CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No. 10201902064X filed March 7, 2019, the contents of it being hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] Various aspects of this disclosure relate to a colored photonic structure. Various aspects of this disclosure relate to a method of forming a colored photonic structure.

BACKGROUND

[0003] Three-dimensional (3D) color printing strategies and techniques are extensively developed for visualization and prototyping designs. Recent colors of 3D objects are all produced by dyes and pigments which could be toxic, unsustainable and not lasting long due to photobleaching. Structural colors arising from the interaction of light with periodic structures are recently investigated due to its potential applications such as paintings and cosmetics to replace the pigments and dyes. For instance, inspired by the brilliant blue colors from the Morpho butterfly, Lexus automobiles now incorporate structural colors to achieve iridescent and high-reflection sapphire blue hue (LC structural blue edition). L’Oreal has also reported investment into engineering methods to produce artificial photonic crystal colors mimicking teal and turkey feathers, aiming to create pigment-free color makeup. These works as well as other extensive investigations on structural colors mainly focus on two-dimensional (2D) surfaces and in colloidal suspensions by controlled self-assembly of nanoparticles, wet-etching and growth of nanoparticles. While these methods enable colors to be produced over large areas, the precise positioning of these structural colors would require a lithographic approach. Recently, structural color printing using localized plasmon and dielectric resonators has demonstrated the ability to print structural color on 2D surfaces with unprecedented resolutions and color gamut. However, there has yet been an attempt to directly 3D color print objects with structural colors. Compared with self-assembly, lithographic direct-laser writing (DLW) method provides more flexibility in the design and control of photonic crystals in three dimensions (3D), enabling production of a series of 3D structural photonic crystal colors by tuning the lattice constants of photonic crystals. Moreover, the fabrication process driven by computer-generated program is more compatible with industrial level automation.

[0004] To reflect vivid colors, the lattice constants of a photonic crystal must be sufficiently small. Existing 3D lithography printing techniques lack the required printing resolution to fabricate photonic structures exhibiting colors due to the diffraction limit. For instance, commercial 3D two-photon lithography (Nanoscribe GmbH Photonic Professional GT) can only achieve down to 500-nm lateral resolution and 800-nm axial resolution, with 200-nm line- width. So far, non-commercial methods have been developed to improve the resolution of 3D two-photon DLW, including stimulated emission depletion (STED) and diffusion-assisted high resolution DLW. STED-DLW achieves sub-diffraction resolution and reduced feature size by utilizing a second inhibition laser beam to inhibit the photopolymerization triggered by the writing laser beam at the inhibition area, but the requirements of this method are stringent, i.e well-aligned writing and inhibition laser beams, high pulsed laser power and short time constants for inhibition laser beam as well as specialized photoresist. To increase the resolution by using a less stringent strategy using single laser beam, diffusion-assisted high resolution DLW has been demonstrated. Woodpile structures lattice constant of 400 nm have been achieved by combination of a mobile quenching molecule with a slow laser scan speed allowing diffusion of the quencher in the scanned area and depletion of radicals. However, laser writing speed range allowed is confined by the diffusion rate of the quencher, and organic- inorganic hybrid photoresist used for this method requires a pre-processing step before photopolymerization. In general, a 3D lithography printing technique that meets the following properties is currently unavailable: (i) spatial resolution better than~300 nm; achieved with (ii) single-beam laser as well as (iii) commercial photo-resist requires no pre-processing step, and (iv) allows high writing speed.

SUMMARY

[0005] Various embodiments may provide a method of forming a colored photonic structure. The method may include forming an initial photonic structure. The method may also include heating the initial photonic structure to reduce a lattice constant of the initial photonic structure, thereby forming the colored photonic structure exhibiting one or more colors. [0006] Various embodiments may provide a colored photonic structure. The colored photonic structure may exhibit one or more colors. A lattice constant of the colored photonic structure may be of a value less than 1 pm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

FIG. 1 is a general illustration of a method of forming a colored photonic structure according to various embodiments.

FIG. 2A is a schematic illustrating the method of forming a photonic structure according to various embodiments.

FIG. 2B is (left) a plot of weight (in percent or %) as a function of temperature (in degree Celsius or °C) and (right) a plot of derivative weight (in percent/ degree Celsius or %/°C) as a function of temperature (°C) of polymerized IP-Dip films according to various embodiments. FIG. 2C shows images of the woodpile micro structure before and after heating according to various embodiments (i) and (v) are brightfield reflection mode optical micrograph before and after heating respectively.

FIG. 3 shows side-view optical micrographs (bright field reflection mode) of photonic structures sitting on strategically-designed pedestals before and after thermal shrinking according to various embodiments.

FIG. 4 shows scanning electron micrographs (SEM) comparing (A) the resultant woodpile structure formed by direct-laser-writing (DLW) and thermal treatment according to various embodiments, and (B and C) the resultant woodpile structures formed with conventional direct- laser-writing (DLW).

