Shallow-etched thin-film lithium niobate waveguides for highly-efficient second-harmonic generation

Efficient Second-Harmonic Generation in Adapted-Width Waveguides Based on Periodically Poled Thin-Film Lithium Niobate

Frequency conversion process based on periodically poled thin-film lithium niobate (PPTFLN) has been widely recognized as an important component for quantum information and photonic signal processing. Benefiting from the tight confinement of optical modes, the normalized conversion efficiency (NCE) of nanophotonic waveguides is improved by orders of magnitude compared to their bulk counterparts. However, the power conversion efficiency of these devices is limited by inherent nanoscale inhomogeneity of thin-film lithium niobate (TFLN), leading to undesirable phase errors. In this paper, we theoretically present a novel approach to solve this problem. Based on dispersion engineering, we aim at adjusting the waveguide structure, making local waveguide width adjustment at positions of different thicknesses, thus eliminating the phase errors. The adapted waveguide width design is applied for etched and loaded waveguides based on PPTFLN, achieving the ultrahigh power conversion efficiency of second harmonic generation (SHG) up to 2.1 × 104%W-1 and 6936%W-1, respectively, which surpasses the power conversion efficiency of other related works. Our approach just needs standard periodic poling with a single period, significantly reducing the complexity of electrode fabrication and the difficulty of poling, and allows for the placing of multiple waveguides, without individual poling designs for each waveguide. With the advantages of simplicity, high production, and meeting current micro-nano fabrication technology, our work may open a new way for achieving highly efficient second-order nonlinear optical processes based on PPTFLN.

High-performance silicon photonic single-sideband modulators for cold-atom interferometry

The laser system is the most complex component of a light-pulse atom interferometer (LPAI), controlling frequencies and intensities of multiple laser beams to configure quantum gravity and inertial sensors. Its main functions include cold-atom generation, state preparation, state-selective detection, and generating a coherent two-photon process for the light-pulse sequence. To achieve substantial miniaturization and ruggedization, we integrate key laser system functions onto a photonic integrated circuit. Our study focuses on a high-performance silicon photonic suppressed-carrier single-sideband (SC-SSB) modulator at 1560 nanometers, capable of dynamic frequency shifting within the LPAI. By independently controlling radio frequency (RF) channels, we achieve 30-decibel carrier suppression and unprecedented 47.8-decibel sideband suppression at peak conversion efficiency of -6.846 decibels (20.7%). We investigate imbalances in both amplitudes and phases between the RF signals. Using this modulator, we demonstrate cold-atom generation, state-selective detection, and atom interferometer fringes to estimate gravitational acceleration, g ≈ 9.77 ± 0.01 meters per second squared, in a rubidium (87Rb) atom system.

Tip-induced nanoscale domain engineering in x-cut lithium niobate on insulator

Nanodomain engineering in lithium niobate on insulator (LNOI) is critical to realize advanced photonic circuits. Here, we investigate the tip-induced nanodomain formation in x-cut LNOI. The effective electric field exhibits a mirror symmetry, which can be divided into preceding and sequential halves according to the tip movement. Under our configuration, the preceding electric field plays a decisive role rather than the sequential one as in previous reports. The mechanism is attributed to the screening field formed by the preceding field counteracting the effect of the subsequent one. In experiment, we successfully fabricate nanodomain dots, lines, and periodic arrays. Our work offers a useful approach for nanoscale domain engineering in x-cut LNOI, which has potential applications in integrated optoelectronic devices.

Octave soliton microcombs in lithium niobate microresonators

Soliton microcombs are regarded as an ideal platform for applications such as optical communications, optical sensing, low-noise microwave sources, optical atomic clocks, and frequency synthesizers. Many of these applications require a broad comb spectrum that covers an octave, essential for implementing the f - 2f self-referencing techniques. In this work, we have successfully generated an octave-spanning soliton microcomb based on a z-cut thin-film lithium niobate (TFLN) microresonator. This achievement is realized under on-chip optical pumping at 340 mW and through extensive research into the broadening of dual dispersive waves (DWs). Furthermore, the repetition rate of the octave soliton microcomb is accurately measured using an electro-optic comb generated by an x-cut TFLN racetrack microresonator. Our results represent a crucial step toward the realization of practical, integrated, and fully stabilized soliton microcomb systems based on TFLN.

