Publications

2018
On-chip platform for a phased array with minimal beam divergence and wide field-of-view
Zadka, Moshe, You-Chia Chang, Aseema Mohanty, Christopher T. Phare, Samantha P. Roberts, and Michal Lipson. “On-chip platform for a phased array with minimal beam divergence and wide field-of-view.” Opt. Express 26 (2018): 2528–2534. Publisher's Version Abstract
Current silicon photonics phased arrays based on waveguide gratings enable beam steering with no moving parts. However, they suffer from a trade-off between beam divergence and field of view. Here, we show a platform based on silicon-nitride/silicon that achieves simultaneously minimal beam divergence and maximum field of view while maintaining performance that is robust to fabrication variations. In addition, in order to maximize the emission from the entire length of the grating, we design the grating's strength by varying its duty cycle (apodization) to emit uniformly. We fabricate a millimeter long grating emitter with diffraction-limited beam divergence of 0.089 degrees.
platform_with_minimal_beam_divergence_and_wide_field-of-view.pdf
On-chip dual-comb source for spectroscopy
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.” Science Advances 4, no. 3 (2018). 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 on-chip dual combs. We report the simultaneous generation of two microresonator combs on the same chip from a single laser, drastically reducing experimental complexity. We demonstrate broadband optical spectra spanning 51 THz and low-noise operation of both combs by deterministically tuning into soliton mode-locked states using integrated microheaters, resulting in narrow (< 10 kHz) microwave beat notes. We further use one 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 high signal-to-noise ratio absorption spectroscopy spanning 170 nm using the dual-comb source over a 20-μs acquisition time. Our device paves the way for compact and robust spectrometers at nanosecond time scales enabled by large beat-note spacings (> 1 GHz).
dutt_dual_comb_sci_adv.pdf
Joshi, Chaitanya, Mengjie Yu Kevin Luke Xingchen Ji Alexander Klenner, Yoshitomo Okawachi, Michal Lipson, and Alexander L. Gaeta. “Counter-rotating cavity solitons in a silicon nitride microresonator.” Optics Letters 43, no. 3 (2018). Abstract
We demonstrate the generation of counter-rotating cavity solitons in a silicon nitride microresonator using a fixed, single-frequency laser. We demonstrate a dual three-soliton state with a difference in the repetition rates of the soliton trains that can be tuned by varying the ratio of pump powers in the two directions. Such a system enables a highly compact, tunable dual comb source that can be used for applications such as spectroscopy and distance ranging.
counter-rotating-solitons-joshi.pdf
Wang, Cheng, Mian Zhang, Brian Stern, Michal Lipson, and Marko Lončar. “Nanophotonic lithium niobate electro-optic modulators.” Optics Express 26, no. 2 (2018): 1547-1555. Abstract
Since the emergence of optical fiber communications, lithium niobate (LN) has been the material of choice for electro-optic modulators, featuring high data bandwidth and excellent signal fidelity. Conventional LN modulators however are bulky, expensive and power hungry, and cannot meet the growing demand in modern optical data links. Chip-scale, highly integrated, LN modulators could offer solutions to this problem, yet the fabrication of low-loss devices in LN thin films has been challenging. Here we overcome this hurdle and demonstrate monolithically integrated LN electro-optic modulators that are significantly smaller and more efficient than traditional bulk LN devices, while preserving LN’s excellent material properties. Our compact LN electro-optic platform consists of low-loss nanoscale LN waveguides, micro-ring resonators and miniaturized Mach-Zehnder interferometers, fabricated by directly shaping LN thin films into sub-wavelength structures. The efficient confinement of both optical and microwave fields at the nanoscale dramatically improves the device performances featuring a half-wave electro-optic modulation efficiency of 1.8 V∙cm while operating at data rates up to 40 Gbps. Our monolithic LN nanophotonic platform enables dense integration of high-performance active components, opening new avenues for future high-speed, low power and cost-effective communication networks.
