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Embedded microball lenses with superior optical properties function as convex microball lens (VMBL) and concave microball lens (CMBL) were fabricated inside a PMMA substrate with a high repetition rate femtosecond fiber laser. The VMBL was created by femtosecond laser-induced refractive index change, while the CMBL was fabricated due to the heat accumulation effect of the successive laser pulses irradiation at a high repetition rate. The processing window for both types of the lenses was studied and optimized, and the optical properties were also tested by imaging a remote object with an inverted microscope. In order to obtain the microball lenses with adjustable focal lengths and suppressed optical aberration, a shape control method was thus proposed and examined with experiments and ZEMAX® simulations. Applying the optimized fabrication conditions, two types of the embedded microball lenses arrays were fabricated and then tested with imaging experiments. This technology allows the direct fabrication of microlens inside transparent bulk polymer material which has great application potential in multi-function integrated microfluidic devices.
© 2015 Optical Society of America
1. Introduction
Recent researches have demonstrated that microlenses (ML) and microlens arrays (MLA) are powerful optical components that have already been utilized in diverse applications, e.g. microelectromechanical systems (MEMS) [1,2 ], wide field/3D imaging systems [3,4 ], microfluidic devices [5–7 ], optical connectors [8], light-emitting diodes (LEDs) [9–11 ], charge-coupled devices (CCDs) [12], optical fibers [9,13 ] and sensors [14,15 ]. Especially, microball lenses which are unique with their spherical shape and micrometer-scale sizes, have been proved to have great application potential in optical super-resolution imaging which a sub-diffraction limited resolution can be achieved (<100 nm) directly under white light illumination [16,17 ]. By combining the advantages of microball lenses and several microfluidic technologies, a recent approach towards microlens integration in microdevices is purposed by Y. J. Fan et. al. that microball lenses were embedded in the pre-designed wells in a microfluidic chip for high throughput multi-color fluorescence detection [18]. This work illustrates a novel direction for multifunction integrated microdevices, e.g. the fabrication of functional units (microchannel, microlens, optical fiber, etc.) inside the microdevices rather than on the surface. A significant advantage of this configuration is that it can effectively avoid the scratches and contaminations caused by physical contact that may degrade the optical properties. In addition, a more compact layout of the functional units will minimize the size of the microdevice, which will considerably reduce the space and the weight when it is employed in the aerospace application.
However, even though there are many advantages in the internal integrations, current methods to fabricate such devices are quite cumbersome, since sophisticated processes and long processing periods are needed [18,19 ]. It is because the manufacturing methods are essentially restrained in surface manufacturing, e.g. 2D structures should be firstly engraved on the surface of several planar substrates; then these substrates were bounded together in a certain order to form a microdevice with 3D internal microstructures. This layer-by-layer fabricating and bonding processes unavoidably prolong and complicate the manufacturing procedures.
Attempts have been made during the past ten years to solve the problems taken by surface manufacturing. Several femtosecond (fs) laser 3D internal manufacturing methods were purposed and utilized in the fabrication of integrated microdevices. By now, microchannels and micro-optical components such as mirrors and microlenses can be fabricated inside photosensitive glasses by using femtosecond laser assisted selective wet etching methods [20–23 ]; Moreover, internal microchannels can be also fabricated in fused silica with water/acid assisted femtosecond laser manufacturing [24–27 ], or in polymethyl methacrylate (PMMA) by femtosecond laser direct writing [28,29 ]. To sum up, techniques for making internal hollow structures such as microchannels are well developed in various materials; however, traditional methods to fabricate microlens such as ink-jet [30], thermal resist reflow [31,32 ], hot-embossing [33,34 ], femtosecond laser multiphoton polymerization [35,36 ], femtosecond laser enhanced local wet etching [37,38 ], etc., are not appropriate for the direct application in the internal microlens fabrication as most of these methods are restrained to surface manufacturing. According to the most recent reports, the only possible techniques to realize the internal microlens fabrication is to use the photosensitive glass, e.g. either by using the photosensitive glass as the substrate material to fabricate a microlens based on fs laser selective etching [39], or additively make the microlenses inside a pre-fabricated microchannel in the photosensitive glass by femtosecond laser induced two-photon polymerization (TPP) [40]. However, there are two main drawbacks for these methods in internal microlens making: (1) Photosensitive glass is less economic that might limited its industrial application due to its relatively higher costs; (2) Although the TPP method is an effective way to fabricate suitable microlenses in the desired position, a microchannel should be always made firstly inside the substrate to “support” the microlenses. This sacrificed region of the material will cause a waste of space in the device which may enlarged the final size of the product. Meanwhile, the 3D distribution of the microlens inside the device will be less flexible due to the fabrication of the sacrificed microchannel.
