1. Introduction

Surface colors in the world are typically classified into pigmentary colors and structural colors. In contrast to pigmentary colors, structural colors are bright, rich, eco-friendly, and never photochemically fade [1]. The photonic mechanisms underlying structural coloration include the plasmonics, as well as diffraction [2–6]; it widely exists in nature, and is also artificially created to produce grating coder and color filter, among other products [7–9]. Plasmonics induced structural coloration occurs on the nanostructured surface, while diffraction-based structural coloration is induced by submicron structured surface, however large-area nanostructured surface fabrication for image rendering is limited by the low effectiveness and huge energy consumption. Structural coloration induced by submicron structures exhibits iridescence, and is a significant method for the production of structural color surfaces via surface texturing [10–12].

In recent decades, structural coloration via laser-induced periodic surface structures (LIPSSs) has been investigated extensively [13], and the femtosecond laser surface structuring technique has been adopted for metal coloration [14–18]. However, the development of the LIPSS coloration technique is limited by its high processing cost, and its difficulty in precisely controlling the shape of the structure. In addition, it is difficult to flexibly and precisely regulate the LIPSSs period for the generation of complex structural color surfaces. Ultra-precision cutting technology creates the submicron structures by removing the material, and it has the advantages of high accuracy in profile control [12]. An elliptical vibration texturing (EVT) with raster scanning was proposed by Guo and Yang for the fabrication of submicron structures [19,20]. To solve the problems of discontinuous cutting and low efficiency in raster scanning of EVT technology, the EVT-assisted turning was proposed [21] and also used to realize the relief effect of structural color [22]. Regular periodic submicron ripples were machined on a metal surface to induce structural coloration, and the period of the ripples could be flexibly controlled by the cutting motion. However, structural color patterns induced by EVT have a low resolution because of the severe distortion of the adjacent area between two rows of ripples, and these EVT-based technologies cannot solve the tool's extrusion deformation of the groove, resulting in low precision of the processed grating structure. [22], Therefore there is an urgent need to develop a new and more flexible method to create submicron structures with high accuracy for structural coloration manufacturing. Limited by the tool fabrication, the shape of tool tip with the required scale less than 1 µm are classified into jagged-shape and the V-shape, which correspondingly generate the grooves with jagged-shape and V-shape respectively on the surface after cutting. For the structural color surface creation, high brightness and good color purity are urgent to be improved, which are dependent on the groove shape and grooves surface quality.

In our previous research, we have demonstrated the manufacturing of single and two-level micro-nano structures [11,12] and arranged the color modules to achieve structural color pattern manufacturing based on a simple grating equation [11]. At present, although some machining methods for high-quality microstructure have emerged [12,23], the optical theory of structural color only considers the influence of grating equation. In fact, the basic grating equation can only calculate the diffraction angle of the diffracted light at a specific wavelength, but cannot analyse the intensity of the diffracted light, which is related to the groove shape, groove quality, material, etc. However, the manipulation mechanism of the profile of the submicron structures to the diffracted light hasn’t been clearly investigated at the submicron meter scale. With the popularization of glass optics, the mass production of structural colors on glass surfaces currently presents a substantial challenge for the machining ability and efficiency of these methods. Precision glass molding (PGM) can be used to achieve the mass production of glass optical elements by replicating the shape of the mold on heated glass preforms without further machining processes [24–26]; thus, methods of optical diffraction manipulation, flexible manufacturing of submicron structures and structural color image construction, are urgent for the production of structural color pattern on the glass surface.

