Optical Thin Films Research Articles

Inverse Design of Distributed Bragg Reflectors Using Deep Learning

Distributed Bragg Reflectors are optical structures capable of manipulating light behaviour, which are formed by stacking layers of thin-film materials. The inverse design of such structures is desirable, but not straightforward using conventional numerical methods. This study explores the application of Deep Learning to the design of a six-layer system, through the implementation of a Tandem Neural Network. The challenge is split into three sections: the generation of training data using the Transfer Matrix method, the design of a Simulation Neural Network (SNN) which maps structural geometry to spectral output, and finally an Inverse Design Neural Network (IDNN) which predicts the geometry required to produce target spectra. The latter enables the designer to develop custom multilayer systems with desired reflection properties. The SNN achieved an average accuracy of 97% across the dataset, with the IDNN achieving 94%. By using this inverse design method, custom-made reflectors can be manufactured in milliseconds, significantly reducing the cost of generating photonic devices and thin-film optics.

Study the Damage Effects of Different Laser Pulses on the Pyrotechnic Bridge

lasers are used more and more widely in daily life, medicine and aerospace fields, which promoted the development of all kinds of laser and laser technology. With the continuous breakthroughs in the research field, the application of high power lasers is also more extensive. When the frequency doubling light is emitted from the laser, different wavelengths will act on the separation film simultaneously. Therefore, it is necessary to investigate the damage of optical thin films by laser of single wavelength or multiple wavelengths. The film was irradiated repeatedly by laser aiming, and the damage radius and damage morphology were studied under the action of 1064nm laser and 532nm laser alone and together, as well as the primary and secondary relationship of the damage induced by the two wavelengths. Then, the distribution of explosive damage temperature field was analyzed by simulation. It is concluded that the damage radius at 532nm is larger than that at 1064nm, and the damage depth is deeper. When the two pulses work together, 532nm laser plays a dominant role in the damage process. This study is helpful to better understand the damage mechanism of bridge film and provide reference for the preparation of damage-resistant explosive bridge and laser ignition.

Sensitive optical thin film sensor based on incorporation of 2-(2′-hydroxynaphthylazo)-benzothiazole in a sol–gel matrix for detection of manganese(II) in environmental samples

A novel selective and precise optical thin film sensor reliant on the incorporation of synthesized 2-(2′-hydroxynaphthylazo)-benzothiazole (HNABT) as a selective ionophore into the nonporous of a clear glass like material via the sol–gel process is examined for its ability to assess Mn(II) ions in aqueous solutions. The sensor was constructed by spin-coating prepared sol onto a glass plate and the influence of sonication time on immobilization of HNABT into silica matrix was demonstrated through calculation of leaching percentage. The results designated that a sonication time of 25 min is the optimum to accomplish more stable thin films without fluctuation in sensitivity and response time of the introduced sensor for a wide period. The offered sensor can be employed to evaluate Mn(II) ions within the range of 6.0 × 10−8 – 1.5 × 10−5 M with detection and quantification limits of 1.7 × 10−8 and 5.5 × 10−8 M, respectively. The relative standard deviation of 2.1 and 0.83% for reproducibility and repeatability, are assessed, along with a rapid response time of ≈3.0 min. The constructed optode is stable in wet circumstances and should be kept for at least four weeks without detecting any variation in its sensitivity. The recommended sensor was successfully performed to estimate Mn(II) in food, saline effluents, tea leaves, biological and water samples, and the results were established by the GFAAS method.

NEUTRON: Neural particle swarm optimization for material-aware inverse design of structural color

SummaryDesigning optical structures for generating structural colors is challenging because of the complex relationship between the optical structures and the color perceived by human eyes. Machine learning-based approaches have been developed to expedite this design process. However, existing methods solely focus on structural parameters of the optical design, which could lead to suboptimal color generation because of the inability to optimize the selection of materials. To address this issue, an approach known as Neural Particle Swarm Optimization is proposed in this paper. The proposed method achieves high design accuracy and efficiency on two structural color design tasks; the first task is designing environment-friendly alternatives to chrome coatings, and the second task concerns reconstructing pictures with multilayer optical thin films. Several designs that could replace chrome coatings have been discovered; pictures with more than 200,000 pixels and thousands of unique colors can be accurately reconstructed in a few hours.

