Cryogenic second harmonic generation in periodically-poled lithium niobate waveguides
Moritz Bartnick, Matteo Santandrea, Jan Philipp Hoepker, Frederik Thiele, Raimund Ricken, Viktor Quiring, Christof Eigner, Harald Herrmann, Christine Silberhorn, Tim J. Bartley
Prospective integrated quantum optical technologies will combine nonlinear optics and components requiring cryogenic operating temperatures. Despite the prevalence of integrated platforms exploiting $χ^{(2)}$-nonlinearities for quantum optics, for example used for quantum state generation and frequency conversion, their material properties at low temperatures are largely unstudied. Here, we demonstrate the first second harmonic generation in a fiber-coupled lithium niobate waveguide at temperatures down to 4.4K. We observe a reproducible shift in the phase-matched pump wavelength within the telecom band, in addition to transient discontinuities while temperature cycling. Our results establish lithium niobate as a versatile nonlinear photonic integration platform compatible with cryogenic quantum technologies.
The absence of the single-photon nonlinearity has been a major roadblock in developing quantum photonic circuits at optical frequencies. In this paper, we demonstrate a periodically-poled thin film lithium niobate microring resonator (PPLNMR) that reaches 5,000,000%/W second harmonic conversion efficiency---almost 20-fold enhancement over the state-of-the-art---by accessing its largest $χ^{(2)}$ tensor component $d_{33}$ via quasi-phase matching. The corresponding single photon coupling rate $g/2π$ is estimated to be 1.2 MHz, which is an important milestone as it approaches the dissipation rat…
We demonstrate efficient first-order quasi-phase-matched second-harmonic generation in a surface periodically poled Ti:indiffused lithium niobate waveguide; 6 mW of continuous-wave blue radiation (wavelength = 412.6 nm) was produced showing the potential of surface domain inversion for efficient nonlinear waveguide interactions.
Lithium niobate (LN), dubbed by many as the silicon of photonics, has recently risen to the forefront of chip-scale nonlinear optics research since its demonstration as an ultralow-loss integrated photonics platform. Due to its significant quadratic nonlinearity ($χ^{(2)}$), LN inspires many important applications such as second-harmonic generation (SHG), spontaneous parametric down-conversion, and optical parametric oscillation. Here, we demonstrate high-efficiency SHG in dual-resonant, periodically poled z-cut LN microrings, where quasi-phase matching is realized by field-assisted domain eng…
The amplified spontaneous emission from a superluminescent diode was frequency doubled in a periodically poled lithium niobate waveguide crystal. The temporally incoherent radiation of such a superluminescent diode is characterized by a relatively broad spectral bandwidth and thermal-like photon statistics, as the measured degree of second order coherence, g$^{(2)}$(0)=1.9$\pm$0.1, indicates. Despite the non-optimized scenario in the spectral domain, we achieve six orders of magnitude higher conversion efficiency than previously reported with truly incoherent light. This is possible by using s…
We report on backward second-harmonic-generation in bulk periodically poled congruent lithium niobate with a 3.2 microns period. A tunable continuous-wave Ti:sapphire laser allowed us exciting two resonant quasi-phase-matching orders in the backward configuration. The resonances were also resolved by temperature tuning and interpolated with standard theory to extract relevant information on the sample.
We report an application of the tri-dimensional pseudo-spectral time domain algorithm, that solves with accuracy the nonlinear Maxwell's equations, to predict second harmonic generation in lithium niobate ridge-type waveguides with high index contrast. Characteristics of the nonlinear process such as conversion efficiency as well as impact of the multimode character of the waveguide are investigated as a function of the waveguide geometry in uniformly and periodically poled medium.
We study the chirality of periodically poled MgO-doped lithium niobate (MgO:PPLN) by electro-optic (EO) effect. It shows that optical propagation is reciprocal in MgO:PPLN when quasi-phase-matching (QPM) condition is satisfied, which is similar to natural optical active material like quartz. The specific rotation of MgO:PPLN by EO effect is shown to be proportional to the transverse electric field, making large polarization rotation in optical active material with small size possible. We also demonstrate that the chirality of MgO:PPLN can be controlled by external electric field.
Second harmonic generation is investigated in lithium niobate channels realized by proton exchange and quasi-phase-matched by surface periodic-poling. The reduction in conversion efficiency at high powers is interpreted in terms of multi-photon absorption via two-color terms, yielding an estimate of the dominating three-photon process.
We have operated a CW triply resonant OPO using a PPLN crystal pumped by a Nd:YAG laser at 1.06 micron and generating signal and idler modes in the 2-2.3 micron range. The OPO was operated stably in single mode operation over large periods of time with a pump threshold as low as 500 microwatts.
We carried out second-harmonic generation in quasi-phase-matched α-phase lithium niobate channel waveguides realized by proton exchange and surface periodic poling. Owing to a limited ferroelectric domain depth, we could observe the interplay between second-harmonic generation and self-phase modulation due to cascading and cubic effects, resulting in a nonlinear resonance shift. Data reduction allowed us to evaluate both the quadratic nonlinearity in the near infrared as well as the depth of the uninverted domains.
