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Integrated photonics on thin-film lithium niobate

Di Zhu, Linbo Shao, Mengjie Yu, Rebecca Cheng, Boris Desiatov, C. J. Xin, Yaowen Hu, Jeffrey Holzgrafe, Soumya Ghosh, Amirhassan Shams-Ansari, Eric Puma, Neil Sinclair, Christian Reimer, Mian Zhang, Marko Lončar

arXiv:2102.11956v1physics.opticsphysics.app-ph

TL;DR

Integrated photonics needs a platform that combines low loss, efficient modulation, and nonlinear functionality while supporting increasingly integrated classical and quantum systems. This Review surveys thin-film LN photonics from materials and passive optics through electro-optics, nonlinearities, acousto-optics, heterogeneous integration, and platform challenges. Thin-film LN has produced integrated devices that outperform bulk counterparts in key communication metrics and has enabled applications including frequency combs, quantum transducers, and piezo-optomechanics.

  • Problem

    Existing integrated photonic platforms cannot simultaneously support ultra-low propagation loss, fast low-loss optical modulation, and efficient all-optical nonlinearities.

  • Method

    The Review synthesizes thin-film LN photonics from basic principles to the state of the art across materials, passive optics, electro-optics, nonlinearities, piezo-optomechanics, heterogeneous integration, and challenges.

  • Results

    Thin-film LN modulators have significantly outperformed bulk counterparts in power consumption, bandwidth, and size, while integrated LN resonators support frequency combs and cavity electro-optics.

  • Takeaways & Limitations

    Thin-film LN integrated photonics is rapidly expanding from telecommunications and nonlinear optics into quantum photonics, cavity electro-optics, and piezo-optomechanics.

  • Takeaways & Limitations

    LN integration remains constrained by CMOS and silicon-photonics incompatibility, lithium-related contamination, and fabrication challenges for low-roughness etching.

Abstract

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Lithium niobate (LN), an outstanding and versatile material, has influenced our daily life for decades: from enabling high-speed optical communications that form the backbone of the Internet to realizing radio-frequency filtering used in our cell phones. This half-century-old material is currently embracing a revolution in thin-film LN integrated photonics. The success of manufacturing wafer-scale, high-quality, thin films of LN on insulator (LNOI), accompanied with breakthroughs in nanofabrication techniques, have made high-performance integrated nanophotonic components possible. With rapid development in the past few years, some of these thin-film LN devices, such as optical modulators and nonlinear wavelength converters, have already outperformed their legacy counterparts realized in bulk LN crystals. Furthermore, the nanophotonic integration enabled ultra-low-loss resonators in LN, which unlocked many novel applications such as optical frequency combs and quantum transducers. In this Review, we cover -- from basic principles to the state of the art -- the diverse aspects of integrated thin-film LN photonics, including the materials, basic passive components, and various active devices based on electro-optics, all-optical nonlinearities, and acousto-optics. We also identify challenges that this platform is currently facing and point out future opportunities. The field of integrated LNOI photonics is advancing rapidly and poised to make critical impacts on a broad range of applications in communication, signal processing, and quantum information.

1. Introduction

Integrated photonics platforms offer scalable optical solutions but generally cannot combine ultra-low loss, fast low-loss modulation, and efficient all-optical nonlinearities. Thin-film LN addresses this gap through improved wafers, nanofabrication, and a broad set of integrated components reviewed from fundamentals to emerging applications.

  • Existing integrated photonics materials cannot simultaneously provide ultra-low propagation loss, fast low-loss modulation, and efficient all-optical nonlinearities.
  • LN combines exceptional electro-optic, nonlinear-optic, and acousto-optic properties with a wide transparency window and relatively high refractive index.Bulk LN already supports fiber-optic modulators, wavelength conversion, and photon-pair generation.
  • Commercially available high-quality thin-film LN wafers and fabrication advances have enabled ultra-low-loss, high-index-contrast nanophotonic waveguides and high-performance integrated components.Reported components include compact modulators and broadband frequency-comb sources.
  • The platform hosts diverse devices, including waveguides, resonators, frequency-comb sources, nonlinear converters, lasers, detectors, electro-optic modulators, acousto-optic modulators, and quantum transducers.Figure 1 presents this device range as a visual illustration rather than an exhaustive inventory.
  • The Review covers thin-film LN materials and passive optics, electro-optics, all-optical nonlinearities, piezo-optomechanics, heterogeneous integration, challenges, and opportunities.Its scope extends from basic principles to the state of the art and includes classical and quantum applications.

