Ablate nanostructure-based high-efficiency nonlinear sol

Abstract

This provides a highly efficient nonlinear optical paste material. [Solution] The setup uses a beaker filled with water placed at a predetermined position, with a pulsed laser positioned on top of the beaker as the light source. First, underwater laser ablation is performed on the surface of a metal plate placed inside the beaker to generate nanostructures. Then, the laser is kept focused inside the beaker to optically trap the structures and control their position or alignment. While the laser is still focused, an additive is inserted into the beaker to induce a phase transition from an aqueous solvent to a sol solvent, improving the viscosity of the solvent. Finally, while continuing to irradiate with the laser, the wavelength conversion ability of the obtained sample is investigated by performing nonlinear spectroscopy using specific optical elements and weak detection equipment. Through this series of operations, a highly efficient nonlinear sol based on ablated metal nanostructures is provided.

Description

Technical Field

[0001] The present invention relates to the trial production of a highly efficient nonlinear optical paste material produced by multi-stage combination of technologies such as laser ablation of metals in water, an optical trap that enables control of the position and arrangement of ablated structures, sol-gel conversion of a solvent from water to a sol, and nonlinear spectroscopy for analyzing wavelength conversion ability, respectively.

Background Art

[0002] Metal nanoparticles are expected to grow rapidly in the next few years and are said to reach $44.87 billion in 2028 at an annual compound growth rate (CAGR) of 11.4%. The application fields are likely to be the automotive and aerospace fields, the textile industry field, the healthcare and biomedical fields, and the electronics field. In particular, in the medical field, the market needs for metal nanoparticles in the pharmaceutical industry are growing, leading to the activation of the entire market. Furthermore, the increasing demand for semiconductors also foresees the expansion of the future nanoparticle market. In any case, the production and application of nanoparticles can be a major research theme in recent years.

[0003] So far, a method of producing metal nanoparticles by irradiating the surface of a metal plate with a pulsed laser in water has already existed (the underwater laser ablation method; see Reference 1). It is known that a large amount of nanoparticles are generated together with plasma generation in water by using this method. The underwater laser ablation method is a useful method when it is desired to harvest a large amount of nanoparticles in water because the particle concentration can be adjusted in proportion to the irradiation time.

[0004] On the other hand, there also exists a method of integrating a high refractive index structure at the focusing point by condensing a laser in water (the underwater optical trap technology; see Reference 2). The optical trap method is an effective method when it is desired to manipulate and control the position of a high refractive index structure in a microscope.

[0005] Furthermore, by adding some kind of solvent to the water, it is possible to change the entire system from low-viscosity water to a high-viscosity sol (sol formation; see, for example, reference 3). A typical viscosity of a sol can be thought of as being similar to that of ketchup or mayonnaise. Sols are easy to use when you want to use them as a paste.

[0006] It is also possible to analyze the nonlinear reflected or transmitted light by irradiating nanostructures with lasers (nonlinear spectroscopy; Reference 4). If the nonlinearity from the nanostructures is high, they themselves are expected to be used as wavelength conversion elements or nonlinear crystals.

[0007] Currently, the four technologies mentioned above are being applied individually in the field, and each has brought significant industrial benefits. However, there has been no opportunity to use these technologies in a multi-layered combination.

[0008] The present invention aims to develop novel nonlinear optical materials by combining the above technologies in a multi-stage manner. Specifically, the objectives are to obtain higher nonlinearity from samples that have not been obtained before, and to discover and apply new value for these nonlinear materials.

[0009] To address these challenges, the inventors quickly performed a series of experiments using a fixed setup, paying attention to minimizing contact between the sample and the outside world, in order to prototype a paste-based, highly efficient, nonlinear material. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 06-269975 (Underwater Laser Processing Device) [Patent Document 2] Japanese Patent Application No. 2015-519972 (Detection device and method for detecting a substance to be detected) [Patent Document 3] Japanese Patent Application No. 2023-540187 (Aerosol Generator) [Patent Document 4] Japanese Patent Application No. 2008-66832 (Light source device, nonlinear spectroscopic measurement system, and method for nonlinear spectroscopic measurement systems) [Overview of the project] [Problems that the invention aims to solve]

[0011] This invention has been made in view of the above-mentioned conventional problems, and aims to produce a laser ablate structure-containing sol in the shortest possible steps and to achieve high efficiency in its wavelength conversion ability. [Means for solving the problem]

[0012] To solve the above problems, the present invention sets up a series of steps as shown in Figure 1. The preferred sequence of steps is underwater laser ablation → optical trapping → solification → nonlinear spectroscopy. (However, the order can be conveniently changed depending on the situation.)

