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Optical and Quantum Electronics

Group index matching for widely tuned SPDC processes in singly poled LNOI waveguide

Muskan Arora, Pranav Chokkara, Jasleen Lugani

Published May 19, 2026• 1 citation
Publisher

Abstract

Abstract We demonstrate the importance of group index engineering and appropriate poling in lithium niobate on insulator (LNOI) waveguide for enabling widely tuned spontaneous parametric downconversion (SPDC) processes.We show that asymmetric and symmetric group index matching (AGIM and SGIM) conditions in such strongly dispersive waveguides allow for phase-matching a wide range of SPDC processes with tunable spectral properties of output photons by changing just the pump wavelength. Condsidering an x-cut LNOI waveguide geometry, we obtain a large pump wavelength tuning range (∼536 nm for AGIM and (∼625 nm for SGIM), thereby facilitating the generation of widely tuned downconverted photons in a singly poled waveguide device. The resultant photon pairs slowly span different wavelength bands, covering NIR and MIR regions, which are important for various quantum information tasks. We also analyze the effect of different waveguide cross-sections on group index engineering capabilities and their influence on the properties of the generated quantum light. The reported work highlights the significance of dispersion properties of the non-linear waveguides and their role in building compact and tunable quantum light sources which can simultaneously cater to a wide variety of quantum optical tasks.

DOI
10.1007/s11082-026-08877-2

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Abstract We demonstrate the importance of group index engineering and appropriate poling in lithium niobate on insulator (LNOI) waveguide for enabling widely tuned spontaneous parametric downconversion (SPDC) processes.We show that asymmetric and symmetric group index matching (AGIM and SGIM) conditions in such strongly dispersive waveguides allow for phase-matching a wide range of SPDC processes with tunable spectral properties of output photons by changing just the pump wavelength. Condsidering an x-cut LNOI waveguide geometry, we obtain a large pump wavelength tuning range (∼536 nm for AGIM and (∼625 nm for SGIM), thereby facilitating the generation of widely tuned downconverted photons in a singly poled waveguide device. The resultant photon pairs slowly span different wavelength bands, covering NIR and MIR regions, which are important for various quantum information tasks. We also analyze the effect of different waveguide cross-sections on group index engineering capabilities and their influence on the properties of the generated quantum light. The reported work highlights the significance of dispersion properties of the non-linear waveguides and their role in building compact and tunable quantum light sources which can simultaneously cater to a wide variety of quantum optical tasks.
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) and photon-pair creation through spontaneous parametric down-conversion (SPDC). 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.
Abstract Polarisation-entangled photon pairs generated through nonlinear optical processes are crucial ingredients for a range of quantum information processing applications, such as quantum sensing and communication. In this work, we report an optimised design of a lithium niobate on insulator waveguide that is capable of directly generating polarisation-entangled photon pairs enabled through two different spontaneous parametric down-conversion processes simultaneously in the telecommunication wavelength band with a single poling period, based on Type II phase matching. The generated photon pairs cover wavelengths in telecom O (1310 nm) and telecom C (1550 nm) bands with the computed spectral brightness of ≈ 2 × 10 9 pairs sec − 1 mW − 1 . We also study the effect of temporal delay on the quality of entanglement, which is minimised by selecting an appropriate length of the waveguide and pump bandwidth. The optimised integrated quantum light source, presented here, eliminates the need for complex domain engineering or intermodal dispersion and will serve as a valuable resource for various quantum-optical experiments.
Abstract Spontaneous parametric down-conversion (SPDC), especially in non-linear waveguides, serves as an important process to generate quantum states of light with desired properties. In this work, we report on a design of a strongly dispersive, singly poled thin film lithium niobate (TFLN) waveguide geometry which acts as a convertible source of photon pairs across telecom band with tunable spectral properties. Through our simulations, we demonstrate that by using this optimized waveguide geometry, two completely different yet desirable type II phase-matched SPDC processes are enabled using a single poling period. One process generates spectrally correlated non-degenerate photon pairs with one photon at 1310nm (telecom O band) and the other at 1550nm (telecom C band). The second SPDC process results in spectrally uncorrelated photon pairs in telecom C band at 1533nm and 1567nm respectively.We attribute this versatility of TFLN waveguide to its strong dispersion properties and make a comparative study with the existing weakly dispersive waveguide platforms.We believe that such a versatile source of photon pairs will serve as an important ingredient in various quantum optical tasks which require photons at different telecom bands and desired spectral properties.
Photonic integrated optical components, notably straight waveguides, serve as pivotal elements for on-chip generation and manipulation of quantum states of light. In this work, we focus on optimizing waveguides based on lithium niobate on insulator (LNOI) to generate photon pairs at telecom wavelength using spontaneous parametric down-conversion (SPDC). Specifically, we investigate lateral leakage for all possible SPDC processes involving type 0, type I and type II phase matching conditions in an X-cut lithium niobate waveguide and provide a recipe to avoid leakage loss for the interacting photons. Furthermore, focusing on type II phase matching, we engineer the waveguide in the single mode regime such that it also satisfies group index matching for generating spectrally pure single photons with high purity (99.33%). We also address fabrication imperfections of the optimized design and found that the spectral purity of the generated photons is robust to fabrication errors. This work serves as a tutorial for the appropriate selection of morphological parameters to obtain lossless, single mode LNOI waveguides for building linear optical circuits and photon pair generation at telecom wavelengths using desired phase-matching conditions.
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.
Abstract Photonic integrated optical components, notably straight waveguides, serve as pivotal elements for on-chip generation and manipulation of quantum states of light. In this work, we focus on optimizing waveguides based on lithium niobate on insulator (LNOI) to generate photon pairs at telecom wavelengths using spontaneous parametric down-conversion (SPDC). Specifically, we investigate lateral leakage for all possible SPDC processes involving type 0, type I, and type II phase matching conditions in an X-cut lithium niobate waveguide and provide a recipe to avoid leakage loss for the interacting photons. Furthermore, focusing on type II phase matching, we engineer the waveguide in the single-mode regime such that it also satisfies group index matching for generating spectrally pure single photons with high purity (99.33%). We also address fabrication imperfections of the optimized design and find that the spectral purity of the generated photons is robust to fabrication errors. This work offers guidance for the suitable selection of morphological parameters to obtain lossless, single-mode LNOI waveguides for building linear optical circuits and photon pair generation at telecom wavelengths using desired phase-matching conditions.