Modal Phase Matching Research Articles

2D sheets of MoSe2 for waveguide based second harmonic generation

Two-dimensional transition metal dichalcogenides show a strong second-order nonlinear response, which can only be exploited if the interaction between light and 2D material is maximized as it happens in waveguiding structures. Such an approach also allows the addition of a second order nonlinear response to linear integrated optics components by just placing a sheet of transition metal dichalcogenides material on top. Here, we analyze the second harmonic generation in MoSe2 sheets combined with silicon nitride ridge and thin-film waveguides for both modal phase matching and quasi-phase-matching. The calculated second harmonic generation efficiency is comparable with that in lithium niobate waveguides and can further be enhanced by increasing the number of MoSe2 layers on top of the waveguides. This also holds true for flux correlated photons generated by spontaneous parametric down conversion as we show by quantitative analysis.

Modal phase-matching in thin-film lithium niobate waveguides for efficient generation of entangled photon pairs

Thin-film lithium niobate (TFLN) waveguides have emerged as a pivotal platform for on-chip spontaneous parametric down-conversion (SPDC), serving as a crucible for the generation of entangled photon pairs. The periodic poling of TFLN, while capable of generating high-efficiency SPDC, demands intricate fabrication processes that can be onerous in terms of scalability and manufacturability. In this work, we introduce a novel approach to the generation of entangled photon pairs via SPDC within TFLN waveguides, harnessing the principles of modal phase-matching (MPM). To address the challenge of efficiently exciting pump light typically in a higher-order mode, we have engineered a mode converter that couples two asymmetrically dimensioned waveguides. This converter adeptly transforms the fundamental mode into a higher-order mode, demonstrating a conversion loss of 1.55 dB at 785 nm with a 3 dB bandwidth exceeding 30 nm. Subsequently, we have showcased the device’s capabilities by characterizing the pair generation rate (PGR), coincidences-to-accidentals ratio (CAR), and spectral profile of the entangled photon source. Our findings present a simplified and versatile method for the on-chip generation of entangled photon sources, which may pave the way for the application in the realms of quantum information processing and communication technologies.

Efficient photon-pair generation in layer-poled lithium niobate nanophotonic waveguides

Integrated photon-pair sources are crucial for scalable photonic quantum systems. Thin-film lithium niobate is a promising platform for on-chip photon-pair generation through spontaneous parametric down-conversion (SPDC). However, the device implementation faces practical challenges. Periodically poled lithium niobate (PPLN), despite enabling flexible quasi-phase matching, suffers from poor fabrication reliability and device repeatability, while conventional modal phase matching (MPM) methods yield limited efficiencies due to inadequate mode overlaps. Here, we introduce a layer-poled lithium niobate (LPLN) nanophotonic waveguide for efficient photon-pair generation. It leverages layer-wise polarity inversion through electrical poling to break spatial symmetry and significantly enhance nonlinear interactions for MPM, achieving a notable normalized second-harmonic generation (SHG) conversion efficiency of 4615% W−1cm−2. Through a cascaded SHG and SPDC process, we demonstrate photon-pair generation with a normalized brightness of 3.1 × 106 Hz nm−1 mW−2 in a 3.3 mm long LPLN waveguide, surpassing existing on-chip sources under similar operating configurations. Crucially, our LPLN waveguides offer enhanced fabrication reliability and reduced sensitivity to geometric variations and temperature fluctuations compared to PPLN devices. We expect LPLN to become a promising solution for on-chip nonlinear wavelength conversion and non-classical light generation, with immediate applications in quantum communication, networking, and on-chip photonic quantum information processing.

Concurrent phase-matchings for multi-wavelength conversion in coupled dual waveguides

Thin film lithium niobate, as a highly promising integrated photonics platform, has emerged as an ideal material platform for wavelength conversion owing to its exceptional second-order nonlinear characteristics. Here, we present the first demonstration of concurrent phase-matchings for multi-wavelength conversion in coupled thin film lithium niobate waveguides without poling technique. In the coupled dual waveguide system, employing modal phase-matching, we validate three effective phase-matching conditions for second harmonic generation, arising from additional momentum compensation due to the coupling of waveguides. Simultaneously, we confirm that phase-matching wavelengths can be tuned by adjusting the gap between waveguides. The concurrent phase-matchings in coupled dual waveguide systems can be readily extended to other nonlinear optical processes, such as difference frequency generation and spontaneous parametric down conversion. Our work contributes to advancing the exploration of efficient on-chip nonlinear optical processes within the realms of nanophotonics and quantum optics.

