Chih‐Hao Chang

Dust contamination is a key challenge for deployment of optics in harsh environments, maintenance of photovoltaics, and the pursuit of sustainable interplanetary exploration. In this work, the dust mitigation properties of nanostructured substrates with thin insulating and conductive coatings are investigated. To examine the contribution of capillary, electrostatic, and van der Waals forces to the surface's overall dust adhesion, the relative humidity is varied to control their relative contributions. Experiments show that samples with conductive coatings can have up to 91.0% less coverage than insulating sample under low humidity. The results indicate that the electrical properties of surface coatings play a significant role in mitigating dust adhesion forces at low humidities, where electrostatic forces dominate. In addition, reduced surface energy and nanostructured features are key for an improved anti-dust performance at all humidities. The results demonstrate that the nanostructure with conductive coatings is highly anti-dust and has less than 2.5% percentage area coverage throughout the humidity range. This research improves understanding of the interparticle forces between substrate and particulate and explores viable alterations of surface geometry and chemistry for passive dust mitigation that are applicable across a broad humidity range.

In this work, the effect of nanostructures and surface coatings on the effective electrostatic force acting on a particle resting on the surface is investigated. A proposed analytical model based on Coulomb’s law predicts that electrostatic forces on a particle decrease with the addition of nanostructures due to increased separation distance. Additional comsol simulations were utilized to predict interactions with a highly charged surface relative to the particle, and an inverse interaction is seen. In this case, the added surface geometry leads to an increased electrostatic force on the particle due to greater surface area and charge concentration at structure peaks. Planar and nanostructured polycarbonate surfaces coated with Al2O3, TiO2, Pt, or left bare are contaminated with lunar dust simulant and charged with an electron beam to confirm these predictions. The number of ejected dust particles when a charge is applied is quantified for each of these samples and compared to the initial predictions. The addition of nanostructures on the highly charging polycarbonate substrate led to a doubling of the frequency of particle removal, whereas the low charging Al2O3 and TiO2 samples saw a tenfold decrease in particle removal. By engineering tunable surface responses to electrostatic forces, dust mitigating surfaces will be designed for enhanced performance in charge inducing enviroments, such as the lunar surface.

In this paper, we investigate the viability of using ZEP520A, a commercially available electron-beam photoresist, as a positive resist for extreme ultraviolet (EUV) lithography at 30 nm wavelength. The resist characterization was performed using a tabletop, high-harmonic-generation EUV source driven by an ultrafast laser. The source produces an elliptical beam at focus, modeled as a Gaussian intensity profile to analyze the exposure dose distribution. A comprehensive protocol was developed, employing confocal microscopy to generate topography maps and extract critical dimensions of the patterned resist. AFM (atomic force microscopy) was subsequently used to measure postdevelopment resist thickness and refine estimates of the clearing dose and contrast, modeled with a binary resist approach. Experimental results show that ZEP520A exhibits a clearing dose of 35.4 mJ/cm2 and a contrast of 2.9 at 30 nm wavelength, supporting its application in EUV lithography. These findings demonstrate that ZEP520A can serve as an effective EUV resist and that a tabletop EUV source provides a practical platform for material characterization in next-generation lithography development.

Achieving higher resolution in high-density chip fabrication requires shorter wavelengths and higher-energy photons, making EUV lithography essential. As EUV becomes the standard, there is growing interest in developing photoresists with high sensitivity, contrast, and etch selectivity. However, testing these materials typically requires costly and limitedaccess EUV sources. Tabletop systems offer a more accessible alternative for accelerating resist development. This work evaluates ZEP520A, a commercially available electron beam resist, as a positive photoresist for EUV lithography at a 30 nm wavelength. Characterization was performed using a tabletop high-harmonic generation (HHG) EUV source driven by an ultrafast laser, with dose distribution estimated by modeling the elliptical beam as a 2D Gaussian. A protocol that combined confocal microscopy and atomic force microscopy enabled the extraction of key metrics using a binary resist model, which assumes a critical clearing dose. Regions exposed to doses above this threshold are completely removed during development, resulting in zero remaining thickness, while regions exposed to lower doses retain their original thickness. ZEP520A exhibited a clearing dose of 35.4 mJ/cm² and a contrast of 2.9. Additional grid tests near the clearing dose may further improve accuracy of the initial results. The developed protocol is broadly applicable for testing both positive and negative-tone resists, such as indium hydrate nitrate, and highlights the practicality of tabletop EUV systems for photoresist development.