Design, Fabrication and Analysis for Al2O3/Ti/Al2O3 Colored Solar Selective Absorbers for Building Applications

A good solar selective absorber (SSA) used in building applications must have good color brightness rendering, an excellent photo–thermal conversion efficiency (PTCE) and a long life. The optical thin film design plays an important role before the coating of the optical thin films. In this study, for fabricating a good colored SSA (CSSA), the optical properties and color distribution of Al2O3/Ti/Al2O3 SSA were calculated to obtain the best design. It was found that the color distribution of Al2O3/Ti/Al2O3 SSA with a PTCE in excess of 90% was wide and included all colors, such as red, orange, yellow, green, blue, purple and white. Five colored Al2O3/Ti/Al2O3 solar selective absorbers were designed and fabricated and their performances were analyzed. The best PTCE of a fabricated sample and its thermal emittance efficiency were 93.2% and 7.1%, respectively. The peak values of the measured optical reflectance in the visible region for the five fabricated CSSAs were all greater than 10%. Their lifetime could be very long because the temperature difference between thermal stability of more than 450 °C and the working temperature of less than 150 °C was up to 300 °C. After annealing at 550 °C, the Al2O3–Ti system formed a compound structure of AlTiO5. The Al2O3/Ti/Al2O3 film performances, including the optical properties in the wavelength range of 0.25 to 25 μm, structure, morphology, adhesion, cross-sectional images, AFM image, PTCE, thermal emittance efficiency and thermal stability, were measured and analyzed in detail, and met the characteristic requirements of colored solar absorber films.

Study the Damage Effects of Different Laser Pulses on the Pyrotechnic Bridge

lasers are used more and more widely in daily life, medicine and aerospace fields, which promoted the development of all kinds of laser and laser technology. With the continuous breakthroughs in the research field, the application of high power lasers is also more extensive. When the frequency doubling light is emitted from the laser, different wavelengths will act on the separation film simultaneously. Therefore, it is necessary to investigate the damage of optical thin films by laser of single wavelength or multiple wavelengths. The film was irradiated repeatedly by laser aiming, and the damage radius and damage morphology were studied under the action of 1064nm laser and 532nm laser alone and together, as well as the primary and secondary relationship of the damage induced by the two wavelengths. Then, the distribution of explosive damage temperature field was analyzed by simulation. It is concluded that the damage radius at 532nm is larger than that at 1064nm, and the damage depth is deeper. When the two pulses work together, 532nm laser plays a dominant role in the damage process. This study is helpful to better understand the damage mechanism of bridge film and provide reference for the preparation of damage-resistant explosive bridge and laser ignition.

Light Field Intensification in Optical Films Induced by Intercoupling of Defects and Organic Contamination.

Based on the finite-difference time-domain method, light field intensification in optical films due to the intercoupling of defects and organic contamination was analyzed. The results show that the intercoupling between the defect and the organic contamination droplet leads to an increase in the local electric field and the coupling effect enhances with the decreasing distance between the defect and the organic contamination droplet and the increasing diameter of the organic contamination droplet. The coupling effect of the defect and the organic contamination layer depends on not only the thickness of the organic contamination layer but also the refractive index of the organic contamination layer. With the thickness and the refractive index of the organic contamination layer increasing, the peak value of the electric field decreases. This work deepens the physical understanding of the degradation mechanism of laser-induced damage in optical thin films used in vacuum.

Concentration Effect of Mixed SnO2-ZnO on TiO2 Optical Properties Thin Films prepared by Chemical Spray Pyrolysis Technique

In this work, the concentration effect of mixed tin dioxide and zinc oxide on optical properties of titanium dioxide thin films was studied. Thin films were prepared by using spray chemical pyrolysis technique on glass substrate at 350 ͦC. The optical results revealed a weak optical transmittance for Un-doped TiO2 at the end of ultraviolet spectrum followed by an increase, more than 48%, at the near IR spectrum. TiO2 thin films revealed more transparent with blue shift in the absorption edge when the SnO2-ZnO mixed increased. A significant decrease in the absorption coefficient with increasing the mixed of SnO2-ZnO. According to the results of the electronic transition of TiO2, thin films have direct and indirect energy gap, about 3.2 eV and 2.11 eV, respectively. An increase in both types of energy gap was observed with the mixing concentrations of SnO2-ZnO increase. In addition, a significant decrease also was in the refractive index and extinction coefficient with the increasing SnO2-ZnO concentration.

