CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority from and benefit of the US Provisional Patent Application No. 63/345,604 filed on May 25, 2022, the disclosure of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT [0002] This invention was made with government support under grant number N68335-21-C-0114 awarded by tire Office of Naval Research. Hie U.S. government has certain rights in tire invention.

TECHNICAL FIELD

[0003] The present invention relates to photoresin-type materials that possess high indices of refraction and that that lend themselves to a 3D-printing of optical components, and to methodologies for producing such photoresin-type materials specifically configured for use in a two-photon lithography processing.

RELATED ART

[0004] Two photon lithography (TPL) is a valuable tool for rapid prototyping of micro-optics and metamaterials via additive manufacturing. (For example, related art demonstrated commercial systems with voxel resolution under 200 run; see nanoscribe.com). Such technology has been used to produce microoptics for various applications including microscopy, endoscopy, metamatenals, and sensors. The nature of 3D printing allows for a wide range of spatial profiles of optical components being manufactured (in the specific example of lenses - various lens profiles, from spherical to those classified as free-form optics) that can be difficult to produce otherwise (that is, using traditional micro-lens fabrication techniques involving 2D lithography and resist reflow). While the process of fabrication of various structures with the use of TPL may be slower than that utilizing conventional 2D lithography, the ability to realize mass production with the use of the TPL-printed structure as a master for nano-imprint lithography presents substantial practical appeal.

[0005] It is recognized that commercial production of resins for TPL that have a refractive index value greater than 1.65 or even 1.55 remains quite problematic - tire indices of most available materials almost universally fall somewhere between 1.45 and 1.55. (Commercial examples of such available TPL photoresins include IP-S, IP-Dip, and IP-nl62 from the IP line of resins from Nanoscribe GmbH.) While related art attempted to increase refractive index of these resins, the only results to date were those demonstrating refractive index increases - with respect to the already known, existing values - of up to and not exceeding about 0.1.

SUMMARY

[0006] Embodiments of the present invention provide a new methodology of production of a photoresin possessing tunable refractive index that can be used in a 3D printing that employs the two-photon lithographic processing of optical components or elements.

[0007] 3D printing (or additive manufacturing) is generally known in related art as the action or process of making a physical object from a three-dimensional digital model, which typically includes laying down many thin layers of a material in succession to realize the construction of a three-dimensional object from a CAD model or a digital 3D model. The term 3D printing is also used to refer to a variety of processes in which material is deposited, joined or solidified under computer control to create a three- dimensional object, with material(s) being added together (such as plastics, liquids or powder grains being fused together), typically layer by layer. Embodiments of the invention provide a method that includes preparing a metal oxide organic compound, and doping a photorcsin with metal oxide particles of tire metal oxide organic compound to form a mixture in which said metal oxide particles are contained in a predetermined concentration, which is substantially optically transparent to light at a first wavelength while being substantially photopolymerizable when exposed to light at a second wavelength (the second wavelength being half of the first wavelength), and which is substantially devoid of agglomeration of said metal oxide particles. In one case, the step of doping includes mixing a said metal oxide particles w ith the photoresin and/or a combination of an epoxide material (or an epoxide-containing material) and metal oxide particles with the photoresin. Additionally or in the alternative - and substantially in every implementation of the metho - such mixture may have a refractive index equal to or exceeding 1.55, and/or equal to or exceeding 1.57, and/or equal to or exceeding 1.6, and/or equal to or exceeding 1.63, and/or equal to or exceeding 1.65, and/or equal to or exceeding 1.7, and/or equal to or exceeding 1.75, and/or equal to or exceeding 1.8, and/or equal two or exceeding 1.82 at at least one wavelength in a visible portion of optical spectrum. Additionally or in the alternative, and substantially in every implementation, the method may include a step of varying viscosity of the mixture by adding a solvent to said mixture and/or a step of removing a solvent from the mixture by evaporation while keeping an unreacted portion of an epoxide in the mixture and/or a process of forming the mixture in which a spatial distribution of the metal oxide particles throughout the mixture is necessarily substantially uniform and/or a step of varying the predetermined concentration of the metal oxide particles in the mixture to tune a refractive index distribution of said mixture. Alternative ly or in addition, an implementation of tire method may include a step of constructing a three-dimensional object by at least in part depositing and/or joining and/or solidifying, under control of a programmable electronic circuitry' and from a three-dimensional model, multiple material layers of the photoresin material formed according to an embodiment of the method, while (or in addition) polymerizing at least a portion of such multiple material layers via a process of two-photon absorption of said light at the second wavelength. (In at least one implementation, such three-dimensional object is not an optical thin-film; and/or such three-dimensional object includes an optical refractive component; and/or such three-dimensional object includes an optical diffractive component.

