ARTICLE

Received 10 Aug 2014 | Accepted 15 Jan 2015 | Published 24 Feb 2015 DOI: 10.1038/ncomms7299

Silicon-chip mid-infrared frequency comb

generation
Austin G. Griffith1, Ryan K.W. Lau2, Jaime Cardenas1, Yoshitomo Okawachi2, Aseema Mohanty1,
Romy Fain1, Yoon Ho Daniel Lee1, Mengjie Yu2, Christopher T. Phare1, Carl B. Poitras1,
Alexander L. Gaeta2,3 & Michal Lipson1,3

Optical frequency combs are a revolutionary light source for high-precision spectroscopy
because of their narrow linewidths and precise frequency spacing. Generation of such combs
in the mid-infrared spectral region (2–20 mm) is important for molecular gas detection
owing to the presence of a large number of absorption lines in this wavelength regime.
Microresonator-based frequency comb sources can provide a compact and robust platform
for comb generation that can operate with relatively low optical powers. However,
material and dispersion engineering limitations have prevented the realization of an on-chip
integrated mid-infrared microresonator comb source. Here we demonstrate a complementary
metal–oxide–semiconductor compatible platform for on-chip comb generation using silicon
microresonators, and realize a broadband frequency comb spanning from 2.1 to 3.5 mm. This
platform is compact and robust and offers the potential to be versatile for use outside the
laboratory environment for applications such as real-time monitoring of atmospheric gas
conditions.

1 School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14850, USA. 2 School of Applied and Engineering Physics, Cornell

University, Ithaca, New York 14853, USA. 3 Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA. Correspondence
and requests for materials should be addressed to M.L. (email: ml292@cornell.edu).

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7299

O
ptical frequency combs are coherent light sources spacing of the resonant cavity. In particular, this enables
consisting of discrete lines that are equally spaced in microresonator combs to be engineered with line spacings in
frequency. Mid-infrared comb sources in particular have the 20–400 GHz range. Microresonator-based frequency comb
proved promising for spectroscopy as their broad bandwidth and sources have been demonstrated in a number of platforms,
narrow frequency linewidths make them ideal for probing narrow including silica, quartz, fluoride glasses, silicon nitride,
molecular transitions1. Recently, mid-infrared frequency combs Hydex glass, aluminium nitride and diamond7–15. Mid-infrared
have been demonstrated in a number of platforms; however, the microresonator comb generation has been achieved in MgF2
realization of a robust on-chip integrated mid-infrared comb crystalline resonators, generating lines up to 2.55 mm (ref. 14). In
source has proven elusive. On-chip integration and particular, on-chip mid-infrared comb generation has not been
miniaturization of the mid-infrared comb are critical, as they realized because of the difficulty of creating a highly confined and
will enable high portability for stand-off atmospheric sensing out high-quality factor microresonator in semiconductor thin films.
in the field, and as well as monolithic integration with other Even though silicon, owing to its CMOS compatibility, wide
necessary components, such as resonant cavities for gas sensing transparency window and high third optical nonlinearity, is an
and photodetectors for measurements. In particular, a ideal platform for on-chip comb generation deep into the mid-
complementary metal–oxide–semiconductor (CMOS) compatible infrared, its linear and nonlinear losses have until now prevented
integrated mid-infrared comb source would be inexpensive and the realization of a silicon microresonator-based comb source.
straightforward for mass production. Mid-infrared combs have Silicon has a wide-transparency window from 1.2 mm to past
been previously demonstrated in platforms such as fibre lasers, 8 mm (ref. 16) and a large third-order optical nonlinearity
mode-locked lasers and optical parametric oscillators2–4, but these (n2 ¼ 10  14 cm2 W  1 at 2.5 mm wavelength17), which makes it
platforms are relatively bulky and cannot be integrated. an excellent platform for mid-infrared nonlinear optics. In etched
Supercontinuum generation represents another means for silicon microresonators, quality factors have been limited by
generating a broad spectrum in the mid-infrared and has been scattering losses because of roughness in the waveguide sidewalls,
realized in a number of platforms including silicon waveguides5. which is made worse by the high index contrast between
However, supercontinuum generation requires a high-peak power- waveguide core and cladding. The dominant nonlinear loss in
pulsed femtosecond source that can generate a broadband coherent silicon in the 2.2–3.3 mm region is three photon absorption
spectrum, and for many applications it is desirable to have comb (3PA)—a process where three photons are simultaneously
spacings much larger than the B100-MHz spacing typically absorbed to excite an electron-hole pair18. The number of
produced by such lasers, so that the individual comb lines can be photons lost directly to 3PA is small (dominated by linear
resolved. Another route to mid-infrared comb generation is waveguide losses), but the generated free-carrier population will
through the use of quantum cascade lasers6, but the active induce significant optical losses19.
materials used make on-chip integration difficult. Here we overcome both silicon’s high-linear and nonlinear
On-chip microresonator-based combs are promising because losses to demonstrate a proof-of-principle silicon microresonator
they can generate a broadband frequency comb in a compact and frequency comb source, and to show the fabrication techniques
robust integrated platform but the reach of microresonator combs necessary to achieve an on-chip integrated microresonator comb
into the mid-infrared has been limited. With a properly phase- source in the mid-infrared.
matched geometry, a frequency comb can be generated with a
high-quality factor microresonator using a single continuous
wave (CW) pump laser7. Using the parametric w(3) nonlinear Results
process of four-wave mixing, energy is transferred from the pump Design and fabrication of silicon resonators. We overcome
laser into frequency sidebands. Comb lines will be generated at silicon’s linear losses using a novel etchless fabrication process
modes supported by the microresonator and lead to an optical based on thermal oxidation to achieve a high-quality factor of
frequency comb with a spacing equal to that of the free spectral 590,000 in a silicon microresonator at a wavelength of 2.6 mm.

