Nanosieves for Photonic Structures

Recently, there has been growing interest in photonic bandgap materials, which have a spatially periodic refractive index with a lattice constant on the order of the wavelength of light, because of potential scientific and technological applications based on their unique light propagation properties [94]. However, experimental surveys of the optical properties of photonic bandgap materials have been limited due to the difficulty in preparing well-constructed samples except in the long wavelength region [95, 96].

Photonic crystals are novel materials with unique optical properties [97]. The crystals have a periodic modulation of the refractive index. As a result, the dispersion of light will be described by a band structure analogous to those of electron wave atomic crystals. Under the right conditions a photonic crystal can exhibit a photonic bandgap: light in a certain range of optical frequencies is forbidden in the crystal [98]. The existence of a photonic bandgap enables an unprecedented control of spontaneous emission and propagation. By locally disturbing the periodicity, a defect-associated photon state is created which can be used to guide light.

A photonic crystal slab is a thin film with a two-dimensionally periodic refractive index modulation in the plane [100, 101]. In a photonic crystal slab the light is confined to the crystal plane by a classical slab waveguide construction. For in-plane wave vectors a bandgap can be created. Thus, the slab has applications in light guiding without the need for a full bandgap in all three dimensions. Optimal performance of the photonic crystal slab is expected when the slab is mirror symmetric in the vertical direction (i.e., when the material on both sides of the slab has equal refractive index at least in the region of the near-field tail of the in-plane propagating light) [102].

Photonic crystals are fabricated by periodically arranging materials with highly dissimilar refractive index. To obtain a bandgap in the visible the periodicity of the index modulation has to be in the submicrometer range, <350 nm. For the fabrication of photonic crystal slabs electron beam lithography has been by far the most frequently used technique, since it is one of the few techniques that fulfills the accuracy requirements on these length scales [103]. However, due to its "direct sequential write" character e-beam lithography is time consuming for large area structures, plus uniformity errors in periodicity can occur due to errors (or quantization roundings) in the beam alignment. It would be highly advantageous to define the entire periodic pattern at once. Laser interference lithography (LIL) has proven its ability to generate uniform periodic submicrometer patterns over very large areas (on the order of square centimeters) [74, 104]. A combination of large area patterning with LIL and local patterning with a direct writing method (e.g., a focused ion beam [105]) is ideal for future fabrication of photonic crystal slabs with arbitrarily shaped light-guiding defects.

Figure 41. Photographs of a membrane made with a self-assembled mask. Left: arrays of nanosized particles on a silicon nitride membrane layer. Right: after chromium deposition over the particles and removal of the particles, the pores in the membrane have been etched. Reprinted with permission from [3], C. J. M. van Rijn, membrane Filter as well as a Method of Manufacturing the Same, PCT Application 95/1386206.

Figure 41. Photographs of a membrane made with a self-assembled mask. Left: arrays of nanosized particles on a silicon nitride membrane layer. Right: after chromium deposition over the particles and removal of the particles, the pores in the membrane have been etched. Reprinted with permission from [3], C. J. M. van Rijn, membrane Filter as well as a Method of Manufacturing the Same, PCT Application 95/1386206.

Figure 43. SEM micrographs of gold spots and pillars evaporated through a nanostencil. Reprinted with permission from [93], M. Kolbel et al., Nanoletters 2, 1339 (2002). © 2002, American Chemical Society.

In Figure 44 a combination of LIL with focused ion beam (FIB)-assisted deposition is presented. Highly uniform freestanding photonic crystal slabs have been fabricated, extending over areas as large as a few hundred micrometers squared, with the possibility to introduce any kind of defect structure. In this way is realized, to our knowledge, the largest photonic crystal slab (100 /m x 4 mm) for visible radiation. Long line defects, extending over more than 1 mm, were also introduced.

photoresist (300 nm) Si3N4 (146 nm) SiO2 (3.2 Mm) Si

LIL on photoresist, double exposure with 60° intermediate relation photoresist dots

Chromium lift off

Definition of membrane:

1) Wet etch on Crto define length

2) Mask lithography to define width

Photoresist (1.2 |m)

Local deposition of platinum with FIB to define defects

1) RIEonSi3N4

2) Removal of all mask materials

Chromium lift off

Definition of membrane:

1) Wet etch on Crto define length

2) Mask lithography to define width

Photoresist (1.2 |m)

Local deposition of platinum with FIB to define defects

1) RIEonSi3N4

2) Removal of all mask materials

Wet etch on SiO2 through porous Si3N4

Figure 44. Schematic representation of the procedure which combines large area patterning through laser interference lithography and local introduction of defect structures through FIB-assisted deposition. The dimensions in these pictures are chosen for illustrative clarity and are not to scale. Note that a slab of 100 /m x 4 mm contains over 4.5 million holes. A photoresist layer on top of a Si3N4 (146 nm), SiO2 (3.2 /m), and Si layer stack (A) is converted into a large area triangular dot pattern with LIL (B). By chromium lift-off a thin, highly selective etch mask containing holes is formed (C). Next, the etch mask is divided into smaller sections by mask lithography (D). Local deposition of platinum is used to define defect structures of any shape in the photonic crystal slabs. Here, we demonstrate the introduction of a line defect (E). By RIE the structure is etched into the Si3N4 layer, after which Pt, Cr, and photoresist are removed (F). By a wet etch through the holes the underlying SiO2 layer is removed and the freestanding Si3N4 photonic crystal slabs are formed (G). Reprinted with permission from [99], L. Vogelaar et al., Adv. Mater. 13, 1551 (2001). © 2001, Wiley-VCH.

