Infrared to Visible Upconversion

Updated 1 December 2025

Infra-red to visible upconversion is a process that transforms IR photons into visible light through mechanisms like SFG, TTA-UC, and rare-earth ladder transitions, enabling enhanced imaging and sensing.
Key mechanisms include nonlinear optical frequency conversion, internal photoemission, and quantum-coherent schemes, each optimizing field enhancement and conversion efficiency through precise phase-matching and resonant phenomena.
Device architectures span nonlinear crystals, semiconductor LEDs, nanophotonic metasurfaces, and atomic systems, offering tailored solutions for real-time imaging, low-light detection, and secure communications.

Infra-red to Visible Upconversion

Infra-red (IR) to visible upconversion encompasses a broad class of processes that transduce electromagnetic radiation from the IR domain (wavelengths ≳0.7 μm) into visible or near-infrared (NIR) photons. These mechanisms, spanning nonlinear optics, semiconductor devices, quantum emitters, and molecular/atomic physics, are central to applications in imaging, sensing, spectroscopy, photodetection, and quantum information. Upconversion enables efficient use of highly developed visible-band photonic technologies for IR photon detection and image formation, overcoming the performance and cost limitations of direct IR detectors.

1. Fundamental Physical Mechanisms

Infra-red to visible upconversion proceeds via diverse microscopic channels, each with distinct carrier/field dynamics, efficiency determinants, and spectral properties:

Nonlinear optical frequency conversion: The dominant paradigm in solid-state and nanophotonic implementations, specifically:

Sum-frequency generation (SFG) and third-harmonic generation (THG) are governed by second- ( $\chi^{(2)}$ ) or third-order ( $\chi^{(3)}$ ) nonlinear susceptibilities, respectively. SFG obeys energy and momentum conservation, with output frequency $\omega_3 = \omega_1+\omega_2$ , and phase-matching enforced by bulk birefringence, quasi-phase-matching, or resonant dispersion in metasurfaces (Demur et al., 2018, Camacho-Morales et al., 2021, Molina et al., 28 May 2024, Wang et al., 25 Sep 2025).

Internal photoemission followed by radiative recombination: In monolithic upconverters such as the HIWIP-LED, incident IR photons generate hot carriers in a heavily doped emitter; carriers surmount an interfacial workfunction and are injected into a quantum-well LED, yielding NIR emission via radiative recombination (Li et al., 2022).

Triplet–triplet annihilation upconversion (TTA-UC): In molecular/organic systems, sensitizer absorption of NIR photons generates triplet excitons that migrate to annihilator molecules, where two triplets annihilate to yield a higher-energy singlet emitting a visible photon (Nienhaus et al., 2019, Hamid et al., 27 Nov 2024).

Rare-earth ion ladder (anti-Stokes) upconversion: Ladder-type transitions in lanthanides (e.g., Er³⁺ in UCNPs) absorb multiple IR photons sequentially, populating high-lying levels that decay radiatively in the visible (Chen et al., 16 Mar 2025).

Optomechanical and Raman-based upconversion: Vibrational modes in molecules (e.g., C–C stretches) are coherently excited by IR fields and interact with optically resonant nanocavities; anti-Stokes Raman scattering or optomechanical mixing modulates a visible field, yielding upconverted sidebands (Chen et al., 2021, Xomalis et al., 2021).

Atomic and quantum-coherent schemes: Ladder-type and Λ-type atomic systems (e.g., Rb, Ba) exploit strong parametric nonlinearities—four-wave mixing or stimulated emission cycles—to efficiently convert IR energy to visible bands, with potential for internal gain exceeding unity via cycling transitions (Ding et al., 2012, Son et al., 16 Nov 2024).