[0008] Embodiments of the invention additionally provide a composite photoresin material produced according to an implementation of the method of the invention and/or an optically-transparent film or a three- dimensional object that comprises a material of a mixture (which includes a photoresin doped with a portion of a metal oxide organic compound such that a distribution of metal oxide particles of said compound in said mixture is substantially spatially uniform, which mixture is substantially optically transparent to light at a first wavelength and substantially photopolymerizable when exposed to light at a second wavelength that is half of the first wavelength, and which mixture is substantially devoid of agglomeration of the metal oxide particles)

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by referring to the following Detailed Description of Specific Embodiments in conjunction with the not-to scale Drawings, of which:

[0009] FIGs. 1A, IB schematically illustrate a process of one-photon absorption and a process of two-photon absorption, respectively.

[0010] FIG. 2: Micro-lens to be printed on a 170 micron thick microscope coverslip designed in Zemax using custom resin based on measured dispersion.

[0011] FIG. 3: Focal length of micro-lenses, fabricated from the photoresin material configured according to an embodiment of the invention, was measured using collimated light from a helium-neon laser with a camera mounted on a translation stage.

[0012] FIG. 4A illustrates respectively, a synthesized nanocomposite solution after removing solvent (ethanol, in this example), a thin film produced from such solution with spin coating, and, in inset, a plot of optical transmittance of such film within the identified region of wavelengths. [0013] FIG. 4B is a plot illustrating refracting index of the material produced according to the idea of the invention as a function of a volume fraction of a nanocomposite dopant present.

[0014] FIGs. 5A, 5B, 5C, 5D illustrate results of TPL 3D printing from a CAD model (FIG. 5A), and optical images (procured with the use of a microscope) of a structure printed with the use of an embodiment of a photoresin mixture configured according to an embodiment of the invention: image with focusing on the bottom of the structure (FIG. 5B), in the middle of the structure (FIG. 5C), and at the top of the structure (FIG. 5D).

[0015] FIG. 6A shows plots of refractive indices for undoped IP-Dip, a GLYME/PO/TiCE nanocomposite, and a GLYME/PO/TiO2/IP-Dip composite. FIG. 6B additionally illustrates dispersion of refractive indices for IP-Dip with various concentrations of the dopant represented by the GLYME/PO/TiCE nanocomposite and identifies the focal lengths of lenslets (such as that discussed in reference to FIG. 2) fabricated from a corresponding version of the doped IP-Dip. FIG. 6C illustrates the dependence of the refractive index of the doped IP-Dip on the volume fraction (doping concentration) of the GLYME/PO/TiO2 nanocomposite.

[0016] FIGs. 7A, 7B: Measured focal length for a micro-lens (FIG. 7A) and ray tracing tolerance of refractive index at design wavelength (FIG. 7B).

[0017] FIGs. 8A, 8B, and 8C illustrate three-dimensional structures printed with the use of a composite resin (photoresin mixure, formed according to the idea of the invention). FIG. 5D illustrates, on a larger scale, a portion of the upper surface of the micro-lens of FIG. 8C showing the relief structure on the lens surface, thereby demonstrating the spatial resolution capabilities of TPL 3D printing process when an embodiment of the photoresin mixture configured according to the idea of the invention is used.

[0018] Generally, the sizes and relative scales of elements in Drawings may be set to be different from actual ones to appropriately facilitate simplicity, clarity, and understanding of the Drawings. For the same reason, not all elements present in one Drawing may necessarily be shown in another.

DETAILED DESCRIPTION

[0019] In accordance with embodiments of the present invention, methods and apparatus are disclosed for forming photoresin-based materials that possess high refractive indices and the properties of which materials lend such materials to 3D printing with the use of two-photon lithography.

