We report on the wafer scale fabrication of single-mode low-loss lithium niobate on insulator waveguides utilizing a chemically amplified resist and an optimized dry etching method. The fabricated single-mode waveguides are free of residuals and re-deposition, with measured losses for straight waveguides around 2 dB/m (0.02 dB/cm). We present a method offering advantages for large-scale production mainly due to its cost-effectiveness and faster writing time. This work holds promise for advancing integrated photonics and optical communication technologies. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Lithium niobate, and more recently thin film lithium niobate on insulator (LNOI, TFLN), is a widely used material in integrated photonics due to its excellent optical and electro-optic properties. With the increasing demand for high-speed, high integration density photonic devices, lithium niobate integrated photonics has become an active research area. The focus of the presented technology development is on low-loss waveguides (below 0.1 dB/cm) that enable the integration of a wide variety of devices, including modulators, resonators and frequency converters, on a single chip for applications in quantum technologies. Implications and challenges of wafer-scale fabrication will be discussed.

We propose an optimized low-temperature ion exchange method for fabricating an erbium-doped lithium niobate on insulator (LNOI) substrate. This method ensures the production of high-quality, crack-free substrates. The erbium-doped substrates are characterized spectroscopically in the near-infrared wavelength range. Additionally, we demonstrate deterministic local doping by using a SiO2 mask. This relatively simple, locally selective doping technology can facilitate the implementation of new and practical active building blocks in the LNOI platform, which could be attractive for several applications, like the realization of integrated lasers and amplifiers.

Spatial engineering of the nonlinear susceptibility χ ( 2 ) in resonant metasurfaces offers a new degree of freedom in the design of the far-field response of second-harmonic generation (SHG). We demonstrate this by applying electric field poling to lithium niobate (LN) thin films, which inverts the spontaneous polarization and thus the sign of χ ( 2 ) . Metasurfaces fabricated in periodically poled LN films reveal the distinct influence of the χ ( 2 ) -patterning on the spatial distribution of the second harmonic. This work is a first step toward far-field engineering of SHG in metasurfaces with electric field poling.

We doped a thin-film lithium niobate on insulator substrate by Er ions, using an ion exchange process. We investigated and characterized the generated fluorescence light from the substrate.

Sources of spectrally engineered photonic states are a key resource in several quantum technologies. Of particular importance are the so-called factorizable biphoton states, which possess no spectral entanglement and hence, are ideal for heralded generation of high-purity single photons. An essential prerequisite for generating these states through nonlinear frequency conversion is the control over the group indices of the photonic modes of the source. Here, we show that thin-film lithium niobate on insulator (LNOI) is an excellent platform for this purpose. We design and fabricate periodically poled ridge waveguides in LNOI to demonstrate group index engineering of its guided photonic modes and harness this control to experimentally realize on-chip group index matched type-II sum-frequency generation (SFG). Also, we numerically study the role of the top cladding layer in tuning the dispersion properties of the ridge waveguide structures and reveal a distinctive difference between the air and silica-clad designs which are currently among the two most common device cladding configurations in LNOI. We expect that these results will be relevant for various classical and quantum applications where dispersion control is crucial in tailoring the nonlinear response of the LNOI-based devices.

We demonstrate resonant metasurfaces with spatiallly engineered nonlinearity based on periodically poled lithium niobate. Such spatial control of the nonlinearity unlocks additional degrees of freedom for controlling the properties of second-harmonic radiation from metasurfaces.

Lithium niobate (LN) is a promising and versatile material platform for implementing various elements essential for realizing integrated photonic technology. We report on integrated entangled photon-pair sources and adiabatic coupler circuits built in bulk/thin-film LN. © 2021 The Author(s)

We present our experimental results on electric field poling of X-cut thin-film lithium niobate with poling periods of 1–4 µm. We used polarization contrast microscopy combined with digital image processing to study the impact of the experimental parameters of the poling process such as electric field strength, poling pulse duration, and layout of metal electrodes on the domain growth and quality of resulting poling patterns. High-quality results with excellent homogeneity of the domain pattern on large areas [i.e., 16 µm along the domain (z axis) and millimeter long domain gratings along the y axis with a period down to 1 µm] allow for the integration of numerous devices and structures.

All-dielectric optical metasurfaces are a workhorse in nano-optics due to both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair production is enhanced up to two orders of magnitude compared to an unpatterned film of the same thickness and material. These results enable flat-optics sources of entangled photons -- a new promising platform for quantum optics experiments.

