Infrared to Visible Upconversion

• Infrared to visible upconversion is a spectral conversion process that uses nonlinear and energy transfer mechanisms to convert IR photons into high-efficiency visible emission.
• It employs diverse platforms such as rare-earth doped fibers, nonlinear crystals, plasmonic cavities, and organic semiconductors to overcome limitations in direct IR detection.
• Advances in phase-matching, adiabatic techniques, and metasurface engineering improve resolution, efficiency, and bandwidth, enabling breakthroughs in imaging, sensing, and quantum systems.

Infra-red to visible upconversion is the process in which photons in the infrared (IR) regime are converted into photons at visible wavelengths through a variety of nonlinear, energy transfer, or optomechanical mechanisms. This spectral conversion is pivotal for overcoming the limitations of direct IR detection—such as poor quantum efficiency, high dark noise, and the need for cooled detectors—by enabling the use of high-performance visible-range detectors (e.g., silicon-based CCD/CMOS), while also facilitating advanced imaging, sensing, and quantum information processing across disjoint frequency domains. Upconversion approaches span inorganic and organic materials, rare-earth-doped fibers, atomic vapors, nonlinear crystals (bulk and metasurface), molecular/plasmonic cavities, and fundamentally distinct upconversion mechanisms including multiphoton, energy transfer, sum/difference-frequency generation, and triplet–triplet annihilation.

1. Fundamental Mechanisms of Upconversion

Several physically distinct mechanisms enable the upconversion of IR photons to visible photons:

1. Multistep Electronic Energy Transfer (Lanthanide-doped hosts):
   • In Tm³⁺-doped silica fibers, IR excitation at 1586 nm populates the ^3F₄ level; subsequent excited-state absorption (ESA) or energy transfer upconversion (ETU, e.g., ^3F₄ + ^3F₄ → ^3H₄ + ^3H₆) leads to emission at ~800 nm (Simpson et al., 2010).
   • The quadratic dependence I₈₀₀ ∝ (I₁₈₀₀)² is a signature of two-photon processes; ETU is confirmed as the dominant mechanism for Tm₂O₃ > 200 ppm, with the population kinetics governed by coupled rate equations involving lifetimes τ₁, τ₂ and a W_ETU coefficient for bimolecular interactions.
2. Nonlinear Optical Frequency Mixing:
   • Four-Wave Mixing (FWM) in Atoms: Off-resonant ladder-type atomic systems (e.g., 85Rb: 5S₁/₂ → 5P₃/₂ → 4D₅/₂) enable high-efficiency IR-to-visible upconversion via FWM, enhanced by large third-order nonlinear susceptibility χ^3 under near two-photon resonance (Ding et al., 2012, Ding et al., 2012).
   • Sum-Frequency Generation (SFG) in Nonlinear Crystals: In bulk/microstructured crystals (PPLN, KTP, AGS, LiNbO₃, GaAs), sum-frequency processes (ω_SF = ω_p + ω_IR) convert IR images to visible with high fidelity, phase matching or QPM crucially determining efficiency, bandwidth, and field of view (Demur et al., 2018, Mrejen et al., 2019, Yang et al., 2019, Camacho-Morales et al., 2021, Ge et al., 2023, Molina et al., 28 May 2024, Han et al., 30 May 2025, Wang et al., 25 Sep 2025).
3. Triplet–Triplet Annihilation in Organic Semiconductors:
   • Bulk or heterojunction films (Y6/rubrene, perovskite/rubrene) absorb NIR photons for triplet sensitization, followed by TTA yielding higher-energy singlets that emit visible light—realized at extremely low NIR intensities and leveraged in energy- and power-constrained passive imaging (Nienhaus et al., 2019, Hamid et al., 27 Nov 2024).
4. Molecular Optomechanics and Raman Interactions in Plasmonic Cavities:
   • Coherent driving of specific molecular vibrational states via MIR absorption in nanogap plasmonic resonators allows visible probe lasers to transduce MIR signals into anti-Stokes Raman sidebands with quantum efficiency boosted by >10¹³-fold via field localization effects (Xomalis et al., 2021, Chen et al., 2021).
5. Nonlinear Metasurfaces Empowered by BIC/Quasi-BIC Modes:
   • CMOS-compatible silicon or GaAs metasurfaces, tailored for symmetry-protected quasi-bound states in the continuum (Q-BIC), enable resonantly enhanced third-harmonic generation (THG) or SFG, supporting high-efficiency direct upconversion imaging with subwavelength spatial resolution (Camacho-Morales et al., 2021, Liu et al., 29 Aug 2025).