2. Device Architectures and Material Platforms

A wide range of architectures and materials have been demonstrated to operationalize IR-to-visible upconversion:

Platform Type	Material System / Example	Key Mechanism
Bulk nonlinear optics	PPLN, KTP, LiNbO₃, GaAs	SFG, THG, QPM, NCPM, metasurface-enhanced SFG
Semiconductor devices	p-GaAs HIWIP-LED	FCA/IVBA + internal photoemission + LED emission
Nanophotonics	Disordered/ordered Si/Al metasurfaces, GaAs, LiNbO₃	Field-enhanced SFG/THG, guided resonances, BICs
Molecular/organic	NaYF₄:Er³⁺ (UCNPs), Y6:rubrene:DBP bulk heterojunction	Ladder-type anti-Stokes, TTA-UC
Quantum emitter/atomic	^85Rb (ladder), Ba-138 (Λ-type), graphite micro-particles	Four-wave mixing, Λ-cycle, multiphoton transitions
Plasmonic nanostructures	Gold disk–nanoparticle hybrids	Cavity optomechanics, SERS-coupled upconversion

Nonlinear crystals and metasurfaces: Periodically poled lithium niobate (PPLN) and KTP are standard for SFG-based imaging, exploiting quasi-phase-matching (QPM) or noncritical phase matching (NCPM) to maximize conversion efficiency and bandwidth (Demur et al., 2018, Wang et al., 25 Sep 2025). Metasurface approaches use dielectric (Si, LiNbO₃, GaAs) or hybrid plasmonic/dielectric nanostructures to enable subwavelength, resonantly enhanced upconversion (Camacho-Morales et al., 2021, Molina et al., 28 May 2024, Chen et al., 16 Mar 2025, Liu et al., 29 Aug 2025).

Semiconductors and LEDs: HIWIP-LED devices use p-GaAs with molecular-beam epitaxy-grown heterostructures to achieve ultra-broadband upconversion and "pixelless" imaging, with carrier dynamics enabling coverage from visible to THz in a monolithic platform (Li et al., 2022).

Organic/inorganic hybrids: Triplet-sensitized upconversion using lead halide perovskites and rubrene and bulk heterojunction systems (Y6/rubrene/DBP) enable efficient, photovoltaic-integratable NIR-to-visible upconversion, with plasmonic and dielectric nanostructures providing absorption and emission enhancements (Nienhaus et al., 2019, Hamid et al., 27 Nov 2024).

Atomic and molecular platforms: Coherent population cycling and stimulated emission in atomic vapors (e.g., ^85Rb, Ba¹³⁸⁾ offer high-efficiency upconversion with the potential for quantum state transfer and amplification (Ding et al., 2012, Son et al., 16 Nov 2024).

3. Conversion Physics and Performance Metrics

Nonlinear conversion efficiency is determined by the overlap of field amplitudes, nonlinear susceptibility, phase- or quasi-phase-matching, and field-enhancement mechanisms:

Bulk SFG and THG processes scale as $\eta\sim |d_{\mathrm{eff}}|^2L^2I_{\mathrm{pump}}$ ( $d_{\mathrm{eff}}$ : effective nonlinearity, $L$ : interaction length, $I_{\mathrm{pump}}$ : pump intensity). For metasurfaces, efficiency is further boosted by high- $Q$ Fano, BIC, or guided-mode resonances increasing local field strength by $\sim \sqrt{Q}$ (Liu et al., 29 Aug 2025, Molina et al., 28 May 2024).
HIWIP-LED devices combine quantum efficiency from photoemission, carrier collection, and LED extraction; upconversion efficiency at 10.6 μm reaches 0.0034% (bias 1.7 V, 4.2 K), limited by LED light-extraction ( $\sim$ 2.4%) (Li et al., 2022).
Triplet UCNPs (NaYF₄:Er³⁺, Y6:rubrene:DBP) and perovskite–rubrene upconversion typically achieve internal quantum yields up to 3–4% under moderate excitation; external efficiency is lower at low absorption or in passive (un-pumped) geometries (Nienhaus et al., 2019, Hamid et al., 27 Nov 2024).
Plasmonic and optomechanical nanocavities can achieve per-molecule enhancements exceeding $10^{13}$ over free space; phonon-to-photon upconversion internal efficiencies in excess of $10^{-4}$ per mW have been demonstrated (Chen et al., 2021).
Internal efficiencies exceeding unity, i.e., photon gain per absorbed IR photon, are possible in atomic $\Lambda$ -systems (Ba-138): $\eta_{\text{int}}=1.49$ measured at room temperature, and theoretical maximum gain $\sim$ 470 (from spontaneous emission cycling) (Son et al., 16 Nov 2024).