FIG. 5 shows optical micrographs of (A) a woodpile structure before heating; (B) the woodpile structure after shrinking by about 50%; (C) the woodpile structure after shrinking by about 68%; and (D) the woodpile structure after shrinking by about 73% according to various embodiments.

FIG. 6A is a plot of reflectance (in percent or %) as a function of wavelength (in nanometers or nm) showing the reflectance of the woodpile structure before heating according to various embodiments. FIG. 6B is a plot of reflectance (in percent or %) as a function of wavelength (in nanometers or nm) showing the reflectance of the woodpile structure after shrinking by about 50%, 68% and 73% upon heating according to various embodiments.

FIG. 6C is a plot of refractive index n as a function of wavelength (in nanometers or nm) showing the variation of refractive index of the woodpile structure before and after thermal treatment according to various embodiments.

FIG. 6D is a plot of extinction coefficient k as a function of wavelength (in nanometers or nm) showing the variation of extinction coefficient of the woodpile structure before and after thermal treatment according to various embodiments.

FIG. 7 shows (i) scanning electron micrograph (SEM) images of woodpile structures before heat treatment; (ii) bright- field micrographs; and (iii) scanning electron micrograph (SEM) images of woodpile structures after heat treatment according to various embodiments.

FIG. 8A is a plot of reflectance (in percent or %) as a function of wavelength (in nanometers or nm) showing the reflectance spectra of woodpiles structures of different lateral periodicities according to various embodiments.

FIG. 8B is a plot of wavelength (in nanometers or nm) as a function of lattice constant (in nanometers or nm) showing variation of positions of maximum reflectance due to as lattice constant changes in the woodpile structures according to various embodiments.

FIG. 9 shows brightfield reflection-mode optical micrographs of woodpiles structures according to various at embodiments at tilt angles of (A) 0 degree, (B) 15 degrees, (C) 45 degrees, and (D) 75 degrees.

FIG. 10 shows (A) schematics and optical micrographs of a first stack of three-dimensional (3D) color voxels a, b and c according to various embodiments, and (B) schematics and optical micrographs of a second stack of the three-dimensional (3D) color voxels a, b and c according to various embodiments.

FIG. 11 shows (A) the Artscience Museum model formed by the direct laser writing (DLW) method, and (B) the Artscience Museum model formed by direct laser writing (DLW) and thermal treatment according to various embodiments.

FIG. 12A shows (left) a shrunken gyroid structure according to various embodiments, and (right) a magnified image of the shrunken gyroid structure according to various embodiments. FIG. 12B shows reflection-mode optical micrographs of a shrunken gyroid structure according to various embodiments taken at tilt angles of (i) 0 degree, (ii) 15 degrees, (iii) 45 degrees and (iv) 75 degrees.

FIG. 12C is a plot of reflectance (in percent or %) as a function of wavelength (in nanometer or nm) showing the reflectance spectra of shrunken gyroid structures according to various embodiments at different tilt angles.

DETAILED DESCRIPTION

[0008] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, and logical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0009] Embodiments described in the context of one of the methods or structures are analogously valid for the other methods or structures. Similarly, embodiments described in the context of a method are analogously valid for a structure, and vice versa.

[0010] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0011] The structure as described herein may be operable in various orientations, and thus it should be understood that the terms“top”,“bottom”, etc., when used in the following description are used for convenience and to aid understanding of relative positions or directions, and not intended to limit the orientation of the structure.

[0012] In the context of various embodiments, the articles“a”,“an” and“the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0013] In the context of various embodiments, the term“about” or“approximately” as applied to a numeric value encompasses the exact value and a reasonable variance. [0014] As used herein, the term“and/or” includes any and all combinations of one or more of the associated listed items.

[0015] Various embodiments may meet these requirements. Various embodiments may combine printing of 3D structures using a commercial lithography tool and resist, with a post exposure thermal shrinking process.

[0016] FIG. 1 is a general illustration of a method of forming a colored photonic structure according to various embodiments. The method may include, in 102, forming an initial photonic structure. The method may also include, in 104, heating the initial photonic structure to reduce a lattice constant of the initial photonic structure, thereby forming the colored photonic structure exhibiting one or more colors.

[0017] In other words, the method may include forming a photonic structure, and heating the photonic structure. By heating the photonic structure, the lattice constant may be decreased, thereby increasing the resolution of the photonic structure, and increasing the reflectance of one or more wavelengths in the visible range of the photonic structure. Accordingly, the photonic structure may exhibit colors in the visible range, e.g. red, orange, yellow, green, blue and/or violet. In the current context, one or more colors refer to one or more colors associated with components of visible light, and do not include greyscale colors, i.e. black, white, or grey. Prior to heating, the photonic structure may not exhibit the one or more colors, and may appear black, white, or grey.

[0018] The photonic structure before heating may be referred to as the“initial photonic structure” or“original photonic structure”, while the photonic structure after heating may be referred to as “colored photonic structure” or“heated photonic structure”. The photonic structure may include photonic crystals, which refer to periodic dielectric structures that are configured to form an energy band structure for photons, which either allows or forbids the propagation of electromagnetic waves of certain wavelengths. After or upon heating, photonic bandgaps allowing certain visible wavelengths to pass through may be formed, thus causing the photonic structure to exhibit colors.

