Mian Zhang, Brandon Buscaino, Cheng Wang, Amirhassan Shams-Ansari, Christian Reimer, Rongrong Zhu, Joseph Kahn, and Marko Loncar. Submitted. “Broadband electro-optic frequency comb generation in an integrated microring resonator.” arXiv:1809.08636. Publisher's VersionAbstract
Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communications and for precision metrology, timing and spectroscopy. To date, wide-spanning combs are most often generated by mode-locked lasers or dispersion-engineered resonators with third-order Kerr nonlinearity. An alternative comb generation method uses electro-optic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability. Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. In this work, we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large electro-optic response, ultra-low optical loss and highly co-localized microwave and optical felds, while enabling dispersion engineering. Our measured EO frequency comb spans more than the entire telecommunications L-band (over 900 comb lines spaced at ~ 10 GHz), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 Hz to 100 MHz), and utilize this feature to generate dual frequency combs in a single resonator. Our results show that integrated EO comb generators, capable of generating wide and stable comb spectra, are a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy to optical communications.
B Machielse, S Bogdanovic, S Meesala, S Gauthier, MJ Burek, G Joe, M Chalupnik, YI Sohn, J Holzgrafe, RE Evans, C Chia, H Atikian, MK Bhaskar, DD Sukachev, L Shao, S Maity, MD Lukin, and M Loncar. Submitted. “Electromechanical Control of Quantum Emitters in Nanophotonic Devices”. Publisher's Version
Linbo Shao, Smarak Maity, Lue Wu, Amirhassan Shams-Ansari, Young-Ik Sohn, Eric Puma, M.N. Gadalla, Mian Zhang, Cheng Wang, and Marko Lončar. Submitted. “High-Q gigahertz surface acoustic wave cavity on lithium niobate.” arXiv:1901.09080. Publisher's Version
Tianhao Ren, Mian Zhang, Cheng Wang, Linbo Shao, Chrisitian Reimer, Yong Zhang, Oliver King, Ronald Esman, Thomas Cullen, and Marko Loncar. Submitted. “An integrated low-voltage broadband lithium niobate phase modulator.” arXiv:1902.09070. Publisher's VersionAbstract
Electro-optic phase modulators are critical components in modern communication, microwave photonic, and quantum photonic systems. Important for these applications is to achieve modulators with low half-wave voltage at high frequencies. Here we demonstrate an integrated phase modulator, based on a thin-film lithium niobate platform, that simultaneously features small on-chip loss (~ 1 dB) and low half-wave voltage over a large spectral range (3.5 - 4.5 V at 5 - 40 GHz). By driving the modulator with a strong 30-GHz microwave signal corresponding to around four half-wave voltages, we generate an optical frequency comb consisting of over 40 sidebands spanning 10 nm in the telecom L-band. The high electro-optic performance combined with the high RF power-handling ability (3.1 W) of our integrated phase modulator are crucial for future photonics and microwave systems.
Lingyan He, Mian Zhang, Amirhassan Shams-Ansari, Rongrong Zhu, Cheng Wang, and Marko Loncar. Submitted. “Low-loss fiber-to-chip interface for lithium niobate photonic integrated circuits.” arXiv:1902.08969. Publisher's VersionAbstract
Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly > 10 dB) dominates the total insertion losses of typical LN photonic integrated circuits, where on-chip propagation losses can be as low as 0.03 - 0.1 dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetric guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7 dB/facet with high fabrication tolerance and repeatability. Our results open door for practical integrated LN photonic circuits efficiently interfaced with optical fibers.
Boris Desiatov, Amirhassan Shams-Ansari, Mian Zhang, Cheng Wang, and Marko Loncar. 3/12/2019. “Ultra-low loss integrated visible photonics using thin-film lithium niobate.” Optica, 6, 3, Pp. 380-384 . Publisher's VersionAbstract
Integrated photonics is a powerful platform that can improve the performance and stability of optical systems, while providing low-cost, small-footprint and scalable alternatives to implementations based on free-space optics. While great progress has been made on the development of low-loss integrated photonics platforms at telecom wavelengths, visible wavelength range has received less attention. Yet, many applications utilize visible or near-visible light, including those in optical imaging, optogenetics, and quantum science and technology. Here we demonstrate an ultra-low loss integrated visible photonics platform based on thin film lithium niobate on insulator. Our waveguides feature ultra-low propagation loss of 6 dB/m, while our microring resonators have an intrinsic quality factor of 11 million, both measured at 637 nm wavelength. Additionally, we demonstrate an on-chip visible intensity modulator with an electro-optic bandwidth of 10 GHz, limited by the detector used. The ultra-low loss devices demonstrated in this work, together with the strong second- and third-order nonlinearities in lithium niobate, open up new opportunities for creating novel passive, and active devices for frequency metrology and quantum information processing in the visible spectrum range.
