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Enhanced optical nonlinearities in ultra-silicon-rich photonic crystal waveguides

Authors:
  • Xiamen University Malaysia
JTu3A.35.pdf Frontiers in Optics 2017 © OSA 2017
Enhanced optical nonlinearities in ultra-silicon-rich
photonic crystal waveguides
Ezgi Sahin1,2,4, Kelvin J. A. Ooi1, George F. R. Chen1, Doris K. T. Ng3, C. E. Png2 and Dawn T. H. Tan1, *
1Photonics Devices and Systems Group, Engineering Product Development, Singapore University of Technology and Design (SUTD), 8 Somapah
Road, 487372, Singapore
2Department of Electronics and Photonics, Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR), 1
Fusionopolis Way, #16-16 Connexis, 138632, Singapore
3Data Storage Institute, Agency for Science, Technology & Research (A*STAR), 2 Fusionopolis Way #08-01 Innovis, Singapore 138634
4ezgi_sahin@mymail.sutd.edu.sg
*dawn_tan@sutd.edu.sg
Abstract: Photonic crystal waveguides (PhCWs) are realized on ultra-silicon-rich nitride platform
which possesses negligible nonlinear losses. An effective nonlinear parameter of 1.97×104(Wm)-1
is estimated from self-phase modulation on PhCW of length 96.6 µm.
OCIS codes: (130.5296) Photonic crystal waveguides; (160.4330) Nonlinear optical materials
1. Introduction
Nonlinear processes allow implementation of ultrafast all-optical signal processing, however nonlinear losses such
as two-photon absorption and free carrier absorption degrade the photon efficiency. To address this, material
platforms such as chalcogenide glasses [1], Hydex glass [2], stoichiometric silicon nitride [3] and silicon rich
nitrides [4] were studied. CMOS-compatible ultra-silicon-rich nitride (USRN) platform [4] possess negligible
nonlinear losses as well as a large Kerr nonlinearity of 2.8×10-13cm2/W and a refractive index of 3.1. Here, we
implement PhCWs, for further enhancement of nonlinearities, on the USRN platform. Effective nonlinear
parameters in excess of 104(Wm)-1 were achieved on a PhCW as short as 96.6µm.
2. Design and fabrication
PhCWs were designed using plane wave expansion method to simulate the group index curves as seen in Fig. 1 (a)
where the first two rows of the photonic crystal were shifted to engineer the group index curve [5]. From the
simulations, for a lattice constant, a=420 nm, a flat band region from 1523 nm to 1540 nm is observed for r=135 nm
for row shifts of s1=30 nm and s2=0. The calculated group delay-bandwidth product, denoted by ng ×∆λ/λ is as high
as 0.28, where ng=25, bandwidth Δλ=17 nm and wavelength λ=1532 nm.
Fig. 1. (a) Simulated group index curves for flat band slow light where lattice constant a=420 nm and row shifts are s1=30
nm and s2=0, insets show the mode profiles from the top and the propagation direction respectively, for radius of 135 nm.
(b) Scanning electron micrograph of the fabricated device before the deposition of SiO2 cladding, showing parameters
radius r, lattice constant a, and, row shifts s1 and s2.
Guided by the simulation results, devices with radii ranging from 110 nm to 150 nm were fabricated to account
for possible fabrication bias (+/-20nm). The fabrication process starts with inductively coupled chemical vapor
JTu3A.35.pdf Frontiers in Optics 2017 © OSA 2017
deposition of 300nm of USRN on a silicon substrate with 3μm buried oxide, followed by electron-beam patterning
of the photonic crystal waveguide using inductively coupled plasma etching. 2μm of SiO2 upper cladding is then
deposited using atomic layer deposition and plasma enhanced chemical vapor deposition. The fabricated device,
before cladding deposition is shown in Figure 1(b).
3. Results
Linear characterization of the PhCWs was performed using a broadband source with a wavelength range of 1520 nm
to 1610 nm. The light was adjusted for TE polarization before coupling into the PhCWs. Transmission
characteristics of the fabricated devices are shown in Fig. 2. (a). These measurements show the band edges to be at
1532 nm, 1568 nm and 1600 nm for devices with parameters r=148 nm s1=30 nm s2=0, r= 150 nm s1= 45 nm s2=5
nm and r=138 nm s1=30 nm s2=0, respectively.
