Immersion Microlenses

Probably the most obvious way to collect illumination from a larger area and to concentrate it to the active area of a photodetector is to use a microlens. Such a lens should have dimensions sufficiently small to be mounted on a detector. Since detector diameters are typically below 1 mm and very often below 0.1 mm, this means that the micro in the name of the microlens only means small, since the dimensions of the lens itself will be of the order of miffimeters. 

In order to avoid reflective losses between the detector and the lens, typically some kind of refractive index-matching immersion will be used. The lenses for the photodetector improvement can be thus called immersion lenses. Obviously, the material of the lens has to be transparent in the wavelength range of interest and is typically used with some kind of antireflective coating. 

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. 

Among spherical microlenses utilized in optoelectronics literature quotes spherical calottes [99] (Fig. 2.4a), hemispheres [100, 101] (Fig. 2.4b) and tmncated spheres [102] (Fig. 2.4e). These lenses may be defined as the refractive surfaces that can be represented as a part of a sphere with a radius r and with its center in the focus of the lens. The immersion lenses are usually formed by bringing into contact the planar surface of such a lens and the incident surface of the detector. Besides the above-mentioned lenses, ball-shaped microlenses are also used [103]. 

The following microlens shapes were also fabricated by microsystem technologies elliptic calottes [104], hemiellipsoids [105], ellipsoids [103], hemicylinders [101, 105] (Fig. 2.4g), and cylinders [106] (Fig. 2.4h). These types of microlenses are especially convenient if the beam shape is elongated. Also among aspherics are conic microlenses with straight or curved generatrix [107], (Fig. 2.4i). In some optical systems, paraboloid or hyperboloid surfaces are sometimes used, usually 

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Figure 2.4 shows some types of refractive microlenses that can be fabricated utilizing the standard microfabrication procedures in materials convenient for the MWIR and LWIR ranges. Most of them are loosely based on the solutions for microlenses used in fiber optics to improve coupling between laser sources and fibers [98]. These immersion lenses were thus intended for the operation with coherent and monochromatic radiation, while most of the microlenses in the field of IR detector technology are intended for incoherent, mono- or polychromatic Lambertian sources and, of course, they operate in different atmospheric windows. 

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