In our previous work, we have proposed and successfully fabricated the cavity microball lenses (CMBLs) inside the low-cost PMMA substrate by femtosecond fiber laser irradiation, which is proved to be function as a micro concave lenses [41]. In this study, a new type of microball lens function as a convex lens was also successfully demonstrated could be manufactured in PMMA as well. The manufacturing conditions of both the convex microball lenses (VMBLs) and concave microball lenses (CMBLs) by using a 50 × objective were investigated. The optical properties of VMBL and CMBL was examined and distinguished by a telescopic imaging experiment. Meanwhile, the refractive index changes of VMBL was observed and explained. To obtain better optical properties, a shape control method was purposed and examined by the experiment. The reason that the optical aberration can be reduced by the shape control method was further illustrated by a ZEMAX analysis. In addition, attempts have been made to fabricate either VMBL or CMBL microlens array. A boundary deformation caused by the stress wavefront was found in our experiment and proved can be cured by an annealing procedure. With the optimized fabrication techniques, both CMBL and VMBL microlens arrays were finally fabricated and tested with imaging experiments.
2. Fabrication of embedded microball lenses
An amplified femtosecond fiber laser (Cazadero, Calmar Laser Inc.) used in this work provides 400 fs pulses at 1030 nm wavelength with repetition rates ranging from 120 kHz to 1 MHz and maximum average power at ~2.5 W. The substrates utilized to fabricate the embedded microball lenses were chosen to be commercially available PMMA slices whose refractive index were 1.49. The substrates were prepared at the size of 20 mm × 10 mm × 2 mm by mechanical cutting. In order to precisely control the laser energy, the average power was measured by a PM100A compact power meter (Thorlabs Inc.). The dimensions of the obtained microball lenses were measured with an inverted metallurgical microscope (IE200M).
The fabrication process of the embedded microball lenses is as follows: the prepared PMMA substrate is first placed on a 3-axis translation stage which is located above an objective lens of inverted microscope (Fig. 1 ). The distance between the objective and the substrate is pre-adjusted in order that the focal point of the objective is located inside the PMMA substrate. Then the symmetric Gaussian beam emits from femtosecond laser system with a laser quality factor M2 < 1.2 and is focused into the PMMA substrate by the aforementioned 50 × /0.6NA long working distance (WD = 5.1 mm) microscopic objective (LMPlan). The laser energy is precisely controlled by adjusting a neutral density (ND) filter to acquire different types of microlenses: 1) When the PMMA substrate is irradiated by laser beam with 90 mW average power at a repetition rate of 120 kHz for 5 s, a refractive index modified spherical region whose refractive index is slightly larger than the base PMMA will be generated at the focal region of the objective lens. This refractive index increased region, as a result, functions as a convex microball lens (VMBL); 2) When the PMMA substrate is irradiated with 400 mW ~1.5 W laser at the same repetition rate and processing time, a spherical cavity surrounded by densified region can be easily obtained inside PMMA substrate and optically functions as a concave microball lens (CMBL). As reported in our earlier work, the refractive index changes of the gas cavity is about 0.49 lower than the PMMA substrate [41]. The hollow ball-like cavity of the CMBL is believed generated when the temperature of the irradiated region is heated up to the random scission temperature of PMMA (350 °C) due to the heat accumulation effect of the laser pulses at a high repetition rate. The irradiation period for fabricating an embedded microball lens in the current experimental setup is chosen as 5 s since the cracks generated at the beginning of the manufacturing disappeared due to the melting of PMMA after successive irradiation.
