# Publications

2017
Yu, Mengjie, Jae K Jang, Yoshitomo Okawachi, Austin G Griffith, Kevin Luke, Steven A Miller, Xingchen Ji, Michal Lipson, and Alexander L Gaeta. “Breather soliton dynamics in microresonators.” Nature Communications 8 (2017): 14569. Publisher's Version Abstract
The generation of temporal cavity solitons in microresonators results in coherent low-noise optical frequency combs that are critical for applications in spectroscopy, astronomy, navigation or telecommunications. Breather solitons also form an important part of many different classes of nonlinear wave systems, manifesting themselves as a localized temporal structure that exhibits oscillatory behaviour. To date, the dynamics of breather solitons in microresonators remains largely unexplored, and its experimental characterization is challenging. Here we demonstrate the excitation of breather solitons in two different microresonator platforms based on silicon nitride and on silicon. We investigate the dependence of the breathing frequency on pump detuning and observe the transition from period-1 to period-2 oscillation. Our study constitutes a significant contribution to understanding the soliton dynamics within the larger context of nonlinear science.
Ji, Xingchen, Felippe AS Barbosa, Samantha P Roberts, Avik Dutt, Jaime Cardenas, Yoshitomo Okawachi, Alex Bryant, Alexander L Gaeta, and Michal Lipson. “Ultra-low-loss on-chip resonators with sub-milliwatt parametric oscillation threshold.” Optica 4, no. 6 (2017): 619. Publisher's Version Abstract
On-chip optical resonators have the promise of revolutionizing numerous fields including metrology and sensing; however, their optical losses have always lagged behind their larger discrete resonator counterparts based on crystalline materials and flowable glass. Silicon nitride (Si3N4) ring resonators open up capabilities for optical routing, frequency comb generation, optical clocks and high precision sensing on an integrated platform. However, simultaneously achieving high quality factor and high confinement in Si3N4 (critical for nonlinear processes for example) remains a challenge. Here, we show that addressing surface roughness enables us to overcome the loss limitations and achieve high-confinement, on-chip ring resonators with a quality factor (Q) of 37 million for a ring with 2.5 {\mu}m width and 67 million for a ring with 10 {\mu}m width. We show a clear systematic path for achieving these high quality factors. Furthermore, we extract the loss limited by the material absorption in our films to be 0.13 dB/m, which corresponds to an absorption limited Q of at least 170 million by comparing two resonators with different degrees of confinement. Our work provides a chip-scale platform for applications such as ultra-low power frequency comb generation, high precision sensing, laser stabilization and sideband resolved optomechanics.
2016
Klenner, Alexander, Aline S. Mayer, Adrea R. Johnson, Kevin Luke, Michael R. E. Lamont, Yoshitomo Okawachi, Michal Lipson, Alexander L. Gaeta, and Ursula Keller. “Gigahertz frequency comb offset stabilization based on supercontinuum generation in silicon nitride waveguides.” Opt. Express 24 (2016): 11043–11053. Publisher's Version Abstract
Silicon nitride (Si3N4) waveguides represent a novel photonic platform that is ideally suited for energy efficient and ultrabroadband nonlinear interactions from the visible to the mid-infrared. Chip-based supercontinuum generation in Si3N4 offers a path towards a fully-integrated and highly compact comb source for sensing and time-and-frequency metrology applications. We demonstrate the first successful frequency comb offset stabilization that utilizes a Si3N4 waveguide for octave-spanning supercontinuum generation and achieve the lowest integrated residual phase noise of any diode-pumped gigahertz laser comb to date. In addition, we perform a direct comparison to a standard silica photonic crystal fiber (PCF) using the same ultrafast solid-state laser oscillator operating at 1 &\#x00B5;m. We identify the minimal role of Raman scattering in Si3N4 as a key benefit that allows to overcome the fundamental limitations of silica fibers set by Raman-induced self-frequency shift.
