2 nonlinear optics - Harvard University

http://mazur.harvard.edu

Coupling padsWe can optimize for insertion losses by fabri-cating low index polymer coupling pads (top). Using an adiabatic taper, we can in-crease our waveguide input coupling e�-ciency by 30% by implementing a coupling pad (bottom).

Waveguide width (nm)

Pow

er c

oupl

ing

300 400 500 600

1

0.8

0.6

0.4

0.2

0

with coupling padwithout coupling pad

TiO2

SiO2

coupling padLaser

Dispersion engineeringNonlinear e�ects cause ultrafast pulses to broaden, disperse and thus attenuate. By tuning the wave-guide geometry, we obtain anoma-lous group velocity dispersion which directly counters these ef-fects and produces optical solitons.

100 200 300 400 500 600 700 800500

0

500

1000

1500

Waveguide width [nm]

2[fs2 /m

m]

Film thickness

1000 nm900 nm800 nm700 nm600 nm500 nm

Input7 pJ

28 pJ46 pJ

wavelength (nm)

norm

aliz

ed p

ower

(dB)

740 780 820 860

10

0

–10

–20

–301450 1500 1550 1600 1650 1700

Input60 pJ

120 pJ231 pJ

wavelength (nm)

A short pulse in a nonlinear waveguide generates new wave-lengths. Here, we show spectral broadening at both 800 nm (left), 1550 nm (middle), and green-light from 1550 nm pulses (right).

Nonlinear properties

input 1550 nm generated

green light

Smooth features are important to achieve low propagation losses in photonic waveguides. We achieve the most consistent etch by using chromium as an etch mask compared to aluminum oxide (left). We also obtain smoother features by using ZEP as a resist in-stead of PMMA (right).

Fabrication materials

500 nm 1 μm

PMMA ZEPAl2O3 Cr

etch mask comparison e-beam resist comparison

Fabrication steps

SiO2

silicon wafer

begin with silicon wafer with thermal oxide film

deposit thin TiO2 film using reactive sputtering

spin on e-beam resist layer

deposit thin metal film using electron beam evaporation

immerse resist in developer to remove exposed regions

write pattern into resist using electron beam lithography

lift-off metal film by dissolving resist

etch through TiO2 film using reactive ion etching

remove remaining metal leaving TiO2 waveguides

TiO2

400 nmλ = 633 nm

300 μm

Once fabricated, TiO2 photonic waveguides have trapezoidal cross sections (left) and are capable of guiding visible light (right).

WaveguidesOverviewTiO2 is a widely transparent, highly nonlinear novel photonic ma-terial that has potential applications in all-optical switching and other nonlinear processes. In order to best exploit it’s large nonlin-earity, we have optimized our fabrication process and have simu-lated design parameters that aid in producing and maintaining large pulse intensities within integrated photonic structures.

EricMazur

OradReshef

ChristopherEvans

JonBradley

SarahGriesse-Nascimento

Harvard University, Cambridge, MAMassachusetts Institute of Technology, Cambridge, MA

Maximizing intensity in TiO2 waveguides fornonlinear optics