2. Material and integrated passive optics

Thin-film LN combines broad, multifunctional material properties with wafer-scale fabrication and high-contrast, low-loss integrated optics. Its waveguides and resonators support compact devices, nonlinear interactions, and applications including modulation, wavelength conversion, and frequency combs, while packaging remains a challenge.

  • Material properties: LN offers a broad transparency window, high refractive index, strong Pockels and third-order nonlinearities, and useful acousto-optic properties.Its transparency spans 350 nm to 5 μm, with r_33 ≈31 pm/V and n_2 = 1.8×10^-19 m^2/W.
  • Thin-film lithium niobate: Ion slicing and direct wafer bonding produce large-area, single-crystalline LN thin films, with commercially available LNOI wafers reaching 6 inches.The process supports doped and stoichiometric LN variants as well as standard LNOI wafers.
  • Waveguides: Low-index-contrast diffused waveguides have weak confinement, millimeter-scale bends, and limited suitability for dense integration, micro-resonators, and dispersion engineering.Their typical mode area is 10 to 100 μm^2, while proton exchange changes the extraordinary index by around 0.1 or less.
  • Waveguides: Dry-etched and chemo-mechanically polished LNOI waveguides have achieved propagation losses of a few dB/m, although sidewall roughness remains a major dry-etching loss mechanism.Dry-etched loss improved from >6 dB/cm in 2007 to 0.027 dB/cm in 2017, while values below 0.1 dB/cm are routinely produced.
  • Waveguides: Hybrid waveguides use deposited materials or bonded LN to combine lithium niobate functionality with other photonic platforms.Rib loading confines part of the mode in the LN slab, whereas bonding makes LN a subsidiary material on platforms such as Si or Si3N4.
  • Integrated passive optics: High confinement reduces device footprint and enhances nonlinear interactions, but creates fiber-interface mismatch and leaves optical packaging as a major practical boundary.End-fire and grating coupling address this mismatch, while resonators and microdisks enable modulation, wavelength conversion, optomechanics, and frequency-comb applications.

3. Electro-optics

Thin-film LN electro-optics supports modulation, sideband generation, frequency shifting, and coupling between optical resonances through strong Pockels interactions and engineered electrodes. The review describes common modulator architectures, governing EO relations, performance metrics, and design trade-offs affecting voltage, bandwidth, and microwave loss.

  • EO principles: The linear EO effect links applied electric fields to changes in LN impermeability and permittivity through the Pockels tensor.The tensor formulation uses crystal-direction indices and LN’s symmetry-reduced contracted notation.
  • EO principles: LN’s dominant EO coefficient r33 is approximately 31 pm/V and produces index modulation along the Z axis when the electric field is applied along z.Most reviewed devices use r33; the corresponding index change is approximately Δn≈−1/2 n_e^3 r33 E_z.
  • Modulator configurations: Thin-film LN EO modulators use phase, Mach-Zehnder intensity, IQ, and Michelson configurations for phase, intensity, amplitude, and phase control.IQ modulators combine two Mach-Zehnder modulators to encode amplitude and phase, supporting coherent transmission systems.
  • Performance metrics: An MZM’s push-pull configuration gives it a half-wave voltage that is half that of a phase modulator.Vπ denotes the voltage for a π phase shift in phase modulators and switching between on and off states in intensity modulators.
  • Performance metrics: 2.1–2.5 V-cm is the typical measured DC/low-frequency VπL range for monolithic dry-etched LNOI MZMs at telecom wavelengths.A value of 1.8 V-cm has been demonstrated with a 3.5 μm electrode gap, while reducing gaps further can increase optical absorption and microwave loss.
  • Electrode engineering: 0.26 dB cm-1 GHz-1/2 microwave loss was demonstrated using segmented CPWs, versus typical LNOI CPW losses of 0.7–1.1 dB cm-1 GHz-1/2.Segmented structures reduce current crowding near the metal edge, but their added capacitance can slow the wave and require lower-permittivity carrier materials for velocity matching.