This research investigates the optical anisotropy and structure-induced birefringence in low-index nanolattices. By designing the unit-cell geometry using 3-dimentional (3D) colloidal lithography, nanolattices can exhibit different refractive indices along orthogonal directions due to the structure geometry. The out-of-plane and in-plane indices are characterized using spectroscopic ellipsometry and agree well with the anisotropic Cauchy material model. Exhibit positive-uniaxial birefringence, the nanolattices can have up to Δ n = 0.003 for nanolattices with low indices that range from 1.04 to 1.12. The birefringence is modeled using the finite-difference-time-domain (FDTD) method, where the reflectance of an anisotropic film is calculated to iteratively solve for the indices. The theoretical model and experimental data indicate that the birefringence can be controlled by the unit-cell geometry based on the relative length scale of the particle diameter to the exposure wavelength. This work demonstrates that it is possible to precisely design optical birefringence in 3D nanolattices, which can find applications in polarizing optics, nanophotonics, and wearable electronics.

Nanostructured materials and nanolattices with high porosity can have novel optical and mechanical properties that are attractive for nanophotonic devices. One existing challenge is the integration of microstructures that can be used as waveguides or electrodes on such nanostructures without filling in the pores. This study investigates the fabrication of TiO2 microstructures on nanolattices using a stencil mask. In this approach, the nanostructures are planarized with a polymer film while the microstructures are patterned in a sequential shadow deposition step. Our results demonstrate the successful fabrication of a “dog-bone” microstructure with 400 μm length, 100 μm width, and 30–560 nm thicknesses on nanostructure with 390 and 500 nm period. The experimental results show that cracks can form in the microstructures, which can be attributed to residual stress and the thermal annealing cycle. A key finding is that the film cracks decrease as the TiO2 layer becomes thinner, highlighting an important relationship between grain size distribution and the film thickness. The mechanical stability of the underlying nanolattices also plays a key role, where interconnected architecture mitigated the crack formation when compared with isolated structures. The demonstrated fabrication process can lead to integrated waveguides and microelectrodes on nanolattices, which can find applications for next-generation photonic and electronic devices.

This study presents the development of a 3D random-modulated pulse lidar based on a gain-switched semiconductor laser with a recirculating delay lines interferometer (RDLI). The random-modulated pulses are generated by homodyning the frequency-shifting gain-switched pulses with multiple self-delays. While they exhibit anti-interference characteristics similar to those in previously developed chaos-modulated lidar, there is no need for external pulse formation and wavelength-sensitive filtering components in the current configuration. By varying the injection currents in gain-switching and the delay lengths in the RDLI, we experimentally investigated the transient dynamics of the generated random-modulated pulses. We demonstrated how these operating parameters influence the modulation in their waveforms and spectra. The detection performance was quantified by calculating the effective bandwidths, signal-to-noise ratios, Cramér-Rao lower bounds, range precision, self-interference peaks, and detection probability. We identified two key operating conditions: the best-precision condition and the precision-interference balanced condition. Compared to a previously investigated delay self-homodyne interferometer (DSHI) scheme, which homodynes the gain-switched pulses with just a single delay, the RDLI scheme achieved a precision as low as 0.46 mm, approximately 1.5 times better than the DSHI scheme. To demonstrate its superior performance and feasibility for detection, we integrated the RDLI into a 3D lidar imaging system and compared its performance to the DSHI and chaos lidar schemes. To highlight its improved precision and robustness to temperature variations, we evaluated its precision under varying average output power and changes in laser temperature. With the developed lidar system, we successfully achieved high-quality face profiling of a person with millimeter-level precision.