Griffith, Austin G., Mengjie Yu, Yoshitomo Okawachi, Jaime Cardenas, Aseema Mohanty, Alexander L. Gaeta, and Michal Lipson. “Coherent mid-infrared frequency combs in silicon-microresonators in the presence of Raman effects.” Opt. Express 24 (2016): 13044–13050. Publisher's Version Abstract
We demonstrate the first low-noise mid-IR frequency comb source using a silicon microresonator. Our observation of strong Raman scattering lines in the generated comb suggests that interplay between Raman and four-wave mixing plays a role in the generated low-noise state. In addition, we characterize, the intracavity comb generation dynamics using an integrated PIN diode, which takes advantage of the inherent three-photon absorption process in silicon.
Joshi, Chaitanya, Jae K. Jang, Kevin Luke, Xingchen Ji, Steven A. Miller, Alexander Klenner, Yoshitomo Okawachi, Michal Lipson, and Alexander L. Gaeta. “Thermally controlled comb generation and soliton modelocking in microresonators.” Opt. Lett. 41 (2016): 2565–2568. Publisher's Version Abstract
We report, to the best of our knowledge, the first demonstration of thermally controlled soliton mode-locked frequency comb generation in microresonators. By controlling the electric current through heaters integrated with silicon nitride microresonators, we demonstrate a systematic and repeatable pathway to single- and multi-soliton mode-locked states without adjusting the pump laser wavelength. Such an approach could greatly simplify the generation of mode-locked frequency combs and facilitate applications such as chip-based dual-comb spectroscopy.
Jang, Jae K., Yoshitomo Okawachi, Mengjie Yu, Kevin Luke, Xingchen Ji, Michal Lipson, and Alexander L. Gaeta. “Dynamics of mode-coupling-induced microresonator frequency combs in normal dispersion.” Optics Express 24, no. 25 (2016): 28794 - 28803. Publisher's Version Abstract
We experimentally and theoretically investigate the dynamics of microresonator-based frequency comb generation assisted by mode coupling in the normal group-velocity dispersion (GVD) regime. We show that mode coupling can initiate intracavity modulation instability (MI) by directly perturbing the pump-resonance mode. We also observe the formation of a low-noise comb as the pump frequency is tuned further into resonance from the MI point. We determine the phase-matching conditions that accurately predict all the essential features of the MI and comb spectra, and extend the existing analogy between mode coupling and high-order dispersion to the normal GVD regime. We discuss the applicability of our analysis to the possibility of broadband comb generation in the normal GVD regime.
Yu, Mengjie, Yoshitomo Okawachi, Austin G. Griffith, Michal Lipson, and Alexander L. Gaeta. “Mode-locked mid-infrared frequency combs in a silicon microresonator.” Optica 3, no. 8 (2016): 854 - 860. Publisher's Version Abstract
Mid-infrared (mid-IR) frequency combs have broad applications in molecular spectroscopy and chemical/biological sensing. Recently developed microresonator-based combs in this wavelength regime could enable portable and robust devices using a single-frequency pump field. Here, we demonstrate a mode-locked microresonator-based frequency comb in the mid-IR spanning 2.4–4.3 μm. We observe high pump-to-comb conversion efficiency, in which 40% of the pump power is converted to the output comb power. Utilizing an integrated PIN structure allows for tuning the silicon microresonator and controlling cavity soliton formation via free-carrier detection and control. Our results significantly advance microresonator-based comb technology toward a portable and robust mid-IR spectroscopic device that operates at low pump powers.