Fig. 2. (a) Simulated group index curves for flat band slow light where lattice constant a=420 nm and row shifts are s1=30
nm and s2=0, insets show the mode profiles from the top and the propagation direction respectively, for radius of 135 nm.
(b) Scanning electron micrograph of the fabricated device before the deposition of SiO2 cladding, showing parameters
radius r, lattice constant a, and, row shifts s1 and s2.
We expect tight modal confinement of PhCWs to further increase the effective nonlinear parameter of 550(Wm)-
1 achievable in non-slow light enhanced USRN waveguides [4]. Therefore, nonlinear characterization was carried
out using 1.9 ps pulses at a repetition rate of 20 MHz generated by a mode-locked fiber laser. Total loss from the
access waveguide, coupling loss [6] and insertion loss [7] were taken into account in calculating the input peak
power. Kerr-induced phase shift of the PhCW possessing a band edge at 1532 nm is demonstrated in Fig. 2 (b) for
different input peak power values. At input peak power of 2.5 W, a phase shift of 1.5 π is observed. From this result,
the effective nonlinear parameter is calculated to be 1.9×104(Wm)-1 from γeffφNL/Pin L. Compared to the non-slow
light enhanced USRN waveguides, we see a scaling by 35 with a PhCW of length 96.6 µm. Since the physical
length of the PhCW is very short, it should be noted that the effective length is nearly identical to the physical length
of PhCW [8].
Design of PhCWs on USRN platform using plane wave expansion method, fabrication, linear characterization as
well as nonlinear characterization of devices were performed. Enhancement of nonlinearities on high nonlinear
figure of merit USRN platform were demonstrated. Tight modal confinement of the PhCW structure enables the
observation of self-phase modulation effects at powers of 2.5W and ultra-short lengths under 100μm.
4. References
[1] B. J. Eggleton, B. Luther-Davies and K. Richardson, Chalcogenide photonics”, Nat. Photon. 5, 141148 (2011).
[2] D. J. Moss, R. Morandotti, A. L.Gaeta, and M. Lipson, New CMOS-compatible platforms based on silicon nitride and Hydex for nonlinear
optics”, Nat. Photon., 7, 597607 (2013).
[3] J. S. Levy, A. Gondarenko, M. A. Foster, A. C. Turner-Foster, A. L. Gaeta and M. Lipson, CMOS-compatible multiple-wavelength oscillator
for on-chip optical interconnects”, Nat. Photon., 4, 37 - 40 (2010).
[4] T. Wang, D. K. T. Ng, S.-K. Ng, Y.-T. Toh, A. K. L. Chee, G. F. R. Chen, Q. Wang, and D. T. H. Tan, Supercontinuum generation in
bandgap engineered, back-end CMOS compatible silicon rich nitride waveguides”, Laser & Photonics Rev., 9: 498506 (2015).
[5] J. Li, T. P. White, L. O’Faolain, A. Gomez-Iglesias, T. F. Krauss, Systematic design of flat band slow light in photonic crystal waveguides”,
Opt. Express, 16, 9 (2008).
[6] K. J. A. Ooi, D. K. T. Ng, T. Wang, A. K. L. Chee, S. K. Ng, Q. Wang, L. K. Ang, A. M. Agarwal, L. C. Kimerling and D. T. H. Tan,
Pushing the limits of CMOS optical parametric amplifiers with USRN:Si7N3 above the two-photon absorption edge”, Nat. Comm. 8, 13878
(2017).
JTu3A.35.pdf Frontiers in Optics 2017 © OSA 2017
[7] Y. A. Vlasov and S. J. McNab, Coupling into the slow light mode in slab-type photonic crystal waveguides”, Opt. Lett., 31, 1, (2006).
[8] G. Agrawal, Nonlinear Fiber Optics, (The Institute of Optics University of Rochester Rochester, NewYork, 2013).
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