To generate structural coloration with high brightness, this paper theoretically studies the diffraction manipulation mechanism of two kinds of typical grooves that can be machined by mechanical cutting, and introduce the axial-feed fly-cutting (AFC) technology and PGM technology to precisely create the designed submicron V-shape grooves and structural color pattern on Ni-P mold and glass surface. Firstly, the brightness of the structural color is characterized by the diffraction efficiency, the diffraction manipulation mechanism of jagged-shape and V-shape groove is studied via theoretical calculation and simulation for the enhancement of diffraction intensity and efficiency. The V-shape groove as the relatively most suitable groove shape is selected and fabricated to create structural coloration. Secondly, the principle of AFC is studied, and the feasibility of the developed experimental platform for the fabrication of submicron optical V-shape grooves is verified. Thirdly, the visible light spectrum manipulated by the submicron grooves is investigated via the experimental detection. Finally, based on the obtained law of the structural color spectrum, structural color patterns are machined on mold surface and glass surface via mold manufacturing and PGM technologies.

2. Diffraction manipulation by submicron grooves

2.1 Theoretical analysis of diffraction manipulation

Grating-diffraction theory can only explain the diffraction angle and order, and the diffraction intensity must be calculated by analysing a single groove. Considering the actual uniform medium and periodic boundary conditions of visible-light propagation, as the special solution of the complex Maxwell’s electromagnetic field theory, Huygens-Fresnel principle is used to calculate the diffraction intensity of two kinds of grooves. According to the Huygens-Fresnel principle, because groove spacing is compatible with the visible spectrum, when the groove length is far greater than the visible light wavelength, the diffraction in the width direction of a groove should be analysed, and the diffraction in the length direction can be ignored. Figure 1 illustrates light with a 0° incident angle on the typical jagged-shape [Fig. 1(a)] and V-shape grooves [Fig. 1(b)], which all has the same width of d, and these groove surfaces as the new wavefront will emit light in all directions.

 

Fig. 1. Manipulation mechanism of the visible light by the submicron grooves: (a) Diffraction of one jagged-shape groove; (b) Diffraction of one V-shape groove.

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To calculate the diffraction intensity in a direction with a diffraction angle of θ, the wavefront of the jagged-shape and V-shape groove are both divided into many small segments along the vertical direction (eg. 10 segments), which makes these segments become the new small light source. The field amplitude at the angle of θ stimulated by each segment is approximately same and the value is a, while the optical path difference induced by the depth change of groove lead to the phase difference between these adjacent segments, which makes that the field amplitude at the θ can be obtained by summing these vectors. Considering that most of the groove-induced diffraction intensity occupies at the first order diffraction, therefore the diffraction angle θ, at which maximum phase difference on the groove surface is less 2π, is analysed and selected to induce structural coloration.

For the jagged-shape groove, the phase difference between the adjacent segments is constant, which forms the resultant vector $\overrightarrow {E^{\prime}G^{\prime}}$, as shown in Fig. 1(a). The length of resultant vector $\overrightarrow {E^{\prime}G^{\prime}}$ determines the diffraction intensity. For the unique V-shape groove, the constant phase difference between the segments is interrupted by the cusp F (termed as the phase turning point). Compared with the jagged-shape, the cusp F in V-shape changes the path of the vector overlay, converting the resultant vector from $\overrightarrow {E^{\prime}G^{\prime}}$ to $\overrightarrow {EG}$, as shown in Fig. 1(b). It can be known that phase turning point existing in the V-shape groove makes the light vector superimpose in the direction where the combined field amplitude increases. The followings calculate the field amplitude in the direction with diffraction angle of θ of the jagged-shape and V-shape.

Dividing the wavefront into an infinite number of equal segments, the vector diagram of jagged-shape becomes an arc E’G’, and the central angle of E’G’ is 2β’, which is equal to the maximum phase difference of light on the jagged-shape surface. The arc of V-shape is divided into two sections (marked as red and blue) from the point F, corresponding to the section EF and FG of the V-shape profile, respectively. And these two arcs are tangent at point F. Central angle corresponding to these two arcs are 2β₁ and 2β_2­, which are equal to the maximum phase difference of light on the two parts, EF and FG respectively.