[0019] In various embodiments, heating may continue after the colored photonic structure is formed. In various embodiments, the one or more colors of the colored photonic structure may change as heating continues. The one or more colors exhibited may be dependent on a wavelength or range of wavelengths in which peak reflectance (i.e. maximum reflectance) occurs. [0020] In various embodiments, forming the initial photonic structure may include depositing a monomer on a surface; and using a lithographic method to polymerize the monomer, thereby forming a polymer. Accordingly, the initial photonic structure may include any suitable polymer that can shrink when heated up either with a heating source or exposed to high power irradiation, i.e. through absorption. For instance, the polymer may be any polymerized acrylate-based photoresist, such as polymerized IP-Dip or polymerized IP-S, while the monomer may be any suitable acrylic resin, e.g. IP-Dip, IP-S, IP-L or IP-G. In various embodiments, the polymer may be any suitable polymerized ultraviolet (UV)-curable acrylate- based photoresist. The colored photonic structure may also include the polymer.

[0021 ] The lithographic method may be any suitable lithographic method for printing three- dimensional (3D) structures. In various embodiments, the lithographic method may be two- photon lithography or multi-photon lithography. In various other embodiments, the lithographic method may be interference lithography. However, there may be disadvantages associated with interference lithography, such as increased difficulties to form uniform regions of a single color and 3D structures with the desired photonic crystal properties. In yet various other embodiments, any other suitable method, such as nanoprinting or self-assembly, may be used to form the initial photonic structure.

[0022] A single beam may be used for writing, i.e. to polymerize the monomer. The photoresist, i.e. the monomer may not require any pre-processing. Various embodiments may achieve a fast writing speed.

[0023] The method may also include, removing a remaining portion of the monomer after polymerization. Monomers that are not exposed to photons during lithography may not be polymerized, and may be removed after polymerization. A suitable solvent such as propylene glycol monomethyl ether acetate may be used to remove the remaining portion of the monomer.

[0024] In various embodiments, the initial photonic structure may be heated in the presence of one or more inert gases, e.g. nitrogen gas or argon gas, to form the colored photonic structure. The one or more inert gases may not react with the initial or colored photonic structure, and may help to provide protection.

[0025] In various embodiments, the lattice constant of the initial photonic structure, i.e. the photonic structure prior to heating, may be of a value more than 1 pm. The lattice constant may be a distance between neighboring unit cells of the photonic structure. The lattice constant may also be referred to as lateral pitch, lateral lattice, lattice size, lateral periodicity or the like. [0026] In various embodiments, the lattice constant of the colored photonic structure, i.e. the photonic structure after heating, may be of a value less than 1 pm, e.g. less than 500 nm, e.g. less than 300 nm.

[0027] In various embodiments, a reflectance of one or more visible wavelengths of the initial photonic structure may be increased after heating.

[0028] In various embodiments, a refractive index contrast of between the initial photonic structure and a surrounding environment may be increased to a value above 0.50, after heating. In various embodiments, a refractive index of the initial photonic structure may be increased, for instance, to a value e.g. above 1.60, e.g. above 1.70, e.g. above 1.80 in a visible range, after heating, which may be desirable for structural color applications. The refractive index contrast between the colored photonic structure and the surrounding environment, may for instance, be of a value above 0.50 in the visible range. The refractive index of the colored photonic structure, may for instance, be of a value above 1.60, e.g. above 1.70, e.g. above 1.80, in the visible range. The refractive index contrast of a material or structure may be defined as the difference in refractive index between the material or structure and its surroundings. The refractive index of a structure or a material may be defined as a ratio of the speed in which light travels in a vacuum relative to that of the structure or the material. An extinction coefficient of the initial photonic structure may also be increased, for instance, to e.g. a value selected from 0.1 to 0.4 after heating, which may enhance scattering of light and increasing brightness of colors. In various embodiments, heating may be carried out by a heating stage. The extinction coefficient value may be measured with respect to electromagnetic waves in the range of 300 nm - 900 nm.

[0029] In various embodiments, the method may include forming or providing a pedestal on a substrate so that the initial photonic structure is formed on the pedestal. In various other embodiments, the method may include forming or providing a sacrificial structure on a substrate so that the initial photonic structure is formed on the sacrificial structure. If the photonic structure is directly provided on the substrate, the photonic structure may experience anisotropic shrinkage, due to large differences in shrinkage between the photonic structure and the underlying substrate. By providing a pedestal or a sacrificial layer, the initial photonic structure may experience isotropic shrinkage.

[0030] The photonic structure may be heated at a temperature selected from a range from 400 °C to 500 °C, e.g. 450 °C. [0031] Various embodiments may provide a colored photonic structure formed by a method as described herein. Various embodiments may provide a colored photonic structure that exhibits one or more colors. The colored photonic structure may have a lattice constant less than 1 pm , e.g. less than 500 nm, e.g. less than 300 nm.

