Mid-Infrared Photonic Devices

• Mid-Infrared Photonic Devices are integrated optical components operating in the 2–15+ μm range, offering robust capabilities for chemical sensing, environmental monitoring, and spectroscopic analysis.
• They utilize diverse material platforms such as group-IV semiconductors, III–V compounds, chalcogenide glasses, and emerging 2D materials to achieve low-loss waveguiding, high confinement, and efficient modulation.
• Innovations in waveguide engineering, nonlinear optics, and heterogeneous integration are advancing chip-scale systems for applications in free-space communications, biomedical sensing, and quantum photonics.

Mid-infrared (mid-IR) photonic devices encompass a diverse class of integrated waveguides, resonators, detectors, modulators, and sources engineered to operate in the spectral region ranging from approximately 2 μm to beyond 15 μm. This region overlaps the so-called molecular “fingerprint” window, where many molecular species exhibit fundamental vibrational and rotational absorption lines, making mid-IR photonics pivotal for chemical sensing, environmental monitoring, spectroscopy, and emerging technologies in quantum photonics and free-space communications. Advances in materials engineering, nanofabrication, and waveguide integration have allowed the migration of mid-IR photonic systems from free-space, bulk-optics platforms to chip-scale architectures compatible with scalable microelectronics processes.

1. Material Platforms for Mid-Infrared Photonics

The performance and feasibility of mid-IR photonic devices are set by the transparency window, refractive index, nonlinearity, and process compatibility of the constituent material platform. Four principal categories have been established:

• Group-IV platforms:
  • Ge-on-Si (GOS) enables integration with CMOS electronics and supports single-mode propagation for 2–8 μm (Marris-Morini et al., 13 May 2025). GOS is limited at longer wavelengths due to increasing overlap of the optical mode with absorbing Si substrate, but increased Ge thickness or suspended structures can extend the cutoff.
  • SiGe on Si allows engineering of graded-index claddings to tailor mode confinement and minimize substrate leakage past 8 μm. Propagation losses as low as 0.5 dB/cm have been achieved in the 5–7 μm range (Marris-Morini et al., 13 May 2025).
  • Ge-on-insulator (GOI) and suspended Ge enable waveguiding to 15 μm, relying on air, subwavelength gratings, or low-loss insulators as claddings.
• III–V compound semiconductors:
  • Suspended AlGaAs on silicon yields low-loss, high-confinement waveguides that support second- and third-order nonlinear interactions, operating from 1.26 to 4.6 μm with propagation losses as low as 0.45 dB/cm and loaded Q factors up to 8.8×10^5 (Chiles et al., 2019).
• Chalcogenide glasses and mid-IR transparent dielectrics:
  • Chalcogenide sulphide glasses (e.g., Ge23Sb7S70, GLS, GCIS) support waveguiding from 3 μm to beyond 10 μm (Rodenas et al., 2011, Lin et al., 2013, Singh et al., 2018) with direct-laser writing fabrication, enabling complex three-dimensional circuits and mid-IR photonic crystal cavities with Q ≈ 2,000.
  • Advanced Z/HBLAN fluoride glasses exhibit large laser-inscribed index contrast (~10⁻²), permitting low-loss, high-NA waveguides directly integrable with ZBLAN fibers (Fernandez et al., 2022).
• Polymeric and plasmonic structures:
  • Dielectric-loaded surface plasmon polariton (DLSPP) waveguides leveraging polyethylene (transparency 2–200 μm, n ≈ 1.48 at 9.26 μm) atop gold can confine and route subwavelength modes for complex integrated photonic circuits over mm-length scales and radii of curvature as small as tens of μm (David et al., 2023).
• Emerging 2D materials:
  • MXenes (e.g., Ti₃C₂Tₓ, V₂CTₓ) provide versatile platforms for photodetection, all-optical modulation, and plasmonics in the 2–20 μm window, offering tuneable work functions, strong optical nonlinearities, and solution-processable integration (Al-Hadeethi et al., 2023).

2. Waveguide Engineering and Passive Device Technologies

Passive mid-IR devices—waveguides, splitters, multiplexers, and resonators—rely on stringent control over mode confinement, scattering, and absorption loss at long wavelengths:

