Up-Conversion Electroluminescence

• Up-conversion electroluminescence is a nonlinear process that converts low-energy excitations into high-energy photon emissions using multi-step and cooperative mechanisms.
• It involves diverse methods such as energy transfer up-conversion, excited state absorption, and plasmonic hot carrier injection, each leveraging unique material interactions.
• Applications include telecommunications, photovoltaics, quantum devices, and bioimaging, with enhancements achieved via nanostructuring and metasurface engineering.

Up-conversion electroluminescence refers to a collection of physical processes in which lower-energy excitations—often from electrical, optical, or tunneling sources—are efficiently converted into higher-energy photon emission via multi-step or non-linear mechanisms. This phenomenon, central for fields ranging from telecommunications to photovoltaics and quantum technology, relies on exploiting cooperative, many-body, or nonlinear interactions in solids, molecules, nanostructures, and hybrid systems.

1. Mechanistic Taxonomy of Up-Conversion Electroluminescence

Up-conversion electroluminescence (UCEL) encompasses distinct physical mechanisms, each with sharply defined characteristics:

• Energy Transfer Up-Conversion (ETU): In rare-earth–doped solids such as Tm³⁺:silica or Er³⁺:TiO₂, two ions are first excited to an intermediate metastable state (e.g., ^3F₄ for Tm³⁺ or ^4I₁₃/₂ for Er³⁺) via absorption of infrared photons. A non-radiative dipole–dipole interaction between the pair then results in one ion being further excited (e.g., to ^3H₄ in Tm³⁺ for 800 nm emission), while the partner relaxes (Simpson et al., 2010). The process is characterized by quadratic intensity dependence and is sensitive to dopant concentration due to ion pair or cluster formation (Simpson et al., 2010, Christiansen et al., 2019).
• Excited State Absorption (ESA): A sequential single-ion process wherein a Tm³⁺ (or similar) ion, having absorbed one photon and residing in a metastable intermediate level, absorbs an additional photon to reach a higher-energy (emissive) state. ESA cross-sections are generally much smaller than ground-state absorption for trivalent lanthanide ions, often rendering ESA a minor contribution in oxide glass hosts compared to ETU (Simpson et al., 2010).
• Auger-Mediated Up-Conversion: In 2D van der Waals heterostructures (e.g., WSe₂/MoS₂), up-converted electroluminescence is mediated by exciton–exciton Auger scattering, where a non-radiative recombination event imparts excess energy to a neighboring interlayer exciton, promoting it into a higher-lying, optically bright state (Binder et al., 2019). This mechanism, enabled by high carrier densities and long-lived excitons, permits up-conversion over energy ranges up to 0.6 eV.
• Inelastic Multi-Electron Tunneling in Molecular/2D Hybrids: Up-conversion is also realized via tunneling-induced sequential excitation of intermediate triplet and higher singlet states in molecules (e.g., PTCDI embedded in hBN with graphene contacts (Svatek et al., 2019)). The process decouples the photon energy from the single-electron bias limit, enabling emission energies up to ~1 eV above the injected electron energy.
• Plasmonic Hot Carrier Up-Conversion: Broadband excitation of metallic nanostructures produces non-thermal distributions of hot electrons and holes. When these carriers are injected into adjacent quantum wells, their radiative recombination results in linearly upconverted emission, fundamentally limited by the Schottky barrier energy and carrier injection efficiencies (Naik et al., 2015).
• Nonlocal Exciton–Photon Coupling in Quantum Wells: Efficient UCPL under weak illumination arises from nonlocal radiative coupling between spatially structured excitonic and photonic modes, as described by Green’s function–mediated mode mixing beyond the long-wavelength approximation. This radiatively induces coherent superpositions able to emit at energies exceeding that of the initial excitation (Matsuda et al., 2015).
• Phonon-Mediated Up-Conversion and Exciton-Polaron Formation: In halide perovskite nanocrystals (e.g., CsPbBr₃), the absorption of below-bandgap photons is assisted through annihilation of specific optical phonons (identified as Pb–Br–Pb bending modes), with subsequent rapid lattice reorganization and exciton–polaron formation enabling up to 75% anti-Stokes quantum efficiency (Abbas et al., 2023).
• Second Harmonic Generation and Intracavity Frequency Doubling: In polar nematic liquid crystal microcavities doped with laser dye, the combination of intracavity lasing and giant χ^2 even-order nonlinearity allows simultaneous lasing and frequency doubling, generating dynamically tunable and highly efficient up-converted electroluminescence (Okada et al., 9 Jun 2024).