[0032] Various embodiments may provide a colored photonic structure. The colored photonic structure may exhibit one or more colors. At least one lattice constant of the colored photonic structure may be of a value less than 1 pm, e.g. less than 500 nm, e.g. less than 300 nm. In various embodiments, the colored photonic structure may include two or more lattice constants.

[0033] In various embodiments, the colored photonic structure may include a polymer, such as a polymerized acrylate-based photoresist. The refractive index contrast between the colored photonic structure and a surrounding environment may be of a value greater than 0.50 in the visible range. The refractive index of the colored photonic structure may be above 1.60, e.g. above, 1.70, e.g. above 1.80 in the visible range. In various embodiments, an extinction coefficient of the colored photonic structure may be of a value selected from 0.1 to 0.4.

[0034] The colored photonic structure may be of any suitable shape, crystal structure and/or 3D arrangement. For instance, the colored photonic structure may be a woodpile structure or a gyroid. The woodpile structure may include a plurality of rods.

[0035] In various embodiments, the colored photonic structure may exhibit a first color when viewed from a first angle, and a second color different from the first color when viewed from a second angle different from the first angle. The colored photonic structure may exhibit the first color when tilted by the first angle, and the second color when tilted by the second angle.

[0036] Various embodiments may provide a plurality of colored photonic structures. Each colored photonic structure of the plurality of colored photonic structures may be formed by a method as described herein. A first photonic structure of the plurality of photonic structures may exhibit a first color, and a second photonic structure of the plurality of photonic structures may exhibit a second color that is the same as the first color, or different from the first color. The first photonic structure may exhibit the first color and the second photonic structure may exhibit the second color when viewed from one angle. The plurality of colored photonic structures may include the same material. [0037] If the second color is the same as the first color, the lattice constant of the second photonic structure may be equal to the lattice constant of the first photonic structure. If the second color is different from the first color, the lattice constant of the second photonic structure may be different from the lattice constant of the first photonic structure.

[0038] Various embodiments may combine printing of 3D structures using a commercial lithography tool and resist, with a post-exposure thermal shrinking process. As an example of the capability of this thermal shrinking process, the fabrication of photonic structures, i.e. photonic crystal woodpiles with lattice sizes below 500-nm is described below. The fabrication has been successfully demonstrated. The thermal process also alters the optical properties of the cross-linked material, producing one with increased refractive index, which is desirable for structural color applications. As evidence of successfully fabricated photonic crystals, photonic bandgaps appear in the visible range, and thus photonic crystal woodpile reflects vivid colors. The reflective color wavelengths are adjustable with the lattice sizes of the woodpiles. In addition, the capability to directly print 3D color objects at the microscale has also been demonstrated. Photonic crystals may be used as 3D color voxels (volumetric pixels) to assemble 3D color objects that reflect vivid colors. Using this printing strategy, a 20-pm full- color prototype of the Artscience Museum , a well-known building in Singapore, has been fabricated to demonstrate the capability of 3D printing full-color devices in the area scale of below 10 pm. Various embodiments may be applicable to make compact photonic crystal devices on chips or directly print photonic crystals on optical fibers to precisely control the outcoming light’s wavelengths and polarizations. Various embodiments may relate to the first full-color 3D printed object that is based on structural colors instead of dyes.

[0039] Fabrication of 3D color voxels

[0040] Photonic crystals may be fabricated as 3D color voxels using a two-step method combining two-photon lithographic polymerization and thermal treatment to induce size reduction.

[0041] In the first step, polymeric photonic microstructures with lattice constants above 1 pm are directly printed by using two-photon lithography system (Nanoscribe GmbH Photonic Professional GT). These microstmctures are a result of cross-linked/polymerized IP-Dip which is an acrylate -based photoresist. In the typical experiment, a droplet of IP-Dip monomer is drop-casted on a glass slide and then polymerized by a computer-assisted femto-second pulsed fiber laser with a center wavelength of 780 nm. Polymeric structures are formed according to the pre-defined graphic programs. The Dip-in Laser Lithography (DiLL) process is performed using an inverted microscope with an oil immersion lens (63x, NA 1.4), and a computer- controlled galvometric stage. The average laser power around 17.5-27.5 mW and a writing speed at 15 mm/s are used for fabricating polymeric structures on glass slides. Then, unexposed photoresist is removed in propylene glycol monomethyl ether acetate (PGMEA) for 10 min. Then samples are immersed into isopropyl alcohol (IPA) for 5 min and immersed into nonafluorobutyl methyl ether for another 5 min. IPA is used to further remove the unexposed photoresist. At the same time, as PGMEA is replaced with IPA, the sample may be dried faster with less surface tension. The immersion in nonafluorobutyl methyl ether may further reduce the surface tension in the drying process. Since the photonic structures include delicate parts, excessive surface tension may induce cohesion, collapse or unwanted shrinking of the structure along with drying. The samples are taken out and dried in the air. Instead of air, the samples may also be dried in nitrogen. In the following thermal treatment step, photonic microstructures were heated under the protection of inert gases. IP-Dip is decomposed under the temperature over 450 °C so that the size of the photonic crystals could be reduced up to about 73%.

