An Introduction to Two-Photon Absorption and Upconversion
Two-photon absorption (TPA or 2PA) is a nonlinear optical process in which a material absorbs two photons simultaneously (or within a very short timespan) to excite an electron from a lower-energy state to a higher-energy state. The total energy of the two photons is equal to the energy difference between the two states.
Proposed by Mary Göppert-Mayer in the 1930s in her PhD thesis, the simultaneous two-photon absorption by the same molecule was first demonstrated in 1961, soon after the discovery of the laser. Two-photon absorption upconversion is a way of accessing a given excited state by using photons of half the energy of the corresponding one-photon transition. Current research focuses on rare-earth-doped nanoparticles and organic chromophores, such as TADF materials, to advance this technology.
Fundamental Principles of Two-Photon Absorption
The two-photon absorption (TPA) process requires the simultaneous absorption of two photons, the probability of two-photon absorption is proportional to the square of the light intensity as a nonlinear optical process. For this to occur, the two photons must hit the molecule within 1 femtosecond of each other (10-15 seconds). TPA is only observed in intense laser beams, particularly focused pulsed lasers, which generate a very high instantaneous photon density. The key principles of TPA are:
- Nonlinear Optical Process: Two-photon absorption is a third-order nonlinear process requiring the simultaneous absorption of two photons. Unlike linear absorption, where a single photon provides sufficient energy for electronic transition, TPA relies on the combined energy of two photons to excite a molecule or material. Its strength depends on the square of the light intensity, and it is several orders of magnitude weaker than linear absorption. Two photon absorption is characteristic of materials namely semiconductor, insulators. When bandgap energy Eg is larger than a photon being incident, there is simultaneous absorption of two photons.
- Photon Density Dependence: With the absorption of two photons of identical or different frequencies, one of the most distinguishing features of two-photon absorption is that the rate of absorption of light by a molecule depends on the square of the light's intensity. High photon densities, achievable through focused laser beams or pulsed lasers, are essential to observe TPA.
- Quantum Mechanical Selection Rules: The intermediate states in TPA are often virtual, meaning the system does not remain in these states. This virtual transition process ensures that TPA can occur even in materials with indirect bandgaps or forbidden transitions in linear absorption.
- Emission and Upconversion: After two-photon absorption, the excited system relaxes to emit light. If the emitted photon energy exceeds the individual absorbed photon energies, it is termed upconversion luminescence.
Two-Photon Absorption vs One-Photon Absorption
The main difference between the two-photon absorption and one-photon absorption is that TPA involves the simultaneous interaction of two photons. As a result it increases with the square of the light intensity, whereas one-photon absorption depends linearly on the intensity.
Lower energy source could be used for two-photon absorption upconversion, yet higher energy source is needed to produce the same emission frequency for one-photon excitation. In the process of two-photon excitation, each photon has half the energy as in the corresponding single-photon absorption event. In other words, two-photon absorption takes advantage of the nonlinear excitation process, where two photons of longer wavelengths combine their energies to excite a fluorophore.
Advantages of Two-Photon Absorption
Two-photon absorption offers great advantages though the laser to create a two-photon absorption makes it an expensive process in comparison to conventional one-photon absorption process. One-photon process collects information only close to the surface while two-photon while two-photon excitation can dive deeper into layers of information with increased imaging depth forming 3D images and reduced phototoxicity. It also can initiate highly localized photochemistry in thick samples. Additionally, two-photon excitation can occur using infrared laser which scatters much less than the light from the visible spectrum and has enough power to excite fluorophores up to around 1 mm in living tissues.
- Photon availability: Two-photon excitation can be advantageous when the desired energy level cannot be directly accessed by a single photon source. The availability of photons at lower energies is generally more common than those at higher energies, making it easier to obtain the required photons. Near-infrared (NIR) wavelengths, particularly those above 1000 nm which could cause less photo-damage, undergo less scattering and allow for deeper penetration (down to 1000 μm) in tissues. Two-photon absorption not only gives access to this NIR wavelengths by doubling the absorption wavelength of dyes, but also increases the possible resolution.