Ring resonator
1.0
Normalized transmission

12 pm
0.5
200 µm

1.4 µm
0.0
Metal contacts 2,588.685 Wavelength (nm) 2,588.720
1.4 µm

Aluminum Nitride mask Aluminum

contact contact
Silicon
P doped N doped

Figure 1 | Profile of the etchless silicon microresonator with integrated PIN diode. (a) Optical microscope image of a ring resonator and metal
contacts fabricated using the etchless process. (b) Normalized transmission spectrum taken at low-input power, showing an overcoupled resonance
with an intrinsic quality factor of 590,000. (c) Simulated optical mode at 2.6 mm, showing high-modal confinement in Si. (d) False coloured cross-sectional
SEM image of silicon waveguide, doped regions and metal contacts.

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7299 ARTICLE

The silicon waveguide is defined in an etchless manner using the spectral-temporal dynamics of the microresonator combs, we
thermal oxidation, instead of dry etching, to form the waveguide use a recently developed numerical approach23,24 based on the
core20,21. We deposit low-pressure chemical vapour deposition Lugiato-Lefever equation25,26. Here we modify the method to
silicon nitride on top of a 500-nm silicon-on-insulator wafer. The take into account multi-photon absorption and the free-carrier
silicon nitride serves as an oxidation mask. We then pattern and dynamics on the generated optical spectra. This adds additional
etch only the silicon nitride, and then thermally oxidize the wafer. loss terms (free-carrier and 3PA) to the Lugiato-Lefever equation,
The silicon underneath the nitride mask does not oxidize, leaving as well as a term for the dispersive effects of the free-carriers.
the silicon waveguide core intact. This method avoids the With the free-carrier population kept at low levels (10 ps lifetime),
roughness and absorption sites that can be introduced by
reactive ion etching22. This method is particularly useful here,
due to the need for a silicon slab for electrical integration. As the 15
slab surface is never etched, we avoid the high losses typically 2.25 µm filter 3 µm 3.25 µm 3.5 µm
associated with active silicon resonators. We characterize the filter filter filter
resonator quality factor by sweeping a narrow linewidth source 0
across the resonance, and measuring the transmission spectrum