[0042] FIG. 2A is a schematic illustrating the method of forming a photonic structure according to various embodiments. The left portion of FIG. 2A shows IP-Dip is being polymerized into photonic microstmctures with lateral constant above 1 pm via direct laser writing. The IP-Dip monomers are deposited on a glass substrate. The middle portion of FIG. 2A shows polymeric microstmctures being heated to temperatures of 450 °C in a closed chamber under inert gas flow. The right portion of FIG. 2A shows the photonic structure after thermal treatment. After the thermal treatment, the dimensions of the photonic structure are reduced by up to about 73% and vivid photonic colors are immediately observed due to the increase of refractive index of the material and the decrease of the lattice constant to 300-700 nm.

[0043] FIG. 2B is (left) a plot of weight (in percent or %) as a function of temperature (in degree Celsius or °C) and (right) a plot of derivative weight (in percent/ degree Celsius or %/°C) as a function of temperature (°C) of polymerized IP-Dip films according to various embodiments. FIG. 2B shows thermogravimetric analysis curves of the polymerized IP-Dip films. The photonic structure is heated at 450 °C because thermogravimetric analysis reveals that the inflection point of the polymerized IP-Dip is 450 °C. In other words, IP-Dip decomposes at the greatest rate at 450 °C. [0044] A woodpile microstructure including a plurality of rods with widths of 330 nm and lengths of 17.4 pm may be printed and heated. The woodpile has lattice constants of 1.7 pm and is composed of 15 repeat units in the vertical direction. FIG. 2C shows images of the woodpile micro structure before and after heating according to various embodiments (i) and (v) are brightfield reflection mode optical micrographs before and after heating respectively (ii) and (iii) are top-view scanning electron micrographs (SEM) of the woodpile microstructure before heating, while (vi) and (vii) are top-view scanning electron micrographs (SEM) of the woodpile microstructure after heating (iv) and (viii) are tilt-view scanning electron micrographs (SEM) of the woodpile microstructure before and after heating respectively. As shown in FIG. 2C, the size of the woodpile is reduced in general. From SEM images of the heated woodpile, the bottom 5 repeat units appear to turn into mesh-like structures adhered to the substrate. The bottom 5 repeat units appear to experience anisotropic shrinkage due to large difference in the shrinking level between the substrate and the polymerized structure. However, the other 10 repeat units on the top shrink isotropically. The 5 bottom repeat units are taken as sacrificial layers, while the upper 10 repeat units are used for characterizations and optical investigations.

[0045] To solve the issue of distortion, a pedestal may be added underneath to decouple the substrate with the structure to obtain homogeneous reduction of size of the photonic crystal. FIG. 3 shows side-view optical micrographs (bright field reflection mode) of photonic structures sitting on strategically-designed pedestals before and after thermal shrinking according to various embodiments. All scale bars in FIG. 3 represent 10 pm.

[0046] During the heating step, the rod widths and lengths are reduced to 144 nm and 4.7 pm, respectively. The lattice constant is decreased to 450 nm. Woodpiles with 500-nm lattice constants are printed using two-photon direct-laser-writing (DLW) and are compared with the thermally treated woodpiles to show the advantage of the improved resolution of the thermally treated woodpiles over samples formed by two-photon DLW.

[0047] FIG. 4 shows scanning electron micrographs (SEM) comparing (A) the resultant woodpile structure formed by direct-laser-writing (DLW) and thermal treatment according to various embodiments, and (B and C) the resultant woodpile structures formed with conventional direct-laser- writing (DLW).

[0048] It may be noted that the woodpile structure resulting from DLW and thermal treatment in FIG. 4(A) is structurally well-defined and neighboring rods are fully resolved. In contrast, SEM images of the woodpile structures formed by DLW are not fully resolved, with unwanted bridging between lines. This issue may not be solved by decreasing the exposure doses, i.e. small laser power. FIG. 4(C) shows the woodpile structure printed at the detectable polymerization threshold of 7.5 mW. The SEM image of FIG. 4(C) shows that the rods are bending and the entire structure has collapsed.

[0049] The lateral and axial resolution enhancement are also characterized. The size (i.e. both diameter D and height H) of an effective writing spot which is polymerized is reduced with the aid of thermal shrinking step. The rod width and height may be counted as diameter and height of the effective writing spot under constant printing rate. The axial Abbe limit is 2.92 times larger than the lateral Abbe limit, and thus the effective writing spot is elliptical. The aspect ratio (H/D) of the effective writing spot is ~2.5. Upon heating, the diameter and height of the effective writing spot are decreased by 56% and 67%, respectively. Due to larger shrinking percentage along the axial direction compared to the lateral direction, the aspect ratio (H/D) is decreased from 2.4 to 1.8, with the height reduced to 260 nm. In fact, thermal treatment may make the shape of the effective writing spot more spherical, which is a state with the smallest surface energy. In this regard, the printing resolutions in all three dimensions are reduced. It is noteworthy that a good axial resolution may be more challenging to be achieved than a good lateral resolution.