Sapphire is an attractive material that stands to benefit from surface functionalization effects stemming from micro/nanostructures. Here we investigate the use of ultrafast lasers for fabricating sapphire nanostructures by exploring the relationship between irradiation parameters, morphology change, and selective etching. In this approach a femtosecond laser pulse is focused on the substrate to change the crystalline morphology to amorphous or polycrystalline, which is characterized by examining different vibrational modes using Raman spectroscopy. The irradiated regions are removed using a subsequent hydrofluoric acid etch. Laser confocal measurements quantify the degree of selective etching. The results indicate a threshold laser pulse intensity required for selective etching. This process was used to fabricate hierarchical sapphire nanostructures over large areas with enhanced hydrophobicity, with an apparent contact angle of 140 degrees, and a high roll-off angle, characteristic of the rose petal effect. Additionally, the structures have high broadband diffuse transmittance of up to 81.8% with low loss, with applications in optical diffusers. Our findings provide new insights into the interplay between the light-matter interactions, where Raman shifts associated with different vibrational modes can predict selective etching. These results advance sapphire nanostructure fabrication, with applications in infrared optics, protective windows, and consumer electronics.

Although there has been significant interest in the novel material properties of bio-inspired nanostructures, engineering them to become mechanically durable remains a significant challenge. This work demonstrates the fabrication of sapphire nanostructures with anti-glare, anti-fogging, anti-dust and scratch-resistant properties. The fabricated nanostructures demonstrated a period of 330 nm and an aspect ratio of 2.1, the highest reported for sapphire thus far. The nanostructured sapphire sample exhibited broadband and omnidirectional antireflection properties, with an enhanced transmission of up to 95.8% at a wavelength of 1360 nm. The sapphire nanostructures also exhibited enhanced wetting performance and could mitigate fogging from water condensation or repel water droplets. Furthermore, owing to their sharp features, the fabricated structures could prevent particulate adhesion and maintain a 98.7% dust-free surface area solely using gravity. Furthermore, nanoindentation and scratch tests indicated that the sapphire nanostructures have an indentation modulus and hardness of 182 GPa and 3.7 GPa, respectively, which are similar to those of bulk glass and scratch-resistant metals such as tungsten. These sapphire nanostructures can be fabricated using high-throughput nanomanufacturing techniques and can find applications in scratch-resistant optics for photonics, electronic displays, and protective windows.

Superhydrophilic periodic nanostructures demonstrate size-dependent nanoparticle separation through wicking, revealing tunable filtering behavior and enabling low-cost detection of nanoparticle size distributions in a fluid.

Sapphire is an attractive material in photonic, optoelectronic, and transparent ceramic applications that stand to benefit from surface functionalization effects stemming from micro/nanostructures. Here we investigate the use of ultrafast lasers for fabricating nanostructures in sapphire by exploring the relationship between irradiation parameters, morphology change, and selective etching. In this approach an ultrafast laser pulse is focused on the sapphire substrate to change the crystalline morphology to amorphous or polycrystalline, which is characterized by examining different vibrational modes using Raman spectroscopy. The irradiated regions are then removed using a subsequent wet etch in hydrofluoric acid. Laser confocal measurements conducted before and after the etching process quantify the degree of selective etching. The results indicate that a threshold laser pulse intensity is required for selective etching to occur. This process can be used to fabricate hierarchical sapphire nanostructures over large areas with enhanced hydrophobicity, which exhibits an apparent contact angle of 140 degrees and a high roll-off angle that are characteristic of the rose petal effect. Additionally, the fabricated structures have high broadband diffuse transmittance of up to 81.8% with low loss, which can find applications in optical diffusers. Our findings provide new insights into the interplay between the light-matter interactions, where Raman shifts associated with different vibrational modes can be used as a predictive measure of selective etching. These results advance the development of sapphire nanostructure fabrication, which can find applications in infrared optics, protective windows, and consumer electronics.

Eliminating the need for multilayer alignment in nanoscale manufactured devices will streamline the lithography process and open up avenues for flexible substrate roll-to-roll (R2R) manufacturing. A system capable of single-exposure 3D holographic lithography with in-line metrology and real-time feedback will revolutionize micro and nano manufacturing. Work towards such developments are demonstrated to show promise in the field of nanopatterning.