[0013] In this experimental procedure of the present invention, the three steps described above—underwater laser ablation, optical trapping, and solification—are classified as sample preparation steps, while the remaining step, nonlinear spectroscopy, is classified as an optical analysis step.

[0014] By performing the above experiments in a multi-stage manner, as previously shown, it becomes possible to sequentially investigate the mass generation of nanoparticles in water, control of their position and arrangement, viscosity of the solution, and wavelength conversion ability.

[0015] The mass production of nanoparticles by laser ablation in water is carried out based on conventional methods (Reference 1). The equipment, materials, and instruments used include a femtosecond pulsed laser, lenses, polarizers, electromagnetic shutters, beakers, pure water, metal plates, and stillers. The pulsed laser used here must be a high-power type with a high peak value, but it is not limited to a specific device. Suitable metal plates include gold (Au), silver (Ag), copper (Cu), and stainless steel (SS), which exhibit plasmon absorption. Scanning electron microscopes (SEM) and transmission electron microscopes (TEM) are used to observe the shape of the metal nanoparticles.

[0016] The position and alignment of the obtained metal nanoparticles are also controlled by optical trapping, based on conventional methods (Reference 2). The equipment used is the same femtosecond pulsed laser, and it is preferable to remove the metal plate from the beaker during the experiment. If it is desired to move the particles as well as trap them, a moving stage equipped with a piezo actuator and a program board will be required. Furthermore, a high-resolution microscope at the SEM level is necessary to confirm the position of the nanoparticles.

[0017] The solification of metal nanoparticle-containing solutions, whose position and arrangement can be controlled, is also performed based on conventional methods (Reference 3). Generally, it is desirable to use a method in which an appropriate solvent is added to water and viscosity is increased through hydrolysis and condensation polymerization reactions. Although the preparation of a pulsed laser is not essential during the solification process, it may be beneficial to keep it irradiated for trapping purposes.

[0018] Nonlinear spectroscopy of metal nanoparticle-containing sol solutions with controlled position and arrangement is also performed based on conventional methods (Reference 4). The light source is the same pulsed laser and is used in the same setup as described above. However, the detection unit requires the appropriate installation and placement of special optical elements (half mirrors, dichroic mirrors, ω-cut filters, etc.) for 2ω-component light extraction, as well as detectors suitable for trace detection of photons (lock-in amplifiers, picoammeters, etc.) on top of the beaker. Polarizers and half-wave plates are also installed depending on the situation.

[0019] In one aspect of the present invention, the resulting metal nanoparticle-containing sol solution is expected to be used as a paste. While the fields of application are primarily focused on aerospace engineering, the scope of application is not limited to healthcare, semiconductors, and other fields. [Effects of the Invention]

[0019] In the present invention, a large amount of metal nanoparticles are produced in water, and further operations such as position arrangement control, improvement of solvent viscosity, and non-linear spectroscopy are added for convenience to obtain a highly efficient non-linear sol sample, and by pasting it on a fighter plane, an airplane, etc., a high-performance chaff assist effect for camouflage is expected.

Brief Description of the Drawings

[0020] [Figure 1] It is a diagram showing the flow of the experimental method according to the first embodiment. [Figure 2] It is a diagram explaining the experimental principle according to the first embodiment. [Figure 3] It is a flowchart of the experiment according to the first embodiment. [Figure 4] It is an application example of an effective chaff assist paste material for camouflage in the first embodiment. [Figure 5] It is a diagram showing the flow of the experimental method according to the second embodiment. [Figure 6] It is a diagram explaining the experimental principle according to the second embodiment. [Figure 7] It is a flowchart of the experiment according to the second embodiment. [Figure 8] It is an example of use of a solar panel paste material with a wavelength conversion function effective in the second embodiment.

