Focal varying microlens array - OSA Publishing

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Letter

Vol. 40, No. 18 / September 15 2015 / Optics Letters

Focal varying microlens array ZHEN-NAN TIAN,1 WEN-GANG YAO,1 JUN-JIE XU,1 YAN-HAO YU,1 QI-DAI CHEN,1,3 AND HONG-BO SUN1,2,4 1

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China College of Physics, Jilin University, Changchun 130012, China 3 e-mail: [email protected] 4 e-mail: [email protected] 2

Received 2 July 2015; revised 10 August 2015; accepted 15 August 2015; posted 17 August 2015 (Doc. ID 244165); published 4 September 2015

We report a novel microlens array with different curvature unit lenses (MLADC) fabricated with femtosecond laser direct writing technology. The MLADC consisted of hexagonal hyperboloid unit microlenses, which have different heights and curvatures from others. The unique optical performance of imaging and focusing capability were demonstrated. An object was imaged at different positions from the MLADC by unit lenses, as the ability of adjusting the curvature of the image plane for overall MLADC. In addition, the experiment had a good agreement with simulation results, which was based on the analysis of the finite element method. The novel MLADC will have important applications in improving the performance of optical systems, especially in field curvature correction and real-time three-dimensional imaging. © 2015 Optical Society of America OCIS codes: (220.3630) Lenses; (230.3990) Micro-optical devices; (220.4000) Microstructure fabrication. http://dx.doi.org/10.1364/OL.40.004222

Today, both military and civilian applications require miniaturized, light, low-cost optical systems, and many novel optical elements with unprecedented performance have been invented [1,2]. As a new generation optical element, microlens and microlens arrays (MLAs) have played an irreplaceable role in micro-optical systems due to their small size, light weight, low cost, and high optical performance [3–5]. In the past few decades, various methods have been explored for fabricating MLAs, including thermal reflow [6], gray-scale photolithography [7], dry etching [8], injection molding [9], and so on. With the increasing of MLAs processing methods, efficiency, cost, uniformity, and tunability as important parameters to evaluate MLAs have been proposed progressively [10,11]. The abovementioned methods and parameters are typical and remarkable in promoting the application of MLAs. Recently, Chen et al. proposed a fast and single-step process, which uses high-speed line-scanning of femtosecond laser pulses, and can fabricate MLAs consisting of millions of units within an hour [12]. Ding et al. introduced a low-cost approach for fabricating large-area concave MLAs by liquid trapping and electrohydrodynamic deformation of the liquid in a microhole array [13]. 0146-9592/15/184222-04$15/0$15.00 © 2015 Optical Society of America

However, the unit lenses of MLAs attained by the above processing methods are all the same, which may not be the most reasonable form for MLAs in various applications. So far, MLAs consisting of different curvature unit lenses have not been proposed, which will possess different optical characteristics and improve performance of optical systems significantly. Here, we propose the microlens array with different curvature unit lenses (MLADC) concept for the first time, whose curvatures of unit lenses are different along with their different positions. Relative to conventional MLAs, the MLADC possesses unique and characteristic optical performance, which will play a significant role in optimizing optical system structure and reducing optical elements, especially in field curvature correction [14]. In real-time 3D imaging system, 3D images are assembled from a focal stack of 2D images acquired using a multifocus imaging system [15]. The MLADC will have a wide range of application in this area. For geometric morphology, the MLADC is a circular microlens array consisting of hexagonal unit microlenses, which have hyperboloid profile. The unit lenses were compactly arranged together, and the fill factor of the microlens array was 100%. The unit lenses have different heights and curvatures from others and the one located at central region is higher than that located at the edge region. Experimentally, the complicated lens array structure was realized by femtosecond laser direct writing (FsLDW), a technology known for its unique high-precision three-dimensional prototyping capability [16–18]. The curvature and position of the unit lens were precisely controlled to guarantee the desired high performance of the microlenses, which were then experimentally confirmed. Field curvature is one of the optical aberrations [19] in which a flat object cannot be brought into focus on a flat image plane instead of a curved plane [Fig. 1(a)]. Due to manufacturing constraints, almost all commercial image detectors are planar, which leads to complex optical systems with multiple lenses in order to achieve a flat image plane [14,20]. The MLADC can be a perfect solution to the problem, whose perspective view and cross-section profile are shown in Figs. 1(b) and 1(d), respectively. The unit lens located at the central region is higher than that located at the edge region, and it has a shorter focal length [Figs. 1(b) and 1(d)]. Cross-section profiles of ordinary MLAs and MLADC are shown in Figs. 1(c) and 1(d), respectively. The initial image plane is curved, which

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Fig. 1. Schematic of microlens array with different curvatures. (a) Schematic of field curvature. The red and blue lines represent different fields of view lights, which cannot be focused by a lens on a flat plane instead of a curved plane. (b) Perspective view of the MLADC. The heights and curvatures of unit lenses were different with their positions. (c), (d) Cross-section profiles of ordinary MLAs and MLADC. The difference of focal plane between ordinary MLAs and MLADC is that the (c) former and (d) latter is flat and curved, respectively.

