Femtosecond Z-Scan Spectrometer
| Return to Research Tool Menu |
Background
The Femtosecond Z-Scan Spectrometer is used to measure extremely fast non-linear absorption and non-linear refraction. This is used to estimate the electro-optic coefficient and Kerr non-linearity.
Significance
Self Focusing or De-focusing
The closed aperture z-scan is based on the self-focusing effect. The index of refraction of a third order material changes as the intensity of the beam changes. As a sample through a focused beam the intensity of incident light increases to a point where the sample changes its index of refraction and alters the focus of the light. The width of the beam determines how much of the light passes through an aperture which can be measured with a simple detector.
Non linear Transmission
Non-linear materials may also exhibit non-linear transmission or absorption meaning that as the intensity of the light pass through increases the transmittance decreases. For this measurement a lens is used (instead of an aperture) to collect all the light passing through the sample. This may be due to two photon absorption or other non-linear processes. The z-scan method can be used for non-fluorescent samples. This property is useful in developing protective goggles that have optical limiting. These would prevent extremely intense laser light from passing through.
Sometimes samples exhibit both non-linear refraction and non-linear transmission. The self-focusing materials with high non-linearity of transmission would have curves that are distorted from those ideal ones presented above.
In this simulation use the green bar to adjust the z position of the sample in the focused laser beam. The curve is representative of a material with a positive nonlinear refractive index; you'd get a curve with a mirror reflection of this for a negative refractive index material.
<swf width="600" height="400">images/6/69/Z-scan.swf</swf>
Operation
The laser beam passes through a pinhole spatial filter to limit eliminate transverse wave modes and attain a tight gaussian beam profile.