lithium-niobate-wang.pdf
2017
Compact narrow-linewidth integrated laser based on a low-loss silicon nitride ring resonator
Stern, Brian, Xingchen Ji, Avik Dutt, and Michal Lipson. “Compact narrow-linewidth integrated laser based on a low-loss silicon nitride ring resonator.” Optics Letters 42, no. 21 (2017): 4541-4544. Publisher's Version Abstract
We design and demonstrate a compact, narrow-linewidth integrated laser based on low-loss silicon nitride waveguides coupled to a III-V gain chip. By using a highly confined optical mode, we simultaneously achieve compact bends and ultra-low loss. We leverage the narrowband backreflection of a high-Q microring resonator to act as a cavity output mirror, a single-mode filter, and a propagation delay all in one. This configuration allows the ring to provide feedback and obtain a laser linewidth of 13 kHz with 1.7 mW output power around 1550 nm. This demonstration realizes a compact sub-millimeter silicon nitride laser cavity with a narrow linewidth.
compact_narrow_linewidth_laser.pdf
Lee, Brian S., Mian Zhang, Felippe A. S. Barbosa, Steven A. Miller, Aseema Mohanty, Raphael St-Gelais, and Michal Lipson. “On-chip thermo-optic tuning of suspended microresonators.” Opt. Express 25 (2017): 12109–12120. Publisher's Version Abstract
Suspended optical microresonators are promising devices for on-chip photonic applications such as radio-frequency oscillators, optical frequency combs, and sensors. Scaling up these devices demands the capability to tune the optical resonances in an integrated manner. Here, we design and experimentally demonstrate integrated on-chip thermo-optic tuning of suspended microresonators by utilizing suspended wire bridges and microheaters. We demonstrate the ability to tune the resonance of a suspended microresonator in silicon nitride platform by 9.7 GHz using 5.3 mW of heater power. The loaded optical quality factor (QL  92,000) stays constant throughout the detuning. We demonstrate the efficacy of our approach by completely turning on and off the optical coupling between two evanescently coupled suspended microresonators.
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.
yu_ncomms_breather_soliton.pdf
Fain, Romy, Felippe Barbosa, Jaime Cardenas, and Michal Lipson. “Photonic Needles for Light Delivery in Deep Tissue-like Media.” Scientific Reports 7, no. 5627 (2017). Abstract
We demonstrate a new platform for minimally invasive, light delivery probes leveraging the maturing field of silicon photonics, enabling massively parallel fabrication of photonic structures. These Photonic Needles probes have sub-10μm cross-sectional dimensions, lengths greater than 3 mm–surpassing 1000 to 1 aspect ratio, and are released completely into air without a substrate below. We show the Photonic Needles to be mechanically robust when inserted into 2% agarose. The propagation loss of these waveguides is low–on the order of 4 dB/cm.
photonic-needles-fain.pdf
Miller, Steven A, Mengjie Yu, Xingchen Ji, Austin G Griffith, Jaime Cardenas, Alexander L Gaeta, and Michal Lipson. “Low-Loss Silicon Platform for Broadband Mid-Infrared Photonics.” arXiv:1703.03517 (2017). Publisher's Version Abstract
Broadband mid-infrared (mid-IR) spectroscopy applications could greatly benefit from today's well-developed, highly scalable silicon photonics technology; however, this platform lacks broadband transparency due to its reliance on absorptive silicon dioxide cladding. Alternative cladding materials have been studied, but the challenge lies in decreasing losses while avoiding complex fabrication techniques. Here, in contrast to traditional assumptions, we show that silicon photonics can achieve low-loss propagation in the mid-IR from 3 - 6 um wavelength, thus providing a highly scalable, well-developed technology in this spectral range. We engineer the waveguide cross section and optical mode interaction with the absorptive cladding oxide to reduce loss at mid-IR wavelengths. We fabricate a microring resonator and measure an intrinsic quality (Q) factor of 10^6 at wavelengths from 3.5 to 3.8 um. This is the highest Q demonstrated on an integrated mid-IR platform to date. With this high-Q silicon microresonator, we also demonstrate a low optical parametric oscillation threshold of 5.2 mW, illustrating the utility of this platform for nonlinear chip-scale applications in the mid-IR.