Fig. 1 Experimental configurations and the schematic for the fabrication of different types of embedded microball lens (VMBL and CMBL): (i) VMBL is fabricated when laser power intensity is adjusted to in a relatively low level at which a refractive index modified region generated after laser irradiation. (b) CMBL is fabricated at a relatively higher power level at which an embedded micro-cavity formed after manufacturing.
By scanning the PMMA substrate horizontally (along X-axis or Y-axis) or perpendicularly (along Z-axis) with the help of a 3D translation stage, it is possible to control the shape of the embedded microball lenses in a certain range. That is, the size of the fabricated microlens can be slight increased or the shape of the lens can be changed along the direction of the vibrated laser spot and result in an ellipsoidal microlens with less optical aberration if well-controlled laser parameters. The movement of electric motors was manipulated by a pre-designed computer program written in Arduino®. The program will be translated to the machine language and transfers to the microprogrammed control unit (MCU) of the translation stage, thus realizes the control of moving patterns of the electric motors.
To obtain the better uniformity of the embedded microball lens, the sample of microball lens was annealed in a vacuum drying oven (Across International Co. Ltd.) after laser irradiation. The temperature of the oven was first raised from the ambient temperature to 90°C at a speed of ~2°C/min and maintained this temperature for 20 min, then slowly cooled down the sample to the room temperature.
3. Results and Discussion
3.1. Development of microball lenses with average power
Figure 2 shows the structures of microball lenses develop with the increasing average power ranging from 30 mW to 400 mW at a repetition rate of 120 kHz for 5 second irradiation. The peak power densities for the fabrication thus vary from 1.8 × 1013 ~2.4 × 1014 W/cm2 accordingly [42]. Figure 2(a) shows a melted region is generated at the center of the focused position after irradiation and combines with random cracks when 30 mW laser beam is focused into PMMA. The melted region is generated due to the heat accumulation effect of the pulsed femtosecond fiber laser and the cracks is explained as a result of the high stress concentration caused by a high thermal gradient [43, 44 ]. When 50 mW average power was applied, the cracks are filled with the melted PMMA during irradiation; however, the edge of the melted region were seen to have sharp burrs as shown in Fig. 2(b). With further increase of the average laser power, the burrs were slowly diminished at 70 mW (Fig. 2(c)) and completely disappeared at 90 mW (Fig. 2(d)). By comparing the results from Figs. 2(a)-2(d), it is clear that the modified structure is a melted-resolidified region where only the refractive index is slightly increased due to the depolymerization and crosslinking as illustrated by A. Baum et. al. [45]. As the modified region possessed higher refractive index than the surrounding PMMA, the fabricated structure thus can be regarded as a convex microball lens (VMBL). If even higher laser power is applied, small bubbles will be generated (Figs. 2(e)-2(h)) inside the melted region since the stronger laser flux provided sufficient energy that raised the central temperature of laser focal point due to the heat accumulated effect, which dissociated PMMA molecules to small gases molecules when it reached the random scission temperature [46]. However, if the laser average is not sufficient enough, the generated bubbles will be random located inside melted region (Fig. 2(e)) or nonuniformly generated (Figs. 2(f) and 2(g)). It is discovered that higher laser average power can somehow stick the bubble in the focused center and a symmetric spherical cavity finally can be obtained when average power is chosen as 400 mW, because the gas bubble tends to move toward the hottest position in the melting pool due to the Marangoni flow [44]. As the internal media in the cavity are gases, the refractive index inside this structure is believed ~1, which is much lower than the suppressed melted-resolidified region (>1.55) calculated based on the Lorentz-Lorenz equation in our previous study [41]. The fabricated structure thus can optically functions as a concave microball lens (CMBL).