Embodiments for Carrying Out the Invention

[0021] (First Embodiment) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or equivalent components, members, and processes are denoted by the same reference numerals, and repeated explanations are omitted as appropriate. FIGS. 1 and 2 show each experimental method and principle found in this embodiment. FIG. 3 is an experimental flowchart. FIG. 4 shows an application example of the sample to the chaff assist material. Note that all the optical elements, solutions, etc. used in the materials of this embodiment are common ones.

[0022] In the first embodiment, the focus is on forming a highly efficient nonlinear sol containing metal nanoparticles. As mentioned above, the experimental method consists of four main stages: underwater laser ablation, optical trapping, sol solution formation, and nonlinear spectroscopy. Details of each stage are described below.

[0023] Details of the mass production of metal nanoparticles by laser ablation in water (Figure 1, left side) are described below (see papers [a][b]). As shown in the upper left of Figure 2, a metal plate (e.g., silver Ag) is placed in a beaker containing pure water, and a high-power femtosecond pulsed laser is focused onto the plate surface using a lens and irradiated. Here, the irradiation conditions are preferably, for example, wavelength: 800 nm, pulse interval: 10-100 fs, repetition period: 1 kHz, fluence: several J / cm2. Plasma is generated at the irradiation spot, and at the same time, nanoparticles of that metal (generally non-spherical nanoparticles) are formed. Since nanoparticle generation occurs continuously during the irradiation time [a][b], the concentration of the solution can be adjusted. The generation of high-concentration nanoparticles may increase the viscosity during subsequent sol formation, or induce high nonlinearity due to cascade plasmons in nonlinear spectroscopy, so it is better to irradiate for as long as possible. A stirring bar called a stiller is placed at the bottom of the metal plate to stir the solution, but the use of this device is limited to laser ablation. On a different note, adding chlorides (such as silver chloride) to water can alter the environment surrounding the particles during laser irradiation, potentially changing the shape of the generated particles to different forms such as nanoprisms and nanorods [a]. This makes it possible for users to recover desired structures in water. Recovery methods may require additional operations such as molecular sieving, centrifugation, or filtering, but while the yield may decrease, selective recovery is not entirely impossible. [a]T.Tsuji, T.Kakita, and M.Tsuji, Preparation of nano-size particles of silver with femtosecond laser ablation in water, Appl.Surf.Sci., 206, 314-320(2003) [b]S.Kodama,R.Yabuuchi,G.Miyaji,andW.Natsu,Fabrication of nanostructure and deposition of metal particles by femtosecond laser ablation in electrolyte,Procedia CIRP 113,439-444(2022)

[0024] Details of the position and arrangement control of the obtained metal nanostructures by optical trapping (Figure 1, center left) are as follows [c]. As shown in the upper right of Figure 2, by focusing the same femtosecond pulsed laser in an aqueous solution in which the nanostructures are diffused, a force F acts at the focal point, trapping the high refractive index metal nanoparticles. In the case of nanostructures such as prisms and rods, the edges become the trapping symmetrical part. If the irradiation power is set to the mW (milliwatt) class, a pN (piconewton) trapping force is generated. Note that this trapping force depends not only on the laser power but also on the polarity and size of the nanoparticles, solvent viscosity, etc. By using piezoelectric elements, it is possible not only to capture but also to move them. This achieves positional movement. Furthermore, by imaging and focusing the beam through a spatial light modulator (SLM) or microelectromechanical system (MEMS) mirror, it becomes possible to control even the arrangement of the nanostructures depending on the image shape. This method is usually called virtual reality (VR) trapping [c] and is attracting attention among researchers [c]. Optical and VR trapping methods can also enhance the trapping force. For example, by placing a retroreflective (RR) sheet at the bottom of a beaker and creating a multi-focus mechanism, trapping can be enhanced. The advantages of optical and VR trapping methods are not limited to simply concentrating the structure at a single point. By rotating the RR sheet itself around the beam irradiation axis (irradiation axis ≠ reflection axis) to create a multi-focus mechanism, trapping at multiple locations should also be possible. These advantages should be implemented as appropriate depending on the situation. [c]OMMarago,PHJones,,,ACFerrari,Optical trapping and manipulation of nanostructures,Nat.Nanotechnology,8,807-819(2013)