Temporal cloaks have inspired the innovation of research on security and efficiency of quantum and fiber communications for concealing temporal events. The existing temporal cloaking approaches possessing ps ~ns cloaking windows employed the third-order nonlinearity mostly. Here we explore a temporal cloak for perpetually concealing pulse events using high efficiency second-order nonlinearity. A temporal pulse event was cloaked as a continuous wave with constant intensity and restored by polarization dependent sum frequency generation processes between the continuous wave and probe. The variet…
Second-order optical nonlinear effects (second-harmonic and sum-frequency generation) are demonstrated in the telecommunication band by periodic poling of thin films of lithium niobate wafer-bonded on silicon substrates and rib-loaded with silicon nitride channels to attain ridge waveguide with cross-sections of ~ 2 μm2. The compactness of the waveguides results in efficient second-order nonlinear devices. A nonlinear conversion of 8% is obtained with a pulsed input in 4 mm long waveguides. The choice of silicon substrate makes the platform potentially compatible with silicon photonics, and th…
Systematic Analysis of Ferroelectric Domain Dynamics in Periodically Poled Lithium Niobate Waveguides Using Two-Photon Microscopy and Digital Imaging Processing
Thin-film lithium niobate on insulator (LNOI) has emerged as a superior integrated-photonics platform for linear, nonlinear, and electro-optics. Here we combine quasi-phase-matching, dispersion engineering, and tight mode confinement to realize nonlinear parametric processes with both high efficiency and wide wavelength tunability. On a millimeter-long, Z-cut LNOI waveguide, we demonstrate ultra-efficient ($1900\pm500 \% $W$^{-1}$cm$^{-2}$) and highly tunable (-1.71 nm/K) second-harmonic generation from 1530 to 1583 nm by type-0 quasi-phase-matching. Our technique is applicable to optical harm…
Quasi-phasematched interactions in waveguides with quadratic nonlinearities enable highly efficient nonlinear frequency conversion. In this article, we demonstrate the first generation of devices that combine the dispersion-engineering available in nanophotonic waveguides with quasi-phasematched nonlinear interactions available in periodically poled lithium niobate (PPLN). This combination enables quasi-static interactions of femtosecond pulses, reducing the pulse energy requirements by several orders of magnitude, from picojoules to femtojoules. We experimentally demonstrate two effects assoc…
The use of laser pulse sequences to drive the cascaded difference frequency generation of high energy, high peak-power and multi-cycle terahertz pulses in cryogenically cooled periodically poled lithium niobate is proposed. Detailed simulations considering the coupled nonlinear interaction of terahertz and optical waves show that unprecedented optical-to-terahertz energy conversion efficiencies > 5%, peak electric fields of hundred(s) of Mega volts/meter at terahertz pulse durations of hundred(s) of picoseconds can be achieved. The proposed methods are shown to circumvent laser-induced damage …
Lithium niobate on insulator (LNOI) is extensively researched for potential applications in telecommunication, sensing and quantum technology. Low-loss waveguides in LNOI have been demonstrated in recent years, with further works investigating their nonlinear optical properties via electric field poling. The fabrication imperfections and minimum feature size of inverted ferroelectric domains currently limit the efficiency and application of nonlinear processes in LNOI. This work looks into focused ion beam poling and demonstrates its potential to overcome the limitations of electric field poli…
In this contribution we investigate the impact of lateral leakage for linear and nonlinear optical waveguides in lithium niobate on insulator (LNOI). Silicon nitride (SiN) loaded and direct patterned lithium niobate cross-sections are investigated. We show that lateral leakage can take place for the TE mode in LNOI ridge waveguides (X-cut lithium niobate), due to the birefringence of the material. This work gives guidelines for designing waveguides in LNOI that do not suffer from the lateral leakage effect. By applying these design considerations, we avoided the lateral leakage effect at the s…
Segment-chirped periodically poled lithium niobate waveguides for broadband supercontinuum generation
National University of Singapore（新加坡国立大学）；Sichuan University（四川大学）；A ∗ STAR Quantum Innovation Centre (Q.InC), Agency for Science, Technology and Research (A ∗ STAR)（A*STAR量子创新中心（Q.InC），科学技术研究局（A*STAR））；Nanyang Technological University（南洋理工大学）
Many technologies in quantum photonics require cryogenic conditions to operate. However, the underlying platform behind active components such as switches, modulators and phase shifters must be compatible with these operating conditions. To address this, we demonstrate an electro-optic polarisation converter for 1550nm light at 0.8K in titanium in-diffused lithium niobate waveguides. To do so, we exploit the electro-optic properties of lithium niobate to convert between orthogonal polarisation modes with a fiber-to-fiber transmission >43%. We achieve a modulation depth of 23.6 +/-3.3dB and a c…
Lithium-Niobate-On-Insulator (LNOI) is emerging as a promising platform for integrated quantum photonic technologies because of its high second-order nonlinearity and compact waveguide footprint. Importantly, LNOI allows for creating electro-optically reconfigurable circuits, which can be efficiently operated at cryogenic temperature. Their integration with superconducting nanowire single-photon detectors (SNSPDs) paves the way for realizing scalable photonic devices for active manipulation and detection of quantum states of light. Here we report the first demonstration of these two key compon…