Lithium niobate on insulator – fundamental opto-electronic properties and photonic device prospects

Abstract Lithium niobate on insulator (LNOI) combines a variety of optoelectronic properties and can meet practical performance requirements that are uncommon in optoelectronic materials. This review introduces the fundamentals and the photonic device concepts that arise from the LNOI materials platform. Firstly, the nonlinear optical response of LNOI is presented, including birefringent phase matching (BPM), modal phase matching (MPM), and quasi-phase matching (QPM). The tunable properties are also introduced, including electro-optical (EO), thermo-optical (TO), and acousto-optical (AO) effects. The structures of nonlinear optical devices, such as ridge waveguides (including periodically polarized inversion waveguides), Mach–Zehnder interferometer (MZI) modulators and micro-resonators (such as disks and rings) are demonstrated. Finally, the future of LNOI devices is discussed. In the already mature and developed optoelectronic material systems, it is rare to find one particular material system supporting so many basic optical components, photonic devices and optoelectronic devices as LNOI does in the field of integrated photonic chips.

Photon-pair generation using inverse-designed thin-film lithium niobate mode converters

Spontaneous parametric down-conversion (SPDC) has become a key method for generating entangled photon pairs. Periodically poled thin-film lithium niobate (TFLN) waveguides induce strong SPDC but require complex fabrication processes. In this work, we experimentally demonstrate efficient SPDC and second harmonic generation using modal phase matching methods. This is achieved with inverse-designed optical mode converters and low-loss optical waveguides in a single nanofabrication process. Inverse design methods provide enhanced functionalities and compact footprints for the converter. Despite the extensive achievements in inverse-designed photonic integrated circuits, the potential of inverse-designed TFLN quantum photonic devices has been seldom explored. The device shows an on-chip conversion efficiency of 3.95% W−1 cm−2 in second harmonic generation measurements and a coincidence count rate up to 21.2 kHz in SPDC experiments. This work highlights the potential of the inverse-designed TFLN photonic devices and paves the way for their applications in on-chip nonlinear or quantum optics.

Ultra-broadband and ultra-high conversion efficiency difference frequency generation in a dual-layer thin-film LiNbO3 waveguide with reversed ferroelectric domain orientations

Although there is a great breakthrough in second harmonic generation based on the birefringence and quasi phase matching (QPM) concepts, the development of an efficient difference frequency generation (DFG) based on modal phase matching (MPM) is still in its infancy at present. In this paper, we propose an ultra-broadband and ultra-high conversion efficiency mid-infrared DFG on the basis of MPM in a dual-layer waveguide consisting of two reversed poled LiNbO3 thin-films on sapphire. The ultra-high conversion efficiency is ascribed to significantly enhanced nonlinear interaction realized by making use of spatial profile of optical nonlinearity of LN crystal produced by two reversed ferroelectric domain orientations in the dual-layer waveguide. The proposed DFG device has been modeled on the basis of a 0.98 μm pump wave and a 1.34–1.44 μm signal wave, the frequency difference of which generates a mid-infrared idler wave centered at 3.32 μm wavelength. The results show that the device exhibits excellent DFG performance, including an ultra-high nonlinear conversion efficiency 434.6 % W−1 cm−2 and an ultra-broad tuning band 340 nm meanwhile, which far exceed the performance of conventional MPM devices, even the QPM ones.

Efficient Parametric Frequency Conversions in Chalcogenide‐Loaded Etchless Thin‐Film Lithium Niobate Waveguides

AbstractParametric frequency conversion based on second‐order nonlinearity (χ(2)) is critical in many applications. The implementation of second‐order nonlinearity on integrated photonic platforms, in particular on the lithium niobate‐on‐insulator platform, has attracted considerable interest due to its low power consumption and small footprint. However, high‐efficiency on‐chip parametric frequency conversion remains challenging due to fabrication complexity. Here, efficient parametric frequency conversions via modal phase matching (MPM) in etchless thin‐film lithium niobate (TFLN)‐chalcogenide glass (ChG) hybrid waveguides fabricated by a simplified process free from domain engineering and etching of TFLN are proposed and demonstrated. An overall conversion efficiency of 25.55% W−1 for second‐harmonic generation is experimentally achieved in a 1‐cm‐long waveguide, significantly over the reach of the etchless counterparts and in the same order of magnitude as those of the etched cases based on MPM. Broadband frequency conversion with a 3‐dB bandwidth of 57 nm is achieved based on cascaded second‐harmonic generation and difference‐frequency generation via MPM in a nanophotonic waveguide using a continuous‐wave pump. This device shows great promise for efficient on‐chip parametric frequency conversion, enabling a wide range of photonic applications, and paving the way for further integration with various advanced ChG photonic devices toward on‐chip multifunctional microsystems.