Dutt, Avik, Chaitanya Joshi, Xingchen Ji, Jaime Cardenas, Yoshitomo Okawachi, Kevin Luke, Alexander L. Gaeta, and Michal Lipson. “On-chip dual comb source for spectroscopy.” arXiv:1611.07673 [physics] (2016). Publisher's Version Abstract
Dual-comb spectroscopy is a powerful technique for real-time, broadband optical sampling of molecular spectra which requires no moving components. Recent developments with microresonator-based platforms have enabled frequency combs at the chip scale. However, the need to precisely match the resonance wavelengths of distinct high-quality-factor microcavities has hindered the development of an on-chip dual comb source. Here, we report the first simultaneous generation of two microresonator combs on the same chip from a single laser. The combs span a broad bandwidth of 51 THz around a wavelength of 1.56 \$\textbackslashmu\$m. We demonstrate low-noise operation of both frequency combs by deterministically tuning into soliton mode-locked states using integrated microheaters, resulting in narrow (\$\textless\$ 10 kHz) microwave beatnotes. We further use one mode-locked comb as a reference to probe the formation dynamics of the other comb, thus introducing a technique to investigate comb evolution without auxiliary lasers or microwave oscillators. We demonstrate broadband high-SNR absorption spectroscopy of dichloromethane spanning 170 nm using the dual comb source over a 20 \$\textbackslashmu\$s acquisition time. Our device paves the way for compact and robust dual-comb spectrometers at nanosecond timescales.
Yu, Mengjie, Yoshitomo Okawachi, Austin G Griffith, Nathalie Picqué, Michal Lipson, and Alexander L Gaeta. “Silicon-chip-based mid-infrared dual-comb spectroscopy.” arXiv:1610.01121 (2016). Publisher's Version Abstract
On-chip spectroscopy that could realize real-time fingerprinting with label-free and high-throughput detection of trace molecules is one of the 'holy grails" of sensing. Such miniaturized spectrometers would greatly enable applications in chemistry, bio-medicine, material science or space instrumentation, such as hyperspectral microscopy of live cells or pharmaceutical quality control. Dual-comb spectroscopy (DCS), a recent technique of Fourier transform spectroscopy without moving parts, is particularly promising since it measures high-precision spectra in the gas phase using only a single detector. Here, we present a microresonator-based platform designed for mid-infrared (mid-IR) DCS. A single continuous-wave (CW) low-power pump source generates two mutually coherent mode-locked frequency combs spanning from 2.6 μm to 4.1 μm in two silicon micro-resonators. Thermal control and free-carrier injection control modelocking of each comb and tune the dual-comb parameters. The large line spacing of the combs (127 GHz) and its precise tuning over tens of MHz, unique features of chip-scale comb generators, are exploited for a proof-of-principle experiment of vibrational absorption DCS in the liquid phase, with spectra of acetone spanning from 2870 nm to 3170 nm at 127-GHz (4.2-cm−1) resolution. We take a significant step towards a broadband, mid-IR spectroscopy instrument on a chip. With further system development, our concept holds promise for real-time and time-resolved spectral acquisition on the nanosecond time scale.
2015
Okawachi, Yoshitomo, Mengjie Yu, Kevin Luke, Daniel O. Carvalho, Sven Ramelow, Alessandro Farsi, Michal Lipson, and Alexander L. Gaeta. “Dual-pumped degenerate Kerr oscillator in a silicon nitride microresonator.” Opt. Lett. 40 (2015): 5267–5270. Publisher's Version Abstract
We demonstrate a degenerate parametric oscillator in a silicon nitride microresonator. We use two frequency-detuned pump waves to perform parametric four-wave mixing and operate in the normal group-velocity dispersion regime to produce signal and idler fields that are frequency degenerate. Our theoretical modeling shows that this regime enables generation of bimodal phase states, analogous to the chi(2)-based degenerate OPO. Our system offers potential for realization of CMOS-chip-based coherent optical computing and an all-optical quantum random number generator.
Johnson, Adrea R., Aline S. Mayer, Alexander Klenner, Kevin Luke, Erin S. Lamb, Michael R. E. Lamont, Chaitanya Joshi, et al.. “Octave-spanning coherent supercontinuum generation in a silicon nitride waveguide.” Opt. Lett. 40 (2015): 5117–5120. Publisher's Version Abstract
We demonstrate the generation of a supercontinuum spanning more than 1.4 octaves in a silicon nitride waveguide using sub-100-fs pulses at 1µm generated by either a 53-MHz, diode-pumped ytterbium (Yb) fiber laser or a 1-GHz, Yb:CaAlGdO4 (Yb:CALGO) laser. Our numerical simulations show that the broadband supercontinuum is fully coherent, and a spectral interference measurement is used to verify that the supercontinuum generated with the Yb:CALGO laser possesses a high degree of coherence over the majority of its spectral bandwidth. This coherent spectrum may be utilized for optical coherence tomography, spectroscopy, and frequency metrology.