Bandwidth: Spectral acceptance is set by phase-matching (bulk), resonance linewidth (metasurface), or energy-level structure (molecular, atomic). Chirped QPM and NCPM enable coverage of 1.3–2.2 μm in bulk KTP (Wang et al., 25 Sep 2025), and adiabatic SFG in chirped PPLN extends conversion across one octave (2–4 μm upconverted to 680–820 nm) (Mrejen et al., 2019).

Spatial/spectral/temporal resolution: Upconversion imaging platforms achieve spatial resolutions limited by optical design, device aperture, and underlying nonlinear process. For example, NCPM KTP enables ~9–19 μm resolution with >6× improvement over QPM, with fields of view up to 6.9° (Wang et al., 25 Sep 2025). Metasurface-based imaging attains diffraction-limited features of ~6 μm, set by unit cell dimension (Liu et al., 29 Aug 2025). Ultrafast pumps (e.g., 800 fs) permit sub-picosecond temporal discrimination (Mrejen et al., 2019).

Noise and sensitivity: Achievable noise-equivalent power falls below $30\,\text{pW/Hz}^{1/2}$ for HIWIP-LEDs (MIR/THz) (Li et al., 2022), and upconversion detectors often reach detectivities $D^*\sim10^{12}$ Jones, surpassing cryogenically cooled InGaAs cameras (Demur et al., 2018). All-passive TTA-UC systems can image at incident NIR powers as low as $10^{-7}$ W/cm $^2$ (Hamid et al., 27 Nov 2024). Plasmonic devices achieve μW/μm $^2$ limits at room temperature (Xomalis et al., 2021).

4. Upconversion Imaging: Methodologies and System Design

Upconversion imaging workflows map spatial, spectral, or spatiotemporal structure from an IR input field into visible images:

Bulk nonlinear SFG imaging: 4- $f$ relay systems place the nonlinear crystal at the Fourier plane. Spatial frequencies are encoded as angles, with phase-matching bandwidth setting the field of view and modulation transfer function (Demur et al., 2018, Ge et al., 2023). Broadband or chirped-pump implementations maximize spatial coverage and FOV, allowing >56×64 resolvable elements (Demur et al., 2018, Mrejen et al., 2019).
Pixelless upconversion and image formation: The HIWIP-LED device converts MIR/THz spatial light patterns directly to visible, emitting a pixel-integrated NIR image onto a CCD without requiring focal-plane arrays. Imaging resolution is governed by carrier diffusion lengths, not pixel pitch (Li et al., 2022).
Metasurface architectures: High- $Q$ metasurfaces (LiNbO₃, Si, GaAs) combine field enhancement and pattern transfer fidelity, enabling ultracompact, diffraction-limited imaging and (in nonlocal metasurfaces) all-optical edge detection by designed grating phase profiles (Molina et al., 28 May 2024, Camacho-Morales et al., 2021, Liu et al., 29 Aug 2025).
Mitigating image fidelity losses: Use of flat-top pumps (vs. Gaussian) in SFG systems improves uniformity and fidelity (correlation coefficient $C=0.95$ vs. $C=0.89$ ), reducing edge attenuation and maximizing transfer accuracy (Yang et al., 2019).

Design trade-offs: Crystal length, phase-matching scheme, pump power, aperture, and optical bandwidth must be co-optimized for maximum efficiency, desired FOV/resolution, and bandwidth. Chirped QPM and NCPM (in KTP) allow for both broad spectral response and large fields of view (Mrejen et al., 2019, Wang et al., 25 Sep 2025). Incoherent illumination enhances spatial bandwidth but reduces absolute conversion efficiency (Ge et al., 2023).