Power (dBm)
using a 10-MHz photodiode. The transmission spectrum is –15
shown in Fig. 1b. The loaded quality factor is 220,000 and the
intrinsic one is 590,000, which corresponds to a propagation loss
of 0.7 dB cm  1. See Methods for full fabrication process. –30
To enable broadband comb generation, we engineer the
etchless waveguide geometry to have anomalous group velocity
–45
dispersion from 2.2 to 3 mm. The geometry of the silicon
waveguide governs the bandwidth of the frequency comb as the
waveguide cross-section determines its dispersion profile9,23. The –60
simulated group velocity dispersion is shown in Fig. 2a. We use
the commercial software Silvaco Athena to simulate the oxidation 2,200 2,400 2,600 2,800 3,000 3,200 3,400
process, to obtain the waveguide profile. From repeated tests, we Wavelength (nm)
have found good agreement between the simulated waveguide
profile and the actual etchless waveguide formed. The simulated Figure 3 | Mid-infrared optical frequency comb generation from an
bandwidth for a frequency comb generated in this geometry is etchless silicon microresonator. Broadband frequency comb generation
shown in Fig. 2b, for 150 mW of optical power in the bus from 2.1 to 3.5 mm in the etchless silicon micro-resonator. This frequency
waveguide and an assumed 10 ps free-carrier lifetime. To simulate comb is generated with 150 mW of optical power in the bus waveguide, and
with a 10-V reverse bias applied on the PIN junction. The frequency spacing
of the comb is 127 GHz. Owing to the limited dynamic range of the optical
50
spectrum analyser, the frequency comb was measured using a series of
optical filters.
ps nm–1 km–1

0
0 No carrier extraction
Power (dBm)

–20

–50
2,000 2,500 3,000 3,500 –40

Wavelength (nm)
2,400 2,450 2,500 2,550 2,600 2,650 2,700 2,750 2,800
0 Wavelength (nm)
Power spectrum (dB)

1 V applied
0
Power (dBm)

–25
–20

–40
–50
2,400 2,450 2,500 2,550 2,600 2,650 2,700 2,750 2,800
Wavelength (nm)
2,000 2,500 3,000 3,500
Figure 4 | Investigation of the effect of carrier extraction on frequency
Wavelength (nm)
comb generation. The effect of three photon absorption on frequency comb
Figure 2 | Dispersion engineering and simulated comb bandwidth for the generation depends strongly on the bias voltage of the PIN structure. The
silicon microresonator. (a) Group velocity dispersion for the etchless broad peaks at 2,510 and 2,685 nm are artefacts of the FTIR. (a) With the
silicon waveguide. We achieve broad anomalous dispersion from 2.2 to pump set at a fixed wavelength and the voltage source off, only a few lines
3.0 mm in a silicon waveguide with 1,400 nm  500 nm cross-sectional are generated near 2,500 and 2,700 nm at this detuning. (b) By applying
dimensions. (b) Simulated comb bandwidth with 150 mW of optical pump even a small voltage in reverse bias to the junction, we can generate
power in the bus waveguide, assuming a 10-ps free-carrier lifetime. hundreds of comb lines across the spectrum.