A Reinforcement Learning Method for Optical Thin-Film Design

Machine learning, especially deep learning, is dramatically changing the methods associated with optical thin-film inverse design. The vast majority of this research has focused on the parameter optimization (layer thickness, and structure size) of optical thin-films. A challenging problem that arises is an automated material search. In this work, we propose a new end-to-end algorithm for optical thin-film inverse design. This method combines the ability of unsupervised learning, reinforcement learning(RL) and includes a genetic algorithm to design an optical thin-film without any human intervention. Furthermore, with several concrete examples, we have shown how one can use this technique to optimize the spectra of a multi-layer solar absorber device.

Preparation of lithium niobate optical thin film and patterned microstructure via chemical modification

In this paper, a stable lithium niobate (LN) sol was prepared using niobium pentachloride, niobium ethoxide, lithium acetate, anhydrous ethanol, and benzoylacetone (BzAcH) as a chemical modifier. The effects of different starting materials and organic solvents on the obtained gel film quality of the sols were studied and the optimization mechanism of BzAcH on the film quality was analyzed in detail. The effects of heat treatment temperature on the structure and performance of the LN thin films were also studied. X-ray diffraction (XRD) spectra showed that LN was more highly crystalline in the heat treatment temperature range of 600–800 °C. The refractive index of the LN thin films first increased and then decreased with increasing heat treatment temperature, and the LN thin film prepared at 700 °C had the largest refractive index. Light transmittance tests showed that as the heat treatment temperature increased, light transmittance decreased. Finally, it was determined that LN gel films modified with BzAcH were more highly sensitive to UV light and that after UV irradiation, it was difficult to dissolve LN gel films in organic solvent. The UV-sensitive property was utilized to realize microstructure processing of the LN thin films. The results indicated that a high-quality, non-destructive, and photoresist-free micro-fabrication method was realized. This method was used to fabricate microscale LN fine patterns with smooth sidewalls while avoiding lateral corrosion. These results are of great significance for LN thin film applications and the development of LN-based optoelectronic devices prepared by chemical methods.

Material ejection and layer peeling-off in HfO2/SiO2 thin-film beam splitters induced by 1ω and 3ω lasers

Nanosecond pulse laser-induced damage is associated with nodular defects and nano-precursors in hafnium oxide (HfO2)/silica (SiO2) multilayer films. In this study, the dynamic evolution of material ejection and layer peeling-off in HfO2/SiO2 thin-film beam splitters used for third harmonic separation were examined by time-resolved shadowgraph technique under the fundamental frequency of 1064 nm (1ω) and third harmonics 355 nm (3ω) laser beam, respectively. The initial material ejection and subsequent peeling-off of layers were studied to clarify most thin films damage events caused by 1ω and 3ω lasers. The peeling-off of films under 1ω laser was mainly caused by thermomechanical effect. In contrast, the peeling-off of films under 3ω laser was mainly affected by laser-induced mechanical damage. The different transmission forms of layers peeled off from the films revealed the influence of mechanical strength and thermal strength on damage progression. The Taylor–Sedov theoretical model was used to analyze the propagation characteristic shock waves after nanosecond laser irradiation of films. In summary, identification of propagation forms and speed of peeling-off layers provides helpful insights into the damage mechanisms during nanosecond laser-induced damage of optical thin films.

Design optimization of broadband extreme ultraviolet polarizer in high-dimensional objective space

With the purpose of designing the extreme ultraviolet polarizer with many objectives, a combined application of multi-objective genetic algorithms is theoretically proposed. Owing to the multi-objective genetic algorithm, the relationships between different designing objectives of extreme ultraviolet polarizer have been obtained by analyzing the distribution of nondominated solutions in the four-dimensional objective space, and the optimized multilayer design can be obtained by guiding the searching in the desired region based on the multi-objective genetic algorithm with reference direction. Compared with the conventional method of multilayer design, our method has a higher probability of achieving the optimal multilayer design. Our work should be the first research in optimizing the optical multilayer designs in the high-dimensional objective space, and our results demonstrate a potential application of our method in the designs of optical thin films.

