Abstract:
Disclosed is a lens fabrication method ( 100 ) which uses a droplet of polydimethylsiloxane (PDMS) solution ( 210 ) cured on a slide ( 214 ) to form a PDMS support layer ( 211 ) having a curved surface ( 211  a). Further PDMS droplet ( 210 ) is then deposited on the curved surface ( 211  a) of the PDMS support layer ( 211 ); the slide ( 214 ) is then inverted to allow gravitational force to pull the uncured, further PDMS  110  PDMS solution ( 210 ) down. The further PDMS solution ( 210 ) on the inverted slide is then cured. Each repetition of depositing, slide-inverting, and curing of the further PDMS droplet ( 210 ) adds an additional layer of PDMS, altering the shape and focal-length of the lens.

Description:
RELATED APPLICATION 
       [0001]    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 Jan. 2014, the content of which is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates generally to the fabrication of lenses and, in particular, to moldless fabrication of a lens using gravitational force. 
       BACKGROUND 
       [0003]    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. 
         [0004]    Thus, a need exists for a method of fabricating lenses that reduces or eliminates waste of raw materials and/or use of molds. 
       SUMMARY 
       [0005]    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. 
         [0006]    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. 
         [0007]    Other aspects of the invention are also disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    At least one embodiment of the present invention is described with reference to the drawings, in which: 
           [0009]      FIG. 1  is a flow diagram illustrating a method of the gravity-assisted additive lens-fabrication in accordance with an embodiment of the invention; 
           [0010]      FIGS. 2A to 2F  show block diagrams illustrating the method of  FIG. 1 ; 
           [0011]      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 
           [0012]      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 
       [0013]    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. 
         [0014]    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. 
         [0015]      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  110  to  130  forming a polydimethylsiloxane (PDMS) support layer  211  on a slide  214  (shown in  FIG. 2A ). For example, the slide  214  may be a polished flat glass slide, such as a coverslip. 
         [0016]    In step  110 , 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 μl) 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. 
         [0017]    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 . 
         [0018]    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  110  then proceeds to step  120 . 
         [0019]    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. 
         [0020]      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  211  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 . 
         [0021]    In step  130 , the deposited PDMS  211  is cured. The deposited PDMS  211  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  211 . The cured PDMS  211  serves as a support layer for subsequent PDMS layers. Step  130  then proceeds to step  140 . 
         [0022]    In step  140 , further PDMS  210  is deposited onto the cured PDMS. Further PDMS droplet  210  is deposited onto the curved surface  211   a  of the PDMS support layer  211 —shown in  FIG. 2C . Step  140  then proceeds to step  150 . 
         [0023]    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  211 . The deposit of further PDMS droplet  210  stays on the curved surface  211   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  211   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  211  due to the gravitational force. Further, since the further PDMS droplet  210  experiences constant forces over the curved surface  211   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  211  depends on the curved surface area  211   a;  a curved surface  211   a  with larger curvature radius is capable of supporting more of the further PDMS droplet  210 . 
         [0024]      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 . 
         [0025]    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 . 
         [0026]    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. 
         [0027]    By repeating the process of steps  140  to  160 , further PDMS droplet  210  is deposited on the PDMS support layer  211 . 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  211 . Lens  220  is a lens with a single layer of further PDMS solution  210  being cured on the PDMS support layer  211 , lens  230  has two layers of further PDMS solution  210  being cured on the PDMS support layer  211 , lens  240  has three layers of further PDMS solution  210  being cured on the PDMS support layer  211 , and lens  250  has four layers of further PDMS solution  210  being cured on the PDMS support layer  211 . The refractive indices of the PDMS support layer  211  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  211  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. 
         [0028]      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 fwlens1  346 , respectively, are shown in  FIG. 3A  only as an example and validation of the enhanced imaging performance using the method disclosed herein. 
         [0029]    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 . 
         [0030]    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 o    342 , away from the image generator, resulting in an intermediate imaging plane  321  located at a distance S i    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 . 
         [0031]    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. 
         [0032]      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 brighffield 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 μm 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 . 
         [0033]    Each captured RGB (Red, Green, or Blue) pixel, shown in images  371 ,  374 ,  378 , and  382 , generated by the LCD  360  is approximately  100  pm 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. 
         [0034]    Images  372 ,  376 ,  379 , and  384  show microscope calibration slides with the non-transparent micrometre graticule  370  of  10  pm 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 . 
         [0035]    In another experiment, a 1 μm 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 μm 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 μm, based on the curve fit value and the PSF being defined by FWHM of an Airy disc. 
         [0036]    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. 
         [0037]    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  211  result in a lens more suitable for collimation as such lenses have shorter focal length. 
         [0038]      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 
       [0039]    The arrangements described are applicable to the lens manufacturing industries. 
         [0040]    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. 
         [0041]    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.