A conventional binary diffraction grating has a periodic structure and splits the incoming light into several beams, called diffraction orders. The intensity distribution of the diffracted light depends on the grating profile (groove width, grating period and height) and the wavelength. Considering a constant wavelength, the number of diffraction orders decline with decreasing grating period. If the grating period becomes smaller than the wavelength, only the zeroth order can propagate. In this case, the grating structure works as a homogenous layer with a laterally constant refractive index. This index can be adjusted between the index of the base material and the index of air by varying the lateral expansion of the subwavelength grating structures (Fig. 1).


Fig.1: Schematic illustration of the effective
        medium approach. (rights: IAP)

DOEs_Freese_2 (rights: IAP)

Fig.2: A conventional DOE surface profile can be
        converted into a binary effective media
        structure. (rights: IAP)

Using conventional approaches the required local phase delay of the diffractive element is generated by a complex height profile within a dielectric optical medium (Fig.2, top). The fabrication of such a surface relief is in most cases a very challenging, time-consuming and expensive multi-layer process. With the help of the effective medium approach this effort can be reduced to a one-step process, due to the binary surface profile (Fig.2, bottom).

Fig.3: SEM image of an effective media DOE structure in fused silica and its far field intensity distribution by laser irradiation at 532nm.
(rights: IAP)

This novel approach enables the design and fabrication of large-scale efficient diffractive elements even suitable for optical applications which demand a high numerical aperture.

[1]  E.-B. Kley, W. Freese, T. Kämpfe, A. Tünnermann, U. D. Zeitner, D. Michaelis
     and M. Erdmann: Large-scale application of binary subwavelength structures,
     Proc. IEEE/LEOS, 148-149 (2009).

[2]  W. Freese, T. Kämpfe, E.-B. Kley and A. Tünnermann: Design of binary
      subwavelength multi-phase level computer generated holograms,
      Opt. Lett. 35, 676-678 (2010).

[3]  T. Weber, T. Käsebier, M. Helgert, E.-B. Kley, A. Tünnermann, Tungsten
      wire grid polarizer for applications in the DUV spectral range,
      Applied Optics 51, 3224-3227 (2012).

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