Cheng Wang, Mian Zhang, Mengjie Yu, Rongrong Zhu, Han Hu, and Marko Loncar. 2/28/2019. “Monolithic lithium niobate photonic circuits for Kerr frequency comb generation and modulation.” Nature communications, 10, 978. Publisher's VersionAbstract
Microresonator Kerr frequency combs, which rely on third-order nonlinearity (χ(3)), are of great interest for a wide range of applications including optical clocks, pulse shaping, spectroscopy, telecommunications, light detection and ranging (LiDAR) and quantum information processing. Many of these applications require further spectral and temporal control of the generated frequency comb signal, which is typically accomplished using additional photonic elements with strong second-order nonlinearity (χ(2)). To date these functionalities have largely been implemented as discrete off-chip components due to material limitations, which come at the expense of extra system complexity and increased optical losses. Here we demonstrate the generation, filtering and electro-optic modulation of a frequency comb on a single monolithic integrated chip, using a thin-film lithium niobate (LN) photonic platform that simultaneously possesses large χ(2) and χ(3) nonlinearities and low optical losses. We generate broadband Kerr frequency combs using a dispersion-engineered high quality factor LN microresonator, select a single comb line using an electrically programmable add-drop filter, and modulate the intensity of the selected line. Our results pave the way towards monolithic integrated frequency comb solutions for spectroscopy data communication, ranging and quantum photonics.
Mengjie Yu, Boris Desiatov, Yoshitomo Okawachi, Alexander L. Gaeta, and Marko Lončar. 2/25/2019. “Coherent two-octave-spanning supercontinuum generation in lithium-niobate waveguides.” Optics Letters, 44, 5, Pp. 1222-1225. Publisher's VersionAbstract
We demonstrate coherent supercontinuum generation (SCG) in a monolithically integrated lithium-niobate waveguide, under the presence of second- and third-order nonlinear effects. We achieve more than two octaves of optical bandwidth in a 0.5-cm-long waveguide with 100-picojoule-level pulses. Dispersion engineering of the waveguide allows for spectral overlap between the SCG and the second harmonic which enables direct detection of the carrier-envelope offset frequency f CEO using a single waveguide. We measure the f CEO of our femtosecond pump source with a 30-dB signal-to-noise ratio.
Mian Zhang, Cheng Wang, Yaowen Hu, Amirhassan Shams-Ansari, Tianhao Ren, Shanhui Fan, and Marko Loncar. 2019. “Electronically Programmable Photonic Molecule.” Nature Photonics, 13, Pp. 36–40. Publisher's VersionAbstract
Physical systems with discrete energy levels are ubiquitous in nature and acre fundamental building blocks of quantum technology. Realizing controllable artificial atom- and molecule-like systems for light would enable coherent and dynamic control of the frequency, amplitude and phase of photons1,2,3,4,5. In this work, we demonstrate a ‘photonic molecule’ with two distinct energy levels using coupled lithium niobate microring resonators and control it by external microwave excitation. We show that the frequency and phase of light can be precisely controlled by programmed microwave signals, using concepts of canonical two-level systems including Autler–Townes splitting, Stark shift, Rabi oscillation and Ramsey interference. Through such coherent control, we show on-demand optical storage and retrieval by reconfiguring the photonic molecule into a bright–dark mode pair. These results of dynamic control of light in a programmable and scalable electro-optic system open doors to applications in microwave signal processing6, quantum photonic gates in the frequency domain7and exploring concepts in optical computing8 and topological physics3,9.
L. Koehler, P. Chevalier, E. Shim, B. Desiatov, A. Shams-Ansari, M. Piccardo, Y. Okawachi, M. Yu, M. Loncar, M. Lipson, A. Gaeta, and F. Capasso. 12/24/2018. “Direct thermo-optical tuning of silicon microresonators for the mid-infrared.” Optics Express, 26, 26, Pp. 34965. Publisher's VersionAbstract

Weuselightfromavisiblelaserdiodetodirectlytunesilicon-on-chipmicroresonators by thermo-optical effect. We show that this direct tuning is local, non invasive and has a much smaller time constant than global temperature tuning methods. Such an approach could prove to be highly effective for Kerr comb generation in microresonators pumped by quantum cascade lasers, which cannot be easily tuned to achieve comb generation and soliton-modelocked states.