Griffith, Austin G., Ryan K. W. Lau, Jaime Cardenas, Yoshitomo Okawachi, Aseema Mohanty, Romy Fain, Yoon Ho Daniel Lee, et al.. “Silicon-chip mid-infrared frequency comb generation.” Nature Communications 6 (2015). Abstract
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.
Fridman, Moti, Yoshitomo Okawachi, Stephane Clemmen, Michael Menard, Michal Lipson, and Alexander L. Gaeta. “Waveguide-based single-shot temporal cross-correlator.” Journal of Optics 17 (2015). Abstract
We describe a novel technique for performing a single-shot optical cross-correlation in nanowaveguides. Our scheme is based on four-wave mixing (FWM) between two orthogonally polarized input signals propagating with different velocities due to polarization mode dispersion. The cross-correlation is determined by measuring the spectrum of the idler wave generated by the FWM process.
Cardenas, Jaime, Mengjie Yu, Yoshitomo Okawachi, Carl B. Poitras, Ryan K. W. Lau, Avik Dutt, Alexander L. Gaeta, and Michal Lipson. “Optical nonlinearities in high-confinement silicon carbide waveguides.” Optics Letters 40 (2015): 4138-4141. Publisher's Version Abstract
We demonstrate strong nonlinearities of n(2) = 8.6 +/- 1.1 x 10(-15) cm(2) W-1 in single-crystal silicon carbide (SiC) at a wavelength of 2360 nm. We use a high-confinement SiC waveguide fabricated based on a high-temperature smart-cut process. (C) 2015 Optical Society of America
Miller, Steven A., Yoshitomo Okawachi, Sven Ramelow, Kevin Luke, Avik Dutt, Alessandro Farsi, Alexander L. Gaeta, and Michal Lipson. “Tunable frequency combs based on dual microring resonators.” Optics Express 23 (2015): 21527-21540. Publisher's Version Abstract
In order to achieve efficient parametric frequency comb generation in microresonators, external control of coupling between the cavity and the bus waveguide is necessary. However, for passive monolithically integrated structures, the coupling gap is fixed and cannot be externally controlled, making tuning the coupling inherently challenging. We design a dual-cavity coupled microresonator structure in which tuning one ring resonance frequency induces a change in the overall cavity coupling condition. We demonstrate wide extinction tunability with high efficiency by engineering the ring coupling conditions. Additionally, we note a distinct dispersion tunability resulting from coupling two cavities of slightly different path lengths, and present a new method of modal dispersion engineering. Our fabricated devices consist of two coupled high quality factor silicon nitride microresonators, where the extinction ratio of the resonances can be controlled using integrated microheaters. Using this extinction tunability, we optimize comb generation efficiency as well as provide tunability for avoiding higher-order mode-crossings, known for degrading comb generation. The device is able to provide a 110-fold improvement in the comb generation efficiency. Finally, we demonstrate open eye diagrams using low-noise phase-locked comb lines as a wavelength-division multiplexing channel. (C) 2015 Optical Society of America
2014
Okawachi, Yoshitomo, Michael R. E. Lamont, Kevin Luke, Daniel O. Carvalho, Mengjie Yu, Michal Lipson, and Alexander L. Gaeta. “Bandwidth shaping of microresonator-based frequency combs via dispersion engineering.” Optics Letters 39 (2014): 3535-3538. Publisher's Version Abstract
We investigate experimentally and theoretically the role of group-velocity dispersion and higher-order dispersion on the bandwidth of microresonator-based parametric frequency combs. We show that the comb bandwidth and the power contained in the comb can be tailored for a particular application. Additionally, our results demonstrate that fourth-order dispersion plays a critical role in determining the spectral bandwidth for comb bandwidths on the order of an octave. (C) 2014 Optical Society of America