3.3. Resonant electro-optic modulators

Resonant electro-optic modulators use voltage-controlled optical resonances to achieve strong modulation in compact thin-film LN devices. Their efficiency and compactness trade off against bandwidth, although coupled-resonator and coupling-modulation designs can tailor this limitation.

  • Voltage tuning shifts a resonator’s frequency and changes transmission near resonance, enabling compact electro-optic modulation in thin-film LN.Ring and photonic-crystal-cavity modulators are representative implementations.
  • Resonant modulators can change both transmitted amplitude and phase, with the response determined by the coupling ratio η = κ_e/κ_i.In the overcoupled, resonant-input regime, transmission is nearly lossless and the output is phase shifted.
  • Q = 10^7 at wavelengths near 1.5 μm corresponds to an electro-optic interaction length of order 1 m, about 10-fold longer than typical non-resonant modulators.Electrode geometry can reduce the effective coverage fraction and therefore the realized interaction length.
  • Resonant modulators trade compactness and high modulation efficiency for reduced electro-optic bandwidth set by the resonator decay rate.For Q = 10^7 near 1.5 μm, the decay rate is κ = 2π × 20 MHz.
  • The efficiency–bandwidth tradeoff can be tailored with multiple coupled resonators or by modulating resonator–waveguide coupling instead of resonance frequency.Relatively low bandwidth can still suffice for optical switch networks and beam steering.

3.4. Electro-optic frequency combs

Electro-optic frequency combs generate equally spaced optical lines by phase modulation, with bandwidth and flatness improved through cascaded or resonant architectures. Thin-film LN enables both broad non-resonant combs and high-line-count resonant combs on chip.

  • Non-resonant electro-optic combs: Phase modulation generates an optical frequency comb whose line spacing equals the microwave modulation frequency.Bessel functions determine individual line amplitudes, while the modulation index controls the overall spectral shape.
  • Non-resonant electro-optic combs: The modulation index is β = πV_p/V_π, relating the peak driving voltage to the modulator half-wave voltage.This parameter is central to shaping the generated comb spectrum.
  • Non-resonant electro-optic combs: Over 40 spectral elements with 30 GHz spacing were generated using a 2 cm X-cut thin-film LN phase modulator driven at approximately 4V_π.The modulator had V_π of 3.5–4.5 V from 5–40 GHz.
  • Non-resonant electro-optic combs: Cascading modulators can broaden non-resonant combs and improve spectral flatness, addressing the limited microwave power available to individual phase modulators.A Mach–Zehnder intensity modulator can be added in series with the phase modulator.
  • Resonant electro-optic combs: Resonant EO combs require the microwave drive to match the optical cavity free spectral range, maximizing cascaded sideband generation.Cavity finesse enhances phase-modulation efficiency through repeated optical passes.
  • Resonant electro-optic combs: A high-Q LNOI racetrack cavity with approximately 10 GHz FSR generated a telecom-band comb containing 900 lines when driven at its FSR.Changing the microwave frequency can generate lines at multiples of the optical FSR.
  • Spectroscopy: EO comb sources support dual-comb spectroscopy, while cascading cavity-based and non-resonant combs produced a 3 GHz comb on X-cut LNOI.Phase-locked microwave drives can provide mutual coherence between EO comb sources.

3.5. Coupled-resonator-based modulators

Coupled resonators and electro-optic modulation create programmable photonic two-level systems and efficient frequency-domain control. Thin-film LN also supports cavity electro-optic transduction between microwave and optical fields.