[0020] The inability of related art to form a material having a tunable or variable (in the process of fabrication of such material) refractive index that exceeds or substantially exceeds 1.55 (at wavelengths of light within the visible and near infrared portion of the optical spectrum) while, at the same time, lending itself to 3D printing of bulk (as opposed to and in comparison with thin-film) optical elements that operate substantially without scattering light (that is, which are substantially free from scattering light) propagating therethrough is solved by fabricating a composite photoresin material containing functionalized particles of metal oxide substantially spatially-uniformly intermixed with a photoresin (and, in a specific case - a two- photon lithography photoresin) and characterized by refractive index the value of which can be readily tuned or varied depending on concentration of such functionalized particles in the photoresin. (As is well known in related art, surface functionalization introduces chemical functional groups to a surface. This way, materials with functional groups on their surfaces can be designed from substrates with standard bulk material properties. Prominent examples can be found in semiconductor industry' and biomaterial research.) [0021] Based on empirical investigations underlying this disclosure it was discovered that the ability to vary (modify, alter, change) a refractive index of a photoresin-based material suitable for TPL-based fabrication processes by intermixing a photoresin with chosen material particles is not ensured - let alone can be made repeatable - but, w hen possible, critically depends on spatial uniformity of such resulting photoresinbased mixed material. In particular, when creating a nanocomposite (or mixture) of material nanoparticles and a polymeric material it is important to arrive a uniform spatial distribution of nanoparticles across the body or volume of such polymeric material. During such empirical investigations it was clearly demonstrated that if and when chosen material nanoparticles agglomerate within the polymer, the resulting composite or mixture would end up scattering light (w hich was easily observed even visually) thereby rendering the resulting mixture less optically transparent.

[0022] For example, it was demonstrated that regular titania nanoparticlcs were not miscible in a liquid monomer (such as MMA) at loading concentrations of 1 wt% or higher. If one managed to dope MMA with titania nanoparticles at lower levels of concentration, the titania would agglomerate w hen the host material is polymerized. This agglomeration is changing the size of the titania dopant elements from that of a nanoparticle (say, smaller than 10 run) to hundreds or even thousands of nanometers. At this large size the dopant elements are worsening the transmission properties of the material by inducing scattering.

[0023] It was unexpectedly discovered that in order to achieve a good dispersion of nanoparticles throughout the host polymer, particularly at higher spatial concentrations, it was necessary to intentionally functionalize the nanoparticles to purposefully improve miscibility' of the resulting mixture. As discussed below, the required functionalization is achieved by using an epoxide, in one instance.

[0024] According to the idea of the present invention, sol-gel methodology is employed to simultaneously synthesize and functionalize metal oxide nanoparticles that are intended to be further used as dopants for commercially available TPL resins (host materials) to produce mixtures with spatially-uniformly distributed dopants. By vaiying the doping concentration, the refractive index of the resulting material is then varied or modulated, as discussed with the use of practical examples below.

[0025] The terms "to scatter light", "scattered light", and related terms as defined and used in for the purposes of this disclosure, may better be understood as that pertaining to optical haze (which is usually defined in related art as the percentage of light scattered at an angle exceeding a specified angle, the latter being typically defined as 2.5 degrees). For example, optical haze may be defined in current context as a ratio of diffuse transmittance to total transmittance, where the diffuse transmittance is defined by light transmitted through a given optical component and deviated, upon such transmission, outside of the predetermined spatial cone (the one defined by an angle of, for example, 2.5 degrees) that is centered on an axis of incidence of such light onto the component. The term "agglomeration of particles" and related terms describe the presence of particle matter, dispersed in the host material, the size of which exceeds a predetermined value (in one non-limiting example - 10 am) as determined with the use of dynamic light scattering (DLS) methodology (described, for example, in en.wikipedia.org/wiki/Dynamic_light_scattering, the disclosure of which is incorporated herein by reference).

[0026] While the specific examples presented below are those utilizing nanoparticles of titania (TiCh), it is appreciated that other materials - for example, functionalized particles of other materials known in related art generally as hard oxides - can be used for the same purpose (such as at least some of metal oxides, for example, ZrCK C1O2, ABO,, to name just a few).