2. Physical and Theoretical Modeling

Upconversion processes are characterized and quantitatively described via:

• Rate Equations: For rare-earth systems, coupled first-order equations for excited state populations, including nonlinear (n₁²) ETU terms, predict double-exponential afterglow kinetics, validated against temporal luminescence data (Simpson et al., 2010).
• Nonlinear Wave Coupling: The SFG process is described by the coupled wave equations for the three fields (signal E₁, pump E₂, up-converted E₃), with solutions under the thin-crystal, slowly-varying envelope, and paraxial approximations yielding explicit forms for the output image intensity as a pointwise product of signal and pump intensities (Yang et al., 2019, Ge et al., 2023).
• Phase-Matching Conditions: Effective upconversion demands precise energy and momentum conservation, with QPM implemented via periodic poling or noncritical angular cutting to maximize conversion bandwidth and angular acceptance (Demur et al., 2018, Wang et al., 25 Sep 2025).
• Nonlocality and Imaging Transfer Function: High-Q metasurfaces exhibit strong nonlocal response (k-dependence of the transfer function), requiring optical designs (e.g., Fourier-plane imaging) to ensure uniform upconversion and preservation of all spatial frequency components (Molina et al., 28 May 2024).
• Density Matrix and Susceptibility Formalism: In atomic FWM schemes, evolution is captured by master equations with optical driving and decay, with the efficiency computed from the third-order polarization p^3 involving detuning, decay rates, and transition dipole moments (Ding et al., 2012).

3. Experimental Architectures and Impact on Performance Metrics

A broad range of optical and material platforms have been engineered for upconversion, each with unique implications for resolution, efficiency, field of view, and system applicability:

• Fiber and Bulk Hosts: Tm³⁺-doped silica fibers with optimized concentrations (>200 ppm) enable efficient ETU, with 800 nm upconversion measured under 1586 nm pulsed excitation. Short fiber lengths minimize amplified spontaneous emission and reabsorption (Simpson et al., 2010).
• Atomic Vapors: Hot Rb vapors in ladder-type FWM systems achieve up to 54% IR-to-visible conversion efficiency, verified with perpendicular pump polarizations. A 4f system ensures phase-matched image transfer and preserves spatial detail, with a similarity R ~ 0.9 between input and upconverted images (Ding et al., 2012, Ding et al., 2012).
• Nonlinear Crystals (Bulk and Metasurface):
  • Narrowband versus broadband-pumped lithium niobate in a 4f geometry demonstrates a trade-off: increased pump bandwidth broadens field of view up to 111 mrad, improving the number of spatial image elements at the expense of per-pixel efficiency (Demur et al., 2018).
  • Noncritical phase-matched (NCPM) KTP bulk crystals (aperture 6×7 mm) support >6× resolution improvement over QPM crystals due to elimination of walk-off and large acceptance angle, achieving <10 μm linewidth resolution and imaging across the 1.3–2.2 μm band (Wang et al., 25 Sep 2025).
  • AGS (AgGaS₂) crystals in noncollinear SFG configurations enable 10 μm-band mid-IR to visible upconversion of incoherent thermal images with analytically modeled and experimentally validated depth of field and astigmatism characteristics (Han et al., 30 May 2025).
• Flat-top versus Gaussian Pump Beams: Employing a flat-top spatial pump profile in SFG upconversion eliminates intensity modulation artifacts present with Gaussian envelopes, thereby increasing the image fidelity and improving subsequent image segmentation tasks (Yang et al., 2019).
• Metasurfaces: CMOS-compatible silicon metasurfaces with unidirectional symmetry breaking access high-Q quasi-BIC modes, delivering THG conversion efficiency of 3×10⁻⁵, and image resolution ~6 μm verified on rich targets (Liu et al., 29 Aug 2025). GaAs (110) nanoantennas support SFG with a χ^2 ≈ 200 pm/V, enabling submicron image transfer (Camacho-Morales et al., 2021).
• Organic TTA Schemes: Y6/rubrene/DBP hybrid heterojunctions with plasmonic gold nanopillar enhancement, combined with dichroic backreflectors for preferential emission directionality, enable all-passive upconversion from NIR (down to 10⁻⁷ W/cm²) to visible with high imaging fidelity and practical use in dual-wavelength telescopic systems (Hamid et al., 27 Nov 2024).

