Visible Photonic Band Gap Waveguide Devices
- PDF / 4,376,413 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 15 Downloads / 217 Views
Mat. Res. Soc. Symp. Proc. Vol. 486 01998 Materials Research Society
as guided mode suppression to the PBG rather than more common optical effects such as scattering or diffraction. Many real applications will also require low loss transmission at wavelengths away from the stop band. To date, there has been very little investigation into the transmission properties of these devices. This is an issue which we start to address here.
Top view Rows of holes written as a strip across waveguide
Cross section
Small PBG area etched completely through the Oxide cover layer (75nm) Nitride core (250nm) Oxide buffer layer (1.81im) Silicon substrate
Figure 1: Waveguide geometry. DESIGN Our devices consist of several rows of air pores arranged on a triangular lattice, plasma etched through the layers of a blanket silicon nitride waveguide structure. The lattice pitch was either 300nm or 260nm. Our experimental approach is to end-fire couple light into the waveguide, and examine the transmission properties of the lattice region. In the design of the devices, we use a threedimensional plane-wave analysis20 '2 ' in combination with conventional waveguide theory to ensure the existence of guided Bloch modes 22 at wavelengths either side of the band gap. This design method is explained in detail in ref [12]. FABRICATION Our waveguide consists of a thermally grown, 1.8p.tm thick silicon dioxide substrate buffer layer (n= 1.46), a 250nm thick silicon nitride core (n=2.02), and a 75nm thick silicon dioxide cladding layer. The core and cladding layers were both deposited by Low Pressure Chemical Vapour Deposition (LPCVD). The wafers were patterned by direct write electron beam lithography using UVIII resist, which was then plasma etched to create narrow isotropic pores (fig 1). Surprisingly, very small pores arranged on a triangular lattice of pitch 260nm, with diameters in the range 50-120nm are easily created by controlling the lithography alone. However, this is still too small to create a reasonably big photonic band gap. If we direct-write wider pores, surface tension in the resist causes the pattern to be obliterated upon development. Our solution to this problem is to initially etch very narrow pores, then expand them up in the nitride core layer, using a nitride selective etch. This also allows us to fine tune the photonic band gap after initial fabrication. (SEMI). Unlike a conventional diffraction grating, the dimensions of the pores are below the Rayleigh resolution limit at wavelengths close to the stop band, and so cannot be properly resolved by the 23 propagating waves . Consequently, an average index approach can be used to give a good 88
SEM 2: Porous honeycomb of silicon dioxide provides support for the waveguide core (lower right). SEM 1: Cross section through waveguide structure. (pitch=260nm, diameter=-150nm)
SEM 3: Cross section through a totally isolated bridge waveguide structure.The oxide buffer layer has been removed beneath the lattice region creating a large air cavity. (pitch=300nm, pore diameter=- 15
Data Loading...