Wang, Cheng, Mian Zhang, Brian Stern, Michal Lipson, and Marko Loncar. “Nanophotonic Lithium Niobate Electro-optic Modulators.” arXiv 1701.06470 (2017). Publisher's Version Abstract
Modern communication networks require high performance and scalable electro-optic modulators that convert electrical signals to optical signals at high speed. Existing lithium niobate modulators have excellent performance but are bulky and prohibitively expensive to scale up. Here we demonstrate scalable and high-performance nanophotonic electro-optic modulators made of single-crystalline lithium niobate microring resonators and micro-Mach-Zehnder interferometers. We show a half-wave electro-optic modulation efficiency of 1.8V-cm and data rates up to 40 Gbps.
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.
1609.08699.pdf optica-4-6-619.pdf
Quantum Interference between Transverse Spatial Waveguide Modes
Mohanty, Aseema, Mian Zhang, Avik Dutt, Sven Ramelow, Paulo Nussenzveig, and Michal Lipson. “Quantum Interference between Transverse Spatial Waveguide Modes.” Nature Communications 8 (2017): 14010. Publisher's Version Abstract
Integrated quantum optics has the potential to markedly reduce the footprint and resource requirements of quantum information processing systems, but its practical implementation demands broader utilization of the available degrees of freedom within the optical field. To date, integrated photonic quantum systems have primarily relied on path encoding. However, in the classical regime, the transverse spatial modes of a multi-mode waveguide have been easily manipulated using the waveguide geometry to densely encode information. Here, we demonstrate quantum interference between the transverse spatial modes within a single multi-mode waveguide using quantum circuit-building blocks. This work shows that spatial modes can be controlled to an unprecedented level and have the potential to enable practical and robust quantum information processing.
quantum_interference_between_transverse_spatial_waveguide_modes.pdf
2016
Yoshitomo, Okawachi, Mengjie Yu, Kevin Luke, Daniel O. Carvalho, Michal Lipson, and Alexander L. Gaeta. “Quantum random number generator using a microresonator-based Kerr oscillator.” Opt. Lett. 41 (2016): 4194–4197. Publisher's Version Abstract
We demonstrate an all-optical quantum random number generator using a dual-pumped degenerate optical parametric oscillator in a silicon nitride microresonator. The frequency-degenerate bi-phase state output is realized using parametric four-wave mixing in the normal group-velocity dispersion regime with two nondegenerate pumps. We achieve a random number generation rate of 2 MHz and verify the randomness of our output using the National Institute of Standards and Technology Statistical Test Suite. The scheme offers potential for a chip-scale random number generator with gigahertz generation rates and no postprocessing.
ol-41-18-4194.pdf
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.
gigahertz_frequency_comb_offset_stabilization_based_on_supercontinuum_generation_in_silicon_nitride_waveguides.pdf
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.
coherent_mid-infrared_frequency_combs_in_silicon-microresonators_in_the_presence_of_raman_effects.pdf
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.
thermally_controlled_comb_generation_and_soliton_modelocking_in_microresonators.pdf
St-Gelais, Raphael, Linxiao Zhu, Shanhui Fan, and Michal Lipson. “Near-field radiative heat transfer between parallel structures in the deep subwavelength regime.” Nat Nano 11 (2016): 515. Publisher's Version Abstract
Thermal radiation between parallel objects separated by deep subwavelength distances and subject to large thermal gradients (>100 K) can reach very high magnitudes, while being concentrated on a narrow frequency distribution. These unique characteristics could enable breakthrough technologies for thermal transport control and electricity generation (for example, by radiating heat exactly at the bandgap frequency of a photovoltaic cell). However, thermal transport in this regime has never been achieved experimentally due to the difficulty of maintaining large thermal gradients over nanometre-scale distances while avoiding other heat transfer mechanisms, namely conduction. Here, we show near-field radiative heat transfer between parallel SiC nanobeams in the deep subwavelength regime. The distance between the beams is controlled by a high-precision micro-electromechanical system (MEMS). We exploit the mechanical stability of nanobeams under high tensile stress to minimize thermal buckling effects, therefore keeping control of the nanometre-scale separation even at large thermal gradients. We achieve an enhancement of heat transfer of almost two orders of magnitude with respect to the far-field limit (corresponding to a 42 nm separation) and show that we can maintain a temperature gradient of 260 K between the cold and hot surfaces at ∼100 nm distance.
st-gelais_raphael_near-field_radiative_heat_transfer_2016.pdf
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.
dutt_2016_on-chip_dual_comb.pdf

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