5. Limitations and Applications

Limitations:

Many high-efficiency nonlinear upconverters require high pump intensities, strict temperature or phase-matching control, or operate at cryogenic temperatures (e.g., HIWIP-LED at 4.2 K) (Li et al., 2022).
The external quantum efficiency of molecular/organic and triplet-based upconversion is still limited by absorption cross-section, exciton diffusion, and extraction (Nienhaus et al., 2019, Hamid et al., 27 Nov 2024).
Spectral acceptance is often locally narrow (a few nm) in phase-matched systems, although chirped or adiabatic methods extend coverage.
Efficient upconversion at multi-micron (mid-IR) regimes is reliant on engineering strong field overlap and minimizing optical losses.

Advantages:

Room-temperature, uncooled operation is feasible in engineered semiconductors, metasurfaces, atomic vapors, and molecular platforms (Son et al., 16 Nov 2024, Camacho-Morales et al., 2021, Molina et al., 28 May 2024).
Ultra-broadband operation from visible to THz is achievable in HIWIP-LEDs, with CMOS camera compatibility (Li et al., 2022). Metasurfaces offer CMOS compatibility and ease of integration in Si photonics (Chen et al., 16 Mar 2025, Liu et al., 29 Aug 2025).
Intrinsically parallel (pixel-by-pixel or pixelless) conversion makes upconversion competitive for real-time imaging and ultra-high-density spatial/spectral data acquisition (Li et al., 2022, Liu et al., 29 Aug 2025, Mrejen et al., 2019).
Passive, low-light-level operation is enabled by TTA-UC, offering human-eye visible response under sub-nW/cm² illumination (Hamid et al., 27 Nov 2024).

Applications: Include MIR/THz/IR imaging and spectroscopy, night vision, stand-off chemical and environmental sensing, quantum frequency conversion, secure classical and quantum communications, integrated biosensing, and remote sensing in hazardous or low-photon flux environments (Li et al., 2022, Hamid et al., 27 Nov 2024, Wang et al., 25 Sep 2025).

6. Recent Advances and Outlook

Significant recent progress includes:

Ultra-broadband, pixelless upconversion in monolithic HIWIP-LEDs spanning visible to THz at sub-50 μm spatial resolution under CCD detection (Li et al., 2022).
Disordered metasurface + UCNP architectures achieving room-temperature silicon responsivity at 1.55 μm exceeding 0.22 A/W (EQE 17.6%), with 2.6× absorption and 3.9× field enhancement compared to ordered structures (Chen et al., 16 Mar 2025).
Adiabatic SFG and chirped QPM (single-shot 2–4 μm to 680–820 nm conversion) with ~20% efficiency, sub-ps time resolution, and multicolor real-time imaging (Mrejen et al., 2019).
NCPM bulk KTP yielding theoretical resolution limits (9–20 μm), 6.9° FOV, and broad upconversion bandwidth (1.3–2.2 μm), surpassing QPM approaches and enabling efficient SWIR imaging at room temperature (Wang et al., 25 Sep 2025).
Silicon and LiNbO₃ metasurfaces leveraging quasi-BIC and high-Q guided resonances for sub-10 μm upconversion imaging with η~3×10⁻⁵ at 10 GW/cm², supporting direct on-chip implementations (Molina et al., 28 May 2024, Liu et al., 29 Aug 2025).
Room-temperature, quantum amplification in atomic Λ-systems (Ba-138) with internal efficiency >1 and theoretically up to 200× higher with cavity enhancement, with the minimum bandwidth set by the excited-state radiative decay (Son et al., 16 Nov 2024).
White-light, broad-band upconversion in graphitic micro-particles via photo-induced multiphoton transitions and lattice hybridization, with a practical efficiency of ~10⁻⁵, offering extreme spectral breadth for broad-spectrum applications (Sharma et al., 2023).

These developments suggest continued rapid progress in IR-to-visible upconversion architectures, particularly in integrated nanophotonics, molecular engineering, and hybrid atomic/solid-state quantum platforms. Applications in real-time imaging, single-photon IR detection, and ultrabroadband sensing are expected to proliferate, with upconversion continuing to erode the performance and cost gap with direct-bandgap IR photodetectors.