The resultant vector $\overrightarrow A$ of V-shape is summation of $\overrightarrow {{A_1}}$ and $\overrightarrow {{A_2}}$. Based on the geometric relationship, field amplitude A₁ and A₂ can be given by:

The angle between vector $\overrightarrow {{A_1}}$ and $\overrightarrow {{A_2}}$ is ${\beta _1} + {\beta _2}$, so the combined field amplitude A can be calculated by law of cosines as follow:

It can be noted that $\beta ^{\prime} = 2{\beta _1}$, so the field amplitude of the resultant vector of jagged-shape can also be expressed as:

Under the first-order diffraction, the arc E’G’ will not close into a circle, which means the $0 < \beta ^{\prime} < \pi$ and $0 < - {\beta _2} < {\beta _1} < \frac{\pi }{2}$. The comparison of value size between the A’ and A shows:

Therefore, the V-shape groove is selected and used when fabricating structural coloration by mechanical cutting method. Meanwhile, According to Eqs. (1), (2), and (3), when the groove width is close to the wavelength (d→λ), sinβ (here β represents the β’, β₁, β₂) is close to β (sinβ→β), which makes $\frac{{{{\sin }^2}\beta }}{{{\beta ^2}}}$ close to 1 ($\frac{{{{\sin }^2}\beta }}{{{\beta ^2}}}$→1). This means that the intensity of diffracted light in all directions of a single groove is uniform, and the diffraction effect is the most obvious. Therefore, the grooves width d should be within the visible spectrum to achieve structural coloration.

Lots of V-shape grooves will induce grating diffraction effect, and the first-order diffraction angle θ increases with the increase of the wavelength [27–29]. For the manufacturing of the structural coloration, the degree of separation of diffracted light by submicron grooves determines the richness degree of the structural coloration, which is defined as the resolution of structural coloration in this paper. The angular dispersive power ($\mathrm{\delta }$) is used to describe the resolution of structural coloration and defined as follow:

It can be concluded that visible light with a larger wavelength will be diffracted into a wider range of diffraction angles. Specifically, the color red occupies a larger range of diffraction angles than does the color violet. A larger dispersive power indicates a higher resolution of structural coloration. Reducing the groove spacing can increase the angle between all the spectral lines of diffracted light, which can then improve the resolution of the entire structural color spectrum from red to violet.

2.2 Diffraction simulation of the submicron grooves

Finite-difference time-domain (FDTD) method is used to simulate the diffraction efficiency of submicron grooves, considering factors of material, groove period and light wavelength. Ni-P and glass, as two representative materials with high reflective and low reflective index respectively, are selected as the groove substrate. Optical property of the Ni-P is set same as the Ni, because the optical properties of Ni-P with a P content of 10% are almost the same as Ni. The visible light is incident on the jagged-shape and V-shape grooves at normal incidence, and the groove period sweep from 300-800 nm is conducted. The analysis group named grating order transmission is used to analyze the simulation result. Being consistent with the common structural color occurring on the reflective surface, the total reflection efficiency and the first-order diffraction efficiency of two kinds of submicron groove surface are obtained in reflection direction. In addition, these efficiencies are all averaged over the visible spectral range of 300-800 nm.

Figures 2(a) and 2(b) show the spectrally (300-800 nm) diffraction efficiency as a function of groove period, for submicron grooves on Ni-P and glass substrates, respectively. With the increase of groove period from 300 nm to 800 nm on Ni-P surface, the total reflection efficiency of V-shape groove fluctuates around 0.36, while this value of jagged-shape groove is just about 0.23, which means the reflection efficiency of Ni-P surface of V-shape groove is 1.5 times of that of jagged-shape groove. Note that this efficiency is the averaged strength and not their summation, because we aim to quantify the influence of groove shape on the reflective performance. The reflective performance on the glass surface decrease drastically, however, compared with the jagged-shape grooves, the total reflection efficiency of the V-shape grooves is increased about 3 times from about 0.003 to 0.01.

 

Fig. 2. Diffraction efficiency simulation of submicron groove on (a) Ni-P and (b) glass by FDTD method.