[0027] As a skilled artisan will readily appreciate, TPL turns on the principle of two-photon absorption (TPA, which is a third-order nonlinear optical phenomenon) and is a microfabrication technique exploiting the nonlinear dependency - of the polymerization rate of a given material - on the intensity of light irradiating such material to produce true three-dimensional structures with feature sizes smaller than (below) the diffraction limit. Here, a photosensitive target material is exposed to photons with energy that is lower than that required for excitation, and thus polymerization can only occur if two photons are simultaneously absorbed: this mechanism is schematically illustrated in FIG. IB (FIG. 1A is provided for comparison purposes and illustrates tire one-photon absorption process). Probability of absorption of light by the photosensitive target material for TPA is proportional to the square of the light intensity and thus requires high intensity values to achieve significant polymerization of the material. By tightly confining the exposure (irradiating) light to a small volume using a microscope objective, for example, polymerization of the target material can be localized to a small voxel at the focal region of the spatial light distribution. Notably, for a resin material to be compatible with TPA such resin must be transparent at the wavelength of the irradiating light in order to be able to be focused through (in the body of) the material, and it must be photopolymerizable at half the wavelength of the irradiating light.

[0028] According to the idea of the invention, a resin-based composite material suitable for a TPL process is formed, which material includes two separate constituent materials with different refractive indices and that, as a result, has an effective refractive index with some intermediate value. A linear change in the effective refractive index of the composite material based on the volume concentrations of the two constituent materials can serve as a first order approximation for the effective refractive index n _e ^ of such composite material. More sophisticated approximations - such as that expressed by the Lorentz-Lorenz Eq. (1) - can be used to estimate the effective refractive index as:

Here, n _x and n ₂ are tire individual indices of tire constituent materials and f is tire volume fraction of tire inclusions of one material into another.

Embodiments of Preparation of a Resin -based TPL Material

[0029] In at least one implementation, synthesis and functionalization of nanoparticles (interchangeably referred to herein as a nanocomposite dopant) was done according to the methodology described by Himmelhuber et al. (see US 8,940,807 and/or Optical Materials Express, 1 June 2011, vol. 1, No. 2, pp 252-258, the disclosure of each of which is incorporated herein by reference). In one case, for example, titanium tetrachloride (TiCL) was added dropwise into the solvent (in one non-limiting case - ethanol) under magnetic stirring inside a fume hood. After the solution cooled to room temperature, excess of water (0.3 M) was added. Following the addition of water, an epoxide material was added. In one specific implementation, a combination of Glycidyl Methacrylate (GLYME; an ester of methacry lic acid and glycidol that contains both an epoxide and an acrylate groups) and Propylene Oxide (PO) was used for this purpose. (Depending on the implementation, the ratio of the two epoxides could be varied to tune the miscibility of the nanocomposite dopant in resin. The reaction with the use of GLYME produced the target hard oxide nanoparticles and coated / functionalized them at the same time. While the highest index was obtained in experiments utilizing only PO, it was unexpectedly discovered that miscibility of tire nanocomposite dopant in resin was significantly better when PO was complemented with some amount of GLYME. In particular, the about 95/5 ratio of PO/GLYME resulted in the operationally reliable miscibility. The total molarity of epoxide in the solvent was preferably kept as 1 ,3M in order to maintain concentration as high as possible before precipitation or aggregation.)

[0030] After synthesis, the solution was aged at room temperature for 5-7 days before processing. To dope the resin with inclusions of the high refractive index metal oxide particles, the contents of solvent, excess water, and reaction byproducts were reduced (and optionally, removed) by evaporation. (In at least one specific implementation, the epoxide was not fully removed before combining the nanocomposite with the commercial TPL resin of choice. Some of the epoxides reacted and attached to the surface of the metal oxide nanoparticles and the unreacted epoxides were left in the solution. Most of the reaction byproducts had boiling points below 150 deg C and were removed during evaporation of the solvent.)

[0031] During evaporation at 150°C, the solution viscosity was observed to increase dramatically.

An acrylate or methacrylate resin designed for two-photon polymerization (2PP) was added towards the end of the evaporation process. This method was tested using IP-Dip (acrylate base) and IP-S (methacrylate base) resins from Nanoscribe GmbH.