Intermodal effective phase matching for second harmonic generation of ultrashort pulses in high index contrast optical fibers

Second harmonic generation (SHG) in glass optical fibers calls for creating a second order susceptibility in the fiber glass and for achieving phase matching between the pump and the second harmonic signal. The latter is very challenging when using ultrashort pulses, given that the group velocities of the pump and the second harmonic should also be matched. We have shown in previous work that it is possible to achieve simultaneous modal phase matching (MPM) and group velocity matching (GVM) when the pump and the second harmonic are propagating in the LP01 and LP02 modes, respectively, in high GeO2-content double-clad optical fibers. However, simultaneous MPM and GVM can only be obtained in optical fibers with dedicated designs and within very tight geometrical tolerances. In this paper, we show that instead of considering the matching of phase and group velocities separately, we can consider a more general or “effective” phase matching approach, in which we consider all the dispersion terms up to the second order in the expressions of the propagation constants of the pump and second harmonic signals. This allows introducing the pulse duration as a controllable parameter that helps to enforce the said effective phase matching in fibers with designs that deviate by as much as 10% from the target, while providing for temporal walk-off lengths in excess of several centimeters. The impact of this finding goes beyond SHG only and can be applied to other ultrashort laser pulse-based nonlinear optical processes in fibers and waveguides.

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.

Frequency comb generation dynamics in χ(2) + χ(3) AlGaAs microresonators

We propose an AlGaAs microring resonator design for the generation of an optical frequency comb by means of the interplay between harmonic generation and Kerr effect. Modal phase matching imposes specific waveguide geometries and, consequently, it impacts the nonlinear efficiency of the system. We show the dynamics of χ(2) + χ(3) comb generation resulting from type-I modal phase matching.

Orbital angular momentum mode femtosecond fiber laser with over 100 MHz repetition rate

Orbital angular momentum (OAM) lasers have potential applications in large capacity communication systems, laser processing, particle manipulation and quantum optics. OAM mode femtosecond fiber laser has become the research focus due to the advantages of simple structure, low cost and high peak power. At present, OAM mode femtosecond fiber lasers have made some breakthroughs in key parameters such as repetition frequency, pulse width, spectrum width, but it is difficult to achieve good overall performance. Besides, the repetition rate is tens of MHz at present. In this paper, a large-bandwidth mode coupler is made based on the mode phase matching principle. In coupler, the first order mode coupler with 3 dB polarization dependent loss is made by the technology of strong fused biconical taper, and the second order mode coupler with 0.3 dB polarization dependent loss is made by the technology of weak fused biconical taper. By combining the nonlinear polarization rotation mode-locking mechanism, OAM mode femtosecond fiber laser with over 100 MHz repetition rate is built. The achievement of the key parameters is attributed to the selection of dispersion shifted fibers that can accurately adjust intracavity dispersion. Comparing with traditional dispersion compensation fibers (DCF), the group velocity dispersion is reduced by an order of magnitude, so it can better adjust intracavity dispersion to achieve the indexes of large spectral bandwidth and narrow pulse width. In addition, the diameter of the fiber is 8 μm, which is the same as that of a single mode fiber. Comparing with DCF, the fusion loss can be ignored, so only a shorter gain Erbium-doped fiber is required, which ensures a shorter overall cavity length and achieves high repetition frequency. The experimental results show that the first order OAM mode fiber laser has 113.6 MHz repetition rate, 98 fs half-height full pulse width, and 101 nm 10 dB bandwidth. Second-order OAM mode fiber laser has 114.9 MHz repetition rate, 60 fs half-height full pulse width, and 100 nm 10 dB bandwidth. Compared with the reported schemes, our scheme has good performance in key parameters such as repetition rate, pulse width and spectral width. We believe that the OAM mode fiber laser with excellent performance is expected to be widely used in OAM communication, particle manipulation and other research fields.

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.

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.

Broadband Second Harmonic Generation in a z-Cut Lithium Niobate on Insulator Waveguide Based on Type-I Modal Phase Matching

We numerically investigate a second harmonic generation (SHG) in a z-cut lithium niobate on insulator (LNOI) waveguide based on type-I mode phase matching (MPM) between two fundamental modes. A mode overlap factor that is close to unity is achieved and the normalized SHG efficiency reaches up to 72.1% W−1cm−2 at the telecommunication band, together with a large spectral tunability of 2.5 nm/K. Moreover, a bandwidth of about 100 nm for the broad SHG in a 5 mm-long LNOI ridge waveguide is demonstrated with this scheme. This stratagem will inspire new integrated nonlinear frequency conversion methods for versatile applications.

Second harmonic generation in modal phase-matched AlGaAs-on-insulator waveguides

We utilize modal phase matching and type-I configuration to achieve second harmonic generation in AlGaAs-on-insulator waveguides. We demonstrate an overall normalized conversion efficiency of 200% W-1 cm-2 at a pump wavelength of 1587 nm in a 1-mm-long waveguide.