Fig. 2 The structural development of microlenses with different average power. (a) A melted region is generated accompanied with cracks at the center of the ablated region (30 mW, 1.8 × 1013W/cm2). (b) and (c) shows nonuniform affected zones are created with sharp edges (50 mW, 3.0 × 1013W/cm2) and will be smoother if further increase the average power (70 mW, 4.2 × 1013W/cm2). (d) A uniform and symmetric VMBL is generated (90 mW, 5.4 × 1013W/cm2). (e) A bubble is generated and formed a cavity at the edge of the affected zone (130 mW, 7.8 × 1013W/cm2). (f) and (g) shows bubble is generated nonuniformly with insufficient energy (150 mW ~170 mW, 9.1 × 1013W/cm2 ~1.0 × 1014W/cm2). (h) Symmetric CMBL is generated when average power reaches 400 mW (2.4 × 1014 W/cm2).
As the fabrication of VMBL has a relatively narrow processing window, the best result shows that a proper average power is 90 mW. Here we mainly focused on the optimization of the experimental conditions to fabricate the CMBLs with the best properties by changing the average laser power (Fig. 3 ). As illustrated above, with laser average power above 200 mW at a repetition rate of 120 kHz irradiated for 5s, micro bubbles can be generated inside a PMMA substrate that provides the possibility to fabricate a CMBL. Nevertheless, the bubble is shaped nonuniformly and located randomly if irradiated with insufficient laser power (< 400 mW). When the average power is increased to 0.4 ~1.5 W (corresponding peak power density ranging from 2.4 × 1014 W/cm2 to 9.1 × 1014 W/cm2), spherical micro-bubble with super-smooth surface can be acquired at the irradiated region which forms the CMBL after the melted region is resolidified. The diameter of the fabricated CMBL displayed a near-linear increase with the average power at a rate of 5.52 μm/ 100 mW. Whereas the optical properties were ruined if the average power exceed 1.5 W when an over-ablated CMBL with dark brown polymer-like matters covers on the surface of the fabricated spherical cavity. These dark brown materials should come from femtosecond laser induced high temperature high pressure conduction and multi-photon decomposition of polymer [47].
Fig. 3 Development of the CMBL sizes when increasing the average laser power. The inset figures shows with processing window for a symmetric CMBL is from 400 mW to 1.5 W (2.4 × 1014 W/cm2 ~9.1 × 1014 W/cm2), otherwise an aspherical cavity or over-ablated CMBL will generated if the laser average power is insufficient or excessive.
3.2. Refractive index change of VMBL after fabrication
The dimension of the fabricated VMBL is found slightly increased with the cooling time as shown in Fig. 4 . The initial diameter of the VMBL after fabrication is 57 μm which grows to 63.5 μm after cooling in room temperature for 80 min. The increase of the diameter is 11.4%. While according to the imaging experiment of the lightened dark field mask with a VMBL as aforementioned, the dimension of the image increased from 78.2 μm to 110.5 μm in which the increase reaches 41.3%. The results show that although there is a slight diameter increase of the VMBL during the cooling process after irradiation, the increase of the image captured by this VMBL is much larger than the diameter increase of the fabricated VMBL. This is mainly because of the stress release caused by the heat diffusion during the slowly cooling process. Highly concentrated heat due to the heat accumulated effect of the fs laser pulses will diffuse to the ambient when the laser irradiation process is finished. The central temperature will then slowly decreases until cooling down to the room temperature. Stress induced by the laser irradiation thus partially released with the cooling process and results in the refractive index decrease of the fabricated VMBL. As a consequence, the size of the image captured by the VMBLs in the similar sizes increases with the cooling time.