[0025] The details of the solification of a metal nanostructure-containing solution with controllable position and arrangement (Figure 1, center right) are as follows [d]. As shown in Figure 2, bottom left, the general method involves inserting a metal alkoxide M(OR)x into an aqueous solution, hydrolyzing it, and then obtaining a sol via a polycondensation reaction of hydroxide M(OH)x. However, a quicker method is to increase the solution concentration by performing laser ablation of the metal in water for a long period of time. The latter is adopted as the initial step. This method is not difficult, but it is uncertain whether the position and arrangement of the structure will be completely maintained in the sol solution. If the surrounding environment of the structure is in a sol state, the position and arrangement of the structure should be less likely to collapse, so it is assumed that the retention rate will be high. For example, in the case of mayonnaise, if the temperature is low, phase separation may form between the oil at the top and the egg white at the bottom in the container. However, it is well known that it can still be used in that state, and this fact supports the effect of maintaining position and arrangement. It is probably safe to assume that in the sol state, although local position control is difficult, global arrangement control can be sufficiently maintained. If you want to ensure accuracy in positional control, it's a good idea to transfer the sample to a slide first and perform the experiment there, rather than in the beaker. Also, although it deviates from the main point of the invention, it might actually be possible to control the positional arrangement of the structure more effectively by introducing gelation rather than solification. [d]M.Toki,Overview of sol-gel method and its application,surface technology,50,161(1999)

[0026] Details of the nonlinear spectroscopy of a sol solution containing metal nanostructures with controlled position and alignment (Figure 1, right side) are as follows [e]. As shown in the lower right of Figure 2, in nonlinear spectroscopy, the investigation should initially focus on second harmonic generation (SHG) spectroscopy. Laser (ω) light is focused onto the sol sample containing the nanostructures at a fluence of approximately mJ / cm2, and the reflected SHG light is detected and analyzed. The method for detecting SHG light involves first collecting the reflected light separately using a half-mirror (HM) (or a dichroic mirror (DM) if available), removing the incident light wavelength with an ω-cut filter, and then using a bandpass filter (BPF) to obtain only the SHG component. It is likely that the output of SHG light obtained from the sample is extremely low at this stage. Therefore, the extraction method should use a lock-in amplifier, picoammeter, or similar detector suitable for detecting minute amounts of SHG photons. During spectroscopy, it is unknown to what extent the position and alignment control is maintained, so polarization analysis may be difficult. Therefore, polarizer insertion is optional. It is certain that SHG light can be obtained from a sample if it contains nanostructures. (Originally, it is thought that there is no nonlinearity from the centrosymmetric structure, but in the case of metal nanoparticles, it is thought that high nonlinearity is produced due to the quadrupole effect). Furthermore, as mentioned earlier, if a large amount of nanostructures are generated in the sol, it is possible that amplified SHG caused by cascade plasmons can be obtained between the nanostructures. The enhancement effect will increase by making the interparticle gap as close to zero as possible. The resulting sample will be a more powerful nonlinear material, but the field excited by cascade plasmons is likely to be limited to the near field, and detection in the far field may require the introduction of a Fresnel propagation mechanism via evanescent. In this case, the experimental system will have to be adjusted for convenience. However, the view that cascade plasmon propagation beam generation ultimately works positively is correct. However, there are also points to be careful about in SHG spectroscopy of sol samples. That is, the degree of nonlinearity that arises from the symmetry elements of the chemical formula of the sol solvent at the time of sol formation must be extremely low compared to the degree of nonlinearity that arises from the metal nanostructures. Otherwise, it becomes dangerous because it becomes impossible to determine whether the SHG originates from a sol solvent or from metal nanostructures.However, in most cases, the asymmetry of nanostructures is significantly higher than that of chemical structures, so a simple test for confirmation is sufficient. The test method involves investigating the reflection SHG response of sols and nonlinear sols. [e]Y.Ogata and G.Mizutani,Control of cross-sections and optical nonlinearity of Ptnanowires and the roughness effect,Phys.Res.Int.,2012,969835(2012)

[0027] This experimental method is carried out in a series of steps as shown in Figure 3. Previously, samples were prepared in a sample preparation room and then transported to a laser darkroom for spectroscopy, which was time-consuming and involved frequent contact of the samples with the outside environment. However, with this linked experiment, all operations can be completed in a single room, resulting in significant time savings and the elimination of external influences, which are major advantages.