On-chip ultraviolet second-harmonic generation in lithium-tantalate thin film microdisk

The compact and reliable ultraviolet (UV) source has attracted remarkable attention for its potential use in optical measurement systems, high-density optical storage, and biomedical applications. We demonstrate ultraviolet generation by frequency doubling in a lithium-tantalate-on-insulator (LTOI) microdisk via modal phase matching. The 50-µm-diameter microdisk was milled by a focused ion beam (FIB) and followed by chemo-mechanical polishing (CMP) to smooth the disk surface and edge, and the Q-factor reaches 2.74×105 in the visible band. On-chip UV coherent light with a wavelength of 384.3 nm was achieved, which shows great promise for using LTOIs in integrated ultraviolet source platforms.

Orbital angular momentum mode femtosecond fiber laser with over 100 MHz repetition rate

Orbital Angular Momentum (OAM) lasers have potential demand in many applications such as large capacity communication systems, laser processing, particle manipulation and quantum optics. OAM mode femtosecond fiber laser has become the research focus with the advantages of simple structure, low cost and high peak power. The current OAM mode femtosecond fiber lasers have made breakthroughs in the repetition frequency, pulse width, spectrum width and other key parameters, but it is difficult to achieve good overall performance. Besides, the repetition rate is currently in tens of MHz. In this paper, a large-bandwidth mode coupler is made based on the mode phase matching principle. Among them, the first order mode coupler with 3dB polarization dependent loss is made by the technology of strong fused biconical taper, and the second order mode coupler with 0.3dB polarization dependent loss is made by the technology of weak fused biconical taper. Combined with the nonlinear polarization rotation mode-locking mechanism, OAM mode femtosecond fiber lasers with over 100 MHZ repetition rate are built. The achievement of the key parameters is attributed to the selection of dispersion shifted fibers that can accurately adjust intracavity dispersion. Compared to traditional dispersion compensation fibers (DCF), the group velocity dispersion is reduced by an order of magnitude, so it can better adjust intracavity dispersion to achieve the indicators of large spectral bandwidth and narrow pulse width. In addition, the diameter of the fiber is 8μm, which is the same as that of a single mode fiber. Compared to DCF, the fusion loss can be ignored, so only a shorter gain Erbium-doped fiber is required that ensure a shorter overall cavity length and achieve high repetition frequency. The experimental results show that the first order OAM mode fiber laser has 113.6 MHz repetition rate, 98 fs half-height full pulse width, and 101nm 10-dB bandwidth. Second-order OAM mode fiber laser has 114.9 MHz repetition rate, 60 fs half-height full pulse width, and 100nm 10-dB bandwidth. Compared with the reported schemes, our scheme has better performance in key parameters such as repetition rate, pulse width and spectral width. We believe that the OAM mode fiber laser with good over performance is expected to be more widely used in OAM communication, particle manipulation and other research fields.

Generation of counterpropagating and spectrally uncorrelated photon-pair states by spontaneous four-wave mixing in photonic crystal waveguides

In this work, we propose and theoretically analyze a new scheme for generation of counterpropagating photon pairs in photonic crystal waveguides through the process of spontaneous four-wave mixing. Using the fundamental properties of periodic Bloch modes in a standard photonic crystal waveguide, we demonstrate how modal phase-matching can be reached between forward-propagating pump modes and counterpropagating signal and idler modes, for generation of degenerate and non-degenerate photon pairs. We then show how this scheme can be used for generation of photon pairs that are nearly uncorrelated in the spectral degree of freedom. Such a source will be highly interesting as a heralded source of single photons, especially as the spectrally separable signal and idler photons are also spatially separated directly at the source. We conduct our investigation based on a design in silicon, yet our design concept is general and can in principle be applied to any nanostructured material platform.

Phase Matching via Plasmonic Modal Dispersion for Third Harmonic Generation.

The overall effectiveness of nonlinear optical processes along extended nonlinear media highly depends on the fulfillment of the phase‐matching condition for pump and generated fields. This is traditionally accomplished by exploiting the birefringence of nonlinear crystals requiring long interaction lengths (cm‐scale). For nonbirefringent media and integrated photonic devices, modal phase matching can compensate the index mismatch. Here, the various interacting waves propagate in transverse modes with appropriate phase velocities, but they suffer from a low refractive index contrast and cm‐scale interaction lengths. This work harnesses modal phase matching for third‐harmonic generation (THG) in plasmonic waveguides using an organic polymer (poly[3‐hexylthiophene‐2,5‐diyl]) as the nonlinear medium. One demonstrates experimentally an effective interaction area as small as ≈ 0.11 µm2 and the phase‐matched modal dispersion results in THG efficiency as high as ≈ 10–3 W‐2 within an effective length scale of ≈ 4.3 µm. THG also shows a strong correlation with the polarization of the incident laser beam, corresponding to the excitation of the antisymmetric plasmonic modes, corroborating that plasmonic modal phase matching is achieved. This large reduction in device area of orders of magnitude is interesting for various applications where space is critical (e.g., device integration or on‐chip applications).