Noncritical birefringence phase-matched second harmonic generation in a lithium-niobate-on-insulator micro-waveguide for green light emission

Lithium niobate on insulator (LNOI) holds great potential for frequency conversion, where a variety of high-performance nonlinear devices based on different structures has been demonstrated. Here, we report on second harmonic generation (SHG) in MgO-doped LNOI ridge micro-waveguides for efficient green light emission, via an exact type-I noncritical birefringence phase matching (BPM). The LNOI micro-waveguide has a cross section of ∼3×4 μm2, featuring low coupling loss with lens fiber. The normalized conversion efficiency from a continuous-wave (cw) pump to its second harmonic is measured to be 37%/Wcm2 in a single-pass configuration. The device shows both relatively high efficiency and a void of periodic poling, offering a potential solution for efficient and scalable green light sources and frequency converters.

Stretchable structural colors with polarization dependence using lithium niobate metasurfaces

Independently tunable biaxial color pixels, composed of isolated nanosquare dimers, are demonstrated in this study. These pixels are capable of displaying a full range of colors under a linear-polarization dependent reflection mode. The metasurface is constructed by arranging LiNbO3 nanodimers on a PDMS substrate. By exciting a strong magnetic dipole (MD) resonance and effectively suppressing other multipolar resonances using surface lattice resonances, the researchers achieved a single reflection peak with a bandwidth of less than 9 nm and a reflective efficiency of up to 99%. Additionally, the stretchability of the PDMS substrate allows for active and continuous tuning of the metasurface by up to 40% strain, covering almost 150 nm of the visible light spectrum and enabling changes in reflection color. This metasurface holds potential applications in various fields, such as color displays, data storage, and anti-counterfeiting technologies.

High-efficiency second harmonic generation in a micro-resonator on dual-layered lithium niobate

High-quality micro-resonators on thin-film lithium niobate (TFLN) have emerged as an ideal platform for on-chip nonlinear optical applications due to their strong light confinement and excellent natural nonlinear optical properties. Here, we present high-efficiency second-harmonic generation (SHG) in micro-resonators on a TFLN based on the modal phase matching and natural quasi-phase matching. By optimizing the phase-matching conditions through thermal tuning, we demonstrate an on-chip SHG efficiency of 149,000%/W in the low power regime. Furthermore, we achieve an absolute conversion efficiency of 10.3% with a 0.3 mW pump power. Our work paves the way toward future efficient on-chip frequency conversion of classical and quantum light without the need for poling of the LN films.

Comparing transmission- and epi-BCARS: a round robin on solid-state materials

Broadband coherent anti-Stokes Raman scattering (BCARS) is a powerful spectroscopy method combining high signal intensity with spectral sensitivity, enabling rapid imaging of heterogeneous samples in biomedical research and, more recently, in crystalline materials. However, BCARS encounters spectral distortion due to a setup-dependent non-resonant background (NRB). This study assesses BCARS reproducibility through a round robin experiment using two distinct BCARS setups and crystalline materials with varying structural complexity, including diamond, 6H-SiC, KDP, and KTP. The analysis compares setup-specific NRB correction procedures, detected and NRB-removed spectra, and mode assignment. We determine the influence of BCARS setup parameters like pump wavelength, pulse width, and detection geometry and provide a practical guide for optimizing BCARS setups for solid-state applications.

Adapted poling to break the nonlinear efficiency limit in nanophotonic lithium niobate waveguides

Nonlinear frequency mixing is a method to extend the wavelength range of optical sources with applications in quantum information and photonic signal processing. Lithium niobate with periodic poling is the most widely used material for frequency mixing due to its strong second-order nonlinear coefficient. The recent development using nanophotonic lithium niobate waveguides promises to improve nonlinear efficiencies by orders of magnitude thanks to subwavelength optical confinement. However, the intrinsic nanoscale inhomogeneity of nanophotonic lithium niobate waveguides limits the coherent interaction length, leading to low nonlinear efficiencies. Here we show improved second-order nonlinear efficiency in nanophotonic lithium niobate waveguides that breaks the limit imposed by nanoscale inhomogeneity. This is realized by developing the adapted poling approach to eliminate the impact of nanoscale inhomogeneity. We realize an overall second-harmonic efficiency of 104% W-1 (without cavity enhancement), approaching the theoretical performance for nanophotonic lithium niobate waveguides. The ideal square dependence of the nonlinear efficiency on the waveguide length is recovered. Phase-matching bandwidths and temperature tuneability are improved through dispersion engineering. We finally demonstrate a conversion ratio from pump to second-harmonic power greater than 80% in a single-pass configuration with pump power as low as 20 mW. Our work therefore breaks the trade-off between the conversion ratio and pump power, offering a potential solution for highly efficient and scalable nonlinear-optical sources, amplifiers and converters.

