Antireflection Microstructures Research Articles

Antireflection microstructures on ZnSe for mid- and far-IR fabricated by femtosecond laser ablation assisted with wet chemical etching

Most infrared materials used in high-power systems, such as optical parametric generators, have high values of refractive indices, which result in high Fresnel losses. The performance of conventional antireflection coatings is limited when used in high-power and ultra-broadband systems. An alternative approach is to fabricate antireflection microstructures (ARMs) that allow for a broadband increase in transmittance without reducing the damage threshold of the material. In this work, ARMs were fabricated on the surface of ZnSe crystals using the femtosecond laser ablation assisted with wet chemical etching method. This allowed to produce high aspect ratio microstructures that increase the transmittance up to 98% in the mid- and far- infrared regions.

Laser induced damage threshold of GaSe with antireflection microstructures at a wavelength of 5 µm.

Large GaSe crystals were grown and various antireflection microstructures (ARMs) were fabricated on their cleaved surfaces using optimized femtosecond laser ablation, which provided the antireflection effect in a wide wavelength range of 4-16µm. The influence of ARMs created on the GaSe surface on the change of the laser-induced damage threshold (LIDT) of the crystal at a wavelength of 5 μm was evaluated. The 5-µm Fe:ZnMgSe laser with the pulse duration of 135 ns was used for the LIDT test in conditions close to single pulse exposure. The measured values of LIDT of 56 ± 6MW/cm2 and 51 ± 9 MW/cm2 for two GaSe substrates, respectively, were comparable with the known data of single pulse LIDT of GaSe. The average LIDT intensities of 54 ± 6MW/cm2 and 52 ± 7 MW/cm2 for the ARMs at two GaSe plates, respectively, were close to LIDT intensities for the corresponding GaSe substrates. The ARMs with lower structural quality had lower LIDT (50-52 MW/cm2) in comparison with the high-quality ARMs (58-60 MW/cm2). High LIDT for high-quality ARMs can be caused by increased selenium content in the ARMs. In any case, all the tested ARMs on the GaSe plates with different surface quality are workable for development of widely tunable mid-infrared nonlinear optical converters.

Efficient fabrication of infrared antireflective microstructures on a curved Diamond-ZnS composite surface by using femtosecond Bessel-like beams.

Antireflective microstructures fabricated using femtosecond laser possess wide-ranging applicability and high stability across different spectral bands. However, due to the limited aspect ratio of the focused light field, traditional femtosecond laser manufacturing faces challenges in efficiently fabricating antireflective microstructures with high aspect ratio and small period, which are essential for antireflection, on curved surfaces. In this study, we present a robust and efficient method for fabricating high-aspect-ratio and basal surface insensitive antireflective microstructures using a spatially shaped Bessel-like beam. Based on theoretical simulation, a redesigned telescopic system is proposed to flexibly equalize the intensity of the Bessel beam along its propagation direction, facilitating the fabrication of antireflective subwavelength structures on the entire convex lens. The fabricated microstructures, featuring a width of less than 2 µm and a depth of 1 µm, enhance transmittance from 75% to 85% on Diamond-ZnS composite material (D-ZnS) surfaces. Our approach enables the creation of high aspect ratio subwavelength structures with a z-position difference exceeding 600 µm. This practical, efficient, and cost-effective method is facilitated for producing antireflective surfaces on aero-optical components utilized in aviation.

SWIR anti-reflective nanostructures on nonlinear crystals by direct UV femtosecond laser printing

Nonlinear infrared (IR) crystals for radiation conversion are of paramount importance for realization of advanced laser spectrometers for medical diagnostics, environmental monitoring, and advanced sensing. However, performance of such crystals suffers from substantial surface reflectivity coming from rather high (over 2.5) refractive index of the key nonlinear materials used. Here, based on the example of promising BaGa4Se7 nonlinear crystal, we attested direct surface patterning with ultraviolet (257 nm) femtosecond laser pulses used to engrave anti-reflective microstructures (ARMs) directly on both output sides of the crystal. Imprinted surface nanotrenches arranged into a fish-net morphology with a periodicity down to 500 nm was found to increase transmittance of the crystals from 65% to 84% within a practically relevant shortwave IR spectral range. Formation of the ARMs with an optimized geometry is expected to weakly reduce the laser damage threshold of a pristine crystal material as it was also evidenced from supporting full-wave simulations and tests.