accumulated by focal points of light from different field of views [Fig. 1(a)]. The focal plane of ordinary MLAs is flat [Fig. 1(c)], which has no effect to change the curvature of the image plane. However, the focal plane of the MLADC is curved [Fig. 1(d)], which can be used to offset the initial field curvature. In addition, the curvature of the focal plane can be adjusted by changing the height and curvature of unit lenses. The MLADCs were fabricated by femtosecond laser-induced two-photon polymerization of the commercial negative photoresist SU-8 (2025, MicroChem). The photoresist has been widely used for fabrication of high-quality three-dimensional photonic crystals and micro-optical devices due to its high transmittance for light from the visible to the near-infrared wavelengths, low polymer volume shrinkage, good mechanical properties, and high thermal stability. It has also been used for FsLDW, a technology that has been successfully employed in producing micromechanical and optical microstructures due to its unique high-precision 3D prototyping capability [21,22]. Femtosecond pulses of a wavelength of 800 nm with a pulse width of 120 fs, mode locked at 82 MHz (from Tsunami, Spectra-Physics), were tightly focused by a high NA objective lens of NA 1.4 into the photoresist. The laser power measured before the objective lens is only 6 mW. The laser focal spot was scanned with a two-galvano-mirror set along the horizontal, and along the optical axis by a piezo stage. The photoresist samples were prepared by spin coating SU-8 films on a glass slide that was cleaned with acetone and absolute ethanol. After a prebaking step by 3 min at 65°C and 30 min at 95°C, the solvent was evaporated and an overall 15 μm thick film was formed. With the control of the fabrication program, the laser focus was scanned in SU-8 film point by point. When the scanning was finished, the slide was laid on the hotplate again for 1 min at 65°C and 10 min at 95°C, namely, postbaking. After this treatment and having cooled down, the slide was immersed in the SU-8 developer for 10 min so that the unpolymerized photoresist would be removed, leaving a solid skeleton. Its surfaces were smooth, and the surface roughness, according to atomic force microscopy (AFM, Dimension Icon with Scan Asyst) measurement, was less than 10 nm [10]. The appearance of the experimentally produced MLADC is shown in Fig. 2. The scanning electron microscopic (SEM, JSM-7500F, JEOL) images of line and circle of arrangement

Fig. 2. Geometric morphology of the MLADC with different arrangements. (a) and (b) are SEM of one-row microlens and five-circle MLADC, respectively. (a1) and (b1), (a2) and (b2), and (a3) and (b3) were taken from top, 60°, and side directions. Note the height differences of lenses in (a3) and (b3). (c1)–(c6) The LSCM images of five-circle MLADC were taken at every 2 μm positions. Scale bar: (a1) 25 μm, (b1) 35 μm, (c1) 40 μm.

were demonstrated [Figs. 2(a) and 2(b)], which were taken from top, 60°, and side directions. Six hexagonal unit lenses were arranged in a row, the diameter of which was 20 μm [Fig. 2(a1)]. Lens height varied from 3 to 10 μm with equal intervals from left to right [Fig. 2(a3)]. The five-circle MLADC consisted of 91 unit lenses, whose diameters were 20 μm, shown in Fig. 2(b). From inside to outer, the lens numbers of each circle were 1, 6, 12, 24, and 48 [Fig. 2(c1)]. Lenses in the same circle have the same height, which changed from 10 to 2 μm [Figs. 2(b2) and 2(b3)]. In order to quantitatively examine the height difference of different circle lenses, a laser scanning confocal microscope (LSCM, OLS3000, EVC electronic) was utilized [Fig. 2(c)]. Rhodamine B (MFCD00011931, Sigma-Aldrich) was doped in SU-8 with an overall concentration of 1 wt. % before fabrication to take a high-contrast LSCM image. The clear fluorescence image was obtained at different positions, and the distance for each picture was 2 μm, shown in Figs. 2(c1)–2(c6). The height differentiation was demonstrated obviously from the change of

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fluorescence images. With the laser focus at low position, the whole lens array can be observed [Fig. 2(c1)]. As the focus moved up, the outer circle unit lenses disappeared gradually [Figs. 2(c2)–2(c5)]. Finally, with the focus 10 μm from the bottom of the lens array, only the center of the lens can be clearly observed. The heights of the lenses, from the inside to the outer circle, decreased gradually. A significant feature of the MLADC, relative to normal MLAs, is the artificial curved image plane array, which can be used to correct inherent field curvature in optical systems. The existing field curvature is a convex surface viewed from the image detector. In order to correct the convex image surface fitting the flat image detector, a MLADC could be placed in front of the curved image plane, which has the ability to convert the convex image plane to a flat plane. The capability of regulating image plane curvature is demonstrated in Fig. 3. Fivecircle MLADC was utilized to characterize the unique capability, whose optical microscope photograph is shown in Fig. 3(a). A letter “F,” used as the imaging object, was fixed on a movable sample stage and placed in front of the MLADC, which is irradiated by a focused sodium lamp. An objective and CCD camera were fixed on a movable stage and placed on the other side of the MLADC. By adjusting the position of objective lens, imaging results at different positions were detected, as shown in Figs. 3(b)–3(f ). Under the conditions of large distance between the MLADC and the objective lens, only the unit lens located

Fig. 3. Optical properties of the MLADC. (a) The optical photographs of five-circle MLADC taken by transmitted-light microscope. (b)–(f ) Imaging pictures were taken at different locations from the MLADC: (b) 33.2 μm, (c) 25.5 μm, (d) 20.5 μm, (e) 18.4 μm, and (f) 17.0 μm, respectively. Scale bar is 35 μm.