Fig. 4 The dimension developments of VMBL and its image with the cooling time. (a) shows that the diameter of VMBL slightly increased after cooling for 80 min, meanwhile the image taken by the cooled VMBL shows distinguished increase. (b) illustrates the diameter development of VMBL and its image with the cooling time.
3.3. Telescopic imaging with microball lens
To simply examine the optical functions of both types of microlenses, a micro-telescopic imaging system is constructed with a PMMA substrate containing an embedded microball lens (both the VMBL and the CMBL) and a microscopic objective lens as illustrated in Fig. 5(a) . An illuminated dark field mask of the inverted microscope is used as an object to be viewed with this micro-telescopic imaging system. Figure 5(b) shows the image taken with a VMBL as the front lens of the micro-telescope in which the shape and details of the mask are perfectly recorded. However, when imaging with a CMBL as in Fig. 5(c), the image of mask is much smaller and blurred. These results shows that the micro-telescope constructed with a VMBL possesses higher resolution but smaller field of view (FOV); whereas, although the details cannot be distinguished with the micro-telescope constructed with a CMBL, this imaging system can provide a super wide FOV reaches ~350° according our previous study [41]. This imaging experiment illustrates that both VMBL and CMBL fabricated with fs laser irradiation possess good optical properties and great application potential.
Fig. 5 (a) A micro-telescope is constructed with microball lens and microscopic objective, enables telescopic imaging of the brightened mask as illustrated. Images captured by a micro-telescopic system with the front lens of (b) VMBL or (c) CMBL.
3.4. Shape control of microball lens fabrication
As the CMBL are fabricated approximate to spherical cavity, it naturally possesses numerous aberration due to its shape [48]. To reduce these aberrations and fabricate microball lens with adjustable focal lengths for different applications, we purpose a method called the shape control method by scanning the PMMA substrate along a certain direction with different speeds. As illustrated in Figs. 6(a) and 6(c) , the PMMA substrate is scanned horizontally (X-axis or Y-axis) for 1~7 cycles in 5 s with a scanning length of 80 μm, in another word, the velocities of the 3D stage moved ranging from 32 μm/s to 224 μm/s. The fabricated CMBL is ellipsoid-shaped which proves that the shape control method can extend the length along a certain direction. The dashed line in Fig. 6(a) indicates the diameter of the CMBL without shape control in the same experimental conditions, which is 104.1 μm. The length of long axis of the ellipsoidal CMBL first increases with the number of scanning cycles (or the scanning speed) from 99.4 μm if irradiated for only 1 cycle in 5 second to the maximum length of 107.7 μm if irradiated for 4 cycles in 5 s. This value will then decreases if further increase the number of cycles to 99.6 μm when irradiated for 7 cycles. The fabricated structure will be less nonuniform or even shaped as separated bubbles when the number of the cycles is larger than 7. This will seriously degrade the optical properties of the CMBL. The shape changes of the CMBL with the increase of scanning speed can be understood like this:
Fig. 6 Shape control of CMBL horizontally (along X-axis or Y-axis) and vertically (along Z-axis) by scanning with the laser for 1~7 cycles in 5s. (a) Scan with a 1.2 W laser power at a repetition rate 120 kHz in a scanning distance for 80 μm. (b) Scan with a 1.4 W laser power at a repetition rate 120 kHz in a scanning distance for 45 μm. (c) An X-axis shape controlled sample. (d) A Z-axis shape controlled sample.
- (i) Low-speed scanning (32 μm/s): when the scanning speed is relatively slow, the earlier heated region will be solidified when the heat resource, the laser focus, moves to other places due to the fast heat diffusion. This solidified region will be re-melted when the laser scans back again, but the later melted region will be solidified in turn. This is the reason for a shorter length of the resulted cavity. In another hand, the volume of the generated gas during this period is limited since the heat generated from laser pulses cannot be effectively accumulated because of a relatively longer heat diffusion period.