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According to the results, the diffraction is dominated by the first-order modes (1st or -1st), which therefore is selected and analyzed among the possible diffraction orders. Note that no matter for V-shaped or zigzag, whether on Ni-P or glass, the diffraction efficiency of the first order increases with the increase of the period, which can be explained by the grating diffraction equation. Light with a wavelength greater than the period will not be diffracted in the first order, so the period increase causes the light with a larger wavelength to be diffracted, thereby increasing the average diffraction efficiency of the first order.

The 1^st and -1^st order of V-shaped has the same function of groove period, which can be explained well by the symmetry of V-shape groove. For the jagged-shape grooves, the difference of 1^st and -1^st order efficiency is caused by the asymmetric blazed surface. Especially, the first-order efficiency of V-shape groove is larger than that of jagged-shape groove even for the -1^st order blazed diffraction in glass. It demonstrates that V-shape groove has better performance being used for structural coloration system based on the first-order visible light diffraction.

3. Submicron V-shape grooves created by fly-cutting

3.1 Process principle of axial-feed fly-cutting

The structural coloration effect is strongly dependent on the quality of the machined submicron structures, which commonly should reach the nanometre level. Fly-cutting technology is a potential process for the generation of submicron optical grooves because of the high quality of the machined structures. To efficiently produce submicron optical grooves on a mold, the feeding of the workpiece in the spindle-axis direction during fly-cutting is initially introduced. This axial-feed fly-cutting technology, abbreviated as the AFC method, is characterized by the flexible control of submicron grooves, and is designed and utilized to fabricate optical structures. The principle of this method is illustrated in Fig. 3(a). A diamond cutting tool is attached to a high-speed rotating spindle, and the rake face of the tool is perpendicular to the rotation direction of the fly-cutting. The workpiece is fed along the Z-axis. One groove is generated per rotation of the cutting tool, and grooves are arrayed while the workpiece is fed perpendicularly to the rotation plane of the cutting tool. This process makes the cross-sectional shape of the groove exactly the same as the shape of the diamond tool. In this paper, being consistent with the theory calculation and simulation, the 90° V-shape diamond tool is used.

 

Fig. 3. (a) Principle of the AFC method; (b) Morphological features of the grooves created by AFC.

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The morphological features of the grooves created by AFC are illustrated in Fig. 3(b). It can be determined from the processing principle that these grooves have variable depths; the depth increases from 0 on the both ends to the maximum h in the middle, R is the rotation radius of the diamond tool, and l_s is the length of the grooves, which is only determined by the maximum h. When grooving on the mold surface using the 90° V-shaped tool, the width of the groove is twice its depth, so the maximum width of the groove is 2h. During the manufacturing of grooves, groove adjacency exists in the middle part of the grooves because the groove spacing d is less than the maximum width 2h; thus, the middle part of the arrayed grooves has the same groove depth t, which is half of the groove spacing d. Additionally, l_w is the length of the part of the groove with the same depth t, and l_D is the length of the part of the groove without groove adjacency. These groove parts without groove adjacency will reduce the diffraction efficiency of the visible light because they reduce the consistency of the groove array. Therefore, to improve the quality of structural coloration, the groove tips without adjacency should be removed when fabricating structural color patterns.

AFC technology can be used to control submicron structures with high quality and efficiency. Groove spacing is the period which is the main factor for the manipulation of the visible light spectrum. Interestingly, groove spacing is only determined by the feed rate of the workpiece during AFC when the rotation speed is given, as given by Eq. (11):

3.2 Experimental setup and V-shape groove cutting

A fly-cutting experimental platform for the efficient manufacturing of submicron optical grooves was established, and was reformed based on an ultra-precision Nanoform X machine (produced by Precitech Corporation, USA), as depicted in Fig. 4. A grooving arbor with a mounting feature on the end was fixed with the spindle axis, and a diamond tool was mounted on the grooving arbor. The peak-to-valley (P-V) value of the dynamic balance was adjusted to be less than 10 nm via the balancing provisions of ultra-precision fly-cutting. The Z-axis that provides the feed motion of the workpiece on the spindle-axis has a moving straightness of 0.1 µm/50 mm. The tool setting process is accomplished by the positioning stage, and has a positioning resolution of 15 nm in the vertical direction.