4. Resolution, Efficiency, and Bandwidth Considerations

Performance optimization and limitations are dominated by:

Platform/Mechanism	Resolution	Efficiency	Bandwidth/Field of View
Tm³⁺–fiber ETU (800 nm emission)	Set by fiber and pump	Upconversion observed at low Tm₂O₃ levels	Spectrally narrow
Atomic vapor FWM	~λ/(NA) (diffraction)	Up to 54% (Ding et al., 2012)	FOV and resolution set by vapor geometry
Bulk crystal SFG (LiNbO₃, KTP, AGS)	Down to ~8–10 μm (Wang et al., 25 Sep 2025)	Up to ~63% (single mode, narrowband) (Demur et al., 2018)	NCPM: up to 6.9° FOV, 1.3–2.2 μm band
Broadband pumped SFG	Up to 56×64 elements	7% at 2.7 nm pump width (cube dilution law) (Demur et al., 2018)	FOV up to 111 mrad
Silicon metasurface (quasi-BIC THG)	~6 μm	3×10⁻⁵ (single pass, 10 GW/cm²)	Limited by metasurface area and pump geometry
Perovskite/rubrene TTA	Not explicitly detailed	>3% internal upconversion (Nienhaus et al., 2019)	NIR-to-vis under 785 nm illumination
Y6/rubrene/DBP, passive, plasmonic TTA	Optical-system-limited	Detectable at 10⁻⁷ W/cm² NIR (Hamid et al., 27 Nov 2024)	700–930 nm input to 610 nm output

In SFG-based platforms, trade-offs between spatial resolution, field of view, and conversion efficiency are governed by the choice of crystal aperture, poling period (or NCPM configuration), pump beam profile, and phase-matching (QPM, adiabatic, or NCPM). In metasurface-based upconversion, field enhancement and sympathetic design of BIC modes yield high efficiency and subwavelength resolution but are limited by the physical patterned area.

5. Advanced Strategies and Functionalities

Beyond basic upconversion, contemporary research illustrates several advanced strategies:

• Adiabatic-Chirped Quasi-Phase Matching: Gradually varying the poling period (Λ(z)) enables multicolor, broadband, and time-resolved upconversion imaging of MIR signals in a single shot, with demonstrated 20% efficiency over the 2–4 μm band and time-of-flight discrimination of ps-scale dynamics (Mrejen et al., 2019).
• Nonlocal Metasurface Engineering: Imaging with high-Q LiNbO₃ metasurfaces placed in the Fourier plane, enabling simultaneous direct upconversion (zeroth order) and edge-enhanced (±1 orders) imaging for multifunctional scene analysis or security applications (Molina et al., 28 May 2024).
• Atomic Amplification Transduction: Λ-systems in Ba-138 vapor facilitate photon-number-amplified, IR-to-visible upconversion, with measured internal efficiency >1 (amplification via rapid cycling transitions) and projected 200-fold improvement in collection with cavity coupling. Polarization-sensitive transduction is theorized through engineered Zeeman substructure and optical pumping (Son et al., 16 Nov 2024).