[0050] Color evolution with the shrinking percentage

[0051] FIG. 5 shows optical micrographs of (A) a woodpile structure before heating; (B) the woodpile structure after shrinking by about 50%; (C) the woodpile structure after shrinking by about 68%; and (D) the woodpile structure after shrinking by about 73% according to various embodiments. The woodpile initially has a lattice constant of 1.7 pm and is heated at about 450 °C. Before heating, no colors are observed from this woodpile and the woodpile appears greyish-white. During the heating process, colors associated with components of visible light are observed. The colors observed change as shrinkage progresses, i.e. the colors shift from yellow to cyan and purple when the woodpile shrinks by about 50%, 68% and 73% from its original size respectively. FIG. 6A is a plot of reflectance (in percent or %) as a function of wavelength (in nanometers or nm) showing the reflectance of the woodpile structure before heating according to various embodiments. FIG. 6B is a plot of reflectance (in percent or %) as a function of wavelength (in nanometers or nm) showing the reflectance of the woodpile structure after shrinking by about 50%, 68% and 73% upon heating according to various embodiments. The observation on reflection-mode optical micrographs tallies with the measured spectra. In the reflectance measurement spectra, a broad peak centered at -600 nm starts to appear when the woodpile shrinks by about 50% and the peak shifts to -500 nm as woodpile shrinks by about 68%. After shrinking by about 73%, the peak shifts to 445 nm and the intensity decreases, coupled with the appearance of another strong peak in the 650 nm to near infrared (NIR) range. The appearance of color indicated the appearance of a photonic bandgap in visible range after thermal shrinking at 450 °C. The phenomenon may be due to the concerted effect of the reduction in periodicity, and the increase in refractive index (n) and extinction coefficient (k) with heating. FIG. 6C is a plot of refractive index n as a function of wavelength (in nanometers or nm) showing the variation of refractive index of the woodpile structure before and after thermal treatment according to various embodiments. FIG. 6D is a plot of extinction coefficient k as a function of wavelength (in nanometers or nm) showing the variation of extinction coefficient of the woodpile structure before and after thermal treatment according to various embodiments.

[0052] The refractive index (n) of the polymerized IP-Dip in visible range is increased from about 1.57 to about 1.82 during the thermal decomposition at 450 °C. Moreover, the extinction coefficient (k) of the polymerized IP-dip increases towards a value selected from 0.1 to 0.4 in the range of 300-900 nm with the decomposition of IP-Dip, likely enhancing the scattering of light, and thus increasing the brightness of colors.

[0053] A series of woodpiles designed with varying lattice constants and rod widths prior to the heatingstep may be printed to achieve multiplexing structural color voxels. FIG. 7 shows

(i) scanning electron micrograph (SEM) images of woodpile structures before heat treatment;

(ii) bright-field micrographs; and (iii) scanning electron micrograph (SEM) images of woodpile structures after heat treatment according to various embodiments.

[0054] The woodpile structures have varied lateral periodicity (a) and line width (w) according to various embodiments. Before heat treatment, lattice constants vary from 1.0 pm to 2.8 pm and rod widths vary from 200 nm to 330 nm. After heat treatment, multiple colors spanning the entire visible range are observed on different substrates and on the same substrate (see FIG. 9 also). These shrunken woodpiles are measured with lattice constants of 330 to 700 nm and rod widths of 107 to 179 nm (see FIG. 7). FIG. 8A is a plot of reflectance (in percent or %) as a function of wavelength (in nanometers or nm) showing the reflectance spectra of woodpiles structures of different lateral periodicities according to various embodiments. [0055] Reflectance measurements on these color voxels demonstrate the continuous shifting of the bandgap positions ranging from 360 to 700 nm.

[0056] FIG. 8B is a plot of wavelength (in nanometers or nm) as a function of lattice constant (in nanometers or nm) showing variation of positions of maximum reflectance due to as lattice constant changes in the woodpile structures according to various embodiments.

[0057] As expected of photonic crystals with incomplete bandgaps, angle-varying colors from these shrunken woodpiles may be observed. To monitor the angle-dependent reflection colors, the substrates are immobilized on a titling stage. Woodpiles are tilted to up to 75 degrees and are observed by using a 50x long-working-distance objective lens. From the reflection images taken at different viewing angles, little color change is observed within 15-degree tilt angles. However, as the viewing angle is increased, drastic color changes are observed. FIG. 9 shows brightfield reflection-mode optical micrographs of woodpiles structures according to various at embodiments at tilt angles of (A) 0 degree, (B) 15 degrees, (C) 45 degrees, and (D) 75 degrees. All scale bars represent 10 pm.

