Simulation Study of Mid-Infrared Electro-Optic Modulators Based on Thin-Film Lithium Niobate Heterogeneously Integrated with Chalcogenide Glass

doi:10.16553/j.cnki.issn1000-985x.2025.0206

Abstract

Abstract: The mid-infrared （MIR） 3~5 μm band spans both an atmospheric transmission window and the molecular fingerprint region，enabling important applications in free-space optical communications，spectroscopic sensing，and thermal imaging. High-speed，low-loss modulators in this band are essential for advancing MIR photonic integrated systems，yet remain challenging to realize. Conventional platforms such as silicon，silicon nitride，aluminum nitride，and gallium arsenide often suffer from carrier absorption，thermo-optic effects，or limited electro-optic （EO） bandwidth in the MIR，making it difficult to simultaneously achieve high modulation efficiency and broad bandwidth. Thin-film lithium niobate （LNOI），featuring a wide transparency window and a strong Pockels effect，is a promising platform for ultrahigh-speed EO modulation；however，commercial LNOI wafers typically incorporate a SiO₂ buried-oxide （BOX） layer whose pronounced absorption in the MIR band introduces excess propagation loss，thereby limiting MIR device performance and system-level integration.In this work，the BOX-absorption-induced loss bottleneck was addressed by aiming to develop a low-loss，high-efficiency，and ultrabroadband MIR EO modulator compatible with standard commercial LNOI wafers. A heterogeneously integrated LNOI/chalcogenide-glass （ChG） platform was proposed and numerically investigated. The key idea is to employ refractive-index-tunable ChG as an optical guiding layer，with a ChG strip waveguide integrated on the LNOI surface. This design pulls the optical mode upward and markedly reduces modal overlap with the SiO₂ BOX layer. This mode-engineering strategy suppresses MIR absorption loss while introducing additional degrees of freedom—via the ChG refractive index and waveguide geometry—for co-optimizing propagation loss，modulation efficiency，and EO bandwidth.A Mach-Zehnder modulator was designed on a representative X-cut LNOI/SiO₂/Si stack，consisting of a ~900-nm-thick lithium niobate film，a ~4.7-μm-thick SiO₂ BOX layer，and a ~500-μm-thick silicon substrate. The device incorporated 1×2 multimode-interference splitters/combiners and a push-pull traveling-wave electrode phase shifter. A multiphysics co-simulation framework was employed for joint optical-RF optimization：on the optical side，the ChG refractive index and key geometric parameters were systematically swept to tailor the modal distribution and minimize propagation loss；on the RF side，electromagnetic simulations were used to optimize electrode dimensions and spacing，enabling optical-microwave group-index matching，reduced microwave attenuation，and favorable impedance matching，thereby enhancing the EO 3 dB bandwidth.After comprehensive optimization，the proposed device achieves，at 4.1 μm，a low propagation loss of 0.67 dB/cm，a half-wave voltage-length product of 14.7 V·cm，and an EO 3 dB bandwidth exceeding 110 GHz，demonstrating a compelling combination of low loss，high efficiency，and ultrabroad bandwidth. Overall，these results establish refractive-index-engineered ChG-assisted mode engineering as an effective route to mitigate BOX absorption on commercial LNOI wafers. Compared with alternative approaches relying on deep etching，BOX removal，or substrate re-engineering，this hetero-integration strategy achieves substantial performance improvements with low process invasiveness，offering enhanced platform compatibility and scalability. These findings provide a transferable design paradigm for compact，high-performance MIR EO modulators and may facilitate further advances in high-speed on-chip modulation for MIR communications，sensing，and quantum information processing.

Key words: thin-film lithium niobate; chalcogenide glass; heterogeneous integration; electro-optic modulator; mid-infrared band; integrated photon technique

CLC Number:

TN256

PAN Kaiyan, CAI Heyi, DU Qingyang, QIN Qi, HU Xiaopeng, ZHU Shining. Simulation Study of Mid-Infrared Electro-Optic Modulators Based on Thin-Film Lithium Niobate Heterogeneously Integrated with Chalcogenide Glass[J]. Journal of Synthetic Crystals, 2026, 55(2): 173-181.

Figures/Tables 7

Fig.1 Optical field confinement factors and absorption loss characteristics in SiO2 layer of ChG waveguide structure with different refractive index. （a） Change with ChG thickness；（b） change with ChG waveguide width

Fig.2 Half-wave voltage-length product and group refractive index matching varying with refractive index of ChG. Inset：zoom in view for 2.18~2.22 refractive index range

Fig.3 Overall structure of MIR thin-film lithium niobate modulator and schematic diagram of MMI structure. （a） Schematic diagram of MIR LNOI EO modulator，inset：cross-sectional of modulation area；（b） schematic diagram of MMI structure，inset：simulated transmissions from two ports and electric field distribution at 4.1 μm

Fig.4 Half-wave voltage and loss varying with spacing between electrodes

Fig.5 Influence of electrode structural parameters on RF characteristics. Simulation analysis of microwave refractive index，characteristic impedance and microwave loss under different electrode gap （a），different electrode widths （b） and different electrode thickness （c）

Fig.6 Frequency response of the MIR LNOI EO modulator. Simulation analysis of micro wave refractive index （a），microwave loss （b），characteristic impedance （c），and electro-optic response at different frequencies （d）

Table 1 Comparison of different heterogeneous platform of EOM in the MIR band

Modulator type	Wavelength/μm	V_πL/（V∙cm）	3 dB EO bandwidth	Reference
Si-on-LN	3.39	26	^a10 GHz	［24］
TiO₂-on-LN	2.5	50	—	［25］
Ge-on-Si	3.8	0.47	^b0.06 GHz	［26］
ChG-on-LN	4.1	14.7	^a110 GHz	This work

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