Material-structure collaborative design for broadband microwave absorption metamaterial with low density and thin thickness

There is a great demand for the metamaterials with low density, thin thickness and broadband microwave absorption in the aeronautic stealth domain. However, it is still a great challenge to achieve satisfactory impedance matching between free space and metamaterials subjected to above constraints. To address this issue, a material-structure collaborative approach was proposed for designing excellent microwave absorption metamaterial with low density and thin thickness, which includes nanomaterial synthesis, unit cell optimal design and absorption mechanism analysis. The 3D Graphene-Carbon nanotubes-Nickel nanomaterial was firstly synthesized by in-situ growth method for fabricating the low-density metamaterial. Inspired by the natural anti-reflection microstructure of Polygonia c-aureum's eye, the unit cell of metamaterial was designed. Size parameters optimization of designed unit cell was conducted with the efficient immune genetic algorithm. The optimized metamaterial is extremely advantageous in effective absorption bandwidth (EAB, reflection loss ≤ −10 dB), low density and thin thickness, which can achieve 29.1 GHz EAB and equivalent density as low as 0.65 g/cm3 (the areal-density is 3.58 kg/m2) with thickness of only 5.5 mm. The proposed material-structure collaborative approach provides an efficient and systematic avenue for designing broadband microwave absorption metamaterials.

Preparation of Antireflection Microstructures on ZnSe Crystal by Femtosecond Burst Bessel Direct Laser Writing

In this work, we fabricated the antireflection microstructures (ARMs) on ZnSe surfaces using a femtosecond Bessel direct laser writing in burst mode. The morphology and transmittance performance of ARMs with different single-pulse energies (from 200 nJ to 500 nJ), different burst modes (burst 1, 3, and 5 modes), different periods (from 3 μm to 6 μm), and different arrangements were investigated. The results revealed that tetragonally arranged ARMs fabricated by 500 nJ of single-pulse energy, the burst 3 mode, and a period of 3 μm show the best transmittance performance. The average transmittance of the ARMs was about 17.13% higher than that of bulk ZnSe in the range of 8–12 μm, and the highest transmittance of 81.75% (an improvement of 18.63% on one side of the ZnSe) was achieved at 12.36 μm. This process makes it possible to enhance ARMs’ transmittance in the infrared wavelength range by using direct laser writing in burst mode.

Antireflection microstructures fabricated on the surface of a LiGaSe2 nonlinear crystal.

LiGaSe2 is a propitious material for nonlinear parametric conversion in the mid-infrared (mid-IR) range. Its refractive index of n = 2.25 in the 2-12 µm wavelength range results in significant losses due to Fresnel reflection. However, the conventional method of increasing the transmittance with antireflection coatings (ARCs) significantly reduces the damage threshold of the material. Fabrication of the antireflection microstructures (ARMs) is an alternative approach for increasing the surface transmittance. In this work, ARMs were fabricated on the surface of a LiGaSe2 crystal using a single-pulse femtosecond laser ablation method. An average transmittance of 97.2% in the 2-8 µm spectral range and the maximum transmittance of 98.6% at 4.1 µm were achieved.

Femtosecond Laser Direct Writing of Antireflection Microstructures on the Front and Back Sides of a GaSe Crystal

The development of antireflection coatings is crucially important to improve the performance of various photonic devices, for example, to increase the efficiency of harmonic generators based on high-refractive index crystals with significant Fresnel losses. A promising technique for the reducing of radiation reflection is to change the refractive index by fabrication of antireflection microstructures (ARM) on the surface. This paper presents the results of ARM direct writing on the surfaces of a nonlinear GaSe crystal (of ε modification, according to Raman and photoluminescence spectroscopy data) using fs laser radiation and a multiples approach. An increase in transmission from 65% to 80% for an ARM fabricated on one side of the crystal and up to 94% for ARMs fabricated on both sides is demonstrated. The increase in transmission with the increasing pulse energy, as well as with an increase in the number of pulses used for the formation of a single crater, is shown. The experimental results of ARM transmission of GaSe are in qualitative agreement with the simulation results based on the measured profiles and morphology of the ARM structures.