- (ii) Medium-speed scanning (64μm/s ~192 μm/s): when the scanning speed is further increased, a melting pool will be maintained at the center of the irradiated region and more gas will tend to be generated because the temperature of the irradiated region is steadily sustained in a high level that is beyond the random scission temperature. As a result, more gas is released and forms an ellipsoidal cavity with its long axis along the scanning direction.
- (iii) High-speed scanning (≥224 μm/s): when laser scans at a speed of 224μm/s, the laser focus will immediately move out from the heated region where the temperature reaches to (or is slightly beyond) the threshold temperature of the gas generation. Thus, the amount of the released gas is limited due to the insufficient heating, which results in a smaller ellipsoidal cavity with its short axis along the scanning direction. If laser scans even faster, the heat accumulated effect will be greatly restrained. As a result, separated tiny bubbles are observed along the scanning route which cannot be precisely controlled to fabricate a single, pre-designed CMBL.
Figures 6(b) and 6(d) shows a PMMA substrate was scanned with 1.4 W laser for 1 to 7 cycles in 5 s for a 45 μm distance. The evolution tendency is similar as discussed in Fig. 6(a) in which the length of the cavity first increase along Z-axis with the increasing cycle numbers from 126.7 μm to 132.4 μm. Then the Z-axis dimension decrease if further increase when the cycle number >4 and reached 116.1 μm when scanned for 7 cycles in 5 s, which is much shorter than the one without Z-axis shape control (124.2 μm). The side view of this CMBL also proves that the shape control long Z-axis could be achieved with this method. Note that the laser power and the scanning distance use in the X-axis and Z-axis shape control is different in the purpose to show a better contrast to the original CMBL. The solid lines in both figures are guides to eyes.
To examine the performance of a shape controlled CMBL, a letter “F” is imaged by using the aforementioned microscope-microball lens telescope system. The microball lenses used in this experiment are CMBLs fabricated without/with the Z-axis shape control, as shown in Fig. 7 . The CMBL without Z-axis shape control is fabricated with a 1.2 W average power at a repetition rate of 120 kHz irradiated for 5 s. A spherical cavity is observed with a diameter of 88 μm in Fig. 7(b). While the CMBL fabricated by scanning for 4 cycles in 5s along Z-axis in the same laser conditions is shaped as an ellipsoid with a 129 μm long axis and 102 μm short axis as shown in Fig. 7(d). It is seen that with the normal CMBL in Fig. 7(a), large distortion is clearly observed, while the image taken by the one fabricated with Z-axis shape controlled CMBL is much clearer and has less aberrations.
Fig. 7 Z-axis shape control of CMBL scanning at a speed of 72 μm/s and a distance 45 μm for 4 cycles while irradiation. (a) The image of a letter “F” taken by a CMBL with a spherical cavity fabricated without Z-axis shape control. (b) The side view of the CMBL with spherical cavity. (c) The image of a letter “F” taken by a CMBL with an elliptical cavity after Z-axis shape control. (d) The side view of the CMBL with elliptical cavity.
The shape control method is explained with a ray tracing simulation taken by ZEMAX®. The optical properties of three kinds of CMBLs are compared, including: 1) one spherical cavity CMBL (Fig. 8(a) ) and two elliptical cavity CMBLs (Figs. 8(b) and 8(c)) whose axis along the Z-direction is 2) longer or 3) shorter than the radius of the spherical cavity. The radius for the spherical cavity of CMBL is set as Ra1 = 50 μm and covered with a refractive index changed PMMA shell (n = 1.55) whose radius Ra2 = 100 μm (e.g. thickness T = 50 μm). In addition, since the significant dimension change of the Z-axis shape control is occurred along the Z-axis direction, for the elliptical CMBLs in Figs. 8(b) and 8(c), the length along Z-axis is supposed to be the same as the radius of the spherical cavity, e.g. b1 (or a2) = 50 μm. Thus, we set these two comparison experiment with elliptical CMBL with Z-axis length at b) a1 = 60 μm and c) b2 = 40 μm, respectively. The covered shell is assumed at the same thickness 50 μm. The radius of curvature ρ at the tips of the long-axis of the ellipse is calculated by ρ = b2/a, where a represents the length of long-axis, b represents the length of the short-axis of the ellipse. Note that it changes to ρ = a2/b when calculating the radius curvature at the tips of the short-axis.