[0032] When and if the solution became overly viscous (as compared to the value of viscosity of the initial resin being modified in every particular case), a small amount of propylene glycol methy l ether acetate (PGMEA) could be added to the generated resin-based material to maintain such a viscosity of this generated material that is suitable for the TPL process. The skilled person will readily appreciate that viscosity of the resulting resin-based composite material (also referred to herein as "mixture") is important at least for the following reasons: when printing in the dip in laser lithography (DiLL) configuration, the microscope objective needs to be able to move freely through tire resin-based material without damaging tire already- printed portion of the structure; if viscosity is too low, the maximum height of the printed structure is likely to be limited (i.e. the resin-based material used for printing will effectively spread over the substrate, reducing the vertical extent thereof above the substrate); the viscosity' is a factor for the amount of oxygen diffusion during the printing process, which can affect the polymerization of the resin-based material and therefore the resolution of the printing process. As preliminarily estimated - and based on the values of viscosities of the IP-Series resins available from nanoscribe.com - viscosity of tire material prepared according to tire idea of the invention with a value in the range between about 2000 and about 14000 mPas (at 20 degrees C) was found to be acceptable for the purposes of 3D printing, as intended. Testing and Characterization.

[0033] 3D printing using two-photon polymerization (2PP) was tested using a Nanoscribe Professional GT lithography system at the exposure wavelength of about 780 nm. While testing new materials, the system was used in an oil immersion configuration, in which the resin-based material configured (or - alternatively - produced, formed, or generated) according to the idea or embodiment of the invention (that is, the resulting material or the synthesized material) was applied to a microscope cover slide and the microscope objective was then immersed in an index matching oil on the opposite side of the slide. Exposure to light was carried out through the cover slip.

[0034] Testing of the dosage of illumination was perfonned by adjusting the scanning speed and tire laser power. For a 20x microscope objective, a suitable dosage of 0.375 mJ/mm was found to reliably polymerize the resin-based material configured according to the embodiment of the invention and could be post-processed to remove the resin that was not exposed following the standard developing procedure for IP resins (for example, as follows: 30 min soak in PGMEA followed by IPA rinse and blow dry with air).

[0035] The refractive index of the configured resin-based material was measured using a Meticon 2010 prism coupler with light at five wavelengths. The sample was prepared by printing a 5mm x 5mm x lOum thin film with the Nanoscribe printer. Due to the limited field of view of the Nanoscribe system, the film was printed by stitching multiple sections sized about 300 um x 300 um. While this way of printing could result in somewhat excessive exposure where the stitching windows overlapped and the boundaries could arguably cause scattering and losses during prism coupling, clear modes were nonetheless easily discernable with tire used instrument. To compare tire results obtained with 2PP and those obtained with one photon polymerization (OPP), a second thin film was fabricated using a spin coater and UV flood exposure. The resin was diluted with ethanol to achieve thin films for spin coating.

Micro-optic design and fabrication.

[0036] Notably, as would be understood by a skilled artisan having the advantage of this disclosure, in advantageous contradistinction with related art the synthesized photoresin material (interchangeably referred to herein as a resin-based material or a composite resin material or denoted with a similar term) lends itself to fabrication of three-dimensional objects that are different from a thin film. (As recognized and accepted in related art, an optical thin film is defined as a thin film layer of material ranging in thickness from nanometers to a few micrometers in thickness and generating interference of visible light upon reflection and/or transmission of such light through such film.) [0037] To demonstrate the application of the photoresin material synthesized according to the idea of the invention for 3D printing of three-dimensional objects such as micro-optical component, a micro-lens - specifically, a convcx-plano lens - was designed and printed with its planar side on a coverslip, as shown in FIG. 2. Freedom of variation of a shape of a surface, allowed by a 3D-printing process, was used to define the convex surface of the lens to be a rotationally symmetric aspheric surface defined as

[0038] Here, C is the surface curvature, k is the conic constant, a^and a ₂ are polynomial expansion coefficients. The curvature, conic constant, and aspheric coefficients were optimized (to values summarized in Table 1) with the use of OpticStudio Zemax optical design software to minimize the spot size of light distribution transmitted through the lens.