Antireflective array on zinc sulfide fabricated by femtosecond laser

Surface antireflection of zinc sulfide (ZnS) is crucial to improve the performance of infrared windows. In this study, femtosecond laser Bessel beam micromachining is proposed to fabricate antireflective microstructures (ARMs) on ZnS to suppress surface Fresnel reflection. The ARM consisting of approximately one hundred thousand holes within 30 × 30 mm2 is uniformly spread on the ZnS surface. The Fabry-Perot (FP) and Mie resonances in the multi-resonant surface work together to reduce reflection and are enhanced with increasing ARM depth. The ARM depth is tunable by adjusting the pulse energy and is utilized to facilely regulate the reflectance spectra as well as wettability. Finally, the fabricated ARM with wide-angle antireflection and hydrophobicity proved the effectiveness of the proposed method.

Highly Water-Repellent and Anti-Reflective Glass Based on a Hierarchical Nanoporous Layer

Optically anti-reflective and water-repellent glass is required for solar cell covers to improve power-generation efficiency due to transparency improvement and dirt removal. Research has been conducted in recent years on technologies that do not use fluorine materials. In this study, we focused on the anti-reflective properties and microstructure of hierarchical nanoporous layer (HNL) glass and used it as a substrate. As a result, we have achieved both strong anti-reflectivity and high water repellency on HNL glass by coating polydimethylsiloxane (PDMS) using baking and thermal chemical vapor deposition (CVD). The surfaces showed a significantly higher sliding velocity of water droplets than the PDMS-treated material on the flat glass plate. They also showed such water repellency that the droplets bounced off the surface.

Direct nanoimprint of chalcogenide glasses with optical functionalities via solvent-based surface softening.

Chalcogenide glasses are attractive materials for optical applications. However, these applications often require patterning of the surface with functional micro-/ nanostructures. Such patterning is challenging by traditional microfabrication methods. Here, we present a new, to the best of our knowledge, approach of direct imprint via solvent-based surface softening, for the patterning of As2Se3 surface. Our approach is based on an elastomeric stamp soaked in an organic solvent. During the imprint, the solvent diffuses into the imprinted substrate, plasticizes its surface, and thereby allows its imprint at the temperature below its glass transition point. Thus, our approach combines the full pattern transfer with the maintenance of the shape of the imprinted substrate, which is necessary for optical devices. By using this approach, we demonstrated functional antireflective microstructures directly imprinted on As2Se3 surface. Furthermore, we showed that our approach can produce imprinted features sized down to 20 nm scale. We believe that our new approach paves the way for more future applications of chalcogenide glasses.

Effect of antireflection microstructures on the optical properties of GaSe

The main application area for GaSe crystals is nonlinear optics, but there are problems with machining and antireflection coatings due to the layered structure of GaSe. Large GaSe crystals with dominant ε-modification were grown. Using various harmonics of a femtosecond laser, antireflection microstructures (ARM) were fabricated by laser ablation on the plate surface and modes providing increased transmission up to 90% in the mid-IR were selected. Using SEM and optical microscopy, as well as EDX and optical spectroscopy, defects on the GaSe surface and their effect on the spectral region and the degree of transmission increase were studied.

All-laser-microprocessed waveguide Cr:ZnS laser

We report the proof-of-concept of an operational laser active medium with a depressed cladding waveguide manufactured in the volume of a Cr2+:ZnS single-crystalline sample and antireflection microstructures fabricated on its facets exclusively by femtosecond laser processing techniques. This allowed us to achieve transmittance in a broad range from 2 to 8 µm, approaching a maximum of over 90% near 2.5 µm, and lasing at 2275 nm at the average output power of 20 mW for the absorbed pump power of 500 mW with the slope efficiency of 5.5 %. This demonstration opens up a route towards the industrial fabrication of compact integrable laser sources and sensors based on II-VI materials.