Ge/BaF2 thin-films for surface micromachined mid-wave and long-wave infrared reflectors

High performance distributed Bragg reflectors (DBRs) are key elements to achieving high finesse MEMS-based Fabry–Pérot interferometers (FPIs). Suitable mechanical parameters combined with high contrast between the refractive indices of the constituent optical materials are the main requirements. In this paper, Germanium (Ge) and barium fluoride (BaF2) optical thin-films have been investigated for mid-wave infrared (MWIR) and long-wave infrared (LWIR) filter applications. Thin-film deposition and fabrication processes were optimised to achieve mechanical and optical properties that provide flat suspended structures with uniform thickness and maximum reflectivity. Ge-BaF2-Ge 3-layer solid-material DBRs have been fabricated that matched the predicted simulation performance, although a degradation in performance was observed for wavelengths beyond 10 μm that is associated with optical absorption in the BaF2 material. Ge-Air-Ge 3-layer air-gap DBRs, in which air rather than BaF2 served as the low refractive index layer, were realized to exhibit layer flatness at the level of 10 to 20 nm across lateral DBR dimensions of several hundred micrometers. Measured DBR reflectance was found to be ≳90 % over the entire wavelength range of the MWIR band and for the LWIR band up to a wavelength of 11 μm. Simulations based on the measured DBR reflectance indicates that MEMS-based FPIs are able to achieve a peak transmission of ≳90 % over the entire MWIR band and up to 10 μm in the LWIR band, with a corresponding spectral passband of ≲50 nm in the MWIR and <80 nm in the LWIR.

STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF NANOCRYSTALLINE NICKEL OXIDE THIN FILMS PREPARED BY HYDROTHERMAL AND SOL-GEL METHODS

The present work focuses on the comparison of the structural, optical and electrical properties nickel oxide thin films synthesized by hydrothermal and sol-gel methods and their characterizations. The films obtained from both methods are having a cubic crystal lattice. The morphological studies have shown that the films are having a uniform coverage with nanocrystalline grains. The NiO film obtained through the sol-gel method possess an optical band gap of 2.86 eV and its electrical resistance tends to decrease from 2.64GΩ to 11MΩ with an increase in temperature (RT to 150°C). The hydrothermally obtained film has an optical band gap of 3.14 eV and its electrical resistance change from 6.83GΩ to 3.86GΩ with an increase in temperature.

Ni-doped metal-organic frameworks as nonlinear optical material used in saturable absorption for Q-switched Tm: YAP laser

Metal-organic frameworks (MOFs) have received significant attention owing to their nanometer-sized void spaces, large surface areas, and tunable bandgaps. The properties of MOFs can be adjusted by doping other metal motifs or organic ligands. MOFs are widely employed in nanodevices and optical thin film materials; however, their third-order nonlinear optical (NLO) properties are rarely investigated, and their application in solid-state laser (SSL) has not been reported. In this paper, the NLO properties of Ni-doped MOFs are investigated comprehensively, wherein the value of the nonlinear refractive index and that of the real part of the third-order nonlinear optical susceptibility are obtained, i.e., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$6.10\times10^{-9}$</tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /W and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$6.20\times10^{-7}$</tex-math></inline-formula> esu, respectively. Consequently, an SSL that coupls Ni-doped MOFs with a saturable absorber (SA) is constructed. An average output power of 0.68/1.10 W is acquired with a 1.5 % or 2.5% transmission of the output coupler (OC) under a passive Q-switch mode operation, corresponding to a per pulse energy of 6.29/10.90 μJ. When coupled with Tm: YAP, the as- constructed SSL exhibits a stable and short pulse (1.97 μs) at 1987.5 nm, indicating that Ni-doped MOFs can be applied to broadband SAs to achieve a fast photonic in the near-infrared spectra.

Multi-layer longitudinally specular optics for distributing diffuse light.

Multi-layer optical thin films can reflect light to guide it along a hollow tube that they internally line. However, reflecting broadband white light for a range of angles requires many precise thin optical layers, which can be too expensive for large-scale applications such as architectural illumination. Here, we present an alternative configuration that requires only one readily achievable aspect of precision-having a substantially constant cross section, perpendicular to the planned longitudinal propagation direction. This aspect conserves a light ray's longitudinal directional component, while the ray's path in the perpendicular direction can be macroscopically chaotic, much like the diffuse reflection characteristic arising from many non-absorbing pigment particles. This macroscopic characteristic is described here as "longitudinally specular" and "transversely diffuse." This new design overcomes two problems that are found with current prism light guides (PLGs), which are a class of hollow light guides that also have a constant cross section. The PLG has two problems: (1) it requires structured surfaces having near-perfect linear prisms that are much larger than the wavelength of light, wasting optical material and increasing absorption; (2) it only works well with light that has been partially collimated, which limits étendue and requires expensive, inefficient input optics. In this paper, a new, to the best of our knowledge, approach, labeled a "multi-layer prism light guide," simultaneously addresses both problems. Although the new structure is more complex than a conventional PLG, it may be simpler to manufacture. One potential application is for guiding sunlight into buildings for illumination purposes, without requiring complex sun-tracking solar collection optics.