 

Fig. 4. Experimental platform of AFC.

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Electroless-plated Ni-P has been demonstrated to have high strength, high hardness, and good performance in micro/nanostructure machining, and it also has been introduced as a mold material in microstructure machining and used for PGM [30,31]. Therefore, a Ni-P coating with a thickness of 200 µm, which was electroless-plated on the stainless steel mold, was used as the workpiece material. To improve the spindle stability and processing efficiency, the spindle speed was set to 3000 rpm. Before groove manufacturing, the flattening of the mold was conducted to ensure the vertical tolerance within 100 nm. As discussed previously, the groove spacing used in structural coloration manufacturing is on the submicron scale. Grooves were fabricated on the mold by AFC, and the submicron groove spacing was controlled by the feed rates, as exhibited in Table 1.

Table 1. Grating period and feed rate parameters of the segment.

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As described in the process principle of AFC method, what we design and manufacture is a 90° symmetrical V-shaped groove and its typical feature is that the period of the groove is twice the depth. Benefiting from the high degree of certainty of the shape brought by the mechanical processing method, the period measurement has been able to confirm the tool motion. Figure 5 presents the SEM images of machined grooves with spacings ranging from 200-1000 nm, which therefore completely covered the visible spectrum. These machined submicron grooves all presented good quality and consistency. In submicron-scale machining, the topographic accuracy of grooves with larger spacing is higher than that of grooves with smaller spacing. Additionally, even the grooves with 200-nm spacing were still highly regularly arrayed and presented a high parallelism, which verifies that the submicron optical grooves machined by AFC and the proposed experimental platform governed the manipulation of visible light for structural coloration.

 

Fig. 5. SEM photographs of the generated periodic submicron structures.

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4. Fabrication of structural color patterns

4.1 Spectrum detection of submicron grooves induced structural color

To evaluate the manipulation of the visible light spectrum by the submicron V-shape grooves, an optical detection platform was used, as depicted in Fig. 6(a). A mold surface machined with submicron grooves was mounted on a two-stage turntable to separately regulate the incident and diffraction angles. White light from a white light-emitting diode (LED) was converted to parallel light beams through a collimating lens, and was then adjusted by an aperture. The spectrogram of the utilized white light source shown in Fig. 6(b) covers the entire visible range from 380 nm to 780 nm, and it is able to analyse the manipulation of every kind of monochromatic light by the periodic submicron grooves. A parallel light beam with a diameter of no more than 3 mm irradiated on the submicron grooves, and the diffraction light reflected from the mold surface was transmitted through the objective lens and then recorded by an image sensor. The location of the beam on the mold surface could be controlled by the aperture. In this research, the incident angle was set to 90°.

 

Fig. 6. Color spectrum detection: (a) Experimental setup; (b) Spectrogram of the white light source.

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Figure 7 presents the detection result of a segment of V-shape grooves with a spacing of 800 nm. It can be observed that the V-shape grooves diffracted the monochromatic light with high brightness and saturation at different angle, which proves that these grooves exhibited a good beam-splitting performance for the white light. The diffracted lights were detected in the angle range of 30°-60°, and the color of the light covered the entirety of visible light from red to violet. Different colors were obtained at different diffraction angles, and the diffracted colors were characterized by high saturation and clear outlines. It is evident that the color red occupied the largest diffraction angle range as compared to other colors; this means that groove-based structural coloration has a higher resolution in light with greater wavelengths. The diffracted spectrum with high uniformity and saturation demonstrates the high quality of the submicron V-shape grooves machined by AFC.

 

Fig. 7. Diffracted spectrum of a grooved segment with a spacing of 800 nm.

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As presented in Fig. 8, the manipulation of the visible spectrum by a grooved segment with a spacing of 700 nm was detected. The diffracted lights were detected in the angle range of 40°-75°. Compared with the spectrum obtained by the groove spacing of 800 nm, it is clear that the diffraction angle of monochromatic light was larger, and the whole color spectrum from red to violet was distributed in a larger range of diffraction angles. The larger angle difference between different kinds of monochromatic light indicates that the resolution of structural coloration was increased, but part of the color red could not be diffracted because of the large wavelength.

