Figure 1 documents the variation in the bandgap energy with
linear refractive index of ICP-CVD-grown silicon-rich nitride
films. As expected from Kramers–Kronig relations and inverse
bandgap scaling laws [53], the measured film bandgap de-
creases as the refractive index increases. A similar phenomenon
can be observed in Fig. 2(b), where the absorption spectrum
of PECVD-grown SRN films is observed to shift to a lower
bandgap energy as the silicon content increases (i.e., refractive
index increases).
C. Design Flexibility for Dispersion Engineering
Engineering for anomalous dispersion has now come to be a
routine for a large number of nonlinear optics applications, in-
cluding optical pulse compression [54–58] and supercontinuum
generation [17,23,59–61]. Consequently, the ability for a plat-
form to provide flexibility in engineering dispersion of varying
signs and magnitude is of great merit. In general, a larger value
of Δn (where Δn n
core
− n
cladding
, where n
core
and n
cladding
re-
fer to the refractive index of the waveguide core and cladding
material, respectively) allows for a greater dynamic range of
dispersion engineering to be achieved. The ability to tailor re-
fractive indices anywhere between n 2.2 and n 3.1 is also
an interesting feature that might allow an additional degree of
freedom in dispersion engineering of waveguides and optical
nanostructures. Indeed, various groups have leveraged tailored
waveguide geometries to achieve dispersion profiles that facili-
tate the nonlinear optical phenomenon under study. Both low-
temperature ICP-CVD-grown films [25] and high-temperature
LPCVD-grown films [24] have been shown to allow for wave-
guides to be dispersion-engineered according to the needs of the
nonlinear optics applications (Fig. 4). In ICP-CVD-grown
ultra-silicon-rich nitride (USRN) films, dispersion engineering
was demonstrated through the tailoring of waveguide geom-
etries. Fourth-order dispersion, which is important for paramet-
ric processes, particularly when the pump is located very close
to the zero-dispersion wavelength, was also demonstrated [25].
In LPCVD-grown silicon-rich nitride films [24], waveguides
have been engineered to possess two zero-dispersion wave-
lengths for the facilitation of supercontinuum generation.
3. NONLINEAR OPTICS APPLICATIONS
The interest in silicon-rich nitride for nonlinear optics is fueled
by its potential for larger nonlinear parameters compared to
stoichiometric silicon nitride, while maintaining a negligible
two-photon coefficient at telecommunications wavelengths.
Early work involving the characterization of achievable nonlin-
ear parameters in silicon-rich nitride devices leveraged both
temporal and spectral measurements. Spectral measurements
involving self-phase modulation experiments in silicon-rich ni-
tride films have previously been used for characterizing the
nonlinear refractive index [18]. Films with a linear refractive
index of 3.1 were observed to possess a nonlinear refractive in-
dex of 2.8 × 10
−13
cm
2
∕W. Using resonance shifts within ring
resonators, Lin et al. extract the nonlinear refractive index
of silicon-rich nitride films with a linea r refractive index
of 2.49 to be 1.6 × 10
−13
cm
2
∕W [20]. Lower-index films
(n 2.1) have been shown to possess a nonlinear refractive
index of 6 × 10
−15
cm
2
∕W [19]. More recently, PECVD-
grown SRN films with a linear refractive index of 2.7 were
characterized to possess a nonlinear refractive index of 2.0 ×
10
−14
cm
2
∕W [28], whereas LPCVD-grown SRN films with
Fig. 3. Refractive index of PECVD-grown silicon-rich nitride films
as the N:Si ratio is varied. Films with higher silicon content result in
larger refractive indices. From Ref. [28].
Fig. 4. (a) Calculated second-order (β
2
) and fourth-order (β
4
) dispersion of ICP-CVD-grown USRN waveguides for different waveguide
widths (W) for a fixed height of 300 nm. (b) Calculated nonlinear parameter of the USRN waveguides as a function of wavelength for various
waveguide widths (W) and fixed height of 300 nm.
Review
Vol. 6, No. 5 / May 2018 / Photonics Research B53