Intrinsic polarity inversion in III-nitride waveguides for efficient nonlinear interactions

III-nitrides provide a versatile platform for nonlinear photonics. In this work, we explore a new promising configuration - composite waveguides containing GaN and AlN layers with inverted polarity, i.e., having opposite signs of the χ(2) nonlinear coefficient. This configuration allows us to address the limiting problem of the mode overlap for nonlinear interactions. Our modelling predicts a significant improvement in the conversion efficiency. We confirm our theoretical prediction with the experimental demonstration of second harmonic generation with an efficiency of 4%W-1cm-2 using a simple ridge waveguide. This efficiency is an order of magnitude higher compared to the previously reported results for III-nitride waveguides. Further improvement, reaching a theoretical efficiency of 30%W-1cm-2, can be achieved by reducing propagation losses.

Low-loss and broadband polarization-diversity edge coupler on a thin-film lithium niobate platform

Fiber-to-chip coupling is an essential issue for taking high-performance integrated photonic devices into practical applications. On a thin-film lithium niobate platform, such a high-performance coupler featuring low loss, large bandwidth, and polarization independence is highly desired. However, the mode hybridization induced by the birefringence of lithium niobate seriously restricts a polarization-independent coupling. Here, we propose and experimentally demonstrate a high-performance and polarization-diversity cantilever edge coupler (EC) with the assistance of a two-stage polarization splitter and rotator (PSR). The fabricated cantilever EC shows a minimal coupling loss of 1.06 dB/facet, and the fully etched PSR structure shows a low insertion loss (IL) of -0.62 dB. The whole polarization-diversity cantilever EC exhibits a low IL of -2.17 dB and -1.68 dB for TE₀ and TM₀ mode, respectively, as well as a small cross talk of <-15 dB covering the wavelength band from 1.5 µm to 1.6 µm. A polarization-dependent loss <0.5 dB over the same wavelength band is also obtained. The proposed fiber-to-waveguide coupler, compatible with the fabrication process of popular thin-film lithium niobate photonic devices, can work as a coupling scheme for on-chip polarization-diversity applications.

Tunable sum-frequency generation in modal phase-matched thin film lithium niobate rib waveguides

In this work, we report a highly efficient and tunable on-chip sum-frequency generation (SFG) on a thin-film lithium niobate platform via modal phase matching (e + e→e). It provides on-chip SFG a solution with both high efficiency and poling-free by using the highest nonlinear coefficient d₃₃ instead of d₃₁. The on-chip conversion efficiency of SFG is approximately 2143%W^-1 with a full width at half maximum (FWHM) of 4.4 nm in a 3-mm-long waveguide. It can find applications in chip-scale quantum optical information processing and thin-film lithium niobate based optical nonreciprocity devices.

Efficient Second- and Third-Harmonic Generations in Er3+/Fe2+-Doped Lithium Niobate Single Crystal with Engineered Surficial Cylindrical Hole Arrays

Herein, significant enhancement of second- and third-harmonic generation efficiencies in a 1 mol% Er³⁺ and 0.07 mol% Fe²⁺-doped lithium niobate single-crystal plate were achieved after ablating periodic cylindrical pit arrays on the surface. Enhanced absorption and reduced transmittance of light were measured when the incident light signal passed through the patterned sample. Enhanced photoluminescence and two-photon-pumped upconversion emission spectra were also explored to obtain more details on the efficiency gains. The excitation-energy-dependent second-harmonic generation efficiency was measured, and an enhancement as high as 20-fold was calculated. The conversion efficiency of second-harmonic generation is 1 to 3 orders higher than that from other lithium niobite metasurfaces and nanoantennas. This work provides a convenient and effective method to improve the nonlinear conversion efficiency in a thin lithium niobite plate, which is desirable for applying to integrated optical devices.