[0039]

Table 1 : Optimized parameters of a convex of an embodiment of the micro-lens

[0040] The micro-lens was designed using the measured refractive index and dispersion of the custom resin. Optimization was performed for a single wavelength of 633 nm. The focal length was measured using a collimated helium-neon laser beam and a CCD camera array with a microscope objective as schematically shown in FIG. 3.

[0041] The camera was initially focused on a surface of the coverslip; images were taken of the focusing beam as the camera was translated away from the coverslip. The relative beam width was determined by taking cross sections of the beam in the images at each z distance and finding the Full-Width- Half-Maximum (FWHM) value. The beam waist and the spot size were determined by fitting the FWHM values to the well-known expression for a Gaussian beam width. Results and Discussion.

[0042] The synthesized nanoparticle solution 404 (the image of which in a bottle is presented on the left side of FIG. 4A) had a reddish-amber hue after evaporation of the solvent (ethanol, in one case) . In one case, the loading of about 0.5 volume fraction of nanoparticles in the solution was demonstrated, as shown in FIG. 4B. The diameters of the suspended in this solution nanoparticles of the metal oxide were measured using Dynamic Light Scattering (DLS) to be smaller than 10 nm. With nanoparticles this small the solution was determined to be highly optically transparent (depending on the particular implementation: in one embodiment, with transmission exceeding 70% across the visible spectrum, in a related case - with transmission exceeding 75 %, and in a preferred case - with transmission exceeding 80%) and did not show any scattering (optical hazing) registrable at a level not exceeding 12%, preferably not exceeding 10%, in a related implementation - at a level not exceeding 5%, and/or substantially in every implementation not visually perceivable. It was evidenced, therefore, that adding the nanocomposite solution to a TPL photoresin resulted in a resin-based material with a reddish hue and little-to-no noticeable scattering FIG. 4A additionally illustrates an embodiment of a material film 410 formed from the solution 404 according to the idea of the invention, as well as the transmittance characteristic of such film.

[0043] Accordingly, a practically-demonstrated embodiment of the invention included a method for fabricating a composite photoresin material, which method contains a step of preparing a metal oxide organic compound, and a step of doping a chosen photoresin with metal oxide particles that have been derived from the metal oxide organic compound to form a mixture in which such metal oxide particles were contained in a necessarily substantially spatially-uniform fashion in a prc-dctcrmincd concentration. Here, necessarily, not only the mixture was substantially optically transparent (generally, in excess of 80%) to light at a first wavelength and substantially photopolymerizable (with the use of commercial equipment such as a 2-photon printer Nanoscribe Photonic Professional GT2, the description of which is available at nanoscribe.com/en/products/photonic-professional-gt2/, which description is incorporated herein by reference) and without the use of an additional, dedicated light source such as a laser) when exposed to light at a second wavelength (the second wavelength being half of the first wavelength, but also such mixture was substantially devoid of agglomeration of the metal oxide particles. (In these experiments and for the purposes of characterization and identification of the mixture, being devoid of agglomeration of the metal oxide particles was determined based on and corresponded to the situation of lack of visually perceived, observable scattering of light and/or lack of scattering of light at a level exceeding 10%, or exceeding 5%, or even exceeding 2%, as would be understood by a skilled artisan. The term substantially polymerizable refers to a resin containing material that forms, as a result of a 2PP process, a solid structure that is not soluble during the development of the resin, where the development process consists of soaking the sample of material for 15-30 minutes in PGMEA, followed by rinsing in an isopropyl alcohol, IP A, bath for about 1 minute to remove the remaining PGMEA.)

[0044] The applicability of such resulting resin-based material to 3D printing using TPL and developing the object was her tested on complex structures like the ones presented in FIGs. 5 A through 5D and FIGs. 8A through 8D. (The surface profile of the micro-lens the image of which is shown in FIG. 8D was not necessarily generated or formed to be ideal: due to the printing resolution and the vertical stitching distance of the printer, the surface of the fabricated lens had a somewhat "stepped" profile. The actual surface profile was measured using a FilmMctric FilmPro3D optical profilometer.)