  • Photonic molecules: Hybrid modes in strongly coupled resonators form photonic two-level systems whose transitions can be driven by dynamic electro-optic modulation.Ultra-low-loss waveguides, high-Q resonators, and strong electro-optic interaction support long-lived photonic states.
  • Photonic molecules: Rabi oscillation, Ramsey interference, Autler–Townes splitting, and Stark shifts were demonstrated, alongside programmable bright-dark photon-pair storage and retrieval.These phenomena arise from electro-optically controlled transitions between hybrid resonator modes.
  • Frequency shifting and beam splitting: ~99% shift efficiency and 0.45 dB on-chip insertion loss were demonstrated for an electro-optic frequency shifter using continuous-wave optical input and sinusoidal microwave drive.The coupled-ring device also supports bidirectional frequency exchange and reconfiguration as a tunable frequency beam splitter.
  • Cavity electro-optics: Cavity electro-optics combines electro-optically active optical resonators with low-loss microwave cavities to strengthen microwave–optical interactions.Microwave resonance enhancement increases modulation strength but narrows bandwidth according to the microwave-resonator linewidth.
  • Cavity electro-optics: On-chip microwave-to-optical photon conversion efficiency reached 1% in thin-film LN cavity electro-optic transducers after mitigating photorefractive effects and increasing pump power.Earlier thin-film devices demonstrated efficiencies as high as 2×10^-5, while near-unity efficiency is expected for optimized devices.
  • Synthetic dimensions: Frequency crystals with dimensions up to four and their one- to four-dimensional densities of states were theoretically and experimentally realized on LNOI.Time-dependent modulation couples optical frequency modes, allowing them to act as lattice points in a synthetic dimension.

4. All-optical nonlinearity

Thin-film LN supports broad all-optical nonlinear processes because of its second- and third-order nonlinearities, wide transparency window, and low propagation loss. The review emphasizes dispersion engineering and phase matching as central tools for these processes.

  • Platform capabilities: LN combines second- and third-order nonlinearities with transparency from ultraviolet to mid-infrared, enabling broad nonlinear optical processes at modest pump powers.For wavelengths above 800 nm, the absence of multiphoton absorption is also advantageous.
  • Phase matching and dispersion: Phase matching is central to thin-film LN nonlinear optics, with progress in engineering group velocity, group velocity dispersion, modal phase matching, and quasi-phase matching.Dispersion engineering and cavity integration are highlighted as complementary routes for nonlinear photonic applications.

4.1. Dispersion engineering

Thin-film LN enables precise geometric and cladding control of waveguide dispersion, supporting nonlinear processes such as frequency combs, supercontinuum generation, and broadband harmonic generation.

  • Geometric and cladding control: Waveguide width, etch depth, film thickness, etch angle, and cladding configuration provide control over thin-film LN group velocity and dispersion.Simulations compare TE-mode dispersion for varied geometries and uncladded, oxide-clad, and suspended structures.
  • Dispersion calculation: Dispersion curves are obtained by simulating effective mode indices and numerically calculating their second derivative.Finite-difference eigenmode simulations and Sellmeier equations were used to evaluate geometry-dependent dispersion.
  • Nonlinear applications: Thin-film LN dispersion engineering has enabled flexible GVD control in microrings and applications including Kerr frequency comb and supercontinuum generation.The sign of GVD can be tuned through waveguide dimensions, with anomalous GVD supporting positive four-wave-mixing parametric gain for positive Kerr nonlinearity.
  • Targeted nonlinear features: Engineering group velocity and higher-order dispersion targets zero group-velocity mismatch, reduced temporal walk-off, and controlled dispersive-wave radiation.These conditions support broadband second-harmonic generation and other nonlinear processes.
  • Phase matching: Phase mismatch for three-wave mixing is Δk = k1 + k2 − k3, where each propagation constant depends on modal effective index and frequency.The phase-matching condition is Δk = 0.

4.2. Phase matching

Thin-film LN nonlinear conversion relies on phase matching through polarization, spatial-mode, domain, and cyclic engineering. These approaches trade bandwidth, efficiency, mode overlap, fabrication complexity, or collection efficiency in different ways.