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms7299

simulations predict that generation of a coherent frequency comb In summary, we have demonstrated an on-chip integrated
in this geometry is possible (see Supplementary Note 1). mid-infrared frequency comb source using an etchless silicon
To mitigate silicon’s nonlinear loss, we embed the silicon microresonator and demonstrated parametric oscillation in a
microresonator in a reverse biased positive-intrinsic-negative silicon platform. We have shown electrical control of the
(PIN) doped junction to sweep out carriers generated from three- frequency comb profile by varying the carrier extraction voltage.
photon absorption. Significant free-carriers accumulate when The etchless silicon microresonator comb source operates with
pumping a passive silicon waveguide with a CW laser, limited low pump power well within the typical output of semiconductor
only by the natural free-carrier lifetime of the structure. Here, we laser diodes making it compatible with on-chip integration of the
counteract the carrier generation while using a CW pump by pump laser. An important next step is to experimentally
extracting the generated carriers using a PIN junction operated in investigate the noise and coherence properties of the generated
reverse bias27. The PIN junction prevents the electrical injection spectra30,31, as well as the effect of free-carrier population
of carriers into the waveguide while allowing generated free on those properties. This platform enables a versatile and
carriers to be swept out—with effective free-carrier lifetimes straightforward source for mid-infrared gas spectroscopy and
demonstrated as short as 12 ps (ref. 28). opens the door for compact and robust mid-infrared gas sensors.
The ability to achieve comb generation in an integrated platform
will enable practical realization of completely integrated
Generation of silicon mid-infrared frequency comb. We
mid-infrared comb-based spectrometers usable in a myriad of
demonstrate the generation of an optical frequency comb environments.
between 2.1 and 3.5 mm under reverse bias conditions. We use as
our source an Argos Model 2400 CW optical parametric oscil-
lator, which is tunable from 2,500 to 3,200 nm with a 100-kHz Methods
linewidth. We couple 1.2 W of optical power at 2.59 mm into a Device fabrication. A thermal oxidation-based ‘etchless’ fabrication process was
lens, which focuses the light into a silicon nanotaper on the chip. used to fabricate the devices. Using a commercial silicon-on-insulator wafer with a
500-nm silicon layer and a 3-mm buried oxide, 200 nm of silicon nitride was
The input coupling loss is roughly 9 dB, leading to 150 mW of deposited using low-pressure chemical vapour deposition. Electron beam litho-
optical power coupled into the bus waveguide on the chip. By graphy was used to pattern the nitride layer, using ma-N-2405 electron-beam
tuning into resonance from the short wavelength side, we achieve resist. The silicon nitride mask was patterned 1.4 mm for the waveguide layer. The
a soft thermal lock29, where cavity heating is compensated by ring resonator is 100 mm bending radius, with a 1.2-mm-long straight section at the
coupling region. The gap between the waveguide and ring is 500 nm. After
diffusive cooling to maintain constant power in the resonator. exposure and development, the silicon nitride was etched in fluorine chemistry.
Once on resonance, we extract 2.7 mA of current, using 10 V of The wafer was then thermally oxidized at 1,200 °C to form the etchless waveguide
reverse bias applied to the PIN junction. This relatively high structure. The wafer was oxidized for 20, 50 and 60 min of dry, wet and dry
current is an indication that three-photon absorption is oxidation, respectively. The slab in the taper region was then etched away. The
significant at these power levels. We collect the output light taper was patterned to be 350 nm wide. The wafer was then clad with 2 mm of
plasma-enhanced chemical vapour deposition silicon dioxide. The oxide was then
from the nanotaper using another lens, and couple this light to a etched away next to the waveguides, and then the slab was doped with boron and
Fourier transform infrared spectrometer (FTIR). We use a phosphorous on either side to form a PIN diode. Finally, the wafer was metalized,
Thorlabs OSA205 FTIR for our measurements. The generated and wires were patterned and etched.
frequency comb has a uniform line spacing of 127±2 GHz, well
below the 7.5-GHz resolution limit of the FTIR. This corresponds
Free-carrier lifetime measurement. For carrier lifetime measurements, we per-
well with the expected free spectral range of the resonator. form pump-probe experiments with a counter-propagating pulsed pump and CW
We measure an oscillation threshold with a low power of probe. An Erbium-doped fibre laser (1,550 nm, 500 fs full-width half-maximum) is
3.1±0.6 mW in the bus waveguide, consistent with a simulation amplified to a peak power of B200 W in an erbium-doped fibre amplifier, then
predicted threshold on the order of 10 mW. To resolve the full coupled via a circulator into one facet of the chip. A CW probe (1,575 nm) is
coupled into the opposite facet and appears on the third port of the circulator,
extent of the frequency comb and overcome the limited dynamic isolated from the pump. Neither laser overlaps with a ring resonance; we measure
range for the FTIR, we filter the output lines using a series of only the bus waveguide. We further improve probe isolation by wavelength filtering
optical bandpass filters. These filters attenuate the pump and the the third port output with a grating filter, then route this fibre to an optical
central part of the comb, allowing us to reach the noise floor of sampling oscilloscope. The oscilloscope is triggered off the pump laser’s fast
sampling photodiode, giving us a time-domain measurement of the transmission of
the FTIR. The comb spectrum as shown in Fig. 3 is then corrected the waveguide after strong pump excitation. Absorption increases sharply when the
for the spectral profile of the filters. pump-generated free-carriers flood the waveguide, then exponentially decreases as
they recombine. We take the time constant of this exponential to be the free-carrier
lifetime.
Discussion
We observe a strong influence on the comb spectral shape from
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