In nonlinear systems, the three-wave equations, sometimes called the three-wave resonant interaction equations or triad resonances, describe small-amplitude waves in a variety of non-linear media, including electrical circuits and non-linear optics. They are a set of completely integrable nonlinear partial differential equations. Because they provide the simplest, most direct example of a resonant interaction, have broad applicability in the sciences, and are completely integrable, they have been intensively studied since the 1970s.^[1]

The three-wave equation arises by consideration of some of the simplest imaginable non-linear systems. Linear differential systems have the generic form

${\displaystyle D\psi =\lambda \psi }$

for some differential operator D. The simplest non-linear extension of this is to write

${\displaystyle D\psi -\lambda \psi =\varepsilon \psi ^{2}.}$

How can one solve this? Several approaches are available. In a few exceptional cases, there might be known exact solutions to equations of this form. In general, these are found in some ad hoc fashion after applying some ansatz. A second approach is to assume that ${\displaystyle \varepsilon \ll 1}$ and use perturbation theory to find "corrections" to the linearized theory. A third approach is to apply techniques from scattering matrix (S-matrix) theory.

In the S-matrix approach, one considers particles or plane waves coming in from infinity, interacting, and then moving out to infinity. Counting from zero, the zero-particle case corresponds to the vacuum, consisting entirely of the background. The one-particle case is a wave that comes in from the distant past and then disappears into thin air; this can happen when the background is absorbing, deadening or dissipative. Alternately, a wave appears out of thin air and moves away. This occurs when the background is unstable and generates waves: one says that the system "radiates". The two-particle case consists of a particle coming in, and then going out. This is appropriate when the background is non-uniform: for example, an acoustic plane wave comes in, scatters from an enemy submarine, and then moves out to infinity; by careful analysis of the outgoing wave, characteristics of the spatial inhomogeneity can be deduced. There are two more possibilities: pair creation and pair annihilation. In this case, a pair of waves is created "out of thin air" (by interacting with some background), or disappear into thin air.

Next on this count is the three-particle interaction. It is unique, in that it does not require any interacting background or vacuum, nor is it "boring" in the sense of a non-interacting plane-wave in a homogeneous background. Writing ${\displaystyle \psi _{1},\psi _{2},\psi _{3}}$ for these three waves moving from/to infinity, this simplest quadratic interaction takes the form of

${\displaystyle (D-\lambda )\psi _{1}=\varepsilon \psi _{2}\psi _{3}}$

and cyclic permutations thereof. This generic form can be called the three-wave equation; a specific form is presented below. A key point is that all quadratic resonant interactions can be written in this form (given appropriate assumptions). For time-varying systems where ${\displaystyle \lambda }$ can be interpreted as energy, one may write

${\displaystyle (D-i\partial /\partial t)\psi _{1}=\varepsilon \psi _{2}\psi _{3}}$

for a time-dependent version.

Formally, the three-wave equation is

${\displaystyle {\frac {\partial B_{j}}{\partial t}}+v_{j}\cdot \nabla B_{j}=\eta _{j}B_{\ell }^{*}B_{m}^{*}}$

where ${\displaystyle j,\ell ,m=1,2,3}$ cyclic, ${\displaystyle v_{j}}$ is the group velocity for the wave having ${\displaystyle {\vec {k}}_{j},\omega _{j}}$ as the wave-vector and angular frequency, and ${\displaystyle \nabla }$ the gradient, taken in flat Euclidean space in n dimensions. The ${\displaystyle \eta _{j}}$ are the interaction coefficients; by rescaling the wave, they can be taken ${\displaystyle \eta _{j}=\pm 1}$. By cyclic permutation, there are four classes of solutions. Writing ${\displaystyle \eta =\eta _{1}\eta _{2}\eta _{3}}$ one has ${\displaystyle \eta =\pm 1}$. The ${\displaystyle \eta =-1}$ are all equivalent under permutation. In 1+1 dimensions, there are three distinct ${\displaystyle \eta =+1}$ solutions: the ${\displaystyle +++}$ solutions, termed explosive; the ${\displaystyle --+}$ cases, termed stimulated backscatter, and the ${\displaystyle -+-}$ case, termed soliton exchange. These correspond to very distinct physical processes.^[2][3] One interesting solution is termed the simulton, it consists of three comoving solitons, moving at a velocity v that differs from any of the three group velocities ${\displaystyle v_{1},v_{2},v_{3}}$. This solution has a possible relationship to the "three sisters" observed in rogue waves, even though deep water does not have a three-wave resonant interaction.

The lecture notes by Harvey Segur provide an introduction.^[4]

The equations have a Lax pair, and are thus completely integrable.^[1][5] The Lax pair is a 3x3 matrix pair, to which the inverse scattering method can be applied, using techniques by Fokas.^[6][7] The class of spatially uniform solutions are known, these are given by Weierstrass elliptic ℘-function.^[8] The resonant interaction relations are in this case called the Manley–Rowe relations; the invariants that they describe are easily related to the modular invariants ${\displaystyle g_{2}}$ and ${\displaystyle g_{3}.}$^[9] That these appear is perhaps not entirely surprising, as there is a simple intuitive argument. Subtracting one wave-vector from the other two, one is left with two vectors that generate a period lattice. All possible relative positions of two vectors are given by Klein's j-invariant, thus one should expect solutions to be characterized by this.

A variety of exact solutions for various boundary conditions are known.^[10] A "nearly general solution" to the full non-linear PDE for the three-wave equation has recently been given. It is expressed in terms of five functions that can be freely chosen, and a Laurent series for the sixth parameter.^[8][9]

Some selected applications of the three-wave equations include:

• In non-linear optics, tunable lasers covering a broad frequency spectrum can be created by parametric three-wave mixing in quadratic (${\displaystyle \chi ^{(2)}}$) nonlinear crystals.^{[citation needed]}
• Surface acoustic waves and in electronic parametric amplifiers.
• Deep water waves do not in themselves have a three-wave interaction; however, this is evaded in multiple scenarios:
  • Deep-water capillary waves are described by the three-wave equation.^[4]
  • Acoustic waves couple to deep-water waves in a three-wave interaction,^[11]
  • Vorticity waves couple in a triad.
  • A uniform current (necessarily spatially inhomogenous by depth) has triad interactions.

These cases are all naturally described by the three-wave equation.

• In plasma physics, the three-wave equation describes coupling in plasmas.^[12]