Microstructuring of the end-surface for silver halide polycrystalline fibers to suppress Fresnel reflection

Silver halide polycrystalline infrared fibers (PIR) have unique properties such as excellent transmittance in the spectral range from 3 to 17 µm, while also being highly flexible, non-toxic, and non-hygroscopic. They are used in industry and medicine for CO2-laser power delivery, flexible infrared imaging, and remote process spectroscopy. While PIR fibers possess a quite low attenuation (0.1-0.5 dB/m) in the 8-12 µm range, their total transmittance is limited by significant Fresnel reflections at the fiber end faces due to the high refractive index of silver halide (>2.1). Functionalization of these surfaces with specially designed Anti-Reflective Microstructures (ARMs) enables a striking enhancement of fiber transmittance. In this work, direct imprinting (or embossing) of microstructures to fiber ends and their profiling with a microstructured knife was applied to fabricate such ARMs. The resulting two-dimensional Moth-eye microstructures and one-dimensional microgrooves at the PIR-fiber ends enable to an increase of fiber transmittance in a broadband range of (5-17 µm) as well as to reach up to 20% improvement for PIR-fiber laser cables used for power delivery of CO2-lasers at 10.6 µm.

The Emirates Mars Mission (EMM) Emirates Mars InfraRed Spectrometer (EMIRS) Instrument

The Emirates Mars Mission Emirates Mars Infrared Spectrometer (EMIRS) will provide remote measurements of the martian surface and lower atmosphere in order to better characterize the geographic and diurnal variability of key constituents (water ice, water vapor, and dust) along with temperature profiles on sub-seasonal timescales. EMIRS is a FTIR spectrometer covering the range from 6.0-100+ μm (1666-100 cm−1) with a spectral sampling as high as 5 cm−1 and a 5.4-mrad IFOV and a 32.5×32.5 mrad FOV. The EMIRS optical path includes a flat 45° pointing mirror to enable one degree of freedom and has a +/- 60° clear aperture around the nadir position which is fed to a 17.78-cm diameter Cassegrain telescope. The collected light is then fed to a flat-plate based Michelson moving mirror mounted on a dual linear voice-coil motor assembly. An array of deuterated L-alanine doped triglycine sulfate (DLaTGS) pyroelectric detectors are used to sample the interferogram every 2 or 4 seconds (depending on the spectral sampling selected). A single 0.846 μm laser diode is used in a metrology interferometer to provide interferometer positional control, sampled at 40 kHz (controlled at 5 kHz) and infrared signal sampled at 625 Hz. The EMIRS beamsplitter is a 60-mm diameter, 1-mm thick 1-arcsecond wedged chemical vapor deposited diamond with an antireflection microstructure to minimize first surface reflection. EMIRS relies on an instrumented internal v-groove blackbody target for a full-aperture radiometric calibration. The radiometric precision of a single spectrum (in 5 cm−1 mode) is <3.0×10−8 W cm−2 sr−1/cm−1 between 300 and 1350 cm−1 over instrument operational temperatures (<∼0.5 K NEDelta T @ 250 K). The absolute integrated radiance error is < 2% for scene temperatures ranging from 200-340 K. The overall EMIRS envelope size is 52.9×37.5×34.6 cm and the mass is 14.72 kg including the interface adapter plate. The average operational power consumption is 22.2 W, and the standby power consumption is 18.6 W with a 5.7 W thermostatically limited, always-on operational heater. EMIRS was developed by Arizona State University and Northern Arizona University in collaboration with the Mohammed bin Rashid Space Centre with Arizona Space Technologies developing the electronics. EMIRS was integrated, tested and radiometrically calibrated at Arizona State University, Tempe, AZ.