[0028] As mentioned earlier, the high-purity nonlinear optical sol sample obtained can be used in the field of aerospace engineering as a chaff assist material for fighter jets. Figure 4 illustrates the situation. Until now, aluminum confetti has been used on the rear of a fighter jet as a decoy to reflect laser beams aimed at missiles. In this invention, the nonlinear optical sol obtained in this invention is automatically applied to the walls of the aircraft to camouflage it, and this effect is used simultaneously with the conventional chaff of scattered aluminum sheets. In this case, the wavelength of the laser reflected from the walls of the aircraft will change, so it will not be detected by enemy aircraft, but enemy aircraft should be attracted to the chaff scattered on the rear. However, this is not the end. If the wavelength of the laser reflected by the aircraft changes, then, conversely, even if the aircraft does nothing, the laser of that wavelength will be directed at the enemy aircraft, and the reflected laser light can be detected on the other side, making a counterattack possible. In other words, by using this material, it is possible to deflect enemy attacks and turn the tables on the enemy aircraft.

[0029] Furthermore, its applications are not limited to the nonlinear field of aerospace engineering, but also extend to the linear fields of healthcare and architecture. For example, it can be applied as a paste to human skin to protect it from solar heat through plasmon optical absorption by metal nanoparticles. It can also be applied to the walls of buildings to absorb solar heat, thereby reducing wall damage. In any case, its non-solid nature allows for numerous applications depending on the specific application.

[0030] (Second Embodiment) Next, a second embodiment of the present invention will be described below. Content that overlaps with the first embodiment will be omitted from the explanation. In the first embodiment, a typical implementation configuration and usage example for forming a highly efficient nonlinear optical sol sample containing metal nanostructures was shown, but in this embodiment, it is limited to metals. Therefore, in the second embodiment, we propose forming semiconductor nanowires using the supercritical liquid solid (SFLS) method with metal nanostructures as seeds, and then subjecting them to optical or virtual reality (VR) trapping, solification, and nonlinear spectroscopy. This will produce a nonlinear sol sample containing semiconductor nanowires. Figures 5 and 6 illustrate the flow and principle of the experiment. Furthermore, Figure 7 shows a flowchart of the experiment. Figure 8 illustrates an example of application to a solar panel with wavelength conversion function.

[0031] In the second embodiment, a semiconductor nanowire-containing nonlinear sol sample is prepared by following the procedure shown in Figure 5. After preparing a large quantity of metal nanoparticles by underwater laser ablation, semiconductor nanowires are synthesized in large quantities from metal seeds using the SFLS method. Subsequently, a semiconductor nanowire-containing nonlinear sol should be possible to prepare by following the same procedure as in the first embodiment, namely, optical trapping → solification → nonlinear spectroscopy. However, this procedure is merely an example, and the order can be changed depending on the situation.

[0032] Details of the SFLS method [f] are illustrated in Figure 6. For example, by continuously adding Au nanoparticles and a precursor such as monophynylsilane (MPS) at a flow rate of 0.7 mL / min for 40 minutes to a toluene solution under ultra-high pressure (approximately 10 MPa) and high temperature (490 K), and allowing the reaction to proceed sequentially, Si-based semiconductor nanowires can be synthesized in large quantities relatively easily. The semiconductor is not limited to Si; by changing the conditions, nanowire synthesis is also possible with Ge. Furthermore, there is some selectivity for the seed, such as Ag and Cu. However, the use of SS as a seed has not yet been done, and it will take time to establish an experimental system. After semiconductor nanowire synthesis, the removal of the metal nanoparticle seed at the tip is deliberately left in mind with an eye towards applications. [f]Y.Ogata,Synthesis of silicon and germanium nanowires,2011 NNIN iREG,p52-53.