Nanoscale periodically poled lithium niobate (LiNbO 3 ) waveguides with cross-sectional area below 1 μm 2 have recently enabled nonlinear frequency up-conversion of light with ultrahigh efficiencies [1] . Such ultra-compact nanophotonic waveguides also allow extensive control over the dispersion properties of their guided modes which has been exploited to realize broadband phase-matched nonlinear optics and supercontinuum generation [2] . In this work we experimentally demonstrate type-II quasi phase-matched sum frequency generation (SFG) in a dispersion engineered nanowaveguide. We explicitly show that the phase matching condition for SFG can be tailored by controlling the group indices of the guided modes through optimal design of the waveguide. Also, we contrast the phase matching condition for this type-II process with that of a type-0 mode-matched SFG process occurring in the same waveguide to clearly distinguish the role of dispersion engineering in nonlinear wavelength conversion processes in nanowaveguides.

We report on the first-time generation of biphotons via spontaneous parametric downconversion driven by Mie-type resonances in subwavelength metasurfaces. The measured biphoton spectrum reveals that emission predominantly occurs at the resonant wavelength of the metasurface.

All-dielectric optical metasurfaces are a workhorse in nano-optics, because of both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair production is enhanced up to 2 orders of magnitude, compared to an unpatterned film of the same thickness and material. These results enable flat-optics sources of entangled photons—a new promising platform for quantum optics experiments.

We report the first demonstration of broadband adiabatic directional couplers in thin-film lithium niobate on insulator (LNOI) waveguides. A three LN-waveguide configuration with each waveguide having a ridge cross section of less than 1 square micron, built atop a layer of SiO2 based on a 500-µm-thick Si substrate, has been designed and constructed to optically emulate a three-state stimulated Raman adiabatic passage system, with which a unique counterintuitive adiabatic light transfer phenomenon in a high coupling efficiency of >97% (corresponding to a >15 dB splitting ratio) spanning telecom S, C, and L bands for both TE and TM polarization modes has been observed for a 2-mm long coupler length. An even broader operating bandwidth of >800 nm of the device can be found from the simulation fitting of the experimental data. The footprint of the realized LNOI adiabatic coupler has been reduced by >99% compared to its bulk counterparts. Such an ultra-compact, broadband LNOI adiabatic coupler can be further used to implement or integrate with various photonic elements, a potential building block for realizing large-scale integrated photonic (quantum) circuits in LN.

We experimentally demonstrate biphoton generation by spontaneous parametric down-conversion in resonant metasurfaces. In our metasurfaces, Mie-type resonances enable over 7 times more efficient biphoton generation compared to an unstructured thin film.

Resonant nonlinear metasurfaces gained a lot of interest in the past decade because of their unique features such as subwavelength thickness and ability to control spatial and temporal properties of light. They have proved to be efficient for classical processes such as second-harmonic generation (SHG), but their potential in quantum nanophotonics – for example for biphoton generation via spontaneous parametric down-conversion (SPDC) – remained mostly unexplored. Recently, SPDC at the nanoscale was established in nonlinear thin films [1] and implied in single nanoresonators [2] , albeit with very low efficiency.

We demonstrate broadband adiabatic couplers using lithium niobate on insulator (LNOI) waveguides with a footprint reduced by 98% relative to bulk LN devices. LNOI couplers with coupling efficiency >95% across telecom S-C-L bands are obtained.

In this work, we present a systematic experimental study of surface electric field poling for creating periodically poled lithium niobate. We want to control the homogeneity and the duty cycle of the periodic structures as well as domain shape and size in general. The created domain patterns are analyzed by selective etching with subsequent scanning electron microscopy. We investigate theoretically and experimentally how the shape of electrodes influences the poling results. Furthermore, the influence of the temporal waveform and magnitude of the applied poling voltage pulse is studied. Our results enable to identify optimal experimental parameters for high-quality surface poling.

We experimentally studied the surface electric field poling method to produce periodically poled lithium niobate on insulator (PPLNOI) structures with micrometer-range period. Our statistical analysis shows the effect of poling field strength and poling duration on the quality of the poled area. Our results help to optimize the poling signal in order to achieve a wide homogeneous area.

Lithium niobate is an excellent and widely used material for nonlinear frequency conversion due to its strong optical nonlinearity and broad transparency region. Here, we report the fabrication and experimental investigation of resonant nonlinear metasurfaces for second-harmonic generation based on thin-film lithium niobate. In the fabricated metasurfaces, we observe pronounced Mie-type resonances leading to enhanced second-harmonic generation in the direction normal to the metasurface. We find the largest second-harmonic generation efficiency for the resonance dominated by the electric contributions because its specific field distribution enables the most efficient usage of the largest element of the lithium niobate nonlinear susceptibility tensor. This is confirmed by polarization-resolved second-harmonic measurements, where we study contributions from different elements of the nonlinear susceptibility tensor to the total second-harmonic signal. Our work facilitates establishing lithium niobate as a material for resonant nanophotonics.