Enhanced laser-induced damage performance of all-glass metasurfaces for energetic pulsed laser applications.

To fabricate optical components with surface layers compatible with high-power laser applications that may operate as antireflective coatings, polarization rotators, or harness physical anisotropy for other uses, metasurfaces are becoming an appealing candidate. In this study, large-beam (1.05 cm diameter) 351-nm laser-induced damage testing was performed on an all-glass metasurface structure composed of cone-like features with a subwavelength spacing of adjacent features. These structures were fabricated on untreated fused silica glass and damage tested, as were structures that were fabricated on fused silica glass that experienced a preliminary etching process to remove the surface Beilby layer that is characteristic of polished fused silica. The laser-induced damage onset for structures on untreated fused silica glass was 19.3J⋅c m -2, while the sample that saw an initial pretreatment etch exhibited an improved damage onset of 20.4J⋅c m -2, only 6% short of the reference pretreated glass damage onset of 21.7J⋅c m -2. For perspective, the National Ignition Facility operational average fluence at this wavelength and pulse length is about 10J/c m 2. At a fluence of 25.5J⋅c m -2, the reference (pretreated) fused silica initiated 5.2 damage sites per m m 2, while the antireflective metasurface sample with a preliminary etching process treatment initiated 9.8 damage sites per m m 2. These findings demonstrate that substrate-engraved metasurfaces are compatible with high energy and power laser applications, further broadening their application space.

Fluorescence lifetime measurements using photon pair correlations generated via spontaneous parametric down conversion (SPDC).

We have used photon pair correlations generated via spontaneous parametric downconversion (SPDC) to measure the fluorescence lifetime of the organic dye rhodamine 6 G, demonstrating that fluorescence lifetime measurements can be achieved using a continuous wave (CW) laser, without pulsed or modulated lasers. Our entangled photon method, quantum fluorescence lifetime (Q-FL) measurements, uses one photon to excite fluorescence and the resulting fluorescence photon is timed and referenced to the arrival time of the other entangled photon. Thus, we can exploit the short timescale of photon pair correlations to conduct experiments that are typically carried out with pulsed lasers and we show that the inherent timing of the photons is fast enough to resolve the nanosecond scale fluorescence lifetime of the sample. This measurement paves the way towards using the time correlations of entangled photons for fluorescence imaging; capitalizing on the presence of fast, sub-100 ps correlations that have not been demonstrated classically.

Wide-field probing of silica laser-induced damage precursors by photoluminescence photochemical quenching.

We describe a wide-field approach to probe transient changes in photoluminescence (PL) of defects on silica surfaces. This technique allows simultaneous capture of spatially resolved PL with spontaneous quenching behavior. We attribute the quenching of PL intensity to photochemical reactions of surface defects and/or subsurface fractures with ambient molecules. Such quenching curves can be accurately reproduced by our theoretical model using two quenchable defect populations with different reaction rates. The fitting parameters of our model are spatially correlated to fractures in silica where point defects and mechanical stresses are known to be present, potentially indicating regions prone to laser-induced damage growth. We believe that our approach allows rapid spatial resolved identification of damage prone morphology, providing a new pathway to fast, non-destructive predictions of laser-induced damage growth.

Birefringent Glass-Engraved Tilted Pillar Metasurfaces for High Power Laser Applications.

Birefringent materials-which are highly needed in high power laser systems-may be limited in usage due to the laser-induced damage threshold of traditional birefringent materials. This work reports here on all-glass metasurfaces, fabricated by angled etching through sacrificial metal nanoparticle (NP) etching masks, for generation of effective birefringence in the formed layer. As a result, a fused silica metasurface, monolithic to the underlying substrate, is demonstrated to exhibit a birefringence of 6.57° under 375 nm illumination. Full-wave analysis shows a good agreement with the measurement and presents potential paths forward to increasing the effective metasurface birefringence. This is the first demonstration, to the best of knowledge, of an etching technique to obtain the resulting tilted pillar-like nanofeatures. The anisotropy of the metasurface nanoelements along the two window in-plane major axes presents different effective paths for the two polarizations and thus generates birefringence in a nonbirefringent material. Additionally, the imparted anisotropy lends itself to manipulation of physical properties of the surface as well, with metasurface feature orientation suppressing water flow along one principal axis and giving rise to water flow steering capabilities.