  • Phase-matching principle: Phase matching is required to prevent accumulated phase mismatch from limiting efficient frequency conversion over the nonlinear medium.Several methods compensate or avoid the momentum mismatch in three-wave mixing.
  • Birefringent phase matching: Birefringent phase matching changes the polarization configuration, but it is rarely used in integrated LN because it excludes the largest χ^(2) tensor component.Using the largest component requires type-0 polarization, which must be phase matched by other methods.
  • Intermodal phase matching: Intermodal phase matching uses a higher-order spatial mode to alter effective index and enable type-0 second-harmonic generation in etched waveguides and resonators.Fundamental and third-order modes are often selected because even–odd mode overlap approaches zero for mismatched symmetry.
  • Intermodal phase matching: Intermodal phase matching can access the largest χ^(2) component but incurs lower mode overlap, so optimal conversion requires matching polarization and spatial modes while satisfying phase matching.The approach therefore presents a direct efficiency trade-off between nonlinear tensor strength and modal overlap.
  • Quasi-phase matching: Quasi-phase matching periodically reverses the relevant χ^(2) sign, adding grating momentum that offsets intrinsic phase mismatch and prevents energy backflow.In LN, periodic poling uses high-voltage pulses to invert ferroelectric-domain orientation.
  • Periodic poling: Thin-film PPLN fabrication is more complicated than bulk PPLN because bulk methods rely on electrodes patterned on LN’s ±z faces.Thin-film LN also generally requires a higher coercive field for domain inversion than corresponding bulk crystals.
  • Cyclic phase matching: Cyclic phase matching broadens the phase-matching bandwidth in ring or microdisk cavities but sacrifices conversion efficiency and produces wavelength-dependent gain profiles.A normalized conversion efficiency of 0.1%/mW was demonstrated in a double-resonant X-cut LN whispering-gallery microresonator.
  • Metasurface-assisted SHG: A dielectric metasurface enabled phase-matching-free SHG with normalized conversion efficiency of 1,660% W^-1cm^-2 at telecom wavelengths.The approach provides broadband enhancement but generally collects higher-order second-harmonic modes inefficiently.

4.3. Second-order nonlinear wavelength conversion

Thin-film LN supports efficient second-order wavelength conversion in traveling-wave and resonant devices. Nanophotonic confinement, periodic poling, modal engineering, and high-Q cavities improve conversion efficiency while introducing phase-matching and fabrication constraints.

  • Traveling-wave processes: Nanophotonic PPLN waveguides can achieve bulk-like conversion efficiency with shorter interaction lengths and broader phase-matching bandwidth.Smaller mode areas raise conversion efficiency, while shorter PPLN sections broaden bandwidth because bandwidth is inversely proportional to interaction length.
  • Traveling-wave processes: 4600%/W·cm2 normalized SHG efficiency was achieved in a 300 μm LN ridge waveguide using active monitoring for optimal poling.Longer ridge waveguides reached below 3000%/W·cm2 when poling duty cycle or uniformity was non-optimal.
  • Traveling-wave processes: Type-I modal phase matching sacrifices efficiency through weaker tensor participation and lower mode overlap but provides a higher thermal tuning range.The passage identifies improved efficiency with broad thermal tunability as a likely direction.
  • Resonant-enhanced processes: Resonant SHG requires simultaneous energy and momentum conservation, strong nonlinear susceptibility and overlap, small mode area, high quality factors, and critical coupling.Periodic poling relaxes the momentum condition by adding a domain-period contribution.
  • Resonant-enhanced processes: PPLN racetracks and microrings reached 230%/mW and 250%/mW SHG efficiencies, respectively, despite differing poling geometries and nonlinear coefficients.The racetrack used d33 with fundamental quasi-TE modes, whereas the radially poled microring used d31 for TE00-to-TM00 conversion.
  • Resonant-enhanced processes: High-Q LN resonators also support cascaded harmonic, sum-frequency, Raman, and down-conversion processes, while photonic-crystal SHG remains limited by 0.078%/W efficiency.Photonic crystals face challenges from simultaneous infrared-visible resonances and photorefractive effects.

4.4. Cascaded second-order nonlinearity

Cascaded second-order processes let thin-film LN emulate tunable third-order nonlinear behavior. Quasi-phase matching and tight optical confinement enable control of nonlinear phase shifts, frequency conversion, and broadband dynamics, while further dynamical regimes remain under study.

