Arrayed microlenses, fabrication

Arrayed microlenses are widely used in a variety of applications that involve miniaturized optical components.172 For example, they can be found at the heart of optical communication systems, facsimile machines, laser printers, and many other kinds of digital information storage or processing devices. In all these applications, the arrayed microlenses simply serve as diode laser correctors, fiber-optic couplers or connectors, and optical scanners. In a set of recent publications, Whitesides and coworkers have also demonstrated that arrayed microlenses could be used as a new platform for photolithography, through which submicrometer-sized structures could be conveniently fabricated as patterned arrays by reducing mm to cm scale features on a photomask.157... 

Biebuyck et al. used selective dewetting of liquid prepolymers on a surface printed with self-assembled monolayers to fabricate arrayed microlenses made of an organic polymer [13]. Due to the low viscosity of the prepolymer required by this process, the microlenses fabricated using this method exhibited relatively small curvatures and thus short focal lengths [12]. 

Hexagonal array of microlenses fabricated by withdrawal method to apply liquid monomer. The lenses consisted of a cross linked poly-alkyl-methacrylate network. Lens diameter = 90 im. Focal length = 870 30 gm. Source Moench, W. and H. Zappe. 2004. Journal of Optics A, 6(4), 330-337. With permission.)... 

There are a number of different geometries convenient for immersion microlenses in photodetection. Probably, the most well known and widely used form is hemisphere. The use of microsystem technologies allows the fabrication of various discrete or arrayed microlenses, with spherical surfaces (calottes, hemispheres and truncated spheres, full spheres), aspheric (ellipsoids, paraboloids, cylinders, cones), toroid, as well as various nonmonotonic surfaces consisting of two or more monotonous segments. Most of the microlenses convenient to increase the incident flux to the detector are plano-convex ones. 

FIGURE 8.14. (A) A schematic illustration of the setup used to project an F-shaped object onto the focal plane through an array of microlenses. (B, C) images formed at the focal plane of an array of mushroom-shaped (B) and hemispherical (C) microlenses. The microlenses used in all of these demonstrations were fabricated by templating 5.7- tm polystyrene beads against 2D arrays of cylindrical holes that were 5 pm in height and diameter. 

An array of 10- i,m microlenses was fabricated from the adhesion of an aminated silicasol on a poly[methyl(phenyl)silane-co-methyl(3,3,3-tri-fluoropropyl)silane] (CF3PMPS) film patterned by UV light irradiation.132 By soaking the UV-patterned polysilane film into the sol-gel solution, a convex xerogel layer adhered only to the UV-exposed poly silane, which was cured to form a glass that functioned as a condensing lens. 

An array of circular or elliptical microlenses made of photoresist was constructed on a chip, as shown in Figure 7.7. The microlenses were fabricated on both the bottom glass plate (for focusing the excitation beam), and on the top glass plate (for collecting the emission). In addition, an array of entrance apertures was formed around the focusing microlenses to limit the excitation beam another array of exit apertures was formed around the collecting microlenses to block off... 

In a somewhat similar fashion, Ishii et alP- have demonstrated inkjet fabrication of polymeric microlenses for optical chip packaging. UV curable epoxy resin is deposited onto optical devices by inkjet printing. When the droplets hit the surface, they form into partial spheres due to their surface tension, and are UV-cured to form the microlens with diameters from 20 to 40 tm with /-numbers of 1.0 to 11.0. Their uniformity in a microlens array was measured to be within 1% in diameter and 3 tm in pitch (total count of 36 lenses). They have also demonstrated hybrid integration of inkjetted microlenses with a wire-bonded vertical-cavity-surface-emitting laser (VCSEL) with coupling efficiencies of 4 dB higher than without the microlens. 

Solid microlenses and microlens arrays have been studied and used for more than twenty years [12], Some applications of microlens arrays such as beam shaping [46], focusing light onto CCD arrays [47], and Shack-Hartmann wave-front sensors [10] have been commercialized. Applications of solid microlens arrays are covered extensively in Daly s book [12]. In this book, we will cover some new applications and fabrications of solid microlenses and microlens arrays in Chapter 4, and will focus on tunable microlenses. 

Lu et al. described a self-assembly approach to fabricating patterned 2D arrays of microlenses with well-controlled lateral dimensions in the range of... 

Experimental procedure used to fabricate two-dimensional arrays of polymeric microlenses on glass substrates, (a) Cross-sectional view of packing cell used to deliver monodispersed polystyrene beads into two-dimensional array of cylindrical holes patterned in thin film of PR spin coated on bottom glass substrate, (b) Fabrication of microlenses with hemispherical and mushroom-shaped profiles by annealing sample at temperatures above glass transition temperature of polystyrene ( 93°C). The formation of a hemispherical shape was driven by the minimization of the surface free energy. (Source Lu, Y., Y.D. Yin, and Y.N. Xia. 2001. Advanced Materials, 13(1), 34-37. With permission.)... 

As described previously, polymer patterning techniques can be used to define LC microlenses. However, it is relatively difficult to control lens shapes with such technique, and the lens surface thus formed is rather rough as a result of the random phase separation between the LC and the prepolymer. Therefore, obtaining microlens arrays with high optical performance while keeping the fabrication process simple remains a challenging task. 

The microlenses of Holmes [17] were bistate types. Chandra et al. utilized a similar mechanism to fabricate a single component, strain-responsive microlens array capable of continuous focus tuning [19]. Figure 6.22 shows the fabrication of the array. A flat PDMS sheet 0.5 mm thick was prepared first. The sheet was clamped at four edges (Figure 6.22a) and then stretched to 20% strain in both planar directions simultaneously (Figure 6.22b). 

The work of Zhu et al. is intriguing in that it combines the benefits of tunable liquid microlenses and the large FOV offered by compound eyes. Figure 6.37 illustrates a timable liquid microlens array with six elements fabricated on a hemisphere. The fabrication of these arrays was described in Example 3 in Chapter 3. The microlenses were made in individual islands connected via thin pol5mier bridges so that the stress from wrapping the whole structure onto the... 

Photostructuring of glasses has been used for the fabrication of arrays of microlenses [20,59]. The reported fabrication method does not reply on geometrical structuring by the last etching step but actually utilises only the thermally induced deformation of partially crystallised and glassy areas in the same sample (see Fig. 9.7b). 

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