• Single-mode operation:
  • Silicon-on-sapphire (SOS) waveguides with 1.8 × 0.6 μm^2 cross-sections confine the TE₀ mode at 4.5 μm (A_eff ≈ 1.1 μm²), with measured propagation loss of ~10.4 dB/cm, dominated by surface roughness and fabrication residues (0911.0949).
  • Direct-laser-inscribed chalcogenide guides support mode field diameters of 43–58 μm at 10.6 μm (Δn up to 0.012), yielding monochromatic interference visibilities of 99.89% in interferometry circuits (Rodenas et al., 2011).
• Resonators and photonic crystal structures:
  • Micro-ring resonators in silicon–CaF₂ or Ge–Si platforms achieve Q factors exceeding 6×10⁴, effective path lengths >5 cm, and finesse up to 190 at 5.2 μm (Chen et al., 2014, Marris-Morini et al., 13 May 2025).
  • 1D photonic crystal cavities in Ge23Sb7S70 attain Q ≈ 2,000 at 5.2 μm; Q is controlled by cavity length and PhC mirror parameters (Lin et al., 2013).
• Dispersion and nonlinearity management:
  • Photonic crystal fiber (PCF) designs, particularly As₂Se₃ with “endlessly single-mode” cross-sections, afford flat, wide dispersion and low loss—crucial for hyper-broadband supercontinuum generation (2–10 μm with 7 μm 20 dB-width after 6 cm) (Yuan, 2013).
  • Suspended III–Vs and SiGe platforms enable dispersion engineering for soliton and parametric effects; e.g., octave-spanning SCG in AlGaAs with pump energies down to 3.4 pJ (Chiles et al., 2019).
• Photonic crystals and 2D band structure optimization:
  • In(Ga,Al)As/InP photonic crystal slabs exhibit modes mapped by angle-resolved reflection spectroscopy with 0.3° angular resolution, exploiting polarization selection rules for mode characterization and Q factor analysis (Chalimah et al., 2021).

3. Active Devices: Modulation, Sources, and Detection

Active mid-IR photonic devices employ material engineering, band structure modification, and innovative architectures for efficient modulation, light generation, and detection:

• Electro-optic modulation:
  • Thin-film lithium niobate on sapphire Mach–Zehnder modulators (operational from 3.95–4.3 μm) achieve >20 GHz bandwidth, 34 dB extinction, and full π-phase shift at Vπ = 22 V·cm, supporting 10 Gbit/s data transmission and frequency comb generation (80 GHz width) (Didier et al., 29 May 2025).
  • GeSn electro-absorption modulators (EAMs) utilize Franz–Keldysh effect, with bandgap reduced via Sn alloying: the direct bandgap empirically described as

  $$E_t(\mathrm{Ge}_{1-x}\mathrm{Sn}_x) = (1-x)E_t(\mathrm{Ge}) + xE_t(\mathrm{Sn}) - b_t x (1-x)$$

  enabling MIR operation (2067–2208 nm) with optimized absorption modulation (Lin et al., 2018).

• Integrated photodetection:

  • Zn-implanted Si PIN diodes operate at 2.2–2.4 μm at room temperature (responsivities up to 87 mA/W; dark current <10 μA), by leveraging sub-bandgap trap states for photon absorption (Grote et al., 2014).
  • Black phosphorus devices (at 3.39 μm) display high photoconductive gain (∼10⁴), external responsivity up to 82 A/W, NEP ~5.6–8 pW/Hz^½, and kilohertz bandwidth due to fast carrier dynamics. Polarization selectivity arises from BP’s low-symmetry crystal structure (Guo et al., 2016).
  • Waveguide-integrated graphene detectors (on GSSe:CaF₂) employ split-gate photothermoelectric effect for bias-free MIR detection up to 5.2 μm with ≈1.1 nW/Hz^½ NEP and GHz-range bandwidth (Goldstein et al., 2021).
  • Room-temperature Ge bolometric detectors on Ge–OI employ heavy p-doping for free-carrier absorption, yielding broadband responsivity (28.35 %/mW, 4030–4360 nm), NEP 4.03×10⁻⁷ W/Hz^½ at 4180 nm, and full CMOS-foundry compatibility (Shim et al., 23 May 2024).
• Mid-IR light sources:
  • Photonic integration with GeSn-based or QCL/ICL sources is ongoing, with emphasis on coupling efficiency, monolithic process compatibility, and platform transparency at target wavelengths (>4 μm) (Marris-Morini et al., 13 May 2025).

4. Nonlinear and Frequency-Conversion Devices

Nonlinear photonic devices exploit the high nonlinearity and engineered dispersion in mid-IR materials to generate new spectral components, increase bandwidth, and allow frequency translation:

• Supercontinuum generation (SCG):
  • As₂Se₃ PCFs deliver SCG from 2 to 10 μm with high spectral flatness using ∼4 μm femtosecond pumping near engineered zero-dispersion wavelengths. The generalized nonlinear Schrödinger equation models broadband spectral broadening, incorporating Kerr, Raman, and higher-order dispersion effects (Yuan, 2013).
  • Suspended AlGaAs and SiGe guides have demonstrated SCG spanning from 3 to 13 μm due to broad transparency and strong χ^2/χ^3 response (Chiles et al., 2019, Marris-Morini et al., 13 May 2025).
• Four-wave mixing (FWM) and parametric gain:
  • Silicon nitride nanophotonic waveguides enable tunable frequency translation (>100 THz detuning), supporting MIR idler generation from 2.6–3.6 μm (100 pJ pump pulses) and >20 dB broadband NIR parametric gain by dispersion engineering (Kowligy et al., 2018).
  • Integrated AlGaAs, with both second- and third-order susceptibility, supports frequency conversion with ultra-low-power operation, enabling prospects for self-referenced combs (Chiles et al., 2019).