Letter at the fifth circle can be imaged clearly [Fig. 3(b)]. With the objective lens moving toward the MLADC, the unit lens located at different circles was imaged gradually, and the central lens was imaged finally [Fig. 3(f )]. The imaging result at different positions demonstrated the curvature image plane capacity of the MLADC clearly and persuasively. For demonstrating the unique focusing characteristic more clearly and numerically, focus image and intensity distribution has been obtained [Fig. 4]. Focus images were taken at different locations from the lens, whose focal lengths were 43.8, 24.5, 19.1, 17.3, and 16.8 μm, respectively. From Figs. 4(a1)–4(e1), the distances decreased from 30.3 to 16.0 μm, and unit lenses focused at different positions. The intensity distribution was obtained from the focus image along the direction of the unit lens arrangement marked with a red dotted line [Fig. 4(a1)]. Characterization of light intensity distribution and focal spot size were displayed more intuitively by the curves [Figs. 4(a2)–4(e2)]. The height of peaks in the same curve indicates the relative intensities of different focus. At the position that one unit lens focused, the relative intensity of other unit lenses was very weak. Focal spot sizes can been obtained from peak widths at half-height marked with two arrows in Fig. 4(a2), which has an opposite relationship with NA. From Figs. 4(a1)–4(e1), NAs were 0.22, 0.35, 0.41, 0.43, and 0.44, and the spot sizes were reduced from 1.7 [Fig. 4(a2)] to 0.9 μm [Fig. 4(e2)] gradually, which has the same changing rule as simulation [Fig. 5]. The unique feature of different focus positions will be used in parallel laser 3D processing systems. Energy distribution behind the lens was simulated by commercial simulation software, COMSOL Multiphysics (COMSOL Inc.), which was based on the finite element method [Fig. 5(a)]. Below the figure, the black curves represent different curvature unit lenses. The refraction index of the lens is 1.59, which equals to that of SU-8 after drying. The parallel beam is incident on the microlens array from the bottom of the lenses, which is 500 nm for the wavelength. Energy

Fig. 4. Focus optical properties of the MLADC. The MLADC consists of five unit lenses arranged in a row, whose height increased gradually from right to left. (a1)–(e1) Focus images were taken at different locations from the lenses: (a1) 30.3 μm, (b1) 24.5 μm, (c1) 20.2 μm, (d1) 17.9 μm, and (e1) 16.0 μm, respectively. (a2)–(e2) Relative intensity distribution of focus image (a1)–(e1). Scale bar is 20 μm (b1).

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Funding. National 973 Program; National Natural Science Foundation of China (NSFC) (2011CB013003, 61127010, 61137001, 61435005, 91423102). Acknowledgment. We thank Xu-Lin Zhang from State Key Lab on Integrated Optoelectronics, Jilin University, for his help in simulation. REFERENCES Fig. 5. Focused beam properties of the MLADC were simulated using the finite element method. (a) Energy distribution behind the MLADC, scale bar is 20 μm. (b)–(d) Relative energy intensity at three different positions from the MLADC: (b) 30 μm, (c) 20 μm, and (d) 16 μm. The three positions were successively through the focus positions of edge, inner, and central lenses.

distributions were different in the rear of each unit lens due to their different curvatures. The height and curvature of the edge lens is smaller than that in the middle, and its NA was changed from 0.22 to 0.44. Energy distribution in normalization through different focal spots is shown in Figs. 5(b)–5(d). With a location far away from the lens, only the edges of the lenses can focus the incident parallel beam [Fig. 5(b)]. Moving toward lens array, detector reached the focal positions of central lenses gradually [Figs. 5(c) and 5(d)]. The peak widths at halfheight were reduced from 1.2 [Fig. 5(b)] to 0.6 μm [Fig. 5(d)], which were associated with the change of NA and had the same changing rule with the experiment. After passing through the MLADC, the parallel beam was focused. And the focuses were not distributed within a flat plane, but a curved surface. The focused beam properties are in good agreement with the experiment. In summary, we report in this Letter a novel microlens array, MLADC, which consists of different curvature unit lenses. Each unit lens could image and focus independently. Images and focal points were located in different planes behind the microlens array, which demonstrates a banding image plane capability for the overall optical element. We demonstrate the unique imaging and focus capability in the experiment, showing a good agreement with simulation results. This microlens array will be useful for improving the performance of an optical system, especially in field curvature correction. In addition, MLADC will have a broad and distinctive application in novel microsystems, such as real-time 3D imaging and parallel laser processing.

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