Channel Scaling Dependent Photoresponse of Copper-Based Flexible Photodetectors Fabricated Using Laser-Induced Oxidation.

Copper (Cu) oxide compounds (CuxO), which include cupric (CuO) and cuprous (Cu2O) oxide, have been recognized as a promising p-channel material with useful photovoltaic properties and superior thermal conductivity. Typically, deposition methods or thermal oxidation can be used to obtain CuxO. However, these processes are difficult to apply to flexible substrates because plastics have a comparatively low glass transition temperature. Also, additional patterning steps are needed to fabricate applications. In this work, we fabricated a metal-semiconductor-metal photodetector using laser-induced oxidation of thin Cu films under ambient conditions. Raman spectroscopy, scanning electron microscopy-energy-dispersive X-ray spectroscopy, and atomic force microscopy were used to study the composition and morphology of our devices. Moreover, the photoresponse of this device is reported herein. We performed an in-depth analysis of the relationship between the channel size and number of carriers using scanning photocurrent microscopy. The carrier transport behaviors were identified; the photocurrent decreased as the length and width of the channel increased. Furthermore, we verified the suitability of the device as a flexible photodetector using a variety of bending tests. Our in-depth analysis of this Cu-based flexible photodetector could play an important role in understanding the mechanisms of other flexible photovoltaic applications.

Laser-Induced Crystalline-Phase Transformation for Hematite Nanorod Photoelectrochemical Cells.

Generally, a high-temperature postannealing process is required to enhance the photoelectrochemical (PEC) performance of hematite nanorod (NR) photoanodes. However, the thermal annealing time is limited to a short duration as thermal annealing at high temperatures can result in some critical problems, such as conductivity degradation of the fluorine-doped tin oxide film and deformation of the glass substrate. In this study, selective laser processing is introduced for hematite-based PEC cells as an alternative annealing process. The developed laser-induced phase transformation (LIPT) process yields hematite NRs with enhanced optical, chemical, and electrical properties for application in hematite NR-based PEC cells. Owing to its improved properties, the LIPT-processed hematite NR PEC cell exhibits an enhanced water oxidation performance compared to that processed by the conventional annealing process. As the LIPT process is conducted under ambient conditions, it would be an excellent alternative annealing technique for heat-sensitive flexible substrates in the future.

Coupling buried etalon layers to an engraved metasurface for durable and large-aperture meta-optics.

Many optical applications that could potentially benefit from the design flexibility provided by the metasurface approach are being prohibited by the limited scalability of the fabrication and the robustness of the end-result structures when using a resonant meta-elements-based approach. An alternative demonstrated approach with superior scalability and robustness is substrate-engraved metasurfaces, based on medium mixing homogenization, yet it suffers from very limited optical response. Here we propose advancing this approach by coupling the metasurface with buried etalon layers, leading to enhancement in the optical response. A transfer matrix analysis is used to study the parameter space, predicting that the patterned reflectance values range of a beam shaper could be raised from only 4% to 30% when the metasurface is engraved in silica, and even up to 66% when engraved into higher-index oxides. Using the method proposed here, the phase difference range across the metasurface could be increased by 0.4 radians beyond the range achievable by a metasurface engraved in silica and could reach even higher values when embedded in higher-index materials. Full-wave numerical simulations are used to demonstrate a cylindrical metareflector and a metalens, further validating the analysis.

Additive Manufacturing of Optical Quality Germania-Silica Glasses.

