All-optical control of light on a silicon chip

1. Vilson R. Almeida, Carlos A. Barrios, Roberto R. Panepucci & Michal Lipson School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA All-optical control of light on a silicon chip

2. Silicon • Dominant material in the microelectronic industry • Challenging to achieve all-optical switch in silicon • Weak nonlinear optical properties • Extremely high powers • Large dimensions • Fast all-optical switching on silicon • Highly light-confining structures to enhance sensitivity to refractive index change • 500ps switch • 25pJ pulses

3. Demonstrated modulation • 300μm long, 1.55μm Mach-Zehnder modulator • 2mJ optical pump pulse energy needed • Δn = -10-3 for 100% modulation • Free carrier absorption • Rectangular waveguide • 450x250nm • 16dB cm-1 absorption for 2mJ optical pulse • 90% modulation depth requires 600μm waveguide

4. Highly confined resonant structures • Low-power light modulation • Δn large effect on transmission response • Modulation depth of 80% in 20μm long structure Ring resonator: 10μm diameter, 450x250nm cross-section

5. Transmission of ring resonator coupled to a waveguide • Greatly reduced at circumference corresponding to integral number of guided wavelengths • 10-ps pump pulse used to inject free carriers through two-photon absorption, tuning the effective refractive index Quasi-TM transmitted spectral response

6. Resonances of the ring resonator λres1 = 1535.6nm λres2 = 1555.5nm Qres1 ≈ λres1/ΔλFWHM1 = 3410 Qres2 ≈ λres2/ΔλFWHM2 = 2290 ΔλFWHM1 = 0.45nm ΔλFWHM2 = 0.68nm Fast temporal response τcav1 = λ2res1/2πcΔλFWHM1 = 1.8ps τcav2 = λ2res2/2πcΔλFWHM2 = 2.8ps

7. Pump • 10-ps pulses with energy less then 25 pJ • Tunable mode-locked optical parametric oscillator pumped by Ti:sapphire picosecond laser at 78-MHz • 1.5-ps pulses pass through Fabry-Perot tunable filter

8. Probe signals Probe 1 below resonance Probe 2 on resonance Probes around: λres1 = 1535.6nm λprobe1 = 1535.2 nm λprobe2 = 1535.6 nm Probes tuned relative to ring resonance in order to maximize modulation depth by setting transmission to low and high levels without pump

9. Temporal response of probe signal to pump excitation Instantaneous spectral shift followed by exponential decay representing free-carrier lifetime Shift: Δλ = -0.36 Relaxation time: τ = 450ps Free-carrier lifetime can be decreased by controlling surface passivation or ion implantation Using pump time much smaller than free-carrier lifetime leaves necessary pump power unchanged

10. Modulation depth (MD) MD = (Imax – Imin)/Imax Imax : Maximum transmitted probe optical power Imin : Minimum transmitted probe optical power MDprobe1 = 94% MDprobe2 = 91%

11. Modulation • Δλ corresponds to a Δneff = -4.8 x 10-4 • Δneff is caused by a free carrier concentration of: ΔN = 1.6 x 1017 cm-3 • Required energy for ΔN is 0.15pJ, other energy of pump scattered • Absorption losses: • Δα = 6.9 cm-1 • αring = 33.6 cm-1 • Low absorption losses indicate modulation due to index change

12. Uses • modulator, switch or router with response as low as 100ps • router: couple ring to two waveguides • input port and through port waveguide • drop port waveguide

13. Control of modulation by fabrication • Minimize temperature effects • induce strain in the silicon waveguide • overcladding deposition conditions • decrease of refractive index with temperature, balancing thermo-optic effect of silicon • Decrease wavelength sensitivity • minimize size of ring • low round trip loss due to high index difference • would require larger Δneff, but smaller size requires similar pump power to obtain higher ΔN