WAVES AND VIBRATIONS IN INHOMOGENEOUS STRUCTURES ...

nanoimprinted patterns are transferred into the top 340 nm thick silicon layer of the SOI wafer
by using an optimized SF6-based inductively coupled plasma (ICP) RIE. The etch-selectivity
is 9:1 (silicon:mr-I T85) [24] which allows for pattern transfer of the imprinted holes through
the device silicon layer of the SOI wafer.
3. Straight photonic crystal waveguides
Figure 1 (left) shows a scanning electron microscope (SEM) image of the central part of a
NIL fabricated SOI device consisting of a W1 PhCW connected to ordinary tapered ridge
waveguides. The photonic crystal part of the waveguide is 10 μm long, the pitch of the
hexagonal crystal lattice 400 nm, and the hole diameter 250 nm. The width of the ridge
waveguides are adiabatically tapered over 450 μm from 4 μm at the end facets of the sample
down to 1 μm at the interface to the crystal waveguide. The etch patterns seen to the right and
left of the PhCW are caused by the controlled flow of excess polymer during the imprint
process. The excess polymer is a result from the large variation in pattern density between the
PhCW area and the surrounding un-patterned regions. The polymer flow does not represent an
issue in the fabrication of high-quality photonic circuits with more complex design. The
excess polymer flow can easily be controlled by adding dummy structures to equalize the
pattern density. Also, the fabrication of more complex and/or high-density photonic circuits
will typically reduce the variation in the pattern density, and thus simplify the control of the
polymer flow.
The fabricated waveguides have been characterized by optical transmission
measurements using quasi-transverse electric (TE) polarized light from a laser source in the
wavelength region from 1520–1620 nm (ANDO AQ4321D) and broadband light emitting
diodes (ANDO AQ4222) covering the wavelength range 1360–1620 nm. Figure 1 (right)
shows the resulting laser transmission spectrum. The spectrum exhibits the characteristics of a
W1 PhCW having a sharp and well-defined transition (around 1590 nm) between the low-loss
guided defect mode and the photonic band gap. The observed sharp cut-off and the high and
uniform transmission level below the cut-off wavelength of the spectrum are similar to results
obtained for PhCWs of similar designs fabricated by EBL [17] and DUVL [6]. The ripples in
the spectrum (zoom-in shown in the inset) are due to Fabry-Pérot oscillations caused by
reflections from the end facets of the sample.
NORMALISED TRANSMISSION (dB)
0
-10
-20
-30
0
-5
-10
1585 1588 1591
-40
1520 1540 1560 1580 1600 1620
WAVELENGTH (nm)
Fig. 1. (Left) SEM image of a photonic wire adjacent to a 10 μm long W1 PhCW fabricated in
SOI by NIL. The etch patterns seen on the outer sides are caused by the controlled flow of
excess polymer during the imprint process. (Right): Measured transmission spectrum for quasi-
TE polarized laser light through the structure. Inset shows a zoom-in on the spectrum.
4. Topology optimized nanophotonic devices
Recently, we have proposed a novel inverse design strategy called topology optimization
(TO), which allows for designing nanophotonic structures with enhanced functionalities [25].
In some cases, this inverse design method proposes optimized designs with feature sizes down
to ~30 nm. Hence, such structures are very challenging to fabricate even with EBL and will
serve as excellent benchmarks for pattern replication fidelity in the NIL fabrication process.
#76773 - $15.00 USD Received 6 November 2006; revised 18 January 2007; accepted 19 January 2007
(C) 2007 OSA 5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1264