Direct ink writing (DIW) three-dimensional (3D) printing provides a revolutionary approach to fabricating components with gradients in material properties. Herein, we report a method for generating colloidal germania feedstock and germania-silica inks for the production of optical quality germania-silica (GeO2-SiO2) glasses by DIW, making available a new material composition for the development of multimaterial and functionally graded optical quality glasses and ceramics by additive manufacturing. Colloidal germania and silica particles are prepared by a base-catalyzed sol-gel method and converted to printable shear-thinning suspensions with desired viscoelastic properties for DIW. The volatile solvents are then evaporated, and the green bodies are calcined and sintered to produce transparent, crack-free glasses. Chemical and structural evolution of GeO2-SiO2 glasses is confirmed by nuclear magnetic resonance, X-ray diffraction, and Raman spectroscopy. UV-vis transmission and optical homogeneity measurements reveal comparable performance of the 3D printed GeO2-SiO2 glasses to glasses produced using conventional approaches and improved performance over 3D printed TiO2-SiO2 inks. Moreover, because GeO2-SiO2 inks are compatible with DIW technology, they offer exciting options for forming new materials with patterned compositions such as gradients in the refractive index that cannot be achieved with conventional manufacturing approaches.

Optical modeling of random anti-reflective meta-surfaces for laser systems applications.

Optical performance of anti-reflective random meta-surfaces are studied numerically for coherent beam propagation systems, such as lasers. A methodology for the modeling of such optics performance is developed and applied to study the reflectivity and laser beam quality degradation. These quantitative metrics and design considerations highlight that reducing the size of the meta-surface period much below the light wavelength is not necessarily required.

Scalable Light-Printing of Substrate-Engraved Free-Form Metasurfaces.

A key challenge for metasurface research is locally controlling at will the nanoscale geometric features on meter-scale apertures. Such a technology is expected to enable large aperture meta-optics and revolutionize fields such as long-range imaging, lasers, laser detection and ranging (LADAR), and optical communications. Furthermore, these applications are often more sensitive to light-induced and environmental degradation, which constrains the possible materials and fabrication process. Here, we present a relatively simple and scalable method to fabricate a substrate-engraved metasurface with locally printed index determined by induced illumination, which, therefore, addresses both the challenges of scalability and durability. In this process, a thin metal film is deposited onto a substrate and transformed into a mask via local laser-induced dewetting into nanoparticles. The substrate is then dry-etched through this mask, and selective mask removal finally reveals the metasurface. We show that masking by the local nanoparticle distribution, and, therefore, the local index, is dependent on the local light-induced dewetting temperature. We demonstrate printing of a free-form pattern engraved into a fused silica glass substrate using a laser raster scan. Large-scale spatially controlled engraving of metasurfaces has implications on other technological fields beyond optics, such as surface fluidics, acoustics, and thermomechanics.

A simple, highly efficient route to electroless gold plating on complex 3D printed polyacrylate plastics.

Compared to tedious, multi-step treatments for electroless gold plating of traditional thermoplastics, this communication describes a simpler three-step procedure for 3D printed crosslinked polyacrylate substrates. This allows for the synthesis of ultralight gold foam microlattice materials with great potential for architecture-sensitive applications in future energy, catalysis, and sensing.

Single Pass Laser Process for Super-Hydrophobic Flexible Surfaces with Micro/Nano Hierarchical Structures.