Fig. 8 Optical properties of CMBL with different shapes. (a) ~(c) Spherical cavity CMBL and elliptical cavity CMBLs; (d) ~(f) The geometric image of a letter “F”; (g) ~(i) The distortion curves when imaging with different CMBLs. The colorful curves represents the distortion curve of the incident light with different wavelengths, in which the blue light (484 nm), the green light (588 nm), and red light (656 nm) are represented by the curves with the same color.
In this simulation, rays transmit from with three different angles 0°, 20° and −20° are focused on the image plane with an objective lens. The best focused geometric images of the letter “F” taken with CMBL 1), 2) and 3) are shown in Figs. 8(d) and 8(f), respectively. To further illustrate the distortion caused by each CMBL, the distortion curves are given as Figs. 8(g)-8(i). According to Figs. 8(d) and 8(g), the spherical CMBL suffers from large positive distortion, which make is seems as a pillow. The maximum distortion is almost reaches 60%. While the elongated CMBL proves that it can reduce the distortion to 40% and makes the letter “F” looks more uniform. Note that all the distortion curves of different wavelengths are in positive range in Figs. 8(g) and 8(h). However, it can be clearly seen that the distortion got some negative value at the higher positions (larger Y value) when the CMBL is shorter along Z-axis. Thus, even though part of the curves are still in the positive range, the slightly distortion which is lower than 12% is not that distinguishable when using for imaging. These results show that the shape control method is a proven method to reduce the aberrations. Besides, the effective focal length (EFL) for the original spherical CMBL is −32.8 μm. It changes to −23.73 μm when the cavity is elongated along the Z-axis direction for 20 μm and varied to −46.5um when it is shorted for 20 μm. Thus, this method is also possible for making CMBLs with different EFLs that is very important in internal microlens fabrication.
3.5. Propagation of the stress wavefront and annealing
During the irradiation process, a wavefront is observed propagating when the cavity is gradually forming and increasing the size as shown in Fig. 9 . It is generated once the laser pulses irradiated on the PMMA substrate and spread quickly ahead the growing melting region. This wavefront is believed as a result of the concentrated stresses generated due to the gas expansion and thermal expansion act on the softened region outside the melting pool, thus it is denominated here as “stress wavefront”.
The stress wavefront has great influence when a fabricate microball lens array. As illustrated in Fig. 10(a) , the boundary of a prior made CMBL (right) is compressed by the stress wavefront generated when fabricating the second CMBL (left), thus affected the uniformity of a microball lens array. As the deformation is caused by the residual stress, the CMBL array is then annealed in the purpose to diminish the stress. The result shows that the deformation is successfully cured after annealing in the oven at 90°C for 20 min. In addition, it is also found that despite the distance of microball lens keep unchanged, the diameter of cavity is increased from 112 μm to 139 μm. Meanwhile, the defects at the boundary of the cavity is disappeared thus lower internal surface roughness is obtained (Fig. 10(b)).
Fig. 10 (a) The boundary deformation caused by the stress wavefront. The defects are partially cured and the boundary of the fabricated CMBL is smoother. (b) The diameters of the cavity regions (CR) and the affected zones (AZ) develop with different annealing temperature ranging from 70 °C to 90 °C. (c) The CMBL sample annealed in 100 °C.