2. Rate Equations, Nonlinearities, and Saturation Effects

UCEL processes are inherently nonlinear, typically requiring multi-photon or multi-particle interaction. The foundational kinetics can be captured by coupled rate equations, for example in Tm³⁺:silica:

$$\frac{dn_1}{dt} = -\frac{n_1}{\tau_1} - W_{ETU} \cdot c \cdot n_1$$

$$\frac{dn_2}{dt} = -\frac{n_2}{\tau_2} + W_{ETU} \cdot c \cdot n_1$$

where $n_1$ and $n_2$ are the normalized populations of the metastable and upconverted states, $W_{ETU}$ is the ETU coefficient, and $c$ is the ion concentration (Simpson et al., 2010).

For lanthanide-doped upconverters, the upconversion quantum yield scales as: $Y_{UCL} = A \cdot f(I/I_{sat}),$ where $f(x)$ smoothly transitions from quadratic (low intensity) to linear (high intensity, saturation) response (Christiansen et al., 2019).

In plasmonic and hot-carrier–based UCEL, the quantum efficiency is governed by the fraction of hot carriers with suitable energy and momentum, injection efficiency, and the need for both electrons and holes per emitted photon (Naik et al., 2015): $$\eta_{UC} = \frac{1}{2}\,\eta_{well}\,\frac{\Delta N(E_F+E_b)}{\Delta N(E_F)} \,\eta_{inj}.$$

These models emphasize the necessity of high excitation densities, local field enhancement, or cooperative effects to reach efficient UCEL.

3. Nanostructuring, Photonic, and Metasurface Enhancement

Achieving practical UCEL quantum yields commonly requires nanostructure-enabled enhancement:

• Plasmonic and Dielectric Metasurfaces: Tailored gold nanostructures and dielectric metasurfaces with optimized resonant modes significantly amplify the local electromagnetic fields, concentrating excitation energy into the upconversion layer. In Er³⁺:TiO₂ systems, field enhancement factors up to 32 and UCEL enhancements of 913× (at 1.7 W/cm²) were observed (Christiansen et al., 2019), while double-layer silicon-based metasurfaces yield a 2.7-fold increase by providing both near-field amplification and light trapping through engineered Rayleigh–Wood anomalies (Manley et al., 2020).
• Resonance Engineering and Polarization Control: By engineering high-Q, quasi-bound states in the continuum and Mie resonances, metasurfaces enable dual-band and polarization-selective upconversion, with experimentally measured polarization purity (DoP) of 0.86–0.91 and enhancement factors exceeding two orders of magnitude (Feng et al., 2023). Spectral overlap of resonances with upconverter emission bands is critical.
• Optical Concentrators and Integrated Devices: Monocrystalline upconverter–Si tandem solar cells with compound parabolic concentrators (CPCs) utilize optics to focus NIR photons and control spatial irradiance profiles within the upconverter, thereby optimizing both absorption and photon outcoupling (Arnaoutakis et al., 2021). Scattering minimization via monocrystalline materials ensures precise control of where upconversion occurs and efficient photon flow into the PV device.

4. Quantum Confinement, Interface Engineering, and Size Effects

UCEL efficiency and spectral characteristics are significantly altered by spatial confinement and local structure:

• Quantum-Confined Nanocrystals: Blue-shifted ("super-bandgap") electroluminescence arises when recombination occurs within nanocrystals (e.g., CsPbBr₃ <3 nm) at interfaces such as PEDOT:PSS. Quantum confinement increases the effective bandgap, resulting in EL peaks up to 0.7 eV above the bulk material's gap (Sculley et al., 13 Oct 2024). Charge injection imbalances and local interfacial morphology control the localization and stability of such upconversion emission.
• Interfacial and Substrate Effects in Molecular Systems: The adsorption geometry of radical molecules with unpaired spins (e.g., vanadyl phthalocyanine on NaCl/Au(111)) determines the upconversion pathway. Reordering of excited states and enhanced transition probabilities switch UCEL on/off, an effect not possible in closed-shell molecules (Rai et al., 15 Aug 2025). This suggests the possibility of controlling molecular-level UCEL via interface structure and local spin interactions.