Review of Surface Modification Technologies for Mid‐Infrared Antireflection Microstructures Fabrication

AbstractMid‐infrared materials antireflection is in high demand for high‐powered or ultra‐broadband coherent light sources, where conventional antireflection coatings cannot be reliably applied. This work provides a critical review of the recent advances in microstructure fabrication technology for mid‐infrared antireflection applications. Several techniques are reviewed, including direct imprinting, wet and reactive ion etching using conventional photoresist masking, novel colloid crystal masks, and maskless etching, laser‐induced periodic structure formation, and multiple laser ablation method modifications, including femtosecond laser direct writing, direct laser interference ablation, and single pulse ablation. The advantages and drawbacks of the different approaches are discussed in detail to highlight the most promising techniques for the fabrication of antireflection microstructures capable of achieving 99% transmittance in the 2–16 m range.

Fabrication of an antireflection microstructure on AgClBr polycrystalline fiber by single pulse femtosecond laser ablation

Polycrystalline infrared (PIR) fibers are used for numerous applications, one of those being power delivery for CO2 −lasers. However, the fiber tip surface’s transmittance cannot be increased with conventional antireflection coatings due to the surface unevenness. Antireflection microstructures (ARMs) offer an alternative way of increasing transmittance. In this work, ARMs were fabricated on the fiber tip surface of an AgClBr fiber by single-pulse femtosecond laser ablation. A single-surface transmittance of 92.8% at 10.6 µm, a CO2 −laser operation wavelength, was achieved. The proposed method can help significantly improve the systems’ efficiency, where power delivery for CO2 −lasers or sources operating in the wide wavelength range is required.

Fabrication of antireflection microstructures on the surface of GaSe crystal by single-pulse femtosecond laser ablation.

GaSe crystals are promising as nonlinear optical converters in the mid- and far-IR ranges. However, it is challenging to increase the GaSe surface transmittance of 77% with conventional antireflection coatings because of poor surface quality, leading to coating adhesion problems. Antireflection microstructures (ARMs) offer an alternative way of increasing surface transmittance. In this work, ARMs were fabricated on the surface of a GaSe plate by single-pulse femtosecond laser ablation. An average GaSe surface transmittance of 94% in the 7-11 µm range and a maximum transmittance of 97.8% at 8.5 µm were obtained. The proposed method can be used to increase the efficiency of GaSe-based nonlinear converters.

Fabrication of antireflection surfaces with superhydrophobic property for titanium alloy by nanosecond laser irradiation

Titanium alloy has been widely used in the aerospace field due to its advantages of low density, high specific strength and high-temperature resistance. Its scope of application, however, has been narrowed down because of its high reflectivity to light. In order to maximize the potential, it becomes necessary to construct a surface light trapping structure of titanium alloy. Therefore, a simple and tunable method was introduced in this paper to fabricate antireflection microstructures on titanium alloy surface with the superhydrophobicity based on nanosecond laser. By optimizing the laser processing speed parameters, microstructured surfaces with low reflectance over the entire visible wavelength range and near-infrared region (400–1000 nm) can be obtained. Results indicate that when the scanning speed is 100 mm/s, the minimum reflectance in the 900–1000 nm wavelength range is less than 1%, and the total reflectance in the 500–1000 nm wavelength range stabilizes below 6%. In contrast, the surface of conventional titanium alloy absorbs only about 50% of the incident light, and the remaining 50% is reflected. Furthermore, treated surfaces demonstrate effective performance concerning hydrophobicity, with a maximum contact angle of 153.03°. The results show that the nanosecond laser can induce the anti-reflection and super-hydrophobic properties, and improve the absorption capacity of the surface.

Numerical calculation of asymmetric antireflection microstructures spectral transmission dependency on polarization

In this paper we performed a numerical analysis of middle infrared antireflection microstructures with elliptical holes. We used COMSOL Multiphysics finite element method analysis commercial software. The calculation results revealed the qualitative influence of structure surface imperfections and non-symmetry on transmission spectra for different polarizations. This fact has to be taken into account for the design of polarization-independent antireflection microstructures.