To investigate the annealing process, CMBLs are annealed in 70°C, 80°C, 90°C and 100°C for 20 min, 40 min and 60 min (Fig. 10(c)). The annealing temperatures are chosen around the Vicat softening point of PMMA (105°C). The results show that a better annealing time for a CMBL sample is 20 min. The diameter of the annealed CMBL remained almost constant even though annealed for longer time. Larger CMBL diameter can be achieved with higher annealing temperature, however, the CMBL structure will be totally destroyed if annealed with 100°C for 20 min as shown in Fig. 10(d).
3.6. Imaging with microball lens array
Finally, VMBL and CMBL microlens arrays are fabricated and tested with imaging experiments. The 5 × 4 CMBL array is fabricated with a 1.4 W laser at a repetition rate of 120 kHz irradiate for 5 s. The center distance of the adjacent two CMBLs is 400 μm to avoid the affection caused by the stress wavefront. The whole process can be completed within 2 minutes, demonstrating its application potential in fabricating large area microlens arrays.
The fabricated CMBL array is then applied to image the word “UT” (represents University of Tennessee) which is written in the size of 2.2 mm × 1.2 mm and located 0.1 mm above the PMMA substrate, as shown in Fig. 11(a) . Some selected images taken by this array at different CMBLs were given in the inset images (i) ~(iv). Although suffered from the image distortion especially at the boundary of the cavity, it is seen that each of the CMBL has a clear field of view larger than 110°. The CMBL array can be another choice to for wide angle imaging.
Fig. 11 (a) Imaging of the word “UT” with 5 × 4 CMBL array. (b) Imaging of a lightened ring mask with a 4 × 3 VMBLs array. (c) A Letter “UT” is imaged with this VMBL array.
Similarly, a 4 × 3 VMBL array is fabricated with a 90 mW average power at a repetition rate of 120 kHz irradiate for 5 s. This array is used to image the aforementioned lightened ring mask (Fig. 11(b)) and a “UT” word (Fig. 11(c)). Although the contrast between the image and the background color is not sharp enough, the shape of the objects can be clearly imaged in CCD. This illustrates that our femtosecond fiber laser processing technique is a proven and facile method to realize internal microlens fabrication in PMMA.
4. Conclusion
In this paper, we successfully fabricated two types of embedded microlens function as VMBLs and CMBLs with a femtosecond fiber laser at a repetition rate of 120 kHz irradiated for 5s which was focused by a 50 × objective. The proper processing window to fabricate VMBL is 90 mW and 400 mW ~1.5 W to fabricate CMBLs. The refractive index change of the VMBL is due to the release of residual stress. Then the optical performance of the VMBL and CMBL were tested with the telescopic imaging by integrating with an inverted microscope. The imaging details show that CMBL is more suitable for wide angle imaging. The aberrations caused by the spherical shape can be partially reduced according to the shape control method, proved by both the experiment and the ZEMAX simulations. The stress wavefront caused by the spreading stresses will induce the boundary deformations of CMBL array, but it can be cured by annealing at 70°C ~90°C for 20 min. Finally a 5 × 4 CMBL array and a 4 × 3 VMBL array were fabricate and used to image remote objects. The presented technique is proved to a powerful protocol to fabricate internal microlenses in PMMA, which is of great application prospect in integrated multifunctional biochemical or electromechanical microdevices.
Acknowledgements
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant 50875007), the Ministry of Science and Technology of China Major Project of Scientific Instruments and Equipment Development (Grant 2011YQ030112), Key Projects of Science and Technology of Beijing Municipal Commission of Education (Grant KZ201210005009 and KZ201410005001), the Beijing Natural Science Foundation (Grant 4132017), the Beijing high level overseas talent project and the international exchange grant of the graduate school of Beijing Institute of Technology. In addition, we appreciate the research initiative funding provided by the University of Tennessee as a new hire package.
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