inc. AB9093 acrylic epoxy matrix, high quantum yield (> 70%) PbSe quantum dots (QDs) and silicon photovoltaic (Si PV) cells. LSCs were tested under a lamp with broadband illumination, photon flux-matched to a standard solar spectrum and verified under a calibrated solar lamp source. The P(LMA-co-EGDMA) sample demonstrated the highest power conversion efficiency of any known LSC fabricated with either QDs or Si PV cells, 4.74%. Additionally, increased temperature was shown to reduce efficiency.

Temperature-dependent optical properties of lead selenide quantum dot polymer nanocomposites.

The optical properties of PbSe quantum dots (QDs) in AB9093 epoxy nanocomposite are examined with respect to temperature over a range of 0°C-80°C, a useful working range for many QD-based sensors and devices, and results are compared to QDs in toluene solution. A complete characterization of QD optical properties is provided as a function of temperature, including the absorption spectrum, first excitonic (1-s) absorption peak intensity and wavelength, fluorescence intensity, and peak wavelength. QD optical properties in toluene were found to be more sensitive to temperature as compared to those in AB9093. Interestingly, 1-s and fluorescence peak wavelength variation with temperature are reversed in AB9093 as compared to those in toluene solution. Results for the fluorescence properties of Lumogen F Red 305 dye in toluene are presented for comparison. The dye was found to have similar sensitivity to temperature to that of the QDs in terms of fluorescence peak wavelength shift, but the fluorescence peak intensity was far less variant. These results can be used to build a temperature sensor or as a guide to building other types of QD-based devices to be more robust against changes in ambient temperature.

Solar thermophotovoltaic system using nanostructures.

This paper presents results on a highly efficient experimental solar thermophotovoltaic (STPV) system using simulated solar energy. An overall power conversion efficiency of 6.2% was recorded under solar simulation. This was matched with a thermodynamic model, and the losses within the system, as well as a path forward to mitigate these losses, have been investigated. The system consists of a planar, tungsten absorbing/emitting structure with an anti-reflection layer coated laser-microtextured absorbing surface and single-layer dielectric coated emitting surface. A GaSb PV cell was used to capture the emitted radiation and convert it into electrical energy. This simple structure is both easy to fabricate and temperature stable, and contains no moving parts or heat exchange fluids.

Design of emitter structures based on resonant perfect absorption for thermophotovoltaic applications.

We report a class of thermophotovoltaic emitter structures built upon planar films that support resonant modes, known as perfectly-absorbing modes, that facilitate an exceptional optical response for selective emission. These planar structures have several key advantages over previously-proposed designs for TPV applications: they are simple to fabricate, are stable across a range of temperatures and conditions, and are capable of achieving some of the highest spectral efficiencies reported of any class of emitter structure. Utilization of these emitters leads to exceptionally high device efficiencies under low operating temperature conditions, which should open new opportunities for waste heat management. We present a theoretical framework for understanding this performance, and show that this framework can be leveraged as a search algorithm for promising candidate structures. In addition to providing an efficient theoretical methodology for identifying high-performance emitter structures, our methodology provides new insight into underlying design principles and should pave way for future design of structures that are simple to fabricate, temperature stable, and possess exceptional optical properties.

Optical and infrared properties of glancing angle-deposited nanostructured tungsten films.

Nanotextured tungsten thin films were obtained on a stainless steel (SS) substrate using the glancing-angle-deposition (GLAD) method. It was found that the optical absorption and thermal emittance of the SS substrate can be controlled by varying the parameters used during deposition. Finite-difference time-domain (FDTD) simulations were used to predict the optical absorption and infrared (IR) reflectance spectra of the fabricated samples, and good agreement was found between simulated and measured data. FDTD simulations were also used to predict the effect of changes in the height and periodicity of the nanotextures. These simulations show that good control over the absorption can be achieved by altering the height and periodicity of the nanostructure. These nanostructures were shown to be temperature stable up to 500°C with the addition of a protective HfO2 layer. Applications for this structure are explored, including a promising application for solar thermal energy systems.

Lead selenide quantum dot polymer nanocomposites.

Optical absorption and fluorescence properties of PbSe quantum dots (QDs) in an Angstrom Bond AB9093 epoxy polymer matrix to form a nanocomposite were investigated. To the authors' knowledge, this is the first reported use of AB9093 as a QD matrix material and it was shown to out-perform the more common poly(methyl methacrylate) matrix in terms of preserving the optical properties of the QD, resulting in the first reported quantum yield (QY) for PbSe QDs in a polymer matrix, 26%. The 1-s first excitonic absorption peak of the QDs in a polymer matrix red shifted 65 nm in wavelength compared to QDs in a hexane solution, while the emission peak in the polymer matrix red shifted by 38 nm. The fluorescence QY dropped from 55% in hexane to 26% in the polymer matrix. A time resolved fluorescence study of the QDs showed single exponential lifetimes of 2.34 and 1.34 µs in toluene solution and the polymer matrix respectively.

