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

As a third order materials passes through an focused light beam its index of refraction changes causing a focusing or defocusing of the beam.

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 absorption.

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.

Z-Scan Spectrometer Simulation

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 an optical parametric amplifier to select for a desired wavelength and then a pinhole spatial filter to limit eliminate transverse wave modes and attain a tight gaussian beam profile.

<embed_document width="100%" height="400">images/c/cd/Zscan.pdf</embed_document>

External Links

• wikipedia:Z-scan technique
• Encyclopedia of Laser Physics and Technology
• Femtosecond Spectroscopy- UCSB