Stiffness tensor estimation of anisotropic crystal using point contact method and unscented Kalman filter

The potential application of Lithium Niobate (LiNbO₃) crystal is immense, specifically in the domain of meta-surfaces and nano-resonators. However, the practical application of LiNbO₃ is impeded due to unreliable experimental techniques and inaccurate inversion algorithms for material characterization. In the current research, material characterization of anisotropic crystal is proposed by exploring the wavefield evolution in the spatial and temporal domains. The presented framework has three major components: a physics-based mathematical model (Christoffel equation), a novel experimental technique, and an inversion algorithm based on Bayesian filtering. An experimental technique based on Coulomb coupling is devised to visualize the propagation of ultrasonic waves in an anisotropic crystal. The crystal is characterized by measuring the directional-dependent acoustic wave velocity from the spatial-temporal information of the wave propagation. The anisotropic constitutive properties of the crystal are estimated by exploring the wave velocity in the Bayesian filtering algorithm. The proposed algorithm is based on the probabilistic framework that integrates the experimental measurement in a physics-based mathematical model for optimal state prediction of stiffness tensor through the Bayesian filtering algorithm. In particular, we utilize the unscented Kalman filter (UKF) in conjunction with the plane-wave Eigen solution to estimate the constitutive parameters. In the presence of measurement uncertainties, the performance of the optimal prediction algorithm is illustrated by comparing the estimated parameter with the corresponding theoretical value. The comparison demonstrates that the proposed inversion algorithm is efficient and robust and performs satisfactorily even with significant measurement uncertainties.

Second-harmonic and cascaded third-harmonic generation in generalized quasiperiodic poled lithium niobate waveguides

Lithium niobate (LN) thin film has recently emerged as an important platform for nonlinear optical investigations for its large χ⁽²⁾ nonlinear coefficients and ability of light localization. In this Letter, we report the first, to the best of our knowledge, fabrication of LN-on-insulator ridge waveguides with generalized quasiperiodic poled superlattices using the electric field polarization technique and microfabrication techniques. Benefiting from the abundant reciprocal vectors, we observed efficient second-harmonic and cascaded third-harmonic signals in the same device, with normalized conversion efficiency of 1735% W^-1 cm^-2 and 0.41% W^-2 cm^-4, respectively. This work opens a new direction for nonlinear integrated photonics based on LN thin film.

Highly efficient, modal phase-matched second harmonic generation in a double-layered thin film lithium niobate waveguide

In this contribution, we numerically investigate second harmonic generation in double-layered lithium niobate on the insulator platform by means of the modal phase matching. The modal dispersion of the ridge waveguides at the C waveband of optical fiber communication is calculated numerically and analyzed. Modal phase matching can be achieved by changing the geometric dimensions of the ridge waveguide. The phase-matching wavelength and conversion efficiencies versus the geometric dimensions in the modal phase-matching process are investigated. We also analyze the thermal-tuning ability of the present modal phase matching scheme. Our results show that highly efficient second harmonic generation can be realized by the modal phase matching in the double-layered thin film lithium niobate ridge waveguide.

Correlated twin-photon generation in a silicon nitride loaded thin film PPLN waveguide

Photon-pair sources based on thin film lithium niobate on insulator technology have a great potential for integrated optical quantum information processing. We report on such a source of correlated twin-photon pairs generated by spontaneous parametric down conversion in a silicon nitride (SiN) rib loaded thin film periodically poled lithium niobate (LN) waveguide. The generated correlated photon pairs have a wavelength centred at 1560 nm compatible with present telecom infrastructure, a large bandwidth (21 THz) and a brightness of ∼2.5 × 10⁵ pairs/s/mW/GHz. Using the Hanbury Brown and Twiss effect, we have also shown heralded single photon emission, achieving an autocorrelation g H(2)(0)≃0.04.

110 GHz, 110 mW hybrid silicon-lithium niobate Mach-Zehnder modulator

High bandwidth, low voltage electro-optic modulators with high optical power handling capability are important for improving the performance of analog optical communications and RF photonic links. Here we designed and fabricated a thin-film lithium niobate (LN) Mach-Zehnder modulator (MZM) which can handle high optical power of 110 mW, while having 3-dB bandwidth greater than 110 GHz at 1550 nm. The design does not require etching of thin-film LN, and uses hybrid optical modes formed by bonding LN to planarized silicon photonic waveguide circuits. A high optical power handling capability in the MZM was achieved by carefully tapering the underlying Si waveguide to reduce the impact of optically-generated carriers, while retaining a high modulation efficiency. The MZM has a [Formula: see text] product of 3.1 V.cm and an on-chip optical insertion loss of 1.8 dB.