Johnson, Adrea R., Yoshitomo Okawachi, Michael R. E. Lamont, Jacob S. Levy, Michal Lipson, and Alexander L. Gaeta. “Microresonator-based comb generation without an external laser source.” Optics Express 22 (2014): 1394-1401. Publisher's Version Abstract
We demonstrate a fiber-microresonator dual-cavity architecture with which we generate 880 nm of comb bandwidth without the need for a continuous-wave pump laser. Comb generation with this pumping scheme is greatly simplified as compared to pumping with a single frequency laser, and the generated combs are inherently robust due to the intrinsic feedback mechanism. Temporal and radio frequency (RF) characterization show a regime of steady comb formation that operates with reduced RF amplitude noise. The dual-cavity design is capable of being integrated on-chip and offers the potential of a turn-key broadband multiple wavelength source. (C) 2014 Optical Society of America
Lau, Ryan K. W., Michael R. E. Lamont, Austin G. Griffith, Yoshitomo Okawachi, Michal Lipson, and Alexander L. Gaeta. “Octave-spanning mid-infrared supercontinuum generation in silicon nanowaveguides.” Optics Letters 39 (2014): 4518-4521. Publisher's Version Abstract
We report, to the best of our knowledge, the first demonstration of octave-spanning supercontinuum generation (SCG) on a silicon chip, spanning from the telecommunications c-band near 1.5 m to the mid-infrared region beyond 3.6 mu m. The SCG presented here is characterized by soliton fission and dispersive radiation across two zero group-velocity dispersion wavelengths. In addition, we numerically investigate the role of multiphoton absorption and free carriers, confirming that these nonlinear loss mechanisms are not detrimental to SCG in this regime. (C) 2014 Optical Society of America
Miller, Steven, Kevin Luke, Yoshitomo Okawachi, Jaime Cardenas, Alexander L. Gaeta, and Michal Lipson. “On-chip frequency comb generation at visible wavelengths via simultaneous second- and third-order optical nonlinearities.” Optics Express 22 (2014): 26517-26525. Publisher's Version Abstract
Microresonator-based frequency comb generation at or near visible wavelengths would enable applications in precise optical clocks, frequency metrology, and biomedical imaging. Comb generation in the visible has been limited by strong material dispersion and loss at short wavelengths, and only very narrowband comb generation has reached below 800 nm. We use the second-order optical nonlinearity in an integrated high-Q silicon nitride ring resonator cavity to convert a near-infrared frequency comb into the visible range. We simultaneously demonstrate parametric frequency comb generation in the near-infrared, second-harmonic generation, and sum-frequency generation. We measure 17 comb lines converted to visible wavelengths extending to 765 nm. (C) 2014 Optical Society of America
Ramelow, Sven, Alessandro Farsi, Stephane Clemmen, Jacob S. Levy, Adrea R. Johnson, Yoshitomo Okawachi, Michael R. E. Lamont, Michal Lipson, and Alexander L. Gaeta. “Strong polarization mode coupling in microresonators.” Optics Letters 39 (2014): 5134-5137. Publisher's Version Abstract
We observe strong modal coupling between the TE00 and TM00 modes in Si3N4 ring resonators revealed by avoided crossings of the corresponding resonances. Such couplings result in significant shifts of the resonance frequencies over a wide range around the crossing points. This leads to an effective dispersion that is one order of magnitude larger than the intrinsic dispersion and creates broad windows of anomalous dispersion. We also observe the changes to frequency comb spectra generated in Si3N4 microresonators due to polarization mode and higher-order mode crossings and suggest approaches to avoid these effects. Alternatively, such polarization mode crossings can be used as a tool for dispersion engineering in microresonators. (C) 2014 Optical Society of America