Extendable neural network and flexible extendable neural network in nanophotonics

To alleviate the formidable training and dataset accumulation workload of the neural network for systems with multiple input/output parameters, i.e., high dimensionality and complexity, we present an extended neural network (ENN) and flexible ENN (FENN) to help modeling the scalable photonics devices and systems. ENN can save from 19.16% to 40% of the database collection cost comparing to the artificial NN (ANN) method when extending the modeling from 4- to 10-layer and 10- to 12-layer cases in the thin optical film studies. And the FENN can generate an appended general network for the iteratively added layers, therefore the decomposed functional sections of the system could be represented by the network, especially for structurally similar components in the photonic circuits.

Design of multilayer optical thin-films based on light scattering properties and using deep neural networks.

Despite limiting the performance of multilayer optical thin-films, light scattering properties are not as yet controllable by current design methods. These methods usually consider only specular properties: transmittance and reflectance. Among other techniques, design of thin-film components assisted by deep neural networks have seen growing interest over the last few years. This paper presents an implementation of a deep neural network model for light scattering design and proposes an optimization process for complex multilayer thin-film components to comply with expectations on both specular and scattering spectral responses.

Influence of eccentricity and tilt of radial GRIN lens on its thickness measurement by chromatic confocal technology

The chromatic confocal technology (CCT) has ultra-high distance measurement resolution and the characteristics of multi-surface tomography. Based on the principle of optical dispersion and confocal, the technology achieves accurate axial position or micro-displacement measurement. The radial gradient index (GRIN) lens is a typical important inhomogeneous material. Its refractive index gradually decreases from the center to the edge along the radial direction. We propose to measure the thickness of the radial GRIN lens based on CCT. The thickness measurement model is established by the ray-tracing method, the optical Lagrange function, and the ray arc differential in the Cartesian coordinate system. The refractive index distribution makes the eccentric tilt of the lens change the propagation of the probe light and affects the extraction of the peak of the spectral response causing the thickness measurement error. The influence of eccentricity and tilt of the radial GRIN lens on its thickness measurement is studied. The precision shift table drives the radial GRIN lens movement to simulate the eccentric tilt state of the lens under the dispersion probe. According to the comparative analysis between the experimental and simulation results, it is concluded that the greater the eccentric distance and inclination degree of the radial GRIN lens is, the greater the influence on the thickness measurement is. The larger the lens thickness is, the greater the measurement error is. For the radial GRIN lens with a thickness of 2.36 mm, the measurement error is about 20 μm when the eccentricity is 0.1 mm and the tilt is 6 deg. The research results have significance for optimizing the system structure and further improving the system performance for the application of the CCT in measuring non-homogeneous materials or optical thin film. And the research will encourage the development of GRIN lens instrument preparation and application technology by improving the thickness measurement accuracy and precision of the GRIN lens.

Study on HfO 2 /SiO 2 high reflective film damage induced by 248nm UV excimer laser

The deep ultraviolet optical thin films play an important role in excimer lasers, deep ultraviolet lithography machine and other laser systems. In this paper, the experiment on damage in HfO2/ SiO2 high reflective film irradiated by 248nm ultraviolet excimer laser was carried out. And the high reflection film is coated with a layer of SiO2 as a protective film. The damage morphology and depth of the samples were observed and analyzed by means of DIC microscope and surface profiler system. In this experiment, the laser-induced damage threshold of HfO2/ SiO2 high reflective film coated with SiO2 protective film was calculated by zero damage probability. Based on the surface characteristics of the damage points, we established the model to analyze the damage mechanism of the high reflectance film by using Finite Element Method(FEM). The experimental results show that the damage threshold of 248nm excimer laser to the highly reflective film is 3.086J/cm2. When the incident laser energy is 3.33J/cm2, stress damage appears on the surface of the highly reflective film. With the increase of laser energy, the high reflective film will appear melting damage and corrugated damage.