Related Application

[0001 A] The present application claims the benefit of the earlier filing date of Australian Provisional Patent Application No. 2014900293 in the name of The Australian National University, filed on 31 January 2014, the content of which is incorporated herein by reference in its entirety.

Technical Field

[0001 ] The present invention relates generally to the fabrication of lenses and, in particular, to moldless fabrication of a lens using gravitational force.

Background

[0002] Existing methods for fabricating lenses (e.g., soft lithography, chemical processing, etc.) often involve multiple steps, e.g., high temperature injection molding into polymer, lapping for glass, etc. Such lens fabrication techniques potentially waste significant amounts of raw materials through excessive use of the raw materials, chemical reactions, etc. Existing techniques also rely on molds to shape the lens - causing defects in the fabricated lens due to imperfection in the molds and not allowing alteration of the lens shape during manufacturing. Such techniques only allow lenses of a certain focal length to be produced.

[0003] Thus, a need exists for a method of fabricating lenses that reduces or eliminates waste of raw materials and/or use of molds.

Summary

[0004] Disclosed is a lens fabrication technique which seeks to address the above problems. The lens fabrication technique uses a droplet of polydimethylsiloxane (PDMS) solution cured on a horizontal slide to form a PDMS support layer having a curved surface. A further PDMS droplet is then deposited on the curved surface of the PDMS support layer; the slide is then inverted to allow gravitational force to pull the uncured, further PDMS droplet down. The further, inverted PDMS droplet is then cured. Each repetition of depositing, slide-inverting, and curing of the further PDMS droplet adds an additional layer of PDMS, altering the shape and focal- length of the lens. [0005] According to a first aspect of the present disclosure, there is provided a method of fabricating a lens using gravity, the method comprising: forming a polydimethylsiloxane (PDMS) support layer on a slide using a needle, the PDMS support layer having a curved surface; depositing further PDMS using the needle onto the curved surface of the PDMS support layer on the slide; inverting the slide; and curing the PDMS on the inverted slide.

[0006] Other aspects of the invention are also disclosed.

Brief Description of the Drawings

[0007] At least one embodiment of the present invention is described with reference to the drawings, in which:

[0008] Fig. 1 is a flow diagram illustrating a method of the gravity-assisted additive lens- fabrication in accordance with an embodiment of the invention;

[0009] Figs. 2A to 2F show block diagrams illustrating the method of Fig. 1 ;

[0010] Figs. 3A to 3D are block diagrams showing an experimental setup for testing four lenses fabricated using the method of Fig. 1 and the performance of those lenses; and

[001 1 ] Figs. 4A to 4C are a block diagram and plots showing the collimation function of a lens fabricated using the method of Fig. 1.

Detailed Description

[0012] Where reference is made in any one or more of the accompanying drawings to steps and/or features, which have the same reference numerals, those steps and/or features have for the purposes of this description the same function(s) or operation(s), unless the contrary intention appears.

[0013] Disclosed is an embodiment of the invention providing a moldless lens fabrication method combining layering and gravity, which efficiently utilizes raw material with little wastage. The disclosed lens fabrication method is also capable of controlling the shape of the lens during manufacturing to produce lenses of varying focal length.

[0014] Fig. 1 shows a flow diagram for a method 100 for gravity-assisted additive lens fabrication, whilst Figs. 2A to 2F provide illustrations of each step of the method 100. The method 100 commences with steps 1 10 to 130 forming a polydimethylsiloxane (PDMS) support layer 21 1 on a slide 214 (shown in Fig. 2A). For example, the slide 214 may be a polished flat glass slide, such as a coverslip.

[0015] In step 1 10, a quantity of PDMS 210 is extracted. A needle 212 (shown in Fig. 2A) extracts a small quantity of the PDMS solution 210 (i.e., about 100 +/- 20 μΙ) by immersing the tip of the needle 212 into the PDMS solution 210. The tip of the needle 212 is typically fine (e.g., about 18 -21 gauge thickness) and is arranged perpendicular to the slide 214 before and after immersion into the PDMS solution.