Shahriar Aghaeimeibodi, Boris Desiatov, Je-Hyung Kim, Chang-Min Lee, Mustafa Atabey Buyukkaya, Aziz Karasahin, Christopher J. K. Richardson, Richard P. Leavitt, Marko Lončar, and Edo Waks. 11/26/2018. “Integration of quantum dots with lithium niobate photonics.” Applied Physics Letters, 113, Pp. 221102. Publisher's VersionAbstract
The integration of quantum emitters with integrated photonics enables complex quantum photonic circuits that are necessary for photonic implementation of quantum simulators, computers, and networks. Thin-film lithium niobate is an ideal material substrate for quantum photonics because it can tightly confine light in small waveguides and has a strong electro-optic effect that can switch and modulate single photons at low power and high speed. However, lithium niobate lacks efficient single-photon emitters, which are essential for scalable quantum photonic circuits. We demonstrate deterministic coupling of single-photon emitters with a lithium niobate photonic chip. The emitters are composed of InAs quantum dots embedded in an InP nanobeam, which we transfer to a lithium niobate waveguide with nanoscale accuracy using a pick-and-place approach. An adiabatic taper transfers single photons emitted into the nanobeam to the lithium niobate waveguide with high efficiency. We verify the single photon nature of the emission using photon correlation measurements performed with an on-chip beamsplitter. Our results demonstrate an important step toward fast, reconfigurable quantum photonic circuits for quantum information processing.
Ruffin E. Evans, Mihir K. Bhaskar, Denis D. Sukachev, Christian T. Nguyen, Alp Sipahigil, Michael J. Burek, Bartholomeus Machielse, Grace H. Zhang, Alexander S. Zibrov, Edward Bielejec, Hongkun Park, Marko Lončar, and Mikhail D. Lukin. 11/9/2018. “Photon-mediated interactions between quantum emitters in a diamond nanocavity.” Science, 362, 6415, Pp. 662-665. Publisher's VersionAbstract
Photon-mediated interactions between quantum systems are essential for realizing quantum networks and scalable quantum information processing. We demonstrate such interactions between pairs of silicon-vacancy (SiV) color centers coupled to a diamond nanophotonic cavity. When the optical transitions of the two color centers are tuned into resonance, the coupling to the common cavity mode results in a coherent interaction between them, leading to spectrally resolved superradiant and subradiant states. We use the electronic spin degrees of freedom of the SiV centers to control these optically mediated interactions. Such controlled interactions will be crucial in developing cavity-mediated quantum gates between spin qubits and for realizing scalable quantum network nodes.
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Cheng Wang, Carsten Langrock, Alireza Marandi, Marc Jankowski, Mian Zhang, Boris Desiatov, Martin M. Fejer, and Marko Lončar. 11/7/2018. “Ultrahigh-efficiency wavelength conversion in nanophotonic periodically poled lithium niobate waveguides.” Optica, 5, 11, Pp. 1438. Publisher's VersionAbstract
Periodically poled lithium niobate (PPLN) waveguides are a powerful platform for efficient wavelength conversion. Conventional PPLN converters, however, typically require long device lengths and high pump powers due to the limited nonlinear interaction strength. Here we use a nanostructured PPLN waveguide to demonstrate an ultrahigh normalized efficiency of 2600%/W−cm^2 for second-harmonic generation of 1.5 μm radiation, more than 20 times higher than that in state-of-the-art diffused waveguides. This is achieved by a combination of sub-wavelength optical confinement and high-fidelity periodic poling at a first-order poling period of 4 μm. Our highly integrated PPLN waveguides are promising for future chip-scale integration of classical and quantum photonic systems.
Cheng Wang, Mian Zhang, Xi Chen, Maxime Bertrand, Amirhassan Shams-Ansari, Sethumadhavan Chandrasekhar, Peter Winzer, and Marko Loncar. 9/24/2018. “Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages.” Nature. Publisher's VersionAbstract
Electro-optic modulators translate high-speed electronic signals into the optical domain and are critical components in modern telecommunication networks1,2 and microwave-photonic systems3,4. They are also expected to be building blocks for emerging applications such as quantum photonics5,6 and non-reciprocal optics7,8. All of these applications require chip-scale electro-optic modulators that operate at voltages compatible with complementary metal–oxide–semiconductor (CMOS) technology, have ultra-high electro-optic bandwidths and feature very low optical losses. Integrated modulator platforms based on materials such as silicon, indium phosphide or polymers have not yet been able to meet these requirements simultaneously because of the intrinsic limitations of the materials used. On the other hand, lithium niobate electro-optic modulators, the workhorse of the optoelectronic industry for decades9, have been challenging to integrate on-chip because of difficulties in microstructuring lithium niobate. The current generation of lithium niobate modulators are bulky, expensive, limited in bandwidth and require high drive voltages, and thus are unable to reach the full potential of the material. Here we overcome these limitations and demonstrate monolithically integrated lithium niobate electro-optic modulators that feature a CMOS-compatible drive voltage, support data rates up to 210 gigabits per second and show an on-chip optical loss of less than 0.5 decibels. We achieve this by engineering the microwave and photonic circuits to achieve high electro-optical efficiencies, ultra-low optical losses and group-velocity matching simultaneously. Our scalable modulator devices could provide cost-effective, low-power and ultra-high-speed solutions for next-generation optical communication networks and microwave photonic systems. Furthermore, our approach could lead to large-scale ultra-low-loss photonic circuits that are reconfigurable on a picosecond timescale, enabling a wide range of quantum and classical applications5,10,11 including feed-forward photonic quantum computation.