  • Mechanism: Phase-mismatched χ(2) processes create an effective Kerr nonlinearity through successive upconversion to the second harmonic and downconversion back to the fundamental wave.The resulting nonlinear phase shift scales with the square of the second-order susceptibility.
  • Engineering: Quasi-phase matching can tune the effective χ(3) positive or negative, enabling nonlinear-dynamics control in periodically poled LN waveguides.The QPM condition is expressed through Δk = k_SH − 2k_FW − k_QPM.
  • Engineering: An effective χ(3) was achieved at 2050 nm in an integrated PPLN waveguide, while thin-film confinement reduces effective mode area by more than an order of magnitude versus weakly confined RPE waveguides.The nonlinear parameter γ scales as χ(3)/A_eff.
  • Dynamics: Group-velocity mismatch sets SHG bandwidth approximately as Δω ∼ 1/(|Δβ′|L), motivating cascaded quadratic soliton compression and related nonlinear regimes.Waveguide dispersion and quasi-phase matching together provide access to further dynamical processes.
  • Dynamics: Supercontinuum spectra were measured at multiple pulse energies in a PPLN waveguide assisted by cascaded second-order nonlinearity.

4.5. Kerr comb

Thin-film LN integrates the components needed for chip-scale Kerr comb systems, but Raman scattering complicates soliton formation. Stable combs have therefore been demonstrated by changing polarization, wavelength, or free spectral range.

  • Kerr comb: Thin-film LN can host an entire Kerr-comb photonic circuit on one chip, including electro-optic tuning and on-chip self-referencing components.The integrated circuit concept includes soliton formation, frequency doubling, and f-2f self-referencing.
  • Kerr comb: 700 nm-spanning Kerr combs were demonstrated on X-cut LNOI, but strong Raman scattering in the Z-polarized TE mode competes with four-wave mixing.The Raman features correspond to scattering from multiple phonon branches.
  • Kerr comb: Stable soliton combs were achieved by generating combs along the non-polar axis, at longer wavelengths with weaker Raman effects, or with larger FSRs that avoid strong Raman gain.
  • Kerr comb: Octave-spanning LNOI combs still face unresolved Raman mitigation and require careful management of competing photorefractive and thermo-optic effects at high power.

4.6. Raman lasing

Raman scattering in LN is both a material-probe technique and a nonlinear-dynamics mechanism. Thin-film devices show fabrication-dependent Raman behavior, with stimulated oscillation often favoring backward propagation.

  • Raman lasing: Raman spectroscopy probes LN crystallinity, surface quality, doping, strain, and vibrational transitions through polarization- and propagation-dependent configurations.
  • Raman lasing: Forward Raman scattering corresponds to a polariton effect in which photons and phonons strongly mix and produce electromagnetic radiation.
  • Raman lasing: Stimulated Raman oscillation in monolithic X-cut LN microrings and racetracks is dominant in the direction counterpropagating to the pump.
  • Raman lasing: Raman oscillation strength, peak position, and linewidth vary across thin-film LN devices, potentially because of annealing, etching, lithography, and wafer-composition differences.These discrepancies make fabrication conditions relevant to interpreting and engineering Raman behavior.

4.7. Supercontinuum generation

Thin-film LN enables coherent, broadband supercontinuum generation by combining tight confinement with χ(2) and χ(3) nonlinearities. These capabilities support on-chip f-2f interferometry and frequency-comb stabilization.