Wetting has been studied in various fields: chemical industry, automobile manufacturing, food companies, and even life sciences. In these studies, super-hydrophobic surfaces have been achieved through complex steps and processes. To realize super-hydrophobicity, however, we demonstrated a simple and single pass laser process for the fabrication of micro/nano hierarchical structures on the flexible polytetrafluoroethylene (PTFE, Teflon) surface. The fabricated hierarchical structures helped increase the hydrophobicity by augmenting the surface roughness and promoting air-trapping. In addition, we employed a low-cost and high-throughput replication process producing numerous polydimethylsiloxane (PDMS) replicas from the laser-processed PTFE film. Thanks to the anti-adhesive characteristics of PTFE and the elasticity of PDMS, the structure perfectly transferred to the replica without any mechanical failure. Moreover, our designed mesh patterns offered the possibility of large area applications through varying the process parameters (pitch, beam spot size, laser fluence, and scan speed). Even though mesh patterns had relatively large pitch (190 µm), we were able to achieve high contact angle (>150°). Through pneumatically deformed structure, we clearly showed that the shape of the droplets on our laser-processed super-hydrophobic surface was spherical. Based on these outcomes, we can expect our single laser pulse exposure process can overcome many drawbacks and offer opportunities for advancing applications of the wetting phenomena.

Thermally ruggedized ITO transparent electrode films for high power optoelectronics.

We present two strategies to minimize laser damage in transparent conductive films. The first consists of improving heat dissipation by selection of substrates with high thermal diffusivity or by addition of capping layer heatsinks. The second is reduction of bulk energy absorption by lowering free carrier density and increasing mobility, while maintaining film conductance with thicker films. Multi-pulse laser damage tests were performed on tin-doped indium oxide (ITO) films configured to improve optical lifetime damage performance. Conditions where improvements were not observed are also described. When bulk heating is not the dominant damage process, discrete defect-induced damage limits damage behavior.

Efficient method for the measurement of lifetime optical damage performance of thin film coatings from laser damage size analysis.

A laser damage test method based on damage size analysis (DSA) is described that simplifies the derivation of the lifetime optical damage threshold of film materials critical in the design of devices used in high-repetition-rate, high-power laser systems. The DSA method presented here is solely based on imaging to measure the damage site size produced from exposure to a known Gaussian-shaped beam with a fixed, systematically selected fluence well above the ablation threshold. The method locates the damage boundary produced from repeated exposures, using images with a high contrast, and maps it to the beam profile to extract a lifetime laser damage fluence threshold value. We validate the DSA approach using a few relevant transparent film material systems and by comparing it to the standard S/1 laser damage test method. The DSA method can be more efficient and accelerate materials development and validation necessary to support the design of high-power repetition-rated lasers and optoelectronic devices.

Synthesis of Nanostructured/Macroscopic Low-Density Copper Foams Based on Metal-Coated Polymer Core-Shell Particles.

A robust, millimeter-sized low-density Cu foam with ∼90% (v/v) porosity, ∼30 nm thick walls, and ∼1 µm diameter spherical pores is prepared by the slip-casting of metal-coated polymer core-shell particles followed by a thermal removal of the polymer. In this paper, we report our key findings that enable the development of the low-density Cu foams. First, we need to synthesize polystyrene (PS) particles coated with a very thin Cu layer (in the range of tens of nanometers). A simple reduction in the amount of Cu deposited onto the PS was not sufficient to form such a low-density Cu foams due to issues related to foam collapse and densification upon the subsequent polymer removal step. Precise control over the morphology of the Cu coating on the particles is essential for the synthesis of a lower density of foams. Second, improving the dispersion of PS-Cu particles in a suspension used for the casting as well as careful optimization of a baking condition minimize the formation of irregular large voids, leading to Cu foams with a more uniform packing and a better connectivity of neighboring Cu hollow shells. Finally, we analyzed mechanical properties of the Cu foams with a depth-sensing indentation test. The uniform Cu foams show a significant improvement in mechanical properties (∼1.5× modulus and ∼3× hardness) compared to those of uncontrolled foam samples with a similar foam density but irregular large voids. Higher surface areas and a good electric conductivity of the Cu foams present a great potential to future applications.

Laser damage mechanisms in conductive widegap semiconductor films.