[0016] PDMS solution 210 is highly viscous, allowing a finite quantity of PDMS solution 210 to easily adhere to the needle tip. In this method, the thickness of the needle tip determines the surface area with which the PDMS solution 210 comes in contact when the needle is immersed, which in turn determines the amount of PDMS solution 210 being extracted. Hence, the size of the needle tip determines the extracted amount of the PDMS solution 210.

[0017] The PDMS solution 210 is created by mixing a PDMS base with a curing agent in a typically 10:1 ratio, as measured by weight. The mixing of the PDMS solution 210 is typically performed by using a Q-tip or other mixing devices. The mixed PDMS solution 210 is allowed to rest, removing residue bubbles during stirring, before the needle 212 is immersed into the PDMS solution 210. Step 1 10 then proceeds to step 120.

[0018] In step 120, the extracted PDMS is deposited onto a slide. The needle 212 with the extracted PDMS solution 210 is held above the slide 214 to allow gravity to pull the PDMS solution 210 until a droplet of the PDMS solution 210 is deposited onto the slide 214. The slide 214 is arranged to be parallel relative to the ground during the depositing of the PDMS droplet 210 onto the slide 214, preventing the deposited PDMS droplet 210 from sliding on the slide 214. That is, the slide 214 is arranged substantially horizontal during the depositing process. The slide 214 is made of materials having a surface that is chemically inert and has low surface roughness, such as glass.

[0019] Fig. 2A shows illustrations of the PDMS solution 210 being deposited on the slide 214. Fig. 2A in 5 steps (1 ) - (5) shows the PDMS solution 210 dropping from the tip of the needle 212 and settling on the slide 214. Fig. 2B shows, in 3 steps (1 ) - (3), settling of the deposited PDMS droplet 21 1 on the horizontal slide 214. Image 280 of Fig. 2A shows photographic images of the PDMS droplet 210 being deposited on the slide 214 and settling on the slide 214. Step 120 proceeds to step 130. [0020] In step 130, the deposited PDMS 21 1 is cured. The deposited PDMS 21 1 on the horizontal slide 214 is placed in an oven at a predetermined temperature for a period of time. For example, the oven is typically set at a temperature of 70°C for a period of 15 minutes to cure the PDMS 211 . However, other appropriate temperatures and period of times can be used to cure the PDMS 21 1 . The cured PDMS 21 1 serves as a support layer for subsequent PDMS layers. Step 130 then proceeds to step 140.

[0021 ] In step 140, further PDMS 210 is deposited onto the cured PDMS. Further PDMS droplet 210 is deposited onto the curved surface 21 1 a of the PDMS support layer 211 - shown in Fig. 2C. Step 140 then proceeds to step 150.

[0022] In step 150, the slide 214 is inverted. To prevent the further deposited PDMS droplet 210 from overflowing onto the slide 214, the slide 214 is quickly inverted (typically within two seconds) (as shown in Fig. 2D) after the further PDMS droplet 210 being deposited on the PDMS support layer 21 1 . The deposit of further PDMS droplet 210 stays on the curved surface 21 1 a, as the slide 214 is inverted, due to the interfacial force existing between the surfaces of the further PDMS droplet 210 and the curved surface 21 1 a. At the same time, gravitational force is pulling the further PDMS droplet 210 toward the ground. The combination of these two forces causes the further PDMS droplet 210 to droop, and any excess further PDMS droplet 210 to drop off from the PDMS support layer 21 1 due to the gravitational force. Further, since the further PDMS droplet 210 experiences constant forces over the curved surface 21 1 a, the fabricated lens exhibits an increased curvature (i.e., decreasing lens radius). Therefore, the amount of further PDMS droplet 210 that can be deposited on the PDMS support layer 21 1 depends on the curved surface area 21 1 a; a curved surface 21 1 a with larger curvature radius is capable of supporting more of the further PDMS droplet 210.

[0023] Fig. 2D also shows the drooping of the further PDMS solution 210 as the slide 214 is inverted. Step 150 then proceeds to step 160.