Smarak Maity, Linbo Shao, Young-Ik Sohn, Srujan Meesala, Bartholomeus Machielse, Edward Bielejec, Matthew Markha, and Marko Loncar. 8/30/2018. “Spectral alignment of single-photon emitters in diamond using strain gradient .” Physical Review Applied , 10, Pp. 024050 . Publisher's Version [pdf]
Shuo Sun, Jingyuan Linda Zhang, Kevin A. Fischer, Michael J. Burek, Constantin Dory, Konstantinos G. Lagoudakis, Yan-Kai Tzeng, Marina Radulaski, Yousif Kelaita, Amir Safavi-Naeini, Zhi-Xun Shen, Nicholas A. Melosh, Steven Chu, Marko Lončar, and Jelena Vučković. 8/21/2018. “Cavity-enhanced Raman emission from a single color center in a solid .” Physical Review Letters, 121, Pp. 083601. Publisher's Version
Li Wang, Cheng Wang, Jie Wang, Fang Bo, Mian Zhang, Qihuang Gong, Marko Lončar, and Yun-Feng Xiao. 6/13/2018. “High-Q chaotic lithium niobate microdisk cavity.” Optics Letters, 43, 12, Pp. 2917-2920. Publisher's Version [PDF]
Srujan Meesala*, Young-Ik Sohn*, Benjamin Pingault, Linbo Shao, Haig A. Atikian, Jeffrey Holzgrafe, Mustafa Gündoğan, Camille Stavrakas, Alp Sipahigil, Cleaven Chia, Ruffin Evans, Michael J. Burek, Mian Zhang, Lue Wu, Jose L. Pacheco, John Abraham, Edward Bielejec, Mikhail D. Lukin, Mete Atatüre, and Marko Lončar. 5/29/2018. “Strain engineering of the silicon vacancy center in diamond.” Physical Review B, 97, Pp. 205444. Publisher's VersionAbstract
We control the electronic structure of the silicon-vacancy (SiV) color-center in diamond by changing its static strain environment with a nano-electro-mechanical system. This allows deterministic and local tuning of SiV optical and spin transition frequencies over a wide range, an essential step towards multi-qubit networks. In the process, we infer the strain Hamiltonian of the SiV revealing large strain susceptibilities of order 1 PHz/strain for the electronic orbital states. We identify regimes where the spin-orbit interaction results in a large strain suseptibility of order 100 THz/strain for spin transitions, and propose an experiment where the SiV spin is strongly coupled to a nanomechanical resonator.
Marc-Antoine Lemonde, Srujan Meesala, Alp Sipahigil, Martin J. A. Schuetz, Mikhail D. Lukin, Marko Loncar, and Peter Rabl. 5/25/2018. “Phonon networks with SiV centers in diamond waveguides.” Physical Review Letters, 120, Pp. 213603. Publisher's Version Main text Supplementary
Young-Ik Sohn*, Srujan Meesala*, Benjamin Pingault*, Haig A. Atikian, Jeffrey Holzgrafe, Mustafa Gündoğan, Camille Stavrakas, Megan J. Stanley, Alp Sipahigil, Joonhee Choi, Mian Zhang, Jose L. Pacheco, John Abraham, Edward Bielejec, Mikhail D. Lukin, Mete Atatüre, and Marko Lončar. 5/22/2018. “Controlling the coherence of a diamond spin qubit through its strain environment.” Nature Communications 9, Pp. 2012. arXiv VersionAbstract
The uncontrolled interaction of a quantum system with its environment is detrimental for quantum coherence. In the context of solid-state qubits, techniques to mitigate the impact of fluctuating electric and magnetic fields from the environment are well-developed. In contrast, suppression of decoherence from thermal lattice vibrations is typically achieved only by lowering the temperature of operation. Here, we use a nano-electro-mechanical system (NEMS) to mitigate the effect of thermal phonons on a solid-state quantum emitter without changing the system temperature. We study the silicon-vacancy (SiV) colour centre in diamond which has optical and spin transitions that are highly sensitive to phonons. First, we show that its electronic orbitals are highly susceptible to local strain, leading to its high sensitivity to phonons. By controlling the strain environment, we manipulate the electronic levels of the emitter to probe, control, and eventually, suppress its interaction with the thermal phonon bath. Strain control allows for both an impressive range of optical tunability and significantly improved spin coherence. Finally, our findings indicate that it may be possible to achieve strong coupling between the SiV spin and single phonons, which can lead to the realisation of phonon-mediated quantum gates and nonlinear quantum phononics.