 

Fig. 8. Diffracted spectrum of a grooved segment with a spacing of 700 nm.

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The color spectrum was also tested for a grooved segment with a spacing of 500 nm, as presented in Fig. 9. The diffraction angle of monochromatic light and the angle difference between different kinds of monochromatic light were all further enlarged, which resulted in only blue-violet light being diffracted in the color spectrum. The diffraction angle of violet light increased to about 55°, and diffracted blue-violet light was distributed in a larger angle range of 55°-80°. The resolution of the structural blue-violet color was also further increased because of the small groove spacing. It can also be predicted that blue-violet light will not be diffracted with the reduction of the groove spacing; it would therefore achieve surface invisibility in the visible range, and could therefore make great progress in stealth technology.

 

Fig. 9. Diffracted spectrum of a grooved segment with a spacing of 500 nm.

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After investigation via theoretical analysis and the structural color spectrum detection of submicron grooves, the law of the structural color spectrum induced by grooves was concluded, as presented in Fig. 10. The incident white light will be split into different kinds of monochromatic light by submicron grooves machined by AFC. Only one diffraction order exists during the visible light diffraction for a groove spacing of 800 nm, even for the shortest optical wavelength of 400 nm. The diffraction angle of monochromatic light is larger, and the entire color spectrum from red to violet is distributed in a larger range of diffraction angles, with the reduction of groove spacing. The light within a greater wavelength has greater resolution when used in structural coloration, and cannot be diffracted with the reduction of groove spacing. Red light cannot be completely diffracted with structures with a 700-nm grating period because the maximum optical wavelength of red light is 760 nm. Only optical wavelengths of violet, blue, and cyan can be diffracted with a groove spacing of 500 nm. The entire visible light spectrum cannot be diffracted by a groove spacing of less than the violet wavelength. Reducing the groove spacing contributes to the improvement of the overall resolution of the color spectrum induced by the grooves. Therefore, during high-resolution structural coloration manufacturing, the groove spacing that is the closest to the wavelength of the target diffracted light according to Fig. 10 is selected and machined to induce structural coloration.

 

Fig. 10. Structural color spectrum induced by grooves with different spacings.

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4.2 Structural color pattern fabrication by mold manufacturing and glass molding

Colorful surfaces are manifested as different colors of different pixels, and these different pixel colors are observed in the same viewing angle. Therefore, instead of using diffracted color induced by the same groove spacing from different diffraction angles, pixel segments composed of grooves with a designed spacing are arranged to create a colorful surface under a given diffraction angle. Based on the obtained structural color spectrum, the groove spacing value can be determined for every pixel segment under a given diffraction angle.

To machine a structural color pattern, the permutation and distribution of the pixel segments were designed. The morphological parameters of the structural color surface are illustrated in Fig. 11. At first, the workpiece is fed along direction I when the fly-cutting tool is rotating; in this way, one row of pixel segments is machined completely. Then, the tool returns to the original position and feeds a distance f_v in direction II before machining the next row of pixel segments. A large area of the structural color pixel segments is fabricated by repeating these steps. As discussed in Section 3.1, the tips of the submicron grooves created by AFC should be removed, as they will reduce the quality of structural coloration. Therefore, to cover the groove tips of the pixel segment, overlap between each row of pixel segments is designed and achieved by making the feed distance f_v in direction II shorter than the length of the groove l_s. During structural color pattern manufacturing, the size of the structural color pixel segment determines the resolution of the machined surface. As discussed previously, the width of pixel segments is equal to the feed distance f_v, which can be controllable in a wide range and is not limited in structural surface manufacturing. The length of the pixel segment is equal to the sum of the spacing of all grooves that make up the pixel segment, so the pixel segment length is determined by both the groove number and spacing. According to the interference theory of grooves, as investigated in Section 2.1, diffracted lights from two parallel grooves will experience interference; thus, the spectrum regulation mechanism studied in this paper is also suitable for pixel segments composed of two grooves. Therefore, the minimum length of a pixel segment is twice the spacing of the submicron grooves, which means it can reach 1 or 2 micrometres. In theory, the pixel size of the machined surface can reach the micron level, and is almost unlimited in high-resolution structural color surface manufacturing.