We report an optical cavity design that combines a distributed feedback (DFB) cavity as the primary feedback element for lasing with a silver mirror acting as a Fabry-Pérot cavity for broadband reflection and mode confinement. To evaluate the design, we studied the effects of the silver mirror by excluding the DFB cavity and compared its amplified spontaneous emission (ASE) properties with the sample without the mirror. In the structure with the mirror, the gain medium undergoes ASE at an excitation fluence of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$17.5\,\,\mathrm { {\mu J c}}\mathrm {m}^{\mathrm {-2}}$ </tex-math></inline-formula> compared to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$37\,\,\mathrm { {\mu J c}}\mathrm {m}^{\mathrm {-2}}$ </tex-math></inline-formula> for the sample without the mirror. This lower ASE threshold is attributed to enhanced mode confinement and photon density of states (PDOS) from the silver mirror increasing the cavity photon lifetime ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ {\tau }_{\mathrm {c}}\mathrm {)}$ </tex-math></inline-formula> . Using this hybrid cavity, a multimode optically pumped laser with a threshold of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$42\,\,\mathrm { {\mu J c}}\mathrm {m}^{\mathrm {-2}}$ </tex-math></inline-formula> is demonstrated. This hybrid cavity design offers an effective solution that can be readily applied to other thin film-based laser devices.

Dielectric mirrors based on Bragg reflection and photonic crystals have broad application in controlling light reflection with low optical losses. One key parameter in the design of these optical multilayers is the refractive index contrast, which controls the reflector performance. This work reports the demonstration of a high-reflectivity multilayer photonic reflector that consists of alternating layers of TiO 2 films and nanolattices with low refractive index. The use of nanolattices enables high-index contrast between the high- and low-index layers, allowing high reflectivity with fewer layers. The broadband reflectance of the nanolattice reflectors with one to three layers has been characterized with peak reflectance of 91.9% at 527 nm and agrees well with theoretical optical models. The high-index contrast induced by the nanolattice layer enables a normalize reflectance band of Δλ/λ o of 43.6%, the broadest demonstrated to date. The proposed nanolattice reflectors can find applications in nanophotonics, radiative cooling, and thermal insulation.

This work reports sapphire nanostructures with broadband and omnidirectional antireflection properties. The structures also mitigate dust adhesion and has increased surface scratch resistance and can find applications in protective windows in extreme environment.

This research involves designing and demonstrating broadband near-100% reflectance in multilayer 3D nanolattice reflectors with stacked layers of alternating high (TiO 2 ) and low (Al 2 O 3 ) refractive indices by varying geometric properties like layer thickness and periodicities.

Extreme ultraviolet (EUV) lithography has pushed the limits of optical lithography techniques, enabling patterning of highresolution feature sizes and driving innovation in the semiconductor industry. Traditional top-down lithography processes face challenges with EUV interference lithography, such as requiring precise incidence angles and specialized, expensive multilayer mirrors for nanostructure patterning. Colloidal nanosphere lithography offers an effective and inexpensive nearfield technique, utilizing self-assembled nanoparticles to create complex 2D and 3D nanostructures. However, existing work primarily uses UV lasers, limiting pattern resolution to above 100 nm. In this study, we propose using colloidal nanosphere lithography combined with a 30 nm wavelength EUV light source to pattern periodic geometric patterns. We describe the proposed system, which consists of a high harmonic generation source pumped by an ultrafast laser at wavelength of 30 nm. The beam shapes and characteristics are characterized and presented. Initial results demonstrate successful patterning using the colloidal assembly serves as a mask, achieving complex geometric patterns with sub-50 nm feature sizes. This work explores a low-cost and scalable approach to studying EUV light interaction through colloidal nanosphere assemblies for 3D nanostructure patterning.

Recent developments in photonic devices, light field display, and wearable electronics have resulted from a competitive development toward new technologies to improve the user experience in the field of optics. These advances can be attributed to the rise of nanophotonics and meta-surfaces, which can be designed to manipulate light more efficiently. In these elements the performance scales are favorable to the index contrast, making the use of low-index material important. In this research, we examine the precise control of refractive indices of a low-index nanolattice material. This approach employs three-dimensional (3D) lithography and atomic layer deposition (ALD), allowing for precise control of the nanolattice geometry and its refractive index. The refractive indices of the fabricated nanolattices are characterized using spectroscopic ellipsometry and agree well with models based on effective medium theory. By controlling the unit-cell geometry by the exposure conditions and the shell thickness by the ALD process, the effective index of the nanolattice film can be precisely controlled to as low as 5 × 10 −4 . The proposed index control technique opens a gamut of opportunities and enables better performance in nanophotonic elements used in displays and other integrated devices.