6. Application Domains and Technological Significance

Upconversion technologies have penetrated or are poised to influence:

• Night Vision and Surveillance: Passive NIR-to-visible upconversion under ambient illumination constraints enables night vision without active illumination or electrical power supply (Hamid et al., 27 Nov 2024).
• Remote Sensing and LIDAR: SFG and adiabatic-based systems are well-suited for atmospheric window imaging, leveraging the low noise and high efficiency of silicon detectors (Demur et al., 2018, Ge et al., 2023, Wang et al., 25 Sep 2025).
• Biological Imaging: The access to biological optical windows (1.3–2.2 μm) (Wang et al., 25 Sep 2025) and deep-tissue NIR-to-vis imaging via TTA (Nienhaus et al., 2019) is significant for label-free and functional contrast imaging.
• Quantum Information Processing: High-fidelity, polarization-sensitive, and amplified IR-to-visible transduction enables bridging of disparate photonic and quantum platforms, critical for hybrid quantum networks (Son et al., 16 Nov 2024).
• Integrated Photonics and On-chip Sensing: Disordered metasurfaces with UCNPs extend silicon photodetection deep into the NIR at room temperature without affecting CMOS compatibility (responsivity up to 0.22 A/W at 1550 nm) (Chen et al., 16 Mar 2025).
• Advanced Optical Devices: Metasurface-based solutions allow for compact, room-temperature, and potentially flexible IR–vis imagers and sensors (Camacho-Morales et al., 2021, Liu et al., 29 Aug 2025).

7. Challenges, Comparisons, and Future Directions

Key challenges and areas of ongoing research include:

• Balancing Conversion Efficiency and Fidelity: Engineering trade-offs among field of view, spatial resolution, and per-pixel efficiency, especially under broadband pumping or limited aperture conditions (Demur et al., 2018, Yang et al., 2019, Wang et al., 25 Sep 2025).
• Aberration and Depth of Field Control: Rigorous analytical modeling of depth of field and astigmatic aberration (DOF: $d_{DoF} = \lambda/NA^2$; astigmatism: $d_a = \ldots$) informs optical design for maximizing imaging contrast in upconversion systems (Han et al., 30 May 2025).
• Bandwidth and Multicolor Conversion: Adiabatic QPM as a strategy to circumvent phase-matching-imposed narrow bandwidth, supporting true hyperspectral, joint spatial-temporal upconversion imaging (Mrejen et al., 2019).
• CMOS and System Integration: Emphasis on metasurface approaches (e.g., Si, GaAs, LiNbO₃) for on-chip, scalable, and robust upconversion (Camacho-Morales et al., 2021, Molina et al., 28 May 2024, Liu et al., 29 Aug 2025, Chen et al., 16 Mar 2025), with advances in nanofabrication, cavity engineering, and nanogap plasmonics.
• Passive and Low-light Operation: Advances in TTA and all-passive upconversion platforms are expanding the operational envelope into low-light, covert, or batteryless applications (Hamid et al., 27 Nov 2024).
• Quantum Information Transfer: Amplified, polarization-preserving transduction mechanisms for interfacing single-photon IR links with visible-range quantum nodes (Son et al., 16 Nov 2024); sub-natural linewidth sidebands in optomechanical nanocavities for highly coherent frequency conversion (Chen et al., 2021).

This domain is thus characterized by diverse materials and physical phenomena, ranging from atomic and molecular nonlinearities to tailored nanostructured fields, supporting an expanding set of real-world and frontier applications in imaging, sensing, photodetectors, and quantum technologies.