Laser damage mechanisms of two conductive wide-bandgap semiconductor films - indium tin oxide (ITO) and silicon doped GaN (Si:GaN) were studied via microscopy, spectroscopy, photoluminescence (PL), and elemental analysis. Nanosecond laser pulse exposures with a laser photon energy (1.03 eV, 1064 nm) smaller than the conductive films bandgaps were applied and radically different film damage morphologies were produced. The laser damaged ITO film exhibited deterministic features of thermal degradation. In contrast, laser damage in the Si:GaN film resulted in highly localized eruptions originating at interfaces. For ITO, thermally driven damage was related to free carrier absorption and, for GaN, carbon complexes were proposed as potential damage precursors or markers.

Facile fabrication of a superhydrophobic cage by laser direct writing for site-specific colloidal self-assembled photonic crystal.

Micron-sized ablated surface structures with nano-sized 'bumpy' structures were produced by femtosecond (fs) laser ablation of polytetrafluoroethylene (PTFE) film under ambient conditions. Upon just a single step, the processed surface exhibited hierarchical micro/nano morphology. In addition, due to the tribological properties of PTFE, polydimethylsiloxane (PDMS) could be replicated from the laser-ablated PTFE surface without anti-adhesive surface treatment. By controlling the design of the ablated patterns, tunable wettability and superhydrophobicity were achieved on both PTFE and PDMS replica surfaces. Furthermore, using fs laser ablation direct writing, a flexible superhydrophobic PDMS cage formed by superhydrophobic patterns encompassing the unmodified region was demonstrated for aqueous droplet positioning and trapping. Through evaporation-driven colloidal self-assembly in this superhydrophobic cage, a colloidal droplet containing polystyrene (PS) particles dried into a self-assembled photonic crystal, whose optical band gap could be manipulated by the particle size.

Directed dewetting of amorphous silicon film by a donut-shaped laser pulse.

Irradiation of a thin film with a beam-shaped laser is proposed to achieve site-selectively controlled dewetting of the film into nanoscale structures. As a proof of concept, the laser-directed dewetting of an amorphous silicon thin film on a glass substrate is demonstrated using a donut-shaped laser beam. Upon irradiation of a single laser pulse, the silicon film melts and dewets on the substrate surface. The irradiation with the donut beam induces an unconventional lateral temperature profile in the film, leading to thermocapillary-induced transport of the molten silicon to the center of the beam spot. Upon solidification, the ultrathin amorphous silicon film is transformed to a crystalline silicon nanodome of increased height. This morphological change enables further dimensional reduction of the nanodome as well as removal of the surrounding film material by isotropic silicon etching. These results suggest that laser-based dewetting of thin films can be an effective way for scalable manufacturing of patterned nanostructures.

Low-cost facile fabrication of flexible transparent copper electrodes by nanosecond laser ablation.

Low-cost Cu flexible transparent conducting electrodes (FTCEs) are fabricated by facile nanosecond laser ablation. The fabricated Cu FTCEs show excellent opto-electrical properties (transmittance: 83%, sheet resistance: 17.48 Ω sq(-1)) with outstanding mechanical durability. Successful demonstration of a touch-screen panel confirms the potential applicability of Cu FTCEs to the flexible optoelectronic devices.

Laser-induced direct graphene patterning and simultaneous transferring method for graphene sensor platform.

General methods utilized in the fabrication of graphene devices involve graphene transferring and subsequent patterning of graphene via multiple wet-chemical processes. In the present study, a laser-induced pattern transfer (LIPT) method is proposed for the transferring and patterning of graphene in a single processing step. Via the direct graphene patterning and simultaneous transferring, the LIPT method greatly reduces the complexity of graphene fabrication while augmenting flexibility in graphene device design. Femtosecond laser ablation under ambient conditions is employed to transfer graphene/PMMA microscale patterns to arbitrary substrates, including a flexible film. Suspended cantilever structures are also demonstrated over a prefabricated trench structure via the single-step method. The feasibility of this method for the fabrication of functional graphene devices is confirmed by measuring the electrical response of a graphene/PMMA device under laser illumination.