[0045] While GLYME itself is known to be able to be photopolymerized with the addition of a photoinitiator, the GLYME- functionalized metal oxide (in one case - l i O2 ) nanocomposite was proven to possess the same ability. However, as is well known and recognized in related art, not only the GLYME- functionalized TiO ₂ nanocomposite has limited practical usefulness in that it allows to only to create very thin films (with thicknesses smaller than about 200 nm) for traditional 2D photolithography using one-photon polymerization (see Optical Materials Express, 1 June 2011, vol. 1, No. 2, pp 252-258), but such material was never even been applied to TPL for 3D printing. While it may, arguably, be possible to apply TPL directly to this material by itself, the results of current research and development demonstrated that TPL with the use of such material had poor printing performance and substantial difficulties developing the structures after printing (specifically, it was found substantially unrealizable to reliably develop a printed structure without also dissolving it).

[0046] In advantageous contradistinction, however, by using the produced composite as a dopant for the commercially available IP resins according to the idea of the invention, a much more reliable printing material was achieved. (Used were IP -Dip, IP-S, and IP-nl62 which are all produced by Nanoscribe, to name just a few; these specific resins are all either acrylate or methacrylate functional groups, and are transparent at 780 nm and photoactive polymerizable at 390 nm, which were the operational wavelengths used in discussed experiments). Adding the IP resin does dilute tire original nanocomposite and lowers tire maximum achievable refractive index. FIGs. 6A, 6B illustrates measured RI for doped and undoped IP-Dip resin. Here, plots of FIG. 6B additionally indicate doping concentrations of the same GLYME/PO/T1O2 nanocomposite.

[0047] As evidenced by the data of FIGs. 6A, 6B, the refractive index of the undoped IP-Dip was increased by 0.13 from 1.55 up to 1.66 (as measured at 633 nm) by adding the nanocomposite dopant. The increase in refractive index based on the volume fraction of the dopant is in good agreement with Eq. (1). see FIG. 6C.

[0048] The focal length of the micro-lens from FIG. 2 had an expected back focal distance (BFD) of

810 um according to the ray tracing model designed for an operating wavelength of 633 nm (which was the same as that of the laser used for testing of the micro-lens) . The refractive index of the material used for the design was based off of the measured refractive index of the resin-based material configured according to the idea of the invention, shown in FIGs. 6A, 6B. The measured beam width as a function of distance from the coverslip is presented in FIG. 7A.

[0049] Hie measured back focal distance (BFD) at the beam waist was 800.1 microns. Uris corresponds to a deviation of 1.2% from the value predicted by the ray tracing. Assuming the surface profile of the printed lens is ideal and matches the design, the refractive index of the lens can be determined by relating the BFD and the refractive index using the ray tracing model as illustrated in FIG. 7B. This method predicts the refractive index of this resin to be 1.668 at 633 nm. From the prism coupler measurements, this value was measured to be 1.6602. The difference between the refractive index as determined by the two methods is 0.0078, corresponding to 0.47% of either refractive index. This error can also be attributed to the surface profile of the micro-lens which was not ideal. Due to the printing resolution and the vertical stitching distance of the printer, the fabricated lens had a stepped profile as evident from the image of the surface of the lens in FIG. 8D.

[0050] Overall, demonstrated was the use of a sol-gel synthesized metal oxide nanocomposite as a dopant for TPL photoresins to raise and tune tire refractive index in 3D printed optical components, hr one specific case, the initial value of the refractive index of the IP-Dip resin was increased by 0.13 from 1.53 to 1.66 at 633 nm by doping such resin, as described, with 52 vol% of the nanocomposite consisting of TiO2, glycidyl methacrylate, and propylene glycol. Even higher indices were achievable by starting with a higher index base resin. The scope of the invention includes additional increase of the index of the nanocomposite by reducing the volume of the epoxides used, provided the molarity of the epoxides remains high enough (in one non-limiting case - of about 1.3 M) to mediate nanoparticle growth. Tuning tire proportions of tire PO and GLYME while maintaining the overall epoxide molarity can also be used to raise the index of the nanocomposite but may affect the miscibility of the nanoparticles with the TPL resin. This non-miscibility could increase scattering of the material, and lead to poor performance during 3D printing. The resulting resins are promising for several applications in optics and photonics. Higher refractive indices can lead to thinner optical elements or higher index contrast for better confinement in photonic applications. [0051] References throughout this specification to "one embodiment," "an embodiment," "a related embodiment," or similar language mean that a particular feature, structure, or characteristic described in connection with the referred to "embodiment" is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment^ "in an embodiment^ and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. It is to be understood that no portion of disclosure, taken on its own and in possible connection with a figure, is intended to provide a complete description of all features of the invention.