[0024] In step 160, the PDMS on the inverted slide 214 is cured. The inverted slide 214 is placed in an oven at a predetermined temperature for a predetermined period of time to cure the further PDMS droplet 210. As described in paragraph [0020] above, the oven can be set at a temperature of 70°C for a period of 15 minutes to cure the further PDMS solution 210. Step 160 proceeds to step 170. [0025] In step 170, the fabricated lens is checked whether the lens has the required focal length. If not (NO), then step 170 proceeds to step 140 and the process of steps 140 to 160 is repeated to add a further layer to the lens. Otherwise (YES), the method 100 is completed.

[0026] By repeating the process of steps 140 to 160, further PDMS droplet 210 is deposited on the PDMS support layer 21 1 . Each added layer increases the curvature, whilst reducing the focal length, of the fabricated lens. Figs. 2E and 2F show the deposit of one to four layers of further PDMS droplet 210 onto the PDMS support layer 21 1 . Lens 220 is a lens with a single layer of further PDMS solution 210 being cured on the PDMS support layer 21 1 , lens 230 has two layers of further PDMS solution 210 being cured on the PDMS support layer 21 1 , lens 240 has three layers of further PDMS solution 210 being cured on the PDMS support layer 21 1 , and lens 250 has four layers of further PDMS solution 210 being cured on the PDMS support layer 21 1. The refractive indices of the PDMS support layer 21 1 and the further PDMS droplet 210 are matched, resulting in the fabricated lens having no abrupt changes in refractive index along the central axis of the fabricated lens - especially between the PDMS support layer 21 1 and the further PDMS droplet 210, or between each further PDMS droplet 210. Abrupt changes in refractive index along the centre axis of the fabricated lens can invoke large spherical aberrations, in addition to other aberrations (e.g, defocusing), which reduces the imaging quality of the fabricated lens.

[0027] Fig. 3A shows the experimental setup of a light transmission imaging system 300 for testing the imaging quality of lenses fabricated using the method 100 (e.g., lenses 220, 230, 240, and 250). The imaging system 300 comprises a complementary metal-oxide- semiconductor (CMOS) imaging sensor 310, an imaging lens 320, the slide 214 with a fabricated lens (e.g., lens 220, 230, 240, or 250), and an image generator (i.e., a liquid crystal display (LCD) 360; or a brightfield light source 390 and a non-transparent micrometre graticule 370; or a fluorescence light source 390 and fluorescent microsphere 380). The different setup for the image generator allows performance of the fabricated lens (i.e., lens 220, 230, 240, and 250) to be assessed over different imaging modalities (e.g., brightfield, fluorescence). Lenses 220 and 250 with corresponding focal length fwlens2 344 and fwlensl 346, respectively, are shown in Fig. 3A only as an example and validation of the enhanced imaging performance using the method disclosed herein.

[0028] The CMOS imaging sensor 310 has a resolution of 3.1 Megapixel, but other resolution are also viable for this experiment. The imaging sensor 310 and the lens 320 are separated by a distance 312, and in this experimental setup, the imaging sensor 310 and the lens 320 are in- built in a camera. A distance fwlensi 326 separates the image generator and the imaging lens 320.

[0029] The slide 214 with the fabricated lens (e.g., lens 220, 230, 240, or 250) is positioned in between the lens 320 and the image generator. The peak of the fabricated lens (i.e., lens 220, 230, 240, or 250) is positioned at a distance, S ₀ 342, away from the image generator, resulting in an intermediate imaging plane 321 located at a distance S, 322 away from the slide 214. Thus, the imaging sensor 310 captures the generated image, after that image passes through the fabricated lens (i.e., lens 220, 230, 240, or 250), the slide 214, and the lens 320.

[0030] The imaging sensor 310, the lens 320, the slide 214, and the fabricated lens (i.e., lens 220, 230, 240, or 250) are horizontally arranged along a principle optical axis 324, so that the generated image does not fall on the sensor 310 at an oblique angle.