[0033] On the other hand, there are some concerns regarding the SFLS method. One concern is the shape of the metal nanoparticles. The particle shape is rarely a perfect sphere and is often irregular. These can be observed with a SEM. Whether SFLS can be successfully performed using such non-spherical nanoparticles as seeds can only be determined by experimentation. This is because the growth surface of the nanowires on the metal nanoparticles is already determined. Another concern is the selection of particle size. While there is a suitable particle size range (several tens of nanometers in diameter) for SFLS synthesis, it is difficult to match this size with underwater laser ablation. Currently, these can only be overcome experimentally by adjusting the irradiation fluence and utilizing filtering techniques.

[0034] As shown in Figure 7, the subsequent experimental procedure is the same as in the first embodiment, but if optical trap position control or solution solification is difficult, these can be omitted without issue. Although the advantages are reduced, SFLS nanowire synthesis using ablated metal particles in water as seeds is still an unprecedented study, and that alone seems to make it valuable. If the above two operations are difficult, but positional alignment control is absolutely necessary, one can manually move each wire one by one using a focused ion beam (FIB), or if one absolutely wants to solify the solution, one can resort to the drastic measure of bunching with air during the SFLS synthesis process. Although it deviates from the main point of the invention, it is possible that introducing a combination of optical trapping and gelation, as in the first embodiment, may actually lead to better wire positional alignment control.

[0035] Similar to the first embodiment, the application of such materials would be a wavelength conversion paste for chaff assistance usable in the field of aerospace engineering. However, here we propose a hybrid paste combining wavelength conversion and solar panels as a novel application. While possessing the wavelength conversion capability shown in the application of the first embodiment, we have newly developed a photovoltaic power generation paste material that utilizes a p / n junction as shown in Figure 8. In principle, it acquires SHG reflected light derived from the quadrupole effect and cascade plasmon effect from nanoparticles, while simultaneously performing a power generation system that generates electromotive force at the p / n junction due to charge transport in the wire itself and vacancy movement between wires. By utilizing the viscosity of the paste, localized wavelength conversion and localized power generation can be performed by partial pasting only where necessary, and simultaneous control of wavelength conversion efficiency and power generation efficiency can be achieved by adjusting the paste thickness. In this case, a condition for constructing the p / n junction is that electrons and vacancies are not affected by the chemical components of the solvent. Of course, charge transport between metal nanoparticles and semiconductor nanowires must also be considered. In order to define the multiple conditions in the hybrid described above, it seems necessary to first calculate the Hamiltonian operator H separately in order to grasp the charge transfer and flow of the entire system.

[0036] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Explanation of Symbols]

[0037] 100. ...Nonlinear sol fabrication method containing positionally controlled metal nanostructures 110. ...Nonlinear sol fabrication method for semiconductor nanowire-containing metal nanoparticle growth with controlled positional alignment

Claims

1. The setup uses a beaker filled with water placed at a predetermined position, with a pulsed laser positioned above the beaker as the light source. First, underwater laser ablation is performed on the surface of a metal plate placed inside the beaker to generate metal nanostructures. Then, while continuing to focus the laser inside the beaker, the structures are optically trapped to control their position or alignment. Furthermore, while continuing to focus the laser, an additive is introduced into the beaker to induce a phase transition from an aqueous solvent to a sol solvent, improving the viscosity of the solvent. Finally, while continuing to irradiate with the laser, the wavelength conversion ability of the obtained sample is investigated by performing nonlinear spectroscopy using specific optical elements and weak detection equipment. The sample obtained through this series of operations is made into a highly efficient nonlinear sol based on ablated metal nanostructures.

2. An assist application for aircraft or fighter aircraft-borne chaff using a highly efficient nonlinear sol based on ablated metal nanoparticles as described in claim 1.

3. A semiconductor nanowire is synthesized using ablated metal nanoparticles as a seed by the supercritical liquid-solid (SFLS) method according to the experimental method described in claim 1. Subsequently, optical trapping, solification, and nonlinear spectroscopy are performed to obtain a semiconductor nanowire-based high-efficiency nonlinear sol.

4. Application of a solar panel paste with wavelength conversion function using a high-concentration nonlinear sol containing metal nanoparticle-grown semiconductor nanowires as described in claim 3.

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