Fabrication of nanostructures on sapphire surfaces can enable unique applications in nanophotonics, optoelectronics, and functional transparent ceramics. However, the high chemical stability and mechanical hardness of sapphire make the fabrication of high density, high aspect ratio structures in sapphire challenging. In this study, we propose the use of optical emission spectroscopy (OES) to investigate the sapphire etching mechanism and for endpoint detection. The proposed process employs nanopillars composed of polymer and polysilicon as an etch mask, which allows the fabrication of large-area sapphire nanostructures. The results show that one can identify the emission wavelengths of key elements Al, O, Br, Cl, and H using squared loadings of the primary principal component obtained from principal component analysis of OES readings without the need of domain knowledge or user experience. By further examining the OES signal of Al and O at 395.6 nm, an empirical first-order model can be used to find a predicted endpoint at around 170 s, indicating the moment when the mask is completely removed, and the sapphire substrate is fully exposed. The fabrication results show that the highest aspect ratio of sapphire nanostructures that can be achieved is 2.07, with a width of 242 nm and a height of 500 nm. The demonstrated fabrication approach can create high sapphire nanostructures without using a metal mask to enhance the sapphire etch selectivity.

Recent advanced in nanofabrication has enabled various opportunities for research and development in photonic crystals, integrated circuits, and nanostructured materials. One interesting class of emerging materials is nanolattices, which consist of hollow-core, thin-shell elements fabricated using thin-film deposition on three-dimensional polymer templates. While many applications of nanolattices have been demonstrated, the residual polymer in the nanolattice can be problematic and is not well understood. This research investigates the effectiveness of different template removal techniques, including oxygen plasma etching, solvent dissolution, and thermal desorption. The rates and effectiveness of resist removal for the different techniques are quantified using spectroscopic ellipsometry, which enables precise measurement of the effective refractive index and calculation of the residual polymer. A three-phase Maxwell–Garnett effective medium model is used to calculate the residual polymer in the nanolattices. This work demonstrates that the temperature treatment is most effective at template removal, which can be used to improve the fabrication of nanolattices for mechanical, optical, and thermal applications.

Dust-mitigating surfaces typically consist of high-aspect-ratio structures that separate particles from resting on the bulk material, thereby limiting adhesion due to short-range van der Waals forces. These surfaces can find uses in solar-panel coatings and a variety of dust-resistant optics. The current method for quantifying surface contamination is optical microscopy, but this method is inadequate for observing particles at the submicrometer scale due to the diffraction limit. Furthermore, regardless of the microscopy technique, particle identification becomes problematic as the particle contaminates approach the same length scale of the surface structures. In this work, we demonstrate a method to identify micro-/nanoparticle contaminates on nanostructured surfaces using electron microscopy and image processing. This approach allows the characterization of particles that approach the length scale of the surface structures. Image processing, including spectrum filters and edge detection, is used to remove the periodic features of the surface nanostructure to omit them from the particle counting. The detection of these small particles using electron microscopy leads to an average of 5.62 particles/100 μm2 detected compared to 0.63 particles/100 μm2 detected for the traditional confocal optical detection method. Beyond dust-mitigation nanostructures, the demonstrated particle detection technique can find applications in nanobiology, the detection of ice nucleation on a structured surface, and semiconductor mask inspections.

In this paper, we investigate the self-assembly of hierarchical nanostructures using monodispersed nanospheres with two different diameters. Our approach is to use a two-step method where the assembly of larger 200 nm nanospheres is used to direct the assembly of smaller 50 nm particles. This self-assembly technique is based on Langmuir–Blodgett assembly and has low equipment cost when compared with traditional lithography methods. We examine the effects of substrate surface treatment, solution concentration ratio, and spin speeds on the quality of the hierarchical assembly. The fabricated samples are examined using optical and scanning electron microscopy to investigate assembly yield. Various defect types are identified and mitigated by process control. The ability to create more complex assembly can result in smaller features and can enhance the performance of photonics and nanostructured surfaces.