  • Mechanisms: Supercontinuum generation in LN uses self-phase modulation, cross-phase modulation, four-wave mixing, and dispersive-wave generation to broaden input pulses.Tight optical confinement supports efficient generation with short pulses and short waveguides.
  • f-2f interferometry: Coherent two-octave supercontinuum generation and fCEO detection were achieved in a single LN waveguide through spectral overlap of SHG and the supercontinuum.Dispersion engineering simultaneously generated a strong second-harmonic signal and broadband coherent supercontinuum.
  • Pulse dynamics: A modeled 0.5-cm dispersion-engineered LN waveguide generated SHG, THG, self-phase-modulation broadening, and a dispersive wave during pulse propagation.The simulation used an 800-nm-thick X-cut LN film, 90 fs pulses centered at 1560 nm, and 107 pJ pulse energy.
  • f-2f interferometry: The LN f-2f interferometer directly detected and stabilized the carrier-envelope offset frequency using a silicon avalanche photodiode and feedback loop.The integrated approach simplified the conventional f-2f interferometer architecture.
  • Spectral extension: Combining dispersive-wave generation, harmonic generation, and difference-frequency generation extended the supercontinuum from 0.35 to 4.1 µm with 240 pJ pulse energy.SHG and sum-frequency generation produced visible components, while cascaded SHG extended generation toward the ultraviolet.
  • Broadband generation: A cascaded χ(2) approach produced a 2.5-octave supercontinuum with 10 pJ pulse energies.Quasi-phase-matched SHG provided a bandwidth exceeding 110 nm and effective self-phase modulation 200 times stronger than intrinsic χ(3) nonlinearity.

5. Piezo-optomechanics

Thin-film LN piezo-optomechanics couples optical, acoustic, and microwave degrees of freedom using LN’s piezoelectric and photoelastic properties. Device performance depends strongly on acoustic confinement, mode design, and substrate choice.

  • Principles: Piezo-optomechanical devices use photoelastic and moving-boundary interactions to interface light with elastic waves, while electrically driven acoustics connect elastic waves to microwaves.LN is attractive because it combines low-loss optical and mechanical operation with a high piezoelectric coefficient.
  • Limitations: PZT and TeO2 could improve piezo-optomechanical efficiency through stronger piezoelectricity or photoelasticity if high-quality, low-loss thin films become available.TeO2 offers strong bulk acousto-optic properties, but its nanofabrication remains challenging.
  • Coupling mechanisms: The secondary electro-optic effect can contribute more than one third of the total microwave-to-optical coupling in LN.It arises from electric fields generated by elastic waves through LN’s piezoelectricity.
  • Platform comparison: Thin-film LN on oxide confines optical and acoustic modes while supporting more than 1 W microwave power, compared with approximately 100 mW for suspended LN under similar IDT structures.The higher power handling follows from simultaneous optical and acoustic confinement by the LN-on-oxide structure.
  • Substrates: LN-on-oxide confines only a few acoustic modes, whereas LN-on-sapphire supports shear modes whose strain can add constructively in the optical overlap.Sapphire bonding remains constrained by bubbles that reduce usable wafer area to less than 50%.
  • Microwave-to-optical conversion: The state-of-the-art single microwave photon to optical photon conversion efficiency is 5% using piezo-optomechanical crystals.Higher mechanical Q factors and smaller mode volumes improve conversion relative to LN cavity electro-optic implementations.
  • Acousto-optic devices: LN acousto-optic frequency shifters achieve frequency-shifting efficiencies 10–100 times higher than the referenced conventional devices.Integrated acousto-optic devices also operate in the gigahertz regime, beyond typical commercial bulk-device frequencies.

6. Heterogeneous integration

Heterogeneous integration supplements thin-film LN with complementary materials and mature fabrication platforms. Demonstrations span hybrid modulators, detectors, quantum emitters, rare-earth devices, and on-chip lasers and amplifiers.