[0058] Two“ladder” structures (three-dimensional (3D) stacked cubes) may be constructed to demonstrate the ability to print 3D color objects as well as multiplex capabilities. The “ladder” structure consists of three-dimensional (3D) color voxels wherein the 3D color voxels are colored photonic structures, which may be thermal-treated woodpile structures. FIG. 10 shows (A) schematics and optical micrographs of a first stack of 3D color voxels a, b and c according to various embodiments, and (B) schematics and optical micrographs of a second stack of the 3D color voxels a, b and c according to various embodiments. The 3D voxels each has a different lattice constant. Component a is put at the bottom in (A) and on top in (B), while component b is put on top in (A) and at the bottom in (B). The reflection-mode optical micrographs are taken at tilt angles of (i) 75 degrees and (ii) 0 degree, with the 0 degree images taken at three focal planes. All scale bars represent 10 pm.

[0059] A 20 pm model of Artscience Museum is also printed by the method as described herein to demonstrate the ability to print 3D color objects at the microscale level. The model is assembled with voxels of thermal-treated woodpile structures. FIG. 11 shows (A) the Artscience Museum model formed by the direct laser writing (DLW) method, and (B) the Artscience Museum model formed by direct laser writing (DLW) and thermal treatment according to various embodiments. As shown in FIG. 11, direct laser writing (DLW) with no heating forms a 120- pm Artscience Museum model with no color observed under microscope. Vivid colors are observed to appear after thermal treatment while the shape and structure remain intact during the shrinking process.

[0060] Versatility

[0061] As a key advantage over self-assembling methods, lithographic methods are able to define a variety of photonic crystal designs other than woodpiles. A gyroid structure with lattice constant of 375 nm has also been formed by thermally shrinking a gyroid structure with l-pm lattice. FIG. 12A shows (left) a shrunken gyroid structure according to various embodiments, and (right) a magnified image of the shrunken gyroid structure according to various embodiments. The scale bar for the left image is 10 pm, and the scale bar for the right image is 1 pm. Before heat treatment, the gyroid is 20 pm in dimension and 1 pm in lateral pitch. After heating, the dimension and pitch shrink to 10 pm and 375 nm, respectively.

[0062] Dimensions of the nanorods included in the gyroid are as small as 94 nm. FIG. 12B shows reflection-mode optical micrographs of a shrunken gyroid structure according to various embodiments taken at tilt angles of (i) 0 degree, (ii) 15 degrees, (iii) 45 degrees and (iv) 75 degrees. The scale bar is 1 pm. The reflection of the gyroid structure is observed with normal incidence and at different viewing angles up to 75 degrees. The entire gyroid photonic crystal exhibits green colors at viewing angles less than 15 degrees. Blue and cyan colors are observed at 45 and 75 degrees. The observation tallies with the reflectance spectra. At 45-degree viewing angle, the purple colors showing defects at the edges may clearly be observed. FIG. 12C is a plot of reflectance (in percent or %) as a function of wavelength (in nanometer or nm) showing the reflectance spectra of shrunken gyroid structures according to various embodiments at different tilt angles. The versatility may enrich the library of 3D artificial pigment pixels.

[0063] Various embodiments may relate to non-toxic 3D photonic crystals producing vibrant structural colors. The photonic crystals may be promising substitutes to traditional pigments used in applications ranging from automobile paints to cosmetics. In comparison to self-assembly methods, 3D printing may provide more flexibility in the design and control of photonic crystals. The major challenge on using direct-laser-writing (DLW) to fabricate 3D photonic crystals with bandgaps in visible range may be to increase the spatial resolution so as to reduce the printable lattice constants.

[0064] A strategy to achieve color 3D Printing using structural colors instead of dyes has been described herein. By enhancing the resolution of two-photon lithography with the aid of thermal-induced size-reduction, polymeric gyroid and woodpile structures exhibiting vibrant colors have been fabricated. In various embodiments, the resolution of two-photon lithography may be enhanced beyond the diffraction limit. Various embodiments may use a single-beam to form structures from commercial UV-cured polymer resin (based on two-photon lithography) with subsequent heating to print colorful 3D photonic crystals. In various embodiments, heating may cause the lattice constants and rod widths of photonic crystal woodpiles to be decreased to 330 nm and 107 nm respectively. The exhibited colors may be tuned in the visible spectrum by varying the lattice constants. Moreover, the versatility of this strategy has also been demonstrated through printing photonic crystal gyroid structures. The lattice constants and feature sizes may be decreased to 375 nm and 94 nm respectively. Compared to conventional methods such as STED and diffusion-assisted DLW, the small lattice constants achieved by various embodiments may be comparable, but the fabrication requirements according to various embodiments may be less stringent compared to the conventional methods. As such, various embodiments may be promising for practical production in this aspect. Using these photonic crystals as voxels, 3D objects/models exhibiting colors in three dimensions may be directly printed out. To demonstrate its applications for prototyping designs in microscale, a 20-pm prototype of Artscience Museum , one of the famous buildings in Singapore, has also been printed as an example. Vivid colors may be observed in varied viewing angles under the microscope. Various embodiments may be able to 3D print complex colors in the prototype without loss of any structural details. Advantageously, no pigments or dyes may be used, so that the printed 3D objects may be more environmentally friendly and stable to bleaching.