Thermally tunable and efficient second-harmonic generation on thin-film lithium niobate with integrated micro-heater

In this Letter, we report thermo-optic tunable and efficient second-harmonic generation (SHG) based on an X-cut periodically poled lithium niobate (PPLN) waveguide. By applying an on-chip heater with thermo-isolation trenches and combining a type-0 quasi-phase matching mechanism, we experimentally achieve a high on-chip SHG conversion efficiency of 2500-3000% W^-1 cm^-2 and a large tuning power efficiency of 94 pm/mW inside a single 5-mm-long straight PPLN waveguide. Our design is for energy-efficient, high-performance nonlinear applications, such as wavelength conversion, highly tunable coherent light sources, and photon-pair generation.

A compact cold-atom interferometer with a high data-rate grating magneto-optical trap and a photonic-integrated-circuit-compatible laser system

The extreme miniaturization of a cold-atom interferometer accelerometer requires the development of novel technologies and architectures for the interferometer subsystems. Here, we describe several component technologies and a laser system architecture to enable a path to such miniaturization. We developed a custom, compact titanium vacuum package containing a microfabricated grating chip for a tetrahedral grating magneto-optical trap (GMOT) using a single cooling beam. In addition, we designed a multi-channel photonic-integrated-circuit-compatible laser system implemented with a single seed laser and single sideband modulators in a time-multiplexed manner, reducing the number of optical channels connected to the sensor head. In a compact sensor head containing the vacuum package, sub-Doppler cooling in the GMOT produces 15 μK temperatures, and the GMOT can operate at a 20 Hz data rate. We validated the atomic coherence with Ramsey interferometry using microwave spectroscopy, then demonstrated a light-pulse atom interferometer in a gravimeter configuration for a 10 Hz measurement data rate and T = 0–4.5 ms interrogation time, resulting in Δg/g = 2.0 × 10⁻⁶. This work represents a significant step towards deployable cold-atom inertial sensors under large amplitude motional dynamics.

Cold-atom interferometers have been miniaturized towards fieldable quantum inertial sensing applications. Here the authors demonstrate a compact cold-atom interferometer using microfabricated gratings and discuss the possible use of photonic integrated circuits for laser systems.

All-optical wavelength conversion of a 92-Gb/s 16-QAM signal within the C-band in a single thin-film PPLN waveguide

Tunable all-optical wavelength conversion (AOWC) within 151 nm bandwidth is demonstrated in a thin-film periodically poled lithium niobate (PPLN) waveguide, which utilizes the cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) process. Also, in the same waveguide, AOWC of a 92-Gb/s 16-ary quadrature amplitude modulated (16-QAM) signal within the C-band is successfully achieved. For Bit-error ratio (BER) measurements, we obtain a negligible optical signal-to-noise ratio (OSNR) penalty (<0.2 dB) for the converted idler wave at a BER of 1e-3.

Technologically feasible quasi-edge states and topological Bloch oscillation in the synthetic space

The dimensionality of a physical system is one of the major parameters defining its physical properties. The recently introduced concept of synthetic dimension has made it possible to arbitrarily manipulate the system of interest and harness light propagation in different ways. It also facilitates the transformative architecture of system-on-a-chip devices enabling far reaching applications such as optical isolation. In this report, a novel architecture based on dynamically-modulated waveguide arrays with the Su-Schrieffer-Heeger configuration in the spatial dimension is proposed and investigated with an eye on a practical implementation. The propagation of light through the one-dimensional waveguide arrays mimics time evolution of the field in a synthetic two-dimensional lattice. The addition of the effective gauge potential leads to an exotic topologically protected one-way transmission along adjacent boundary. A cosine-shape isolated band, which supports the topological Bloch oscillation in the frequency dimension under the effective constant force, appears and is localized at the spatial boundary being robust against small perturbations. This work paves the way to improved light transmission capabilities under topological protections in both spatial and spectral regimes and provides a novel platform based on a technologically feasible lithium niobate platform for optical computing and communication.