  • Rationale: Heterogeneous integration addresses missing LN functions, including memories, detectors, deterministic sources, lasers, amplifiers, and driving electronics.It also adds χ(2) nonlinearity or quasi-phase matching to mature platforms such as silicon and silicon nitride.
  • Hybrid modulators: Hybrid LN/Si modulators combine silicon photonics scalability with LN electro-optic performance, including demonstrated 3 dB bandwidth exceeding 100 GHz.Designs use mode transition and LN confinement for modulation, while dry-etched approaches can transition fully into the LN waveguide.
  • Hybrid modulators: Rib-loaded hybrid modulators avoid LN etching and reduce some process-compatibility issues while achieving similar performance to monolithic approaches.The approach has been implemented with SiNx, Ta2O5, chalcogenide glasses, and Si.
  • Detectors: Direct integration of SNSPDs on thin-film LN waveguides has been demonstrated despite fabrication challenges from etching, cleaning, and superconducting-film deposition.The partially etched waveguide provides approximately 0.1 dB/μm TE-mode absorption, requiring nanowires hundreds of micrometers long for near-unity on-chip efficiency.
  • Detectors: Amorphous-silicon metal-semiconductor-metal photodetectors on thin-film LN show responsivities of 22 mA/W to 37 mA/W from 635 nm to 850 nm.Waveguide-integrated detectors reduce fiber-to-chip coupling loss and support compact arrays.
  • Quantum emitters: InAs quantum dots transferred onto LN circuits provided telecom-band single-photon sources with 40.1% coupling efficiency.Focused-ion-beam pick-and-place transferred the quantum dots from an InP nanobeam to the LN photonic circuit.
  • Rare-earth integration: Rare-earth characterization on thin-film LN found optical absorption and polarization behavior consistent with bulk material, while resonance fluorescence showed a 170 GHz linewidth.A measured fluorescence lifetime of 3.2 ms exceeded the 2.0 ms bulk population lifetime reference.
  • Active integration: Er3+-doped thin-film LN enabled a 3.6 cm spiral waveguide amplifier with maximum net gain of 18 dB near 1530 nm.The amplifier was demonstrated in the small-signal-gain regime.

7. Challenges and Opportunities

Thin-film LN photonics offers new capabilities across quantum, microwave, nonlinear, and nonreciprocal photonics, while facing stability, fabrication, and integration challenges.

  • Stability challenges: Charge-carrier effects can limit LN device stability and power handling, with photorefractivity stronger and faster in LNOI than in comparable bulk devices.Dielectric relaxation may cause bias drift; thermo-optic control avoids drift but trades power dissipation against tuning bandwidth.
  • Quantum photonics: Thin-film LN supports quantum applications through low propagating loss and low-loss electro-optic phase shifters.Electro-optic phase shifters could reduce power consumption compared with thermal phase shifters.
  • Nonlinear photonics: Its strong χ(2) and χ(3) nonlinearities and phase-modulation capability enable new frequency-comb, soliton, and topological-photonics directions.The review highlights chimera states, band solitons, co- and counter-propagating mode coupling, and high-dimensional topological effects.
  • Nonreciprocity: Magnetic-free optical isolation can use directional difference-frequency generation in periodically poled LN waveguides or phonon-mediated acousto-optic modulation.Difference-frequency generation favors co-propagating signal and pump waves through stringent phase matching, while counter-propagating waves are blocked by filtering.
  • Microwave photonics: LNOI microwave photonics combines ultralow loss, >100 GHz electro-optic modulators, few-volt half-wave voltage, <0.5 dB on-chip insertion loss, and ~100 dB Hz^2/3 SFDR.The platform also offers MHz resonant filters, broadband frequency comb sources, and magnetic-free isolators, but lacks native light sources, amplifiers, and detectors.
  • Fabrication and scale-up: Low-roughness LN etching remains insufficiently available, so most thin-film LN devices are still chip-scale proof-of-concept demonstrations relying on electron-beam lithography.Monolithic 4-inch and 6-inch wafer-scale processing offers a possible lower-complexity, lower-cost alternative to bonded fabrication.

8. Conclusion

The review presents thin-film LN as a rapidly expanding integrated-photonics platform with broad application potential, while emphasizing unresolved material, fabrication, and integration challenges.

  • Review scope: The review surveys thin-film LN materials, passive and active components, nonlinearities, piezo-optomechanics, heterogeneous integration, and remaining challenges.Its scope extends from foundational material properties to opportunities for future integrated devices.
  • Field trajectory: Thin-film LN is rapidly expanding from telecommunications and nonlinear optics into quantum photonics, cavity electro-optics, and piezo-optomechanics.The platform builds on a half-century legacy of discrete LN-crystal components.
  • Future priorities: Future progress depends on optimizing material preparation, device fabrication, and optical and RF packaging while addressing photorefraction, charge accumulation, power handling, and loss.These studies are described as essential for extreme nonlinearities, high-power electro-optic operation, harsh environments, and long-term stability.
  • Outlook: The review expects thin-film LN to become a strong contender in integrated photonics and affect everyday applications.This outlook is linked to the platform’s versatility and complexity.
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