[0052] Within this specification, embodiments have been described in a way that enables a clear and concise specification to bet written, but it is intended and will be appreciated that embodiments may be variously combined or separated without parting from the scope of the invention. In particular, it will be appreciated that all features described herein at applicable to all aspects of the invention.

[0053] For the purposes of this disclosure and the appended claims, the use of the terms "substantially", "approximately", "about" and similar terms in reference to a descriptor of a value, element, property or characteristic at hand is intended to emphasize that the value, element, property, or characteristic referred to, while not necessarily being exactly as stated, would nevertheless be considered, for practical purposes, as stated by a person of skill in the art. These terms, as applied to a specified characteristic or quality descriptor means "mostly", "mainly", "considerably", "by and large", "essentially", "to great or significant extent", "largely but not necessarily wholly the same" such as to reasonably denote language of approximation and describe the specified characteristic or descriptor so that its scope would be understood by a person of ordinary skill in the art. In one specific case, the terms "approximately", "substantially", and "about", when used in reference to a numerical value, represent a range of plus or minus 20% with respect to the specified value, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2% with respect to the specified value. As a non-limiting example, two values being "substantially equal" to one another implies that the difference between the two values may be within the range of +/- 20% of the value itself, preferably within the +/- 10% range of the value itself, more preferably within the range of +/- 5% of the value itself, and even more preferably within the range of +/- 2% or less of the value itself.

[0054] The use of these terms in describing a chosen characteristic or concept neither implies nor provides any basis for indefiniteness and for adding a numerical limitation to the specified characteristic or descriptor. As understood by a skilled artisan, the practical deviation of the exact value or characteristic of such value, element, or property from that stated falls and may vary within a numerical range defined by an experimental measurement error that is typical when using a measurement method accepted in tire art for such purposes. [0055] While the invention is described through the above-described exemplary embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. For example, and while not necessarily discussed in detail in the above disclosure, a specific embodiment of the optical imaging system of the overall lens-free optical system of the invention may be configured such that the optical detector is disposed to face the mask layer directly (that is, without any tangible component or element therebetween); in a related embodiment, however, the optical spectral filter may be utilized therebetween if certain degree of spectral discrimination is required during the image acquisition. Similarly, there may be optionally employed a non-lcns optical component between the laser source of the embodiment of the overall optical system and the optical imaging system such as, for example, an optical reflector.

[0056] The term “and/or”, as used in connection with a recitation involving an element A and an element B, covers embodiments having element A alone, element B alone, or elements A and B taken together.

[0057] While embodiments of the invention were not necessarily described as including and/or employing a processor (such as programmable electronic circuitry) controlled by instructions stored in a memory, a person of skill appreciates that the operation of the optical system and/or collection and processing of imaging information may be, and preferably is indeed governed by such a processor. The memory may be random access memory (RAM), read-only memory (ROM), flash memory or any other memory, or combination thereof, suitable for storing control software or other instructions and data. Those skilled in the art should also readily appreciate that instructions or programs defining the functions of the present invention may be delivered to a processor in many fonns, including, but not limited to, information permanently stored on non- writable storage media (e.g. read-only memory devices within a computer, such as ROM, or devices readable by a computer I/O attachment, such as CD-ROM or DVD disks), information alterably stored on writable storage media (e.g. floppy disks, removable flash memory and hard drives) or information conveyed to a computer through communication media, including wired or wireless computer networks. In addition, while the invention may be embodied in software, the functions necessary to implement the invention may optionally or alternatively be embodied in part or in whole using firmware and/or hardware components, such as combinatorial logic, Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other hardware or some combination of hardware, software and/or firmware components.

[0058] Disclosed aspects, or portions of these aspects, may be combined in ways not listed above. Accordingly, the invention should not be viewed as being limited to the disclosed embodiment(s).