An all-silicon passive optical diode.

Abstract

A passive optical diode effect would be useful for on-chip optical information processing but has been difficult to achieve. Using a method based on optical nonlinearity, we demonstrate a forward-backward transmission ratio of up to 28 decibels within telecommunication wavelengths. Our device, which uses two silicon rings 5 micrometers in radius, is passive yet maintains optical nonreciprocity for a broad range of input power levels, and it performs equally well even if the backward input power is higher than the forward input. The silicon optical diode is ultracompact and is compatible with current complementary metal-oxide semiconductor processing.

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www.sciencemag.org/cgi/content/full/science.1214383/DC1




Supporting Online Material for

An All-Silicon Passive Optical Diode
Li Fan, Jian Wang, Leo T. Varghese, Hao Shen, Ben Niu, Yi Xuan, Andrew M. Weiner,
Minghao Qi*


*To whom correspondence should be addressed. E-mail: mqi@purdue.edu


Published 22 December 2011 on Science Express
DOI: 10.1126/science.1214383


This PDF file includes:
Materials and Methods
Table S1

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Materials and Methods
Sample Fabrication

We fabricated the microrings on a silicon-on-insulator wafer (from SOITEC) with a
250 nm-thick top silicon layer and 3 μm of buried oxide. The single-crystalline Si
microrings and waveguides have a rectangular cross-section of 250 nm in thickness and
500 nm in width, which supports a low-loss single-mode quasi-transverse magnetic (TM)
mode for the filters at near infrared. The device was patterned with high resolution
electron-beam lithography (Vistec VB6) which has a beam step size of 2 nm. The diode
was formed after reactive-ion etching with chlorine/argon gas mixture in an inductive-
coupled plasma tool. No cladding was applied over the silicon waveguides or microrings.
Titanium micro-heaters with 5.3 kΩ resistance were evaporated on top of the buried
dioxide next to only the micro-ring resonator in the NF. Fabricated chip was manually
cleaved and has a width ~5 mm. No polishing or other treatments were applied to the
facets.

The key prerequisite for our optical diode is the matching of the resonant
wavelengths of the two high-Q filters when they are operating in linear regime, i.e. with
very low incident power. It is well known that as-fabricated high-Q microrings cannot
match exactly in their resonant wavelengths due to limited precision in nanofabrication.
We targeted the radii of the two microrings in the NF and ADF at 5 μm and 5.002 μm,
respectively. In most cases, this will make the resonant wavelength of the NF slightly
shorter than that of the ADF. A titanium micro-heater was then placed to the side of the
microring in the NF so that we can red-shift the resonant wavelength of the NF to match
that of the ADF, through thermo-optic effect (19) of silicon. Compared to the
conventional way of placing the heater above the microring and cladding, depositing the
heater to the side of the microring reduces the heating efficiency. However, it preserves
the high Q and the thermal isolation of the microrings, which are critical for low-power
optical nonreciprocity.

Experimental Setup and Measurement Details

A continuous-wave tunable laser source with 1 pm resolution was guided into the
device with the help of single mode tapered lensed fiber and the output was coupled out
using another single mode tapered lensed fiber and fed into an optical power meter. The
fibers were position on xyz nanopositioned stages and butt coupled to the waveguides on
either facets of the device. Even though the fabricated device and method of coupling has
a rather high insertion loss of ~21.4 dB due to the facets, no amplifiers were used since
optical nonreciprocity was achieved at low power. A fiber-based polarization controller
was used to control the polarization of light input into the device to obtain maximum
extinction of the NF. A fiber based variable power optical attenuator was used to control
the amount of optical power fed to the device.

To tune the resonance of the NF to the ADF, the microheater to the side of the NF
was heated using a constant voltage source so that the dip of the NF overlaps the peak of

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the ADF. Forward and backward spectra were obtained by switching the input and output
fiber connectors. Two scan modes were used to fetch the spectra: continuous-mode scan
where the laser sweeps from the beginning wavelength to the end wavelength without
stops while the power meter takes a moving average of the received optical power, and
stepped-mode scan where the laser goes to each wavelength and stops while the power
meter averages for a reasonably long time.

For point measurements, the wavelength was fixed at the desired operating
wavelength, which typically is the resonance wavelength of the NF in the backward
direction, and transmitted power was taken 10 seconds after the laser was turned on. The
standard deviation of the nonreciprocal transmission ratio includes contributions from
both the forward and backward transmissions and a Fabry-Perot uncertainty of ±0.4 dB.
Table S1.
Optical diode performance for different laser source power levels at 1630.011 nm.

Laser source
power
(dBm)
(μW)
power (dBm)
5 ~270 -49.8 ± 1.5
10 ~850 -51.7 ± 1.5
Input power
at the diode
Backward
transmitted
Forward
transmitted
power (dBm)
-27.5 ± 1
-24.8 ± 1
Nonreciprocal
transmission
ratio (dB)
22.3 ± 1.8
26.9 ± 1.8
14 ~2,100 -52 ± 1.5 -23.6 ± 1 28.4 ± 1.8

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Numerical Simulation

A model based on coupled mode theory (21, 22) was developed to describe the
nonlinear response of the side coupled microring resonator. Nonlinear effects considered
include third order nonlinearity χ(3), such as two-photon absorption(TPA) and Kerr effect;
free carrier effect (FCE); and thermal effect resulted from Joule heating from TPA
process, FCE and linear absorption process. In our case, the thermal effect dominates all
other effects and caused the red-shift of the resonance at elevated temperature. With
various coefficients taken from literature, including an estimated thermal dissipation time
of 2 μs (21-23), as well as effective nonlinearity volumes calculated accurately through
finite-difference time domain (FDTD) simulation (20, 24), we can simulate the nonlinear
responses of the NF and ADF using MATLAB.



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