Lithium niobate is a promising platform for integrated quantum optics. In this platform we aim to efficiently manipulate and detect quantum states by combining superconducting single photon detectors and modulators. The cryogenic operation of a superconducting single photon detector dictates the optimisation of the electro-optic modulators under the same operating conditions. To that end, we characterise a phase modulator, directional coupler, and polarisation converter at both ambient and cryogenic temperatures. The operation voltage $V_{π/2}$ of these modulators increases due to the decrease…
Quantum networks are likely to have a profound impact on the way we compute and communicate in the future. In order to wire together superconducting quantum processors over kilometer-scale distances, we need transducers that can generate entanglement between the microwave and optical domains with high fidelity. We present an integrated electro-optic transducer that combines low-loss lithium niobate photonics with superconducting microwave resonators on a sapphire substrate. Our triply-resonant device operates in a dilution refrigerator and converts microwave photons to optical photons with an …
We report intracavity Bragg scattering induced by photorefractive (PR) effect in high-Q lithium niobate (LN) ring resonators at cryogenic temperatures. We show that, when a cavity mode is strongly excited, the PR effect imprints a long-lived periodic space-charge field. This residual field in turn creates a refractive index modulation pattern that dramatically enhances the back scattering of an incoming probe light, and results in selective and reconfigurable mode splittings. This PR-induced Bragg scattering effect, despite being undesired for many applications, could be utilized to enable opt…
We obtained an optical-to-terahertz (THz) energy conversion efficiency of 0.5% using the tilted-pulse-front technique in lithium niobate at a cryogenically cooled temperature of 100 K pumped by amplified Ti:sapphire laser pulses with ~150 fs pulse duration at 800 nm wavelength. Compared with the optimized conversion efficiency of 0.18% achieved at room temperature, we achieved more than 2.5 times enhancement in conversion efficiency upon cryogenically cooling the crystal due to reduction of THz absorption. Further improvements to the conversion efficiency can be made by optimizing the out-coup…
Chip-integrated nonlinear photonics holds the key for advanced optical information processing with superior performance and novel functionalities. Here, we present an optimally mode-matched, periodically poled lithium niobate nanowaveguide for efficient parametric frequency conversions on chip. Using a 4-mm nanowaveguide with subwavelength mode confinement, we demonstrate second harmonic generation with efficiency over $2200\%~W^{-1}cm^{-2}$, and broadband difference frequency generation with similar efficiency over a 4.5-THz spectral span. These allow us to generate correlated photon pairs ov…
Low-noise quantum frequency conversion in periodically-poled nonlinear crystals has proved challenging when the pump wavelength is shorter than the target wavelength. This is - at least in large part - a consequence of the parasitic spontaneous parametric down-conversion of pump photons, whose efficiency is increased by fabrication errors in the periodic poling. Here we characterise the poling quality of commercial periodically-poled bulk potassium titanyl phosphate (ppKTP) by measuring the sum-frequency generation (SFG) efficiency over a large phase mismatch range from 0 to more than 400$π$. …
We introduce a general technique to generate compressed broadband terahertz pulses based on difference frequency generation of optical pump radiation using aperiodically poled structures. The pump pulse format and poling of the crystals conceived are such that the emergent terahertz pulse is compressed. The approach circumvents pump pulse distortions that result from non-collinear approaches and the need for external compression. It is particularly efficient for the generation of pulses of intermediate durations of few to tens of cycles. For instance, conversion efficiencies in the few percent…
Integrated quantum optics becomes a consequent tendency towards practical quantum information processing. Here, we report the on-chip generation and manipulation of photonic entanglement based on reconfigurable lithium niobate waveguide circuits. By introducing periodically poled structure into the waveguide interferometer, two individual photon-pair sources with controllable phase-shift are produced and cascaded by a quantum interference, resulting in a deterministically separated identical photon pair. The state is characterized by 92.9% visibility Hong-Ou-Mandel interference. Continuous mor…
Quantum light sources in the mid-infrared (MIR) band play an important role in many applications, such as quantum sensing, quantum imaging, and quantum communication. However, there is still a lack of high-quality quantum light sources in the MIR band, such as the spectrally pure single-photon source. In this work, we present the generation of spectrally-pure state in an optimized poled lithium niobate crystal using a metaheuristic algorithm. In particular, we adopt the particle swarm optimization algorithm to optimize the duty cycle of the poling period of the lithium niobate crystal. With ou…
We describe a technique for surface domain engineering in congruent lithium niobate single crystals. The method is based on conventional electric-field poling, but involves an intentional overpoling step that inverts all the material apart from a thin surface region directly below the patterned photoresist. The surface poled structures show good domain uniformity, and the technique has so far been applied to produce domain periods as small as 1 μm. The technique is fully compatible with nonlinear optical integrated devices based on waveguide structures.