5. Sensing, Spectroscopy, and Biomedical Applications

Integrated mid-IR photonic devices enable compact, high-sensitivity spectroscopic platforms targeting vibrational and rotational transitions:

• Label-free chemical and biological sensors:
  • On-chip silicon micro-ring resonators (Q ≈ 6×10⁴) coupled to analytes via enhanced field overlap achieve sub-0.1 ng detection limits, leveraging MIR vibrational absorption strengths two–three orders of magnitude higher than in the near-IR (Chen et al., 2014).
  • Si and Ge-based ATR and slot waveguide sensors resolve molecular signatures (e.g., toluene, BSA, cocaine, CO₂, aerosols), achieving low-parts-per-million detection with on-chip referencing and robust miniaturization (Marris-Morini et al., 13 May 2025, Ottonello-Briano et al., 2019, Shim et al., 23 May 2024, Singh et al., 2018).
• Astrophotonics and interferometry:
  • Three-beam combiners fabricated via femtosecond direct laser writing in chalcogenide glass demonstrate 99.89% monochromatic interference visibility at 10.6 μm, providing robust, fully-integrated beam recombination for stellar interferometry (Rodenas et al., 2011).
• Lab-on-chip spectroscopy platforms:
  • Spiral Ge23Sb7S70 chalcogenide waveguides increase the particle–light interaction path for broadband aerosol fingerprinting (1–10 μm), complementing resonator-based particle counting for comprehensive environmental and bio-sensing (Singh et al., 2018).

6. Integration, Fabrication Strategies, and System-Level Considerations

Achieving scalable, robust mid-IR photonic devices depends on advances in micromachining, transfer, and integration protocols:

• Heterogeneous integration:
  • Adhesive-free transfer processes relocate Si devices from SOI onto CaF₂ and sapphire, yielding low-loss high-index-contrast waveguides and resonators ready for mid-IR applications where buried SiO₂ is opaque (Chen et al., 2014, Didier et al., 29 May 2025).
  • Suspended structures (air or subwavelength grating) minimize substrate loss and maximize transparency and field overlap (Marris-Morini et al., 13 May 2025).
• Direct laser writing and ultrafast inscription:
  • One-step fs-laser writing in chalcogenide and tailored fluoride glasses enables robust, three-dimensional embedded waveguides with engineered dispersion and index contrast, fully compatible with fiber-pigtailing (Rodenas et al., 2011, Fernandez et al., 2022).
• Polymer and plasmonic patterning:
  • Photolithographic processes with polymeric ridges atop noble metals form DLSPP platforms for subwavelength integrated photonics, supporting mm-scale propagation and <2 dB bend losses at mid-IR wavelengths (David et al., 2023).
• System-level integration:
  • Mono- and heterogeneous integration of III–V lasers, graphene/MXene/2D detectors, and Si/Ge-based passive components is progressing. The emergence of 200 mm wafer platforms (e.g., for GOS) and precise edge or inverted taper couplers supports industrial-scale production (Marris-Morini et al., 13 May 2025).

7. Challenges and Future Outlook

Despite significant progress, further development is needed:

• Materials: Innovations in low-loss, mid-IR transparent cladding materials (Al₂O₃, CaF₂, HfO₂, polymers) are being pursued to replace lossy SiO₂ (Marris-Morini et al., 13 May 2025). Minimizing lattice mismatch (e.g., via graded SiGe buffers) mitigates dislocation-induced loss in Ge-on-Si.
• Device integration: Monolithic or heterogeneous integration strategies for mid-IR sources (QCLs, GeSn), efficient couplers, and on-chip frequency combs are active areas.
• Noise and thermal management: Bolometric and photothermoelectric detectors must balance bandwidth, noise-equivalent power, and scalability while maintaining compatibility with standard CMOS workflows (Shim et al., 23 May 2024, Goldstein et al., 2021).
• Fabrication repeatability and industrial scaling: Transitioning from laboratory demonstrations to high-yield, cost-effective fabrication depends on advanced wafer thinning, dicing, and integration of high-quality dielectric and passivation layers.
• Emerging platforms: Further control of surface terminations and scalable patterning for 2D materials such as MXenes will shape future device architectures for mid-IR photonics (Al-Hadeethi et al., 2023).

A plausible implication is continued convergence towards versatile, multi-functional, and scalable integrated photonic platforms that harness the molecular fingerprint region for sensing, spectroscopy, nonlinear optics, and quantum photonic applications. The accumulating developments in materials, device architectures, and fabrication approaches are poised to underpin the next generation of mid-infrared photonic systems.