Direct-write laser ablation is an effective manufacturing method for etching complex microscale patterns, especially on hard ceramics such as sapphire that are difficult to machine using traditional mechanical or micromachining methods. However, the variability of the laser–matter interaction causes inconsistencies that prevent this process from moving beyond the research realm. This work presents the real-time monitoring of the ablation process in sapphire using optical emission spectroscopy to assess the key wavelengths that exhibit strong correlations to the fabricated features. In this process, a focused ultrafast laser is used to create microscale features and morphological changes in sapphire substrates, which are studied by a subsequent wet etching in a hydrogen fluoride solution. The etched sapphire samples are observed to have amorphous sapphire removed, resulting in microstructures with higher profile fidelity. Furthermore, principal component analysis of the measured spectral obtained during the etch process indicates that the emission from a few key wavelengths exhibits strong correlations to the etched sapphire patterns. This result indicates that the use of data-driven techniques to assess the spectral emissions of direct-write laser ablation can be a useful tool in developing in situ metrology methods for laser-matter interactions.

In this work, we present a binary assembly model that can predict the co-assembly structure and spatial frequency spectra of monodispersed nanoparticles with two different particle sizes. The approach relies on an iterative algorithm based on geometric constraints, which can simulate the assembly patterns of particles with two distinct diameters, size distributions, and at various mixture ratios on a planar surface. The two-dimensional spatial-frequency spectra of the modeled assembles can be analyzed using fast Fourier transform analysis to examine their frequency content. The simulated co-assembly structures and spectra are compared with assembled nanoparticles fabricated using transfer coating method are in qualitative agreement with the experimental results. The co-assembly model can also be used to predict the peak spatial frequency and the full-width at half-maximum bandwidth, which can lead to the design of the structure spectra by selection of different monodispersed particles. This work can find applications in fabrication of non-periodic nanostructures for functional surfaces, light extraction structures, and broadband nanophotonics.

Periodic nanostructures have important applications in nanophotonics and nanostructured materials as they provide various properties that are advantageous compared to conventional solid materials. However, there is a lack of metrology techniques that are suitable for large-scale manufacturing, as the traditional tools used in nanotechnology have limited throughput and depth resolution. In this work, we use spectroscopic scatterometry as a fast and low-cost alternative to characterize the porosity of three-dimensional (3D) periodic nanostructures. In this technique, the broadband reflectance of the structure is measured and fitted with physical models to predict the structure porosity. The process is demonstrated using 3D periodic nanostructures fabricated using colloidal phase lithography at various exposure dosages. The measured reflectance data are compared with an optical model based on finite-difference time-domain and transfer-matrix methods, which show qualitative agreement with the structure porosity. We found that this technique has the potential to further develop into an effective method to effectively predict the porosity of 3D nanostructures and can lead to real-time process control in roll-to-roll nanomanufacturing.

Dust accumulation is detrimental to optical elements, electronic devices, and mechanical systems and is a significant problem in space missions and renewable energy deployment. In this paper, we report the demonstration of antidust nanostructured surfaces that can remove close to 98% of lunar particles solely via gravity. The dust mitigation is driven by a novel mechanism, whereby particle removal is facilitated by the formation of particle aggregates due to interparticle forces, allowing the particles to be removed in the presence of other particles. The structures are fabricated using a highly scalable nanocoining and nanoimprint process, where nanostructures with precise geometry and surface properties are patterned on polycarbonate substrates. The dust mitigation properties of the nanostructures have been characterized using optical metrology, electron microscopy, and image processing algorithms to demonstrate that the surfaces can be engineered to remove nearly all of the particles above 2 μm in size in the presence of Earth's gravity. Compared to the 35.0% area coverage on a smooth polycarbonate surface, the particle coverage on nanostructures with 500 nm period is significantly reduced to 2.4%, an improvement of 93%. This work enhances the understanding of the particulate adhesion on textured surfaces and demonstrates a scalable, effective solution to antidust surfaces that can be broadly applied to windows, solar panels, and electronics.