Graded-index structures for high-efficiency solar thermophotovoltaic emitting surfaces.

This Letter presents a highly efficient emitter structure for solar thermophotovoltaic systems. The structure consists of a graded index on tungsten, shows a spectral efficiency of 59%, or 70% with the use of a back reflector, and is compared to other state-of-the-art emitter structures. The effects of different structures and periodicities on the efficiency of the emitter are explored, as well as the effect of a protective oxide coating. The causes of the antireflection properties of these structures are also explored.

Exceptionally low thermal conductivities of films of the fullerene derivative PCBM.

We report on the thermal conductivities of microcrystalline [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) thin films from 135 to 387 K as measured by time domain thermoreflectance. Thermal conductivities are independent of temperature above 180 K and less than 0.030 ± 0.003 W m(-1) K(-1) at room temperature. The longitudinal sound speed is determined via picosecond acoustics and is found to be 30% lower than that in C(60)/C(70) fullerite compacts. Using Einstein's model of thermal conductivity, we find the Einstein characteristic frequency of microcrystalline PCBM is 2.88 × 10(12) rad s(-1). By comparing our data to previous reports on C(60)/C(70) fullerite compacts, we argue that the molecular tails on the fullerene moieties in our PCBM films are responsible for lowering both the apparent sound speeds and characteristic vibrational frequencies below those of fullerene films, thus yielding the exceptionally low observed thermal conductivities.

Micro texturing of silicon using pulsed N(2)-laser and formation mechanism.

A low-cost pulsed N(2)-laser has been used to successfully demonstrate the formation of self-organized conical microtexture in Si. The process is demonstrated in vacuum environment to avoid the use of SF(6) gas and sulfur incorporation. The microtexture is formed with an average structure height of ~15 um, base diameter ~10 µm, and tip-to-tip separation ~8 µm. Energy dispersive x-ray spectroscopy of individual conelike structure shows that the material remains free from impurity incorporation. We have shown that the laser-induced-damage-related absorption can be successfully restored after an hour annealing at 1000 °C, making the material an ideal candidate for photovoltaic and other photonic applications.

Ultrafast laser-induced microstructure/nanostructure replication and optical properties.

This paper demonstrates replication of ultrafast laser-induced micro/nano surface textures on poly(dimethylsiloxane) (PDMS). The surface texture replication process reduces the processing steps for microtexturing while improving light trapping. Two methods are demonstrated to replicate surface microtexture, a simple mold method and an embossing method. The laser microtextured silicon and titanium surfaces with micro to nanoscale features have been successfully replicated. Optical characterization of the replicated microtextured PDMS surfaces is performed and the results agree with model predictions. The replicated microtextured PDMS film is applied on a silicon surface and optical characterization shows that surface reflectance can be suppressed over 55% compared to the control value.

Optical properties of solution-processable semiconducting TiOx thin films for solar cell and other applications.

The optical properties of solution-processable semiconducting titanium suboxide (TiOx) thin films were investigated as a function of wavelength (350-800 nm) using ellipsometric and optical reflection technique. The variation of refractive index under different thermal annealing conditions (room temperature to 900 °C) was studied. The increase in refractive index with high-temperature thermal annealing process was observed, allowing the opportunity to obtain refractive index values from 1.77 to 2.57 at a wavelength of 600 nm. The x-ray diffraction and atomic force microscopy studies indicate that the index variation is due to the TiOx phase, density, and morphology changes under thermal annealing. The TiOx thin films have applications in organic and inorganic solar cells as well as other optical and photonic devices. We show that TiOx thin films can be used as an effective antireflection layer for Si solar cells.

A constant-frequency ultrasonic phase method for monitoring imperfect adherent/adhesive interfaces.

A primary mechanism of adhesive bond failure is a degradation of the adherent/adhesive interfacial stiffness from unwanted contamination or exposure to those environmental factors, which reduce adhesion quality. Substantial research has been conducted on the assessment of adhesively bonded structures and the detection of "kissing" bonds. Advanced ultrasonic assessment methods to interrogate bonded joints and measure interfacial stiffness using a distributed spring interface model have been developed. Amplitude-based ultrasonic methods have traditionally been used in adhesive bond quality assessment, but recent advancements in ultrasonic phase measurements allow for high measurement resolution with low-uncertainty. In this work, an ultrasonic phase technique for the monitoring of adhesively-bonded interfaces is demonstrated. Constant frequency measurements are obtained from the ultrasonic phase of the reflection coefficient from the adhesive bond with a glass adherent, where the degree of cure is controlled by exposure to ultraviolet light. A peak in the phase of the reflection coefficient, as predicted by the interfacial spring model, is measured experimentally. It is shown that the peak phase predicts the interfacial stiffness when some frequency dependent threshold value is crossed. With knowledge of the acoustic impedances of both materials at the interface, the interfacial stiffness is determined by an inverse algorithm involving measurements of ultrasonic phase shifts of bonded joint reflections. By monitoring the interface of bonded structures and coatings, this method permits a nondestructive inspection of bond strength from structural construction through its service life.