 

Fig. 11. Tool planning of the structural color surface.

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A mold with a structural color pattern of the monogram “BIT” (which stands for the Beijing Institute of Technology) was fabricated via a pixel segment arrangement and the adjustment of the processing parameters. As depicted in Fig. 12(a), the three primary colors of blue, green, and red were respectively presented on the three letters at the viewing angle of 60°, which were respectively induced by the groove spacings of 500 nm, 600 nm, and 800 nm. The color of each single letter was vibrant and bright, and, in theory, any complex pattern can be fabricated by AFC.

 

Fig. 12. Ni-P mold and molded glass component with structural color patterns: (a) Machined structural color pattern on the mold; (b) Molded structural color pattern on the glass.

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Glass molding has been demonstrated to be the most effective and efficient method for the high-precision fabrication of micro/nano-structures on glass surfaces in our previous research [32,33]. During the molding process, glass is softened via heating and then solidified via annealing. The glass is processed into the desired shape after undergoing four processes in the molding machine, namely heating and maintaining, pressurizing at a high temperature, annealing at low pressure, and cooling and demolding. The mold can be divided into the upper mold, lower mold, and sleeve. The vertical distance between the upper and lower molds is in line with the designed thickness of the glass, and it can be controlled by regulating the sleeve height. The glass preform is placed into the mold core, and is then transferred into the mold chamber for heating. The mold chamber is filled with a protective nitrogen gas to prevent the oxidation of the glass and mold under the high temperature. When the glass preform is heated to 30-40 °C higher than the softening point of the glass, the upper and lower molds are closed and loaded to compress the glass preform to achieve the complete filling of the replication of the micro/nano-structures. The mold temperature is slowly reduced to approximately 200 °C, during which a small load is still applied. Finally, the mold leaves the mold chamber and is taken out after being cooled to room temperature.

A PGM experiment was conducted with the mold of the “BIT” pattern, and the molded glass component is depicted in Fig. 12(b). The observed colors of the three letters were the same as those on the mold. The information of the structural color surface was therefore successfully transferred from the mold to the glass via PGM. As studied in simulation part, reflection efficiency of grooves on glass surface is lower than that on Ni-P surface, which makes the“BIT” letters on the glass shows lower diffraction intensity than that on the Ni-P surface.

SEM photographs of periodic grooves with a spacing of 500 nm were used to verify the formation capacity of PGM in submicron structure replication. Figure 13(a) presents the grooves machined on the mold, and Fig. 13(b) presents the grooves on the molded glass. It is demonstrated that the molded periodic grooves maintained good regularity and uniformity although there are some stains on the molded submicron grooves of the glass. According to the study on the PGM process, it is presumed that these stains left on the molded glass results from the air bubbles of interface between mold and glass. However, the regularity and uniformity of the molded submicron grooves is not affected by these submicron stains, which validates the feasibility of the replication of the submicron grooves and structural color pattern on the mold via PGM.

 

Fig. 13. SEM photographs of periodic submicron grooves with a spacing of 500 nm: (a) On the mold; (b) On the molded glass

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5. Conclusions

This paper theoretically studies the diffraction manipulation mechanism of two kinds of typical grooves, and introduce the axial-feed fly-cutting (AFC) technology and PGM technology to precisely create the designed submicron V-shape grooves and structural color pattern with high brightness on Ni-P mold and glass surface. The main conclusions of this research are drawn as follows:

Methods of optical diffraction manipulation, flexible manufacturing of submicron structures and structural color image construction in this paper for the production of structural color pattern are benefit to a wide range of fields, such as anti-fake and stealth technologies.