Calculate two-photon absorption cross-sections, excitation wavelengths, action cross-sections, and fluorescence rates. Covers GM units, power density, and comparison of TPA fluorophores.
Two-photon absorption (TPA) is a nonlinear optical process where two photons are simultaneously absorbed to excite a molecule to a higher electronic state. The energy of the two photons together equals the transition energy: E = hν₁ + hν₂. In practice, two identical photons at twice the wavelength of the one-photon absorption are used, enabling deep-tissue imaging with near-infrared light.
The TPA cross-section (σ₂) is measured in Göppert-Mayer units (GM), where 1 GM = 10⁻⁵⁰ cm⁴·s/photon. Typical fluorescent proteins have σ₂ of 1–100 GM, while specially designed organic fluorophores can exceed 10,000 GM. The action cross-section (σ₂ × φ, where φ is quantum yield) determines practical brightness for microscopy.
This calculator converts between one-photon and two-photon wavelengths, computes photon flux and TPA rates from laser parameters, calculates action cross-sections, and provides a comparison database of common TPA fluorophores used in multiphoton microscopy and photodynamic therapy.
For best results, combine calculator output with direct observation and periodic check-ins with a veterinarian or qualified advisor. Small adjustments made early usually improve comfort, safety, and long-term outcomes more than large corrective changes made later.
Calculate TPA excitation parameters, action cross-sections, and compare fluorophores for multiphoton microscopy, photodynamic therapy, and nonlinear optical applications. This two-photon absorption calculator helps you compare outcomes quickly and reduce avoidable mistakes when making day-to-day care decisions. Use the estimate as a planning baseline and confirm final decisions with a qualified professional when risk is high.
TPA wavelength: λ_TPA = 2 × λ_one-photon\nPhoton energy: E = hc/λ\n\nPhoton flux: F = P_avg / (E_photon × f_rep × τ_pulse × A)\n where P_avg = average power, f_rep = repetition rate, τ = pulse width, A = focal area\n\nTPA rate: R_TPA = σ₂ × F² × C\n where σ₂ = TPA cross-section, C = concentration\n\nAction cross-section: σ₂φ = σ₂ × quantum yield\n1 GM = 10⁻⁵⁰ cm⁴·s·photon⁻¹ This keeps planning practical and lowers the chance of preventable errors.
Result: TPA λ = 976 nm, action σ₂φ = 90 GM
For a fluorophore with one-photon peak at 488 nm: TPA wavelength = 976 nm (near-IR). With σ₂ = 100 GM and QY = 0.9, the action cross-section is 90 GM. Using 10 mW average power at 80 MHz rep rate with 100 fs pulses, the instantaneous photon flux and TPA rate are computed.
TPA is a third-order nonlinear optical process. The transition probability is proportional to the square of the light intensity (I²), which is why it only occurs at the laser focal point where photons are most concentrated. The TPA cross-section tensor connects to molecular properties through the imaginary part of the third-order susceptibility χ⁽³⁾.
High σ₂ values come from extended π-conjugation, donor-acceptor-donor (D-A-D) architecture, and planar molecular geometry. Push-pull chromophores with strong intramolecular charge transfer have the largest TPA cross-sections. Quantum dots and conjugated polymers can exhibit σ₂ > 10⁴ GM per particle.
Two-photon absorption enables 3D microfabrication (two-photon polymerization), optical data storage, photodynamic therapy with deeper tissue penetration, upconversion lasing, and optical power limiting for eye/sensor protection against intense laser pulses.
Named after Maria Göppert-Mayer who predicted TPA in 1931, 1 GM = 10⁻⁵⁰ cm⁴·s·photon⁻¹. It measures the probability of simultaneous two-photon absorption. Values range from <1 GM for simple molecules to >10,000 GM for designed chromophores.
TPA uses near-IR light (~700-1100 nm) which penetrates tissue deeper (up to ~1 mm), causes less photodamage, and provides inherent 3D sectioning because absorption only occurs at the tight focal volume where photon density is highest. This keeps planning practical and lowers the chance of preventable errors.
Ti:sapphire lasers (tunable ~680-1080 nm, ~80 MHz, ~100 fs pulses) are standard. Newer options include OPO systems for extended wavelength range and fiber lasers for specific fixed wavelengths.
TPA rate scales linearly with concentration (like OPA), but quadratically with photon flux. Doubling the laser power quadruples the TPA signal, unlike one-photon where it only doubles.
Action cross-section = σ₂ × quantum yield (σ₂φ). It's the practical figure of merit for fluorescence microscopy because it combines absorption probability with emission efficiency.
Yes, TPA-PDT uses near-IR light that penetrates deeper into tissue. The photosensitizer absorbs two photons to reach the excited state that generates reactive oxygen species. Requires high σ₂ values (>100 GM) for practical efficacy.