- Background suppression: Two-photon excitation can offer improved background suppression compared to single-photon excitation. Since the two photons must interact simultaneously and in a specific manner, background noise from other energy levels or sources that do not satisfy the two-photon conditions can be significantly reduced.
- Enhanced penetration depth and reduced photobleaching: Two-photon excitation is advantageous for point measurements conducted within limited volumes like cells. In this technique, only the confocal volume is susceptible to bleaching, minimizing undesired effects. Unlike one-photon absorption, two-photon absorption lacks linear absorption of the laser beam by fluorophores above the plane of focus, resulting in significant reduction of excitation light intensity before it reaches fluorophores within deeper tissue regions. Also, two-photon absorption minimizes out-of-focus excitation to reduce photobleaching and phototoxicity, increases photon collection efficiency to rule out pinholes by the collection of full emission peak, and extends the depth of imaging because NIR photons are more than 10-fold less scattered than visible light.
- Broad absorption spectra: The broad absorption spectra associated with two-photon excitation can be leveraged as an advantage in applications like tissue profiling through autofluorescence, enabling detailed examination of biological samples, cross-correlation measurements since the excitation volume is precisely determined by the same excitation beam, ensuring accurate and reliable results.
Two-Photon Absorption Upconversion
Two-photon absorption (TPA) upconversion is a nonlinear optical process where two photons of lower energy (longer wavelength) are absorbed simultaneously by a material. As a result, the material becomes excited, reaching a higher energy state. Emission from this state produces a photon with higher energy (shorter wavelength) than the absorbed photons. Essentially, it converts low-energy photons (e.g., infrared) into high-energy photons (e.g., visible or ultraviolet).
TPA upconversion involves a molecule being excited from the ground state (S0) to the excited singlet state (S1) by simultaneous absorption of two photons via a virtual state. The two absorbed photons can have identical or different frequencies. The process requires high excitation power density, and it is only achievable by an ultrafast laser.
TPA upconversion shows quadratic light intensity dependence therefore if the intensity of incident light is doubled the TPA rate increases by a factor of four. TPA also possesses the overwhelming advantages of high spatial resolution, as TPA only happens where a laser beam is pointed for example. The longer wavelength of the light allows TPA to penetrate deeper into materials and biological tissues. Due to its unique characteristics, this phenomenon has shown numerous potential applications in photonics, fluorescence bioimaging, photodynamic therapy, optical data storage and sensing, frequency upconverted lasing, displaying technology and energy conversion.
Materials for Two-Photon Upconversion
Two-photon absorption upconversion has been observed in push-pull semiconducting copolymers which a delocalized π-system consisting of alternating electron-rich (donor) and electron-deficient (acceptor) repeating units. Other TPA type materials include rare-earth-doped nanoparticles, quantum dots, organic chromophores such as dibenzylideneacetones, and organic-inorganic nanohybrids.
Optical functional organic/inorganic hybrid materials usually show high thermal stability and excellent optical activity. Metal complexes offer opportunities for a wide range of metals with different ligands, which can give rise to tuneable electronic and TPA properties. The metal ions, including transition metals and lanthanides, can serve as an important part of the structure to control the intramolecular charge-transfer process that drives the TPA process. The common types of materials are discussed in more detail:
Organic Chromophores
Great effort has been dedicated to developing organic materials with large two-photon absorption cross sections. Especially for applications such as optical imaging and photodynamic therapy. Studies show that the TPA cross sections (σ2) of donor-acceptor (D-A) dipoles and D-π-D and D-A-D quadrupoles can be increased by using strong D-A groups while maintaining the planar structure to facilitate intramolecular charge transfer (ICT).
Common examples of organic chromophores include pyrimidine, pyridinium and triphenylamine derivatives. Among the various cores, triphenylamines appear to be an efficient core. It is of great importance to optimize the core, donor-acceptor pair, and conjugation-bridge to obtain a large σ2 value since the two-photon absorption cross section of organic fluorophores increases with the extent of charge transfer. Large values of σ2 are observed for extended conjugation length and moderate donor-acceptors in the near-IR wavelengths. The σ2 value of the three-arm octupole is larger than that of the individual arm, if the core has electron accepting groups that allow significant electronic coupling between the arms.
The simple octupolar chromophore, tris(4′-nitrobiphenyl)amine, has three nitro peripheral groups attached to a triphenylamine core via biphenyl linkers. The molecules exhibits comparable two-photon absorption cross section values, reaching 1330 GM at 730 nm and 900 GM at 820 nm in toluene and displaying potential as a promising two-photon probe for bioimaging.
Semiconductors
Quantum dots (QDs) and other semiconductor nanostructures such as single or few-layers of transition metal dichalcogenides (TMDCs) exhibit strong nonlinear optical responses, making them suitable for TPA-based upconversion. Biocompatible water-soluble CdS quantum dots show low toxicity to human liver hepatocellular carcinoma (HepG2) cells. CdS QDs induced two-photon absorption red fluorescence emission can avoid overlapping with the autofluorescence emission of biological samples. AS a result, HepG2 cells incubated with CdS QDs emit bright red upconversion fluorescence and the fluorescence brightness is 38.2 times of that of the control group without the QDs.
Rare-Earth-Doped Nanoparticles
Inorganic materials capable of photon upconversion often contain ions of d-block or f-block rare-earth elements. Lanthanide ions pairs, such as erbium-ytterbium (Er3+, Yb3+) or thulium-ytterbium (Tm3+, Yb3+), are widely used due to their sharp energy levels, long lifetimes, and efficient upconversion properties. In such combinations ytterbium ions are acting as energy antennas to absorb light at around 980 nm and transfer it to the upconverter ions.
Based on the excellent characteristics of rare-earth ions, such as their low photobleaching, various absorption and emission wavelengths, and low energy losses, rare earth ions can exhibit extremely sharp absorption and emission lines when they are doped into a host structure. While organic fluorescent dyes and quantum dots (QDs) are subject to photobleaching and photodegradation, upconversion nanoparticles (UCNPs) are chemically stable and never bleach. The emission wavelengths of UCNPs do not dependent on crystal size and the multicolour emission can easily be accomplished by varying host crystal and rare-earth dopant.
Nanoparticles are often used in complex multicomponent systems. One examples is a nanocomposite of core–shell structured rare-earth doped up-conversion nanoparticles (UCNPs) of NaGdF4:Yb,Tm@NaGdF4 on graphitic-phase carbon nitride (g-C3N4) nanosheets (photosensitizer). This nanocomposite was used in conjunction with carbon dots (photosensitizer) and the metal-organic framework ZIF-8. Without sacrificing its efficacy under ZIF-8 shell protection, the rare-earth doped UCNPs convert low-energy near-infrared light into higher-energy ultraviolet–visible emission. This is a great match to the absorption range of the photosensitizers for reactive oxygen species (ROS) generation. The UV light generated by the upconversion of the rare-earth doped nanoparticles allows successive activation of g-C3N4 and carbon dots. The visible light from carbon dots upon UV light excitation once again activate g-C3N4 to produce ROS, allowing the maximized use of the light. According to the report, this dual-PDT system exhibits excellent antitumor efficiency superior to any single modality, verified vividly by in vitro and in vivo assay.
Applications of Two-Photon Absorption Upconversion
Two-photon absorption (TPA) materials are widely used in various applications. In biomedical imaging, they are valued for their high penetration depth with near-infrared (NIR) excitation, excellent spatial resolution, strong signal-to-noise ratio, and low photobleaching tendency. They are also used to detect pollutants and toxins, in optical power limiting, and for 3D data storage due to their unique luminescent responses. Additionally, TPA materials enhance photovoltaic performance by converting sub-bandgap photons into higher-energy photons and improving the absorption of low-energy light for solar fuel production. Upconversion nanoparticles are applied in security inks for anti-counterfeiting, as their luminescence activates only under specific conditions. Key applications of these materials are outlined below:
Deep Tissue Imaging
To ensure effective light energy delivery to deep-seated diseases, the light source should be capable of penetrating biological tissues. To improve the light penetration, photosensitizers with absorption in the red and near-infrared (NIR) regions are greatly desired. TPA enables imaging at near-infrared (NIR) wavelengths, which penetrate deeper into biological tissues with minimal scattering and absorption. Upconversion fluorescence provides high-resolution imaging.
Photodynamic Therapy
Photodynamic therapy (PDT) relies on the administration of photosensitizers (PSs) that are activated by light at a specific wavelength to produce cytotoxic reactive oxygen species (ROS) to kill cancer cells and other pathogens. TPA materials can activate photosensitizers for targeted cancer therapy, minimizing damage to surrounding tissues. Two photon photodynamic therapy (TP-PDT) enables the photosensitizer to absorb the energy of two photons concurrently, thereby generating ROS and producing therapeutic effects on cancerous cells and tissues. Compared with conventional single-photon PDT, TP-PDT allows for the utilization of longer light wavelengths, which achieves deeper tissues penetration depth.
TADF OLEDs
Two-photon absorption is normally associated with limited absorption rate and low photoluminescence quantum efficiency for OLEDs. TADF materials with two-photon absorption characteristics can potentially offer a solution to such problems. Nonlinear optical (NLO) TADF ternary exciplex system of a two-photon absorbing acceptor material 3TPYMB, a two-photon absorbing donor material TAPC, and a linear optical acceptor material T2T exhibits enhanced two-photon excited fluorescence under long-wavelength laser excitation. The TADF exciplex system of the high-energy exciplex TAPC:3TPYMB and the low-energy exciplex TAPC:T2T generates an effective synergy effect on the basis of their reverse intersystem crossing (RISC) channels to achieve further utilization of triplet excitons. The ternary exciplex system 3TPYMB:TAPC:T2T demonstrates a high photoluminescence quantum yield (PLQY) of ∼77.8%, which is a great improvement compared to those of the binary exciplex TAPC:T2T (∼39.6%), and the exciplex TAPC:3TPYMB (∼3.7%).
The unique TADF emitter TPCz2NP possessing two-photon absorption characteristics can be obtained by the introduction of a terephthalonitrile unit into a sterically hindered donor-π-donor structure, inducing a hybrid electronic excitation character. The main π-conjugated donor-acceptor-donor backbone, in line with locally excited feature, demonstrates a large oscillator strength and transition dipole moment, rendering a high two-photon cross-section (σ2) value. The ancillary N-donor-acceptor-donor with charge transfer character highly balances the TADF phenomenon by minimizing ΔEST. A near-unity photoluminescence quantum efficiency value with a large radiative decay rate over an order of magnitude higher than the intersystem crossing rate and a high horizontal orientation ratio of 0.95 are simultaneously achieved for the TADF emitter. The organic light-emitting diodes based on TADF emitter TPCz2NP exhibit a high maximum external quantum efficiency (EQEmax) of 25.4 % with an elevated TPA cross-section (σ2) value up to 143 GM at 850 nm. TTT-3HMAT, a donor–acceptor compound based on the tris(triazolo)triazine, exhibits deep blue emission with CIE(x, y) < (0.16, 0.05) and a photoluminescence quantum yield (PLQY) of 0.94 in toluene. The locked-planarity of the three hexamethylazatriangulene (HMAT) arm donors induces a large two-photon cross-section σ2 of 2928 GM.
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- M. Pawlicki et al. (2009); Two-Photon Absorption and the Design of Two-Photon Dyes, Angew. Chem. Int. Ed., DOI: 10.1002/anie.200805257.
- W. Kaiser et al. (1961); Two-Photon Excitation in CaF2: Eu2+, Phys. Rev. Lett. DOI: 10.1103/PhysRevLett.7.229.