In the photonic crystal slabs Si3N4 (n = 2.16 at A = 670 nm) and air acted as high and low index material, respectively. A triangular lattice of air holes in the Si3N4 layer was fabricated in order to obtain a bandgap for TE modes (polarization in the plane of the crystal). By making the slab freestanding (i.e., embedded in air on both sides), we obtain the desired vertical mirror symmetry of the crystal slab. See Figures 45 and 46.

The advantage of LIL is that the entire periodic pattern is created at once. A high accuracy and flexibility of FIB deposition to introduce line defects in the extended crystals has been used. In principle, any defect is possible. The combination of LIL with FIB deposition opens avenues for more optical applications of photonic crystals. Due to its simplicity and scope, FIB-assisted LIL is a promising alternative for e-beam lithography in the fabrication of photonic materials. See Figure 47.

Alumina Photonic Crystals [106] Anodic porous alumina with an ideally ordered air-hole array with a 200 to 250 nm lattice constant exhibits a two-dimensional (2D) photonic bandgap in the visible wavelength region. The airhole preparation process was based on Al pretexturing by imprinting with a mold (cf. Fig. 66 in Section 4.3), followed by the anodization of Al in acid solution.

The depth limit of the ordered air holes is due to the disturbance of the arrangement of the air holes along with the growth of the oxide layer during anodization. For the application of 2D photonic crystals to a wide range of scientific and technological fields, an air-hole array with a higher aspect ratio is required. Anodization in a phosphoric acid solution under a constant voltage condition of 195 V yielded an air-hole configuration with a lattice constant of 500 nm. See Figures 48 and 49.

Figure 50 shows the typical transmission spectra of the anodic porous alumina with a 500 nm lattice constant. The filling factor of the air holes was 0.1. In Figure 50a, which presents the transmission spectra for H polarization light, distinct dips in the transmission were observed for both the J and X directions. The observed position and width were in good agreement with the theoretical prediction based on the plane-wave method for the triangular lattice of air holes in the alumina matrix [107].

Figure 45. Left: SEM image of triangular photoresist dot pattern with a period of 300 nm after laser interference lithography. The uniform pattern extends over an area of the order of a centimeter squared. Right: FIB image of an approximately 50 nm thick platinum line covering holes in a 20 nm thin chromium mask. The platinum line defines a defect structure in the photonic crystal slab and is deposited locally with a focused ion beam. Courtesy of Adv. Materials.

Figure 45. Left: SEM image of triangular photoresist dot pattern with a period of 300 nm after laser interference lithography. The uniform pattern extends over an area of the order of a centimeter squared. Right: FIB image of an approximately 50 nm thick platinum line covering holes in a 20 nm thin chromium mask. The platinum line defines a defect structure in the photonic crystal slab and is deposited locally with a focused ion beam. Courtesy of Adv. Materials.

Figure 46. Left: SEM micrograph of the end face of a photonic crystal slab between the support bars. This 146 nm thin slab is freestanding across 100 fxm and has a length of 4 mm. Right: SEM micrograph, close-up of the defect line in the photonic crystal slab. The highly uniform triangular hole pattern has a period of 297 nm. The axial widths of the elliptical holes are 212 and 164 nm. The shape of the holes appears unharmed by the presence of the defect line. The defect is expected to form a multimode waveguide for visible light (estimated central wavelength 670 nm). Right: Courtesy of Adv. Materials. Left: Reprinted with permission from [99], L. Vogelaar et al., Adv. Mater. 13, 1551 (2001). © 2001, Wiley-VCH.

perforations

Figure 46. Left: SEM micrograph of the end face of a photonic crystal slab between the support bars. This 146 nm thin slab is freestanding across 100 fxm and has a length of 4 mm. Right: SEM micrograph, close-up of the defect line in the photonic crystal slab. The highly uniform triangular hole pattern has a period of 297 nm. The axial widths of the elliptical holes are 212 and 164 nm. The shape of the holes appears unharmed by the presence of the defect line. The defect is expected to form a multimode waveguide for visible light (estimated central wavelength 670 nm). Right: Courtesy of Adv. Materials. Left: Reprinted with permission from [99], L. Vogelaar et al., Adv. Mater. 13, 1551 (2001). © 2001, Wiley-VCH.

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