Encyclopedia … combined with a great Buyer's Guide!

Critical Phase Matching

Definition: phase matching of a nonlinear interaction by adjustment of a propagation direction

Alternative term: angle phase matching

More general term: phase matching

Opposite term: noncritical phase matching

German: kritische Phasenanpassung, Winkel-Phasenanpassung

Category: nonlinear opticsnonlinear optics


Cite the article using its DOI: https://doi.org/10.61835/sms

Get citation code: Endnote (RIS) BibTex plain textHTML

critical phase matching of LBO
Figure 1: Critical phase matching of second-harmonic generation in LBO. The polarization directions of fundamental (red) and second-harmonic wave (green) are perpendicular to the beam direction, and to each other.

Critical phase matching (also called angle phase matching) is a technique for obtaining phase matching of some nonlinear process (usually nonlinear frequency conversion) in a birefringent crystal. The interacting beams are aligned at some angle(s) to the axes of the index ellipsoid. In almost all cases, there are one or two waves linearly polarized along one axis of the index ellipsoid (ordinary beam), while another one or two waves are polarized at some variable angle with the plane spanned by the other two axes (extraordinary beams). Adjustment of the propagation angle affects the refractive index of the extraordinary beam (called extraordinary refractive index), whereas the ordinary index stays constant. For some angular position, phase matching may be achieved, i.e., the phase mismatch vanishes. Effectively, this means that the birefringence exactly cancels the phase mismatch which would result from chromatic dispersion. Note that this would not be possible if all involved beams had the same polarization direction, since one would have no effect of the birefringence.

As an example, Figure 1 shows the beam direction and the polarization directions for phase-matched second-harmonic generation (frequency doubling) in LBO based on the type I scheme with polarizations oo-e in the XY plane. This means that the beam propagates within the XY plane, the fundamental polarization is ordinary (o, here in the Z direction) and the second-harmonic polarization is extraordinary (e, with an angle <$\varphi$> to the Y axis). Figure 2 shows the required parameters. For example, for a pump wavelength of 800 nm the phase-matching angle <$\varphi$> would have to be 31.6°, and the effective nonlinearity would be 0.72 pm/V. The walk-off angle of the second-harmonic beam (not shown in the diagram) would be 16.5 mrad.

critical phase matching of LBO
Figure 2: Phase-matching angle (red, left axis) and effective nonlinearity (blue, right axis) for critical phase matching of frequency doubling in LBO at room temperature, configuration oo-e in the XY plane.

The angle shown is the angle <$\varphi$> in Figure 1. This phase matching scheme is suitable for frequency doubling from 1064 nm to 532 nm, but less so for shorter wavelengths due to the decreasing nonlinearity and the increasing walk-off angle (not shown).

The attribute “critical” comes from the fact that this technique (in contrast to noncritical phase matching) is relatively sensitive to misalignment of the beams. There is only a finite range of beam angles (the acceptance angle, also called angular phase-matching bandwidth) where critical phase matching works; in the example above, it is 0.67 mrad for a 1 cm long crystal. This also implies that the beam divergence must be limited, and that often forces one to use beams with a large beam radius. Efficient nonlinear conversion then requires a higher peak power.

A related problem is the spatial walk-off between ordinary and extraordinary beams, which limits the effective interaction length and may affect both the conversion efficiency and the beam quality. With sufficiently high optical powers, walk-off effects can be minimized by using a short nonlinear crystal and/or large beam diameters.

Nonlinear crystals are often cut such that critical phase matching is possible for close to normal incidence on the input surface. It may be less convenient to use a crystal which needs a substantially non-normal incidence, e.g. because it was designed for use with noncritical phase matching.

A major advantage of critical phase matching is that the crystal temperature can often be close to room temperature, so that a crystal oven is not required. One may only need to stabilize the crystal temperature near room temperature, e.g. using a Peltier element. That also minimizes the required warm-up time.

The above-mentioned limitations of critical phase matching can be severe in some cases, but get less relevant in cases where beams with a high peak power are converted.

Note that critical phase matching (in fact any kind of birefringent phase matching) can work only if the polarization directions of fundamental and second-harmonic light are different. This is not always the case, but there are many configurations where it is so, based on the properties of the nonlinear tensor.

More to Learn

Encyclopedia articles:

Questions and Comments from Users


In Figure 1, phi appears two times, but not theta. Is that correct?

The author's answer:

Yes, it is. The diagram indicates that <$\varphi$> is the angle between the X axis and the beam direction, and also between the Y axis and the polarization of the second-harmonic wave. In this case, we have <$\theta$> = 90°.


You refer to this diagram as illustrating critical phase matching, even though the phase matching angle theta here is 90 degrees. This is, in fact, what is known as non-critical phase matching.

The author's answer:

No, it is critical phase matching. While <$\theta$> = 90°, we also have <$\varphi$> not being 0 or 90°.

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

Spam check:

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.


Share this with your network:

Follow our specific LinkedIn pages for more insights and updates: