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Cited by 171Photorefractive and Nonlinear OpticsPhotonic and Optical DevicesFerroelectric and Piezoelectric Materials

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High density lithium niobate photonic integrated circuits

Open access
Li, Zihan, Wang, Rui Ning, Lihachev, Grigory, Zhang, Junyin, Tan, Zelin, Churaev, Mikhail, Kuznetsov, Nikolai, Siddharth, Anat, Bereyhi, Mohammad J., Riemensberger, Johann, Kippenberg, Tobias J.·2023·Nature Communications
DOI 10.1038/s41467-023-40502-8PMC PMC10415301

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Abstract

Abstract Photonic integrated circuits are indispensable for data transmission within modern datacenters and pervade into multiple application spheres traditionally limited for bulk optics, such as LiDAR and biosensing1. New applications and higher performance are enabled by the diversification of optical waveguide materials past silicon-on-insulator. Of particular interest are ferroelectrics such as Lithium Niobate, which exhibit a large electro-optical Pockels effect enabling ultrafast and efficient modulation, but are difficult to process via dry etching2. For this reason, etching tightly confining waveguides - routinely achieved in silicon or silicon nitride - has not been possible. Diamond-like carbon (DLC) was discovered in the 1950s 3 and is a material that exhibits an amorphous phase, excellent hardness, and the ability to be deposited in nano-metric thin films. Its use today is pervasive, ranging from applications for hard disk surfaces 4 and medical devices 5 to low friction coatings for automotive components 6. It has excellent thermal, mechanical, and electrical properties, making it an ideal protective coating. Here we demonstrate that DLC is also a superior material for the manufacturing of next-generation photonic integrated circuits based on ferroelectrics, specifically Lithium Niobate on insulator (LNOI). Using DLC as a hard mask, we demonstrate the fabrication of deeply etched, tightly confining, low loss photonic integrated circuits with losses as low as 4 dB/m and Q-factor as high as 10 · 106. In contrast to widely employed ridge waveguides 7,8, this approach benefits from a more than 1 order of magnitude higher area integration density while maintaining efficient electro-optical modulation, low loss, and offering a route for efficient optical fiber interfaces. As a proof of concept, we demonstrate a frequency agile hybrid integrated III-V Lithium Niobate based laser with sub-kHz linewidth and tuning rate of 0.7 Peta-Hertz per second with excellent linearity and CMOS-compatible driving voltage. Our approach can herald a new generation of high density ferroelectric photonic integrated circuits, in particular for applications in coherent laser based ranging 9 and beamforming 10, optical communications 7, and classical 11 and quantum computing networks 12.

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Fig. 1 Tightly confining lithium niobate photonic integrated circuit platform. a Scanning electron micrograph (SEM) of a LiNbO3-based photonic integrated circuit with high resolution insets of microring coupling section ((b, orange), curved waveguide (c, blue), and inverse taper for fiber coupling (d, red)). e SEM of a fully etched optical waveguide with cross section 2 μm × 0.7 μm. f SEM of a partially etched ridge waveguide 1.5 μm × 0.6 μm, 350 nm etch depth and 250 nm LiNbO3 slab. g, h Optical mode field distribution of curved waveguides corresponding to (e, f) with 20 μm bend radius. i, j Electrical field distribution in log-scale for slab and ridge waveguides with electrode spacing 0.75 μm and 2.5 μm, which are selected to feature equal metal-induced optical losses. k FEM simulations of LN waveguide bending loss for different slab heights and waveguide cross-section 2 μm × 0.6 μm. l FEM simulation of the voltage-length product of electro-optical modulation for strip and ridge waveguides as a function of metal-induced waveguide loss.
Fig. 1 Tightly confining lithium niobate photonic integrated circuit platform. a Scanning electron micrograph (SEM) of a LiNbO3-based photonic integrated circuit with high resolution insets of microring coupling section ((b, orange), curved waveguide (c, blue), and inverse taper for fiber coupling (d, red)). e SEM of a fully etched optical waveguide with cross section 2 μm × 0.7 μm. f SEM of a partially etched ridge waveguide 1.5 μm × 0.6 μm, 350 nm etch depth and 250 nm LiNbO3 slab. g, h Optical mode field distribution of curved waveguides corresponding to (e, f) with 20 μm bend radius. i, j Electrical field distribution in log-scale for slab and ridge waveguides with electrode spacing 0.75 μm and 2.5 μm, which are selected to feature equal metal-induced optical losses. k FEM simulations of LN waveguide bending loss for different slab heights and waveguide cross-section 2 μm × 0.6 μm. l FEM simulation of the voltage-length product of electro-optical modulation for strip and ridge waveguides as a function of metal-induced waveguide loss.CC BY · PubMed Central
Fig. 2 Fabrication process based on diamond-like carbon (DLC) hardmask. a Process flow of the LNOI waveguide fabrication including diamond-like carbon (DLC) hard mask deposition via plasma enhanced chemical vapor deposition (PECVD) from methane precursor, DLC dry etching via oxygen plasma, lithium niobate etching via argon ion beam etching (IBE), followed by redeposition and mask removal. LiNbO3 is illustrated in green, SiO2 in light blue, DLC in dark blue and Si in gray. b Photo of a deposited DLC film on the silicon wafer. SEM images of the taper pattern and the cross section with 250 nm width after DLC dry etching (c) DLC mask and the LiNbO3 waveguide after redeposition cleaning (d) and the LN waveguide with 0.7 μm height and 2 μm width after mask removal (e). In the SEM images, the LiNbO3 is colored in green, DLC in black, Si3N4 in purple and SiO2 in light purple. f The schematic and the result of indentation hardness measurement for DLC. g Hardness and etching selectivity chart for different materials. h Raman spectrum and fitting result for DLC film.
Fig. 2 Fabrication process based on diamond-like carbon (DLC) hardmask. a Process flow of the LNOI waveguide fabrication including diamond-like carbon (DLC) hard mask deposition via plasma enhanced chemical vapor deposition (PECVD) from methane precursor, DLC dry etching via oxygen plasma, lithium niobate etching via argon ion beam etching (IBE), followed by redeposition and mask removal. LiNbO3 is illustrated in green, SiO2 in light blue, DLC in dark blue and Si in gray. b Photo of a deposited DLC film on the silicon wafer. SEM images of the taper pattern and the cross section with 250 nm width after DLC dry etching (c) DLC mask and the LiNbO3 waveguide after redeposition cleaning (d) and the LN waveguide with 0.7 μm height and 2 μm width after mask removal (e). In the SEM images, the LiNbO3 is colored in green, DLC in black, Si3N4 in purple and SiO2 in light purple. f The schematic and the result of indentation hardness measurement for DLC. g Hardness and etching selectivity chart for different materials. h Raman spectrum and fitting result for DLC film.CC BY · PubMed Central
Fig. 3 Highly confining X-cut and Z-cut LiNbO3 photonic integrated microresonator characterization. a The most probable value κ0/2π of 9 chips measured on each of the fields shown on the 100 mm (4 in.) wafer. The reticle design contains 16 chips in each of the 9 fields uniformly exposed over the x-cut LiNbO3 wafer. b Microresonator transmission (blue) and linewidth measurement of a single resonance at 192.97 THz from the racetrack resonator with 3 μm width waveguide using frequency-comb-assisted diode laser spectroscopy results in κ0/2π = 18.2 MHz. c Histogram of TE00 resonances from a 2 μm (red) and 3 μm (blue) wide single racetrack x-cut LiNbO3 resonator showing the most probable value of κ0/2π = 45 MHz. d SEM image of microresonator with a radius of 20 μm from a z-cut LiNbO3 wafer (corresponding to a ca. 1 THz FSR). e Measured integrated dispersion of the x-cut LiNbO3 microresonator with free spectral range about 81 GHz, anomalous dispersion D2/2π = 105.72 kHz for 2 μm waveguide width microresonator and normal dispersion of D2/2π = −291.23 kHz for 3 μm waveguide width. Histograms of intrinsic cavity linewidths for TE00 resonances of z-cut LiNbO3 microresonators with FSRs of (f) 102 GHz, (g) 504 GHz, (h) 967 GHz. i Intrinsic cavity linewidth vs free spectral range of the microresonators. Red circles show the mean values from distributions in (f, g, h). Vertical bars show min and max values.
Fig. 3 Highly confining X-cut and Z-cut LiNbO3 photonic integrated microresonator characterization. a The most probable value κ0/2π of 9 chips measured on each of the fields shown on the 100 mm (4 in.) wafer. The reticle design contains 16 chips in each of the 9 fields uniformly exposed over the x-cut LiNbO3 wafer. b Microresonator transmission (blue) and linewidth measurement of a single resonance at 192.97 THz from the racetrack resonator with 3 μm width waveguide using frequency-comb-assisted diode laser spectroscopy results in κ0/2π = 18.2 MHz. c Histogram of TE00 resonances from a 2 μm (red) and 3 μm (blue) wide single racetrack x-cut LiNbO3 resonator showing the most probable value of κ0/2π = 45 MHz. d SEM image of microresonator with a radius of 20 μm from a z-cut LiNbO3 wafer (corresponding to a ca. 1 THz FSR). e Measured integrated dispersion of the x-cut LiNbO3 microresonator with free spectral range about 81 GHz, anomalous dispersion D2/2π = 105.72 kHz for 2 μm waveguide width microresonator and normal dispersion of D2/2π = −291.23 kHz for 3 μm waveguide width. Histograms of intrinsic cavity linewidths for TE00 resonances of z-cut LiNbO3 microresonators with FSRs of (f) 102 GHz, (g) 504 GHz, (h) 967 GHz. i Intrinsic cavity linewidth vs free spectral range of the microresonators. Red circles show the mean values from distributions in (f, g, h). Vertical bars show min and max values.CC BY · PubMed Central
Fig. 4 III-V self-injection locked laser based on tightly confining LiNbO3 photonic integrated circuits. a Schematic of hybrid LNOI laser. DFB laser is self-injection locked to an LNOI photonic chip-based racetrack resonator. The bulk and surface scattering inside the LNOI waveguide induces back-scattered light and triggers the laser self-injection locking and frequency noise reduction. b SEM of the racetrack resonator with electrodes including high resolution insets of the waveguide with electrodes (c, red) and the coupling section (d, blue) (e) Schematic of the optical setup for laser characterization. Heterodyne beatnote measurement using free running Toptica CTL as a reference allows to measure laser frequency noise using a fast photodetector (FPD) and electrical spectrum analyzer (ESA). Optical spectrum analyzer (OSA). f Optical spectrum of the hybrid LNOI laser emission. The side mode suppression ratio (SMSR) is greater than 60 dB. g Single-sided PSD of frequency noise of the hybrid LNOI laser upon self-injection locking to racetrack resonator with 81 GHz FSR (red) and in free-running regime (green). h Spectrogram showing laser frequency change upon the linear tuning of the diode current, dashed areas correspond to the 2.5 GHz range where the laser is self-injection locked with minimal lasing frequency fluctuations and reduced laser linewidth.
Fig. 4 III-V self-injection locked laser based on tightly confining LiNbO3 photonic integrated circuits. a Schematic of hybrid LNOI laser. DFB laser is self-injection locked to an LNOI photonic chip-based racetrack resonator. The bulk and surface scattering inside the LNOI waveguide induces back-scattered light and triggers the laser self-injection locking and frequency noise reduction. b SEM of the racetrack resonator with electrodes including high resolution insets of the waveguide with electrodes (c, red) and the coupling section (d, blue) (e) Schematic of the optical setup for laser characterization. Heterodyne beatnote measurement using free running Toptica CTL as a reference allows to measure laser frequency noise using a fast photodetector (FPD) and electrical spectrum analyzer (ESA). Optical spectrum analyzer (OSA). f Optical spectrum of the hybrid LNOI laser emission. The side mode suppression ratio (SMSR) is greater than 60 dB. g Single-sided PSD of frequency noise of the hybrid LNOI laser upon self-injection locking to racetrack resonator with 81 GHz FSR (red) and in free-running regime (green). h Spectrogram showing laser frequency change upon the linear tuning of the diode current, dashed areas correspond to the 2.5 GHz range where the laser is self-injection locked with minimal lasing frequency fluctuations and reduced laser linewidth.CC BY · PubMed Central
Fig. 5 Fast electro-optical tuning of a self-injection locked III-V LiNbO3 based laser. a Experimental setup for electro-optical response characterization of the LNOI microresonator laser using an external laser (Toptica CTL), a photoreceiver (PD) and a vector network analyzer (VNA). The beatnote is recorded on a fast oscilloscope (DSO) and analysed with short-time Fourier transforms. b Measured response of the electro-optic actuation for the 81 GHz FSR microresonator showing flat actuation bandwidth up to the cavity cutoff frequency. c Time-frequency spectrogram of the heterodyne beatnotes for triangular chirp repetition frequencies from 10 kHz to 1 MHz. The frequency excursion is 761 MHz at 10 kHz tuning rate, 730 MHz at 100 kHz and 721 MHz at 1 MHz chirp rate. Bottom row: residual of least-squares fitting of the time-frequency traces with symmetric triangular chirp pattern.
Fig. 5 Fast electro-optical tuning of a self-injection locked III-V LiNbO3 based laser. a Experimental setup for electro-optical response characterization of the LNOI microresonator laser using an external laser (Toptica CTL), a photoreceiver (PD) and a vector network analyzer (VNA). The beatnote is recorded on a fast oscilloscope (DSO) and analysed with short-time Fourier transforms. b Measured response of the electro-optic actuation for the 81 GHz FSR microresonator showing flat actuation bandwidth up to the cavity cutoff frequency. c Time-frequency spectrogram of the heterodyne beatnotes for triangular chirp repetition frequencies from 10 kHz to 1 MHz. The frequency excursion is 761 MHz at 10 kHz tuning rate, 730 MHz at 100 kHz and 721 MHz at 1 MHz chirp rate. Bottom row: residual of least-squares fitting of the time-frequency traces with symmetric triangular chirp pattern.CC BY · PubMed Central