All-Dielectric Structural Colors with Lithium Niobate Nanodisk Metasurface Resonators

Lithium niobate (LN) is a promising optical material, its micro–nano structures have been applied to fields such as photonic crystals, nonlinear optics, optical waveguides, and so on. At present, lithium niobate structural colors are rarely studied. Although the nanograting structure was researched, it has such large full width at half-maximum (fwhm) that it cannot achieve red, green, or blue pixels or other high-saturation structural colors, thus, its color printing quality is poor. In this paper, we design and simulate lithium niobate nanodisk metasurface resonators (LNNDMRs), which are based on Mie magnetic dipole (MD) and electric dipole (ED) resonances. In addition, the resonators yield very narrow reflection peaks and high reflection efficiencies with over 80%, especially the reflection peaks of red, green, and blue pixels with fwhm around 11 nm, 9 nm, and 6 nm, respectively. Moreover, output colors of different array cells composed of single nanodisk in finite size are displayed, which provides a theoretical basis for their practical applications. Therefore, LNNDMRs pave the way for high-efficiency, compact photonic display devices based on lithium niobate.

Broadband second-harmonic generation in step-chirped periodically poled lithium niobate waveguides

Periodically poled lithium niobate (PPLN) structures on a chip enable efficient second-order nonlinear optical effects, benefiting from the tight light confinement and the utilization of d₃₃. Here, we report a broadband second-harmonic (SH) generation in a step-chirped PPLN waveguide on X-cut lithium niobate on insulator (LNOI). The high fidelity of the poling period is demonstrated over the whole length of 7 mm using a non-destructive technique of piezoresponse force microscopy. The SH signal was continuously observed in the step-chirped PPLN waveguides while scanning the wavelength of the pump laser from 1550 nm to 1660 nm. The SH conversion efficiency was measured to be 9.6 % W^-1 cm^-2 at 1642 nm. This work will benefit wavelength conversions of light sources with wideband spectra.

Study on the single-mode condition for x-cut LNOI rib waveguides based on leakage losses

Lithium niobate-on-insulator (LNOI) has recently emerged as a promising material platform for high-density and advanced photonics integrated circuits (PICs). And single-mode waveguides (SMW) are the most basic building blocks for structuring various PICs. In this paper, single-mode conditions (SMCs) for shallowly etched LNOI rib waveguides in x-cut LNOI wafer are investigated with the finite element method (FEM) in consideration of the lateral leakage and the magic width for the first time, to our best knowledge. Our results indicate that due to the lateral leakage and the magic width these shallowly etched x-cut LNOI rib waveguides have unique and complex SMCs. Our method and results provide a guidance in designing low-loss LNOI SMW and high-performance PICs.

“Seeing Is Believing”—In-Depth Analysis by Co-Imaging of Periodically-Poled X-Cut Lithium Niobate Thin Films

Nonlinear and quantum optical devices based on periodically-poled thin film lithium niobate (PP-TFLN) have gained considerable interest lately, due to their significantly improved performance as compared to their bulk counterparts. Nevertheless, performance parameters such as conversion efficiency, minimum pump power, and spectral bandwidth strongly depend on the quality of the domain structure in these PP-TFLN samples, e.g., their homogeneity and duty cycle, as well as on the overlap and penetration depth of domains with the waveguide mode. Hence, in order to propose improved fabrication protocols, a profound quality control of domain structures is needed that allows quantifying and thoroughly analyzing these parameters. In this paper, we propose to combine a set of nanometer-to-micrometer-scale imaging techniques, i.e., piezoresponse force microscopy (PFM), second-harmonic generation (SHG), and Raman spectroscopy (RS), to access the relevant and crucial sample properties through cross-correlating these methods. Based on our findings, we designate SHG to be the best-suited standard imaging technique for this purpose, in particular when investigating the domain poling process in x-cut TFLNs. While PFM is excellently recommended for near-surface high-resolution imaging, RS provides thorough insights into stress and/or defect distributions, as associated with these domain structures. In this context, our work here indicates unexpectedly large signs for internal fields occurring in x-cut PP-TFLNs that are substantially larger as compared to previous observations in bulk LN.

Electro-optic reconfigurable two-mode (de)multiplexer on thin-film lithium niobate

We propose and demonstrate a compact electro-optic reconfigurable two-mode (de)multiplexer using the configuration of cascaded Mach-Zehnder interferometers formed on thin-film X-cut lithium niobate on silica. Our fabricated device, which is 9.5-mm long, can spatially switch between the two transverse-electric modes with an efficiency higher than 98% from 1530-1560 nm and beyond at an applied voltage of 6.5 V. The switching speed is faster than 30 ns. Our proposed mode switch could find applications in fiber-based and on-chip mode-division-multiplexing systems.

Second-Harmonic Generation in Resonant Nonlinear Metasurfaces Based on Lithium Niobate

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.