[0065] Experimental Section

[0066] Materials

[0067] IP-Dip photoresist with refractive index n» 1.52 (Nanoscribe Inc, Germany) was used as negative photoresist for two-photon lithography in dip-in laser lithography (DiLL) configuration propylene glycol monomethyl ether acetate, isopropyl alcohol, nonafluorobutyl methyl ether were purchased from Sigma-Aldrich. All chemicals were used without further purification. Glass slides (fused silica, 25 mm squares with a thickness of 0.7 mm) were purchased from Nanoscribe GmbH and used without further surface modification.

[0068] Fabrication of polymeric photonic crystal structures on glass slides

[0069] Polymeric nano and/or microstructures were fabricated using direct laser writing system (Nanoscribe Inc., Germany). In the typical experiment, a droplet of IP-Dip monomer was drop-casted on a glass slide and then polymerized by a computer-assisted femto second pulsed fiber laser with a center wavelength of 780 nm. Polymeric structures were formed according to the pre-defined graphic programs. The Dip-in Laser Lithography (DiLL) process was performed using an inverted microscope with an oil immersion lens (63 X, NA 1.4), and a computer-controlled galvo stage. Pre-defined graphic programs determine the position of the laser spots and thus the shape of the polymerized structures. The average laser power around 17.5-27.5 mW and a writing speed at 15 mm/s were used for fabricating polymeric structures on glass slides. Then, unexposed photoresist was removed in propylene glycol monomethyl ether acetate for 10 min. Then samples were immersed into isopropyl alcohol for 5 min and immersed into nonafluorobutyl methyl ether for another 5 min. The samples were taken out and dried in the air.

[0070] Thermal treatment of polymeric photonic crystal structures

[0071] The polymeric photonic crystals were heated by using a temperature-controlled stage, Linkam heating stage. The sample was put in the closed chamber of the heating stage with N2 flow. The temperature in the chamber was heated from 26 °C to 450 °C at a ramp rate of 10 °C/min and was maintained at 450 °C for about 17 min. Then, heating to the stage was stopped and the chamber was cooled down to room temperature by a water chiller.

[0072] Reflectance measurement

[0073] Optical micrographs and spectra (reflectance mode) were taken using a Nikon Eclipse LV100ND optical microscope equipped with a CRAIC 508 PV micro spectrophotometer and a Nikon DS-Ri2 camera. Samples were lit by halogen lamp illumination and measured/imaged in reflection though a 50x/0.4NA long working distance objective lens. The angle-varying reflectance measurements were performed on a tilt stage. The tilt angle of the stage could be tuned from 0 to 90 degrees with 15-degree interval. When at tilt angle of 0 degree, illuminating and reflected light paths were normal to the substrate.

[0074] Characterization

[0075] Scanning electron microscopy (SEM) was performed using a JEOL-JSM-7600F with an accelerate voltage of 5 kV. Thermogravimetric analysis (TGA) was performed using thermal gravimetric analyzer TA Q50, with a platinum (Pt) boat to held the sample and the entire TGA analysis was processed in a closed chamber with nitrogen (N2) flow (60 ml/min).

[0076] Various embodiments may relate to a method to directly print 3D microscale colored objects in which the colors are a result of the structures in 3D photonic crystals. By combining two-photon lithography and thermal treatment at 450 °C, polymeric photonic crystals that constitute the 3D microscale objects may be fabricated with lattice constants (repeat units) down to 300 nm and structures with sub-100-nm dimensions. Such a capability may allow the printing of photonic crystals such as woodpiles and gyroids with tunable bandgaps in the visible range. Photonic crystals may be used as 3D color voxels to assemble 3D colorful objects with micro-meter sizes. These 3D color voxels may be used to replace dyes for 3D printing applications. The colors exhibited may be tuned in the visible range and the multiplexing capability may be promising for applications like anti-counterfeiting, spectroscopy, color displays, etc.

[0077] Various embodiments may relate to directly printing 3D color objects at microscale for applications such as visualization and prototyping designs and to increase the spatial resolution of 3D objects printable with two-photon direct-laser-writing technique. The colors in 3D objects maybe structural colors in photonic crystals so they are non-toxic, sustainable and stable to photobleaching as compared with colors in dyes and pigments.

[0078] Various embodiments may be used for prototyping designs due to the achievable colors. Moreover, various embodiments may be used in anti-counterfeiting security labels due to that microscale objects and color features observed under the microscope are not visible using naked eyes. With the benefit of the capability of 3D printing full color devices in the area scale of below 10 pm, various embodiments may be applicable to make miniature pressure sensors on the basis that pressure-induced changes in lattice constants of photonic crystals make the color changes. These miniature sensors may be printed on chips to inspect actuated responses from fine areas of micro-meter sizes. Photonic crystal devices may be directly printed on end of optical fibers acting as filters to precisely control the wavelengths and polarizations of outgoing light.

[0079] Non-homogeneous shrinking at the bottom of the photonic structures may be solved by adding a sacrificial layer or a pedestal beneath the photonic structures to be printed.

[0080] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.