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Research works printed but not linked: 2

  1. Wang, C. et al. Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages. Nature 562, 101–104 (2018). · DOI · Paper page
  2. He, M. et al. High-performance hybrid silicon and lithium niobate mach–zehnder modulators for 100 Gbit s−1 and beyond. Nat. Photonics 13, 359–364 (2019). · DOI · Paper page

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Authors
Li, Z., Wang, R. N., Lihachev, G., Zhang, J., Tan, Z., Churaev, M., Kuznetsov, N., Siddharth, A., Bereyhi, M. J., Riemensberger, J., & Kippenberg, T. J.
Affiliations
École Polytechnique Fédérale de Lausanne (CH)
Corresponding author
Kippenberg, Tobias J.
ORCID
Venue
Nature Communications
Publisher
Springer Science and Business Media LLC
Published
2023-08-10
Volume
14
Issue
1
Pages
4856-4856
ISSN
2041-1723 (ISSN-L)
DOI
10.1038/s41467-023-40502-8
PMC
PMC10415301
OpenAlex
W4385727101
Cited by
169
References
50
Citations by year
2026: 35 · 2025: 80 · 2024: 51 · 2023: 3
FWCI
19.62
Type
journal article
Language
en
Open access
Licence
Locations
  • Nature Communications (publishedVersion, cc-by)
  • Research Square (acceptedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • Infoscience (Ecole Polytechnique Fédérale de Lausanne) (submittedVersion, cc-by-nc-nd)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
  • National Science Foundation
  • European Commission (HORIZON2020, 835329, 812818)
  • École Polytechnique Fédérale de Lausanne
  • Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (201923, 186364)
  • Defense Advanced Research Projects Agency (HR0011-20-2-0046)
  • Advanced Research Projects Agency
  • Horizon 2020 Framework Programme (812818, EU‐H2020)
  • HORIZON EUROPE Framework Programme
  • HORIZON EUROPE European Innovation Council (101113260)
Topics
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Photorefractive and Nonlinear Optics
  • Physical Sciences › Engineering › Electrical and Electronic Engineering › Photonic and Optical Devices
  • Physical Sciences › Materials Science › Materials Chemistry › Ferroelectric and Piezoelectric Materials
Keywords
lithium niobate, photonic integrated circuits, deep etching, low-loss waveguides, electro-optic modulation, Mach-Zehnder modulator, half-wave voltage, CMOS-compatible photonics, optical fiber interface, integration density

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