5. Experimental Signatures and Device Engineering

UCEL's diagnostic features include:

• Quadratic (or higher order) Intensity Dependence: Observed in rare-earth–doped and molecular upconverters, confirming multi-particle upconversion (Simpson et al., 2010, Christiansen et al., 2019).
• Double Exponential Decay Dynamics: ETU-mediated upconversion in Tm³⁺/Er³⁺ systems demonstrates decay profiles with characteristic time constants matching metastable and upper emitting state lifetimes (Simpson et al., 2010).
• Photon Energy Exceeding Driving Energy: In molecular tunneling diodes, observing $h\nu > eV_{SD}$ unambiguously demonstrates upconversion, confirming the operation of multi-electron, sequential tunneling and energy pooling (Svatek et al., 2019).
• Spectral and Polarization Selectivity: Polarization-resolved, dual-band emission facilitated by metasurfaces enable both fundamental spectroscopy and advanced encryption/lasing applications (Feng et al., 2023).

Device engineering leverages:

• Field and Optical Concentration: Electromagnetic engineering (plasmonics, metasurfaces, CPCs) to boost local excitation densities and photon recycling (Christiansen et al., 2019, Manley et al., 2020, Arnaoutakis et al., 2021).
• Dynamic Modulation: Use of liquid crystal phases, electric fields, or temperature for signal switching and wavelength tuning, providing platform versatility for adaptive photonics (Okada et al., 9 Jun 2024).

6. Applications and Implications Across Disciplines

UCEL technologies underpin key advances:

Application Area	Physical Principle/Platform	Notes
S-band optical amplification	ETU in Tm³⁺-doped silica fibers	Quadratic scaling; telecom relevance (Simpson et al., 2010)
Silicon-based photovoltaics	Upconverter layer, field enhancement	Surpass Shockley–Queisser limit (Christiansen et al., 2019, Arnaoutakis et al., 2021)
Bioimaging, single-particle microscopy	UCNPs, LED/TGL readout	High SNR under low-power NIR excitation (Cao et al., 2020)
Quantum and spin-based optoelectronics	Multi-electron/molecular upconversion	Triplet state access; spin coherence (Svatek et al., 2019, Rai et al., 15 Aug 2025)
Optical cryptography, photonic lasers	Metasurface-enhanced UCNPs	Polarization dual-band encoding and low-threshold lasing (Feng et al., 2023)
Tunable and coherent sources	Intracavity lasing+SHG in PNLC	Dynamic switching/wavelength control (Okada et al., 9 Jun 2024)

This broad applicability, grounded in precisely engineered nonlinear optical and many-body processes, marks UCEL as a crucial intersection of condensed matter physics, quantum optics, materials science, and device engineering.

7. Challenges and Prospects

UCEL research faces several technical and conceptual challenges:

• Low Intrinsic Absorption/Emission Cross Sections: Many rare-earth upconverters or lanthanide-based nanoparticles exhibit forbidden transitions resulting in low upconversion efficiency, necessitating enhancement strategies such as nanostructuring and field concentration (Christiansen et al., 2019, Manley et al., 2020).
• Intensity Thresholds and Saturation: Nonlinear processes demand high local intensities; translational scaling to low-irradiance (e.g., solar) applications requires further innovation in field enhancement and host materials (Arnaoutakis et al., 2021).
• Spectral Tuning and Stability: Traditional compositional tuning in halide perovskites is hampered by instability; interface-driven quantum confinement offers a route to robust, stable color control for EL (Sculley et al., 13 Oct 2024).
• Tailoring Many-Body and Spin Effects: Understanding and engineering the role of spin statistics, excited state ordering, and substrate effects are crucial for controlling UCEL in single-molecule and radical systems (Rai et al., 15 Aug 2025).

The field advances toward embedding UCEL functionality in next-generation optoelectronic, energy, and quantum devices, leveraging a synergy between materials design, nanostructure engineering, and non-trivial many-body phenomena.