[0031 ] Fig. 3B shows the fours lenses 220, 230, 240, and 250 fabricated using the method 100. The images 371 , 374, 378, and 382 of Fig. 3C show the images processed by the imaging sensor 310 from images generated by the LCD 360 passing through lenses 220, 230, 240, and 250, respectively. Images 372, 376, 379, and 384 of Fig. 3C show the image processed by the imaging sensor 310 from images generated by the brightfield light source 390 and the non- transparent micrometre graticule 370 passing through lenses 220, 230, 240, and 250, respectively. The parallel gridlines of the non-transparent micrometer graticule 370 used in this example are separated by a distance of 10 μιη from each other. Lastly, the images of Fig. 3D are images processed by the imaging sensor 310 after the image generated by the fluorescence light source 390 passes through a fluorescent microsphere 380 and the fabricated lens 240.

[0032] Each captured RGB (Red, Green, or Blue) pixel, shown in images 371 , 374, 378, and 382, generated by the LCD 360 is approximately 100 μιη wide. As seen in the images 371 , 374, 378, and 382, lens 250 has the highest magnification compared to the other lenses 220, 230, and 240 based on the magnified LCD pixels resolved by the imaging sensor 310. As expected under the thin lens approximation, decreasing radius of curvature of the fabricated lens (i.e., lens 220 to 250) leads to a proportional decrease in focal length, resulting in increasing magnification and resolving power of the lenses (i.e., increasing numerical aperture). Hence, a highly curved PDMS lens has higher optical magnification and imaging resolution.

[0033] Images 372, 376, 379, and 384 show microscope calibration slides with the non- transparent micrometre graticule 370 of 10 μιη per division being magnified and processed by the imaging sensor 310. Similar to the images 371 , 374, 378, and 382, these images show lens 250 having the highest magnification and greatest resolving power compared to the other lenses 220, 230, and 240 based on the magnified image of the non-transparent micrometer graticule 370 processed by the imaging sensor 310.

[0034] In another experiment, a 1 μηη fluorescent microsphere is used as a point spread function (PSF) to measure image resolution provided by the lens 240. Fig. 3D shows a cross- section light intensity plot and a two-dimensional light intensity image processed by the imaging sensor 310. The cross-section light intensity plot and the two-dimensional light intensity image is from the image of a 1 μηι fluorescent microsphere being illuminated by a fluorescence light source 390, after passing through the fabricated lens 240, falling on the imaging sensor 310. As seen from the images of Fig. 3D, the lens 240 is capable of resolving an image with a full- width-half-maximum (FWHM) of 2.5 μιη, based on the curve fit value and the PSF being defined by FWHM of an Airy disc.

[0035] The advantages of the lens-fabrication method 100 are the simplicity and reproducibility of the manufacturing method. The lens-fabrication method 100 also minimises lens defect that typically exists in existing lens-fabrication methods due to asymmetry or deformation of the molds used. Furthermore, a lens fabricated using the method 100 can be shaped - by adding PDMS layers - to achieve a focal length of between 10 mm to 5 mm (i.e., lens 220 to 250, respectively) resulting in significantly different optical magnifications. Lenses of differing magnification can be used for different purposes, e.g. imaging and collimation.

[0036] Lenses 220 and 230 shown in Fig. 3C are particularly useful for imaging applications, whilst lenses 240 and 250 are useful for light collimation applications. Typically, lenses fabricated with three or more layers of the further PDMS solution 210 deposited on the PDMS support layer 21 1 result in a lens more suitable for collimation as such lenses have shorter focal length.

[0037] Fig. 4A illustrates the confocal light measurement setup to determine that the lens 240 is capable of collimating/redirecting light emitted from a single light emitted diode (LED) 430. An optical fibre 434 connected to a photo-detector (not shown) is used to measure two-dimensional light intensity distribution being emitted by the LED 430 with and without the lens 240 to generate the images shown in Figs. 4B and 4C, respectively. Fig. 4B shows the light (produced by the LED 430 without the lens 240 attached) varying in intensity along the vertical axis. Conversely, Fig. 4C shows the light produced by the LED 430, after passing through the lens 240, having an almost uniform illumination along the vertical axis. Industrial Applicability

[0038] The arrangements described are applicable to the lens manufacturing industries.

[0039] The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.

[0040] In the context of this specification, the word "comprising" means "including principally but not necessarily solely" or "having" or "including", and not "consisting only of". Variations of the word "comprising", such as "comprise" and "comprises" have correspondingly varied meanings.